I read that a massive portion of usable energy is wasted and turned to heat instead of being used to turn the crankshaft — would there be like any way of reducing the heat/cooling the engine so you could get 50-70% thermal efficiency?
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Others have approached this but missed the point – there are commercial engines which are far over 20-30% in routine use.
20-30% is about right for crappy 4-stroke.
For example – this normally aspirated 160cc honda engine https://www.justgenerators.co.uk/honda-gx160-qhq4-engine.html uses 313g/kWh.
1kg of petrol is around 44MJ/kg, so this is 13.7MJ input per hour, and 3.6MJ out.
This is 26%, and this is a mediochre 160cc 4 stroke single cylinder engine with nearly no economy measures.
To hit 20%, you need a really badly malfunctioning engine, a very old one, or one operating far from its ideal operating point. Or a 2-stroke.
https://en.wikipedia.org/wiki/Brake-specific_fuel_consumption is the page you want.
Some highlights.
The first ever functional diesel engine hit 26%. (1931)
The engines of the B29 hit 35% on avgas (35%)
Napier Nomad (experimental aero engine) hit 40% in 1949.
On a more commercial note the 2000s Volkswagen 3.3 V8 TDI got to 41%.
Gasoline in the Toyota 1NZ-FXE (prius) at about the same time being a few points down at 36%. Cars have not meaningfully improved since then.
The top of the pile are large marine engines that are hitting 55%, and stationary gas turbines at 60%.
But, we’ve had >30% internal combustion engines in routine use since the 1920s, >40% since the 30s and 60% for around a decade. (the last being jet engines)
These are the best fuel efficiencies at any RPM, often 60% max power output or so for gasoline, a bit more for diesel.
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