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Comment by manholio | Hacker News Reader

Unless you build a rocket or submarine, all engines are air cooled. Using an intermediate transfer medium with high heat capacity seems like a design tradeoff, it helps you achieve higher power density in the combustion chamber at the expense of total mass of the propulsion system and some extra complexity.

Given the specifics of aircraft operation, large amounts of available coolant air, extra mass of cooling gear with available materials etc., I wouldn't be surprised if the result of that optimization problem is always against liquid cooling; at least for ICE engines where you can only achieve limited efficiency gains with higher temperatures and power densities.

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> [...] at least for ICE engines where you can only achieve limited efficiency gains with higher temperatures and power densities.

More generally: all heat engines need high temperatures for efficiency. By Carnot's Theorem the best energy conversion efficiency you can hope for is (T_hot - T_cold) / T_hot

Where T_hot is the temperature of your combustion, and T_cold is the lowest temperature you can cool it too, ie ambient temperature at best.

Sure, in principle, but in practice you are limited by the actual fuel you need to burn, and it seems the existing designs get close enough to those limits and do not stand to gain significantly from higher energy densities. Fuel efficiency and range is a primary target for optimization for decades, I doubt there are vast reserves of untapped efficiency just waiting for a hotter engine.

Engine temperature effect on efficiency is about heat loss, which isn't accounted for in the Carnot cycle (it assumes perfectly reversible cycle, and heat conduction across a temperature difference is not reversible: it's heat bleeding from one point to another and averaging out, kind of like mixing two substances).

If the metal walls of your combustion chamber are cold, there will be more heat transferred and more entropy generated. Looking more physically, heat lost after combustion reduces the temperature and therefore pressure of the gas on the power stroke, reducing power output for a given heat input (i.e. less efficient).

Yes. Carnot's Theorem gives merely an upper bound on the efficiency you can achieve.

In practice, you can easily do much worse. Eg via heatloss as you say.

However, you can never do better.

Most (perhaps all) marine engines are water cooled, not just submarines.

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