| Gasoline Compression Ignition (CI) mode is the frontier and hot topic of internalcombustion engine researches, and shows a great energy saving and emission reductionpotential compared to gasoline Spark Ignition (SI) mode. Gasoline Partially PremixedCompression Ignition (PPCI) mode has disadvantages of high maximum pressure riserate and high combustion noise, as well as high NOx emissions if the EGR rate is nothigh enough. This dissertation proposed a gasoline Multiple Premixed CompressionIgnition (MPCI) mode, whose dominant feature is the two-stage low and hightemperature premixed combustion with a “spray–combustion–spray–combustionâ€sequence around the compression top dead center. The heat release rate and peakcombustion temperature of the MPCI mode can be tuned properly by multiple injectionand combustion, leading to a controllable, efficient and clean combustion.The CI mode with conventional high-octane gasoline has issues like cold startdifficulty and running unstability at low load. In this dissertation, the low-octanegasoline-like fuels such as straight-run naphtha are adopted in the MPCI mode to have astable running at part load, and high fuel efficiency and low emissions at middle andhigh load. A temporally separated two-stage injection and spatially separated two-zonecombustion strategy with high pressure fuel injection is utilized in order to realize thesecond premixed combustion process in the MPCI mode, which is validated by thevisualization experiment and numerical simulation.Experimental study on a common-rail diesel engine with18.5compression ratioshows that an ideal MPCI mode can be realized if the low-octane blended gasoline fuel(70v%RON93gasoline+30v%n-heptane, G70H30) is firstly injected at50°CA BTDCand secondly injected at TDC. In addition, compared to diesel single injection mode, theG70H30MPCI mode has1%lower ISFC,47%lower NOx,90%lower soot,11%lowerCO, and37%lower maximum pressure rise rate, but81%higher THC emission. TheMPCI mode can achieve simultaneous reduction of NOx and soot emissions, breakingthe trade-off relationship between NOx and soot in conventional combustion engines.Moreover, the MPCI mode indicates the same level of fuel efficiency as the PPCI modewithout boosting and EGR, but about70%lower NOx emissions, and about40%lowermaximum pressure rise rate. In a constant-volume vessel test with high back temperature and high backpressure, the straight-run naphtha and G70H30fuels realize the two-stage premixedcombustion process at100MPa common rail pressure. But diesel fuel only appearstwo-stage diffusion compression ignition under the same condition. In a single-cylinderoptical engine measurement, the ignition delay of the straight-run naphtha and G70H30is prolonged with the increase of the common rail pressure. However, the ignition delayof the diesel fuel shows an opposite changing trend.Numerical analysis of the mixture formation and combustion process in the MPCImode using KIVA-CHEMKIN indicates that the first compression ignition of the MPCImode is a bulky low temperature combustion triggered by the local fuel rich area, andoccurs at the peripheral area of the combustion chamber. However, the second hightemperature premixed combustion mainly appears at the central area of the combustionchamber, and is governed by the local turbulenc induced by the second fuel injection.In a word, the temporally and spatially distributed combustion process in the MPCImode has a high thermal efficiency due to high compression ration and low heat transferloss, and also low NOx and soot emissions due to the low peak combustion temperatureand premixed combustion. |
