Effect Of EGR On The Thermal Efficiency In Sidi Engines

With the increase of world’s vehicle production and sales year byyear, problems in oil resources and environment become even more urgent.Energy-saving and emission-reduction has become the focus of nationalattention. Oil consumption of the engines accounts for a large proportion.Therefore, to improve the fuel economy of gasoline engine becomes a hotspot. Downsizing has become an important trend for automobile industry.However, to meet the power needs, boosting technique should be adoptedin the downsizing engine. The heat-load of the downsizing boosted engineraises which results in the increase of knocking tendency. External cooledEGR technology is a very efficient technique to suppress knocking.This paper first studies the mechanism of external cooled EGR effecton the engine thermal efficiency, and then analyzes how the compressionratio influences the EGR effect. The research based on the first law ofthermodynamics and the second law of thermodynamics. A combination of factors result in improved thermal efficiency with introduction of EGR atfull load engine operating conditions, including eliminated fuel enrichmentrequirement, decreased in-cylinder heat losses, increased specific heat ratioand degree of constant volume. However, the combustion irreversibilitybased on the second law of thermodynamics increases. At medium load,the improvement in thermal efficiency with EGR not only result fromfactors that have been talked above, but also from reduced pumping losses.At low load, although a reduction in degree of constant volume is observedwith external EGR, the improvement in the factors above lead to aincreased thermal efficiency. For combustion irreversibility under mediumand low load, it increases firstly followed by a decrease when EGR levelexceeds16%. At full load, the increase of compression ratio will reducethe degree of constant volume which has a negative effect on the fueleconomy. At part load, the increase of compression ratio can improve thebrake efficiency. The raise of compression ratio reduces the combustionirreversibility.