This thread got me looking at the tanks that France was working on like the
AMX 40, including variations of the
Char G1 with lower height and three man turrets.
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The G1 above looks interesting, but I would ditch the mini turret on top and focus instead on crew optimization, communications and fighting ability. As for an engine, what can we put into this 1939 era beast? There were several variants of the G1, including this turretless one below.
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I have redone and continued the research started by the journalist Stéphane Ferrard who wrote multiple articles on the Char de 20 tonnes and G1 program.
Initially the ARL attempted to use a 3-man 75mm turret on the Char G1s, but the extra weight and the fact that many projects exceeded the limit of 35 tonnes for "easy" rail transport meant that they had to go for a lighter 2-man turret. That said I a suspect that the French overestimated the amount of room needed for 3-man crews, as the turret is quite big and so was the turret ring (1.45m diameter was free for the crew, which is more than the Churchill turret for example).
The MG cupola was present on both the Renault and ARL turret as it was preferred that the MG be operated by the commander-gunner, and it was also fitted for observation and, in the case of the ARL at least, fitted for a hunter-killer system where the rest of the turret would slew in the direction of the cupola, so the commander had a lot of assistance compared to a regular 2-man turret. A simplified version of the ARL turret had a simple fixed cupola with a panoramic periscope on top of the episcopes in the cupola.
Automotively, the G1s were quite different in the end.
The Renault entry used a modified Cletrac controlled differential (conservative) with a new transversely-mounted 350 hp 17.6 L straight-six engine able to run on 65 octane petrol. It was of a more robust construction than the engine in the B1 Bis and didn't run on 72 octane fuel which wouldn't necessarily be available in wartime.
The BDR entry used an electric transmission with 2-speed reduction unit and the Potez 12D flat-12 engine, 350hp for a little over 17L as well, but it was air-cooled so the power losses to cooling were uncertain. The electric transmission, like many other such tanks, crippled the design from a weight-efficiency perspective (37-38 tonnes). It wouldn't have been practical beyond a testbed.
The final Fouga entry used a 4-speed mechanical unit coupled to a 2-speed reduction unit, and some form of clutch/brake steering, but it's not very clear. The engine was a 450 hp Flat-8 of 24L, which I suspect was a cut-down version of the Hispano-Suiza Type 86 railcar engine. A good compromise but the project was released in July 1939 so too late before the cancellation of the G1 project after the outbreak of WW2.
The Lorraine entry used twin Cotal units (essentially Wilson characteristics, but with electromagnetic clutches), coupled to a bulky Panhard V12K4F with an output of 350-400-450hp depending on the document, nothing is definitely confirmed as I have yet to find the provisional description manual. The engine was powerful, but bulky and the installation was even bulkier, which drove the weight up to 36 tonnes. Doable, but not efficient.
The SEAM that you posted at the bottom had a lighter but less proven electric transmission mated to a compact 300hp Hispano-Suiza V12. Was relegated to the role of testbed as it could not be modified for the 75mm turret.
In any case the French went beyond that for tank engines programs by 1939-40. They had compact engines (Talbot V12 and Brandt fuel-injected V12) at 340 hp, a Renault 20L V12 at 420hp, a Talbot 19.9L engine at 450-500hp, and an Aster 2-stroke diesel giving 450 to 500hp. Most to be put in mass production in 1941 or 42. All of them were designed from stratch for tank use and are meant to be sturdy and tolerant of low octane fuel as a result.
Though other countries went for even more powerful engines from 1941 onwards, this was a pretty sound lineup which could have served as the basis for bigger and more powerful types.
