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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• The coordinated control of fuel reactivity and in-cylinder charge conditions can realize efficient and clean combustion. • The indicated thermal efficiency under low and medium loads was improved by using the high-reactivity fuels (G0 and G50 fuels). • The indicated thermal efficiency of low-reactivity fuel (G100 fuel) is higher than high-reactivity fuels (G0 and G50 fuels) under the high load. • The highest indicated thermal efficiency under three loads was over 53%. High-octane fuel (such as gasoline) does not easily self-ignite, which is conducive to the formation of homogeneous mixture. However, gasoline compression ignition (GCI) under low load is difficult, and some problems arise, such as rough combustion under high load, which limit the application of high-octane fuel in the compression ignition (CI) combustion. The synergistic effect of the fuel characteristics and boundary conditions represents the key for achieving efficient and clean combustion. In this study, based on a modified single-cylinder engine, fuels with different reactivity (diesel G0, gasoline and diesel blended fuel G50, and gasoline G100) are studied under different loads. In the test, the engine operates at 1300 rpm, and the loads were 6, 10, and 15 bars gross indicated mean effective pressure (IMEPg) respectively. Meanwhile, the injection mode of the three fuels was direct injection with one injector. The results indicate that in-cylinder charge conditions of the engine largely vary under different loads. Under a low load (IMEPg: 6 bar), the in-cylinder temperature and pressure are low, and the gross indicated thermal efficiency (ITEg) of the high-reactivity fuels (G0 and G50) are higher. Under a medium load (IMEPg: 10 bar), high ITEg (52.1%) and low emission can be achieved using a G50 fuel. Under a high load, higher charging pressure and temperature lead to the high ITEg of the G100 fuel (fuel reactivity is the lowest), which reaches up to 53.1%. [ABSTRACT FROM AUTHOR]