Combustion System Optimization Of A Stoichiometric Combustion Natural Gas Engine

With the scarcity of gasoline and diesel and the growing problem of atmospheric pollution,more and more researchers start to look for clean energy that can replace oil.The main component of natural gas is CH₄,which has the advantages of abundant storage,low price and clean combustion.Therefore,it has been valued by researchers all over the world and has become the first choice for alternative fuels.With the implementation of the China VI emission legislation,the natural gas engine on stoichiometric operation mode only needs a three-way catalytic converter（TWC）to meet the emission legislation.In this paper,the effects of camshaft,intake port and combustion chamber on the combustion,emission and air-exchange process of natural gas engine were studied systematically.Based on this,the effects of spark ignition timing and EGR on the combustion characteristics and performance with stoichiometric operation were studied under different speed and load conditions,which is of great importance for achieving high efficiency and clean combustion for natural gas engine on stoichiometric operation mode.Since the thermal efficiency of port injection stoichiometric combustion natural gas engine is mainly limited by knocking,most natural gas engines use late intake valve closing（LIVC）to expand the knock limit.Therefore,the effects of different camshafts on the engine performance were studied firstly.The results show that the camshaft with higher valve lift and retarded closing angle has better performance in terms of air-exchange process,fuel economy and emissions.On this basis,the effects of intake port and combustion chamber on combustion and emission characteristics were further studied.The results show that the optimized intake port can increase the thermal efficiency by 0.96%compared with the original vortex intake port.Due to the long combustion duration,the potential of the reentrant combustion chamber to increase thermal efficiency is limited.Based on the above work,the combustion optimization control strategy based on spark ignition timing and EGR were studied under different speed and load conditions,and the influence of above parameters on the engine fuel economy,combustion and emission characteristics were analyzed systematically.The results show that the optimal engine brake specific gas consumption can be reduced to 198.46g/kW·h.At 50%load,the thermal efficiency increases at first and then gets worse with the retarding of spark ignition timing.When the CA50 is in the range of 5°CA ATDC⁸°CA ATDC,the thermal efficiency is the highest.At 100%load,as the spark ignition timing increases,the thermal efficiency decreases.At this time,the spark ignition timing can better characterize the change of thermal efficiency.Under the premise of ensuring engine reliability,the spark ignition timing should be as close as possible to the knock boundary.Therefore,in the part loads condition,the CA50 should be controlled to between 5°CA ATDC⁸°CA ATDC for higher thermal efficiency,while at full load condition,CA50 should be as close as possible to the knock boundary under the premise of ensuring safety,which implied that expanding the knock boundary could be very effective for ITE improvement at high load.Finally,the indicated thermal efficiency at50%load and full load can be improved to 40.79%and 40.65%,respectively.