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
