Here is an article I wrote for EAA Sport Av mag but they don't act
like they are going to publish it. I will upload this to the
ACRE NL web site soon.
HOW TO COOL YOUR WANKEL copyright by Paul Lamar
Review of basic principals and history.
Heat balance
HP is a force times a velocity, Or put another way, force times
a distance traveled per second. Watts (heat per unit time) and
HP are interchangeable (746 watts equals one HP). It takes HP at
the power plant to generate the electricity that is used to light
your 100 watt light bulbs.
The critical thing when designing a cooling system is the amount
of continuous HP the engine is called on to develop.
A cooling system that works for a light duty car application
might not work for the same 200 HP engine when used as an aircraft
engine. Cars generally average around 40 HP continuously and
are occasional required to climb a long steep hill where
the HP required might climb to 100 HP. The same engine used
in an aircraft might be expected to generate 200 HP max power
for as long as it takes to climb to cruising altitude.
Then it may be called on to generate 150 HP continuously.
All engines have a heat balance. Heat balance is a list of the
portions of the total energy in the fuel that is distributed to
varies purposes. The intrinsic efficiency of the engine
determines what fraction of the heat must be rejected. The heat
rejected is the sum of what goes out the exhaust pipe and what
goes out the cooling surfaces.
A Rolls Royce Merlin used in the P51 has a heat balance of:
Mechanical HP out 29.6%
Waste cooling heat 18.9%
Waste exhaust heat 51.6%
One of the most efficient aircraft engines ever designed was the
liquid cooled O-200 Continental aircraft engine as used in Dick
Rutan and Jeana Yeager's around the world non stop and un-
refueled Rutan Voyager. Continental claimed this engine had a
heat balance of:
mechanical HP out 35%
waste cooling heat 21%
waste exhaust heat 44%
For the early 1981 Mazda 12A rotary engine it was
about:
Mechanical HP out 28%
Waste cooling heat 24%
Waste exhaust heat 48%
This differs little from any other intermittent internal
combustion engine but small improvements continue to be made in
the Mazda wankel rotary.
The large amount of energy 'thrown away' as waste heat is an
inescapable result of the laws of thermodynamics. Engineers try
to keep a high combustion temperature for maximum overall
efficiency and a high coolant temperature (by minimizing the
heat rejected to the cooling system), while not melting or
softening the metal the engine is made of!
There is no cooling but air cooling. Ultimately all waste heat
must be dissipated to the air surrounding the aircraft.
There are three main ways of transferring heat. Conduction,
convection and radiation. Conduction takes place between solids
or within the same solid while convection happens between a
solid and a fluid such as air or water. Radiation takes place
by one object "seeing" another hotter object. No not nuclear
radiation. You can be warmed by the sun no matter how cold it
is out side. You will not be warmed if you are in the shade.
This is a classic example of heat transfer by radiation. We are
mainly concerned with conduction and convection when cooling an
engine.
This is a bit subtle and concentrated so it will take some
cogitation and re-reading for this to sink in.
The HP per cubic inch of displacement and the geometry of the
combustion chamber combine to determine how much heat must be
rejected per unit time through the walls of the combustion
chamber. The higher the HP per cubic inch the more severe the
cooling problem. If your engine has 20 square inches of
combustion chamber surface to transmit 100 HP worth of waste
heat that is easier to do than transferring 200 HP worth of
waste heat with the same 20 square inches. This is the very crux
of the situation in getting the heat out of the metal and into the
coolant when dealing with high continuous HP per cubic inch
engines.
Aluminum combustion chambers melt at a lower temperature than
cast iron of course but they conduct heat better than cast iron.
Most really heavy duty piston engines use cast iron and or steel
combustion chambers, heads and cylinders. As you can see there
is a severe compromise in the power to weight ratio of any heavy
duty piston ICE. It is also one of the developmental
differences between a heavy duty truck engine, aircraft engine
and a light duty car engine. A heavy duty over the road tractor
trailer turbo diesel engine can weigh 2000 pounds while
generating only 350 HP. The life is expected to be up to 500,000
miles. A 350 HP air cooled aircraft engine has a similar duty
cycle and life but it weighs only about 500 pounds with a
remarkable power to weight ratio. That is one reason why they
are so expensive.
Air cooled aircraft engines have relatively low HP per cubic
inch partly for this reason. In an air-cooled engine waste heat
is transferred by the fins on head and cylinders directly to the
air. This is done first by conduction and then by convection.
The rate of heat transfer is proportional to the difference in
temperature between the fins and the surrounding air. This
difference, initially, is about 400 degrees. The metal surface
surrounding the combustion chamber is 500 degrees and the
ambient air temperature surrounding the fins is about 100
degrees F. If the engine gets hotter the rate of heat transfer
increases. In a sense this is a stable cooling system. The fin
density or number of fins per inch of head or cylinder surface
must be such that the rate of heat transfer to the air keeps the
hottest part of the combustion chamber below the melting
or softening temperature of the metal. Usually aluminum.
Temperature gradients in the metal are very important. A
temperature gradient is defined here as a large change in
temperature from one part of a solid to another. The air cooled
engine by its very nature incorporates high temperature
gradients. High temperature gradients lead to cracking as part
of the solid wants to expand and this expansion is resisted by
the cooler parts.
Pistons, of necessity are almost always made of light weight
aluminum by the fact that they are starting up and stopping
twice per engine revolution. Consequently they must be cooled by
oil or they too will soften, fail from over stress and may melt.
Heavy duty aluminum pistons sometimes have steel or cast iron
inserts in the high stress, high temperature ring land area.
The current air cooled aircraft piston engine's power to weight
ratio will be very very difficult to improve upon based on its
hundred year detail development. No helpful material
breakthroughs have occurred in the last 70 years. However
strict attention must be paid to the cylinder head temperature
of an aircraft engine or failure will occur.
In a water cooled engine waste heat is first transferred to the
surface of the coolant jacket by conduction and then to a
coolant by convection. It is stored in the coolant and moved to
a heat exchanger where it is then transferred to the air by
convection. In a water cooled engine the water temperature is
limited to about 250 degrees due to boiling. The metal
temperature can be 500 degrees so the difference is only about
250 degrees initially and the rate of heat transfer is lower. If
the engine metal gets hotter the rate of heat transfer will at
first be increased by a process called nucleate boiling.
Nucleate boiling is a benign form of small scale boiling. Tiny
bubbles of vapor are formed and they work their way to a vapor
pocket at the very top of the cooling system by buoyancy.
Bubbles rise. If they cannot make their way to the top of the
cooling system steam pockets may occur which is very bad as the
metal cooling in that area is greatly reduced.
If the temperature of the bulk coolant goes beyond the boiling
temperature overall boiling occurs of course. All hell breaks
loose and the coolant is forced out of the water jacket through
the pressure cap overflow valve by massive amounts of hot vapor
or steam. Once boiling of the coolant starts it is all over in
an instant. Less coolant, hotter metal, more steam. One might
say the cooling system pops like a pop corn. Thus the cooling
system becomes very unstable indeed. Something we all have
probably experienced in our cars. This is a major difference
between the air cooled engine and the liquid cooled engine.
Needless to say this cannot be allowed to happen in an aircraft
engine.
A few bad experiences like this during climb out can be very
discouraging indeed. Forget all you have learned about dealing
with car overheating. You cannot just quickly reduce power and
pull over to the side of the road and stop. It is best to test
your new water cooled airplane at low use uncontrolled airports
so you can get it back on the ground right now! Otherwise you
may damage your engine.
Furthermore the internal shape of the water jacket around the
combustion chamber must be such that no local severe boiling
takes place at the maximum continuous power the engine is likely
to generate. This is regardless of the bulk temperature of the
coolant. If this rule is violated parts of the combustion
chamber may melt or crack. This is the limiting factor on how
much continuous HP you can get out of any liquid cooled ICE.
Many people have given up on water cooled aircraft engines at
this point. It takes an extra amount of perseverance and
determination to continue.
Having said all that and you are going forward anyway it is best
to suppress your creative urges and copy a system that someone
else has used and that you know works. Later on, after you have
flown your airplane for awhile, you can do a little
experimenting.
A Mazda wankel rotary engine is indeed a small displacement
engine as it displace only about 75 cubic inches while
generating 200 HP therefore its cooling requirements are severe.
Unfortunately it is liquid cooled as it is very difficult to
design a multi-rotor air cooled wankel because of the geometry.
Unlike a piston engine however the aluminum combustion chamber
or rotor housing of a Mazda wankel engine is lined with steel.
The steel is chrome plated which of course is shiny and reduces
heat transfer by radiation from the white hot combustion gases.
Metal temperature measured near this surface are much the same
as temperatures in piston engine combustion chambers. About 500
degrees F. The rotor is cast iron and nearly impossible to melt.
(See the side bar the Rotor Verses the Piston.)
In a Mazda wankel rotary engine one third of the cooling
requirements is oil and two thirds is water or glycol/water mix.
This differs from most piston engines. The piston must be cooled
by the oil of course but the piston makes up a smaller portion
of the total metal surface exposed to the hot combustion gases.
The rotor face makes up roughly half of the total surface area
in contact with the hot combustion gases during the entire
expansion cycle. A piston top is a decreasing proportion of the
total area exposed to the hot gases as the piston moves down the
bore. Oil cooling in a wankel rotary is therefore very very
important.
Pistons, of necessity are almost always made of light weight
aluminum for reasons mentioned above. The wankel rotary engine
on the other hand can and does use a heavy cast iron, impossible
to melt rotor as the rotor never stops and starts up in the
opposite direction. It moves in a perfect circle at all times.
In a typical water cooled automotive type engine the water
jacket inside the cylinder head is inaccessible. You can not
open up the head and make modifications and improvements to the
water jacket. Fortunately this is not the case with a Mazda
wankel engine. The rotor housing is a separate part and when the
engine is un-assembled the interior part of the rotor housing
water jacket is readily accessible for modifications. The
development of the exact shape of the internal water jacket
around the wankel combustion chamber is still in a state of
change after only 40 years of development. Some turbo charged
Mazda rotary engines used in drag racing cars burning alcohol
and nitrous oxide are generating up to 400 HP per rotor or 800
HP for a two rotor engine. The four rotor Mazda engine that won
the 1991 Le Man 24 hour road race out right generated about 172
HP per rotor for a total of 690 HP. The engine was in excellent
to new condition after the race. Racing Beat in Anaheim CA has
about 200 hours on the dyno with a turbo three rotor Mazda
engine generating 900 HP on 100LL aviation fuel. That of course
is 300 HP per rotor. This engine is intended for use in an
aircraft.
Several modifications have been tried such as grooving the walls
of the water jacket to improve the heat transfer from the metal
to the liquid coolant. We have discovered that this is
unnecessary if you limit your engine to around 200 HP for take
off and cruise at 175 HP. However this feature may come in handy
in the future when we get more experience with higher HP turbo
charged Mazda two and three rotor engines in aircraft use.
We do know the two rotor Mazda engine will currently generate
about 200 HP continuously with out melting or cracking for a
bare engine weight of 235 pounds with PSRU for an excellent
power to weight ratio. Tracy Crook near Bell Florida has over
1000 hours in his Mazda rotary powered RV4 with no metal
cracking or housing warping. Ken Welter near Portland Oregon has
run hundreds of pounds of nitrous oxide through his Mazda wankel
engine increasing the power by 100 HP and helping him get his
2900 pound modified Coot amphibian on the step and off the
water.
Once the heat is in the coolant it must be transferred to the
air by the heat exchanger (unfortunately called a radiator) by
convection. In this case the heat exchanger is 250 degrees F and
the air is 100 degrees F or a difference of only 150 degrees so
the rate of heat transfer is also unfortunately still lower.
Small differences in ambient air temperature make relatively
large differences in the cooling of liquid cooled aircraft
engines. This is much more pronounced and noticeable than it
would be with air cooled aircraft engines. Air cooled engines
historically work better in the desert.
Here is where most builders have difficulty. It is not easy to
design a low aerodynamic drag aircraft liquid cooling system.
The cooling drag can be as much as 30% of the overall drag on
the airplane. The scoops and ducts must be designed properly.
The job of the duct is to slow the air velocity and to raise the
pressure to the point that it optimizes radiator performance.
Many lessons were learned in world War I & II on this subject.
Some important improvements did not have time to be applied
during the war so some lessons can not be learned by studying WW
II aircraft. The P51 seems to be the ultimate liquid cooled
aircraft of WW II yet its duct system has been improved upon by
some work published after the war. There is an out of print
book written by Kuchemann and Weber published in 1953 called The
Aerodynamics of Propulsion wherein much of this has been
presented. Used copies are still available on the Internet.
Prior to this work most duct designers thought it necessary to
design the duct so there was a gradual transition from scoop
opening to heat exchanger. See figure x. This is not the case
and a more bell shaped transition is called for such as figure
y. The coordinates of such as duct are given in table a.
Another improvement has been made in heat exchangers themselves.
It has been found that the thinner the heat exchanger is the
better it works given a constant volume. In other words a heat
exchanger that is twenty inches by twenty inches by one inch
thick works better than a heat exchanger that is ten inches by
ten inches by four inches thick. Both heat exchangers have the
same four hundred cubic inches of volume. The rule of thumb is
the first half of the exchanger does three quarters of the work.
This is because as the cooling air passes through the heat
exchanger it warms up and the difference between the air
temperature and the metal at the back of the heat exchanger is
reduced and so too is the the rate of heat transfer. Another
advantage of the thin heat exchanger is the air pressure
required to push air through the exchangers is lower as the
exchanger gets thinner and thinner. This pressure is an out
growth of the dynamic pressure caused by the aircraft moving
through the air at high speed. All modern cars use thin, large
area heat exchangers mounted perpendicular to the air flowing
through the grill as they need to cool while climbing hills at
only sixty or so MPH.
Unfortunately a large area thin heat exchanger is difficult to
install perpendicular to the air stream in an aircraft as it
increases the aircraft's frontal area. Let me define the term
frontal area so there is no mis-understanding. Technically
frontal area is the maximum cross sectional area of the aircraft
perpendicular to the air stream regardless of where along its
length this occurs. It could occur half way back. Some people
mis construe this term to mean the shape of the front of the
airplane.
Thick heat exchangers were successfully used in fast airplanes
during WW II as the dynamic pressure available is proportional
to the square of the air speed. At two hundred MPH the dynamic
pressure is four times what it is at one hundred miles per hour.
Most home built airplanes climb out at about one hundred MPH so a
thin heat exchanger is called for. A P51's heat exchangers were
a total of eleven inches thick and at four hundred MPH the
dynamic pressure was sixteen times higher than it is at one
hundred MPH. Never the less P51 and other liquid cooled WW II
airplanes had problems cooling while taxing for takeoff.
Obviously a thin heat exchanger is called for in home built
aircraft. The problem is how do we fit them in? The solution was
mentioned in Kuchemann & Weber book Aerodynamics of Propulsion
and another book still in print published after WW II by Kays &
London simply called Compact Heat Exchangers.
The heat exchanger is installed at an oblique angle to the
center line of the aircraft as shown in figure z. This was
referred to by K & M as an oblique cooling block. Unfortunately
it is difficult to apply the bell shaped duct principle to such
an arrangement so a modified shape was developed by K & M and
Kays & London shown in figure r. An approximation to this shape
that works fairly well is a simple wedge shaped duct as shown in
figure s. The distribution of the air flow through the heat
exchanger can be checked with a test rig. This consist of a
blower, a heat exchanger and a prototype duct. The air velocity
distribution coming out of the back side of the heat exchanger
can be measured by a hand held digital wind speed meter.
Why not place the thin rad in the back of the fuselage
perpendicular to the air stream similar to the P51 arrangement?
This will work of course but the frontal area of the the
aircraft will be increased by the P51 type scoop required. A
flush duct such as a NACA duct is not recommended for feeding
air to a flow restriction such as found in a heat exchanger. The
NACA duct will fail to work under those circumstances. The long
plumbing lines required might tax the capabilities of the water
pump. Not to mention the extra weight of coolant required to
fill the necessary long and large diameter coolant lines.
It is better to fit obliquely mounted heat exchangers under the
cowl. This is facilitated by the small size of the Mazda wankel
engine compared to a air cooled aircraft piston engine. The two
rotor engine is twelve inches wide by 14 inches high by eighteen
inches long not including the PSRU. This is much smaller than
any piston engine of comparable HP.
It is also very important to make sure the air going through the
exchanger is a cool as possible so it is is unwise to pass the
air over the hot engine and then through the heat exchanger. It
is better to duct the cool air directly to the heat exchanger
and then let it flow over the engine and out the exit.
When the air arrives at the heat exchanger it should be slowed
considerably and the pressure should be about 80% of dynamic
pressure if the duct and scoop were designed properly. As the
air passes through the heat exchanger the pressure will be
reduced still further and so too the air velocity. The air
temperature will increase up to about 10 degrees F above ambient
air if the heat exchanger is designed properly. The air will
then go on to cool parts of the engine before exiting through
the cowl outlet openings. As the air passes through these
relatively small openings it must speed back up to near or above
the slipstream velocity and directed to the rear of the aircraft
to recover as much energy from the cooling system as possible.
This energy recovery phase reduces the over all cooling drag.
If an opening is at the bottom or edge of the firewall a large
radius must be used on the firewall just before the opening in
the cowl. This minimizes the energy loss of the high speed
cooling air exiting the cowling. This has been well known for at
least 70 years.
Cowl Flaps
Cowl flaps must be used as the dynamic pressure varies widely
over the speed range of an airplane. At 100 MPH it is 26 pounds
per square foot and at 200 MPH it is over 100 pounds per square
foot. The most severe cooling requirements are during a max
angle max HP climb out at about 100 MPH. At 200 MPH the dynamic
pressure is four times higher than it is at 100 MPH and so too
is the potential volume of air available to cool the engine.
Therefore the cowl flaps are nearly closed increasing the
pressure in the entire cooling system and diverting the air flow
around the scoop and reducing the internal air flow to roughly
what was needed at 100 MPH. This greatly reduces the cooling
system drag. Probably worth about five or ten MPH additional
speed around 200 MPH. More as aircraft speed increases.
In a car a thermostat is used to control the engine temperature.
This is a bad idea in an aircraft engine as it is better to use
the cowl flaps for a reduction in drag as stated above.