Lot of talk about high stresses on piston aero engines running at 75-100% of rated output for long periods, but let’s look at this from another point of view.
My 4 cyl O360 is rated at 180 bhp. 360 cu in = approx 5904 cc, or 1476 cc per cyl. Now how many modern cars engines of 6 liters are rated at a maximum output of only 180 bhp?
Most are producing better than 400, and go up from there.
All of them would have no difficulty producing 180 bhp at 2700 rpm almost indefinitely!
In short we must accept that even running at max rated output, most N/A aero engines are not mechanically highly stressed, but there are a couple of caveats, driven by the KISS principle:.
Modern car engines make their max rated outputs at 5-7000 rpm, much too fast for a propeller where tip speed is the limiting factor. In order to reduce the prop speed a gearbox or other type of reduction mechanism would be needed, and here we run into the KISS thing. In GA most propellers are bolted directly to the crankshaft thus avoiding all that extra complication and the added weight of a reduction mechanism. The disadvantage is we are now restricted (normally) to a maximum of approx 2700 rpm to make all the power we need for takeoff and flight.
Cooling is the major hurdle. It is much more efficient to liquid cool a piston engine. It provides even temperatures throughout the engine, effectively controls hot spots, and because it is more efficient at carrying away unwanted heat, it allows the engine to make higher horse power for a given size vs. air cooling. It’s disadvantages are high weight, added complexity, leaks can be catastrophic, and radiators are prone to FOD.
Air cooling fits the KISS principle again, nothing to leak, no pumps, no radiator, and very little added weight. The problem is air cooling is not as efficient; it has difficulty dealing with hot spots, is difficult to control, and allows vast temperature differences to exist throughout the engine. Careful monitoring of engine cylinder head and oil temperatures is therefore needed to ensure long engine life. Because engine temperatures are difficult to control most GA aero engines are not rated for very high outputs given thier capacty vis a vis automotive engines.
(One should also mention detonation, which is a real issue for aircraft fitted with C/S props. Detonation cannot be heard by the pilot in an air cooled aircraft engine, therefore careful adherence to the POH or the use of a sophisticated engine monitor is needed to avoid it.)
So there you have the two main differences between aero engines and car engines, both dictated by the KISS principle: The need to make reliable power and torque at a relatively low rpm, coupled with the simplicity and low weight of air cooling.
Horses for courses, one might say.
Regards,
White Bear.