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Fig. 11. In-cylinder gas temperature and heat transfer losses in free-piston and conventional engines. The fast power stroke expansion of the free-piston engine reduces the time which the gases spend in the high-temperature parts of the cycle, and this possibly reduces in-cylinder heat transfer losses and temperature-dependent emissions formation. Using the simulation model, the predicted heat transfer rate from the cylinder and in-cylinder gas temper- ature was compared for the free-piston engine and for a similar conventional engine. — Figure 11 In-cylinder gas temperature and heat transfer losses in free-piston and conventional engines. The fast power stroke expansion of the free-piston engine reduces the time which the gases spend in the high-temperature parts of the cycle, and this possibly reduces in-cylinder heat transfer losses and temperature-dependent emissions formation. Using the simulation model, the predicted heat transfer rate from the cylinder and in-cylinder gas temper- ature was compared for the free-piston engine and for a similar conventional engine.

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An engine as shown in figure 1 is proposed. The main parts of the engine are a combustion cylinder, a bounce chamber cylinder and a linear electric ma- chine. The engine will have only one moving part, rigidly connecting the two pistons and the translator of the linear alternator. The piston will move freely between its two endpoints, its motion being deter- mined by the instantaneous balance of cylinder gas forces, electric machine force and frictional forces.

The basic performance parameters of the free- piston engine are summarised in table 2. The engine is operating at a fuel-air equivalence ratio of 0.60. The efficiency values presented below refer to the ‘shaft’ fuel efficiency of the combustion engine, and losses in the electric machine are therefore ignored.

Fig. 2. Simulated combustion cylinder and bounce chamber pressure for one engine cycle. (TDC at piston position 0) Simulated engine basic performance.

(a) Simulated piston motion for the free-piston engine compared to a conventional engine running at the same speed. (Piston position shown as distance below TDC.)

(c) Simulated piston speed profile for the free-piston engine compared to a conventional engine running at the same speed.

(d) Simulated piston acceleration profile of the free-piston engine compared to a conventional engine running at the same speed.

(b) Simulated piston motion close to TDC for a free-piston engine and a conventional engine, normalised around TDC position.

Fig. 4. Effects of changes in moving mass on engine perfor- mance.

Fig. 5. Effect of varying nominal compression ratio on engine performance.

Fig. 6. Effects of changing exhaust back pressure on engine operation.

Fig. 7. Effects of varying free-piston engine stroke to bore ratio with constant swept volume and constant equivalence ratio.

Fig. 8. Effect of changing effective compression ratio during engine operation at constant load. The free-piston engine has the potentially valu- able feature of variable stroke. This means that the compression ratio of the engine can be altered dur- ing operation, to optimise engine performance for any operating condition. This has been mentioned by a number of authors, and suggested advantages include increased part load efficiency and multi-fuel operation possibilities.

Fig. 9. Effect of total time for combustion (TTFC) on engine performance predictions. As discussed above, some uncertainty exists re- garding the accuracy of the combustion modelling as some authors have reported an increased fuel burn rate in free-piston engines. Furthermore, one of the claimed advantages of the free-piston engine is its multi-fuel possibilities, and it is therefore worth- while to investigate how fuel burn rate influences engine performance.

Fig. 10. Efficiency of the free-piston engine compared to a conventional engine for varying loads.