Simulation Research On Lean-Burn Characteristics Of HCNG Engine With High Compression Ratio

With the urgent situation of energy crisis and environmental pollution, it is urgent to develop energy-saving and green alternative fuel. As a kind of vehicle alternative fuel, COG(coke oven gas) can relieve energy crisis and reduce vehicle emissions. After demethanized, the COG can be conversed to HCNG fuel which is composed of hydrogen(55% by volume) and methane(45% by volume). In this paper, the lean-burn and backfire characteristics of an HCNG engine with high compression ratio was studied.(1)In order to study the influence of compression ratio on engine performance and combustion process and avoid backfire at the same time, based on the original engine?s valve lift, two valve lift curves of 14 oCA and 0oCA valve overlap were redesigned. The engine performance simulaton model and in-cylinder combustion simulation model were established at the conditions of ε=10,11,12 and 0oCA valve overlap. The indicator diagrams and heat release rate of rates for different engine working conditions were calculated. And then, the data were processed and the values of MBT(minimum advance for best torque) and wibe parameter m were calculated, which is the base for engine performance simulation. The engine performance simulation model and in-cylinder simulation model were used to study the influence of compression ratio on lean-burn performance and combustion chractersitics.The engine performance simulation results show that, the higher compression ratio, the higher torque, indicated thermal efficiency and NOx emissions, and the lower BSFC(brake specific fuel consumption). With the excess air ratio increases from 1.0 to 2.0, the torque decreases; the indicated thermal efficiency increases first and then decreases, and the peak value is in the range of φa=1.6~1.8; NOx emissions also increase first and then decrease, when φa=1.2 reaches the maximum value; BSFC shows an upward trend after the first fall. When ε=10 and ε=11, dp/dφ<0.25MPa/(oCA), within the normal range; when ε=12, dp/dφ>0.25MPa/(oCA). Through the performance analysis, it is more reasonable to increase the compression ratio to 11.The in-cyclinder combustion process simulation results show that, the higher compression ratio, the faster flame propagation, the higher in-cylinder temperature and NOx emissions. With approaching the end of combustion, cylinder temperature decreases; and the higher compression ratio, the faster temperature decreases.(2)In order to study the backfire of lean-burn HCNG engine with high compression ratio, an experiment of backfire conditions was carried out. A 3-D model of HCNG engine was built up based on the engine specifications and experimental data.The simulation results show that, large valve overlap and high compression ratio lead waste gas with high temperature back to the intake ports, resulting backfire. The invalid ignition, closing to the TDC of exhaust stroke, increases the gas temperature near the intake valve, which raises the backfire trend.At the condition of n=1600r/min, MAP(manifold absolute pressure)=90kPa, θig=16oCA BTDC, φa=1.6, ε=11, when valve overlap angle is 30 oCA, the power is 42.74 kW, the volumetric efficiency is 70.83%. When valve overlap angle is 0oCA, the power is 40.60 kW, decreases by 5.01%, the volumetric efficiency is 68.93%, decreases by 2.68%. When valve overlap angle is 0oCA, it can avoid the backfire.