Cooling Fins vs. Water Jackets

[BronxFigs]

Just a general question:

If a model engine is designed for using air-cooling fins, or, for a water jacket, can either method be used?

I have noticed many old, Hit and Miss engine designs using water jackets/tanks mounted to the cylinders. Many small, air-cooled vertical, 4-stroke engines use air cooling, but can also sometimes be built using a water jacket. So, does it matter how the cylinder is cooled off? Is water more efficient in carrying away extra heat? Are actual fins necessary, or can a hunk of Aluminum, profiled to the called for dimensions, be mounted onto a cylinder without the fins being machined into it? I would imagine that fins add surface area for better cooling efficiency, but are they necessary. How about just increasing the size of the "heat-sink", and forget about machining plunge-cutting/sawing cooling fins?

Let's take for example, the popular Nemett-"LYNX". The drawings call for some cooling fins on the cylinders. Why? Can I just machine a donut-shaped piece of aluminum, having the same size, and shape as the original set-up, but without the fins?

Just wondering....


Frank

 

[Jasonb]

Alot will depend on how hard and how long you want to run the engine for. If you are just going to have it lightly loaded and ticking over with the odd blip of the throttle it won't get anywhere near as hot as if its hooked upto a large prop pulling an aircraft behind it and inside a confined engine cowel.

A lot of the hopper cooled hit and miss engines can be run for 15-20mins witout putting water in the hopper if they are just ticking over and firing every 6-8 cycles, but if you make them run say a dynamo or saw bendch they will start to fire more often so more heat is generated

This is a good example of how hot a Lynx can get if you keep it at full throttle

[ame]http://www.youtube.com/watch?v=wTq0ogyTFEc[/ame]

 

[n4zou]

A big hunk of aluminium will collect heat but there is little surface area available to dissipate it to the air surrounding it. Cutting cooling fins provides a larger surface area in that big hunk of aluminium which increases the surface. This allows more heat to flow into the air surrounding the cylinder and head. Adding a fan so hot air in the fins is blown away increases heat transfer. Cooling fins give an engine a classic look and it makes operation of the engine simple. One problem with air cooling with fins is the inability to control the temperature of the engine. In cold weather it takes longer for the engine to warm up and in very hot weather you may have inadequate cooling of the engine. The next step up is a hopper cooled engine. A hopper cooled engine with an adequate volume of water surrounding the cylinder with water passages in the head provides the easiest way to regulate the cooling of the engine. A pump is not required and temperature of the engine is regulated at 212 degrees F or 100 degrees C. This happens automatically because the energy required to heat water above the boiling point is almost infinite as the water vapour carries most of the excess heat as it boils away. All that is required is the addition of water in the hopper as it boils away. This is the best cooling system for a model engine that is going to operate for long periods of time but it loses the classic look of cooling fins. The most complicated systems use thermostats to regulate temperature and require the use of pumps, radiators or cooling tower, and possibly a fan to provide air flow through the radiator or cooling tower. This system adds complication but can add display value as the cooling equipment adds visual value even better than cooling fins on a 'classic' air cooled model.

 

[Lew Hartswick]

Also note that all the fins in the world won't do "much" good without a good stream of
air flowing over them. (as in an aircraft installation)
...lew...

 

[BronxFigs]

Thanks to all who responded. Now I know the answers to my questions.

It all seems so logical when it's explained in the way that your information was presented. I had not considered any loading variables when operating the engine...since, they are models. But like I just said...now I know.

Appreciate the thoughts and explanations.


Frank