Cooling Fins sizes Air-Cooled Gas Engines

[BronxFigs]

Is there a simple relationship between bore diameter and cooling-fin diameter?

If I want to air-cool a cylinder with 1.500" bore X 3.00" stroke, how large a diameter should the cooling fins be, how many, and made from what materials?

Surely, cooling fins made from Aluminum will radiate heat faster than cast iron/steel cooling fins, yet, the old, air-cooled stationary engines, and compressors used cast-iron cooling fins.

I need a simple formula/ratio to correctly size the cooling-fin diameter for a given bore of "X" diameter.

Thanks for the help.


Frank

 

[Jasonb]

You will also want to figure in RPM, a slow runner will generate less heat than a fast one.

Type of engine a hit and miss will be sucking in cold air on the miss stroke to cool the engine, throttle governed fires all the time.

Static or moving engine, one that will get a flow of air over it is not going to need such large fins, also could have a crankshaft driven fan, many compressors have a fan blade in the pully

Fuel type as they burn at different temperatures

 

[Brian Rupnow]

Frank---I don't think there is a set method of calculating this. Basically, the more cooling fins the better, but if you get the fins too close together, air won't be able to circulate between the fins, defeating the purpose. Cast iron engines had fewer fins because of the difficulty in casting thin fins--as a consequence, the fins were thicker and the space between the fins was about equal to the fin thickness, perhaps a little greater. The most efficient cooling fins are part of the parent material of the cylinder itself. That way no efficiency is lost as the heat of the cylinder has to transfer from the cylinder material across to the different material which the fins are made of. Nowadays, many people use a thin cast iron liner, fitted with an aluminum outer barrel into which the fins are cut. This does seem to work, and the tighter the fit between the aluminum barrel and the cast iron liner, the more efficient they are in terms of thermal transfer. In a situation like this, the width and depth of the spaces between the fins is only limited by how bold you are with your cut-off tool which will be used to cut the slots between the fins. There are a certain amount of aesthetics involved also. Probably for a cylinder the size you are contemplating, a slot width and fin width of .125" would not look out of place. To cut these fins any greater than 5/8" deep presents a huge problem, because the aluminum will want to grab the cutter and make it deflect sideways or break off, ruining the fin on either side. Use flood coolant and hold your breath, and say a short prayer with every groove you cut.---Brian

 

[BronxFigs]

Many thanks.

Once again, my mentors have gotten my head straight, and pointed out many of the problems that need to be addressed when trying to properly cool the cylinder(s) on a gas engine.

If the truth be told, I had thought about machining an Aluminum outer barrel, onto which would have a series of tightly fitted, Aluminum cooling fins (discs) and spacers stacked onto it. This built-up, construction, would hopefully eliminate the need of taking the deep plunging cuts that would have to be done with a cut-off tool, when trying to machine cooling-fins. I'm getting too old for this level of anxiety, and stacking seems easier than plunge-cutting. After this cooling-fin assembly is fabricated, I was going to tightly press the whole shebang onto the iron/steel cylinder. The I.D. of the cooling-fin assembly would be machined to match the O.D of the cylinder, using a press fit/shrink fit.

Whether this idea has any merit is debatable. I must admit that I cannot seem to think things through without the input by others. I like to consider this forum as a substitute for classroom in a Technical School, and all the experienced members/engine builders, as my teachers.

I always welcome your comments and suggestions, and thank you for taking the time to consider, and offer answers to my naive questions.


Frank

 

[vederstein]

Other ways to cut fins (if you have the equipment):

1. Set up your part on a mill (a horizontal mill would be better than a vertical) chucked into a rotary table. Setup a slitting saw on the arbor. Rotate the part around the slitting saw.

2. Set up your part on a rotary table in a bandsaw or power hacksaw. Set the saw with a stop to prevent cutting the part in half. Rotate the part with the rotary table as the bandsaw cuts. (I may be wrong, but I've heard somewhere this was one of the methods used during WWII to cut cooling fins in the aircraft radial engine cylinders. Once set up, an idiot can run the machine).

...Ved.

 

[Longboy]

Frank, make your radiator in one piece. This stacking of fin/spacer on a 3 inch stroke engine gonna drive you nuts. An aluminum round 2.25in. with a narrow cyl. liner for a 1.5 inch bore will give you enough cooling and you can go with many narrow or fewer broader finning. Have at it on the lathe with a cut off tool bit.

 

[dave-in-england]

Personally, I don't see any point at all with putting cooling fins on a stationary MODEL engine.
A nice smooth cylinder will look just as good.

The problem with horizontal fins is that they actually trap hot air between them, in stagnant zones, there is no smooth path for the rising hot air to escape along.

The only way to clear the hot air from between the fins is to have a fan on the crankshaft and air guide ducting around the cylinder, like those engines on Lambretta scooters.
But then that spoils the view of the engine.

The model engine will not be producing Horse-Power output in anger, but just ticking over, using very little input energy, and it will not be running for hours at a time.

The heat in the hottest part of the cylinder will soak away down through all the connecting parts of the engine, so it will take a long time to become really hot.