5 ms·

Ooooohhh, the air-cooled vs. water-cooled argument is a great one. The mechanical simplicity of air-cooled engines is a very strong point in their favor, but it

Ooooohhh, the air-cooled vs. water-cooled argument is a great one. The mechanical simplicity of air-cooled engines is a very strong point in their favor, but it's arguably the only one. Water-cooled engines can be run leaner (i.e. more efficiently) because they don't need to run rich to keep cool, don't have uneven cooling issues, aren't susceptible to thermal shock when going to idle during a cruise descent, have better power-to-weight ratios, and don't have as big of a problem dealing with the dilemma of having the slowest/warmest airflow when the most engine power is needed (during takeoff and go-around conditions)--all for the relatively modest risk of a pump, radiator, or hose failure. In an alternate reality where we had seen the same amount of R&D go into general aviation piston engines as automotive ones, I wouldn't be surprised if nearly all light aircraft used hybrid-electric liquid-cooled engines with reduction gearboxes.

jabl 4y ago
I don't know. If you look at the swan song of big aviation piston engines just before turbines took over (immediate post-WWII era), it was all air-cooled radials. If the WWII experience would have proved liquid cooled engines to be superior, I guess all those Convairs, Constellations, DC-6's etc. would be using Griffons, Sabres or such.
- VLM 4y ago
  Classic scalability issue, those engines were HUGE compared to general aviation so all the rules about shock cooling and surface area to volume ratios don't apply unchanged to engines a thirtieth the size.
  - jabl 4y ago
    I'm not sure. Surface vs. volume is certainly and interesting thing that explains many scaling phenomena, but in this case, those huge radials had huge fins as well (correspondingly huger than fins on smaller engines in order to cool the much higher mass) to cool them under normal operations. So those same fins would equally rapidly cool the engine when descending quickly at idle power. So shock cooling is certainly an issue with big radials as well. Now I don't know the exact scaling relation, but I'd guess it's more similar to a smaller engine than a straightforward surface/volume scaling would imply. Or to put it another way, the surface area of the cooling fins is not proportional to the surface area of the cylinders without any fins, but to the power output of the engine.
- consp 4y ago
  That's survivor bias and sheer production numbers of us bombers, only the us models mainly used air cooled radial engines (bombers and carrier fighters), neither the german nor the UK ones did (mostly, exceptions always exist).
  - jabl 4y ago
    > That's survivor bias and sheer production numbers of us bombers To an extent yes, but the US certainly produced liquid cooled aircraft engines as well, like the Allison's and Packard Merlins powering many famous WWII aircraft. If they would have considered liquid cooled engines obviously superior, I'm sure they would have preferred those in post-WWII piston aircraft. > neither the german nor the UK ones did (mostly, exceptions always exist). The UK produced nearly 60000 Bristol Hercules radials, practically all used in multi engined aircraft (Beaufighter, Wellington, Stirling, Halifax being the major ones AFAIK). Sure, a much lower production numbers than the famous Merlin at 150000 engines, but far from insignificant. Similarly, Germany produced over 60000 BMW 801 radials, mostly used in the FW190A fighters but some were also used in some Ju88 variants. In comparison the DB 600 engines powering mostly Bf 109 was about 60000 as well, and about 70000 Jumo 21X series used mostly in bombers. So very broadly speaking, of total aircraft engine production, both UK and Germany produced about 1/3 radials and 2/3 liquid cooled.
    - p_l 4y ago
      Allisons and Packard Merlins happened IIRC due to RAF orders and were at least partially related to British Merlin engine. Briston Hercules was fitted to sever plane types due to lack of production capability for the more complex liquid cooled engines, this was specifically a case with Beaufighter which suffered badly for it.
      - jabl 4y ago
        The Packard Merlin was a license produced version of the Rolls-Royce Merlin, famous for powering the P-51. The Allison, OTOH, was AFAIK an entirely US design.
- samatman 4y ago
  I worry about people who use a word like 'superior' without any dimensions. I can think of plenty of reasons the simpler design with one less failure mode might dominate production during the war, without being the obviously better choice given peacetime applications and seventy years of technical advances.
  - Gibbon1 4y ago
    I worry when there is a hidden assumption that people were dummies for the choices they made. That tends to be more true for policy than engineering. Distilled impression. Water cooled inline engines. Small frontal area means lower drag. Water cooled provides a forgiving flexible operating envelope. What you want in a fighter. Downside, more complicated. Upside better performance means kill enemy better. Air cooled radials. The drag penalty is less as the engine size increases. More reliable as long as you stay inside the design envelop. Scales up better than inline water cooled engines. Better suited for multi-engined bombers with a well defined mission profile. At the end of the war though it was obvious that turbojets were the future for fighters. Which is why water cooled engines went away. Radials though are more efficient at lower speeds than early turbojets which is why they persisted.
    - jabl 4y ago
      I don't think it's that simple. Inline engines have less frontal area, yes, but then you need a radiator which will add back quite a bit of drag. Further, as Kurt Tank demonstrated with the, at the time, revolutionary FW 190, it's possible to make a very low drag radial engine installation. These ideas were then used in the Tempest II and Sea Fury, which were among the fastest piston engined planes ever made. Further, I'd say in history there's a lot of path dependence here (in aviation engine development as well as elsewhere, obviously) as well rather than 'pure' arguments based on the best option for a from scratch design. As it wasn't clear at the time which engine type would 'win', major powers developed both lines in parallel, if for no other reason but to not be so much behind their competitors in case some development would cause one of the engine types to clearly pull ahead. WWII showed that both air cooled radial and water cooled inlines could be used for top of the line fighters for the entire conflict. After the war, it was clear that for speed jets were going to be the future and there wasn't the wartime pressure to keep any and all production lines going at maximum capacity, so aircraft designers had more leeway in choosing powerplants. Perhaps it was because US multi-engine aircraft designers were familiar with radials and it was clear to them they were plenty good enough, perhaps it was better reliability, but radials they chose (with some exceptions, obviously).
- dTal 4y ago
  A) the highest performance aircraft were all liquid cooled B) combat aircraft have to deal with bullet holes
  - jabl 4y ago
    > A) the highest performance aircraft were all liquid cooled Well, how do you define high performance? Judging by speed at least, Tempest Mk II, Sea Fury, Bearcat, and P-47M are among the fastest piston engined planes ever made. > B) combat aircraft have to deal with bullet holes Sure. But an engine that lacks a critical subsystem by design (say, a liquid cooling system with pumps, pipes, radiators etc.), all else being equal, is less failure prone than an engine that has it. Whether that failure happens due to shells or mechanical failure.
raverbashing 4y ago
I think people are trying to balance in a false dychotomy here You could have liquid cooling, but it could be different from a car liquid cooling. For example, maybe you don't need pumps and could rely on a sturdy passive convection system. (Also include some fail-safe aspects)
- upsidesinclude 4y ago
  The concept doesn't need to be married to modern automobile design, agreed. Much like the transition motorcycle engine manufacturers made, moving away from air cooling in the late 70's. The first innovation being under piston oil jet cooling and then moving progressively to water cooling. Oil cooling offers little additional risk or complication over air cooled designs, but there are limitations on liner/piston temperatures and additional vapors which increase maintenance that don't exist with water cooling. It really comes down to whether it seems reasonable to trade on some risk for economy, but ultimately recip engine flight isn't efficient anyway
manholio 4y ago
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.
- eru 4y ago
  > [...] 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.
  - manholio 4y ago
    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.
  - HPsquared 4y ago
    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).

jabl 4y ago

I don't know. If you look at the swan song of big aviation piston engines just before turbines took over (immediate post-WWII era), it was all air-cooled radials. If the WWII experience would have proved liquid cooled engines to be superior, I guess all those Convairs, Constellations, DC-6's etc. would be using Griffons, Sabres or such.

VLM 4y ago
Classic scalability issue, those engines were HUGE compared to general aviation so all the rules about shock cooling and surface area to volume ratios don't apply unchanged to engines a thirtieth the size.
- jabl 4y ago
  I'm not sure. Surface vs. volume is certainly and interesting thing that explains many scaling phenomena, but in this case, those huge radials had huge fins as well (correspondingly huger than fins on smaller engines in order to cool the much higher mass) to cool them under normal operations. So those same fins would equally rapidly cool the engine when descending quickly at idle power. So shock cooling is certainly an issue with big radials as well. Now I don't know the exact scaling relation, but I'd guess it's more similar to a smaller engine than a straightforward surface/volume scaling would imply. Or to put it another way, the surface area of the cooling fins is not proportional to the surface area of the cylinders without any fins, but to the power output of the engine.
consp 4y ago
That's survivor bias and sheer production numbers of us bombers, only the us models mainly used air cooled radial engines (bombers and carrier fighters), neither the german nor the UK ones did (mostly, exceptions always exist).
- jabl 4y ago
  > That's survivor bias and sheer production numbers of us bombers To an extent yes, but the US certainly produced liquid cooled aircraft engines as well, like the Allison's and Packard Merlins powering many famous WWII aircraft. If they would have considered liquid cooled engines obviously superior, I'm sure they would have preferred those in post-WWII piston aircraft. > neither the german nor the UK ones did (mostly, exceptions always exist). The UK produced nearly 60000 Bristol Hercules radials, practically all used in multi engined aircraft (Beaufighter, Wellington, Stirling, Halifax being the major ones AFAIK). Sure, a much lower production numbers than the famous Merlin at 150000 engines, but far from insignificant. Similarly, Germany produced over 60000 BMW 801 radials, mostly used in the FW190A fighters but some were also used in some Ju88 variants. In comparison the DB 600 engines powering mostly Bf 109 was about 60000 as well, and about 70000 Jumo 21X series used mostly in bombers. So very broadly speaking, of total aircraft engine production, both UK and Germany produced about 1/3 radials and 2/3 liquid cooled.
  - p_l 4y ago
    Allisons and Packard Merlins happened IIRC due to RAF orders and were at least partially related to British Merlin engine. Briston Hercules was fitted to sever plane types due to lack of production capability for the more complex liquid cooled engines, this was specifically a case with Beaufighter which suffered badly for it.
    - jabl 4y ago
      The Packard Merlin was a license produced version of the Rolls-Royce Merlin, famous for powering the P-51. The Allison, OTOH, was AFAIK an entirely US design.
samatman 4y ago
I worry about people who use a word like 'superior' without any dimensions. I can think of plenty of reasons the simpler design with one less failure mode might dominate production during the war, without being the obviously better choice given peacetime applications and seventy years of technical advances.
- Gibbon1 4y ago
  I worry when there is a hidden assumption that people were dummies for the choices they made. That tends to be more true for policy than engineering. Distilled impression. Water cooled inline engines. Small frontal area means lower drag. Water cooled provides a forgiving flexible operating envelope. What you want in a fighter. Downside, more complicated. Upside better performance means kill enemy better. Air cooled radials. The drag penalty is less as the engine size increases. More reliable as long as you stay inside the design envelop. Scales up better than inline water cooled engines. Better suited for multi-engined bombers with a well defined mission profile. At the end of the war though it was obvious that turbojets were the future for fighters. Which is why water cooled engines went away. Radials though are more efficient at lower speeds than early turbojets which is why they persisted.
  - jabl 4y ago
    I don't think it's that simple. Inline engines have less frontal area, yes, but then you need a radiator which will add back quite a bit of drag. Further, as Kurt Tank demonstrated with the, at the time, revolutionary FW 190, it's possible to make a very low drag radial engine installation. These ideas were then used in the Tempest II and Sea Fury, which were among the fastest piston engined planes ever made. Further, I'd say in history there's a lot of path dependence here (in aviation engine development as well as elsewhere, obviously) as well rather than 'pure' arguments based on the best option for a from scratch design. As it wasn't clear at the time which engine type would 'win', major powers developed both lines in parallel, if for no other reason but to not be so much behind their competitors in case some development would cause one of the engine types to clearly pull ahead. WWII showed that both air cooled radial and water cooled inlines could be used for top of the line fighters for the entire conflict. After the war, it was clear that for speed jets were going to be the future and there wasn't the wartime pressure to keep any and all production lines going at maximum capacity, so aircraft designers had more leeway in choosing powerplants. Perhaps it was because US multi-engine aircraft designers were familiar with radials and it was clear to them they were plenty good enough, perhaps it was better reliability, but radials they chose (with some exceptions, obviously).
dTal 4y ago
A) the highest performance aircraft were all liquid cooled B) combat aircraft have to deal with bullet holes
- jabl 4y ago
  > A) the highest performance aircraft were all liquid cooled Well, how do you define high performance? Judging by speed at least, Tempest Mk II, Sea Fury, Bearcat, and P-47M are among the fastest piston engined planes ever made. > B) combat aircraft have to deal with bullet holes Sure. But an engine that lacks a critical subsystem by design (say, a liquid cooling system with pumps, pipes, radiators etc.), all else being equal, is less failure prone than an engine that has it. Whether that failure happens due to shells or mechanical failure.

raverbashing 4y ago

I think people are trying to balance in a false dychotomy here You could have liquid cooling, but it could be different from a car liquid cooling. For example, maybe you don't need pumps and could rely on a sturdy passive convection system. (Also include some fail-safe aspects)

upsidesinclude 4y ago
The concept doesn't need to be married to modern automobile design, agreed. Much like the transition motorcycle engine manufacturers made, moving away from air cooling in the late 70's. The first innovation being under piston oil jet cooling and then moving progressively to water cooling. Oil cooling offers little additional risk or complication over air cooled designs, but there are limitations on liner/piston temperatures and additional vapors which increase maintenance that don't exist with water cooling. It really comes down to whether it seems reasonable to trade on some risk for economy, but ultimately recip engine flight isn't efficient anyway

manholio 4y ago

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.

eru 4y ago
> [...] 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.
- manholio 4y ago
  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.
- HPsquared 4y ago
  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).