Radial engines favored for powering the tanks & AFVs, 1935-45

I thought that the French didn't consider radial engines at all for tanks, but turns out that a report from the Inspector of Tanks in 1931 mentions the study of an air-cooled 250 CV Lorraine engine. The only Lorraine engine with these two characteristics is the 7M Mizar 7-cyl radial: Lorraine 7M Mizar - Wikipedia

For the actual gearheads: how workable a thing might've been a 'cropped radial' - something with the genes from the Jupiter/Pegasus/Bramo 323 engine, but without the lower 4 cylinders? Sorta 5 cyl W engine?
Jupiter was making more than 500 HP on '73-77' octane fuel; shaving 4 cylinders = about 300 HP remaining. Having 300 HP from a 5 cyl engine of under 400 kg is not too shabby. Height of the engine drops down by 1/3rd? (from 1.4m to under 1m), and the prop shaft is now as low as it gets. Yes, a new crankshaft will be needed (doh).

There were some pre WWI 3 cylinder fan engines (Anzani) and there were a few experimantal engines using more(?). One French inventor Robert Esnault-Pelterie even built engines with multiple banks, like 3 cylinders in one bank and 4 in the other for a 7 cylinder engine.

A major problem with large displacement, high powered engines with a small number of cylinders is the vibration. Companies, like Bristol, offered them, like the 8 liter 3 cylinder Lucifer.
They were light weight and cheap, but they may have been unpleasant to fly behind.
The 'fan' engines needed more counterweighting than the 3 cylinder "Y" engine and were heavier for the same power.

What 'worked' in early aviation (just getting the plane to fly for more than a few minutes) was not what was wanted in the late 20s or early 30s, aside from homebuilders or companies looking for small, cheap engines.

50hp engine, Company offered 5 cylinder and 7 cylinder versions.

The US went for air cooled radial engines in tanks because they had them, in quantity. The US also had large truck, marine, industrial gasoline/diesel engines that were rugged and long lived. They were also heavy. The US didn't want to spend the money to develop a special tank engine when they were only ordering small numbers of tanks in the late 30s.

It took the US around 3 years to build over 300 of these plus the M1 combat cars. Spending hundreds of thousands of dollars on a "special" tank engine meant fewer tanks purchased when you are ordering under 200 tanks per year.

There are all kinds of things you can do from an engineering perspective. Wither they should be done is subject to question.

Shortround6 said:

It took the US around 3 years to build over 300 of these plus the M1 combat cars. Spending hundreds of thousands of dollars on a "special" tank engine meant fewer tanks purchased when you are ordering under 200 tanks per year.

There are all kinds of things you can do from an engineering perspective. Wither they should be done is subject to question.

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French and Germans and Soviets were making hundreds of tanks a year from 1935-ish, Soviets even more, and the British and Italians were also catching up (granted, the best part of the Italian production were not real tanks).
The reasons of why were stated several times in this thread. Just cutting the tank's length due to having a shorter engine is a major boon. Or, having more volume under armor for the same external dimensions and weight.

tomo pauk said:

The reasons of why were stated several times in this thread. Just cutting the tank's length due to having a shorter engine is a major boon. Or, having more volume under armor for the same external dimensions and weight.

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Those are the advantages. Now balance them with some of the negatives. Brand new engine..........trouble free or troublesome?
Air cooled engines in a steel box with a fan? Please note that the British failed in rather spectacular fashion to figure out how to use a fan (or fans) to cool a liquid cooled engine, twice. People had been using fans on liquid cooled engines in cars and trucks for over 30 years. Granted on the Crusader it was the 'details' like the fan drive system failing/breaking in under 1000 miles (miles not hours) as much the more common "leaks" but still. They had changed the fan/fan drive on the Crusader from the system used on the older Cruiser tanks to use two small fans vs one large one. Maybe they got enough airflow, I don't know. But using an open to the air chain drive (with oil) around sprockets in a tank operating in the dessert turned out to be a bad idea.
Note that often the radiators/ducts and fan/s took up more space than the engine did.
Oddball cylinder layouts are not a guaranteed failure, but they increase the odds.
Many 1930s and early 40s tanks did NOT pay attention to servicing the engine in place.
Do you know why that 3 cylinder engine in the aircraft was located with the single cylinder up the other two splayed out to the sides instead of the other way around(single straight down)? It is because a single cylinder pointed straight down has a bigger chance of oil fouling the spark plug/s.
Granted cutting off the bottom cylinders of a radial solves that problem

If you need more room to work on an engine than a 'proper' 6 cylinder or V-8 (most everything should have been arranged (often wasn't) to be reached from the top with a W or multi bank engine some of the space saving disappears.
Airplane engines don't usually have flywheels. They depend on better balancing, low rpm, vibration dampers, layout and yes, the propeller, acting as a flywheel to average out the power pulses and smooth out the engine. In the 1930s cars and trucks also used heavy construction to help dampen out vibration, or at least help survive it.
A lot of what we know now was unknown in the 1920s and 30s. In the 1950s and 60s nobody was making a 4 cylinder car over 2.2-2.4 liters. The engine vibrated too much to be comfortable. Mitsubishi refined the idea of the balance shaft from Frederick W. Lanchester of 1909 in 1975 (using computers) to make a smooth engine in the 2.4 liter range.
Worrying about crew comfort in a 1930s tank seems a little silly at first. But how well do you want the crew to fight after a 6-8hr road march? or even trying to tend to end of the day maintenance? For a less human perspective, and engine with less vibration doesn't vibrate things loose or crack/break things on the engine or tank.
This is one reason for using 6 cylinder engines in light tanks even though it might be possible to use a large 4. V-6s don't balance as well as inline 6s.
In the 1950s and 60s they knew a lot more about vibration. How to design better rubber mounts, how to design certain parts of the engine, how to balance moving parts better and so on. What they could do in the 1950/60s may be different from what would have been a good design in the 1930s.