Combustion analysis of a hydrogen-diesel fueloperated DI diesel engine with exhaust gas recirculation

Abstract

The rapid depletion of fossil fuel and growing demand necessitatesresearchers to find alternative fuels which are clean and sustainable.The need for finding renewable, low cost and environmentally friendlyfuel resources can never be understated. An efficient method of generationand storage of hydrogen will enable automotive manufacturers to introducehydrogen fuelled engine in the market. In this paper, a conventionalDI diesel engine was modified to operate as gas engine. The intakemanifold of the engine was supplied with hydrogen along with recirculatedexhaust gas and air. The injection rates of hydrogen were maintainedat three levels with 2 L/min, 4 L/min, 6 L/min and 8 L/min and 10L/min with an injection pressure of 2 bar. Many of the combustionparameters like heat release rate (HRR), ignition delay, combustionduration, rate of pressure rise (ROPR), cumulative heat release rate(CHR), and cyclic pressure fluctuations were measured. The HRR peakpressure decreased with the increase in EGR rate, while combustionduration increased with the EGR rate. The cyclic pressure variationalso increased with the increase in EGR rate.

Keywords

hydrogen / exhaust gas recirculation(EGR) / diesel / combustion / heat release rate (HRR) / combustion duration

Introduction

The rapid growth in vehicle populationdepletes resources and the concern for environment has put additionalrequirement in engine performance. While research has been done onfinding clean alternate fuels, there is also a demand for researchersto design efficient technologies which increase performance and reducepollutants for the conventional fuels like diesel and gasoline. Therecent conference on climate has put restrictions on the emissionof gases like NO_x, SO_x andparticulate matter from industrial processes and IC engines. Hence,it has become a necessity rather than convenience to find alternativeclean fuels. While there are many alternatives available, hydrogenis the most suitable fuel to replace or to supplement existing fuels.The main attraction for hydrogen as a fuel lies in the fact that existingengines can be converted with little modification to use hydrogenfuel [1]. One of the criticalmodifications is the additional introduction of the manifold injectiontechnology [2,3].

The main attraction for the hydrogenas a fuel lies in the fact that they emit H₂O as emission. But hydrogen is not available freely and the separationprocess from its compounds is still expensive [4]. Moreover, there should be robustsafety mechanism implemented at the actual usage point as hydrogenis highly inflammable. Some of the existing technologies like usingEGR in the engine are no longer attractive as they used to be, becauseof their increased soot emission. As soot emission standards havebecome more stringent, some other technologies must be adopted toreduce NO_x and soot [5]. Whilemany researchers have successfully modified the existing gasolineengines to run with hydrogen [6], still these gasoline engines have not been implemented in automobileson a commercial scale. As the self-ignition temperature of hydrogenis high, no researchers have been able to modify the existing dieselengine to use hydrogen and derive power from it just as a diesel engine[7].

But hydrogen fuelled engines haveother problems like high NO_x emission, if they are operated on lean mixture. Thehigher NO_x emission from the modified engine is caused by the higher flametemperature of hydrogen. Heffel et al. used hydrogen and cooled EGRin the modified ford ZETEC engine. For the hydrogen rate of 20 L/min,they reported a reduced NO_x level compared to non-EGR mode [3]. Compared to the incylinder EGRreplenishment method, the external cooled EGR can reduce NO_x levels much morecompletely [8].

In order to use hydrogen to operatein the HCCI engine, the cylinder gas temperature should be increasedsufficiently high so that the hydrogen might reach a self-ignitioninside the cylinder just at the appropriate time [9]. Injecting a pilot diesel fuel duringthe compression stroke of a diesel cycle will ensure a pre-combustionprobably around 10^o to 60 ^o CA before TDC, which will ensure a temperature riseto auto ignite hydrogen. Several studies have been conducted withhydrogen on modified gasoline engines [10–13]. Hydrogen is known for its high diffusivity. Hence, injecting asmall quantity of hydrogen either in the manifold or directly insidethe cylinder can reduce the heterogeneity of diesel spray and makethe subsequent combustion a more premixed one [14]. Hydrogen engines exhibit almostthe same brake thermal efficiency for their entire load range [15]. Other methods of hydrogen introductionlike continuous carburetion, continuous manifold injection, pulsedmanifold injection and low pressure direct injection have been proposedin previous researches [16,17]. Previous studiesshow a brake thermal h enhancement of 1.29% with the supplement of 0.15 kg/h of hydrogen.Hydrogen enriched engines produce approximately the same brake power,a higher thermal h and a lower emission compared to diesel engine of the same loadranges [18,19]. But some of the problems likecombustion knock, higher values of pressure rise, and temperatureshould be carefully studied and controlled [20,21].

Hydrogen in diesel engines

The idea to use hydrogen in internalcombustion engines is not new. As the self-ignition temperature ofhydrogen is 858 K, it cannot be used in CI engine without air pre-heating[4]. Another method forutilizing hydrogen is to use it with diesel fuel, where hydrogen isinducted along with intake air and a small quantity of diesel injectedas pilot fuel during the compression stroke.

Hydrogen combustion is vastly differentfrom hydrocarbon fuel combustion. Hydrogen has more extensive flammabilitythan diesel fuel. Some important properties of diesel and hydrogenlike flammability and burning speed are given in Table 1. As the flamevelocity of hydrogen is high, extremely quick combustion usually takesplace [4]. Hydrogen requiresa minimum ignition energy of 0.02 mJ, which enables the engine thatuses hydrogen to run on even extremely lean mixtures.

Exhaust gas recirculation

One of the effective and tested methodsto control NO_x emission is the use of exhaust gas recirculation (EGR) in engines.Nitrogen, NO_x, HC, CO and particulates are the main components of exhaust gas.As the temperature of the exhaust gas is higher than the normal intakeair, it reaches the volumetric efficiency of the engine. Recirculationof exhaust gas displaces some quantities of fresh air entering thecombustion chamber. The level of oxygen in the combustion chamberis reduced, which, in turn, effects the exhaust emission. The additionof exhaust gas in the engine cylinder increases the specific heatof the mixture. This results in a reduced combustion rate of air-fuelmixture. The combination of reduced oxygen content of air and reducedcombustion rate results in a lower peak temperature. Thus, NO_x formation in theengine is reduced. The percentage of EGR (h (EGR)) in the engine is defined as

(1)\eta (\mathrm{EGR})={\displaystyle \frac{\mathrm{Volume of EGR}}{\mathrm{Total charge into the cylinder}}}\times \mathrm{100}.

Yet, another method for specifyingthe percentage of EGR is the specification of CO₂ concentration [15],

(2)\eta (\mathrm{EGR})=\mathrm{ratio}\times {\displaystyle \frac{{[{\mathrm{CO}}_{\mathrm{2}}]}_{\mathrm{intake}}-{[{\mathrm{CO}}_{\mathrm{2}}]}_{\mathrm{ambient}}}{{[{\mathrm{CO}}_{\mathrm{2}}]}_{\mathrm{exhaust}}-{[{\mathrm{CO}}_2]}_{\mathrm{ambient}}}}.

Due to increased concentration ofinert gases in combustion chamber, the adiabatic flame temperatureis reduced [8]. UsingEGR full loads has its own problems like reduced diffusion combustion,which may result in higher soot and particulate emissions. But atlower loads, part of the unburned HC gets burned in the next cycle,which reduces the HC emissions at part loads. Apart from that, usingEGR in diesel engine at full loads increases the wear and tear ofthe cylinder due to increased production of soot and particulate emission.In this paper, the engine was tested at different EGR rates with optimizedflow of H₂. The EGR rate is varied from 0%to 10%, and to 20%. The combustion parameters were analyzed for differentEGRs.

Experimental setup

The experimental setup and the schematicdiagram of experimental setup are shown in Fig. 1 and Fig. 2. In thispaper, a four stroke, direct injection diesel engine was used forinvestigation. The particulars of the test engine are listed in Table2. Hydrogen was stored at a pressure of 150 bar and was regulatedto an outlet pressure of 2 bar through a pressure regulator. The hydrogenflow rate was adjusted by a fine control gas value. The flow rateof hydrogen was measured with a digital mass flow meter. In orderto prevent the flow reversal in the system, the hydrogen was meteredthrough a non-return value (NRV). As an additional safety measureto prevent the flame from travelling through the system, a flame arrestorwas provided next to the NRV. The flame arrestor works on the fundamentalprinciple that if sufficient heat is removed from the gas, the energylevel available to initiate auto ignition is reduced. There was alsoa diaphragm type valve inside the flame arrestor, which would be rupturedin the event of severe flash back or adverse pressure rise, therebypreventing explosion. The flame trap had a sliding sleeve which suppressedthe flame with some quantities of water to extinguish the flame, ifit all developed inside the hydrogen system. A gas carburetor wasfitted next to the flame trap to regulate the flow of hydrogen. Thismethod of mixture preparation with fuel (hydrogen) and air is calledmixture enrichment. The hydrogen flow rate was varied from 2 to 10L/min with a 2 L interval. The exhaust gas was supplied with the inletmixture (air and hydrogen) with 10% EGR and 20% EGR. The emissionsand performance of the hydrogen enriched engine with and without EGRwere published in the previous paper. In this paper, the combustionparameters were analyzed.

Results and discussion

In this paper, combustion characteristicsof a DI diesel engine were studied by using hydrogen enrichment withoutEGR, with 10% EGR and with 20% EGR. The flow rate of hydrogen wasset at optimum 2 L/min.

Heat release rate

Figure 3 shows the variation of heatrelease rate (HRR) with crank angle at different values of hydrogenenrichment without EGR, with 10% EGR and 20% EGR. It is noticed thatthe HRR is 13.10% higher for hydrogen operation than the diesel fuelmode. This might be caused by the higher flame speed of hydrogen andthe immediate burning. The HRR in the premixed burning zone is higher,which demonstrates the expanded weight ascend in the ignition chamber.The HRR obtained for 2 L/min hydrogen enrichment neat diesel fuelof 66.10 J/ºCA compared to that obtained without EGR is 59.75J/s at full load. The HRR increases with the increase in EGR flowrate and it is higher than that of neat diesel at full load. With10% EGR, it is 72.50 J/ ºCA and with 20% EGR it is 72.85 J/ ºCA.

Cylinder pressure

Figure 4 depicts the variation ofcylinder pressure with crank angle at different percentages of EGRwith hydrogen fuel. The cylinder pressure with 10% EGR and 20% EGRis 69.99 bar and 69.70 bar, which are higher than the neat dieselvalue (65.22 bar). The rise in cylinder pressure with the increasein EGR rate is due to the substitution of fuel air mixture by inertgas, which, in turn, increases the combustion temperature [8]. In compression ignition (CI) engine,the cylinder pressure depends upon the fuel-burning rate during thepremixed burning phase.

Ignition delay

Figure 5 demonstrates the variationof ignition delay with brake power at different percentages of EGRwith hydrogen fuel. It is observed from Fig. 5 that the ignition delayof all the fuels decreases with an increase in engine load. The ignitiondelay formation is high in neat diesel, which is 18.4 °CA, comparedto that without EGR of 18.06 °CA. The ignition delay for 10% and20% EGR is 18.04 °CA and 17.76 °CA respectively. The ignitiondelay is longer for diesel, compared to that with 10% EGR, with 20%EGR and without EGR. The longer ignition delay is caused by the chemicalreaction during the injection time, which slows down the physicaldelay period.

Combustion duration

Figure 6 illustrates the variationof combustion duration with the brake power of the engine at differentEGRs. This time duration starts from the beginning of the heat releaseto the end of heat release. The combustion duration in neat dieselfuel at full load is 50.44 °CA, while with 20% EGR, it is 54.3°CA and 53.08 °CA, and with 10% EGR and without EGR it is52.45 °CA. When the load increases, the combustion duration increasesdue to the high mass of fuel injected. At any particular load, thecombustion duration increases as the EGR rate also increases.

Rate of pressure rise

Figure 7 exhibits the variation ofROPR with the brake power of the engine at different EGRs. The ROPRwithout EGR is 4.23 bar compared to neat diesel fuel of 4.00 bar atfull load. ROPR is indicative of noisy operation of an engine. TheROPR depends on the amount of heat released in the initial stage ofcombustion. The ROPR with 10% EGR is 4.51 bar and with 20% EGR itis 4.68 bar.

Cumulative heat release rate

Figure 8 displays the variation ofCHR with the crank angle of the engine at different EGRs. The CHRincrease with the increase in the engine load for all powers becauseof the increase in the amount of fuel infused into the chamber. TheCHR in neat diesel fuel at full load is 695.96 kJ/m, while with 20%EGR it is 766.53 kJ/m and 730.74 kJ/m with 10% EGR. A negative CHRis seen at the start of the curve, because of the vaporization ofthe fuel accumulated at the time of ignition delay. When the burningis started, the CHR becomes positive. Figure 8 demonstrates that thereis an earlier CHR for sole fuel compared with 10% EGR and 20% EGR.

Cylinder gas temperature

Figure 9 presents the variation ofCGT with the brake power of the engine at different values of hydrogenenrichment without EGR, with 10% EGR and 20% EGR. The CGT in neatdiesel fuel at full load is 1312 °C, while with 20% EGR it is1427.26 °C and 1384.13 °C with 10% EGR and without EGR itis 1367 °C. When the load increases, the combustion duration increasesdue to the high mass of fuel injected and hence the gas temperatureincrease.

Cyclic variation of cylinder pressure

Figure 10 shows the cyclic variationof cylinder pressure with brake power. In the cylinder, pressure isa critical marker of cyclic variations. Since the estimation of theair-fuel proportion for the individual cycle is unrealistic, the incylinder pressure of 100 cycles were analyzed for peak pressure variation.It is found that the peak pressure is higher for 20% EGR followedby 10% EGR compared with that without EGR and diesel. This is causedby the uneven combustion with hydrogen and EGR.

Conclusions

The following conclusions are drawnfrom the experimental results.

The heat release rates are higherfor 10% and 20% EGR compared to diesel fuel at full load. The ratesof pressure rise and cylinder pressure with both EGR rates are higherthan those with diesel fuel. The ignition delay period is higher fordiesel blends at full load compared with 20% EGR, 10% EGR and withoutEGR. The combustion duration is higher for 10% EGR and 20% EGR atfull load compared with diesel fuel. The CHR value with 10% EGR and20% EGR increases as compared with diesel fuel and that without EGR.The cylinder gas pressure and cyclic variation of pressure increasewith 10% EGR and 20% EGR.

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