Influence of cetane number improver on performance and emissions of a common-rail diesel engine fueled with biodiesel-methanol blend

Abstract

In this paper, the effect of cetane number (CN) improver on performance and emissions, including particulate number concentration and size distribution, of a turbocharged, common-rail diesel engine fueled with biodiesel-methanol were studied. Two volume fractions (0.3% and 0.6%) of CN improver were added to BM30 (30% of methanol in the biodiesel-methanol blend) in the experiment. The results show that, compared with those of biodiesel-methanol blend, the peak value of cylinder pressure increases, the second peak of heat release rate decreases, the start of second heat release are advanced, and the fuel economy and thermal efficiency are improved when CN improver is added to biodiesel-methanol blend. Besides, CO and HC emissions decrease, NO_x emission varies little and smoke emissions increase slightly. Moreover, exhaust particles of BM30 mainly distribute in nano-size range. Furthermore, particle number concentration decreases and peak of size distribution profile shifts toward large size direction.

Keywords

biodiesel / methanol / common-rail (CR) diesel engine / cetane number (CN) improver

Introduction

Because of the increasing concerns over energy shortage and environmental pollution, the requirement in decreasing the emissions of internal combustion engine is reinforced. Diesel engine has been widely used to power automobiles and construction machineries due to its efficiency and reliability, but its high emissions of NO_x and particulate matter (PM) need to be restricted. To reduce the emissions of diesel engine, many technologies have been adopted, such as high-pressure and electronically controlled injection, exhaust after-treatment, exhaust-gas recirculation and utilization of alternative fuels, among which, utilization of alternative fuels, especially renewable oxygenated fuels have a promising aspect on account of its little modification to the original engine with an appreciable reduction in engine exhaust emissions. Biodiesel, derived from methyl esters of animal fats or plant oils, is a good alternative fuel for diesel engine. Previous studies [1–3] showed that using biodiesel in diesel engine could effectively reduce HC, CO and smoke emissions primarily because of its high oxygen content. However, NO_x emission has to be mitigated by some means or other. Besides, the high viscosity of biodiesel is unfavorable for its spray atomization in combustion chamber [4].

Methanol is another kind of oxygenates, with high oxygen content, high heat of evaporation, and low viscosity, making it a potential additive for biodiesel fuel [5]. Cheung et al. [6–8] and Yu et al.<FootNote>

Performance and emissions of a turbocharged, common-rail diesel engine fueled with biodiesel-methanol blends. Proceedings of the Institution of Mechanical Engineers. Part D, Journal of Automobile Engineering (In press)

</FootNote> studied the influence of methanol addition on the performance and emissions of engine fueled with biodiesel in a pump-line-injector diesel engine and a common-rail (CR) diesel engine respectively. Simultaneous reduction of NO_x and PM emissions was realized in both studies. However, methanol addition in biodiesel decreases the cetane number (CN) of fuel blend and increases the ignition delay time and engine noise. To compensate the CN of fuel blend, CN improver is usually used. Previous investigations [9–12] in diesel-ethanol blends indicated that the ignition timing could be controlled by adding a CN improver. However, few studies report the effects of the CN improver on performance and emissions in turbocharged, CR diesel engine fueled with biodiesel-methanol blends. As CR diesel engines will gradually replace the pump-line-injector diesel engines, the study on biodiesel-methanol blend with a CN improver is worthwhile. The objective of this study is to investigated engine combustion and emissions of a turbocharged, CR diesel engine fueled with biodiesel-methanol blend combined with a CN improver.

Experimental apparatus procedures

A turbocharged, CR diesel engine was used. Table 1 lists the main specifications of the engine while Fig. 1 shows the schematic diagram of the test bench. The engine, whose speed and load were controlled by an engine control system (Powerlink FC 2000), was connected to an eddy-current dynamometer. The cylinder pressure of the engine was measured by using a piezoelectric transducer (Kisterler 6055c) and a crank angle (CA) encoder (Kistler2614A) providing a 0.1 CA resolution data acquisition of cylinder pressure. The signals of cylinder pressure and CA were recorded by a data acquisition system (Yokogawa DL750). The heat release rate was calculated from cylinder pressure on the basis of a zero-dimensional thermodynamic model [13]. The fuel flow rate was measured using an electronic balance with a measurement precision of 0.1g. The HC and CO were measured by a Horiba MEXA-554JA exhaust gas analyzer, while the NO_x were measured by a Horiba MEXA-720 NO_x analyzer. The exhaust smoke was detected by a SV-5Y opacity smoke meter. An electrical low pressure impactor (ELPI) was used to measure the exhaust particle number concentration and size distribution. The sample gas was diluted with two Dakati diluters in series before passing through the ELPI. The ELPI was set to measure particles with sizes between 7 nm and 10 μm in 12 measurement stages. Detailed descriptions of the ELPI and diluter can be found in Refs. [14,15].

The fuels used in this study were diesel, soybean derived biodiesel and BM30 (30% (vol) methanol in biodiesel-methanol blend). The CN number of BM30 is lower than that of biodiesel due to the presence of methanol, and 2-ethylhexyl nitrate (2-EHN, C8H17NO3) is used as a CN improver. 0.3% (vol) and 0.6l% (vol) of CN improver were added to BM30, named as BM30C0.3 and BM30C0.6, respectively. The physicochemical properties of the components are given in Table 2.

An engine speed of 1600 and 2600 r/min and five engine loads with brake mean effective pressure (BMEP) of 0.159, 0.313, 0.467, 0.621 and 0.776 MPa were selected in the experiment. The variation of engine speed and torque were controlled within 5 r/min and 0.1 N·m. Each measurement was repeated 3 times, and the average data were used for the analysis.

Results and discussion

Engine performance

Figure 2 displays the cylinder pressure and heat release rate of different fuels at two engine speeds and loads. The heat release rate curves show two peaks, which reflect the split (pilot and main) fuel injection strategy in the engine. BM30 gives a higher second peak of heat release rate due to more premixed fuel available as ignition delay is increased and gives a postponed second heat release compared with those of biodiesel. When the CN improver is added to the blended fuel, the maximum heat release rate and the beginning of heat release in the second heat release are recovered approximately to those of biodiesel. Beside, adding the CN improver moves the start of combustion closer to the top dead center, thus increasing the peak cylinder pressure.

Figure 3 depicts the brake specific fuel consumption (BSFC) and brake thermal efficiency (BTE) of different fuels. The BSFC of BM30 is higher than that of biodiesel because of its low heating value. The BTE of these two fuels are almost the same. When the CN improver is added to BM30, the BSFC decreases slightly while the BTE increases slightly. This is because that, adding the CN improver will move the start of combustion closer to the top dead center, thus improving fuel economy and thermal efficiency.

Engine emissions

The CO and HC emissions fueled with different fuels are illustrated in Figs. 4 and 5. BM30 has higher CO and HC emissions than biodiesel. The presence of methanol in fuel decreases the cylinder temperature due to the higher latent heat of evaporation and fast evaporation of methanol, and slows down the oxidation rate of CO and HC. With the addition of the CN improver, the CO and HC emissions of BM30 are decreased. This is because that, the ignition delay is decreased when CN improver is added, which leads to a higher combustion temperature and thus enhances the oxidation of CO and HC.

Figure 6 prsents NO_x emissions fueled with different fuels. The formation mechanism of NO_x formation is mainly related to the temperature. Compared to those of biodiesel, the NO_x emission of BM30 is lower at low and moderate loads and higher at high load. The fast evaporation and low heat of evaporation of methanol tend to reduce the combustion temperature while the improved spray atomization and combustion and the increased fuel burned in the premixed phase by methanol addition tend to increase the combustion temperature. These factors counteract with each other, leading to the variations of NO_x emission. When the CN improver is added, although combustion temperature increases, the variation of NO_x emission is small.

The opacity level of smoke emissions is demonstrated in Fig. 7. The engine smoke decreases with the increase of oxygen fraction in fuels. When the CN improver is added, the combustion temperature and the amount of fuel burned in the diffusive combustion phase are increased, leading to a moderate increase of engine smoke emission.

Figure 8 gives the exhaust particulate number concentration and size distribution of different fuels. Here, D_i represents the geometric mean of the D_p (the cut diameter represents the particle size with a 50% collection efficiency). Compared with biodiesel, BM30 has a lower particle number concentration and its peak occurs when the size is smaller. The increase of oxygen content in the fuel reduces the particle number and inhibits the coagulation of the small particles to form larger particles. When the CN improver is added, the particulate number concentration of BM30 increases and unimodal shaped particle number-size distribution appears again, the explanation for which is similar to that for smoke emission.

Conclusions

Influences of the CN improver on performance and emissions of a turbocharged, CR engine fueled with biodiesel-methanol blend were studied. The main conclusions are summarized as follows.

1) Addition of the CN improver increases the peak of cylinder pressure, decreases the second peak of heat release rate, advances the beginning of second heat release and improves the fuel economy and thermal efficiency.

2) In the presence of the CN improver, CO and HC emissions decrease and NO_x emission varies little. However, the engine smokes have a moderate increase with the presence of the CN improver.

3) Particle number concentration decreases and peak of particle number-size distributions shifts to large size with the addition of the CN improver.

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