A tribo-dynamic solution for the flexible piston skirt and liner conjunction
thesisposted on 18.06.2014, 11:43 by Bryn 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.
- Mechanical, Electrical and Manufacturing Engineering