The Impact of Miller Valve Timing on Combustion and Charging Performance of an Ethanol- and Methanol-Fueled Heavy-Duty Spark Ignition Engine

Combustion engines and liquid fuels are likely to continue playing a central role in freight transportation with renewable fuels reducing carbon emissions. Ethanol and methanol are future renewable fuels with a knock resistance that make them suitable for heavy-duty (HD) spark ignition (SI) engines. This simulation work focuses on the potential for improving the efficiency of an ethanol- and methanol-fueled HD SI engine using early intake valve closing Miller valve timing. With Miller valve timing, the expansion ratio and thermodynamic efficiency can be increased while maintaining the same effective compression ratio. However, Miller timing requires increased boost pressure to retain the same trapped air mass and also suffers from reduced in-cylinder turbulence. Unlike previous simulation studies, a validated semi-predictive combustion model was used to resolve the implication of turbulence reduction on burn rate and its impediment in extracting higher thermodynamic efficiency with Miller timing discussed. The observed increase in burn duration adversely affected knock and the overall efficiency benefit from Miller timing. At stoichiometric conditions, a 2-3% increase in brake efficiency was observed with Miller timing by increasing the geometric compression ratio even with a relatively low turbocharger efficiency of 49%. At lean conditions, the increase in burn duration and pumping loss was significant for both fuels demanding a minimum turbocharger efficiency of 55% to gain an improvement in brake efficiency from Miller timing. If the degree of Miller timing is constrained by a single-stage turbocharger, Miller timing showed only a 0.7% point efficiency increase at lean conditions due to the reduced burn rate. If the burn rate can be increased, ~2.5% increase in brake efficiency can be achieved using Miller timing leading to over 48% brake efficiency for both fuels thus making the HD SI engine competitive to HD diesel engines.