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On account of their high power, thermal efficiency, good reliability, safety, and durability, low-speed two-stroke marine diesel engines are used as the main drive devices for large fuel and cargo ships. Most marine engines use heavy fuel oil (HFO) as the primary fuel, however, the physical and chemical characteristics of HFO are not clear because of its complex thermophysical properties. The present study was conducted to investigate the effects of fuel properties on the spray and combustion characteristics under two-stroke marine engine-like conditions via a sensitivity analysis. The sensitivity analysis of fuel properties for non-reacting and reacting simulations are conducted by comparing two fuels having different physical properties, such as fuel density, dynamic viscosity, critical temperature, and surface tension. The performances of the fuels are comprehensively studied under different ambient pressures, ambient temperatures, fuel temperatures, and swirl flow conditions. From the results of non-reacting simulations of HFO and diesel fuel properties in a constant volume combustion chamber, it can be found that the increase of the ambient pressure promotes fuel evaporation, resulting in a reduction in the steady liquid penetration of both diesel and HFO; however, the difference in the vapor penetrations of HFO and diesel reduces. Increasing the swirl flow significantly influences the atomization of both HFO and diesel, especially the liquid distribution of diesel. It is also found that the ambient temperature and fuel temperature have the negative effects on Sauter mean diameter (SMD) distribution. For low-speed marine engines, the combustion performance of HFO is not sensitive to activation energy in a certain range of activation energy. At higher engine speed, the difference in the effects of different activation energies on the in-cylinder pressure increases. The swirl flow in the cylinder can significantly promote fuel evaporation and reduce soot production.

In spark ignition engines, gasoline direct injection (GDI) is surely the most attractive technology to achieve the demand of high energy efficiency by directly injecting fuel into combustion chamber. This work, as a preliminary study, investigates the effect of retarded injection timing on knock resistance and cycle-to-cycle variation in gasoline engine by experimental method. The retarded injection timing during compression stroke coupled with increased intake air temperature was employed to concentrate on suppressing knock occurrence with stable combustion. Based on the great advantage of injection timing retard on knock suppression, intake temperature was used in this work to reduce cycle-to-cycle variation. In addition, piezo-electrically actuated injector was employed. The results show that injection timing retard during compression stroke can significantly suppress the knock tendency, but combustion becomes unstable and cycle-to-cycle variation is larger than 10%. Thus, increasing intake temperature decreased the cycle-to-cycle variation but increased significantly the knock tendency, as expect. Meanwhile, rich fuel–air mixture in this work also had the same effect as intake temperature did. It can be concluded that retarded injection timing is of significant potential to suppress the knock in GDI engine, although the high intake temperature causes high probability of large knock occurrence. The percentages of knock at the spark timings of 24 °CA before top dead center (BTDC) and 26 °CA BTDC were significantly reduced from approximately 40% to 7% and from approximately 60% to 10%, respectively. Furthermore, the retarded injection timing not only reduced the probability of knock occurrence, but also decreased the knock intensity obviously.

The combustion of a direct injection spark ignition engine is significantly affected by the fuel injection strategy due to the impact this strategy has on the gas-mixture formation and the turbulence flow. However, comprehensive assessments on both knock and engine performances for different injection strategies are generally lacking. Therefore, the main objective of the present study is to provide an experimental evidence of how a single injection strategy and a split injection strategy compare in terms of both knock tendency and engine performances like thermal efficiency, torque and combustion stability. Starting from the optimization of a single injection strategy, a split injection strategy is then evaluated. Under the present operating conditions, an optimum secondary injection timing of 100 CAD BTDC is found to have significant improvements on both the knock resistance and the overall engine performances. It should be noted that the present results indicate that the relationship between double injection and anti-knock performance is not monotonous. In addition, the double injection shows superior potential in improving fuel economy and power performance in contrast with the single injection thanks to a more stable combustion when a late injection timing is applied.