Research And Application Of Optimum Valve Timing Based On High Speed Diesel Engine

People have to demand to improve the comprehensive performance of diesel engine further due to global energy and environment issues. Variable Valve Timing System (VVT) improves the engine performance through fine tuning the parameters of the valve system, and it gets more and more attention. Adjusting valve timing parameters is a effective way to realize the miller cycle. Miller cycle can not only increase thermal efficiency, but also reduce the emissions. Traditional way, either rule of thumb or experiment, cannot meet the requirements of optimizing modern engine variable valve timing. Therefore it has become the first aim for practitioners to find an effective and fast way to optimize the valve timing and miller cycle.In this thesis, the whole working process of a high speed diesel engine which contains variable valve timing system was simulated by the CFD method. The simulation is then verified by experiment data. In order to investigate the effects of variable valve timing affect on engine power characteristics, fuel economy and emission performance, and proceed to optimize it, the effective power, specific fuel consumption, quantity of NOx and Soot, and mass of fresh charge were calculated as valve timing parameters were modulated at6speed running conditions. A method of calculating CRG (coefficient of residual gas) was suggested to quantitative analysis the sweep efficiency. The calculation result indicates that exhaust advance angle has a little effect on sweep and emission performance, but a big effect on total loss power (include power loss of expansion and push). The optimum exhaust advance angle should increase as engine speed rises to reduce the expansion loss and forced exhaust loss. Increasing intake lag angle in low engine speed could increase coefficient of charge, thereby increase power characteristics and fuel economy performance. Comprehensive performance could be increased by decreasing intake lag angle in high engine speed.Based on the research of optimizing valve timing, miller timingcycle was realized and optimized by employing the variable valve timing way. The method which was used to analyze the valve timing could be used to realize and optimize the miller cycle, the effects that miller cycle affect on comprehensive performance of diesel engine were calculated as changed the effective compression ratio running conditions, adjusted the turbocharged pressure, modulated miller timing parameters in3speed working conditions. The calculation results suggest that, take the air flow back point for reference, and ensure injection quantity and excess air coefficient unchanged, intake valve was delayed close in low speed and advanced close in high speed, the temperature and pressure level of initial and maximum combustion in cylinder can decreases, and the NO quantity was significantly reduced as effective power little descend.