Despite challenges with poor emissions and fuel economy, gasoline two stroke engines
continue to be developed for a number of applications. The primary reasons for the choice
of a gasoline two stroke engine includes its low cost, mechanical simplicity and high specific
power output. Some applications for the gasoline two stroke engine include small capacity
motorcycles and scooters, off road recreational vehicles, hand held power tools and
unmanned aerial vehicles.
New technologies, which are already mature in four stroke engines, are now being applied to
two stroke engines. Such technologies include direct fuel injection, electronic engine
management and exhaust gas after treatment. To implement these new technologies
computation models are being continuously developed to improve the design process of
engines. Multi-dimensional computational fluid dynamics modelling is now commonly
applied to engine research and development, it is a powerful tool that can give great insight
into the thermofluid working of an engine. Multi-dimensional tools are however
computationally expensive and quasi-dimensional modelling methods are often better suited
for the analysis of an engine, for example in transient engine simulation.
This thesis reports the development of a new quasi-dimensional combustion model for a loop
scavenged two stroke engine. The model differs from other quasi-dimensional models
available in the literature as it accounts for a bulk motion of the flame front due to the
tumble motion created by the loop scavenge process. In this study the tumble motion is
modelled as an ellipsoid vortex and the size of the vortex is defined by the combustion
chamber height and a limiting elliptical aspect ratio. The limiting aspect ratio has been
observed in experimental square piston compression machines and optical engines. The new
model also accounts for a wrinkled flame brush thickness and its effects on the interaction
between flame front and combustion chamber.
The new combustion model has been validated against experimental engine tests in which
the flame front propagation was measured using ionization probes. The probes were able
determine the flame front shape, the bulk movement of the flame front due to tumble and
also the wrinkled flame brush thickness.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University. This is a copyright-cleared version of the Thesis.