As part of the effort to improve thermal efficiency, engines are being
significantly downsized. A common issue in gasoline engines which limits
thermal efficiency and is further exacerbated by downsizing, is low speed pre ignition (LSPI). This thesis uses a Multiphysics approach, initially using a
validated 1D engine performance model of a GTDI engine, to define realistic
boundary conditions. A strong emphasis on validating each simulation
methodology as much as possible is maintained at each stage.
A hydrodynamic model of the ring-liner and Lagrangian CFD model are used
to investigate the impact of engine oil fluid properties on the mass of oil
transported from the crevice volume to the combustion chamber. A heat
transfer and evaporation model of a single droplet inside an engine
environment was developed for alkanes of chain lengths representing the
extremes of the chain lengths present in engine oil. It was found the droplet
generally evaporates at a crank angle which is close to the point where LSPI
is observed. The hydrocarbon study ends with a CFD constant volume
simulation to understand why engine oil like hydrocarbons ignite in rig tests
but not in an engine.
This research then proceeds to develop a single particle detergent model in
an engine environment, to initially understand why ignition occurs when a
calcium Ca based detergent is present but not in the case of a magnesium Mg
detergent. It was found from simulation that the common theory of calcium
oxide CaO resulting from thermal degradation from the previous cycle then
reacting with Carbon dioxide CO2 late in the compression stroke is unlikely.
There is a stronger case for the CaO particle causing ignition as it is present
in fresh engine oil sprayed onto the liner. As predicted by the hydrocarbon
evaporation model the oil will cover and protect the CaO particle until late in
the compression stroke when the oil will evaporate, exposing the CaO particle
to CO2.