Improvement Of The Combustion In Vehicle Diesel Engine Based On Key Initial Boundaries

With the increased energy crisis and the deteriorated environmental pollution, theenergy saving and emissions reduction have become the theme of our times. Diesel engineshave become the main driving force for transportation and power facilities due to thehigher thermal efficiency and better reliability; however, the NO_xand soot emissions fromdiesel engines have an important impact on the environment and human health. Moreover,there is a trade-off relationship between NO_xand soot emissions in the conventionalcombustion mode of diesel engines. In this paper, while the effects of key initialboundaries on the combustion for vehicle diesel engine were investigated firstly, theincylinder mixture formation quality of vehiclediesel engine was improved by optimizingthe key initial boundaries, so that when the mixture burned, it could avoid the NO_xandsoot generation regionsandimprove the trade-off relationship between NO_xand sootemissions. Due to the lower cetane number, lower sulfur and PAH content andlowerdistillation temperature for the CTL (Coal to Liquid) fuel, it was used to substitute forpetroleum diesel fuel, and the fuel injection parameters and EGR rate were optimized tofurther improve the trade-off relationship between NO_xand soot emissions in order toachieve the clean and high-efficiency combustion.In this paper, a diesel engine combustion test bench has been established byreconstructing the key systems of a mass production CA6DL2-33E3F diesel engine. Somedevelopments are as follows:the mechanical in-line fuel system was replaced by the highpressure common rail fuel system. The cylinder head with the intake air swirl ratio of2.1was replaced by the one with the intake air swirl ratio of1.3.The EGR system with theintake air throttle valve was developed to ensure that enough exhaust gas could be inducedinto the intake manifold, and the shell-and-tube heat exchanger was developed to ensurethe induced exhaust gas could be cooled to the target temperature.An exposure portcombustion chamber of low compression ratio and large throat diameter was alsodeveloped. Also, the key parameters test equipments have been installed in the dieselengine combustion test system. The fuel injection rate testing bench has been established, and the fuel injection rateof both the impingingspray nozzle and the conventional spray nozzle were measured. Thefuel injection test results showed that the maximum fuel injection rate was increased withthe increased fuel injection pressure, which meant that the fuel injection duration could beshortened by increasing the fuel injection pressure at the same cyclic fuel quantity. Themaximum fuel injection rate first increased and then remained unchanged with theincreased fuel injection pulse width. Compared with the conventional spray nozzle, theimpinging spray nozzle has a higher fuel injection rate and shorter fuel injection duration,which increased the possibility of injecting the cyclic fuel into the cylinder completelyprior to the ignition.The spray characteristics testing bench has been established and the spray photoprocessing program has been developed using Matlab codes. The spray photos of both theimpinging spray nozzle and the conventional nozzle wererecorded in the spraycharacteristic testing bench, and the spray photos were processed by the spray photoprocessing program. The spray characteristics test results showed that the spray tippenetration, the spray projected area and the spray volume decreased, and the spray coneangle increased with the incrased back pressure for both the impinging spray nozzle andthe conventional spray nozzle. The spray tip penetration, the spray cone, the sprayprojected area and the spray volume increased with the increased fuel injection pressure forboth the impinging spray nozzle and the conventional spray nozzle. Compared with theconventional spray nozzle, the impinging spray nozzle had shorter spray tip penetration,larger spray cone angle, smaller spray projected area and spray volume.The effects of combustion system parameters on combustion for vehicle diesel enginehave been investigated on thenewly-built combustion test bench. The test results showedthat the heavy EGR is necessary to significantly reduce the NO_xemissions. Compared withthe reentrant combustion chamber, the exposure port combustion chamber of lowcompression ratio and large throat diameter can improve the trade-off relationship betweenNO_xand soot emissions, and even improve the HC and CO emissions as well as the BSFC.The conventional spray nozzle was more beneficial for improving the trade-off betweenNO_xand soot emissions than the impinging spray nozzle, because the spray tip penetrationof the impinging spray nozzle was shorter than that of the conventional spray nozzle,which was not well matched with the exposure port combustion chamber, resulting in the heterogeneous mixture in the cylinder.The increased fuel injection pressure can promotethe fuel-air mixing process and shorten the fuel injection duration; however, theexcessively high fuel injection pressure may result in the over-lean fuel-air mixture in thelocal areas at low load, which can lead to the incomplete combustion. Thus, there exists anoptimum fuel injection pressure at low load, and the fuel injection pressure can be raisedappropriately at high load.Thenear TDC late fuel injection strategy was used to control themixture combustion phase, which could prevent the violent combustion of diesel engine atthe relatively high load. The relatively ideal clean and high-efficiency combustion modecan be achieved by combining the exposure port combustion chamber of low compressionratio and heavy EGR as well as the optimal fuel injection parameters.After the relatively ideal clean and high-efficiency combustion mode has beenachieved by optimizing the injector-combustion chamber parameters, fuel injectionparameters and EGR rate, when vehicle diesel engine was fuelled with petroleum dieselfuel. Due to the lower cetane number, lower sulfur and PAH content and lower distillationtemperature for the CTL fuel, petroleum diesel fuel was replaced by the CTL fuel.And theeffects of fuel injection parameters and EGR rate on the combustion for vehicle dieselengine have been investigated in the newly-bulit combustion test bench.Moreover, the fuelinjection parameters and EGR rate were optimized in order to further improve thetrade-offrelationship between NO_xand soot emissions.The experimental resultsshowed that thepremixed combustion mode could be realized with lower EGR rate when vehicle dieselengine fuelled with CTL fuel, comparaed to petroleum diesel fuel. The gradient of NO_xemissions with EGR rate for the CTL fuel was nearly the same with that for petroleumdiesel fuel. However, the gradient of soot emissions with EGR rate was much smaller thanthat for petroleum diesel fuel. Moreover, the CTL fuel could indeed improve the trade-offrelationship between the NO_xand soot emissions while petroleum diesel fuel had no sucheffects.Compared to original vehicle diesel engine with petroleum diesel fuel, the NO_xandsoot emissions of new vehicle diesel engine fuelled with the CTL fuel were simultaneouslyreduced in all the tested operation conditions. Especially in the25%and35%loads, theNO_xand soot emissions decreased more than50%, while the BSFC was not obviouslydeteriorated. Moreover, the maximum supercharging ratio of original vehicle diesel enginewas2.6, which meets national stage Ⅲ emission standard. Thus, there is a remarkable improvement for the new vehicle diesel engine compared to original vehicle diesel engine.AVL-Fire software was used to simulate the combustion process of the vehicle dieselengine, the simulation results validated that the exposure port combustion chamber of lowcompression ratio was beneficial forimproving the mixing quality of the cylinder mixture,thus contributing to solving the trade-off between the NO_xand soot emissions.