EFFECTS OF BIODIESEL ON BUS DIESEL ENGINE CHARACTERISTICS

Transcription

1 EFFECTS OF BIODIESEL ON BUS DIESEL ENGINE CHARACTERISTICS Breda Kegl, Stanislav Pehan University of Maribor, Faculty of Mechanical Engineering, Slovenia Abstract. This paper discusses the influence of biodiesel on the injection, spray, and engine characteristics with the aim to reduce harmful emissions. The considered engine is a bus diesel engine with mechanically controlled diesel fuel injection M system. The considered fuels are neat biodiesel from rapeseed oil and some of its blends with mineral diesel (). The density, viscosity, surface tension, and sound velocity of tested fuels are determined experimentally and compared to those of. The obtained results are used to analyze the most important injection, fuel spray, and engine characteristics, which are determined experimentally. The injection, fuel spray, and engine characteristics, obtained with biodiesel and its blends with, are compared to those obtained with mineral diesel () under various engine operating regimes. The results indicate that, by using biodiesel, the harmful NO x, CO, smoke, and HC emissions can be reduced effectively by some minor engine adjustments. Key words: biodiesel, injection, spray and engine characteristics, engine emission. Introduction Biodiesel offers a potentially very interesting alternative fuel to mineral diesel regarding harmful emissions in addition to engine wear, availability and cost [1, 2]. Additionally, biodiesel does not contain carcinogens, such as poly-aromatic hydrocarbons and nitrous poly-aromatic hydrocarbons and therefore produces pollutants that are less detrimental to human health when burned [3]. However, high viscosity, high molecular weight, low volatility etc. of biodiesel fuels may in some cases lead to problems like severe engine deposits, injector cooking and piston ring sticking. In general, the viscosity of biodiesel increases by reducing the fuel temperatures. Investigations show that the viscosity of biodiesel from rapeseed oil is not so temperature-dependent as the biodiesel from waste cooking oil [4, 5]. Furthermore, the influencing parameters are the chain length, position, number, and nature of double bonds, as well as the nature of oxygenated moieties [6]. At this time, most working diesel engines have been developed for operation with mineral diesel fuels. For these engines biodiesel fuels can obviously not be used without any precautions. For this reason, many investigations are necessary to prevent or at least mitigate different engine or environmental problems. Due to above arguments, many current investigations are related to the influences of biodiesel fuel on combustion and exhaust emissions [1, 7, 8]. Many researchers agree that neat biodiesel generally causes a decrease of unburned HC, CO and PM emissions with respect to mineral diesel and an increase of NO x emission [9, 1]. Although the investigations show some general trends, one must say that the obtained results vary significantly in dependence on the used fuel, engine, engine operating regime, and injection system. This paper deals with the influences of biodiesel on the processes in mechanical inline fuel injection M system and on engine characteristics. For this purpose, the fuel properties of used mineral diesel and biodiesel as well as their blends are determined. The influences of fuels on injection, fuel spray and engine characteristics are investigated. Finally, the importance of injection pump timing in order to reduce all harmful emissions is presented. Tested fuel properties The tested fuels are: (i) neat diesel fuel, conforming to European standard EN 59, (ii) neat biodiesel fuel, conforming to European standard EN and (iii) their blends BXX, where XX denotes vol % of biodiesel in fuel (e.g. B2 consists of 2 vol % of biodiesel and 8 vol % of mineral diesel). Some measured properties of these fuels are given in Table 1. The biodiesel fuel is produced from rapeseed by Pinus, Slovenia. Table 1. Biodiesel and diesel properties Fuel Kinematic 3 o C, mm 2 /s Surface 3 o C, N/m Calorific value, kj/kg Cetane number >

2 The fuel density is measured with Density meter DMA 35 PAAR. The fuel density obtained by our experiment at ambient pressure is presented in Figure 1. In this figure the density for neat mineral diesel, neat biodiesel and for their blends is presented. The density increases by increasing the content of biodiesel and with lower fuel temperatures. The measurement of sound velocity in fuel is based on the principle of pressure wave propagation on the specified length of the HP tube. The sound velocity is measured at different pressures up to bar using different fuels. Figure 1 shows the dependence of fuel sound velocity. It increases with both the fuel pressure and biodiesel content. Fuel density (kg/m 3 ) Fuel temperature T ( o C) B8 B6 B4 B2 Fig. 1. Density and sound velocity of fuels at different conditions With respect to our experimental results and the numerically determined dependencies, the bulk modulus of elasticity is determined as E = a 2. Figure 2 shows how the bulk modulus increases when the pressure increases and the fuel temperature decreases. It also shows that the slopes of curves of used fuels from to are practically the same. The dynamic viscosity of fuels was measured for different fuels at two fuel temperatures, Figure 2. The results show that with lower fuel temperature the viscosity increases progressively with higher content of biodiesel in the fuel. Sound velocity a (m/s) T = 2 o C 2 3 Pressure p (bar) Bulk modulus E (MPa) T = -2 o C T = 4 o C 2 3 Pressure p (bar) Dynamic viscosity of fuel. (mpas) T = 4 C T = 3 C B8 B6 B4 B2 Fuel Fig. 2. Bulk modulus and dynamic viscosity of fuels at different conditions Fuel properties, like density, viscosity, sound velocity, bulk modulus, cetane number, oxygen content and so on, have significant effect on the start of injection, start of combustion, and premixed and diffusion burn peak, and over these on the emission and other engine performances. Influence of biodiesel on injection characteristics The injection M system was mounted on Friedman-Maier type 12H_h test bed for a conventional fuel injection pump Bosch PES 6A 95 D 41 LS 2542, fuel pipes and injection nozzles with one nozzle hole of.68 mm diameter. The data acquisition and processing system was based on a personal computer equipped with an I/O interface and a 12-bit A/D converter with 1 MHz sustained sampling rate. Here it needs to be noted that throughout this paper it is assumed that the pump load is not determined by the fuelling, but by the rack position. This means that at a single operating regime, the use of different fuels will cause the fuelling to be slightly different. The experiments are performed for neat diesel, neat biodiesel and their blends,, at peak torque and rated conditions at fuel temperature of 2 o C and at constant pump injection timing, Figure 3. The injection timing is advanced and the maximum of injection pressure p II increases by increasing the content of biodiesel. The deviations in viscosity and consequently in bulk modulus, 159

3 which affects the speed of sound, are responsible for the difference in injection timing. A higher bulk modulus, caused by increased content of biodiesel, leads to more rapid pressure wave propagation from the pump to the nozzle and an earlier needle lift. Higher viscosity of biodiesel leads to reduced fuel leakage during injection process, to faster evolution of pressure, and thus to advanced injection timing. Pressure pii [bar] 3 2 T =2 o C FL 8 rpm Camshaft angle [ o CAM] Pressure pii [bar] 6 5 T =2 o C FL 1 rpm Camshaft angle [ o CAM] Needle lift hn [mm] Camshaft angle [ o CAM] Needle lift hn [mm] Camshaft angle [ o CAM] Fig. 3. Fuel influence on injection pressure p II and needle lift h n at peak torque and rated condition In order to get a better comparison between injection characteristics with,,, and fuels, a comparison of some important injection characteristics at different operating regimes is performed. At constant pump injection timing and at constant rack position one can observe a gradual increase of fuelling with increased biodiesel content at lower speeds regimes and at low partial loads. The injection duration of biodiesel blends deviates from those when using mineral diesel, especially at low load regimes. For almost all tested regimes the largest difference is obtained at 5% partial load when using. Only at low speed regime (irrespective of load), the injection duration of biodiesel and its blends is shorter then that of mineral diesel. Shorter injection duration at relative high fuelling leads to higher mean injection rate, as can be seen at lower speed regimes at all tested loads, Figure 4. The difference between the injection pressures of different fuels arises due to different fuel density, viscosity, bulk modulus and sound velocity. The maximum injection pressure p II for and all its blends is higher than for, Figure 4, at all tested operating regimes. The maximum differences are observed at lower speed regimes at full load. Relative mean injection rate / / / / 12 FL, 5 rpm 118 FL, 8 rpm 116 FL, 1 rpm PL 75%, 5 rpm 11 PL 75%, 8 rpm 18 PL 75%, 1 rpm 16 PL 5%, 5 rpm PL 5%, 8 rpm PL 5%, 1 rpm 98 / / / / Fig. 4. Relative mean injection rate and maximum pressure using different fuels at different regimes The experimental results show that the injection timing advances with higher content of biodiesel because of the already mentioned reasons. This means that the injection delay (time interval between start of delivery and start of injection) becomes gradually smaller. For all operating regimes the injecttion characteristics have shown that with increasing content of biodiesel the injection delay decreases. Meanwhile, the injection timing, the mean injection rate, and the maximal injection pressure increase. It is known that with the advanced injection timing, the pressure in cylinder increases and the NO x emissions increase also. For this reason the NO x emission will increase by increasing the content of biodiesel, if the injection timing will remain unchanged. Relative maximum pressure. 16

4 Fuel spray characteristics The different properties of and lead to different injection characteristics and consequently to different fuel spray. As an example, Figure 5 shows the spray comparison for and at peak torque condition. One can see that the biodiesel spray angle is narrower and the penetration length is larger. Fig. 5. Fuel spray development of and at peak torque condition In fact, spray leads to a narrower and longer spray tip penetration in comparison to at most tested operating regimes. Some of the most important reasons for that are low fuel vaporization, worse atomization, and higher injection pressure of. Worse atomization is a consequence of high surface tension and viscosity of. This leads to higher spray tip penetration and to higher SMD of biodiesel spray. Higher injection pressure causes higher injection rates, contributing also to higher spray velocities of. This also contributes to a narrow spray and longer spray tip penetration. Higher injection pressure also annihilates the effect of increased friction between the biodiesel and nozzle surface due to higher viscosity of. This means that higher injection pressure is a necessity in the case of a designed diesel engine fueled with, if one wants to achieve good combustion performance. For our tested injection system, the injection pressure of is higher than that of at the same operating regime. The higher injection pressure of has a positive effect on the SMD and consequently on the combustion process. Influence of biodiesel on engine characteristics The experimental system consists of an engine and electro-dynamometer Zöllner A-35AC, 3kW, air flow rate meter RMG, fuel consumption dynamic measuring system AVL, UHC analyser Ratfisch, NO x chemoluminiscent analyzer Thermoelectron, O 2 analyzer Programmelectronic, CO analyzer Maihak and smoke meter AVL. The tested engine is 4 stroke, 6 cylinder in line, water cooled MAN D 2566 MUM with M injection system. This engine is used from exploitation after 5 km and after general renovation. The influence of fuel on engine characteristics is tested by running the engine with injection pump timing i = 23 o CA BTC, prescribed by the engine producer (for use with ). The comparison of some engine characteristics of and on idle and full load conditions (EC characteristics) is shown in Figure 6. Pe [kw] , _ EC, i = 23 o CA BTC EC, i = 23 o CA BTC 8 M e T g,e P e g e n [min -1 ] Me [Nm], ge [g/kwh], Tg,e [ o C] Brake thermal efficiency [%]. Fig. 6. Influence of fuel on engine performance at idle and full load conditions At full load, the engine effective torque M e and power P e decrease by about 5% using while the effective specific fuel consumption g e (for the actual fuel mass) increases by about 1% in the n [min -1 ] CO, HC, NOx [ppm] , _ CO HC EC, i = 23 o CA BTC smoke NOx n [min -1 ] Smoke [BU]

5 whole engine speed regime. On the other hand, the temperatures of exhaust gases T g,e are lower by about 3 o C, which may be due to the lower calorific value of. To compare the fuels with different calorific values, the brake thermal efficiency is presented in Figure 6 also. This efficiency is defined as the actual effective power, divided by the amount of fuel chemical energy (fuel consumption rate x calorific value). In spite of higher effective specific fuel consumption g e for at all engine speeds, the brake thermal efficiency is practically the same for both fuels because of different calorific values. By comparing the emissions of NO x, smoke, CO and unburned HC, it is evident from Figure 6 that the NO x emission increases at full load when using, meanwhile it decreases at idle. An opposite effect is observed for the smoke. The lower CO, HC and smoke emissions, when using, are primarily due to the fact that biodiesel contains more oxygen, which helps to oxidize these combustion products in the cylinder. The influence of injection pump timing on engine characteristics is tested for fuel. The tested values of pump timing I ( o CA BTC) in degree of cranckshaft angle before top center are 23, 21, 2, 19, 18 and 17. For retarded pump injection timing, the injection occurs closer to the end of compression stroke. Therefore, the peak in-cylinder pressure decreases with retarded injection pump timing and the peak is moved away from the top center. As it is evident, the maximum in-cylinder pressure decreases by about 2 bar when pump injection timing changes from 23 CA BTC to 17 o CA BTC. Figure 7 presents a comparison of at prescribed pump timing with at different pump timing values at rated conditions. Compared quantities are P e, g e, thermal efficiency, T g,e as well as emissions of CO, HC, NO x and smoke. 12 Rated conditions % 8 6 4, 23 o BTC, 23 o BTC, 21 o BTC, 2 o BTC, 19 o BTC, 18 o BTC, 17 o BTC 2 thermal P e g e effeciency T g,e CO HC NOx Smoke Fig. 7. Influence of injection pump timing and fuel on engine characteristics at rated condition The obtained results shown, that the compromise with respect to harmful emissions, engine power, specific fuel consumption as well as exhaust gas temperature has to be searched for by optimization of pump injection timing. According to the European 13 Modes ESC test, the individual characteristic points are weighted by corresponding factors that take into account the importance of individual engine regimes. By considering the ESC weighting factors, the reduction of emissions of with optimized injection pump timing i = 19 o CA BTC compared to with standard i = 23 o CA BTC is as shown in Figure 8. Evidently, all of the emissions reduced essentially. The results indicate that the employed biodiesel could represent a very attractive fuel, at least for engines similar to the one used on the test. 8 [%] 6 4 2, 23 o BTC, 19 o BTC CO HC NO x Smoke Fig. 8. Emissions of and considering ESC test weighting factors Finally, it is worth to make some notes on the engine behavior during the test. In order to perform all the above described tests, the engine has run on fuel for about 6 hours. During this period the engine performed completely normally. There were no difficulties regarding engine starting. After 162

6 the tests, the critical components of the engine were carefully examined. All of them were in a normal condition. Conclusions The influence of biodiesel on the most important injection characteristics of the fuel injection M system as well as on the most important engine characteristics was investigated. From the results, the following conclusions can be made: the fuelling, injection duration, injection timing as well as injection pressure increase with increasing the part of biodiesel at most considered operating regimes. When increasing the part of biodiesel, the injection delay and advanced injection timing are a consequence of higher sound velocity and higher bulk modulus. By higher content of biodiesel the injection timing is advanced. Because the advanced injection timing leads to higher combustion pressure and temperatures, and consequently to higher NO x emission, the injection timing needs to be retarded accordingly; compared to, biodiesel forms a narrower and longer spray under most tested operating regimes. The SMD is not as high as it could be expected due to the physical properties of. This is due to higher mean injection pressure and mean injection rate of (compared to ); Because of the lower heating value of, the engine effective power is reduced by about 5% and the specific fuel consumption is increased by about 1%, while the thermal efficiency is practically the same for both fuels; it is possible to find the optimal injection pump timing for so that all harmful emissions are reduced while keeping the effective power, effective specific fuel consumption, temperature of exhaust gases, cylinder pressure, and other important engine characteristics at acceptable levels. The optimized injection pump timing of i = 19 o CA BTC for fuel offers a reduction of CO and NO x emissions by about 25%, a reduction of HC emissions by about 3% and a reduction of smoke emissions by about 5% with respect to ECS test and the corresponding weighting factors. Acknowledgment. This research was supported by the European Community s Sixth Framework Programme in the scope of the Civitas II Mobilis project. References 1. Dorado, M. P., E. Ballesteros, J. M. Arnal, J. Gomez, F. J. Lopez, Exhaust emissions from a diesel engine fueled with transesterfified waste olive oil, Fuel 82 (23) Senda, J., N. Okui, T. Tsukamoto, H. Fujimoto, On board measurement of engine performance and emissions in diesel vehicle operated with biodiesel fuel, SAE , Lin C.Y., H.A. Lin, Diesel engine performance and emission characteristics of biodiesel characteristics of biodiesel produced by the peroxidation process, Fuel 85 (26) Kerschbaum, S., G. Rinke, Measurement of the temperature dependent viscosity of biodiesel fuels, Fuel 83 (24) Krisnangkura K., T. Yimsuwan, R. Pairintra, An empirical approach in predicting biodiesel viscosity at various temperatures, Fuel 85 (26) Knothe, G., K. R. Steidley, Kinematic viscosity of biodiesel fuel components and related compounds, Influence of compound structure and comparison to petrodiesel fuel components, Fuel 84 (25) Kegl B., Experimental investigation of optimal timing of the diesel engine injection pump by using biodiesel fuel, Energy & Fuels 2 (26) Dorado M.P., E. Ballesteros, J.M. Arnal, J. Gomez, F.J.L. Gimenez, Testing waste olive oil methyl ester as a fuel in a diesel engine, Energy and Fuels 17 (23) Nabi, N., S. Akhter, M. Z. Shahadat, Improvement of engine emissions with conventional diesel fuel and diesel biodiesel blends, Bioresource Technology 97 (26) Kegl, B., Numerical analysis of injection characteristics using biodiesel, Fuel 85 (26)