Natural gas (NG) engines have very broad application prospects. The pilot-ignited NG diesel engine can be organized into two combustion modes according to the sequence of oil and gas injection: (1) High-pressure direct injection, where NG is mainly diffused combustion; and (2) partially premixed compression ignition, where NG is mainly premixed combustion. In this study, we used CONVERGE to explore the influence of the NG injection timing on the distribution of the mixture equivalence ratio, ignition characteristics, thermal efficiency, emission, and combustion reaction rate under the two combustion modes. We also used a multi-step soot model to analyze the particle mass and quantity. We showed herein that the NG injection timing significantly affects the mixture distribution in the cylinder, thereby consequently affecting the combustion process. Both very early and very late injection times were not conducive to NG combustion. In addition, the mass, quantity, and diameter of the soot produced by diffusion combustion were larger than those produced with premixed combustion.

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The gasoline compression ignition (GCI) is deemed to be one of the effective ways to achieve efficient and clean combustion. However, the low in-cylinder temperature under idle and low-load conditions leads to certain problems, such as difficulties in the ignition and unstable combustion. In this paper, based on a refitted single-cylinder diesel engine, the combustion stability of the GCI mode under idling speed and low-load conditions was studied. The combustion stability and thermal efficiency were improved by adjusting the intake temperature, the coolant temperature and the injection strategy. During the test, the engine speed was set to 800 rpm at the idle condition and 1300 rpm at the low-load condition, and the gross indicated mean effective pressure was 5.5 bar. The injection mode of the fuel was direct injection with one injector. The results indicated that when the injection timing was −15°CA ATDC, and the intake temperature was increased from 50°C to 60°C under the idle condition; the gross indicated thermal efficiency (ITEg) was 39.7%, an increase of 8.5%. Under the low-load condition, increasing the coolant temperature can improve the combustion stability. Specifically, when the coolant temperature was increased from 70°C to 90°C and the injection timing was −27°CA ATDC, an ITEg of 52.1% can be realized.

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