| As one of new clean energy, natural gas engine is being promoted in many countries. Buta series of problems arisein the engine actual application, such as power decline, high heatload, and low efficiency which prevented the popularization and application of natural gasengine. Meanwhile,engines use a conventional throttle to control the engine load, causingsubstantial pumping losses at part load. Aiming at these problems, this thesis analyzes anovel approach for improving the performance of CNG engine based on the Miller cycle.One aspect of this work is studying on the impact of Miller cycle parameters on the CNGengine’sperformance.The parameters containthe Miller cycle timing, geometry compression,and the turbocharger. Complete the geometric compression ratio selection, turbochargingsystem adjustment and Miller timing matching.Through researchingthe impact of the geometric compression ratio on the knock tendency,raise the geometric compression ratio up to13.5.And the turbocharging system is optimizedfor the application of Miller cycle. Reduce the turbine nozzle ring exit flow area to adjust thejoint operation curve. According to the impact law of Miller cycle timing on engineperformance,when the engine speed is below1000rpm the intake valve closing angle choseEIVC=20°CA (IVC=561°CA) to optimize the low speed torque. On the other speed Theintake valve closing angle chose LIVC=32°CA (IVC=613°CA) to reduce the knock index.The Comprehensive optimization result is the engine torque at low engine speeds increasemore than5%,the engine exhaust temperature is reduced by an average of20K, rduced theengine heat load. Solved the problem of deflagration which caused by the improvement ofthe engine power performance.Another important aspect of this work is to compare the fuel consumption of different loadcontrol mode (VVT, VVL, VVTL and throttle).Analyzes a novel approach for teduce thepumping losses and Improve fuel economyat part loadof CNG engine.the simulationcondition is idlingcondition and20%load condition.According to the researchrelying solely on delaying the intake valve closing angle cannot achieve all operating conditions’s unthrottle control.Under idling condition, the higherinlet pressure and lower pumping losses of the Miller cycle load control cannot make upfor the pulling outwork loss generated whenthe pistonupstream. The BSFCis23%higherthan the throttle load control way and the volume efficiency is lower. When compare theperformance of different load control mode at20%load, The pumping loose of the VVLload control modeis even higher than the original machine,that is because gas flowthrough the valve seat caused heavy throttling loss.The corresponding BSFC increasedby5%. While VVT and VVLT load control methods improved the CNG endineperformance,greatly reduced engine pumping loss. During The final optimization,theumping lossesreduced more than50%, the BSFC reduced20g/Kw.h and the economyof natural gas engine is increased by2.6%. |
