The Low Load Limit of Gasoline Partially Premixed Combustion (PPC) - Experiments in a Light Duty Diesel Engine

Abstract: The decreasing oil supply, more stringent pollutant legislations and strong focus on decreasing carbon dioxide emissions drives the research of more efficient and clean combustion engines. One such combustion engine concept is Homogeneous Charge Compression Ignition (HCCI) which potentially achieves high efficiency and low NOx and soot emissions. One practical realization of HCCI in SI engines is to use a variable valve train to trap hot residual gases in order to increase the temperature of the fresh charge to auto-ignite around top dead center. The limited operating region of HCCI and lack of immediate control actuator, which makes feedback control of the combustion difficult, are two contributing reasons to change focus from HCCI to gasoline Partially Premixed Combustion (PPC). The advantage with using gasoline is a longer ignition delay which enhances the mixing of the fuel and air before combustion. But the attainable operating region is limited at low load with high octane number fuels and the main part of the thesis is devoted to extending the low load limit of gasoline PPC. The goal is to extend the operating region of the engine towards low load using the variable valve train system, the glow plug and more advanced injection strategies. The thesis is based on experimental investigations performed in a single cylinder research engine. During the first part of the thesis, results from a fundamental experimental study on HCCI combustion in comparison with SI combustion using different variable valve timing strategies is presented. Residual gas enhanced HCCI with negative valve overlap (NVO) or rebreathing has higher efficiency compared to SI combustion. To extend the operating range of the NVO HCCI engine, a combustion mode switch from SI combustion to HCCI combustion using NVO, was investigated. In the first gasoline PPC investigation, a comparison between diesel and two gasoline fuels with different octane numbers was performed. It is shown that the low octane number gasoline (69 RON) can be operated without using a high fraction of trapped hot residual gas down to 1 bar IMEPn. But the operating range of the 87 RON gasoline fuel was limited and could be run down to approximately 2 bar IMEPn using a high fraction of trapped hot residual gas. The rest of the gasoline PPC work is devoted to the 87 RON gasoline fuel. Experimental investigations on the effects of the hot residual gas using NVO and rebreathing, more advanced fuel injection strategies and effects of the glow plug are presented. In order to minimize fuel consumption while maintaining combustion stability, the suggested gasoline PPC low load operating strategy is to use the NVO valve strategy with a fuel injection during NVO at low engine load up to approximately 2 bar IMEPn and then switch to the rebreathing valve strategy using a split main fuel injection strategy at higher engine load.