Jerry E. Howell Model Project Plans & Kits
Outpost Enterprises, LTD - 695 Godfrey Road - Hollansburg - Ohio 45332 - USA
(World Wide Supplier of Quality Model Project
Plans & Kits Since 1992)
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!
© Copyright 1999 - 2007 Jerry
E. Howell - All rights reserved