| As China’s energy structure is “poor-oil,less-gas,relatively rich-coal”,the development and promotion of the coal-based methanol fuel as a traditional fuel alternative fuel is one of the feasible options.The application of methanol in the engine is a development direction of new energy vehicles in China.In the 1990 s,the southwest research institute proposed a high compression ratio spark-ignition methanol engine program which provides a new idea for the development of high efficiency methanol engine.Based on the methanol’s advantages is“high Octane number,good antiknock property,fast flame propagation,and wide dilute limit”,a control strategy of lean dilution combustion combined with high compression ratio is presented in this paper.It was applied in a high compression ratio(ε=17.5)methanol engine which was modified from a diesel engine.In this paper,the knocking suppressing mechanism,fuel economy,thermal efficiency and emissions were studied in a spark-ignition methanol engine by using this control strategy.This control strategy,which provides experimental and theoretical basis for high-efficiency and clean utilization of methanol in engine,has a useful reference value.Firstly,in this paper,by using software CFD,a multi-dimensional simulation was established which based on experimental prototype’s air inlet and the combustion chamber.Through the established model,from the perspective of microcosmic to explore the knocking combustion process and explain the reason and the position of knocking.Knocking combustion under various engine operating conditions were simulated,and the effects of spark timing,EGR(exhaust gas recirculation)techniques for suppressing knock in a spark-ignition methanol engine were studied based on the multi-dimensional simulation analysis.It can provide adequate theoretical guidance to the test of the methanol engine.In order to verify the accuracy of simulation calculation,the effects of spark timing,EGR dilution,EGR temperature,and air dilution for suppressing knock in a spark-ignition methanol engine were studied based on test of the methanol engine.The results showed that the knocking combustion is easier occur at the low speed and heavy load of high compression ratio methanol engine,and the methanol engine can work stability at full load range by using later ignition timing,and both EGR dilution and air dilution had certain inhibitory effect to knock suppression in methanol engine.Secondly,the effects of cold EGR,hot EGR,and air dilutions for fuel economy and emissions were studied of a high compression ratio spark-ignition methanol under the condition of homogeneous combustion.The results showed that three kinds of dilution improved the fuel economy and emission of methanol engine to some extent,and air dilution had the best fuel economy,cold EGR was worst.The methanol engine’s NOx emissions achieve low level when the dilution ratio was a certain dilution degree under all of three dilutions.Comprehensive evaluation,the use of hot EGR had a better thermal efficiency on the knock suppression as compared to the use of cold EGR and air dilutions,when the dilution ratio at the rage of 1.5~1.6.In order to reduce pumping losses,improve thermal efficiency,and reduce probability of knocking at high load,this paper,using EGR to control load of a spark-ignition methanol engine with wide open throttle(WOT)and stoichiometric mixture have been investigated experimentally,and its performance and emission characteristics were compared with the traditional control method by throttle.The experimental results indicated that at engine speed of 1400 rpm and full load,the methanol engine can run stably without knock when the ignition timings are later 18°CA BTDC.The load range of the control method with variable of EGR and ignition timing was carried out from 36% to 100% load.According to the analysis of fuel consumption,when the output torque is higher than 62% load,fuel consumption can be reduced by using EGR to control load,and fuel consumption increased at the other load.Thus,EGR combine with throttle to control load was used at the medium and high load to study of influence on fuel economy.And the results show that using EGR combine with throttle control load had a better economy than the original machine and using EGR control load.In order to realize the high efficiency and clean combustion in full load,this paper presents using EGR and excess air coefficient coordinated control load at the medium and low load.The influence on fuel economy and emissions was studied by using EGR and excess air coefficient coordinated control load.The results show that methanol engine’s thermal efficiency reached to 34% at 25% load,and the methanol engine obtained higher thermal efficiency and low NOx emissions to achieve efficient clean combustion at the EGR ratio 20% and the excess air coefficient between 1.4~1.6.Based on the above study,this paper presents a load control strategy for high efficiency and clean combustion of methanol engine in full load.At low load,EGR and excess air coefficient coordinated control load should be adopted to attain relatively high thermal efficiency.In middle-high load,using EGR combine with throttle control load should be adopted to achieve clean combustion with high efficiency and low NOx emissions.At high load,using EGR control load should be adopted to achieve clean combustion with higher efficiency and low NOx emissions than throttle control load.This load control strategy,which provides theoretical and experimental basis for high-efficiency and clean utilization of methanol in engine,has a useful reference value.In order to extend further engine load range by using the EGR control load,reduce the cycle-by-cycle variations(CoV),improve EGR tolerance,this paper presents a stratified EGR strategy by intubation or adding helical partition in the intake port.The effects of EGR pressure and EGR inlet time for EGR stratification in methanol engine were studied based on the CFD with detailed aerodynamic kinetics.The results showed that if only turning EGR pressure up,average EGR rate in cylinder increases but stratified gradient is less than 5% when EGR is not stop.Reasonably EGR pressure and inlet time can reach the ideal stratified effect.By intubation,there is no EGR stratification in bowl,and EGR ratio of central part in lateral section upon bowl is higher when EGR is stopped later,on the contrary,EGR ratio of periphery part in lateral section upon bowl is higher when EGR is stopped earlier.By adding helical partition in the intake port,turning EGR pressure up,at the same time shortening EGR inlet time,EGR stratification will get better in cylinder,and stratified gradient will increase too.For example,when EGR pressure at 130 kPa,inlet time was 97°CA,the average EGR rate is up to 40%,with 53% highest EGR rate and 23% lowest EGR,and stratified gradient reach to 30%.In order to verify the two stratified EGR strategy’s effects of the methanol engine in practical application,the methanol engine’s intake port is modified.The intubation guided mode is easier to implement than adding helical partition in the intake port by simply modify.In order to add a helical partition in the intake port,3D printing technology was adopted to make a helical partition which modified a single intake port to double inlet.The effects of intubation and adding helical partition in the intake port for improving the potential of EGR tolerances in methanol engine were studied.The results showed that both stratified EGR strategy can improve EGR tolerance to a certain extent,improve the dilution limit of methanol engine.The use of add a helical partition in the intake port had a better fuel economy,emissions and stability compare to use of intubation guided,and had a wider dilute limit. |
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