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The direct injection of natural gas (NG), which is an important research direction in the development of NG engines, has the potential to improve thermal efficiency and emissions. When NG engines operate in low-load conditions, combustion efficiency decreases and hydrocarbon (HC) emissions increase due to lean fuel mixtures and slow flame propagation speeds. The effect of two combustion modes (partially premixed compression ignition (PPCI) and high pressure direct injection (HPDI)) on combustion processes was investigated by CFD (Computational Fluid Dynamics), with a focus on different injection strategies. In the PPCI combustion mode, NG was injected early in the compression stroke and premixed with air, and then the pilot diesel was injected to cause ignition near the top dead center. This combustion mode produced a faster heat release rate, but the HC emissions were higher, and the combustion efficiency was lower. In the HPDI combustion mode, the diesel was injected first and ignited, and then the NG was injected into the flame. This combustion mode resulted in higher emissions of NOx and soot, with a diffusion combustion in the cylinder. HC emissions significantly decreased. Compared with PPCI combustion, HPDI had a higher thermal efficiency.

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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