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mdkcrf250r

Why do 4 strokes run hotter than 2 strokes?

6 posts in this topic

Why do 4 strokes run hotter than 2 strokes? Is it a single factor or a combination? More parts? With one combustion per 2 revolutions the 4 stroke has more time between combustion providing time to cool but yet it runs hotter than a 2 stroke?

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If you touch the pipe right after you start a two stroke, it's a little warm, doesn't get too hot at all when it's ideling.

When you touch a four stroke pipe right after it starts, you'll burn your hand, bad.

This tells me that the combustion on four strokes is more efficient than two strokes, thus the hotter exhaust gasses. At least at idle this is the case..

Maybe I'm wrong on that, just my observation.

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There are a number of reasons for this. The first is that two stroke combustion efficiency is really rather poor at idle, so less of the heat that could be extracted from the fuel actually is. Also, quite a bit of that escapes halfway through the power stroke when the exhaust port opens.

A four stroke head is covered wall to wall with valves, leaving little of the combustion chamber surface available for direct contact with the coolant. That means that the four stroke must rely more on the upper cylinder for cooling than the two stroke does, and since the head is exposed to the combustion chamber more than the upper cylinder, heat is more quickly carried away, before it builds up so high.

The high exhaust temp at idle is usually due to aggressive cam timing.

At speed, there isn't much difference, and in fact two-strokes will actually tend toward running hotter. At idle, the situation is often reversed.

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My hypothsis is that at idle and off idle speeds, piston port 2-stroke engines run very inefficiently and blow a ton of excess air through the cylinder which helps keep things cool.

But, it is not correct to say 2-strokes run cooler. The easiest example to state is the old air cooled engines. Compare the surface area of cooling fins on a 2-stroke engine to that of 4-stroke of equivalent horsepower. The 2-stroke has much more cooling area because it has a much greater ability to generate heat. Even compared to modern MX bikes, the 2-stroke still has a greater ability to generate heat. It's just the 2-stroke makes more heat when going fast where the radiators are efficient about rejecting heat. At low speeds, the excess air blowing through the engine keeps things cool while the radiators are not working so well.

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My hypothsis is that at idle and off idle speeds, piston port 2-stroke engines run very inefficiently and blow a ton of excess air through the cylinder which helps keep things cool.

But, it is not correct to say 2-strokes run cooler. The easiest example to state is the old air cooled engines. Compare the surface area of cooling fins on a 2-stroke engine to that of 4-stroke of equivalent horsepower. The 2-stroke has much more cooling area because it has a much greater ability to generate heat. Even compared to modern MX bikes, the 2-stroke still has a greater ability to generate heat. It's just the 2-stroke makes more heat when going fast where the radiators are efficient about rejecting heat. At low speeds, the excess air blowing through the engine keeps things cool while the radiators are not working so well.

+1

two strokes are very well air cooled by their own scavenging at low speeds. even in detroit diesels that have the same valve covered head as a 4 stroke, because they are supercharged and have so much air blowing through they almost never even reach the thermostat temperature at idle.

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