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The Medium Tank M4E1 was an experimental version of the Sherman that used a diesel version of the Wright G200 Cyclone air-cooled radial engine.
The original Wright Cyclone was a petrol powered engine, but the Caterpillar Tractor Company managed to modify the engine so it could also use diesel or even crude oil. The modified engine was designated the Caterpillar D200A and used a mix of Wright and Caterpillar components, with the pistons, cylinder heads, crankcase, fuel injection system and lubrication coming from Caterpillar. The new engine also had a transfer case that increased the rotational speed of the drive shaft, and also lowered it. One of the reasons for the high profile of the M4 Sherman was the high position of the crankshaft in the original design. The D200A could produce 450hp at 2,000rpm.
In 1942 the Ordnance Department authorised the production of 28 D200As, of which twenty were to be installed in the M4. In November 1942 the M4 with D200A was given the designation M4E1.
The M4E1 was based on the M4A4, which had a longer hull to make room for its Chrysler multi-bank engine. A series of standard M4A4s were delivered to Caterpillar. The new engine was still slightly too large for the engine compartment, and so a larger square bulge had to be installed on the rear deck, replacing the narrow rectangular bulge on the M4A4. The M4E1s had a mix of the three piece and single piece nose.
The first M4E1 was ready in December 1942, and underwent tests at the Caterpillar Proving Grounds. The second went to Fort Knox in January 1943. The third went to the General Motors Proving Ground. The fourth went to Fort Knox in May 1943.
The tests at Fort Knox revealed problems with the standard clutch, which had to be replaced with a Lipe clutch. There was also a problem where the pistons scored the cylinder walls, which needed extra work to fix. The gear box in the single piece nose also turned out to be less reliable than the earlier model, and the specification had to be improved. Tests with 72 and 80 octane petrol proved that the new engine worked on those fuels, as well as on diesel.
These early tests suggested that the D200A engine was a very promising design. It was accepted for larger scale production as the Ordnance Engine RD1820, and 775 were ordered for use in the Medium Tank M4A6. This model of the Sherman entered production at the Detroit Tank Arsenal, and the first was completed on 28 October 1943. Production ended after only 75 of the 775 had been completed, mainly because the US Army decided to concentrate on petrol powered tanks.
The engines of the M4E1 and M4A6 weren't interchangeable, and so the M4E1 was withdrawn in March 1944. Fort Knox kept two for further tests with petrol fuel, and one remained at the Aberdeen Proving Grounds, but the rest were scrapped or expended as targets.