Emissions reduction effects of diesel combustion with low compression and high expansion ratios

For suppressing NO_x and smoke simultaneously, an alternative diesel combustion strategy is required to avoid local over-rich regions in the mixtures at EGR conditions. In the present study, a variable valve timing (VVT) mechanism is applied in a light duty HSDI diesel engine. By means of lowering effective compression ratios, compressed gas temperatures near the top dead center are lowered, thereby preventing too early ignition and increasing ignition delay to enhance fuel-air mixing. At the same time, the expansion ratio is kept constant to ensure thermal efficiency. It is recognized that the optimum control of EGR and effective compression ratios can simultaneously reduce NO_x and smoke.