Numerical Study On The Knock Combustion Of High-pressure Direct Injection Natural Gas Marine Engine

Energy shortage and environmental pollution have always been the common issues internationally.Ocean-going ships,as efficient and economical means of transportation,discharge a large amount of pollutants into the ocean at the same time.Additionally,emission regulations of ships being increasingly strict,emission reduction technologies have become the key of research.An effective way of solving the problem of energy shortage and reducing ship emission is to use natural gas,which is abundant and clean,as the alternative fuel of diesel and heavy oil.Engines using the technology of natural gas direct injection with diesel as pilot fuel,known as HPDI(High Pressure Direct Injection)mode,can have lower emissions while keeping the same thermal efficiency and power as diesel engines.However,strong detonation combustion phenomenon may occur in cylinder if injection parameters of pilot diesel and nature gas are mismatched under some conditions,which can cause damage to the engine.Firstly,in the present paper,a three-dimension model of a constant volume combustion chamber was established based on CONVERGE software(version 2.3)and validated based on experimental data.Consequently,the dual-fuel spray combustion process and the effects of pilot diesel mass and the natural gas injection timing on the ignition and combustion process were studied.Results show that there are specific distributions of species in different temperature and concentration areas of the gas jet flame.And there are the optimal premixed and diffusion combustion of natural gas in terms of the given pilot diesel mass and injection timing of natural gas.These results give a theoretical fundament for studying the combustion process of the HPDI dualfuel marine engine.Secondly,a three-dimension numerical model of marine engine was built in CONVERGE software(version 2.3)and validated.The effects of natural gas injection timing,pilot diesel injection timing,pilot diesel mass and scavenge air temperature on the knock combustion of dual-fuel marine engine with HPDI mode were studied.Results show that when the natural gas injection timing is advanced,the premixed combustion proportion of natural gas is increased.As a result,the knock combustion is intensified.While advancing the pilot diesel injection timing causes abnormal combustion for two fuels simultaneously and obviously increases pressure oscillation intensity.When the pilot diesel injection timing is retarded too much,compression ignition of natural gas is occurred,which alleviates the intensity of the violent knock combustion caused by prolonged ignition delay period of diesel combustion during the earlier stage.In terms of the pilot diesel mass,the pressure oscillation intensity is the maximum when the diesel ratio is 7.5% and is the minimum when the diesel ratio is 5.3%.Scavenge air temperature has little influence on cylinder mean pressure.When the scavenge air temperature is 407 K,the instantaneous pressure oscillation intensity is the maximum.However,the scavenge air temperature becomes 347 K,the pressure oscillation intensity is the minimum and the duration is the shortest.Based on the previous studies,further research of effects of Miller Cycle and double injections of natural gas on knock inhibition were studied.Results show that,under the combined consideration of knock inhibition effect and keeping power performance,the exhaust valve closing time should be retarded by 4 °CA.The reduction of pre-mixture amount caused by double injections of natural gas has a significant effect on the knock combustion inhibition.To summarize,this paper systematically studied the spray and knock combustion process of the dual-fuel marine engine with HPDI mode.The results have practical value for the development of new natural gas marine engines.