Simulation Optimization Of Miller Cycle Coalbed Methane Engine

Nowadays,energy and environment have become an important factor restricting China's economic security and sustainable development.As a kind of unconventional natural gas resources,coalbed methane has been widely used,and the development and utilization of coalbed methane has many benefits-increasing the supply of renewable energy,improving production safety and protect the global environment.In this paper,a 4103 turbocharged diesel model is established by using GT-POWER simulation software and modified into coalbed methane engine model.At the full load condition,the simulation results show that the power performance of the engine is reduced due to the low calorific value of the coalbed methane and the small compression ratio.Therefore,we proposes an measures to improve the performance of the engine-using Miller cycle to expand the knock limit,while improving thermal efficiency and reduce NOx emissions,and then optimize the supercharging ratio,compression ratio,air-fuel ratio to improve power performance.Firstly,the influence of different Miller cycle on the engine performance is studied.And then,based on the limitation of the knocking index,the influence of the supercharging ratio,the compression ratio,the air-fuel ratio and the Miller cycle is studied using the multi-parameter optimization method.Finally,the optimal design scheme of Miller cycle combined with each parameter is determined,which provides a reference for the actual modification of coalbed methane engine.At the partial load condition,in order to reduce the throttling losses,the Miller cycle are used so that we can control the load by reducing flow rate.At the 75%,50%and 25%load,we compared the performance of the CBM engine with throttle control method and Miller cycle control method.The results show that:(1)When the diesel engine is modified into CBM engine,its power,intake-air volume and combustion pressure are decreased.The thermal efficiency is improved at low speed and decreased at high speed.NOx emissions decreased,but CO,HC emissions increased.(2)Miller cycle,compression ratio,supercharging ratio,air-fuel ratio have a variety of cooperation cases to improve the engine's dynamic performance.For example,when the intake valve is early 20° CA,the supercharging ratio is 2.72,air-fuel ratio is 6.91,compression ratio is 10.69,The power can be increased by 19.55%,but its thermal efficiency decreased by 2.52%,NOx emissions is 4.02 times of the original machine.Due to the low cost of coalbed methane fuel,a small decline in economic performance is acceptable.And for the increase of NOx emissions,post-processing measures can be taken.(3)Under the premise of power increase,most of the cases make the thermal efficiency reduce,the maximum decline rate is 3.53%.And most of the program also reduce the NOx emissions.Therefore,in the actual design of coalbed methane engine,we should both take care of the power,economy and emission performance,and choose the design according to the actual demand.(4)Both EIVC(Early Intake Valve Closing)and LIVC(Late Intake Valve Closing)Miller cycle can replace the throttle to control the engine load,reducing the PMEP(Pumping Mean Effective Pressure),exhaust gas temperature,combustion pressure,cylinder temperature,NOx emission,and increasing the brake efficiency.But when the early intake closing angle is too large,the reduction of the valve lift will cause a bigger loss than the throttle,and the PMEP will increase.When the late intake closing angle is too large,the piston will push some of the mixture outside the cylinder,resulting a increase of the PMEP.(5)The results of the partial load condition of the Miller cycle control scheme are as follows:at 75%load,using LIVC 80°CA,the thermal efficiency is improved by 1.95%,the pump loss is reduced by 35.74%,the NOx emission is reduced by 18.37%;at 50%load,using EIVC 40° CA,the thermal efficiency increased by 4.02%,pump loss decreased by 44.64%,NOx emissions decreased by 41.98%;at 25%load,using EIVC 60 ° CA,the thermal efficiency increased by 7.32%,pump loss decreased by 38.10%,NOx emissions decreased by 63.08%.