An Experimental Study of Different Effects of EGR Rates on The Performance And Exhaust Emissions of The Stratified Charge Piston Direct Injection Compressed Natural Gas Engine

Exhaust Gas Recirculation (EGR) is one of the principal techniques used to control spark ignition NOX. A fraction of the exhaust gas is recycled through a control valve from the exhaust to the engine intake system. However, EGR has different effect on performance, combustion and emissions production that are difficult to distinguish such as increase of intake temperature, delay of Rate Of Heat Rrelease (ROHR), decrease of peak heat release, decrease in oxygen concentration etc. Therefore the impact of EGR on the aforementioned engine parameters (i.e., performance, combustion and exhaust emission) is not perfectly understood, especially under high EGR rates. An experimental study has been conducted to analyze various effects of EGR rates on the performance and emissions of the stratified charge piston direct injection compressed natural gas engine and to determine the stable operating limit of the engine at different excess air ratios ( = 0.9, 1.0, 1.1 and 1.2) which represents rich, stoichiometric, slightly lean and moderately lean mixture respectively. The results showed that as the EGR is increased, the brake torque, brake specific fuel consumption decreased, while nitric oxide emissions (NO) reduced drastically at various fraction of EGR, just as Unburnt Hydro Carbon (UHC) increased. EGR has no significant effect on carbon monoxide (CO) emission. The addition of EGR also reduces cylinder’s gas temperature and pressure. It can be concluded that in introducing EGR in DI-CNG engines, there is a tradeoff between the engine’s performance and NOX emission, while it is difficult to realize stable combustion at high temperature.