PRME Engines User Community

[pe reivers]

In an internal combustion engine, roughly 1/3rd of the energy in the fuel is available as power, 1/3rd goes out of the exhaust system, and 1/3rd must be carried off by cooling.
The math for the amount of cooling needed, for simplicity, is reduced to a 1000 Watt engine, and assuming that the amount of energy that needs to be removed by cooling is the same as the energy delivered to the crankshaft. (all values are quite rounded, and air expansion is not accounted for)
Data:
specific mass of air is 1.29 grams/liter
specific heat of air = 0.237 cal = 0.99joules (common set to 1 joule/grK )
temperature difference over the fins is assumed 100C (from 20 to 120 C)
Watts=dT*mass/sec*specific heat of cooling medium.
mass per second needed is: 1000/(100*1) = ~ 10 grams/second.
10 grams of air is ~8.4 liter (about 2 gallons each second! , with no air wasted in bypassing the fins)
For an engine running at 2000 watts mean power (MVVS 45cc, DA50) that would be about 17 liter/second (somewhere between 3 and 4 gallons of air per second) that is needed for cooling.

Now that is known how much air is needed, it might be good to look again at the design of the engine cowl. The simple rule, that the air outlet should be at least twice as large as the air inlet is not good enough, especially for gas engines. Glow engines tend to get away with itbecause they have so much additional internal cooling by the methanol fuel.
Only two things are realy needed:

1] The air should pass throug the engine cooling fins and not take the easy route around the engine with less resistance. In order to achieve this, one has to use:
1.1) engine baffles
1.2) jackets to guide and force the air through the fins.

2] To achieve an air flow, there must be a pressure difference.
To set up a pressure difference a high pressure is needed in front of the baffle, and a low pressure after the baffle.
2.1) High pressure: The centre of the prop is a very lousy fan design, and all it does is stir up the air. This turbulent air is however good for cooling the front of the engine. If the plane has a decent forward speed, the incoming air will provide a pressure high enough. At low speeds and in long verticals, this speed is very low, and thus the high pressure will be absent.
2.2) Low pressure. This can be achieved by carefully looking at the flow around the cowl and making use of the high speed prop blast that hugs the fuselage.

In the following post I will try to explain how to achieve this.
But until then, take a look at what BME fliers reported here: http://www.geistware.com/rcmodeling/articles/baffles.htm
_________________
PRME - Pe Reivers Model Engines
Dealer for MVVS, Mejzlik, MTW
http://www.mvvs.nl
http://www.prme.nl
http://www.mvvs-nl.com

[louis.louis]

Please do as I am eager to learn.

[pe reivers]

It took some while, but here are the drawings:













For radial cowls it looks like this:
If the cowl rear edge flares to a slightly larger diameter than the fuselage, it will greatly enhance the cooling. Black is the basic baffle. The added purple jacket is the ultimate in cooling.
A good example can be seen here: http://www.mini-iac.se/forum/topic.asp?TOPIC_ID=277

_________________
PRME - Pe Reivers Model Engines
Dealer for MVVS, Mejzlik, MTW
http://www.mvvs.nl
http://www.prme.nl
http://www.mvvs-nl.com

[pe reivers]

Peter Folmer, who at first sent me in an engine with severe overheating symtoms, gave me permission to publish his data and experience, so others might benefit.
Here goes:
Repair engine, run in "normal glow engine fashion with cowled engine" in a 3D airplane.
The cylinder had turned all shades of yellow. purple, blue and gray, indicating engine temperatures in excess of 300°C. Piston and cylinder were OK, but crankshaft, conrod and all bearings needed to be replaced.
The cylinder head anodize had lost a few shades of hue (color had bleached) due to the heat.

After repairs, he installed a Venom temperature gage, and these are the results:

No cowl:
prop 18x6, rpm 9000 static, max temp. 132°C (static and 3D)
prop 16x12, 8000 rpm static, max temp 118°C (Aerobatics)

Peter then prepared baffling and a cylinder jacket made from aluminium air ducts. For air extraction from the cowl, louvres were made, and they proved very effective.
In flight: max temperature of 95°C (outside air was at 10°C, so quite cool)
ground running: max temp 100°C
Ground running, changing from 1/2 to 1/1 throttle vice versa, the temperature rose to 85 degrees. when the engine returned to idle, the temp. dropped very quickly to 69°C
For Images of his setup, see next post (soon)
_________________
PRME - Pe Reivers Model Engines
Dealer for MVVS, Mejzlik, MTW
http://www.mvvs.nl
http://www.prme.nl
http://www.mvvs-nl.com

[pe reivers]

Pictures of the cooling baffles and jackets made from aluminium ducting tubes. Nice job you did Peter!








_________________
PRME - Pe Reivers Model Engines
Dealer for MVVS, Mejzlik, MTW
http://www.mvvs.nl
http://www.prme.nl
http://www.mvvs-nl.com