Experiment And Numerical Simulation On Low Temperature Combustion Of Gasoline Homogeneous Charge Induced Ignition By Diesel

Diesel engine has higher thermal efficiency than gasoline engine, but the emissionproblems are difficult to solve due to the trade-off between NO_xand soot. HCII(Homogeneous Charge Induced Ignition) combustion mode uses a port injection ofgasoline to form a homogeneous charge and a direct injection of diesel to ignite, whichhas the potential to achieve the combustion targets for high efficiency and lowemissions. The thermal efficiency and emission characteristics of HCII combustionmodes were studied by experimental investigation and numerical simulation in thispaper.A combustion concept named low temperature combustion of HomogeneousCharge Induced Ignition (LT-HCII) was proposed based on the original HCIIcombustion. The effects of the engine control parameters including gasoline/diesel ratio,EGR ratio and diesel injection timing on the combustion and emission characteristics ofLT-HCII were studied in detail by experimental investigation in this paper. The resultsshow that the combustion phasing of LT-HCII can be effectively controlled; thetrade-off between NO_xand soot emissions is not apparent, and the NO_xand sootemissions can be reduced to an extremely low level simultaneously; the indicatedthermal efficiency of LT-HCII is significantly higher than the tranditional gasoline sparkignition engine, and equal or even higher than the conventional diesel engine. Aiming atthe high Hydrocarbon (HC) emissions of HCII combustion at low load, hot EGR wasapplied to increase the intake temperature and reduce the HC emissions. Meanwhile, thediesel dual-injection method was used to increase the gasoline reactivity and reduce theHC emissions.Three modes were discovered in HCII combustion, which were slow single-stagedheat release, two-staged heat release and fast single-staged heat release. Two-staged heatrelease can significantly reduce the maximum pressure rise rate due to the staged andsequenced combustion, and maintain the thermal efficiency to a high level, which canbe used to suppress the excessively rapid heat release of HCII at high load. Two-stagedcombustion can be achieved by adjusting the parameters such as the fuel ignitionproperties, equivalent ratio, EGR rate and the diesel injection timing. The numerical model was established using three-dimensional computational fluiddynamics coupled with chemical kinetics, and the emission formations of LT-HCII andtwo-staged combustion were numerically studied. The results show that the NO_xemissions are mainly generated from the high temperature combustion region of dieselspray region, and the HC emissions are mainly from the wall and crevice quenching.When two-staged combustion is achieved, the first peak of heat release rate is from therapid multipoint autoignition of diesel and the gaoline in the combustion region. Then,the rapid multipoint autoignition of the gasoline-air mixture occurs with flamepropagation, which results in the second peak of heat release rate.Gasoline diesel blend fuels (GDBF) is another mode of utilizing gasoline anddiesel. The comparative study on the combustion characteristics, emissions, and thermalefficiency between HCII and GDBF combustion modes were carried out. Both HCIIand GDBF combustion modes have shorter combustion duration and higher constantvolume ratio, which is beneficial to the indicated thermal efficiency. Meanwhile, thesoot emissions of both HCII and GDBF are significantly lower than conventional dieselcombustion. However, the HC emissions of HCII are higher than GDBF, while the sootemissions of HCII are lower. This research provides guide for future development ofgasoline/diesel unified engines.