Co-optimizing Engine Hardware and Fuel Chemistry for Improving Thermal Efficiency and Reducing Pollutant Emissions of Gasoline Fueled Direct Injection Internal Combustion Engines

Abstract

This thesis addresses the drawbacks of current generation downsized boosted gasoline powered engines through experiments that were designed to push the limits of thermal efficiency. Experiments were conducted on both spark ignited and compression ignited engine platforms to demonstrate potential solutions. The drawbacks of light-duty downsized gasoline spark ignited engines that are addressed in this thesis are the limited thermal efficiency, high knock propensity and high particulate matter emissions. All the experiments on the spark-ignited platform were conducted on a production, multi-cylinder spark ignited engine that was equipped with both port and direct injectors as well as a twin independent variable camshaft timing. The drawbacks of light-duty downsized gasoline compression ignited engines that are addressed in this thesis are the limited combustion efficiency and limited combustion stability at low loads and low temperatures. All the experiments on the compression-ignited platform were conducted on single cylinder research engine. The first part of the dissertation addresses the limited thermal efficiency of downsized spark ignited GDI (gasoline direct injection) engines by implementing Miller cycle operation with the assistance of a prototype floating nozzle turbocharger. Valve timing tables were generated using a GT-Power model which predicts the camshaft phasor positions for lowest fuel consumption. The results of the simulation study indicate that the engine can operate with higher thermal efficiency than the baseline strategy by adopting a Miller valve timing. The excess boost pressure required to implement the changes to the valve timing was provided by the prototype turbocharger. Implementation of Miller cycle resulted in a 4% improvement in fuel economy and a 30% reduction in NOx emissions over the FTP75 (Federal Test Procedure) standard drive cycle. The second part of the dissertation addresses the knock propensity and particulate matter emissions from spark ignited GDI engines. Three independent studies are conducted to investigate the knock-soot correlation first discovered by Han. The results show that Miller timing, direct injection and high-octane rated fuels lower knock propensity and improve thermal efficiency. The results also indicate that there is a significant increase in accumulation mode particles generated in the engine during knocking and the concentrations of the particles are proportional to knock intensity. This correlation is confirmed to exist on a multi-cylinder platform across hardware changes, injection strategy changes and changes in fuel chemistry. The last part of the dissertation delves into the issue of reduced combustion robustness of gasoline compression ignition at low loads and low temperatures. High cetane rated gasoline fuel was formulated using small percentages of peroxide compounds. Cetane enhanced gasoline was shown to reduce manifold pressure requirements up to 60 kPa and increase combustion efficiency up to 8% compared to the baseline gasoline. The implementation of a prototype dual fuel injector presented an opportunity for optimizing the consumption of cetane enhanced gasoline by switching to regular gasoline when combustion stability is high. This combination of CEG (cetane enhanced gasoline) and a dual fuel injector can potentially overcome the challenge of GCI (gasoline compression ignition) operation at engine-idle conditions and cold starts. The studies presented in this thesis provide potential pathways for enhancing thermal efficiency and reducing emissions of future production light-duty gasoline engines. The observations made from the experiments in this thesis can directly impact the decision making of future light-duty downsized gasoline engine manufacturers.