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!
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