The Influences Of Variable Compression Ratio Mechanism On Gasoline Engine Performances

Compression ratio, which is a principal parameter to evaluate the performance ofgasoline engine, plays an important role to improve the thermal energy efficiency. Thetraditional engine figures out the value of compression ratio based on some conditions,it does not completely draw out its potential. Variable compression ratio (VCR) cansolve this problem to improve the power performance and fuel economy of gasolineengine.This study discusses the VCR mechanism based on an eccentric mechanism incrank connecting rod, analyzes the kinematics of piston movement and cylindervolume changed profile, and explains how the compression ratio and the length offour strokes change with the key parameters. The range between0°and90°of theangle eccentric mechanism starts with is the most practical interval due to the largercompression ratio. Moreover, four piston strokes in engine cycle could diversify andthe expansion ratio could be greater than the compression ratio with this VCRmechanism.According to the experimental data of a1.6L naturally aspirated gasoline engine,a simulation model was set up in GT-POWER platform and calibrated to predict howthis VCR mechanism affects the engine performances. The thermal efficiency, themaximum temperature and the peak pressure in cylinder increases with increase of thecompression ratio, especially in small and medium load working condition. In thetypical working condition rotate speed3000r/min, BSFC improved by8%whencompression ratio is increased to13.2and expansion ratio is increased to13.9. Thepumping loss would be reduced in small load working condition but deteriorated inheavy load working condition due to the higher speed of gas expansion.Knocking situation is a key problem in gasoline engine when the compressionratio increases. A model was designed based on the auto ignition theory to get theknocking index and predict its variation. The simulation results show that knockinglevel increases with increase of the load and compression ratio but decreases withincrease of the engine speed, and knocking tendency increases much faster in lowspeed working condition. If the expansion of gas achieves a certain condition, it willlower the temperature in cylinder unburned zone and restraint the knocking tendencywhich is activated by surface ignition. Due to the method which guarantees the knocking tendency not to exceed the original engine full-load level, the mostappropriate compression ratio in every certain condition was calibrated. Thus, thepotential of the engine could be known, this VCR mechanism could reduce the enginefuel consumption rate by5%to10%in most area of the universal characteristic.