Gasoline Engine HCCI Combustion - Extending the high load limit

There is an increasing global focus on reducing emissions of greenhouse gases.For the automotive industry this means reducing CO2 emissions of the vehiclesmanufactured, which is synonymous with reducing their fuel consumption oradapting them for using renewable fuels.This thesis is based on a project aimed at improving the efficiency of gasolineengines in the lower load/speed region. The focus was mainly on a combustionstrategy called homogeneous charge compression ignition (HCCI), but also onhomogeneous lean and stratified lean spark-ignited combustion. In contrast totraditional stoichiometric spark-ignited combustion, HCCI can operate withdiluted mixtures, which leads to better cycle efficiency, smaller pumping lossesand smaller heat losses. However, at relatively high loads, HCCI combustionbecomes excessively rapid, generating in-cylinder pressure oscillations (ringing),which are perceived as noise by the human ear. The main objective ofthe project was to identify ways to avoid this ringing behaviour in order toincrease the upper load limit of HCCI. This is vital to avoid the need for modeswitches to spark-ignited combustion at higher loads and to operate the engineas much as possible in the more effective HCCI mode.The strategy for reducing ringing investigated most extensively in the projectwas charge stratification, achieved by injecting part of the fuel late in the compressionstroke. Available literature on effects of this strategy gave conflictingindications, both positive and negative effects have been reported, dependingon the type of fuel and engine used. It was soon found that the strategy iseffective for reducing ringing, but with resulting increases of NOX emissions.Further, in order for the strategy to be effective, global air/fuel ratios mustnot be much leaner than stoichiometric. The increases in NOX emissions werecountered by shifting the ratio towards stoichiometric using exhaust gas recirculation(EGR), allowing a three-way catalyst to reduce the excess NOX.Intake air boosting was also experimented on and is discussed as an alternativemethod or as a method to use in combination with charge stratification.During the project, experiments have been conducted with a productionlikemulti-cylinder engine and a single-cylinder research engine to investigatethe potential of various strategies for raising the high load limit of HCCI whenusing gasoline or gasoline-like fuels. To explain observed phenomena, opticalexperiments were conducted in which high-speed video was used to capturelight from the combustion and the residuals. A method was developed toextract pressure oscillations from these measurements and to correlate themto the combustion. Laser-based experiments were further used to analyse fueland temperature distributions before the combustion to investigate their effectson combustion and pressure oscillations.Based on the acquired data, plausible reasons why charge stratification canreduce ringing, and the circumstances in which it can do so, are presented. Thethesis also shows the extent to which the load can be increased using the strategy,and the resulting efficiency penalties, observed in both the production-likegasoline engine and single-cylinder research engine.Finally, the various strategies for load extension using combinations ofcharge stratification, EGR and boosting were compared to operating the enginein two-stroke HCCI mode. Although two-stroke operation was investigatedvery briefly, in an engine not designed for it, indications were obtainedthat this might be a much better alternative, since it provided higher loads,more stable combustion, less ringing, low NOX levels and higher efficiency thanany of the other tested load extension strategies