Numerical modelling of the gas dynamics of a prototype free-piston engine

Publication Type:
Thesis
Issue Date:
2011
Metrics:
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Free-piston internal combustion engines found commercial success as air compressors in the 1920's and 1930's, and afterward as gas turbine gasifiers for stationary applications. Since that time they have failed to see commercial application, however in the last decade or so there has been a resurgence of interest in free-piston engines because of their ostensible simplicity and in the flexibility afforded by an unconstrained piston. This thesis reports the testing and modelling on a free-piston engine by Pempek Systems Pty. Ltd. It is an opposed cylinder, electric machine, operating on a two stroke cycle with direct fuel injection. Analysis of experimental cylinder pressure shows that while compression ignition is suitably fast and reliable, the Pempek engine suffers from (among other things) low charging efficiency. The aim of the modelling work is to understand the reasons for this, and to investigate design options for improvement. A comprehensive, generally applicable 1D gas dynamics engine model has been developed. The important features of this model are described in some detail. While the model builds on existing methods, a number of unique contributions have been made. A chemical equilibrium code was developed which is computationally efficient and flexible. The lD gas dynamics method is based on a method developed at Queens University, Belfast (QUB) in the early 1990's but has been thoroughly reworked in the way it handles friction, gas property changes and heat transfer. The originally first order accurate method has been changed to second order, and a way of preserving full mass conservation has been developed. An unsteady heat transfer model is proposed. A comprehensive boundary solution is presented, which has relevance to all 1D gas dynamics models. The gas dynamics model is validated against extensive single shot data from QUB, and also against some experimental engine-run data. The lD gas dynamics engine model is used to assess the viability of utilising exhaust pipe tuning to drive the charging process of the Pempek engine. Simulation results show that it is possible to charge the engine using exhaust gas dynamics alone.
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