Discussion and Conclusions Engine Design Review

Unfortunately, at this time the construction of the engine has not been completed due to the author having a tight deadline and unforeseen delays in the fabrication of the engine and as a result of this it is not possible to comment on its performance. At this stage all of the outer casing and wheeled base is completed, the piston and crankshaft is assembled, the generator and motor mounts fabricated, the displacer shaft and framework is complete, and the polypropylene gaskets and cylinder liners have been cut. Tasks yet to be completed include assembly of the heat exchangers (all parts have been made), finishing the displacer by pouring in the expanding foam that makes up its bulk, welding in the generator and motor mounts and basically bolting everything together.

In most respects the construction of the engine achieved the goals that it set out to — namely to build a large engine using relatively cheap and easy-to-pressurise piping that will operate with an electronically actuated displacer. The motor/driver/controller system was set up and tested, so once construction of the engine is completed it should be a straightforward task to transfer this equipment into the engine.

One area in which a large improvement could be potentially made in the operation of the engine is by reducing the dead space volume associated with the heat exchangers and regenerator. One possible way to reduce dead space volume is to implement a device such as that pictured Figure 90, where the flat face of the displacer is replaced with comb-like fingers that interlock with the heat exchangers to eliminate most of the dead volume inside them. This idea was not used in this prototype due to time and budget constraints hindering the fabrication process. A side benefit of this idea is that the combs (which would ideally be made of something thermally conductive such as aluminium) would take heat from the exchangers when they are interlocked and then transfer that heat to (or from) the working gas when the displacer moves, effectively increasing the heat transfer area. In turn this could lead to reduction in the size of the heat exchangers and further benefits from reduced dead space.

Another way to improve performance would be to increase the size of the piston. This would mean a larger displacer chamber could be used while retaining the same compression ratios. This could be achieved by simply reducing the size of the displacer wedge, from 120° down to virtually nothing so that essentially just a ‘flap’ with a sliding seal would become the displacer. This would also reduce the weight of the displacer, allowing it to be actuated faster by the motors (although it would have further to travel).

Discussion and Conclusions Engine Design Review

Figure 90: Alternate displacer design with ‘combs’ to reduce dead space in heat exchangers

A significant improvement that could be made to the prototype engine would be the ability to adjust phase difference purely in the software of the control system. This would speed up the testing procedure considerably as it would eliminate the need to de-pressurise and partially disassemble the engine in between tests to make changes. This had been the initial plan however the capabilities of the stepper motor controller did not allow for this. Given more time the controller would have ideally been replaced by a PLC or laptop running more customisable software and allowing for easy changes in parameters such as phase difference. The phase difference would be calculated as a time delay between receiving the signal from the crankshaft-mounted opto-sensor, and starting the motion of the displacer. This would have to account for the engine rotation speed, meaning that rotary encoder feedback would be necessary. If these improvements were to be made it would also make possible the use of a closed-loop control system, resulting in greater precision in displacer motion.

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