| The diesel combustion processes in homogeneously premixed highly lean methane or hydrogen-air mixtures were investigated both experimentally and numerically. Such processes were realized through the intake admission in small amounts of methane or hydrogen in a fully operated diesel engine. They also represent the special and uncommon dual fuel combustion processes where the diesel fuel is the main fuel while the methane or hydrogen acts merely as a supplement. A modified experimental setup based on an IDI diesel engine with a swirl chamber together with a modified 3-D predictive computational model based on KIVA-3 was employed. The model incorporates detailed chemical kinetics for the oxidation of methane, hydrogen, n-heptane, toluene, and diesel fuel while accounting for the turbulence-chemistry interactions during combustion.;In the experimental part of the research, the effects of employing highly lean methane or hydrogen-air mixtures in varying concentrations on engine performance, emissions, and combustion in the IDI engine were investigated. The corresponding results were further used to validate the predicative model. The simulation provides spatial and temporal distributions of cylinder pressure, temperature, velocity, species mass fraction, which are key data for the interpretation of the combustion processes. Analysis showed that the oxidation of methane or hydrogen occurs mainly around regions of high temperature. The incompleteness of flame propagation and quenching effects may be mainly responsible for some of the methane or hydrogen remaining unburned. The effects of changes in a number of relatively not so easy to investigate and control experimental parameters on such diesel combustion processes were investigated computationally. As examples, it was shown that changes in the values of the combustion chamber surface temperature have a significant effect on engine combustion and emissions. Low concentration of methane addition could probably used as a tracer to indicate the quality of the liquid fuel injection characteristics. Similar trends were found for a DI diesel engine operating similarly with a homogeneously premixed highly lean methane-air mixture except for a lower methane conversion rate which could have resulted from the effects of factors such as a correspondingly lower cylinder pressure, less intense charge motion, and longer flame propagation distances. |
