Originally posted by f7ben
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I imagine if you tried running most any automotive engine at 75% of full power for 2,500 or 3,500 hours, with thousands of cycles from zero to extended full power runs in between, and with the vibration and stress of a propeller in turbulence attached to the front, you would not be impressed. Auto engines are designed a lot closer to their limits, and are designed to run in tightly controlled conditions. Because they normally run at much lower power settings and the consequences of failure are basically nil, this is a viable design choice.
I think that reliability is possibly the single biggest factor behind any aircraft power plant design. The consequences of failure are dire. When I am 60NM away from land or over rocky peaks behind my engine, the cost, power output, and efficiency are all secondary considerations.
What's more, ICE technology stagnated a long time ago. Things haven't changed in a material way for decades. Maybe aircraft engines just found their sweet-spot a little sooner because of the pressures of flight and limits of certification. If not for the marketing department and oil companies, I think there would be a lot more criticism levelled at "modern" ICEs. I agree that experimental ones can easily be improved a wee bit, but the margins aren't very big.
An afterthought on vibration and harmonics, I am not sure whether the PSRU reduces the risk there, maybe it does - I know there are a range of benefits of a PSRU. However, what I learned recently is there is no warning about incompatible prop-engine combinations. The vibration frequencies are so high that you can't even tell if there's a problem; until the prop or engine physically fails - normally a catastrophic failure - that's the first you learn about it, unless you do extensive factory testing. Of course wooden and composite props (like the MT) are not particularly vulnerable to this kind of harmonic or vibration failure.
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