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> An air-cooled engine willing to run at much hotter temperatures... Are they "willing to run at a higher temperature?" I feel you're begging the question, but

by bch 11y ago

> An air-cooled engine willing to run at much hotter temperatures... Are they "willing to run at a higher temperature?" I feel you're begging the question, but I'd be delighted to be shown I'm wrong. The engine has cooling requirements, and they're met by careful ducting and a fan that the engine runs directly, or indirectly. VW Beetles and early buses ran a fan off a generator or alternator via a belt from the engine, and VW type 3 and 4 engines (and at least later, if not all air-cooled Porsche 911s, (and 912s?)) are run off the engine crank. The engine needs to be cooled, needs airflow (ask a knowledgeable BMW airhead motorcycle owner). So, "why not put then engine up front" could be answered with "we've contained all cooling requirements in the engine itself, so we put it where we want." As I read more about this there are lots of interesting considerations[0] but I don't see "willing to run at a higher temperature" as a consideration, yet. [0] "Internal combustion engine cooling" on @Wikipedia: https://en.wikipedia.org/wiki/Internal_combustion_engine_cooling?wprov=sfia1 https://en.wikipedia.org/wiki/Internal_combustion_engine_coo... Edit: s/chilling/cooling/

sandworm101 11y ago
If you want to get all definitional, then one might say the porsche/VW were also liquid-cooled. They had oil which did move some heat around. So the line is really whether or not the engine has a separate cooling loop, usually filled with water. There are a couple different temperatures to speak of. There is the overall engine temp, which may well be very similar to a modern engine. But there is also the temperature of the cylinder walls deep inside. An air-cooled engine relies on metal to conduct heat away from the heart of the engine. A water-cooled engine puts a pipe of cold water right alongside the cylinders to physically move heat away. So the internals of an air-cooled engine go through a much wider temperature range during normal operation. With differential metals that means gaps... gaps between the cylinder wall and the piston. That in turn means increased wear and emission-related problems. An air-cooled engine is willing to suffer these issues for the advantage of decreased weight and complexity. An air-cooled engine not willing to suffer this internal heating would be a very slow and heavy engine. Having a hotter cylinder wall can also impact choice of fuels (need higher octane) and compression ratios (need less) as the hot walls might ignite things before the spark.

by bch 11y ago

sandworm101 11y ago

If you want to get all definitional, then one might say the porsche/VW were also liquid-cooled. They had oil which did move some heat around. So the line is really whether or not the engine has a separate cooling loop, usually filled with water. There are a couple different temperatures to speak of. There is the overall engine temp, which may well be very similar to a modern engine. But there is also the temperature of the cylinder walls deep inside. An air-cooled engine relies on metal to conduct heat away from the heart of the engine. A water-cooled engine puts a pipe of cold water right alongside the cylinders to physically move heat away. So the internals of an air-cooled engine go through a much wider temperature range during normal operation. With differential metals that means gaps... gaps between the cylinder wall and the piston. That in turn means increased wear and emission-related problems. An air-cooled engine is willing to suffer these issues for the advantage of decreased weight and complexity. An air-cooled engine not willing to suffer this internal heating would be a very slow and heavy engine. Having a hotter cylinder wall can also impact choice of fuels (need higher octane) and compression ratios (need less) as the hot walls might ignite things before the spark.