Influence Of Negative Valve Overlap On Combustion And Emissions Of Homogeneous Charge Compression Ignition Engine Fueled With The Hydrogen-mixed Natural Gas Blends

Excessive consumption of traditional energy sources and increasingly strict emission restrictions make finding new energy sources and new combustion methods become the research focus in the field of vehicle engine combustion and emission.The engine uses natural gas and hydrogen mixed fuel and adopts homogeneous compression ignition,which is a working mode with good clean combustion characteristics and relatively high heat efficiency in theory.However,due to the high octane number of the fuel and the combustion fluctuation of homogeneous compression ignition,the ignition and combustion rate of the engine are difficult to be stable in operation.In addition,although the mixture of hydrogen in the fuel reduces the carbon content of the whole components before and after the combustion reaction,during homogeneous compression ignition,Excessive combustion temperature will stimulate the increase of nitrogen oxides.These unfavorable factors make it necessary to improve the combustion and emission characteristics of natural gas-hydrogen mixed fuel homogeneous compression ignition engine.Negative valve overlap can promote spontaneous combustion by using the internal exhaust gas circulation effect,and control the combustion rate by using inactive components,thus reducing the combustion end temperature in cylinder,which is a convenient and effective improvement method.To study the influence of negative valve overlap on combustion emissions of natural gas hydrogen-mixed homogeneous compression ignition engine,based on bench test and coupling simulation of ventilation dynamic process,the cylinder pressure,heat release rate,combustion temperature and nitrogen oxide formation under three negative overlap schemes were analyzed at the speed of 1100r/min,excess air coefficient of 1.4,hydrogen-mixed ratio of 7%and intake pressure of 0.101MPa.The results show that the internal exhaust gas circulation formed by overlapping negative valves can heat the intake working medium,improve the spontaneous combustion characteristics of natural gas-hydrogen homogeneous compression ignition engine,and the remaining exhaust gas can slow down the combustion heat release rate,reduce the in-cylinder pressure rise rate and the highest combustion temperature,and reduce the emission of nitrogen oxides.Among the three negative valve overlap schemes,the pressure loss caused by the strategy of delaying the opening time of the intake valve alone is 9.2%.Meanwhile,changing the symmetric negative overlap strategy of closing the exhaust valve and opening the intake valve can achieve the best intake heating effect,with a combustion temperature drop of 20.7%and the largest NOx emission reduction.According to the scheme of obtaining better comprehensive performance(EVC is 25℃A BTDC and IVO is 25℃A ATDC),the influence of changing operating parameters under working conditions is studied.The results show that under the selected negative valve overlap scheme,when the equivalence ratio decreases from 1 to 0.5,the NOx production decreases,the pressure peak decreases and the heat release slows down.The addition of hydrogen can make up for the loss caused by the internal circulation of waste gas,but when the mixing amount exceeds 9%,the heat release is excessively advanced.In-cylinder dynamic process is insensitive to the change of rotational speed,and intake air warming can obviously improve ignition phase,but increase nitrogen oxides.Inlet supercharging is beneficial to homogeneous compression ignition,but it increases from 0.13MPa to 0.16MPa,and excessive supercharging causes negative work loss in compression.When the compression ratio is excessively increased,the heat release is advanced before the top dead center,which is not conducive to the power output of the engine.