Interactions between charge conditioning, knock and spark-ignition engine architecture
thesisposted on 16.06.2011, 08:08 by James W.G. Turner
There are currently many factors motivating car manufacturers to reduce the tailpipe CO2 emissions from their products. One of the major routes to achieving reduced CO2 emissions in spark-ignition 4-stroke engines is to ‘downsize’ the swept volume which, among other advantages, reduces the proportion of fuel energy expended on pumping losses. The full-load performance deficit caused by reducing the swept volume of the engine is normally recovered by pressure charging. One of the limits to pressure charging is combustion knock, which is the unintended autoignition of the last portion of gas to burn in the combustion chamber after combustion has been initiated. This thesis presents results from investigations into a number of methods for suppressing knock, including (1) tests where the density of the intake air is closely controlled and the effect of charge air temperature is isolated, (2) where the latent heat of vaporization of a fuel is used to reduce the outlet temperature of a supercharger, and (3) where the engine architecture is configured to minimize exhaust gas residual carryover to the benefit of stronger knock resistance. Extensive comparison of this resulting engine architecture is made with published data on other strategies to reduce the effect of the knock limit on engine performance and efficiency. Several such strategies, including cooled EGR, were then investigated to see how much further engine efficiency (in terms of brake specific fuel consumption) could be improved if they are adopted on an engine architecture which has already been configured with best knock limit performance in mind. Within the limits tested, it was found that if the charge air density is fixed then the relationship between knock-limited spark advance and air temperature is linear. This methodology has not been found in the literature and is believed to be unique, with important ramifications for the design of future spark-ignition engine charging systems. It was also found that through a combination of an optimized direct-injection combustion system, an exhaust manifold integrated into the cylinder head, and a 3-cylinder configuration, an engine with extremely high full-load thermal efficiency can be created. This is because these characteristics are all synergistic. Against the baseline of such an engine, other technologies such as excess air operation and the use of cooled EGR are shown to offer little improvement. When operating a pressure-charged engine on alcohol fuel, it was found that there exists a maximum proportion of fuel that can be introduced before the supercharger beyond which there is no benefit to charge temperature reduction by introducing more. Strategies for reducing the amount of time when such a system operates were developed in order to minimize difficulties in applying such a strategy to a practical road vehicle. Finally, a new strategy for beneficially employing the latent heat of vaporization of the fuel in engines employing cooled EGR by injecting a proportion of the fuel charge directly into the EGR gas is proposed. This novel approach arose from the findings of the research into pre-supercharger fuel introduction and cooled EGR.
- Mechanical, Electrical and Manufacturing Engineering