Radial engines favored for powering the tanks & AFVs, 1935-45 (4 Viewers)

[marathag]

We can take a look at the Valentine tank. The engine bay was long enough for the 6-cyl diesel to fit, that went at 79 in long? The R-670 was under 35 in of lenght. Let's be conservative and have the radial engine of good power being 40 inches shorter - a full meter. In the time when tanks were 5+- meter long, that is a major thing. Even saving of 50cm leaves a lot of weight allowance to be used on other stuff.

Valentine Engine Compartment

 

[tomo pauk]

Me neither. Just pointing out that engines, even from the same manufacturer, had wildly varying costs. So I don't think in general you can make the argument that a radial would be cheaper than a liquid-cooled inline, all else being equal.

I've tried to make a comparison between a 9 cyl radial (often used on the US tanks/AFVs) vs. a 12 cyl liquid cooled engine (standard on German, Soviet and British tanks/AFVs worth speaking about) tanks, not just any radial vs. any liquid cooled engine.
In this comparison, my opinion is that the radial is a less costly choice, while also offering the knock-on effects wrt. tank weight/price/armor etc.

I don't think you can attribute that 500kg (rough difference between BMW 132 and Maybach HL 230, if we're going by wikipedia numbers?) solely to cast iron vs aluminum. Or radial vs inline, for that matter.

I'm not sure that I've simply attributed to the materials chosen the weight difference between the two engines. Iron/steel was possibly 'guilty' for 200+- kg weight creep on the HL 230 (that went to 1300 kg)?
1-row radials were very light for the power they were making, though. The BMW 132 was far lighter than the HL 230, possibly up to 900 kg when we also account for the cooling system. Germans can also use the Mercury engine made in Poland as the base for their tank engine for the 1940s (Mercury was with a smaller diameter). Or make their own 'Mercury equivalent' with the Bramo 323 or BMW 132 base.

Maybe? But maybe you instead lose by needing a higher hull, which adds weight by itself as well as being an easier target. If you look at the volume of the powerplant instead of focusing on any one particular dimension, I'm not sure the radial wins by that much, if at all, anymore. Also keep in mind the radial will need a large fan with assorted ducting for cooling, which will increase the length somewhat (of course the liquid cooled engine will need radiators and fans as well, but has a little more flexibility how these are placed). With a long and narrow engine like a V-12, maybe you can compensate with a lower hull, and by putting things like fuel tanks beside the engine bay?

Seems like that Valentine tank, that used the humble inline 6 engine, was tall to the roof same as the M3 light. It was also much wider. German tanks were also not known for their small height no width, especially the 'big cats'.
We can also see that a bit smarter packaging, like what the people that designed the M-18 Hellcat did, keeps the height within the modest proportions. It used the same engine as many of the M3s/M4s, yet it was a lot lower because there was the gearing added that kept the propeller shaft low.
In case that gearbox is at the back, as it was common on the French, British and Soviet tanks, even that gearing is not needed.

 

[marathag]

In this comparison, my opinion is that the radial is a less costly choice, while also offering the knock-on effects wrt. tank weight/price/armor etc.

The biggest downside was low torque output at low RPMs, that was overshadowed by the great Horsepower to weight ratings.

Aircraft engine sit at cruise RPMs for most operations, where that low torque just never came into play.

The way to avoid all that, is adding the complexity of electric or even hydraulic drive, or what the Germans did with their high rpm, low torque V-12s, add more gears.

 

[tomo pauk]

A table from the Russian book about the tanks, FWIW:

 

[Shortround6]

M3A3 light tank.

M5A1

Two V-8s side by side.
Added 1250lbs to the tank to max out at 33,000lbs. Also includes the Hydro-matic transmission.
You may have to look at situation on case by case basis. Blanket always good or always bad might not apply.