Hot-Air Engine Cooling Systems

Of all the hot air engines I have seen in magazines, books, on web sites and at the model engineering shows across the country, very few have adequate cooling systems. Some actually have no cooling system at all!

For those engines, the mass of the engine (heat sink) is the cooling system! Once the engine gets hot it will gradually slow down and come to a stop because the temperature difference between the hot end and the cooler (in the case of stirling engines) becomes too small to sustain operation. In the case of atmospheric engines the cylinder gets so hot that the flame which is drawn into it doesn't cool enough to create enough of a partial vacuum for operation.

That is fine if the designer and builder does not intend the engine to be ran for more than just minutes at a time. Those of us who want our engines to be able to operate for extended periods of time without getting hot need to provide our engines with adequate cooling systems. For sustained operation, all the heat that is applied to operate the engine must be radiated, conducted or convected away to the extent that the engine operates as near ambient temperature as practical.

Before we go on, lets brush up a little on the science of heat so we can better understand what we need to do. Radiation is heat that leaves (emitted from) an object the same way as it leaves the sun and warms our face. A dark colored object radiates heat faster than a light colored object. Dull is better than shiny. If it is a good reflector, it is a bad radiator. A dull black object is the best radiator and a shiny metallic object is the worst. Conduction is how heat moves from one object to another when they are in direct contact with each other. Convection is when a fluid (liquid such as water or a gas such as air) moves past a warm object. This is really conduction but with a moving medium rather than stationary contact. Objects warmer than the ambient temperature will always loose heat in one or more of these three ways.

Another factor is that the hotter an object is above ambient, the faster it will loose heat. If you heat a block of metal with a torch it will only get to a certain temperature and it won't get any hotter. The reason is that the hotter it gets, the faster it looses heat. At some point, it is loosing heat just as fast as the torch can provide it. Of course size, surface configuration, color and other factors will determine the maximum temperature obtained when equilibrium is reached.

Liquid cooling is the most compact and effective but can be messy and requires either a constant new supply of coolant (water faucet) or a method to cool a fixed amount of recirculating coolant. This generally means a water to air radiator. The latter will usually also require a pump to circulate the coolant because thermo-syphons don't work well in small sizes. Effective cooling is easily achieved with a small water jacket around an engine cylinder. The temperature of the coolant and the rate of circulation will be the primary determining factors. It would be wise to use brass or stainless steel for the inner and outer surfaces of the water jacket to prevent corrosion. In passing, keep in mind that water can absorb more heat per unit volume than any other liquid you will have access to.

Air cooling takes up more space on the engine but requires no plumbing or further cooling methods as does liquid cooling. Fins on the cylinder are required to increase the surface area in contact with air because air cannot carry away heat as fast as water can for a given area. Use all the length of the cylinder as possible for fins. A good rule of thumb for fin diameter is that fin diameter should be 2-1/2 times the diameter of the cylinder bore. A fin thickness of around 1/7 of the depth of the fin space works well. Spacing between fins should be 1/16" to 3/32" on engines of from 1/2" to 1" cylinder bore. Air blown across the fins by a fan greatly increases the cooling effect as well as the visual one! A sheet metal shroud to direct the air over the fins adds to the cooling. If the cylinder is fan cooled, the position of the cylinder is of no consequence. Without fan cooling, a horizontal cylinder will tend to cool by convection as the warm air between the fins rises, but a vertical cylinder with conventional fins will not be very effective unless there is horizontal air movement. Air cooled engines also derive some cooling by radiation so blackened fins will be a benefit, especially on non fan cooled engines. The black should be a chemically applied process instead of paint because the thickness of the paint film is a layer of insulation and the net result may actually be less cooling!

The best material for an air cooled cylinder is aluminum (be sure it is sleeved with some other metal if it has a piston in contact, see "Hot-Air Engine Pistons") and brass is next best because they are excellent conductors of heat. We want the fins to conduct heat all along their surface so there is maximum hot surface area in contact with the air. Steel and cast iron are not so good.

A really good argument for providing an adequate cooling system is that the greater the difference in temperature between the hot cap and cooler of a stirling engine and the greater the difference between the flame temperature and the cylinder wall temperature of an atmospheric engine, the more power your engine will produce! In the case of the vacuum engine, the cylinder must operate warm to the touch or water vapor (which definitely is created by burning any hydrocarbon fuel) will condense on the cylinder wall. This will cause a drag on the piston which will more than likely prevent the engine from running.

Other than the tiny Sideshaft Vacuum engine, all of the hot-air engines shown on this site have adequate cooling and they can operate continuously from 9:00 AM to 5:00 PM at the shows, and at closing time they are little more than just luke warm. The Duplex Vacuum Engine which has horizontal fins does operate at a somewhat higher temperature during extended operation than the others if there is not a gentle breeze across the fins. Being a scale model, I didn't want to make changes. The originals probably should have had either vertical fins or a belt driven fan!