The flywheel serves as more than indication the engine is working. It acts as something of an energy storage device. A well-balanced flywheel takes the energy created during the power stroke of the motor and stores it. When energy is needed to push the displacer down the flywheel provides its stored energy to overcome friction and other forces. Without a good flywheel the displacer would simply rise to the top of the chamber and stay there.
Larsen said having a well-balanced flywheel is key to efficiency. If the wheel is not balanced the engine has to work harder to move it. "You don't want the engine to do more work than it has to," he said.
The displacer in a hot air engine serves to displace the air within the pressure chamber. Remember, the engine cannot run without the thermodynamic cycle where is air is heated and cooled, causing expansion and contraction. If the pressure chamber was simply heated, without anything within it to displace the air, the air inside would heat up and expand, but never contract.
With the heat source at the bottom, the hot air engine also uses cooling at the top, usually ice or cold water, to cool the air. As the air is heated it expands moving the displacer to the near the top of the pressure chamber. At the top of the chamber the air is cooled, contracts, and moves the displacer down. This all happens with the aid of the drive mechanism, crankshaft and flywheel.
The displacer is most commonly a rolled piece of steel wool with a light wire running through the center. Remember when Larsen spoke about needing to think like a watch maker? This is one of those times. The displacer needs to be able to slide freely within the pressure chamber, while at the same time filling most of it. It needs to allow for the free flow of air, while restricting some of the flow. The idea is to minimize friction and maximize effectiveness. This theme is a constant throughout the construction of the engine.
The heat box is simply a stand the engine sits on. The heat source is placed below the engine.
This seems like a lot of work for not a lot of return. But there is a tangible feeling when you finish the engine, troubleshoot the works to get it going, and see it puffing along on its own. For Larsen, his fascination began more than half a decade ago, while yours could begin in as little as a few days from now.