AbstractTechnologies for improving engine performance and fuel economy have proven effective and promising to reduce emissions. However, owing to the increased pressure and temperature of the mixture by the turbocharging system, one negative result is a higher risk of knocking. This is not conducive to the safety of the engine or a reduction in fuel consumption. Low-temperature combustion by a diluted mixture in the cylinder has been validated to effectively reduce the knock occurrence and NOx emission. Because the ignition timing advances by suppressing the knock caused by the mixture dilution, the fuel economy could be improved in the spark-ignition (SI) turbocharged engines. In this study, the intake valve closing (IVC), exhaust valve closing (EVC), and exhaust gas recirculation (EGR) strategies as different dilution methods were applied to investigate their effects on combustion, emission, and fuel consumption. The experimental results showed that all these dilution methods were beneficial for decreasing the maximum burned gas temperature, thereby reducing emissions of NOx. The external EGR showed more benefits to suppress the knocking, and thus the reduced fuel consumption compared to the inner EGR (valve overlap). The external EGR method had showed larger effects on the combustion phase and fuel economy than IVC and EVC. In addition, the difference between the IVC and EVC was small when external EGR was added.

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