Carnot efficiency. 30% is pretty typical in land-based motive systems. Shipboard combustion engines and coal-fired power plants (of all things) are rated as high as 45% efficiency.

This is based on input and output temperatures, more than anything.

F1 "power units" are at 47% thermal efficiency but there a complicated hybrid system.

https://www.formula1.com/content/fom-website/en/latest/inter...

Got it, so a 1.5x improvement on 30% lines right up with 45%. That's where they're getting their 50% gains number from.

Good eye, though it's not immediately clear that that's strictly edging in on Carnot efficiencies or if other aspects (two-stroke power cycle, valveless port-based design, eliminating cylinder-head heat loss -- on which, doesn't that eventually reach equilibrium temp anyway?, etc.) are to credit.

I've seen a few other radical reciprocating energy designs, there are some interesting YouTube vids I should track down. I think "radial engine" might reveal something. New Zealand, possibly Australian engineering firm, IIRC.

Most IC engines are nowhere near the carnot limit, which is around 60%-70% in most cases. The majority of the efficiency losses come from excessive heat rejection in the engine block and also mass transfer in the exhaust. Most engine blocks are (in)conveniently made out of aluminum, which has a very high thermal conductivity, meaning more energy gets transferred to the cylinder walls and wasted during combustion.

As a mechanical engineer, this engine design seems much more promising, however: http://liquidpiston.com/

Engine blocks used to be made of cast iron, but they've pretty much all moved to aluminum because of 1) weight, and 2) that higher thermal conductivity. IANAME, but IIRC the problem with iron's lower thermal conductivity is that it created hot spots which could cause pre-ignition, so you had to use lower compression ratios to avoid this. Aluminum has more uniform temperature so you can boost the compression ratio, which gets you more efficiency and power.

Carnot limit is over 60-70 percent in fixed load power plants. 70% is something only could be attained with very exotic materials(expensive and difficult to manage) that could handle high temperatures.

In a mobile system you could get 50% with a fixed load. The problem with the car is that is has to work in different regimes.

That is the reason hybrid engines are more efficient. They work at a fixed load and store the energy in a battery or super cap.