Study Of Variable Compression Ratio On GDI Engine Performance Improvement And Design Of Variable Compression Ratio Mechanism

As the market of automotive continues to expand, the number of automotivevehicles is increasing persistently, non-renewable resources have been consumedseverely, and the greenhouse effect caused by carbon dioxide has brought negativeeffect to the global climate, acting as an important energy user, vehicles consume a lotof resources, if the thermal efficiency of the engine can be improved and carbondioxide emissions reduced, it will make a great difference in promoting energy savingand emission reduction. Compared with diesel engines, gasoline engines’ compressionratio can not be quite large because of the problem of knock combustion, only inaccordance with the design of compression ratio of full load, resulting in gasolineengine thermal efficiency at low load is relatively low,however in city conditions, thegasoline engine operating conditions are often under middle and small load, variablecompression ratio can make a contribution to the engine fuel economy, using highcompression ratio at small and middle load will improve the thermal efficiency, whileunder high load,using low compression ratio can avoid detonation. In this paper, aGDI engine model was built based on a1.4L engine using the simulation softwareGT-Power, engine operating condition were at1500r/min,3000r/min,4500r/min, underthe load of20%,50%,80%,and under the condition of homogeneous equivalent fuelair ratio, quantitatively, the performance improvement made by variable compressionratio was studied. Besides, engine speed at1500r/min,3000r/min,4500r/min, and loadat20%,50%,in the case of homogeneous lean combustion, the performanceimprovement made by variable compression ratio was studied. In addition, sometypical variable compression ratio mechanisms were compared, and based on thecomparison, a new mechanism was put forward, the key parts were designed, somerelating checking calculation was finished.3-D model of the key parts was made bythe CAD/CAE/CAM software CATIA, what’s more, the mechanism assembly wasdesigned. The main conclusions are as follows:1. Under the condition of homogeneous stoichiometric, low engine speed, low load, it is not the case that the larger the compression ratio is, the more excellent thepower and economic performance can be, as a matter of fact, there is a certaincompression ratio value that can make engine performance best, as the compressionratio value goes up, mechanical loss will increase and this will lead to the drop ofbrake efficiency. And rising of engine speed increases the adaptability of highercompression ratio.2. Under the condition of homogeneous stoichiometric, power and economicperformance improvement by variable compression ratio is relatively obvious whenthe load is low, with the increase of load, the probability of knock increases, limitingthe further improvement by variable compression ratio.3. Under the homogeneous lean combustion condition, power and economicperformance improvement by variable compression ratio is relatively obvious whenthe load is low, and with the increase of air-fuel ratio, the improvement gets less. Withthe air fuel ratio increases, the indicated work is greatly reduced, the mechanical lossdoes not change much, after the compression ratio increases to a certain value, themechanical efficiency will decrease, the adaptability of high compression ratio is nothigh.4. This paper carried out a variable compression ratio scheme that can change thechange the crankshaft axis position.5. In this paper, key parts of the variable compression ratio mechanism weredesigned, and the relating checking calculation was finished, as to the key partcrankshaft bearing beam, relating simulation was done using CAE method, and it wasproved that the mechanism could meet the intensity of use requirements, and theassembly design was finished.