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Let's keep this simple...we need to cool engines because they create
heat during the combustion process. The more heat generated, the more
difficult it becomes to get rid of that heat. As you raise compression
ratios and run the engine at higher rpm's, at some point with an
air-cooled engine, you just can't make the cooling fins any larger,
can't fit any more on, and you can't blow enough air over those fins to
get rid of the heat faster than it is being produced. This is when the
dreaded "piston seizure" usually takes place. While most people believe that the welding of the piston to the cylinder wall was caused by excessive expansion of the piston (this too can happen), but it is usually the failure of the lubricant to keep the piston and rings separated from the cylinder. As the oil gets hotter it gets thinner, at some elevated temperature it stops being a lubricant...the lubricating film breaks down and the destructive metal to metal contact begins. Almost all air cooled engines have a considerable cooling safety factor built in. The designer of an air cooled engine makes allowances for long up-hill runs, hot days and even provides for some out-of-tune conditions. There is no reason to be wary of air cooled engines...unless you start doing things to your engine that start cutting into this safety margin. The makers of bolt-on performance equipment rely on this safety factor. Because, when their products squeeze more horsepower from your engine, they also generate a lot more heat. What some of these go-fast products do is take away any over-heat safety margin. If we fully understand what we might be getting into, OK... but the swami has never seen an advertisement for horsepower increasing products with the notification: "this equipment will probably shorten the life of your engine and also might lead to catastrophic failure, and by the way, good luck with your engine warranty" So when you see an air-cooled scooter engine putting out surprising horsepower, you are looking at an engine living a risky life in the fast lane. The swami can report from experience that the romance of souped up engines fades in direct proportion to the distance you are from civilization when the seizure occurs. But the swami digresses...what about the water? At the very early stages of engine design, the engineer decides how much power he wants from a given displacement. He can play it safe, keep the compression ratio, (and a lot of other parameters) conservative, generate less heat and comfortably cool with air...or...make the decision to squeeze some more horsepower out of the engine. At some point they just can't put enough cooling fins on the engine and it's time to go to another way to move the heat away from the cylinder ... water. Water is a whole lot better at conducting heat than air. Have you ever seen a blacksmith drop a red-hot bar into a tub of water to cool it off quickly? Did you ever see him wave it around in the air to cool it off quickly? NO! The water does a much better job of conducting the heat. DON'T TRY THIS AT HOME! JUST IMAGINE THIS AS THE SWAMI EXPLAINS IT. A pizza maker will put his entire arm into a pizza oven with air inside that is around 450 degrees F. He does not get burned immediately because the air is such a poor conductor of heat, it would take time to begin to cook his arm. Now, what would happen if he put his arm into water that is boiling (which is only 212 degrees F.) The results would be immediate and terrible. Yes, water is a better conductor of heat than air. So, the engine designer loses the fins and puts a watertight jacket around the head and cylinder to contain the water that will carry away the heat. The water is circulated through tubes in a loop from the cylinder to a radiator, where the water is cooled by... AIR!, then the cooled water is returned to the engine to pick up some more heat. After all this, the engine is still cooled by AIR? What's going on here? The water is the vehicle for carrying the heat from a place where the engine designer can't put enough cooling fins, to a place where he can put enough cooling fins. Looking closely at a radiator you will see many small tubes and thin fins. This gives a very large surface area exposed to the air, where the heat is carried away. As scooter designs become more sophisticated with tightly fitting body panels that restrict air flow to the engine and as riders demand more horsepower without increasing engine displacement (in some parts of the world you pay taxes base on displacement) you will begin to see more water cooled engines. So, if you have a water cooled engine, there's some more weight, and a few more components to check and care for. And, most importantly, keep that radiator free of bugs and debris because, after all, it's still an air cooled engine.
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