posted on 2014-06-18, 11:43authored byBryn Littlefair
The internal combustion engine is still at the heart of the vast majority of vehicles
manufactured worldwide today. For these applications reciprocating pistons are typically
employed to convert the pressures generated by internal combustion into mechanical work
required by the vehicle. Of the energy supplied to the engine as a whole approximately 17%
is lost by means of mechanical friction. The piston ring - liner and piston skirt - liner
conjunctions contribute approximately 30% of the overall friction losses in almost equal
proportions. It is, therefore, important to note that reduction in piston assembly friction would
have a significant effect on the fuel consumption and, therefore, performance of engines
manufactured today.
In order to reduce the effect of friction it is of critical importance that the model and
predictions made alongside the design of engine components accurately represent the real incycle
conditions encountered in practice. Much of the published research to date has excluded
the effects of global thermo-elastic distortions on the lubrication of the piston skirt. In cases
where this effect has been studied, it has been for relatively low engine speeds or loads on
relatively stiff conjunctions. In motorsport applications the expected component lifespans are
much shorter than in the usual OEM production vehicles. Reduction in component mass,
particularly in reciprocating components has been at the centre of these recent gains. The
effect of mass reduction coupled with the increased BMEP observed in high performance
engines emphasises the importance of underlying mechanisms of lubrication.
This thesis develops the modelling methodology for piston skirt – cylinder liner conjunction
for the motorsport and high performance engine applications. It presents a multi-body, multiscale
approach to the prediction of the lubrication conditions of the skirt – liner conjunction,
incorporating realistic measured boundary conditions. It highlights the effect of inertial
loading observed at high speeds in such applications. Using the methodology developed in
this work, future improvements in friction may be accurately predicted though the use of the
modular boundary and component contributions used throughout. Crucially though, the
models created have been scrutinised and verified using instantaneous ultrasonic film
thickness measurements non-invasively from the conjunction.
One of the key findings of the thesis is that the component stiffness profiles have a significant
effect on the dynamics of the piston assembly. The shape of the conjunction at a given
instant, and thus the contact condition, is largely governed by the interaction between the
themo-mechanical distortion of the contiguous solids, as well as changes in lubricant
characteristic responses. The iso-viscous elastic mechanism of lubrication has been identified
as being the dominant mechanism of lubrication.
Funding
EPSRC
History
School
Mechanical, Electrical and Manufacturing Engineering