Air-cooled inline engines with forced cooling

kool kitty89 · Jul 27, 2015

Huh, I wonder if the front mounted exhaust is less cooling related and more related to practical intake and exhaust manifold routing.

Shortround6 · Jul 27, 2015

A lot of times yes. Bristol poppet valve radials had a lot of 'legacy' engineering since they trace back to the Cosmos Jupiter engine of WW I (Bristol bought Cosmos when Cosmos went bankrupt at the end of WW I.) On radials trying to route the intake pipes from the rear mounted superchargers (convenient since the supercharger doesn't interfere with the propshaft or reduction gear) to front side of the cylinders gets rather difficult. Especially on a two row radial.

gumbyk · Jul 27, 2015

Yes, most radials have some form of supercharging, with the supercharger mounted on the back of the engine. To route the intake manifold from the rear of the engine to the front of the cylinders would block a lot of your cooling airflow, especially on a two-row radial, so having the exhaust exiting on the front of the cylinder makes sense (unless the exhaust and intake are both mounted on the rear).

Balljoint · Jul 27, 2015

There's an aspect of air cooling that may or may not have been considered in air cooled aero engines.

Air when heated becomes more viscose. This increases the boundary layer forming a relatively insulating thermal boundary layer. Actually, the hotter air has a greater heat transfer coefficient but the conductive heat transfer mode is much less effective than the convective mode at lower temperature. This phenomenon is quantified in the Prandtl number if I recall correctly.

Succinctly, higher temperature air can have a negative feedback effect on heat rejection efficiency beyond ΔT per se.

GregP · Jul 27, 2015

Is that the same as saying hot air is a bad thing?

Balljoint · Jul 28, 2015

If so this forum is in trouble.

Koopernic · Jul 28, 2015

Balljoint said:
There's an aspect of air cooling that may or may not have been considered in air cooled aero engines.

Air when heated becomes more viscose. This increases the boundary layer forming a relatively insulating thermal boundary layer. Actually, the hotter air has a greater heat transfer coefficient but the conductive heat transfer mode is much less effective than the convective mode at lower temperature. This phenomenon is quantified in the Prandtl number if I recall correctly.

Succinctly, higher temperature air can have a negative feedback effect on heat rejection efficiency beyond ΔT per se.

As part of my undergraduate degree in electrical engineering I had to study "thermodynamics and heat transfer" for the sake of understanding the heat sinks used to keep electronic devices cool and that lead to a study of the airflow over heat sinks and so that I would not feel like an idiot talking to a steam turbine expert in a power station. Here was a fascinating and mind blowing world of Reynolds, Nusselt and Prandle numbers that helped predict where the boundary layer would become turbulent and how much transfer could be expected by conductivity and convection etc. Smaller objects obviously retain laminarity over a greater proportion of their length.

My lecturer was Earl Baker Junior. A brilliant man who solved the heat transfer problems of the coaxial cable used in the Safegard ABM radar (it was 300 yards long and under ground and had been melting) and the Telstar TV relay satellite. He never stopped teaching undergraduates.

kool kitty89 said:
.

Originally Posted by Koopernic
Less area to volume on a larger piston means less material to protect.

Internal volume or surface area doesn't matter in terms of combat vulnerability, at least not in this context. It's overall external diameter, length, and general engine geometry that limits vulnerable area. (that and oil cooler capacity)

I had actually meant less surface area to volume ratio means there is less surface area to protect from heat stress for a given combustion volume. The reason you cool a cylinder is to protect the metal from getting so hot it gets damaged. You don't need to cool the contents of the cylinder, just the walls. In fact you don't want to cool the contents. Some areas might be problematic, such as piston cooling, but I imagine oil splash is the main source of that.

I would say an air cooled in line is less vulnerable than an liquid cooled in line simply because it has less plumbing, less high pressure plumbing and radiators. As was shown with the BMW801 an armoured oil cooler was possible.

Balljoint · Jul 28, 2015

Air cooling has the advantage of being distributed. When a water cooled system is damaged enough to affect a cylinder, the whole engine goes down –though less immediately if the system is valved to limit fluid loss. An air-cooled engine can lose a cylinder but may well keep running indefinitely.

There an interesting film of –I think- a Hellcat landing on a carrier during the Battle of the Philippine Sea. When arrested and stopped a flaming fluid cascades from the cowling. At first blush this appeared to be lubricant or maybe hydraulic fluid. But closed inspection shows it to be rather viscous and most likely molten metal –perhaps even magnesium from the reactive appearance of a large puddle on the deck. But it could be aluminum destructively distilling the wooden deck. In any event, landing successfully with the engine literally melting is a pretty good definition of tough. Not likely with a water cooled engine.

kool kitty89 · Jul 28, 2015

Koopernic said:
I had actually meant less surface area to volume ratio means there is less surface area to protect from heat stress for a given combustion volume. The reason you cool a cylinder is to protect the metal from getting so hot it gets damaged. You don't need to cool the contents of the cylinder, just the walls. In fact you don't want to cool the contents. Some areas might be problematic, such as piston cooling, but I imagine oil splash is the main source of that.

Internal heat thermal stresses would certainly be worse for a smaller surface area to volume ratio, but external heat dissipation improves similarly with higher surface area to volume ratios. With similar cylinder and cooling fin designs, shouldn't the R-1830 be easier to cool than the R-1820.

This likely differs more with more drastically different cylinder proportions (longer or shorter stroke to bore ratio).

I would say an air cooled in line is less vulnerable than an liquid cooled in line simply because it has less plumbing, less high pressure plumbing and radiators. As was shown with the BMW801 an armoured oil cooler was possible.

I'd actually think more compact conventional oil coolers would be easier to protect than the BMW type, but less aerodynamically efficient than using the cowl as a heat-sink.

MiTasol · Nov 24, 2019

Balljoint said:
The Liberty engine was successfully converted to air-cooling – by Alison if I recall correctly. But, like multirow air-cooled radial engines, higher outputs tend to run into heat rejection problems

Yes but the Liberty already had wide spacing between cylinders and each cylinder was separate. Most liquid cooled engines post WW1 were much shorter monoblocks like the Hispano WW1 engines

Shortround6 · Nov 24, 2019

MiTasol said:
Yes but the Liberty already had wide spacing between cylinders and each cylinder was separate. Most liquid cooled engines post WW1 were much shorter monoblocks like the Hispano WW1 engines

And even at that Allison used a smaller bore on the air cooled engines to allow more space for fins.

Admiral Beez · Nov 25, 2019

gjs238 said:
Could the use of forced cooling, ala BMW 801, have permitted air-cooled inline engine types such as those manufactured by Ranger and Argus to potentially achieve power levels and reliability of their water cooled inline cousins, or air cooled radial cousins?

Interestingly, BMW experimented with changing the air cooled 801 into the 3,847 hp BMW 803 liquid cooled radial. Liquid cooled radials being a rare thing.

BMW 803 - Wikipedia

Air-cooled inline engines with forced cooling

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