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A phenomenological model for high-pressure direct injection natural gas-diesel dual-fuel marine engine was developed, which includes natural gas mixing process using Musculus discrete control volume transient diesel jet model, combustion process using quasi-steady model and Woschini heat transfer model, NO generation using Zeldovich mechanism. Effects of natural gas injection pressure and the start of injection timing on the mixing and combustion process were investigated. The results indicated that increasing the injection pressure with fixed injection mass, the NO emission decreased. While the start of injection timing was before TDC, retarding the injection start timing will increase NO generation.

Low-speed two-stroke dual-fuel engines has been paid more attention due to the energy efficiency design index and Tier III emissions limitations issued by International Marine Organization. Although the dual-fuel engines have strong merits on emissions reduction, which can reach the IMO Tier III without aftertreatment, the power output is much lower than that of diesel engines. Therefore, the dual-fuel engine is also needed to improve continuously. However, the mixing and combustion processes in the engine have not been fully understood. In this study, a 3D-CFD model of the dual-fuel engine was established using CONVERGE to explore the mixing and combustion processes. Locally embedding fine grids are considered at scavenging ports, natural gas injection ports, pre-chamber. The model was validated by experimental in-cylinder pressure.

Multiple-injection strategy widely used in modern diesel engines can control the in-cylinder mixing process and adjust the combustion process to fit the engine operating conditions. This study uses a one-dimensional spray model with improved injection velocity calculation and determination of spray tip to simulate the diesel spray with multiple-injection. Then the calculated spray penetrations are compared to the experimental data to validate the developed spray model. Analysis is performed on mixing process of diesel spray with multiple-injection using entrainment rate, equivalence ratio, velocity over cross-section, with the spray-to-spray interaction revealed and the entrainment wave effects on spray-to-spray interaction discussed.

Effects of measurement method, coolant temperature and fuel composition on soot emissions were examined by engine experiments. By reducing the pressure fluctuation in the sampling line, the measured soot emissions with better stability and reproducibility could be obtained. With lower coolant temperatures, larger soot emissions were yielded at much advanced fuel injection timings. Compared to gasoline, soot emissions with a blend fuel of normal heptane, isooctane and toluene were significantly decreased, suggesting the amounts of aromatic components (toluene or others) should be increased to obtain a representative fuel for the predictive model of particulate matter in SIDI engines.

To understand the mechanism of the combustion by torch flame jet in a gas engine with pre-chamber and also to obtain the strategy of improving thermal efficiency by optimizing the structure of pre-chamber including the diameter and number of orifices, the combustion process was investigated by three dimensional numerical simulations and experiments of a single cylinder natural gas engine. As a result, the configuration of orifices was found to affect the combustion performance strongly. With the same orifice diameter of 1.5mm, thermal efficiency with 7 orifices in pre-chamber was higher than that with 4 orifices in pre-chamber, mainly due to the reduction of heat loss by decreasing the impingement of torch flame on the cylinder linear. Better thermal efficiency was achieved in this case because the flame propagated area increases rapidly while the flame jets do not impinge on the cylinder wall intensively.

The influence of fuel properties on the operational range and the thermal efficiency of premixed diesel combustion was evaluated with an ordinary diesel fuel, a primary reference fuel for cetane numbers, three primary reference fuels for octane numbers, and two normal heptane-toluene blend fuels in a single-cylinder DI diesel engine. The fuel injection timing was set at 25°CA BTDC and the maximum rate of pressure rise was maintained below 1.0 MPa/°CA when lowering the intake oxygen concentration by cooled EGR. With increasing octane numbers, the higher intake oxygen concentration can be used, resulting in higher indicated thermal efficiency due to a higher combustion efficiency. The best thermal efficiency at the optimum intake oxygen concentration with the ordinary diesel fuel is lower than with the primary reference fuels with the similar ignitability but higher volatility.

The influence of fuel volatility on the thermal efficiency of premixed diesel combustion was evaluated with three ordinary diesel fuels with different distillation temperature distributions and also with a primary reference fuel with an octane number of 20 (PRF20) as a high volatility fuel. The experiments were conducted on a single-cylinder DI diesel engine for the premixed diesel combustion with a single injection at 11% intake oxygen concentration and conventional diesel combustion with a pilot fuel injection at 21% intake oxygen concentration. With the premixed diesel combustion, the indicated thermal efficiencies with the ordinary diesel fuels were lower than with PRF20 although the shapes of the rate of heat release and the combustion efficiencies calculated from the exhaust gas components were almost unchanged. With the conventional diesel combustion, the indicated thermal efficiencies with the ordinary diesel fuels and PRF20 were similar.