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Measurements of performance and emissions of a Detroit Diesel 1-71 engine fueled with natural gas have been made using high-pressure direct-injection (HPDI). Natural gas is injected late in the compression cycle preceded by pilot injection of conventional liquid diesel fuel. With 6 nozzle holes for both natural gas and diesel pilot there was instability in engine operation at low load and wide scatter in emission measurements. Guided by numerical simulation results it was found experimentally that data reproducibility and engine operating stability could both be much improved by using unequal jet numbers for injection of natural gas and pilot diesel. In the range of 100 to 160 bar, combustion rate and NOx emissions increased with gas injection pressure. Best thermal efficiency results were obtained for a gas pressure of 130 bar. By adjusting beginning of injection, NOx reductions of up to 60 % from the diesel baseline could be obtained, while preserving conventional diesel efficiency.

Pilot-ignited high-pressure direct injection (HPDI) of natural gas in diesel engines results in lower emissions while retaining high thermal efficiency. As a study of HPDI technique, three-dimensional numerical simulations of injection, ignition and combustion were conducted. In particular, the effects on engine combustion of the injection interlace angle between the pilot diesel sprays and natural gas jets were investigated. Numerical simulations revealed ignition and combustion mechanisms in the engine with different injection interlace angles. The results show that altering the interlace angle changes the contact areas between the pilot diesel sprays and the natural gas jets; this affects the heat release rate. Statistical analysis was done to evaluate the expected value and variance of “closeness” between diesel sprays and natural gas jets for different injector tip configurations.

This paper investigates the effects of the variations of turbulence characteristics and initial flame kernel center position on the cyclic combustion variations by means of quasi-dimensional turbulent entrainment combustion model. The turbulence intensity and turbulence integral length scale at spark ignition time in the model are determined by maximizing the agreement between the predicted and measured results such as pressure diagrams, mass fraction burned etc. With different values of the turbulence intensity and turbulence integral length scale at spark ignition time, the calculation of the cyclic combustion variations for the engine is carried out. In addition, the prediction of the effect of different flame kernel center positions on the cyclic combustion variations is also studied. Finally, some conclusions are drawn out about the importance of turbulence and initial flame kernel center position on the cyclic combustion variations for spark-ignition engine.

A simple diaphragm injection carburettor (DIC) is developed and patented by the author in the paper. The principle is that, as the function of the diaphragm mechanism, the pressure difference between two sides of the metering hole equals the vacuum of the Venturi, so it can realize the pressurized fuel metering, by the fuel metering hole and vacuum from the Venturi, and fuel injection. The fuel is injected into the feed port and mixes with the small fraction of air through the port, the main port of fresh air from the crankcase is injected to the cylinder with the scavenging organized so that a separating layer is created between the exhaust and feed port, it can substantially reduce short-circuit. According to the characteristics of low short-circuit at low delivery ratio, the low load fuel supply system of conventional carburettor is preserved and combined with DIC which functions at high load operation.

Proper fuel supply system is the crux to realize the stratified scavenging for small type of two-stroke gasoline engine. A simple and effective diaphragm fuel injection system (DFI) is developed in the paper, which mainly consists of diaphragm pump and injector, the DFI utilizes the crankcase pressure which reflects the inlet flow rate to meter and inject fuel. As low short-circuit at low load operation, stratified scavenging seems not necessary, so the conventional carburetor is preserved to function at low load. This makes the whole fuel system simple and effective. The paper describes the operation principle of the DFI, and results of test on a 30cm3 modified engine are also presented, including the performance of fuel supply and contrast of complex engine performance with the original carbureted engine.