Alternative Combustion Strategies for Gasoline Engines

Licentiate thesis, 2009

Due to increasingly strict legislative demands and customer concerns regarding CO2 emissions, there is a need to develop and implement new combustion strategies for gasoline (Otto) engines. In attempts to meet this need, most attention is focusing on the low load/speed range where the classic spark ignition strategy (SI) has poor efficiency, mainly due to the throttling of the intake air causing pumping losses. The work this thesis is based upon is part of an ongoing project, in which alternative combustion strategies are being compared and evaluated experimentally in terms of fuel efficiency (as indicated by CO2 emissions) and emissions of legally-regulated toxic species. Two novel combustion strategies, homogeneous charge compression ignition (HCCI) and stratified charge spark ignition (SCSI), are being implemented and compared in these respects to stoichiometric and lean variants of the classic SI strategy. In addition, efforts are being made to reduce the rapid combustion rates that often cause ringing (or knock) and hence are the load-limiting factors for HCCI combustion. The adopted strategy for this is to reduce the heat-release rates by using a late injection in the compression stroke to create charge stratification. A multi-cylinder Volvo engine capable of running (and switching between) all the combustion strategies investigated in the work to date has been used in the evaluations, providing an equal basis for comparisons. In addition, a single-cylinder research engine has been used in both metal engine and optical engine configurations to investigate the effects of charge stratification on ringing in more detail. Results from the experiments clearly indicate that both HCCI and SCSI are capable of delivering large (~10-20%) reductions in fuel consumption during stationary operation in low load/speed operating conditions, and SCSI could deliver potentially further reductions if the combustion chamber geometry can be optimised. Both HCCI and SCSI are combustion strategies in which excess air creates problems for aftertreatment of NOX emissions. However, while use of SCSI would require a lean NOX after-treatment system, it may be possible to operate engines in HCCI mode without such a system, because of the inherently low engine-out levels it generates. The introduction of a late injection in the compression stroke in HCCI combustion mode has shown great potential for reducing ringing intensities at the high load limit, albeit at the expense of increases in NOX emissions to levels requiring lean NOX aftertreatment for this combustion strategy too.