Eco-diesel engine fuelled with rapeseed oil methyl ester and ethanol. Part 3: combustion processes

Transcription

1 Eco-diesel engine fuelled with rapeseed oil methyl ester and ethanol. Part 3: combustion processes A Kowalewicz Technical University of Radom, al. Chrobrego 45, Radom, , Poland. andrzej.kowalewicz@pr.radom.pl The manuscript was received on 28 October 2005 and was accepted after revision for publication on 12 May DOI: / JAUTO Abstract: In Part 1 of the paper efficiency and emissions of the engine fuelled with rape oil methyl ester (RME) and ethanol were presented and discussed. In the second part of the paper a comparison of emissions and brake fuel conversion efficiency of the engine for two base fuels, diesel fuel and rape oil ethyl ester, was carried out. In this paper, Part 3, combustion processes are investigated. Experiments were carried out on the same single-cylinder direct injection compression ignition engine as in Part 1. In the course of the experiments the pressure time history of the engine cylinder and instantaneous parameters of injection of the RME fuel were measured and used to calculate the rate of heat release and the fraction of fuel burnt versus crank angle, which showed that the higher the energy fraction of ethanol in the total fuel, the longer the ignition delay but the shorter the total time of combustion. It was found that shorter combustion results in higher efficiency. Keywords: rate of heat release, fraction of fuel burnt, RME, ethanol 1 INTRODUCTION measured with the AVL sensor QL21D. The highspeed measurements were synchronized with crank In the first part of the paper [1], brake fuel conversion angle measured using the Introl sensor. All measured efficiency and emissions of CO, CO, HC, and smoke 2 quantities were transmitted to the high-speed were measured and analysed. In the second part measurement system developed in the Department of the paper [2] a comparison of emissions and of Internal Combustion Engine and Automobiles [3]. efficiency of the engine fuelled either with diesel fuel Pressure versus crank angle (CA) and the lift of or with ester as a base fuel with and without injection injector needle versus CA both from 200 cycles of ethanol into an inlet port was carried out and were input data to the programmes worked out at analysed. In this paper the diagrams of pressure in Politechnika Radomska [3], with whatever rate of the cylinder, heat release rate, and fraction of fuel heat release and fraction of fuel burnt versus CA was burnt are analysed and discussed from the point simulated. Also, in the course of computation, the of view of the influence of the ethanol fraction in period of the start of injection to the end of comthe total fuel. It is worth remembering that the aim bustion (at the 98 per cent fraction of fuel burnt) of ethanol injection into the inlet port was the was determined. These data were used to calculate formation of a homogeneous mixture with air, which, ignition delay, time of combustion, and total time of after ignition by self-ignited droplets of rape oil combustion, i.e. the period from the moment of the methyl ester (RME) should accelerate their burning, start of injection to the end of combustion. In this resulting in shorter combustion of the total fuel. paper combustion processes in the engine cylinder During the course of experiments described in the for representative working conditions with speeds of first part of the paper instantaneous engine para- 1200, 1800, and 2200 r/min, two loads of 20 and meters were measured. Pressure in the cylinder 40 N m, and the angle of the beginning of pressing was measured with the AVL transducer 8QP505 at 30 CA BTDC (before top dead centre) are analysed. inserted in the cylinder head. Injector needle lift was (Between the beginning of injection and pressing measured with the inductive sensor CL80 of Polish there is an injection delay, dependant on engine production. Fuel pressure before the injector was speed.)

2 1284 A Kowalewicz 2 INFLUENCE OF ETHANOL INJECTION ON TEMPERATURE OF THE CYCLES Ethanol injected into the inlet port quickly evaporates and forms a homogeneous gaseous mixture in the cylinder before self-ignition of RME droplets. The evaporation process decreases temperature of the charge, especially when the ethanol fraction is high. Moreover, due to the high velocity of air, especially at high engine speeds (n= 2200 r/min), the temperature of the charge decreases. The total drop in static temperature is expressed by the equation Dt= w2 + L 2c ll c p o p where w=air velocity in the inlet port c =specific heat of the air at constant pressure p l=air excess coefficient L =stoichiometric air (kg/kg fuel) o L=heat of evaporation of ethanol The charge temperature in the function of the ethanol fraction is shown in Fig. 1 at low load and in Fig. 2 at high load. Because of the cooling effect of ethanol evaporation, the exhaust gas temperature also decreases with the ethanol fraction V E, but only at high load, while at low load it is rather independent of V E (Figs 3 and 4). The decrease in temperature at the inlet and at the exhaust of the engine with the ethanol fraction at Fig. 2 Temperature of inlet charge as a function of the ethanol energy fraction in the total fuel at high load (p =0.512 MPa) e Fig. 3 Temperature of exhaust gases as a function of the ethanol energy fraction in the total fuel at low load (p e =0.256 MPa) Fig. 1 Temperature of inlet charge as a function of the ethanol energy fraction in the total fuel at low load (p =0.256 MPa) e high load means that the cycle temperature is also decreased as a result of the cooling effect of ethanol evaporation, visible for the high ethanol fraction. Another reason for the reduction of the exhaust gas temperature at high load is that the combustion is advanced and in-cylinder pressure of the gases is higher, which results in an earlier temperature rise and, as a result, a higher heat transfer to the walls and more heat loss (especially at low speeds).

3 Eco-diesel engine fuelled with rapeseed oil methyl ester and ethanol. Part not shown here, the following conclusions may be drawn: 1. At low load maximum pressure (Fig. 5): (a) is the highest for fuelling only with RME (V E =0) (P maxrme ); (b) decreases with an increase in the ethanol fraction; (c) decreases with an increase in engine speed; (d) shifts to the right according to TDC (top dead centre) with an increase in the ethanol fraction. The results above are caused by the cooling effect of ethanol evaporation resulting in lower temperature of the cycle at low load. Fig. 4 Temperature of exhaust gases as a function of the ethanol energy fraction in the total fuel at high load (p =0.512 MPa) e 3 INFLUENCE OF ETHANOL INJECTION ON COMBUSTION CHARACTERISTICS 3.1 Analysis of cylinder pressure Pressure versus CA diagrams measured at high speed and at low and high loads are shown in Figs 5 and 6. From these and other pressure diagrams 2. At high load the maximum pressure (Fig. 6): (a) is the lowest for fuelling only with RME (V =0); E (b) increases with an increase in the ethanol fraction; (c) decreases with an increase in engine speed; (d) indicated by the shape of the pressure diagrams, suggests a long ignition delay (shown in the diagrams of the heat release rate and the fraction of fuel burnt). This behaviour of pressure is the result of the compensation of the cycle temperature drop by the temperature increase at high load (more fuel is burnt) and more and more rapid combustion Fig. 5 Pressure versus CA at low load, p e =0.256 MPa, speed 2200 r/min, and injection timing of

4 1286 A Kowalewicz Fig. 6 Pressure versus CA at high load (p e =0.512 MPa), speed 2200 r/min, and injection timing of with an increase in the ethanol fraction. Values of (b) burning is rapid, controlled by chemical maximum pressure P, CA of maximum pressure kinetics (burning premixed fuel vapours and max a, appropriate ethanol fraction V, P, and air). Pmax E maxrme adequate CA a are given in Table 1. PmaxRME 2. For high load (Figs 8 to 10): 3.2 Analysis of the relative rates of heat release (a) the higher the ethanol fraction, the higher the maximum of heat release rate; Representative diagrams of the relative rates of (b) the diffusional character of burning changes heat release, corresponding to the pressure diagrams into a kinetic one with an increase in the shown in Figs 5 and 6, are shown in Figs 7 and 8 and ethanol fraction. additionally in Figs 9 and 10. From these and other With an increase in the ethanol fraction, the fraction diagrams (for 1200 and 1800 r/min) the following of fuel burnt increases slowly at the beginning of conclusions may be drawn: combustion and very quickly at the end for all cases 1. For low load (Fig. 7): of engine operation (Figs 11 and 12). (a) the higher the ethanol fraction, the lower the An explanation of these results is as follows. Ethanol maximum of heat release rate and its maximum injected into the inlet port evaporates very quickly is more delayed; and forms a homogeneous (premixed) mixture with Table 1 Maximum cylinder pressure P max, angle of its occurrence a Pmax, appropriate V E, maximum cylinder pressure for fuelling with RME only P maxrme, and angle of its occurrence a PmaxRME n (r/min) T (N m) P max (MPa) a Pmax ( OWK) V E P maxrme (MPa) a PmaxRME ( CA)

5 Eco-diesel engine fuelled with rapeseed oil methyl ester and ethanol. Part Fig. 7 Heat release rate versus CA at low load (p e =0.256 MPa), speed 2200 r/min, and injection timing of Fig. 8 Heat release rate versus CA at high load, speed 2200 r/min, and injection timing of air during the intake stroke. Also droplets of RME injected during the compression stroke evaporate during ignition delay. Vapours of both these fuels form the premixed mixture. For low load, after ignition the rate of heat release is typical for premixed/rapid (chemical kinetics controlled) combustion. For high load and fuelling only with RME and with both fuels but with a low ethanol fraction, the diffusional character of burning (diffusion controlled combustion) is more significant and the hump (or peak) at the curve of the rate of heat release is visible. This hump (peak) disappears with an increase in the ethanol fraction. This means that mixing/ diffusion controlled combustion changes into rapid combustion (chemical kinetics controlled) with an increase in the ethanol fraction. 3.3 Analysis of the fraction of fuel burnt Representative diagrams of the fraction of fuel burnt versus CA corresponding to the pressure diagrams (Figs 5 and 6) and the heat release rate diagrams (Figs 7 and 8) respectively are shown in Figs 11 and 12. In all cases, even if at the beginning burning of the fuel

6 1288 A Kowalewicz Fig. 9 Heat release rate versus CA at high load, speed 1800 r/min, and injection timing of Fig. 10 Heat release rate versus CA at low high, speed 1200 r/min, and injection timing of is lower, the higher the ethanol fraction, the shorter efficiency (reference [1], Fig. 4), lower NO emission x the combustion. For a high ethanol fraction the curve (reference [1], Fig. 11), and higher HC and CO x(a) is very steep in the later phase of combustion. emission (reference [1], Figs 16 and 17). On the contrary, To sum up, under low load, when the cooling under high load, when the cooling effect of effect of ethanol evaporation is high in comparison ethanol evaporation is low in comparison with the with the effect of low temperature of the cycle effect of high temperature of the cycle corresponding corresponding to a low amount of heat evolved, to a large amount of heat evolved, the increasing the increasing fraction of ethanol results in lower fraction of ethanol results in higher pressure (and pressure (Fig. 5) and heat release rates (Fig. 7), low temperature), better efficiency (reference [1], Fig. 4),

7 Eco-diesel engine fuelled with rapeseed oil methyl ester and ethanol. Part Fig. 11 Fraction of fuel burnt versus CA at low load, speed 2200 r/min, and injection timing of Fig. 12 Fraction of fuel burnt versus CA at high load, speed 2200 r/min, and injection timing of lower emissions (reference [1], Figs 5 to 11), except 3.4 Ignition delay and combustion time NO (reference [1], Fig. 12), especially CO and x 2 smoke, and change in the character of combustion Measured data of the beginning of needle lift, the from diffusional (for neat RME) into kinetic. Similar point of the start of combustion (the heat release rate results have been obtained for fuelling a CI engine increases from zero), and the point of the end of with diesel fuel with an ethanol injection into the combustion (the fraction of fuel burnt x=0.98) were inlet port [4, 5] and for diesel fuel alcohol blends used to compute ignition delay and the time of [6 8]. combustion. The total time of combustion is the sum

8 1290 A Kowalewicz of ignition delay and combustion time, and is very important for diagnostics of combustion processes. All these characteristic times (in degrees of CA) for ignition timing at 30 CA, three speeds, and two loads versus ethanol fractions are shown in Table 2. The time of combustion and total time of combustion are presented as a function of the ethanol fraction V E in Figs 13 to 15. From these figures (and also from diagrams of the heat release rate and fraction of fuel burnt) the following conclusions may be drawn. For all speeds and loads: 1. In spite of a longer ignition delay, the total combustion time is shorter with increasing ethanol fraction, V E. 2. The combustion time decreases with an increase in the ethanol fraction, V E. Fig. 13 Combustion time t (CA deg) and total combustion time t +t (CA deg) as a function of id 4 CONCLUSIONS id c the ethanol energy fraction V at 1200 r/min E and two loads As far as combustion processes in DI CI engines are concerned, an ethanol injection into the inlet port Additional injections of ethanol into the inlet port during the suction stroke accelerates combustion, result in lower emissions of smoke and carbon changing the diffusional character of burning into dioxide and, at low load, also nitric oxide. The rapid combustion typical of a premixed mixture. ethanol energy fraction in the total fuel energy V E Although the ignition delay increases with the may reach 50 per cent at low load and 30 per cent ethanol fraction, the combustion time decreases, at high load but is limited by the occurrence of resulting in a much shorter total combustion time. diesel knock. Table 2 Ignition delay t id, combustion time t c, and total time of combustion t id +t c versus speed, load, and ethanol fraction V E n (r/min) T (N m) V E t id (CA deg) t c (CA deg) t id +t c (CA deg)

9 Eco-diesel engine fuelled with rapeseed oil methyl ester and ethanol. Part REFERENCES 1 Kowalewicz, A. Eco-diesel engine fuelled with rapeseed oil methyl ester and ethanol. Part 1: efficiency and emission. Proc. IMechE, Part D: J. Automobile Engineering, 2005, 219(D5), Kowalewicz, A. Eco-diesel engine fuelled with rapeseed oil methyl ester and ethanol. Part 2: comparison of emissions and efficiency for two base fuels: diesel fuel and ester. Proc. IMechE, Part D: J. Automobile Engineering, 2006, 220(D9), (this issue). 3 Różycki, A. Microcomputer system for measurement of high speed parameters for CI engines. In 8th European Automotive Congress, Bratislava, Kowalewicz, A. and Pajączek, Z. Eco-diesel engine with additional injection of ethanol. In Archivum Combustionis, Vol. 23, 2003, No. 3 4 (Polish Academy of Sciences). 5 Kowalewicz, A. Emission characteristics of compression ignition engine fuelled with RME/DF and ethanol. J. KONES, Int. Combust. Engines, 2004, Fig. 14 Combustion time t (CA deg) and total com- c 11(1 2), bustion time t +t (CA deg) as a function of id c 6 Lü, X., Huang, Z., Zhang, W., and Li, D. The influence the ethanol energy fraction V at 1800 r/min E of ethanol additives and the performance and comand two loads bustion characteristics of diesel engines. Combust. Sci. Technol., 2004, 176, Huang, Z. H., Lu, H. B., Jiang, D. M., Zeng, K., Liu, B., Zhang, J. Q., and Wang, X. B. Engine performance and emissions of a compression ignition engine operating on the diesel methanol blend. Proc. Instn Mech. Engrs, Part D: J. Automobile Engineering, 2004, 218(D4), Huang, Z. H., Lu, H. B., Jiang, D. M., Zeng, K., Liu, B., Zhang, J. Q., and Wang, X. B. Combustion characteristics and heat release analysis of a compression ignition engine operating on a diesel/methanol blend. Proc. Instn Mech. Engrs, Part D: J. Automobile Engineering, 2004, 218(D9), Fig. 15 APPENDIX Notation n engine speed (r/min) T torque (N m) x fraction of fuel burnt a Combustion time t (CA deg) and total com- c bustion time t +t (CA deg) as a function of id c the ethanol energy fraction V at 2200 r/min E and two loads t id t c V E CA of beginning of injection of RME (deg BTDC) ignition delay (CA deg) combustion period ratio of ethanol energy to total fuel energy (ethanol energy fraction) Abbreviations BTDC before top dead centre ACKNOWLEDGEMENT CA crank angle CI compression ignition The work was sponsored by a grant of the Polish DI direct injection Committee for Scientific Research 5T12D2922. RME rape oil methyl ester