Application Of Miller Cycle In Conventional Gasoline Engine

Increasingly stringent fuel consumption regulations challenge the development of conventional Internal Combustion Engine.In Miller cycle,the compression ratio is decoupled with the expansion ratio,making the expansion ratio is higher than the compression ratio,resulting in higher thermal efficiency and better fuel economy compared to the conventional engine.Currently,relative studies of Miller cycle attract more and more attention.Whereas,compared to foreign car companies,Chinese independent companies are still in its infancy in the research of Miller cycle engines.Therefore,simulation and experimental researches of Miller cycle are carried out in this paper to investigate the influences of Miller cycle on engine performance,which can provide theory and test support for the application of Miller cycle in the current engine platform,decreasing the risk and cost in the development of the Miller cycle engine.Firstly,the effects of Miller cycle including early and late valve closing(EIVC or LIVC)control strategies on the engine performance of fuel consumption,knock and combustion characteristic at different operating loads are investigated based on one and three-dimensional simulation.The results show that both Miller cycle strategies can reduce the engine fuel consumption by 7% averagely at full operating loads.At low load operation point,the EIVC strategy allows larger throttle opening angle and higher intake pressure,which results in less pumping loss than the LIVC strategy.At high load operation point,the turbulence kinetic energy with EIVC strategy is lower than that with LIVC strategy.On the consequence,the EIVC cycle’s velocity of flame propagation decreases and the crank angle(CA)of combustion 50% fuel is postponed by 1.8°CA than LIVC cycle.All the results indicate that at high load operation,the fuel consumption and the suppression of knock with the LIVC strategy is better than that with the EIVC strategy.Then,based on the simulation study,improvement of the Miller cycle engine with the LIVC control strategy is carried out and relative performance parameters and structures are determined.The performance parameters include the maximum valve lift,the valve open duration and the compression ratio.Relative structures include the intake camshaft,the piston and the exhaust pipe.Finally,an experimental study of the modified engine is conducted.The results show that in order to maintain sufficient output power,the engine uses the Otto cycle at WOT(wide-open throttle),decreasing the fuel efficiency.However,the output power is better than the original engine due to the high compression ratio.At part load operation point,the test results suggest that Brake Specific Fuel Consumption could be reduced by 7-8% over a wide range of loads and engine speeds.In summary,the research about the development of the Miller cycle engine in this master thesis is of important significance in engineering.It provides theoretical and experimental support for the application of Miller cycles at existing engine platforms and reduces the risk and cost of the development of the Miller cycle engine.