Air cooled inline engines - a missed opportunity?

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z42

Senior Airman
647
416
Jan 9, 2023
I was thinking of air cooled inline engines, that in principle could combine the low frontal area of liquid cooled inlines, the lack of a heavy and battle damage prone radiator, and the simplicity of air cooling. There were some developments in this direction in the interwar years, though as I'm sure we all know the major aviation powerplant types in WWII were liquid cooled inlines and air cooled radials.

I did find an earlier thread from 2015 on this topic: Air-cooled inline engines with forced cooling

What piqued my interest in this was stumbling upon this video of the Tatra air cooled V-8 engines used in auto racing in 1950'ies:
View: https://www.youtube.com/watch?v=Hle2_Dy-UW8 .

What was particularly ingenious about this was that it used something they called 'exhaust ejector cooling', meaning essentially using the exhaust gases to drive a venturi type device that pulls the cooling air through the cylinders without needing a fan. See the diagram at 4:35 and some pictures of the engine following that.

That being said, ingenious as that system was, I'm not sure it would be that useful on an aircraft vs. using forced cooling with fans? IIRC the BMW 801 used about 60hp do drive the fan during takeoff, however when the aircraft gained speed the fan would essentially freewheel in the ram air, and thus consume no power from the engine. Looking at the pictures of the engine, the cooling ducts with the exhaust venturi system is relatively compact and doesn't make the entire engine that much wider, so maybe it could have been feasible in an aircraft after all?

Summarizing arguments from the previous thread:
  • Design of cooling ducts, fans, baffles etc. critical to ensure sufficient cooling to all cylinders with minimum drag. In the Tatra design it seems air entered between the cylinder banks, vent laterally past the cylinders, and then exited to the plenum on the outside of the cylinder banks, where also the exhaust headers were installed to create this Venturi effect.
  • Need longer bore pitch to make room for all the fins, making the engine itself longer than an equivalent liquid cooled engine.
  • Almost certainly needs separate cylinder barrels as casting a monoblock cylinder block with all the fins is probably not feasible. Though the crankcase, and if using overhead camshafts, the cylinder head block might be possible to cast as monoblocks. Though in the latter case the question becomes the cooling of the cylinder heads.
  • The reduced cooling capacity of air vs water means the power limit of each cylinder is lower. To be competitive with a liquid cooled V-12, might need to go to a, say, H-24 layout. The chunky H-24 crankcase would also help a bit with the stiffness issues due to lack of monoblock cylinders/heads.
 
I was thinking of air cooled inline engines, that in principle could combine the low frontal area of liquid cooled inlines, the lack of a heavy and battle damage prone radiator, and the simplicity of air cooling. There were some developments in this direction in the interwar years, though as I'm sure we all know the major aviation powerplant types in WWII were liquid cooled inlines and air cooled radials.

I did find an earlier thread from 2015 on this topic: Air-cooled inline engines with forced cooling

What piqued my interest in this was stumbling upon this video of the Tatra air cooled V-8 engines used in auto racing in 1950'ies:
View: https://www.youtube.com/watch?v=Hle2_Dy-UW8 .

What was particularly ingenious about this was that it used something they called 'exhaust ejector cooling', meaning essentially using the exhaust gases to drive a venturi type device that pulls the cooling air through the cylinders without needing a fan. See the diagram at 4:35 and some pictures of the engine following that.

That being said, ingenious as that system was, I'm not sure it would be that useful on an aircraft vs. using forced cooling with fans? IIRC the BMW 801 used about 60hp do drive the fan during takeoff, however when the aircraft gained speed the fan would essentially freewheel in the ram air, and thus consume no power from the engine. Looking at the pictures of the engine, the cooling ducts with the exhaust venturi system is relatively compact and doesn't make the entire engine that much wider, so maybe it could have been feasible in an aircraft after all?

Summarizing arguments from the previous thread:
  • Design of cooling ducts, fans, baffles etc. critical to ensure sufficient cooling to all cylinders with minimum drag. In the Tatra design it seems air entered between the cylinder banks, vent laterally past the cylinders, and then exited to the plenum on the outside of the cylinder banks, where also the exhaust headers were installed to create this Venturi effect.
  • Need longer bore pitch to make room for all the fins, making the engine itself longer than an equivalent liquid cooled engine.
  • Almost certainly needs separate cylinder barrels as casting a monoblock cylinder block with all the fins is probably not feasible. Though the crankcase, and if using overhead camshafts, the cylinder head block might be possible to cast as monoblocks. Though in the latter case the question becomes the cooling of the cylinder heads.
  • The reduced cooling capacity of air vs water means the power limit of each cylinder is lower. To be competitive with a liquid cooled V-12, might need to go to a, say, H-24 layout. The chunky H-24 crankcase would also help a bit with the stiffness issues due to lack of monoblock cylinders/heads.

Liquid- cooled engines have an advantage od cylinder & cylinder head temperature stability. Other than motorcycles, how many air-cooled engines are still in production? Tatra, VW, Porsche, Corvair, etc are gone or have switched to liquid cooling.
 
Liquid- cooled engines have an advantage od cylinder & cylinder head temperature stability. Other than motorcycles, how many air-cooled engines are still in production? Tatra, VW, Porsche, Corvair, etc are gone or have switched to liquid cooling.

Largely you're correct, but just to nitpick Tetra still makes military trucks with air cooled diesel engines. The latest iteration in this line being Tatra 815-7 - Wikipedia first introduced in 2007.

Also noteworthy is that the Porsche 911 switched to liquid cooling as late as 1998, AFAIU largely to meet tightening emissions regulations. Not much of a concern for WWII military aviation engines.
 
I replaced the air cooled flat 4 in my Vanagon with a water cooled inline 4. About the same displacement and nearly double the horsepower. HP correlates to heat and you have to do something with that heat. I could have gotten the same power and stayed air cooled, but it would have been more complicated and expensive than water.
 
I replaced the air cooled flat 4 in my Vanagon with a water cooled inline 4. About the same displacement and nearly double the horsepower. HP correlates to heat and you have to do something with that heat. I could have gotten the same power and stayed air cooled, but it would have been more complicated and expensive than water.
I'm sure the general merits of air cooling vs liquid cooling for aircraft piston engines have been debated ad nauseam in this forum and elsewhere. In the context of this thread I'm more interested why specifically air cooled inlines never became that successful whereas air cooled radials did. It seems to me that an inline, even with the air scoops for cooling airflow, would have much lower frontal area and less drag than an equally powerful radial.
 
IMO the primary reasons for the liquid-cooled engine being left by the wayside post-WWII were:

1. turbojet and turboprop engines coming along fairly 'nicely'
2. For the US (and allies if they desired) radial engines were already available in 'large' sizes (ie R-3350, Centaurus, etc) with the power needed, and the R-4360 was in the pipeline.

While there is reason to think that a large liquid-cooled engine in the same class would be more efficient in several ways, IIRC the only 'large' one anywhere near successful development completion was the RR Eagle XXII (H24-cylinder of 2807 in3, 3200 BHP at its premature development end). IIRC it was intended for the Wyvern and Gannet, but was replaced by the Python and Double Mamba turboprops respectively.

"Rolls-Royce Eagle (1944) - Wikipedia"
 
Ranger V-770 was a 520hp air-cooled V-12. They had a lot of trouble getting it to cool enough for operational use; as you can imagine, baffling required a lot of development work. Not saying it couldn't be done but the war might have ended before you got a much more powerful air-cooled V-12 developed.
 
Aircooled in lines had the disadvantage of having their cylinders in a row unlike the radial which presented their cylinders up front (in theory), which allowed better airflow across the cylinders and heads.

In order to cool an aircooled inline (like a Ranger), there had to be a substantial increase in airflow across and to the rear of the engine, which would require either enlarged inlet/ducting or design the cowling to have the cylinders exposed. Both options would impose a considerable drag penalty over that of a radial cowling.
 
I'm sure the general merits of air cooling vs liquid cooling for aircraft piston engines have been debated ad nauseam in this forum and elsewhere. In the context of this thread I'm more interested why specifically air cooled inlines never became that successful whereas air cooled radials did. It seems to me that an inline, even with the air scoops for cooling airflow, would have much lower frontal area and less drag than an equally powerful radial.
Shoving a lot of air, and a powerful engine needs a lot of air, through a small frontal area creates a lot of drag. It also isn't going to work well if there isn't a lot of air, such as climbing at slower speed.
 
In the 20s/30s/40s, designers were learning a lot about airflow management/cooling. For the air cooled engine, "just hanging jugs out in the airflow like cauliflower leaves" no longer worked. Air, at the higher speeds of the newer airplanes, went around the engine rather than across the cooling fins. Also, if the air is moving too fast, it doesn't absorb as much heat as it could.

So, you need to get the air to enter, slow it down to the optimum speed, baffle the head/cylinder to ensure air goes over the cooling fins, cast/machine the optimum cooling fins, then extract the heated air. Note: optimum air volume changes depending on speed/power i.e. climb/cruise/combat/descent/etc.

de Havilland Albatross with de Havilland Gipsy Kings is how you do it. We note how small the intake ducts are on the wing leading edge and how small/simple the exhaust duct is.

The challenge is the V-12 gets really long - the Gipsy King 82" long (4.646" pistons) vs Merlin 69" (5.4" pistons). The Pennine (5.4" pistons, but X-24) is 106" long! And an engine that long doesn't "swap" into an existing airframe - you need an engine swap ala reverse of P36 to P40/Fw.190A to Fw.190D, where you can demonstrate the superiority of your engine e.g.
Eliminating liquid cooling by using R-R Exe in a Barracuda. Poppet valves on the Exe probably desired given some 20/20 hindsight. Note: the air intake on Battle with Exe installed is far from optimal.
 
The Germans also developed diesel aircraft engines but those were an evolutionary dead-end, too.

All high power aviation piston engines were evolutionary dead ends, thanks to turbines.

In defense of the German aviation diesel engines, they did eventually evolve into the superb Napier Deltic.
 
Ranger V-770 was a 520hp air-cooled V-12.
Ranger (Fairchild) probably as much as anybody did in studying air cooling. Like the size and spacing of the fins and how far away the air could be and still do any cooling.
They also spent a lot of time and money just trying to get that 520hp engine to cool in aircraft.
Basic cooling diagram here.
003.jpg

A problem with V-12s is that you are trying to turn the air twice through around 90 degrees. It has very little to do with the temperature of the air at rear cylinders, Well it kind of does but every cylinder is supposed to be getting fresh air and not air that already gone by another cylinder.
A lot of detail here.

You can play games with reverse cooling at lower speeds. When you try for 350-400mph changing the direction of the air introduces losses.

Also note that the air passages on the DH Albatross are for just over 500hp engines. You need a air passages about 3 times larger for 1500hp engines.
 
Ranger (Fairchild) probably as much as anybody did in studying air cooling. Like the size and spacing of the fins and how far away the air could be and still do any cooling.
They also spent a lot of time and money just trying to get that 520hp engine to cool in aircraft.
Basic cooling diagram here.
View attachment 726112
A problem with V-12s is that you are trying to turn the air twice through around 90 degrees. It has very little to do with the temperature of the air at rear cylinders, Well it kind of does but every cylinder is supposed to be getting fresh air and not air that already gone by another cylinder.
A lot of detail here.

You can play games with reverse cooling at lower speeds. When you try for 350-400mph changing the direction of the air introduces losses.

Also note that the air passages on the DH Albatross are for just over 500hp engines. You need a air passages about 3 times larger for 1500hp engines.
Aero Digest or Aviation Week had an article by a Ranger engineer about the V-770 development. He said that air 1/16 inch away from the cooling fins contributed nothing to cooling. They needed excellent tight fitting baffles and one wonders how long they would stay in good condition in service.
 

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