ME 200 Stirling Engine Project
What went wrong?
Regardless of our immense efforts, we were not able to make the engine run. Below we discuss some of the problems and potential solutions.
How to improve design?
The diaphragm is too tight: A large amount of force is required to turn the shaft & flywheel configuration. The diaphragm is compressed too much when the shaft turns.
Proposed solution: Currently, even with manually turning the flywheel, the person must apply a large force to get the crankshaft to turn a full rotation. To make it so that the flywheel and crankshaft turn with little to no effort, the diaphragm must be slack. This can be achieved by using a bigger balloon so that it is not as stretched when put on over the lid.
Crankshaft and flywheel are too heavy: Our calculations were made assuming the flywheel & pulley system would be massless. However, the wheel has a large amount of mass concentrated at the center.
Proposed solution: Replacing the crankshaft and flywheel with lighter materials will lower their moment of inertia. Also, designing and building a real flywheel, with its mass concentrated on the outsides to decrease its moment of inertia.
What if we were given more time and money?
To complete this project we were given a time limit of 16 weeks with a $10 budget for the entire semester. With this limited budget, we could only afford cheap or recycled materials. We also had to work quickly and build and test rapidly to make sure we would meet the deadlines. Had we been given more time, we could devote more time to each prototype, and really understand what the problem was before just making a new engine. Also given a larger budget we could have used higher quality materials. The coat hanger we used for the shaft was extremely difficult to bend as it would snap if too much force was applied. With more money, we could have bought a thinner, more malleable wire. We could have also bought many more sizes of balloons to create a more flexible diaphragm.
Potential Solutions to Designs.
More localized heat source: Since the most important part of the engine is getting a large temperature difference, getting the bottom of the can as hot as possible is crucial. Right now, the engine sits on top of a stand that has openings to the surroundings. These openings allow heat from the candle to escape. This waste of heat can be prevented by closing the gaps of the stand. This can be done by making the stand shorter and making the openings smaller. However, we have to be careful not to make it so short that the engine touches the flame and extinguishes it. Also, if the openings to the surroundings are too small, not enough oxygen can be supplied to the candle for it to burn to its maximum potential.​
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Shorter Stroke Length: The net force the engine generates depends on the cycle distance, which is twice the stroke length. If stroke length is shortened, then net force is increased. This could be an alternative solution to reducing the weight of the crankshaft and flywheel, or, for maximum improvement, doing both would greatly increase the force of the engine. Stroke length is determined by the size of the cranks on the crankshaft, so we would simply have to decrease the radius of the cranks.
Most important aspects to consider when making a Stirling Engine.
When building a Stirling Engine there are many important factors to consider, such as heated air flow & friction. When making this engine initially we used a thin aluminum can. This was causing the air that was heated by the candle to travel outside the can and be dissipated to the surroundings instead of traveling through the diaphragm. So for our second design, we used a thicker can. This helped trap more heat inside the can. For the initial can the temperature gradient from the bottom of the can to the top was 1°C while for the second design it was 3°C. Along with changing the can we also checked all the seals to make sure no air or heat was escaping. But the second problem we encountered, was too much friction while turning the shaft and the flywheel. In order to fix this, we tried a different design and using different balloons. Although this trial and error method produced an engine that required less force to turn, it still does not spin freely without a heat source.