Control of the combustion behaviour in a diesel engine using early injection and gas addition

Transcription

1 Applied Thermal Engineering 26 (2006) Control of the combustion behaviour in a diesel engine using early injection and gas addition A.P. Carlucci *, A. Ficarella, D. Laforgia University of Lecce Research Center for Energy and Environment (CREA), via per Arnesano, Lecce, Italy Received 10 January 200; accepted 31 March 2006 Available online 30 May 2006 Abstract Multiple injections and natural gas addition were investigated as ways to modify combustion behaviour, and therefore pollutant emissions and specific fuel consumptions, inside a direct injection Diesel engine equipped with a common rail injection system. During the experimental tests, engine efficiency, in terms of fuel consumption, and pollutant emissions, in terms of nitric oxides, opacity, carbon monoxide and total hydrocarbons, have been measured. The tested multiple injection strategy consisted of the simultaneous use of early and pilot injections. This strategy has been compared with the more traditional techniques based on the use of either pilot or early injections. During the tests, the effects of several injection parameters were analysed, like duration and timing of early, pilot and main injections. Results show that, mainly for medium values of engine torque and speed, the injection of a small fuel quantity during the early stage of the compression stroke, coupled with the pilot injection, may be effective in reducing specific fuel consumption if compared to the only pilot or only early injection strategies. Furthermore, this result is obtained whit a simultaneous reduction in nitric oxides and particulate. However, unburned hydrocarbons levels remain constant or usually increase. Early injection is in effect a way to obtain a very lean premixed charge, both globally and locally, inside the combustion chamber. Therefore, it has been shown that nitric oxides and soot, deriving respectively from an inhomogeneous distribution of temperatures and a locally rich mixture, both decrease performing the early and pilot before the main injection. Concerning the natural gas addition, it has been premixed with the engine intake air before the turbocharger and used in small percentages, in order to improve the engine combustion and to reduce pollutant emissions, in particular the soot produced during the mixing-controlled combustion phase. Experiments underlined that, using the natural gas as an additive fuel, while performing the Diesel fuel main injection, leads to keep practically unchanged engine efficiency with respect to the traditional Diesel fuel operation mode. Concerning the emission levels at the exhaust, the use of small quantities of gas (10 30% respect to the total fuel energy) improves the oxides soot trade-off; however, at the same time, total hydrocarbons and carbon monoxide emissions are characterized by higher values. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Combustion behaviour; Gas addition; Injection 1. Introduction Future developments of direct injection diesel engines will aim at the reduction of specific fuel consumption and exhaust emissions. Such a reduction can be obtained either directly controlling the combustion development, for example using multiple fuel injections, optimising the design of * Corresponding author. Tel.: ; fax: address: paolo.carlucci@unile.it (A.P. Carlucci). the combustion chamber, controlling the exhaust gas recirculation, or using alternative fuels or additives, or acting at the engine exhaust, for example using the DeNO x catalyst, particulate trap or secondary air at the outlet. Previous works focused on the effectiveness, among direct techniques, of multiple injections and natural gas addition as ways to improve the diesel combustion efficiency without increasing significantly the engine complexity and costs. With the common rail injection system, and using the potentialities of the electroinjectors, it is possible to /$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi: /j.applthermaleng

2 2280 A.P. Carlucci et al. / Applied Thermal Engineering 26 (2006) Nomenclature CA degrees crank angle CABTDC degrees crank angle before top dead center AFR air to fuel ratio Ai main injection timing [ CABTDC] AiE early injection timing [ CABTDC] AiP pilot injection timing [ CABTDC] BSFC brake specific fuel consumption [kg/h] CO carbon monoxide [vol.%] EM early main injection strategy EMA early main after injection strategy EPM early pilot main injection strategy EtE early injection duration [ls] EtP pilot injection duration [ls] FSN filter smoke number GHRR gross heat release rate [J/ CA] NO x nitric oxides [ppm] p inj fuel injection pressure [MPa] PM pilot main injection strategy TDC top dead center THC total hydrocarbons [ppm] manage the fuel rate-of-injection, in particular to perform the fuel main injection coupled with the so-called pilot injection, early injection, post injection or, more in general, to perform a multiple injections strategy. The pilot injection is a small quantity of fuel injected before the main injection [typically CABTDC]. This strategy is effective in the reduction of the combustion noise characterising direct injection diesel engines. In any case, the variation in the pressure history inside the combustion chamber, and consequently in the heat release rate, determines an effect of the pilot injection on pollutant emission levels and in specific fuel consumption as well [1 6]. The early injection is an injection of a widely variable quantity of fuel, extremely advanced with respect to the main injection (typically CABTDC). Performing the early injection, the fuel fraction burned in premixed phase is greater than in the traditional operations, thanks to the prolonged ignition delay. This contributes to obtain a more homogeneous temperature and AFR distribution within the combustion chamber, so limiting NO x and soot production [7]. Yokota et al. [8] compared the injection strategy obtained coupling early and main injections (EM strategy) with the one coupling pilot and main injections (PM strategy), and with the simultaneous use of early, main and after injections (EMA injection strategy, where the After injection is performed late during the expansion stroke), and their effectiveness in reducing specific fuel consumption and pollutant emissions, for different values of the main injection advance. Results have shown the possibility to decrease NO x emissions levels moving from PM to EM and to EMA, obtaining in the same time an increase in particulate levels. Yamane and Shimamoto [9] have compared NO x, particulate and specific consumption of a standard diesel engine with different injection strategies based on the use of the early injection. Results show that, increasing early injection advance, NO x levels abatement is observed without a significant increase in particulate emissions. However, because of a very advanced ignition of the lean mixture, fuel consumptions tend to increase noticeably with respect to those of a traditional diesel engine. Among alternative fuels, natural gas is considered particularly promising, thanks to its chemical and physical characteristics. That is why the literature proposes many works dealing with the use of natural gas and the best conditions of its utilization in the internal combustion engine field, in order to decrease both specific fuel consumption and pollutant emission levels. In fact, coupling the natural gas properties with the potentials of common rail high pressure injection system, it is possible to reach performance and efficiency levels similar to those of a traditional diesel engine. Plus, using the natural gas, particulate emissions are considerably reduced, thanks to the reduced diesel fuel consumption. However, such engines also show higher unburned hydrocarbons and carbon monoxide levels at the exhaust, which are generally caused by lower temperatures and therefore combustion rates inside the combustion chamber. This phenomenon is more evident with lower loads, when the air/natural gas mixture is very lean and therefore more difficult to ignite and propagate the flame [10]. In the field of dual fuel engines, which means engines fuelled with both diesel fuel and natural gas, two operating modes can be generally distinguished. In the first one, most of power (80 9%) is provided by the natural gas. Only a pilot injection of diesel fuel is therefore used in order to ignite the natural gas. In the second operating mode, the natural gas is simply used as additive, in order to clean the combustion. Consequently, power provided by the natural gas is generally in the range of 10 30% of the power delivered by the engine. An effective way to control the performance and emissions of a dual fuel engine is to vary the mass ratio, which is defined as the percentage of natural gas flux over the full mixture flux: _m natgas x ¼ 100ð%Þ ð1þ _m dieselfuel þ _m natgas In [11], for example, it has been found that increasing the mass ratio produces a lower heat release rate during the diffusive phase of combustion if compared with the traditional diesel engine. This phenomenon is related to the increase in the specific heat of the air/natural gas mixture

3 A.P. Carlucci et al. / Applied Thermal Engineering 26 (2006) and therefore to its lower combustion rate with respect to that of the air/diesel fuel mixture. This causes, consequently, an increase in the combustion duration with increasing mass ratio, and therefore, an increase in brake specific fuel consumption as well. Generally, it has been observed a decrease in particulate levels at the exhaust without NO x penalization increasing the mass ratio. Particulate decreases due to reduced diesel fuel quantity. THC levels, as previously observed, increase with increasing the mass ratio. This increase is mainly evident with low loads, while, at medium-high loads, the increasing temperature promotes their oxidation. CO emission levels present the same behaviour of THC, also due to lower temperatures. These results are generally proposed in other works as well [12 16]. 2. Experimental set-up The apparatus used for the experiments is schematised in Fig. 1. During experiments, a 2-liter, 4-cylinder, direct injection Diesel engine type FIAT 4 D1.000 was used, supercharged by a GARRETT TD 02 turbocharger. The engine was equipped with a BOSCH common rail fuel injection system, characterized by a maximum injection pressure of 1 MPa. The engine was coupled by a flexible joint to an eddy-current dynamometer. An electronic control unit (ECU), connected to a PC, controlled the common rail injection system; injection strategy and related injection parameters, such as durations, timings, and injection pressure, were fixed running the management software. Natural gas, initially stored in a cylinder at a pressure of 22 MPa, was supplied to the engine intake at ambient pressure, obtained using a pressure valve control. The data acquired during the experimental tests were: combustion chamber pressure, BSFC, and pollutant concentrations in exhaust gases. In-cylinder pressure was measured using a piezoelectric pressure transducer. The signal of the cylinder pressure was digitised each 0.1 CA. A mean combustion cycle was obtained averaging 0 cycles acquired in sequence. The GHRR was obtained adding the net heat release, obtained by using the single-zone first law equation, with the heat transferred through the cylinder walls, estimated by the Woschni model. Engine performance have been characterized in terms of exhaust emissions and fuel consumption. In particular, NO x and THC emissions have been measured by sampling the exhaust gases and analysing them by means of an exhaust gas analyser. Smoke emissions have been characterized estimating the FSN by the exhaust gas opacity measurements. BSFC measurements were obtained averaging the instantaneous gravimetric fuel consumptions measured during an observation time of 30 s. 3. Experimental results with multiple injections 3.1. Combustion behaviour In the following, any test characterized by different injection strategies will be indicated as follows: Pilot and main (PM) injection strategy: PMðAiP value½ CABTDCŠ=EtP value½lsš= Ai value½ CABTDCŠÞ Early and main (EM) injection strategy: EMðAiE value½ CABTDCŠ=EtE value½lsš= Ai value½ CABTDCŠÞ PC Based Data Storage and Post-processing System PC Based Injection Control and E.C.U. Monitoring Opacimeter NOx, THC Analysers Data Acquisition Board E.C.U. Compressed Natural Gas Angular Reference Cylinder Pressure Fuel Meter System Air Carburetor Pressure Valve Control Eddy-Current Dynamometer PC Based Dynamometer Monitoring and Control Fig. 1. Experimental set-up.

4 2282 A.P. Carlucci et al. / Applied Thermal Engineering 26 (2006) Early, pilot and main (EPM) injection strategy: EPMðAiE value½ CABTDCŠ=EtE value½lsš= AiP value½ CABTDCŠ=EtP value½lsš= Ai value½ CABTDCŠÞ It has to be pointed out that advances and durations refer to the activation of the energizing current driven to open the injector. GHRR are compared in Figs. 2 and 3, when both early and pilot injections are performed, for different values of fuel injected during the early injection and for operating condition 1 in Table 1. InFig. 2 pilot injection is characterized by an advance and a duration of respectively 16.0 CABTDC and 0 ls, while in Fig. 3 they are equal to 16.0 CABTDC and 0 ls. In both figures the results are reported for both operating conditions 1 (case a ) and 2 (case b ). Injection pressure was always equal to p inj = 100 MPa, while main injection advance was fixed equal to 3. CA. It is clear, by comparing Figs. 2 and 3, that the combustion behaviour strongly depends on the pilot injection tendency to burn. In fact, as already shown in [2], a pilot injection 0 ls long, coupled with only the main injection, does not burn. In this case, an increase of the early duration causes the increase of heat released before the main combustion too (small combustion humps are already observed for 20 CABTDC), and consequently bulk temperature inside the combustion chamber is increased Gross Heat Release Rate [J/deg] 6 4 EtE = 0 micros EtE = 0 micros EtE = 300 micros EtE = 40 micros Gross Heat Release Rate[J/deg] 6 4 EtE = 0 micros EtE = 0 micros EtE = 300 micros EtE = 40 micros Fig. 2. Gross heat released for EPM 63.0/EtE/16.0/0/3. (EtE = ls) for operating condition 1 and 2. Gross Heat Release Rate [J/deg] 6 4 EtE = 0 micros EtE = 0 micros EtE = 300 micros EtE = 40 micros Gross Heat Release Rate[J/deg] 6 4 EtE = 0 micros EtE = 0 micros EtE = 300 micros EtE = 40 micros Fig. 3. Gross heat released for EPM 63.0/EtE/16.0/0/3. (EtE = ls) for operating condition 1 and 2. Table 1 Engine operating conditions related to the multiple injections mode n [rpm] T [Nm] Engine operating conditions too, leading to a decrease in the ignition delay of the main injection. The comparison between the two cases with EtE = 0 and 0 ls infig. 2a underlines that an increase of the premixed combustion peak is observed with a reduction of the main ignition delay. This behaviour is opposite to the expected; in fact, the premixed peak is generally proportional to the ignition delay. Further increasing the early injection duration, the premixed peak decreases, as expected, as a consequence of decreasing main ignition delay. The mixing-controlled combustion shows no differences with varying the early duration, being basically regulated by the fuel injection rate of the main injection. Increasing the duration of the pilot injection to 0 ls leads to an evident phase of heat released before the main combustion, as plotted in Fig. 3a. However, the peak of GHRR related to the combustion of early + pilot injections decreases as the early duration increases, contrary to what expected and found in [2], where only pilot injection was performed. Furthermore, for EtE = 40 ls, a variation of the combustion nature of the early pilot fuel is observed, changing from premixed to mixing-controlled.

5 A.P. Carlucci et al. / Applied Thermal Engineering 26 (2006) These effects are observed with no apparent variation in the early + pilot injection delay, but only in the initial combustion rate. From Fig. 3a it is evident that the peak of the premixed phase behaves in a different way if compared to the previous case. In particular, the increase in the peak value is still observed increasing EtE from 0 to 0 ls, but increasing more, peak is almost unchanged with respect to the 0 ls case. The variation of fuel quantity burnt before the main combustion seems to have a negligible effect on main ignition delay too. The more pronounced combustion of the early pilot mixture before the main combustion leads to an appreciable reduction of the ignition delay of the main injection, and consequently to a sensibly lower premixed combustion peak and a more pronounced mixing-controlled combustion phase, as one can see by comparing Figs. 2a and3a. Comparison between these two plots also reveals that pilot combustion always starts around 12 CABTDC, apart from its duration. In Figs. 2b and 3b, GHRR are compared, related to different values of fuel injected during the early injection, for the operating condition 2 in Table 1. Compared with the previous case, now the pilot combustion is more advanced, showing small humps around CABTDC and starting around 20 CABTDC with longer early durations. About the main combustion, it seems to not be sensibly influenced by the pilot combustion, always starting around TDC. The early pilot combustion development strongly depends on the early duration. In fact, increasing EtE, the CA with the GHRR peak tends to be advanced. Plus, the maximum value shows a decreasing value increasing the early duration from 0 to 0 ls, but increasing further early duration, the peak trend becomes increasing. Premixed peak of the main injection is almost constant for null or lowest tested early duration, while further increasing early duration it is lower and advanced. When increasing pilot duration (Fig. 3b), an evident increase of the fuel burning before the main combustion is observed. By comparing Figs. 2b and 3b, the pilot injection seems to not have any effect on early pilot ignition delay. However, in this last case, GHRR peak before the main combustion is less dependent on the early duration. Moreover, for higher early durations, the related GHRR plots show a more complex shape of the early pilot combustion, with two visible peaks before the main injection. Main combustion is characterized by a lower premixed peak if compared to cases in Fig. 2b, and a slight decrease of peak value when increasing the early duration can be observed. Data related to the idle condition (operating condition 3 in Table 1), obtained with an injection pressure equal to p inj = 30 MPa, and a main injection timing equal to 3. CA showed that, for lower pilot durations, no combustion can be observed related to early + pilot before the main injection. The only observable effect of increasing the early duration on GHRR histories is a premixed peak slightly decreasing and advancing. Increasing the pilot Gross Heat Release Rate [J/deg] injection until 0 ls, a small hump in GHRR plots is observed at 10 CABTDC without performing the early injection. In this case the main combustion takes place earlier, leading also to a lower premixed peak. But, performing the early injection, the main combustion appears more retarded and characterized by an higher premixed peak. As before, the peak is decreasing and advancing with increasing the early duration. In Fig. 4, a comparison among M, EM and EPM injection strategies is underlined. In this case, the engine speed and torque were those related to the operating condition 1. Injection pressure was kept equal to p inj = 100 MPa and main timing was always equal to 3. CABTDC. Performing only the main injection (M 3. case), combustion starts around TDC showing a strong premixed combustion followed by a mixing-controlled phase. Coupling the main injection with the early injection (EM 63/40/ 3. case), in particular with a duration of 40 ls, combustion humps appear around 20 CABTDC and after 13 CABTDC. The main combustion is therefore sensibly advanced if compared with the previous case. Adding also the pilot injection (EPM 63/40/16.0/0/3. and EPM 63/ 40/16.0/0/3. cases) with variable pilot injection duration, the humps previously described remain unchanged, while a reduction of both main injection delay and related premixed peak can be appreciated Performance M 3. EM63/40/3. EPM 63/40/16.0/0/3. EPM 63/40/16.0/0/3. Fig. 4. M, EM and EPM injection strategies comparison for operating condition 1. A massive experimental analysis was performed in order to compare different injection strategies obtained performing the EM, PM and EPM strategies, varying several injection parameters like early duration, pilot timing and duration and main timing, for the engine operating conditions 1. BSFC data are compared in Fig.. THC emission levels showed a behaviour quite similar. It is evident that the injection strategy characterized by a lower fuel consumption is the EPM with EtE = 0 ls. Nevertheless, the early duration is more effective in controlling both BSFC and THC than the pilot injection parameters. Main advance

6 2284 A.P. Carlucci et al. / Applied Thermal Engineering 26 (2006) Bsfc [kg/h] EPM(63/X/24.3/0/X) EPM(63/X/16.0/0/X) EPM(63/X/24.3/0/X) EPM(63/X/16.0/0/X) PM(24.3/0/X) PM(16.0/0/X) PM(24.3/0/X) PM(16.0/0/X) EM(63/X/X) 0/3. 0/0.0 0/-3. 0/3. 0/0.0 0/ /3. 300/ /-3. Early Duration [micros] / Main Timing [ CA] 40/3. 40/0.0 40/-3. Fig.. BSFC data for EPM strategy compared with related PM and EM injection strategies for operating condition 1. is effective for high early durations, when an abrupt decrease in BSFC and THC levels is observed when retarding the main injection. The effects of each injection variable on engine emissions are shown in Fig. 6. EPM and EM injection strategies both reduce NO x emissions with respect to the PM strategy, as reported in Fig. 6a. In this case, as expected, both main timing and early duration are effective in the same way in Exhaust NOx [ppm] Opacity [%] /3. 0/0.0 0/-3. 0/3. 0/0.0 0/-3. 0/3. 0/0.0 0/ /3. 300/ /-3. EPM(63/X/24.3/0/X) EPM(63/X/16.0/0/X) EPM(63/X/24.3/0/X) EPM(63/X/16.0/0/X) PM(24.3/0/X) PM(16.0/0/X) PM(24.3/0/X) PM(16.0/0/X) EM(63/X/X) Early Duration [micros] / Main Timing [ CA] EPM(63/X/24.3/0/X) EPM(63/X/16.0/0/X) EPM(63/X/24.3/0/X) EPM(63/X/16.0/0/X) PM(24.3/0/X) PM(16.0/0/X) PM(24.3/0/X) PM(16.0/0/X) EM(63/X/X) 0/3. 0/0.0 0/ /3. 300/ /-3. Early Duration [micros] / Main Timing [ CA] 40/3. 40/0.0 40/-3. 40/3. 40/0.0 40/-3. Fig. 6. NO x and opacity data for EPM strategy compared with related PM and EM injection strategies for operating condition 1. reducing NO x levels. This reduction is remarkable for higher early duration (EtE = ls). Opacity values measured at the engine exhaust are plotted in Fig. 6b. In this case the behaviour is different considering low or high early duration. For low duration, in fact, EPM strategy shows opacity similar to those related to EM and PM strategies. With high EtE values, otherwise, the opacity difference between the three strategies is more evident. For low EtE value, the EPM strategy reduces the opacity levels, compared to the PM one. For the opacity levels, tests with EtE = 0 ls present the lowest particulate emissions ranging on all experimental tests. In this case too it is noticeable a sensible reduction when delaying the main injection and using long early durations. In this case this behaviour is particularly interesting because no corresponding increase in NO x levels is observed. Finally, from the analysis of all the experimental tests, EPM strategy with EtE = 0 ls globally leads to a reduction in BSFC, THC, NO x and particulate emissions compared to EM and PM strategies. In Fig. 7, THC vs. BSFC and NO x vs. opacity levels are plotted for engine operating condition 2. Increasing the output power with respect to the operating condition 1, EPM injection strategy is less performing than PM strategy concerning both THC and BSFC levels. Such behaviour can be explained considering that the increase of the engine power will cause the increase of the temperatures of residual gas andcombustionchamberwalls.atthesametime,theturbocharger causes an increase of intake air pressure and temperature. These effects accelerate chemical and physical THC [ppm] PM strategy EM strategy EPM strategy (EtE = 0 micros) 10 EPM strategy (EtE = 300 micros) EPM strategy (EtE = 40 micros) Bsfc [kg/h] opacity [%] PM strategy EM strategy EPM strategy (EtE = 0 micros) EPM strategy (EtE = 300 micros) EPM strategy (EtE = 40 micros) NOx [ppm] Fig. 7. THC vs. BSFC and NO x particulate emissions trade-off for operating condition 3.

7 A.P. Carlucci et al. / Applied Thermal Engineering 26 (2006) phenomena leading to spray autoignition. Consequently, combustion will occur early during the compression stroke, as already underlined by comparing Fig. 3a and b, penalizing BSFC significantly. As seen in results related to the engine operating condition 1, THC levels increase when EtE increases, for both EM and EPM strategies. Concerning the NO x -particulate trade-off, in this case the best injection strategy for the reduction of emission levels is the EPM one with medium-high values of early injection duration. Moreover, comparable levels of pollutant emissions, in terms of NO x and particulate, characterize the EM injection strategy. Measurements taken operating the engine in condition 3 showed that, from a general point of view, EM strategy represents the best compromise for the pollutant reduction. 4. Experimental results with natural gas used as additive As previously underlined, the use of natural gas as additive is mainly addressed to a decrease of particulate emissions. This is why, in order to test the engine operating in dual fuel mode, the chosen modes were characterized by a medium-high values of output power. The tested operating conditions are reported in Table 2. The natural gas energy flux has been fixed equal to 0%, 20% and 30% of the engine output power for operating condition 2 0 (n = 2000 rpm, T = 40 Nm), while, for operating condition 3 0 (n = 2000 rpm, T = 80 Nm), the natural gas flux has been fixed equal to 0%, 10% and 20%. Pilot injection was not performed, and the main injection timing and pressure were fixed, as reported in Table 3, while main injection duration was automatically tuned by the test bench. In Fig. 8 and Table 4, results obtained using the natural gas in additive mode are reported, in terms of GHRR histories, NO x, THC, CO, particulate and efficiency levels. Fig. 8 highlights that substantial variations in GHRR plots have been observed only with high mass ratios values (30% in operating condition 2 0 ). In particular, as previously underlined, higher natural gas ratios cause an increase of ignition delay of the main injection and, at the same time, an increase in the premixed phase. However, in this case, the expected simultaneous increase of NO x levels depending on the increased Table 2 Engine operating conditions related to the AG mode n [rpm] T [Nm] Engine operating conditions Table 3 Injection parameters related to the AG mode Injection parameters Ai main inj. timing [ CA before TDC] 3.0 P inj rail mean injection pressure 100 GHRR [J/ ] GHRR [J/ ] w/o NG 30 % NG 20 % NG CA [ ] 20 % NG 10 % NG w/o NG CA [ ] Fig. 8. GHRR for different natural gas mass ratios ( a operating condition 2 0 ; b operating condition 3 0 ). Table 4 THC, NO x, particulate, CO levels and efficiency for different natural gas mass ratios at the operating conditions 2 0 and 3 0 n = 2000 [rpm]; T = 40 [Nm] Mass ratio THC [ppm] NO x [ppm] CO [vol.%] FSN Efficiency n = 2000 [rpm]; T = 80 [Nm] Mass ratio THC [ppm] NO x [ppm] CO [vol.%] FSN Efficiency premixed peak is not observed, as can be observed based on the results reported in Table 4. Then, it is possible the decrease in NO x levels to be due to the oxygen concentration drop in the intake mixture. Particulate level behaviour is decreasing with mass ratio reduction. Globally it can be said that, using the natural gas as additive, the classic trade-off between NO x and particulate is not observed, and, on the contrary, both pollutants decrease with increasing natural gas mass ratio. THC levels, however, such as for CO emissions (Table 4), tend to increase. From the same table, it is interesting to

8 2286 A.P. Carlucci et al. / Applied Thermal Engineering 26 (2006) notice that for both the tested operating conditions, low percentages of the mass ratio do not lead to significant decreases of efficiency (operating condition 2 0 ), and even increase it for operating condition Conclusions An experimental activity was performed to test the effects of early injection and natural gas addition on a direct injection diesel engine. The experiments were performed for different engine operating conditions in order to understand the influence of different parameters on combustion, pollutant emissions and specific fuel consumption. Combustion analysis when performing the early injection showed that the effect of the fuel quantity injected during the early varies with varying the pilot duration. For high output power operating conditions, the combustion of the early and pilot mixture has been always observed. This combustion is advanced when increasing the early duration. In the same time, GHRR peak can either increase (with short pilot injection), or shows an almost constant value (long pilot injection). Decreasing the output power to medium and low values, the combustion of early + pilot fuel tends to be less evident or even to disappear. In the same time, combustion of the main injection shows a premixed combustion phase more pronounced. BSFC analysis, with medium output power, conducted with varying early duration, pilot duration and timing and main timing showed that the early + pilot + main strategy with the shortest early duration is characterized by the lowest fuel consumption. THC levels, generally show the same behaviour if compared to BSFC. The above considerations can be generalized to lower power operating conditions, while, for the higher, pilot injection seems to be the best compromise in decreasing both BSFC and THC. NO x levels show a decreasing behaviour when increasing the early duration for low and medium output power. Opacity levels generally show an opposite behaviour. With high output operating conditions, the observed trends do not show an evident and regular trend. It may be generalized that EM and EPM injection strategies with medium-high values of early duration, represent the best compromise between NO x and particulate production. An experimental analysis has been carried out to analyse the potentialities of a traditional diesel engine operated in dual fuel mode. In particular, natural gas has been used in low quantities to clean the diesel fuel combustion. Substantial variations in cylinder pressures and GHRR curves are evident only with high mass ratios values. In this case, an increase of ignition delay of the main injection and, at the same time, an increase in the premixed phase is observed. Introducing low natural gas quantities (10%, 30%), reduction in particulate emissions and NO x emissions can be observed at the same time, but increases in HC and CO levels have been observed as well. Operating in dual fuel mode, the engine efficiency slightly increases, thanks to the higher pressure and temperature values inside the combustion chamber, facilitating the THC oxidation process. Acknowledgements This work has been supported by MIUR (Italian Research, University Education Ministry), PRIN 2001 Project. The authors would also thank Paolo Passabí, Domenico Camarda and Pasquale Bianco for their helpful assistance during the test bench experiments. References [1] M. Ishida, Z. Chen, G. Luo, H. Ueki, The effect of pilot injection on combustion in a turbocharged D.I. diesel engine, Society of Automotive Engineers Paper , [2] A.P. Carlucci, A. Ficarella, D. Laforgia, Effects of pilot injection parameters on combustion for common rail diesel engines, Society of Automotive Engineers Paper , [3] M. Dürnholz, H. Endres, P. Frisse, Preinjection: a measure to optimize the emission behaviour of DI-diesel engine, Society of Automotive Engineers Paper , [4] T. Minami, K. Takeuchi, N. Shimazaki, Reduction of diesel engine NO x using pilot injection, Society of Automotive Engineers Paper 90611, 199. [] S.K. Chen, Simultaneous reduction of NO x and particulate emissions by using multiple injections in a small diesel engine, Society of Automotive Engineers Paper , [6] M. Badami, F. Millo, D. D Amato, Experimental investigation on soot and NO x formation in a DI common rail diesel engine with pilot injection, Society of Automotive Engineers Paper , [7] T. Ryan, T.J. Callahan, Homogeneous charge compression ignition of diesel fuel, Society of Automotive Engineers Paper , [8] H. Yokota, Y. Kudo, H. Nakajima, T. Kakegawa, T. 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9 ID Title Pages Control of the combustion behaviour in a diesel engine using early injection and gas addition 8