Numerical modelling of the gas dynamics of a prototype free-piston engine
- Publication Type:
- Thesis
- Issue Date:
- 2011
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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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