Potential impacts of ammonia/hydrogen on engine lubricants: A review

Abstract

As intrinsically carbon-free molecules, ammonia and hydrogen are considered as fuels for internal combustion engines, mainly for long-distance or off-road applications. These alternative fuels have different combustion characteristics, reactivity, and exhaust gas compositions compared to conventional fuels, raising questions about the suitability of lubricants in engines operating with them. The impact of ammonia, hydrogen, and their blends on lubricants in internal combustion engines is a relatively new topic, with few reference studies available. However, degradation processes of lubricants have been studied in the context of hydrocarbon fuels, and in compressors using ammonia as a refrigerant, for example. This work presents a review of the literature on engine oil degradation phenomena in relation to ammonia and hydrogen combustion characteristics. In particular, it highlights the current state of knowledge regarding compatibility with unburnt gases, elevated nitrogen oxide levels, and water. Additionally, it summarizes the latest insights into the contribution of lubricants to pollutant emissions.

Graphical abstract

Keywords

lubricant / engine oil / hydrogen engine / ammonia engine

1 Introduction

With 8.24 Gt CO₂ emitted in 2019 (pre-COVID), the transport sector is responsible for around 25% of total CO₂ emissions worldwide [1]. This sector also contributes to NO_x and particles emissions which are harmful to both the environment and human health [2]. In this context, research into electrification and alternative fuels is drawing increasing attention from both scientists and industries. Although batteries and electrical engines seem a promising solution for individual vehicles, internal combustion engines with carbon-free fuels remain more suitable for certain applications, such as ships or off-road machinery [3]. These engines not only eliminate the need for precious metals, but also easily meet the demand for fast refueling and long operational autonomy.

Hydrogen, as a versatile energy carrier and carbon-free fuel, is considered a key element for decarbonization, although it faces significant challenges, such as storage and transportation [3,4]. Ammonia, a hydrogen-based compound, is increasingly considered as an efficient hydrogen carrier [4,5]. It can be stored in a much easier, thus cheaper, way, either at −33 °C at atmospheric pressure, or at 10 bar at ambient temperature. Compared to hydrogen, it has a higher volumetric energy density, with 11.3 GJ/m³ at 1.1 MPa/300 K, compared to 4.7 GJ/m³ at 70 MPa [6]. Ammonia can also be used directly as a fuel itself, either pure or blended with co-fuels, in gas turbines and internal combustion engines [7]. For these reasons, it is gaining attention as a potential means of reducing emissions in specific energy sub-sectors, particularly in power generation and maritime applications. It is currently the second most commercialized chemical product in the world [8], which means that substantial infrastructure for its production, storage, and transport is already available. These characteristics make it a fuel of particular interest for the marine sector.

In the “Net Zero Carbon” scenario of the International Energy Agency (IEA), ammonia is projected to account for 25% to 50% of the total energy demand in the shipping industry by 2050 [9]. Major engines manufacturers are planning to commercialize ammonia-powered two-stroke engines for containerships by the end of 2025/2026. To date, the production of both hydrogen and ammonia has primarily relied on fossil fuels. However, various decarbonized production pathways are currently under development, as detailed in the review by Valera et al. [5]. State-of-the art and current advances in electrochemical ammonia synthesis technologies are reviewed in Cao et al. [10].

Hydrogen and ammonia can be directly used as fuels, but with several differences compared to conventional hydrocarbon fuels [5]. Existing internal combustion engines have to be adapted to accommodate these new fuels. While an increasing number of studies have focused on optimizing engine design, combustion conditions, and injection parameters [6,11,12], some compatibilities must be assessed for these molecules with a particular reactivity, as with the lubricant.

In internal combustion engines, lubrication is a key challenge to ensure moderate fuel consumption, high energetic yield, and durability of the engine. Engine oils are complex products, with formulations tailored to specific types of engines. Their lifetime is usually determined by their resistance to oxidation and consumption rate. Oxidation, the major degradation mechanism, occurs due to oil exposure to oxygen and combustion by-products [13]. This process introduces impurities, degrades lubricant integrity, and modifies its friction-reducing ability. New fuels impose new constraints, and their effect on lubricant integrity and properties must be studied to guarantee proper function and reasonable drain intervals.

The effect of ammonia, hydrogen and their blends on lubricants in internal combustion engines is a relatively new area of study [14,15]. Yet, lubricants degradation processes have been explored in other contexts, including engines running on conventional fuels [13,16] and compressors using ammonia as a refrigerant [17]. Measurements at the exhaust of ammonia test engines conducted by Lhuillier et al. [6], Mercier et al. [18], Dupuy et al. [19], Mounaïm-Rousselle et al. [20] show high levels of NO_x and unburnt ammonia, in addition to substantial water content. Hence, lubricants are likely to be in contact with these species on the cylinder walls or in the crankcase due to blow-by mechanism. This can contribute to the degradation of the lubricant [14,21,22].

Indeed, similarly to oxygen-driven oxidation, NO_x can solubilize in oils and react with hydrocarbons, generating radicals [22]. This “nitro-oxidation” process leads to functionalization of hydrocarbon chains with ester nitrate groups (RONO₂) [23], which can affect the viscosity and result in sludge and deposits. Even though this phenomenon is recognized by lubricants manufacturers and is controlled with specific standards [24], it is yet not fully understood and deserves to be further studied. Nitro-oxidation could be reduced by choosing conditions where NO_x emissions are the lowest. Unfortunately, several studies show that limiting NO_x and unburnt ammonia levels at the exhaust is a trade-off [6,25].

The interaction between oil and ammonia was mainly studied in the context of ammonia compressors in refrigeration systems [17,26]. It appears that ammonia, as a polar molecule, can be solubilized in oils according to their polarity, causing a viscosity decrease [26]. Furthermore, dissolved ammonia may act as a base and react with acidic compounds in the oil—acidic functions of the base oil itself, acidic additives, or oxidation products—and form sludge and deposits, as reported by manufacturers [17,27]. Recent studies [14,28] examined this potential reactivity between engine oil and ammonia under conditions closer to those in engines. Agocs et al. [14] reported aminification and increased deposit tendencies of engine oils exposed to air contaminated with 1000 ppm and 21.7% NH₃.

Additionally, water produced during combustion can travel to the crankcase and condensate on cooler surfaces. Emulsification issues were reported in hydrogen and ammonia engine prototypes, though the consequences for lubricant integrity and friction properties remain unclear [29]. In the marine sector, water contamination is well-known as it can degrade the lubricant and damage engine itself through oxidation and corrosion, bacterial growth, and cavitation [30]. If water droplets enter the oil circulation flow, they can destabilize the oil film and decrease the friction-reduction ability of the lubricant [30]. Like ammonia, water uptake seems to depend on the polarity of the base oil and its additives [29]. All these factors are likely to affect lubricant properties, reducing their service life.

Lastly, since lubricants are partially burned and contribute to particles emissions [31], changes in their properties could influence overall pollutant emissions, which is a key concern for the development of new, cleaner engines.

This review outlines the role of lubricants, how they are solicited and exposed to fuels and exhaust gases in standard engines, and the specific challenges posed by hydrogen and ammonia. High levels of nitrogen oxides, residues of unburnt gases, compatibility with water, and pollutants emissions are identified as the main challenges for lubricants in engines powered by blends of ammonia and hydrogen. The state-of-the-art regarding each degradation mechanism is successively detailed.

2 Lubricants in internal combustion engines

2.1 Roles of lubrication

In internal combustion engines, the lubrication system operates as a closed loop, with engine oil continuously circulating. It is collected in the crankcase, filtered through a strainer, and pumped to the top of the engine, from where it returns by gravity. A fraction of the oil is also injected directly under the piston crown through a special nozzle. A heat exchanger can be installed in the crankcase to maintain an average oil temperature of around 100 °C (Fig.1).

In addition to preventing wear, lubricating oils play several critical roles essential for the optimised operation of engines (Fig.1) [33,34]:

• Preventing impurities deposition in the form of sludge or varnish

• Protecting surfaces against corrosion

• Cooling the engine

Lubricants are thus high-technology products. They must maintain effective friction-reducing properties under severe mechanical stress and varying pressure and temperature conditions, while also fulfilling these complementary key roles for a long period of time. Some of these properties are directly linked to the hydrocarbon nature of lubricants, while others have to be fulfilled by additives [34].

2.2 Fuel-oil interactions

An internal combustion engine is not a completely sealed system. Several oil and gas transport mechanisms lead to contacts between lubricants and fuels or combustion gases [35].

On the one hand, combustion gases can pass through the sides of the piston and enter the crankcase, carrying oil droplets on the way [35]. This blow-by phenomenon is due to the pressure difference between the combustion chamber and the oil pan (Fig.2(c)). As a result, nitrogen oxides, oxygen, and hydrocarbon residues are transported into the crankcase. Once solubilized, these gases can take part in reactions in the liquid phase, such as oxidation and nitro-oxidation, which degrade the lubricant, as will be discussed further.

On the other hand, as it is distributed near the pistons crown and next to the injection valves, lubricant can be in contact with fuels and exhaust gases, and may even contribute to combustion if transported into the combustion chamber. Several oil consumption mechanisms were identified (Fig.2) [35,36]:

• Spraying of oil into the chamber due to piston inertia (a)

• Drainage of oil accumulated in the piston rings by reverse blow-by (b)

• Evaporation of the oil film (d)

• Leakage around intake valves (e)

Oil droplets can also be carried back to the crankcase by blow-by gases, as depicted in Fig.2(c).

Various parameters that affect the relative contribution of each mechanism under different operating conditions, such as regime, and temperature, have been identified. The amount of lubricant entering the combustion chamber depends on the thickness of the film oil on the cylinder walls, which in turn is influenced by the viscosity of the oil [36]. Besides, evaporation occurs mainly at high loads, probably due to an increase in wall temperature under these conditions [36]. The evaporation rate is also related to the oil’s volatility and the residence time of the oil film on hot surfaces, as might be expected [37].

Any change of these properties can affect oil consumption, engine operation, and pollutant emissions [38,39]. Because of these oil and gas transport mechanisms, engine oils are likely to be in contact with both fuels and combustion gases. Fuel-oil compatibility therefore needs to be addressed at two levels: chemical reactivity in the crankcase (liquid phase), and co-combustion in the chamber. When exploring alternative fuels, fuel-oil compatibility seems to be a critical issue.

2.3 Formulation of lubricants

Lubricants usually consist of 80% of base oil, complemented by 20% of additives, with each component fulfilling one or more specific functions [34].

2.3.1 Base oils: origins and classification

Several types of oils can be used to formulate lubricants. Base oils can have a mineral or a synthetic origin. Mineral oils are petroleum products obtained from the heaviest fractions of atmospheric distillation, while synthetic oils are produced through petrochemical processes. To ensure shared standards, the American Petroleum Institute (API) classifies mineral oils into three types according to their final content of aromatics, sulfur and nitrogen, and their final viscosity [40]. In addition to these three groups, polyalphaolefins (PAO) and other types of synthetic oils, such as esters, alkylbenzenes, polyalkylene glycols, silicons, vegetable oils, are classified in two other groups.

The choice of a base oil determines essential properties of lubricants, such as viscosity, polarity, and chemical stability. These, in turn, are respectively key properties for friction and wear reduction ability, volatility, solvency, and oxidation stability [34]. Yet, performance in terms of tribology, detergency, and lifetime stability can be further improved by incorporating specific additives.

2.3.2 Additives

To improve the performance of base oils and add new functionalities to lubricants, additives are commonly incorporated in their formulations. According to the classification proposed in Minami [41], these additives can be grouped into three categories: those that improve the tribological properties of lubricants, those that enhance their rheological behavior, and those that extend their service life by introducing new functionalities, as summarized in Tab.1. A detailed review of the chemistry and functions of these additives can be found in Refs. [41–46].

Additives are tailored to meet the specific requirements of each application, with some compounds exhibiting multifunctional properties. In engine oils, the main additives usually include aminic and phenolic antioxidants, along with zinc dithiophosphate (ZnDTP) [42]. Oils dedicated to marine engines also contain dispersants and overbased detergents to neutralize acidic species due to the high sulfur content of heavy fuels used in large ships, and deal with water contamination [47].

In this section, the role of lubricants in engine applications and the mechanical, thermal and chemical constraints they endure during their service-time are presented. These constraints are directly related to the fuel used and its combustion byproducts. A brief overview of different oil formulations and types of additives is also provided, pointing out that lubricants’ formulations are complex and adapted to each type of applications. Therefore, the development of alternative fuels raises new issues of fuel/oil compatibility. To adapt lubricant formulations to hydrogen and ammonia, it is necessary to first understand the unique aging mechanisms involved in this type of engine. The following section examines the combustion characteristics of these gaseous fuels and how they differ from conventional fuels, aiming to identify potential malfunctions they can cause in terms of proper engine operation and pollutant emissions.

3 Effect of hydrogenated based fuels combustion on aging of lubricant

3.1 Potential issues related to hydrogen and ammonia combustion characteristics

Tab.2 summarizes the main combustion characteristics of ammonia and hydrogen in comparison to standard gasoline.

Gasoline, ammonia, and hydrogen have markedly different properties in terms of energy density and combustion properties (Tab.2). Therefore, extensive research has been conducted and continues to be undertaken to adapt engine design and operation for ammonia and hydrogen [6,11,48,50]. To achieve maximum engine performance, lubricant formulations must also be tailored to accommodate the unique characteristics of these alternative fuels.

Among the data from Tab.2, several features deserve emphasis. The combustion of hydrogen is characterized by a wide flammability range [11], a high burning velocity [51] and a high flame temperature. In contrast, the combustion of ammonia is more restricted by a narrower flammability range [49] and a higher resistivity to ignition. Hydrogen is known to facilitate ammonia ignition and stabilize its combustion, which is why these two fuels are often considered as complementary co-fuels [6,52–55].

Compared to gasoline, high temperatures and short quenching distance [11] in the case of hydrogen, might increase the thermal degradation of the lubricant film on engine walls, thereby accelerating lubricant degradation and promoting reactivity in the liquid phase.

Another issue with these alternative fuels is the composition of their combustion products. First, for both fuels, water vapor is the principal combustion product, comprising more than 30% of exhaust gas volume under stoichiometric conditions. Second, they can produce large quantities of nitrogen oxides, particularly NO and NO₂ which are highly toxic molecules causing health and environmental issues such as respiratory diseases, and acid rains (e.g., NO₂ reacts with water to form nitric acid, HNO₃) [56]. When in contact with the lubricant, they can also solubilize and react with hydrocarbons leading to sludge accumulation in the crankcase [21], as further detailed in Section 3.4.2. Another relevant emission is nitrous oxide (N₂O), which, despite its limited reactivity with hydrocarbons, has a high global warming potential [2].

The formation of nitrogen oxides occurs mainly from several pathways [57]:

• Thermal-NO_x: Arises from the reaction N₂ + O₂ → 2NO, involving nitrogen atoms from the air. This reaction is initiated by the nitrogen decomposition due to the effect of oxygen radicals. Nitrogen being a highly stable molecule, this can only happen at high temperatures, typically above 1800 K [11].

• Prompt-NO_x: Results from N₂ reacting with CH free radicals formed at very high temperatures, leading to nitrogen containing functions on hydrocarbons and N radicals. Oxides are then formed as fuel-NO_x [57].

Moreover, in the case of nitrogenated fuel:

• Fuel-NO_x: Involves nitrogen atoms directly contained in the fuel (amines, nitriles), which are more reactive than nitrogen molecules, but less concentrated. This reaction can happen at temperatures below 1200 K [58].

In the case of ammonia combustion, De Soete [59] proposed a global kinetic mechanism for NO production from NH₃ oxidation [59] with reactions NH₃ + O₂ → NO + H₂O + 1/2H₂ and NH₃ + NO → N₂ + H₂O + 1/2H₂. According to the calculated reaction rates, this mechanism is responsible for significant NO production even at temperatures below the 1800 K threshold required for the thermal route. More detailed pathways involve NH₂ and NH radicals, and HNO as intermediate species [60,61]. Besides, N₂O can be formed from the reactions NH₂ + NO₂ → N₂O + H₂O, or NH + NO → N₂O + H at temperatures below 1400 K [61]. Westlye et al. [62] and Rahinov et al. [63] showed that the first reaction was dominant in the case of abundant NH₂ production from ammonia pyrolysis. On the contrary, Sako et al. [60] showed that the second reaction was dominant in the case of ammonia/methane/air combustion.

In the case of hydrogen combustion, high temperatures can promote thermal-NO formation. Although in the case of ammonia, one origin of NO_x is attributed to the fuel-NO pathway [50], the addition of H₂ in NH₃ combustion strongly increases NO in lean conditions in Zhang et al. [52] and Hu et al. [53] when the hydrogen fraction reaches 10% and 15%.

Even if optimized post-treatment systems can reduce final NO_x emissions, lubricants are exposed to these species inside the engine, during different phases of the combustion development. Therefore, lubricant degradation due to increased nitrogen oxide concentrations needs to be addressed. Additionally, some unburnt ammonia may also come into direct contact with the lubricant.

In summary, ammonia and hydrogen combustion present several challenges: degradation of the lubricant due to its exposure to higher temperatures, degradation due to high exposure to nitrogen oxides, compatibility with residual hydrogen or ammonia, and compatibility between water formed and engine oil. These concerns are addressed in detail in the following sections, which review the current state of knowledge on each topic.

3.2 Liquid phase oxidation

3.2.1 Oxidation mechanism

The auto-oxidation of hydrocarbons is a free radical chain reaction happening in the liquid phase, with its mechanism differing depending on whether it occurs at low temperature or high temperature. At temperatures below 120 °C, the mechanism can be depicted as follows:

1.\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{s}\mathrm{t}\mathrm{e}\mathrm{p}:\mathrm{R}\mathrm{H}+{\mathrm{O}}_2\to \mathrm{R}•+\mathrm{H}\mathrm{O}\mathrm{O}•

2.\mathrm{P}\mathrm{r}\mathrm{o}\mathrm{p}\mathrm{a}\mathrm{g}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{p}:\mathrm{R}•+{\mathrm{O}}_2\to \mathrm{R}\mathrm{O}\mathrm{O}•

\mathrm{R}\mathrm{O}\mathrm{O}•+\mathrm{R}\mathrm{H}\to \mathrm{R}\mathrm{O}\mathrm{O}\mathrm{H}+\mathrm{R}•

\mathrm{H}\mathrm{O}\mathrm{O}•+\mathrm{R}\mathrm{H}\to {\mathrm{H}}_2{\mathrm{O}}_2+\mathrm{R}•

3.\mathrm{C}\mathrm{h}\mathrm{a}\mathrm{i}\mathrm{n}\mathrm{b}\mathrm{r}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{h}\mathrm{i}\mathrm{n}\mathrm{g}:\mathrm{R}\mathrm{O}\mathrm{O}\mathrm{H}\to \mathrm{R}\mathrm{O}•+ \mathrm{H}\mathrm{O}•

\mathrm{R}\mathrm{O}•+\mathrm{R}\mathrm{H}\to \mathrm{R}\mathrm{O}\mathrm{H}+\mathrm{R}•

\mathrm{H}\mathrm{O}•+\mathrm{R}\mathrm{H}\to {\mathrm{H}}_2 \mathrm{O}+\mathrm{R}•

4.\mathrm{T}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}:\mathrm{R}• +\mathrm{R}•\to \mathrm{R} -\mathrm{R}

\mathrm{R}•+\mathrm{R}\mathrm{O}\mathrm{O}•\to \mathrm{R}\mathrm{O}\mathrm{O}\mathrm{R}

\mathrm{R}\mathrm{O}\mathrm{O}•+\mathrm{R}\mathrm{O}\mathrm{O}•\to \mathrm{R}\mathrm{O}\mathrm{O}\mathrm{R}+{\mathrm{O}}_2

At low temperatures, hydroperoxides production is the rate-limiting step in the oxidation process, typically represented by the reaction ROO• + RH → ROOH + R• [13,64]. Above this temperature, both hydroperoxides production and decomposition are much faster. The oxidation process can then be divided into two distinct stages [13]:

• Primary phase: Initiation and propagation reactions accelerate compared to low temperatures. This phase is characterized by the proliferation of HO• radicals and formation of light products such as carboxylic acids, esters, and ethers (the latter formed via intramolecular reactions)

• Secondary phase: This stage involves the polycondensation of bifunctional oxygenated intermediates and subsequent polymerisation, leading to the formation of high-molecular-weight degradation products.

Fig.3 provides a schematic summary of these different processes [13].

The liquid-phase oxidation process produces a variety of products, including ketones, acids, aldehydes, alcohols, heavy hydrocarbons. While light oxidation products can be formed and evaporate, heavy products and low energy interaction between polar products can increase the viscosity of the oil and/or lead to the formation of sludge, varnishes, or deposits on the hottest surfaces [13]. Acid compounds formed tend to make the oil corrosive toward the metallic high-temperature engine surfaces [42,64]. Suspended metal particles can then act as catalysts, self-accelerating the process. To prevent this phenomenon and increase the lubricant effectiveness, different kinds of antioxidant additives with complementary mechanisms of action are employed, as detailed in Ref. [42–44].

3.2.2 Oxidation parameters

Different parameters influence the oxidation rate of lubricants, including the reactivity of the base oil, temperature, oxygen availability and the presence of catalysts [13,42,64,65]. Therefore, hydrogen and ammonia combustion characteristics might change some of these parameters influencing the oxidation rate.

Mathai et al. [66] found that the addition of 18% hydrogen into natural gas resulted in a significant change in the viscosity of the lubricant compared to the reference conditions over a 60-h running time in an engine test bench. They also observed a faster increase in the total acid number (TAN), which means a faster accumulation of acidic oxidation products. These changes were accompanied by a higher concentration of wear metals, which the authors attributed to an increase in combustion temperature.

High temperatures, high exposure to oxygen, and increased mechanical wear result in a higher oxidation rate. While the chemical nature of the base oil does not change with alternative fuels, all other parameters are likely to be different from conventional fuels and modify the oxidation kinetics. Moreover, lubricant aging due to oxidation can even be more severe in the case of engine running on hydrogen or ammonia, due to the presence of nitrogen oxides as combustion by-products.

3.3 Nitro-oxidation

3.3.1 NO_x production

Nitro-oxidation is another free-radical degradation reaction caused by the reactivity of nitrogen oxides with hydrocarbon chains, with ester nitrates (RONO₂) as key intermediates, acting similarly as hydroperoxides in oxidation [21]. Nitrogen oxides can react with unburned fuel residues around the injectors, promoting sludge formation [67], and can also react directly with the lubricant in the cylinder or if they are transported by blow-by gases (Section 2.2). The detrimental impact of nitrogen oxides on both fuel and oils, as well as on the accumulation of deposits have long been recognized by lubricant and engine manufacturers, as it has been studied since the first half of the 20th century [68]. However, knowledge is missing on the precise reaction mechanism and relative contribution compared to oxidation.

As hydrogen combustion induces high temperatures (Tab.2), this can lead to high thermal NO_x production (Section 3.1). However, this can be mitigated by operating under very lean equivalence ratio, (i.e., with a very large excess of air compared to hydrogen [11]) or by using high dilution strategies. Doing so, the flame temperature is reduced and nitrogen oxide production can be almost reduced to zero, by preventing N₂ dissociation. Verhelst and Wallner [11], found that for equivalence ratios below 0.5, NO_x emissions were almost negligible, but increased exponentially above this threshold in correlation with the rising combustion temperature. With hydrogen, nitrogen oxides can then be reduced or even avoided by selecting appropriate combustion conditions.

Meanwhile with ammonia, as the fuel itself contains nitrogen atoms, the production of fuel-NO_x could be significant and difficult to reduce in the context of engines [50,52,53,69], without an excess of unburnt ammonia. It has to be noticed that in ammonia-diesel dual-fuel engines, NO and NO₂ emissions are less important than with diesel fuel, when the ammonia fraction remains below 40% [61,69]. This reduction is attributed to both decreased combustion temperature with ammonia addition and potential de-NO_x reactions. Tab.3 summarizes NO_x emissions reported in different studies, with measurements conducted using in-line Fourier transform infrared (FTIR) spectroscopy at exhaust [6,19,20,52,70] and dedicated gas analyzers [48,50,58].

Although absolute NOₓ emission values obtained under different operating conditions and engines types cannot be directly compared, a consistent trend across studies shows that lower NO emissions are achieved with rich fuel-air mixtures (ϕ > 1) for premixed ammonia/air SI engines. Valera-Medina et al. [71] attributed this to the promotion of the termination reaction NH₂ + NO → H₂O + N₂, which suppresses radical chain propagation and limits NO formation. Besides, Li et al. [50] showed that although fuel-bound NO_x becomes the dominant source as the ammonia content increases in ammonia/hydrogen blends, the overall amount of NO_x emissions decreases as the flame temperature decreases. Similar conclusions were reported by Lhuillier et al. [6], who observed a consistent reduction in NOₓ across a wide range of NH₃ contents in the mixture (from 20% to 100% by volume).

It has to be reminded that comparable NOₓ concentration ranging between 200 and 2000 ppm could also be observed in gasoline and diesel engines under varying conditions, as reported by Pochopien in 2012 [22]. Mounaïm-Rousselle et al. [20] compared the maximum NO_x level of 3750 ppm for a NH₃-H₂/90-10 fuel blend with ϕ = 0.9, to three times what is emitted with a similar engine fueled with gasoline. Hiraoka et al. [69] measured NO + NO₂ levels 2.5 times higher with a 5-95/diesel-ammonia blend compared to neat diesel.

Regarding the speciation of these nitrogen oxides, several studies show that NO is dominant at the exhaust with ammonia. For instance, in Mercier et al. [18], among the 4200 ppm of the total NO_x measured with pure NH₃ fuel, only 70 ppm was NO₂ and 35 ppm was N₂O. Similarly, in Mounaïm-Rousselle et al. [20], the NO₂/NO ratio ranged between 0.02% and 0.75% of the total NO_x emissions at ϕ = 0.9 and 1.1 respectively. Xiang et al. [61] recorded a maximum NO₂ fraction of 10.9% compared to NO in a diesel-ammonia dual-fuel engine, while Westlye et al. [62] reported a fraction of 3%–4% NO₂ in a hydrogen/ammonia engine.

3.3.2 Structure of nitrogen oxides

NO and NO₂ have a radical structure by nature (Fig.4), which explains their high reactivity, especially in interactions with other radicals [72].

In contrast, N₂O does not have a radical structure. Controlling its emissions is a challenge, as this gas has a global warming potential 265 times higher than CO₂ [20], but no studies have investigated its potential reactivity with hydrocarbons, especially lubricants.

3.3.3 Nitro-oxidation reaction

The chemical mechanism by which nitrogen oxides degrade lubricants is not yet fully understood. Many studies assess the degradation consequences of nitrogen oxides on lubricants [14,16], but few directly explore the underlying chemical reactions involving NO_x and hydrocarbons. Some possible reactions involving NO, NO₂, hydrocarbons and oxidation intermediates can be found in Ref. [21,22,74,75].

From kinetics and thermodynamic standpoints, authors as Johnson and Korcek [21] and Coultas [23] proposed theoretical models. Johnson and Korcek [21] conducted experiments in a batch reactor at 160 °C, using a continuous gas flow of 20% O₂ and 0 to 792 ppm NO₂ over hexadecane for up to 3 h, and analyzed exhaust gases. They also monitored the hydroperoxide concentration in the liquid phase by iodometry. However, they did not identify reaction intermediates or products analytically, limiting the validation of their mechanism. Coultas [23] used FTIR to quantify nitration products in lubricants used in real engines at different temperatures. However, this methodology was not able to identify key species precisely.

Later, the theoretical mechanisms have been confronted to experimental results and product identification thanks to the work of Pochopien [22] and Harris [72]. Pochopien [22] studied the reactivity of phenolic and aminic antioxidants in squalane, using an autoclave at 180 °C, exposed to 100% O₂, 1000 ppm NO₂/N₂, and 1000 ppm NO/N₂. Reaction products were identified using GC × GC (gas chromatography) coupled with a nitrogen chemiluminescence detector (NCD) [22]. Harris [72] studied the reactivity of squalane directly at 150 °C, using a similar methodology [72]. Squalane (a C₃₀ hydrocarbon) is considered a good surrogate for lubricants as it has a high molecular weight and a similar ratio of tertiary carbons [22]. Recently, Slavchov et al. [67] explored the reactivity between fuel and nitrogen oxides. Their work enabled a better understanding of nitro-oxidation process [67]. They chose isooctane as a surrogate, as their work was focused on fuel reactivity, and used a pressurized autoclave at 160 °C, 10 bar to reproduce the injector nozzle conditions. Conclusions from these different studies are presented below, according to the different stages of the radical reaction.

3.3.3.1 Initiation

As for oxidation, the radical chain reaction in nitro-oxidation starts with the initiation of a radical, by proton abstraction from a hydrocarbon chain. This initiation step is rate-limiting, as it has a high activation energy [72]. Whether the abstraction is initiated by O₂ or NO₂, the most likely proton to be removed is the one which will result in the formation of the most stable radical, i.e., on tertiary carbons atoms (Section 3.2.2) [22,72].

There seems to be consensus on the fact that proton abstraction is caused by NO₂ rather than NO directly under engine-relevant conditions [67,72]. However, NO can still indirectly contribute to the initiation, by being oxidized to NO₂ via the reaction 2NO + O₂ → 2NO₂, as exhaust gases travel to colder, oxygen-rich zones [21]. Besides, according to Slavchov et al., [76], NO₂ has a higher rate of radical initiation compared to O₂ in the temperature range of 100–220 °C. This is consistent with the reduction in inhibition time observed in the presence of nitrogen oxides in Johnson and Korcek [21], both with and without antioxidants.

The nitrous acid formed during this proton abstraction is highly unstable [21] and readily decomposes into two radicals, thereby propagating the chain reaction.

The initiation step can then be written as

\mathrm{R}+{\mathrm{N}\mathrm{O}}_2\to \mathrm{R}•+\mathrm{H}\mathrm{O}\mathrm{N}\mathrm{O}

\mathrm{H}\mathrm{O}\mathrm{N}\mathrm{O}\to \mathrm{H}\mathrm{O}•+\mathrm{N}\mathrm{O}

3.3.3.2 Propagation and branching

For the propagation step, based on the products identified by gas chromatography, Harris [72] proposed the reaction R• + NO₂ → RONO → RO• + NO. Although alkyl nitrites (RONO) are highly unstable and could not be detected in his study, the subsequent formation of RO• radicals due to its decay is supported by the identification of secondary oxidation products such as ketones. These ketones arise from alkoxy radicals which undergo β-scission.

Based on the measurement of the concentration of NO and NO₂ in the exhaust gas of a batch reactor at 150 °C, Johnson and Korcek [21] found that this reaction R• + NO₂ → RONO or RNO₂ was likely to happen, but only without oxygen. With oxygen in the gas flow exposed to hexadecane, the measured consumption rates of NO and NO₂ no longer aligned with this reaction pathway, or only to a minor extent. As a radical reaction rate is determined by the probability of a collision between the radicals, R• are more likely to meet O₂ molecules, which are more concentrated, than NO₂ in the liquid phase. Then, in the presence of oxygen, they propose that propagation reaction is dominated by R• + O₂ → RO₂•, resulting in peroxy radicals.

This would mean that radicals initially generated by nitrogen oxide initiation also contribute to hydroperoxide formation, thereby accelerating the oxidation process. This hypothesis was followed by Coultas, who proposed the following simplified mechanism, showing how oxidation and nitro-oxidation are entangled [23].

1.\mathrm{R}\mathrm{H}+{\mathrm{N}\mathrm{O}}_2\to \mathrm{R}•+{\mathrm{H}\mathrm{N}\mathrm{O}}_2

2.\mathrm{H}\mathrm{O}\mathrm{N}\mathrm{O}\to \mathrm{H}\mathrm{O}•+\mathrm{N}\mathrm{O}

3.\mathrm{R}•+{\mathrm{O}}_2\to \mathrm{R}\mathrm{O}\mathrm{O}•

4.\mathrm{R}\mathrm{O}\mathrm{O}•+\mathrm{R}\mathrm{H}\to \mathrm{R}\mathrm{O}\mathrm{O}\mathrm{H}+\mathrm{R}•

5.\mathrm{R}\mathrm{O}\mathrm{O}\mathrm{H}\to \mathrm{R}\mathrm{O}•+\mathrm{H}\mathrm{O}•

6.\mathrm{R}\mathrm{O}•+{\mathrm{N}\mathrm{O}}_2 \to {\mathrm{R}\mathrm{O}\mathrm{N}\mathrm{O}}_2

Recent experimental findings support this hypothesis. Agocs et al. [14] found that adding 1000 ppm of nitrogen dioxide to synthetic air and exposing commercial oil to this gas flow at 170 °C resulted in accelerated aging of the oil compared to exposure to air alone. This was evidenced by higher concentration of acidic compounds and a greater increase in viscosity, even though nitrated products could not be identified.

Field tests conducted by Coultas [23] at varying oil temperatures further supported this conclusion. He found that the nitration peak decreased between experiments conducted at 130 and 150° C, while the oxidation peak continued to increase. He proposed the following temperature-dependent mechanism, depicted in Fig.5:

• Below 110 °C, only few RO• radicals are formed, resulting in slow ester nitrates production.

• Around 130 °C, a lot of ester nitrates are formed, and have a long lifetime, leading to their accumulation in the oil.

• Above 150 °C, a lot of RO• radicals are generated, accelerating ester nitrate formation. However, these compounds decompose quickly due to the high temperatures, making them more difficult to detect with IR spectroscopy.

Precaution must be taken as tests at different temperatures were conducted with different engine types. Nevertheless, Besser et al. [77] also found that the nitration peak was decreasing after the oil was heat-treated at 170 °C for 2 h, while oxidation continued to increase. This finding is consistent with the hypothesis that ester nitrates decompose rapidly at higher temperatures, whereas oxidation products are more thermally stable. In Dörr et al. [16], tests conducted in a passenger car for 20000 km with oil samples collected every 1000 to 3000 km, showed that oxidation and nitration increased in parallel. It might be due to the fact that engine temperatures remain below 150 °C during the field test, thereby allowing ester nitrates to accumulate without rapid thermal degradation.

While NO is generally considered unreactive toward hydrocarbons in standard engine conditions, it can still participate in radical chain reactions by reacting with peroxy radicals (RO₂•), regenerating NO₂ through the reaction: RO₂• + NO → RO• + NO₂.

This pathway is supported by Slavchov et al. [67], who studied degradation of isooctane in an autoclave at 160 °C and 10 bar. Their gas chromatography analysis revealed the presence of primary and secondary alcohols as degradation products. If these alcohols were derived from peroxy radicals formed via R• + O₂ → RO₂•, they would likely be tertiary alcohols, due to the selectivity of the initial proton abstraction. However, as the selectivity toward tertiary alcohols is much higher with NO₂ as oxidizing gas than with NO, it suggests that alkoxy radicals were produced primarily through the reaction RO₂• + NO → RO• + NO₂.

Johnson and Korcek [21] also found evidence of this reaction, as they could not observe any measurable NO accumulation despite the nitrous acid decomposition. This suggests that nitrogen monoxide is actively consumed in the system, likely through its reaction with RO₂•. In parallel, HO• radicals formed by decomposition of nitrous acid can be involved in the oxidation reaction HO• + RH → H₂O + R•, thereby propagating the radical chain reaction on another alkyl chain.

3.3.3.3 Termination

The termination products formed during nitro-oxidation depend heavily on the previous reaction pathways and experimental conditions. R• radicals may recombine to form alkanes of various chain lengths, but they can also react with alkoxy and peroxy radicals, or with nitrogen dioxide, or nitrogen monoxide, producing a wide range of functionalized molecules. Seeing that almost all of the nitrogen dioxide (180 ppm NO₂, 10 ppm NO in pure N₂) was retained by hexadecane, Johnson et al. [21] concluded that it was retained in nitrogen-containing termination products such as alkyl nitrites and ester nitrates, but could not be isolated or definitively identified.

Interestingly, Slavchov et al. [67] did not find any nitrogen-containing species after 2 h at 150 °C when they used a gas flow composed of 5% O₂ and 100 ppm NO₂. They could only identify nitrogen-containing products when 500 ppm of NO was added to the gas flow. Using GC-MS (mass spectrometry), they identified nitromethane, methylethyl and 3,3-dimethlypropylesters of nitric acid, 2,2,4-trimethyl-4-nitropentane, nitromethane and 2-methyl-2-nitropentane. Nitroalkanes are supposed to come from the reaction R• + NO₂ → RNO₂, while ester nitrates likely from the reaction ROO• + NO → ROONO → RONO₂ [67,78]. They concluded that this termination reaction is particularly important, as it scavenges two radicals simultaneously. It could have a strong impact on the nitro-oxidation rate. However, this is compensated by the decomposition ROO• + NO → ROONO → RO• + NO₂, which regenerates new radicals and perpetuates the chain reaction [67].

Using GCxGC-TOF-MS (Time-of-flight) to study the liquid phase of squalane exposed to 1000 ppm NO₂ and 10% O₂ for 2 h, Harris [72] was not able to detect nitrogen-containing products but only found alkanes, alcohols, ketones, and lactones. However, nitroalkanes could be detected despite the absence of NO thanks to a nitrogen chemiluminescence detector (NCD) which has a detection level below ppb. This highlights the challenge of detection limits and choosing appropriate detection techniques. This result questions the fact that Slavchov et al. [67] did not find any nitrogen-containing products with NO₂ with the protocol they used.

Interestingly, Slavchov et al. [67] also observed that degradation occurred in the presence of 100 ppm NO₂ and 5% O₂, whereas 5% O₂ alone did not result in any degradation. This confirms that NO₂ still participates in oil accelerated degradation compared to pure oxygen, even if no nitrogen-containing products are identified in the end [67]. Combining the findings of Harris and Slavchov, it appears that products with nitro groups are less abundant than oxygenated products in the presence of NO₂ only, making them hard to detect. This might be explained by the fact that termination products containing nitrogen involve mainly NO molecules, which have to be produced by the reaction 2NO₂ → 2NO + O₂, in the absence of oxygen [21].

Thus, termination reactions directly involving NO₂, such as NO₂ + R• → RONO or RNO₂ and NO₂ + RO• → RONO₂ are unlikely to occur. Nonetheless, NO₂ still participates in initiating radicals and accelerates the oxidation rate. Interestingly, Harris [72] found that squalane exposed to NO₂ without oxygen resulted in less degradation products than when NO₂ and O₂ are combined. Fig.6 shows the results obtained, the large spot corresponds to unreacted squalane, while small spots to degradation products. This is consistent with the fact that NO₂ reacts with oxidation intermediates and accelerates the reaction, even though no nitrogen compounds can be found in the end.

In summary, the results of the studies reviewed above show that NO₂ readily abstracts protons from hydrocarbon chains to initiate radical reactions. It participates in the formation of ester nitrates as intermediates and can also participate in forming hydroperoxides. As the reaction proceeds, oxidation products accumulate rapidly, which suggests that oxidation takes over, even if nitrogen dioxide can be responsible for the chain initiation. Meanwhile, NO can react with intermediates to form nitroalkanes and ester nitrates, or regenerate NO₂.

Overall, oxidation and nitro-oxidation appear to be tightly coupled processes, and should be studied in conjunction rather than in isolation.

Despite progress, several challenges remain in fully understanding the nitro-oxidation mechanism. Precise kinetics and energy calculations would allow for better predicting the reactions and intermediates species. Experimental studies should consider both NO and NO₂, as they seem to play different roles. Besides, it is useful to have a good understanding of the combustion kinetics to predict their relative composition in the blow-by gases. Finally, analytical techniques are a key issue to study this topic, as detecting unstable intermediates or products with low concentrations determines the hypothesis which can be made.

Together, addressing these points will enhance the predictive power of degradation models and support the development of more robust lubricant formulations for engines operating with alternative fuels.

3.3.4 Impact of oxidation and nitro-oxidation on additives

Nitro-oxidation and oxidation degrade lubricant integrity via reactions involving hydrocarbons. Depending on the conditions, these processes can lead to changes in the viscosity, sludge or varnish deposition, altering the performance of oils. Beyond hydrocarbon degradation, several studies have also attempted to evaluate whether nitro-oxidation could also affect tribo-improver additives, such as molybdenum dithiocarbamate (MoDTC) and zinc dialkyldithiophosphate (ZnDTP) in parallel to altering hydrocarbon chains.

De Bouchet et al. [79] studied the effects of nitro-oxidation and oxidation consequences on MoDTC and ZnDTP additives using X-ray photoelectronic spectroscopy (XPS) and liquid chromatography. Tribological performances were assessed with a rolling ball on a disk tribometer. They showed that these additives were also affected by oxidation and nitro-oxidation, which led to ligand exchange between the two additives. After 12 h of exposure in a 140 °C bath under an oxygen and nitrogen dioxide atmosphere, additives concentration decreased by 80%. Higher friction coefficient but medium wear was detected. They suggested that some compounds had lost their sulfur atoms, cancelling their friction reduction ability. After 16 h, additive depletion was completed which led to poor performance of the lubricant.

More recently, and Dörr et al. [16] and Besser et al. [77] studied bench and in-field aging of ZnDTP in gasoline engines. Using GC-MS, they identified more precisely ZnDTP and degradation products over different time scales. They confirmed that this component was also oxidized, leading to sulfur substitution by oxygen atoms and alkyl chains loss, as suggested in De Barros et al. [79]. Additionally, they showed that free sulfur atoms could recombine to form sulfuric acid (Fig.7). Fresh oil had the lowest friction coefficient and wear, while aging below 8 h led to reduced wear compared to oil in the fresh state. However, extensive wear was observed after two days of artificial aging, attributed to the corrosive effects of phosphoric and sulfuric acids formed by additive depletion.

In related experiments, aged lubricants originally containing MoDTC had the same friction coefficient as their base oil without any additives after similar aging protocol as reported in Fuller et al. [80]. This confirms that additive depletion, especially the loss of sulfur-containing functionalities, leads to substantial loss of friction-reducing capability.

Numerous studies on this topic concluded that friction modifiers such as MoDTC and ZnDTP act as antioxidants and are gradually consumed during lubricant aging. The tribological performance of the lubricant is then degraded with time, as these additives can no longer form low frictions films. One study tried to isolate the role of oxidation versus nitro-oxidation in this process by varying the relative concentration of gases in NO_x/air atmospheres [82]. Interestingly, it is found that at high temperatures (aging at 160 °C), NO_x concentration had no significant influence on the rate of friction loss, suggesting oxidation is the major cause of degradation under these conditions. In contrast, NO_x concentration was highly correlated to the loss of friction properties at 100 °C [82]. This means that at low temperatures, radical generation by NO_x species is a relative important degradation mechanism, while at high temperatures, oxidation happens so fast that nitro-oxidation is no longer dominant.

Pochopien [22] studied the impact of NO₂ on phenolic and aminic antioxidants by exposing squalane enriched with antioxidants to 1000 ppm NO₂ in N₂ at 180 °C. She found that the phenolic antioxidant octadecyl 3-(3,5-di-tert-butyl-4-oxo-cyclohexa-2,5-dien-1-ylidene) propanoate used alone provided the longest inhibition period (130 min), outperforming the aminic antioxidant 4.4’-dioctyl diphenylamine (60 min) and even a mix of both (90 min) [22]. The reaction between NO₂ and the phenolic antioxidant resulted in quinone derivatives due to NO₂ addition followed by β-scission, as nitro-phenols were unstable at 180 °C. The aminic antioxidant resulted in nitrated amines and cyclo-imines. Two new products were obtained when both kinds of antioxidants were combined, suggesting simultaneous inhibition pathways.

Both nitro-oxidation and oxidation have negative effects on base oils and their additives. However, the relative contribution of these processes to overall degradation is not fully understood, and in the case of nitro-oxidation, no consensus exists on the exact chemical mechanism. Considering that large amounts of NO_x are expected to be in contact with lubricants with hydrogen and/or ammonia combustion, a better understanding of nitro-oxidation process is essential.

3.4 Compatibility with unburnt fuel

3.4.1 Compatibility with ammonia

Nitro-oxidation could be reduced by choosing operating conditions where NO_x production is the lowest. Unfortunately, several studies showed that limiting NO_x and unburnt ammonia levels at the exhaust remains a trade-off: increasing the equivalence ratio decreases NO_x emissions, while increasing the fraction of ammonia in the exhaust [6,18,20,52,69,83]. According to Westlye et al. [62], NH₃ emissions are due to the accumulation of the gas in the crevice volume of the combustion chamber. Liu et al. [84] also suggest that the ammonia measured at the exhaust might come from its desorption from the lubricant film.

Observed concentrations of residual ammonia were found to vary from 1000 to 16000 ppmv in Lhuillier et al. [6], depending on conditions: NH₃/H₂-40/60 fuel blend, ϕ = 0.6 and NH₃/H₂-95/5 fuel blend, ϕ = 1.2, at the exhaust of a monocylinder at 1500 r/min. Consistently, NH₃ at the exhaust was found to be between 0.7% (NH₃/H₂-90/10 fuel blend, ϕ = 0.9) and 1% (pure NH₃ ϕ = 1.1) in Mounaïm-Rousselle et al. [20]. In another study with a monocylinder operated at 1000 r/min [19], NH₃ at the exhaust was between 1% from equivalence ratios between 0.7 and 1 with ammonia/diesel-98/2 blends, and reached 5.5% at ϕ = 0.45 and with 2.5% diesel. In Zhang et al. [52], adding 15% of hydrogen led to cut NH₃ emissions in half, ranging from 7000 to 3000 ppm, in a single-cylinder at 1000 r/min.

Ammonia concentrations in the exhaust are an indicator of the ammonia which can be in contact with lubricants. Obrecht et al. [28] highlighted this in a 1.5 L 4-cylinder engine test bench fueled with an 85/15 ammonia/diesel blend, where they measured concentrations of 8910 ppm of ammonia in the exhaust and 22890 ppm in the blow-by circuit at 2000 r/min [28].

Unfortunately, unburnt ammonia in the exhaust is a major challenge for the development of ammonia engines due to its adverse environmental and health effects [2]. Besides, it might induce new reactivity for lubricant degradation, as studied in the field of compressors using ammonia as a refrigerant [17]. The effect of ammonia on engine lubricants is not an abundant topic in literature. To date, only one academic study from 2023 has focused on this concern [14]. In parallel, engine manufacturers and lubricant suppliers have recently begun to develop in-house protocols to determine whether ammonia engines require new lubricant formulations as in Refs. [85,86].

3.4.1.1 Oil/ammonia interactions in compressors

The compatibility between gaseous ammonia and lubricants has mainly been studied in the context of compressors using ammonia as a refrigerant [17].

One of the key concerns is the solubility of gaseous ammonia in oils. Ammonia is a polar molecule, while most base oils are non-polar. Takahashi and Kaimai [17] found that ammonia could become miscible in oils enriched with polar compounds, such as esters or polyalkylene glycols. They evaluated miscibility by measuring phase separation temperature of oil/ammonia mixtures at a 5 to 1 ratio, and observed that miscibility resulted in viscosity reduction at −50 °C.

More recently, Feja et al. [26] studied viscosity changes in oils exposed to ammonia under different pressure and temperature conditions (up to 50 bar and 140 °C). Miscibility was measured by vapor pressure measurements to prevent bias due to emulsification. In accordance with the findings of the previous study, they found that ammonia could dilute oils, resulting in viscosity decrease. The maximum observed ammonia solubility was around 6% at 50 bar. At 40 °C and 15 bar, the viscosity reduction reached 80% for alkylbenzene (with 4% ammonia uptake), 50%–55% for mineral oil, and 45%–50% for polyalphaolefin (with 3% ammonia uptake) (Fig.8). At 90 °C and 10 bar, which seems reasonable conditions in an engine, the ammonia solubility in polyalphaolefin was around 1%, leading to a 11% viscosity decrease (Fig.8). The authors explained that the higher miscibility in alkylbenzene was attributed to the increased polarity brought to the oil by aromatic rings.

Although polyolefin-based oils are often considered by manufacturers to be the most suitable lubricants for ammonia compressors due to their low miscibility [27], some ammonia might still dissolve the oil.

The second issue is the alkalinity of ammonia. Once solubilized in the lubricant, ammonia reacts with acidic components such as additives, oxidation products, or the acid functions of the base oil. Takahashi and Kaimai [17] observed the formation of sediments in oils treated under ammonia atmosphere in autoclaves, which they tentatively identified as amides, although no chemical analysis was shared to support this claim. There seems to be a consensus that ammonia solubilization is linked to the formation of amide sludge in compressors, although no specific scientific studies have been conducted. Only Eurofins and Fluitec study supported this with examples of sediments analysis showing the identification of amides could be found [87,88].

Reports of floodings next to ammonia compressors leading to large amounts of sludge tend to suggest that reactivity might be increased with the coupled presence of water [27]. More detailed knowledge on the reactivity of ammonia and identification of the degradation products then appears as an important challenge for ammonia engines.

3.4.1.2 Oil/ammonia interactions in engines

The interaction between oil and ammonia under typical engine conditions has only begun to be studied in recent years, with the emergence of ammonia-fueled marine engines. Liu et al. [84] measured NH₃ concentrations higher than 100 ppm in lubricants used in gasoline/ammonia and hydrogen/ammonia dual-fuel engines, showing that some ammonia could accumulate in the lubricant during engine operation. Agocs et al. [14] further studied the effect of ammonia exposure on the aging of lubricant with a 2 L reactor, where a Group II mineral oil with an unspecified additive package was subjected to a constant gas flow containing synthetic air and ammonia for up to 100 h, at 170 °C.

In addition to conventional oxidative degradation, which could be monitored using FTIR, the authors reported the accumulation of aminic compounds in the oil exposed to both trace (1000 ppm in air) and stoichiometric (21.7% in air) concentrations of ammonia. This accumulation contributed to an increase in viscosity [14]. Furthermore, oils exposed to ammonia had a higher propensity to form deposits in static micro-coking tests (Fig.9), even at low concentrations, and showed lower load-bearing capabilities compared oil exposed only to air. These results suggest that ammonia is likely to react with lubricants in conditions reproducing engine environment, leading to both accelerated chemical aging and diminished tribological performance.

Using the same experimental setup, several lubricant types with different additives packages were tested in the study by Obrecht et al. [28]. The evolution of kinematic viscosity at 40 °C with increasing ammonia exposure times appeared to be highly dependent on both base oil chemistry and additive composition (Fig.10). A similar trend was observated when the failure loads were measured. All tested oils had a lower failure load after being exposed to ammonia instead of pure air, though the extent of this reduction varied significantly between formulations. The authors also observed a correlation between the failure load and micro-coking behavior: oils which tend to produce more deposits have lower failure loads [28].

The study also reported measurements of the total acid number (TAN) and total base number (TBN) at different aging times, which yielded unexpected results, such as a decreasing TAN and increasing TBN. Indeed, with oxidation, TAN should increase along with the accumulation of acidic products in the oil, while TBN, measuring the basic reserve due to additives, should decrease as additives are consumed [16]. According to the authors, these results could be explained by the accumulation of some ammonia in the liquid oil, which increased the basic reserve and neutralized acidic products. These findings suggest that, unlike in conventional fuels, TBN and TAN do not seem to be reliable indicators of lubricant degradation when ammonia is present. Overall, this study highlights significant chemical interaction between ammonia and lubricants under these experimental conditions.

A similar experimental setup was used by Chevron Oronite to study the impact of ammonia on lubricants, as presented in Rik et al. [86]. They exposed a Group I mineral oil and a plant-based branched hydrocarbon oil, each formulated with additives leading to a TBN of 40, to gaseous ammonia at 180 °C until the nitrogen content of the oil reached 600 ppm. They also measured the deposit tendency, using the Komatsu hot tube test at 330 °C, the kinematic viscosity, and the oxidation onset temperature using differential scanning calorimetry (DSC). The Komatsu deposit test involves flowing oil into a tube at elevated temperature and rating the lacquer formation. This method is different from the micro-coking test performed by Agocs et al. [14], which involves heating a small volume of oil at 230 °C (center of the plate in Fig.9) and 285 °C (edges of the plate in Fig.9) for 90 min and rating the lacquer formed under static conditions. Compared to the fresh oil, these characteristics were not significantly different for the aged oils. The difference between the conclusions of those two studies might come from differences in aging duration, ammonia concentration, or the specific additive packages used.

A different experimental setup has been developed by MAN [85], who designed a test rig comprising pressurized cylinders capable of exposing engine parts or lubricants to liquid or gaseous ammonia at ambient temperature. They exposed different lubricants in liquid ammonia for one week. After the ammonia was allowed to evaporate, the oils were analyzed for changes in color, deposit formation, cold flow properties, and kinematic viscosity. The tested oils included mineral-based system oils, base oils, and lubricants for steam turbines, refrigeration compressors, gas turbines, and piston engines. Among these, a synthetic oil formulated for ammonia refrigeration compressors exhibited the most stable performance, showing minimal to no observable interaction with ammonia. [85]. No quantitative results were provided, but this finding implicitly suggests that other types of lubricant were, in contrast, affected by ammonia.

To evaluate lubricant behavior under more realistic conditions, Obrecht et al. [28] performed long-duration engine tests using a 1.5 L 4-cylinder engine fueled with diesel/ammonia blend (15/85 vol.%) with three lubricants, tested over 250 h at 2000 r/min and high load. Kinematic viscosity at 40 °C, TAN, and TBN did not appear to follow any clear trends, indicating limited degradation of lubricant or inappropriate characterization protocols, as suggested by the authors. The impact on tribology was monitored by targeting specific trace metals with inductively coupled plasma optical emission spectrometry (ICP-OES):

• Copper, usually found in engine bearing alloys such as conrod bearings or turbocharger bearings, was considered as a sign of corrosive wear.

• Iron, considered as a sign of adhesive or abrasive wear.

• Nickel, used as a sign of wear in the ring/piston/cylinder region, as it is used in cylinder liner coatings.

Obrecht et al. [28] observed increasing concentrations for those three metals with rates function of oil chemistry. The copper concentration reached 600 ppm in the worst case, that is, one order of magnitude higher than the results obtained with neat diesel. This is consistent with the corrosiveness of ammonia toward copper and brass [5]. The iron concentration did not surpass the values expected with neat diesel, and the content did not depend on the oil chemistry. This also agrees with the literature, as iron theoretically passivates in the presence of ammonia [5]. Both metals can act as catalysts for liquid-phase oxidation. They observed that the increase of the nickel content was correlated with an increase in the blow-by flowrate, indicating a reduced sealing of the engine around the piston rings due to significant wear in this region. Visual inspection of the engine after the tests confirmed this hypothesis [28]. But it is unclear if it is due to the direct interaction of gaseous ammonia with engine parts, or to ammonia uptake in the oil. The results from artificial aging could not be exactly reproduced with the engine test bench. This discrepancy might be due to the fact that the blow-by gas mixture contains less ammonia than stoichiometric proportion, but water, NO_x and diesel combustion by-products in addition. In engines, oil also undergoes varying temperatures and might contain metals acting as catalysts. Moreover, they also indicated that some oil samples were found to contain water, which may alter the measured properties.

In summary, the collective findings from artificial aging and engine tests clearly indicate that ammonia interacts chemically and tribologically with lubricants in engine-like conditions. Depending on the additive packages, this affects their properties and the engine operation, which highlights the need for developing ammonia-compatible engine oils.

3.4.2 Compatibility with hydrogen

Although hydrogen is a highly diffusive gas which can easily travel to the crankcase with blow-by gases, its uptake into the lubricant seems unlikely in an engine. The reactivity of hydrogen toward unsaturations is well-established and widely exploited in refineries for hydrotreatment processes, but such reactions occur at very high temperature and pressure, typically around 400 °C and up to 200 bar, along with specialized catalysts.

Obrecht et al. [89] also studied the effect of hydrogenation on the main properties of lubricants. They exposed 800 g of a commercial engine oil to a constant flow of 2 mL/min of hydrogen at 20 bar and 80 °C for 48 h and performed standard characterizations, including kinematic viscosity, oxidative stability, TBN, and micro-coking behavior. No measurable changes were observed in any of these properties before and after hydrogenation. Furthermore, GC-MS analysis revealed no evolution of the oil at the molecular level [89].

To the best of our knowledge, no other studies have addressed this topic, and no additional data are available to validate or challenge these findings.

3.5 Compatibility with water

Like ammonia, water, a polar molecule produced during ammonia and hydrogen combustion, can partially dissolve into engine lubricants and travel to the crankcase by blow-by gases. However, in contrast to ammonia, water can condense and exist in the liquid state under the temperature and pressure conditions of the crankcase. It can first dissolve in oils, up to a threshold where emulsification happens, resulting in water droplets dispersed and stabilized in the oil phase [30]. Over time, this emulsion can destabilize, leading to phase separation and the accumulation of free water at the bottom of the crankcase [34].

The problem of water contamination arises in ships where poor storage conditions in large ships can lead to water accumulation in large tanks, and leaks of refrigerating systems can happen. Here is a list of possible bad consequences associated with water contamination reported from Refs. [30,90]:

• Oxidation promotion and antioxidant neutralisation leading to sludge and deposits

• Hydrolysis of lubricant additives into acids (especially esters in detergents, dispersants, and friction modifiers)

• Rust and corrosion, with increased abrasive wear due to metal particles

• Impaired lubrication from a discontinuous oil film

• Overflowing due to emulsification and/or foam

• Cavitation, if vapor bubbles form in critical regions

• Hydrogen embrittlement from reduction of water into hydrogen and diffusion through metal causing blistering and cracking

• Bacterial growth, which further increases acids formation and oil degradation [91]

The Chevron branch dedicated to industrial marine lubricants recommends a complete drain when the water content is greater than 0.5 wt% [90].

In controlled conditions (80°C), Cen et al. [29] observed that the water content in oils decreased from 1 wt% to less than 0.4 wt% in two weeks, suggesting that water uptake was partially reversible [29]. This timescale is long considering the possible repetitive exposure to water in an engine, but higher temperatures (especially above 100 °C) would probably lead to faster evaporation. The uptake and evaporation rate depend on the polarity of the base oils: with ester bases, a larger quantity of water is dissolved, but it takes longer to evaporate than with PAOs, as expected based on polarity [29].

In the same study, water uptake correlated with increased TAN and accelerated depletion of ZnDTP additives, suggesting that water promotes oxidation and other ways of acids formation (ester hydrolysis, bacteria). The same results were obtained by Shishigin and Bel’ganovich [92], who measured a total base number decrease of 58% after 100 h on engine test benches with a constant 1 wt% water content in the oil, versus only a 22% drop without water. Under these conditions, no significant difference was obtained with tap water, sea water, and distilled water. In the same study, viscosity increased by 190% after a 36-h laboratory wear test with ball bearings, and the mass loss from bearing inserts was 70% higher than with fresh oil [92]. On the other hand, Cen et al. [29] did not measure any impact of water contamination on the evolution of the viscosity. Using a rolling ball on a disk, they measured similar friction coefficients with and without water for a polyester and a PAO oil with the same ZnDTP additive, but aging with water was associated with slightly higher wear for both oils. They attributed this result to the accelerated degradation of ZnDTP with water, evidenced by the depletion of the bands associated with P=S, P–O and S–O in FTIR.

In marine oils, additives are used to limit these processes [30,42]. Overbased detergents increase the initial basic reserve, demulsifiers promote phase separation, and anti-foaming agents counterbalance foam formation caused by emulsifiers. These additives are usually not found in engine oil formulations for heavy duty or passenger cars.

As shown earlier, water uptake seems to be influenced by the base oils and its additives, like ammonia. The accumulation of acid in the oil with time might also facilitate further water and ammonia absorption. Experimental results suggest that water may contribute to oil aging with conventional fuels, but the effect of larger amounts of water in the case of hydrogen and/or ammonia combustion is not well explored. Besides, water uptake can amplify the accumulation of ammonia, considering the high solubility of this gas in water, or nitric oxides, leading to the accumulation of nitric acid. This issue was never addressed.

In conclusion, previous sections have highlighted the potential chemical and rheological changes in lubricants used in internal combustion engines fueled with ammonia-hydrogen blends, particularly when exposed to nitrogen oxides, unburnt ammonia and water at high temperatures. However, lubricants are known to contribute to particle emissions, and their chemical composition and viscosity influence emission characteristics. The next section will clarify the role of lubricants in particle emissions from hydrocarbon-fueled engines and present recent studies on emissions from carbon-free fuels. It emphasizes that particle emissions must be investigated, even in “zero carbon” scenarios, especially as both changes in lubricant chemistry and variations in exhaust gas composition can impact emissions.

3.6 Particles emissions

With increasingly stringent regulations on the emission of pollutants, researchers have explored all potential sources of particulate matter and found that lubricants contributed to the particle emissions [31]. This contribution is expected to be greater with carbon-free fuels, as they are the primary source of carbon in the system [31,89,93]. Oil-derived particles are believed to come from lubricant droplets which are transported into the combustion chamber, where they are partially oxidized, leading to the formation of carbonaceous particulates [31,93,94].

3.6.1 Oil contribution to soot formation with hydrocarbon fuels

Engine exhaust particle formation was thoroughly described in Refs. [95,96]. The lubricant’s contribution to particle formation is thought to be linked to partial combustion and pyrolysis of the oil film wetting the cylinder walls and piston during post-combustion reactions [95,97]. A portion of this film can combust to generate nuclei, while the vaporized fraction can condensate either on these nuclei or on mineral particles in the exhaust, where temperature decreases [95].

Several studies have concluded that lubricants mostly contribute to the soluble organic fraction (SOF), of particles formed during combustion with conventional fuels, as a high proportion of unburnt heavy hydrocarbon from the oil can condensate on soot precursors [98–101]. Sulfur and phosphrous atoms contained in oil additives in the lubricant further contribute to the SOF, with sulfates and phosphates being detected in particle residues [100,101].

Oil-derived particles also seem to have a more disordered structure. Raman and XPS characterizations show higher proportions of tertiary carbons and C–H groups, indicating a less graphitic character [101], which is consistent with microscope observations showing larger interlayer distances between carbon crystallite layers [99]. The disordered structure offers more reactive sites for particles post-oxidation, as confirmed by oxidation studies [99,102].

Particles characteristics also vary with lubricant formulation. Wang et al. [99] compared the emissions when using two different base oils, and found that it led to variations in the size of particles, structure, volatile fraction, and oxidation reactivity (Fig.11). Higher viscosity oils seem to result in particles with in a higher volatile fraction. Lubricants with similar viscosity values also produced similar particle number distributions, but different size distributions [103]. Similarly, six oils with similar viscosity but different additive packages resulted in different PN₁₀ emissions from a hydrogen-fueled engine [89], confirming that both lubricant characteristics and composition had an influence on particulate matter emissions.

A more in-depth analysis can be found in the recent review by Lyu et al. [104], focusing on this topic of the influence of lubricants on the emission of particulate matter from internal combustion engines.

3.6.2 Oil contribution to particulate emissions with zero-carbon fuels

Lubricants also contribute to particulate emissions in engines fueled with carbon-free fuels such as ammonia and hydrogen. A recent study by Apicella et al. analyzed the SOF of particles collected using GC-MS at the exhaust of a hydrogen-fueled monocylinder engine, revealing the presence of large polyaromatic hydrocarbons (PAHs), clear evidence that some oil underwent severe thermal degradation in the combustion chamber [93]. Similarly, Thawko et al. [94] studied particulate emissions from a hydrogen-rich reformate blend (75% mol H₂ and 25% mol CO₂), focusing on the number distribution and chemical composition of particles. They found a higher number of accumulation-mode particles with the lubricant containing the highest metal content, although the total number of ultrafine particles (< 100 nm) was greater with the oil containing fewer metals. Particle mass distribution peak was found to be twice as high with the metal-rich oil. They also found that all trace elements from the lubricants did not have the same recovery yield in the particles: significant amounts of silicon, iron, sodium and potassium were found in the particles compared to calcium, zinc, phosphorous, and molybdenum. The authors suggested that elements with higher boiling points tend to remain in the combustion zone and act as precursor for nucleation [94].

This hypothesis is consistent with findings by Miller et al. [31], who studied particulate emissions in hydrogen-fueled engines in 2007 to isolate the effect of the lubricant in particulate matter by filling the engine with a carbon-free fuel. They classified four different types of particles by size and morphology: nanoparticles (< 50 nm), core-shell particles (core < 20 nm), dense spheres (~100 nm), and agglomerates (~300 nm). They studied the elemental composition of each type of particles and showed that agglomerates were mostly made of carbon, dense spheres mostly calcium (along with Na, K, S, P), iron dominated the particle cores, and shells contained P, K, Zn, and Na [31]. Their hypothesis posits that combustion evaporates the most volatile elements (P, K, Na, Zn), while high-boiling-point elements (e.g., Fe: Tₑb = 2860 K; Ca: Tₑb = 1484 K) act as precursors for nucleation. During the expansion stroke and subsequent cooling of the chamber, vapors condensate on nucleation cores, or nucleate independently. Dense spheres would then result from oil droplets from which it would only remain the non-volatile residues, while core-shell structures would originate from condensation of vapor on iron and/or calcium nuclei [31].

To summarize, lubricants are a significant source of particulate matter, not only in conventional engines but also in hydrogen-based engines [31,89,93,94]. Although no equivalent studies were reported for ammonia-fueled engines, it is reasonable to assume that lubricants can contribute to particle formation in these engines as well. Lubricant composition affects the number, mass distribution, and the structure of particles. In addition, the contribution of the lubricant depends on the oil transport mechanisms and injection type. Aging of the lubricant might induce physical and chemical changes and thus change the particle emissions. One can think of increased metallic content due to corrosion and the addition of detergents, and/or viscosity changes. Addressing these questions is essential for the development of hydrogen and ammonia engines that meet emission regulations while delivering on their promise as authentic carbon-free fuels.

3.6.3 Effect of NH₃ and H₂ on soot formation

Additionally, as exhaust gases can condensate on particles precursors, variations in their composition can affect particle structure, mass, and morphology. In their fundamental work on soot formation, Haynes and Wagner [105] investigated the influence of various fuel additives, including H₂, NH₃, and H₂O in premixed flames. They reported that the addition of NH₃ to ethylene decreased the total amount of particles, while water vapor had negligible effects at concentrations below 10% [105]. In another more in-depth study in which the soot volume fraction, the mean particle size, and the particle number density were measured on flat premixed flames of ethylene air at atmospheric pressure, they found that NH₃ reduced the soot volume fraction while H₂ promoted soot formation under these conditions [106]. Although H₂ promoted the onset of soot formation, it did not appear to affect its aspect or number density at molar fractions below 3%. They also found a shift in the soot production zone, with the onset of soot formation occurring farther from the burner when ammonia was present [106]. More recently, Boyette et al. [107] studied soot formation in ethylene/nitrogen turbulent flames with varying nitrogen substitution degrees. They concluded that replacing nitrogen with ammonia reduced the soot volume fraction by a factor of three in flames containing 75% C₂H₆ by volume. Their data (Fig.12) also showed that soot production started in fact farther downstream in the case of ammonia substitution. According to Haynes et al., this shift was associated with a slight increase in flame temperature, between 10 to 20 K, due to ammonia substitution [105]. However, H₂ substitution resulted in a higher soot volume fraction along the flame centerline (Fig.12). Interestingly, when H₂ and NH₃ were mixed, soot volume fraction still decreased, suggesting that the soot-inhibiting effect of ammonia is more efficient than the soot-promoting effect of hydrogen.

However, the effect on particle growth dynamics appears to differ between studies. Haynes et al. [106] found that there is no effect on surface growth rate or coagulation of particles in their ethylene flat flames, while Boyette et al. [107] observed that ammonia substitution in ethane flames (0–25% NH₃) resulted in the formation of smaller particles.

Zaher et al. [108] studied soot obtained in laminar co-flow flames with ethylene doped with up to 50% NH₃, using XPS and Raman spectroscopy, and concluded that nitrogenated species interact with the surface of soot particles, preventing soot growth by blocking carbon addition pathways. Their analysis revealed that the nitrogen content on the particle surface increase linearly with the increasing NH₃ concentration (Fig.13). This was attributed to the ability of nitrogenated species to bond with graphitic carbon defects, since the sp²/sp³ carbon ratio was higher when ammonia was added, meaning that the particles have a more organized structure overall, with fewer defect sites [108]. In their recent review, Chen and Liu [109] summarized current knowledge on the effect of ammonia addition on soot formation, supporting these conclusions.

Burner studies clearly demonstrated that the addition of hydrogen to hydrocarbon fuels, and even more importantly, ammonia, affects soot formation. Although the experimental conditions in these studies differ substantially from those of internal combustion engines, the influence of unburnt gases on particulate emissions should be considered when studying oil-derived particles. Exploring particle formation in ammonia-fueled and dual-fuel engines is therefore essential for the deployment of these technologies.

4 Conclusions

Lubricant oils are essential to ensure the optimal efficiency and durability of internal combustion engines. They are chemically complex products, containing a wide range of additives tailored to specific applications and operating conditions. The current transition toward alternative fuels, particularly ammonia and hydrogen, which are increasingly studied as these carbon-free fuels gain attention for long-distance and off-road applications, raises challenges regarding new fuel-oil compatibility.

This review highlights several distinctive characteristics of ammonia and hydrogen combustion that may introduce new degradation mechanisms, according to current knowledge, for lubricants. Drawing on existing knowledge from related domains, it compiles current insights on four key topics: compatibility with unburnt ammonia, reactivity with nitrogen oxides, water contamination, and the impact of the lubricant on pollutants emissions. The main takeaways and perspectives can be summarized as follows:

Combustion characteristics with alternative fuels, such as increased flame temperatures and higher residual oxygen concentrations, may affect the oxidation kinetics of lubricants

High nitrogen oxide concentrations measured in the exhaust gases, and by implication, in contact with lubricants, can accelerate nitro-oxidation, reducing the lifetime of the engine oils. Future research is needed to examine more precisely the role of NO and to identify reaction intermediates involved in the reaction mechanism.

Lubricants can also be in contact with unburnt ammonia. This could be associated to new degradations mechanisms involving base oils and/or additives which have yet to be fully investigated.

Water contamination might also necessitate the use of new additives. Additionally, the solubilization of ammonia and nitrogen oxide in accumulated water could be investigated to address the potential production of nitric acid into the oil.

Although ammonia and hydrogen are carbon-free fuels, lubricant-derived soot and particulate matter can still be emitted. Future studies should be conducted to characterize particulate emissions from engines operating on these fuels and to evaluate the influence of lubricant properties.

As ammonia and hydrogen engine technologies remain in early stages of deployment, most conclusions herein are extrapolated from adjacent research areas. Comprehensive studies under engine-representative conditions are still required to fully understand the implications for lubricant degradation and long-term engine performance.

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