1942 Caterpillar RD-1820 Radial Diesel

In today’s world, a Caterpillar diesel in a tank seems like a naturally good idea. In World War II, however, diesel-powered tanks were not common. Certainly not with American ground forces. There are many reasons why and we’ll touch on them here a little, before highlighting an engine developed to dieselize the armored forces.

Gassy Grunts

The primary reason so few diesel tanks were used in WWII is standardization. The American military and it’s supply infrastructure was set up on gasoline, as was just about all motor transport in the U.S. Diesels just did not have a large foothold in U.S. transportation systems back then and diesel technology was only just evolved enough to start making a foothold. Steam still ruled ship and rail transportation and the vast majority of trucks, even the biggest ones, were still powered by gasoline.

One of the big reasons the United Stated did so well in WWII was that we became masters at logistics, standardization and asset allocation. Speaking just of tanks, it was certainly possible to push the research and develop suitable diesels for tanks. That was done as you will read here but after some very good gasoline engines were developed, the very practical choice made to not allocate many resources to a changeover and keep the logistics running smoothly.

The forward facing part of the engine again with the cooling fan, flywheel and part of the clutch exposed. As with the other rear view of the engine, this is a test engine, so the details were not exactly like the production engine.

Some of you may be shouting, “But gasoline powered tanks can burn!” That is true, but if you look at the true causes of tanks burning after being hit, you will find it was mostly the ammunition propellant burning. Midway in the war, our tanks were equipped with wet ammo stowage and tanks so equipped seldom burned when hit. Of the nations fighting in WWII, the Soviet Union was the largest user of diesel tanks, with nearly all of their medium and heavy tanks so equipped. Almost all of Japan’s tanks were diesel and many Italian tanks used diesels, but neither of those nations produced tanks any better than marginal at best and their armies operated mostly in warn climates. With a few exceptions, virtually all German tanks were gasoline powered That may seem odd since Germany was pretty close to being at the forefront of diesel technology, but their choice was a deliberate one, just like ours.

Finally, one of the bigger technical issues with diesels in the European theater was cold weather. Gasoline engines started in subzero weather with minimal effort (relative to the era) and the diesels of those days didn’t. Also, the power to weight and package size was an issue. The gasoline engines of the day produced more power and torque for their weight and size.

With the cooling fan removed, you can see the nine individual injection pumps that operated from a radial cam on the crankcase. The RD-1820 was indirect-injected with a removable prechamber. The injection pump supplied fuel to what Cat called a fuel injection valve, which appears to be little more than a check valve and spray nozzle into the precombustion chamber. We looked at one of the repair manuals for the engine (TM 9-1756A) but the injection pressure wasn’t listed in that one. Most likely it was low by today’s standards.

Tanks and Planes

As the U.S. entered the war, our tanks, both light and medium, were powered by aircraft engines. Radial, air-cooled aircraft engines! In fact, both light and medium American tanks were designed around those radial engines just before the war began. There was good justification for this. First, aircraft engines were proven, the factories ramped up for production and they developed a lot of power and low-end torque in relatively light (if awkwardly shaped for a tank) packages. They were very reliable (had to be in aircraft) and with air cooling, many of the ancillary needs were simplified.

There rearward part of the engine, accessed by panels at the back of the tank, shows the accessory drive, 165 GPM fuel transfer pump and under all that, the supercharger. Yes, the Cat radial had a centrifugal supercharger driven at 10 times engine speed. We couldn’t find a boost spec for it. Again, this doesn’t represent a production engine as mounted in a tank.

Our early light tanks, the M3 Stuart series, used a 7-cylinder, 668 cubic inch, Continental W670 engine that made 262 horsepower and 590 lbs-ft. The medium M3 and M4 series (Grant and Sherman respectively) used a Wright-designed, Continental-built 973 cubic inch R975 air cooled radial that made 350-400 net horsepower and 800-940 lbs-ft, depending on the variant. The midwar versions of both tank types were converted to newer and better gasoline powerplants (V8s in both) but radial powered medium tanks remained in production to nearly the end of the war and were in service for a very long time after.

Diesel American Tanks

Bearing in mind what was said above, the only U.S. branch that expressed a need for diesel tanks was the United Stated Marine Corps. The Marines were the nation’s primary amphibious force and their main job in WWII was to take back every island in the Pacific held by the Japanese. The Navy fleets that delivered them had lots of diesel fuel, but gas had to be brought in separately so USMC logistics worked in reverse from the Army. If the Marines could have dieselized every vehicle in their fleet, they’d have been happy. The Navy too, as diesel powered equipment carried in the hold of a ship is less scary than a hold full of vehicles full of gasoline.

This cutaway shows the intricacies of the radial engine. Not a casual rebuild.

Early in the war, the USMC used M3 series tanks with Guiberson radial diesels (See Diesel World, January 2018 issue). The Guiberson was designed to replace the Continental W670 with very few alterations to the tank. The Guibersons worked reasonably well but were in short supply, so it messed with parts and supply logistics and they faded away. Some of those Guiberson-powered light tanks went to allied countries that were more diesel centric… or desperate enough to take anything.

On the medium tank front, Guiberson also designed and built a larger radial diesel, the T-1400 (1402 ci, 350 hp/935 lbs-ft), designed for the M4 medium and a future heavy tank, but wasn’t accepted. Along the same lines, and at nearly the same time, GM designed and built the 6046 power unit, which was a pair of 6-71 Jimmies on one gearbox. The 6046 setup worked very well and more than 10,000 M4A2 Jimmy-powered tanks were built and the USMC used a lot of them. Many went to the British and the Soviets under Lend-Lease. The two Jimmies together delivered 410 horses and 1000 lbs-ft (gross) and the engines could be operated independently if one was damaged or had a mechanical failure.

The final contender in the American WWII diesel tank development is the subject of our story. As with the Guiberson in light tanks, it was thought an air-cooled radial diesel could be easily installed into new hulls designed for radials and even retrofitted later if needed. The aforementioned Guiberson T-1400 was a contender in this but a Caterpillar engine aced it out.

Cat RD-1820/D200A

It started in July of 1941, when Caterpillar was approached to offer input on developing a radial diesel. Not just a diesel but a multifuel engine. This was likely inspired by Guiberson’s engines and the Ordnance Department (who was in charge of tank development) always wanted another bidder and potential supplier. Guiberson was a small outfit as well, so as a bigger company, Cat was viewed as potentially more reliable.

It isn’t clear if the Wright R-1820 “Cyclone” was provided as inspiration by the Ordnance Department or came from Caterpillar but the prototype engine that emerged in January of 1942 was based on it. The initial results were very positive and the project continued. Before 1942 ended, a goodly number of test engines had been built and two were actually powering tanks in field tests. Its original intent was to power a new American heavy tank, something that never came to pass in WWII, but it was found to also fit in the M4 Sherman medium tank hull.

The final variant of the RD-1820 Ordnance Engine (called the D200A by Cat) was a 1823 cubic inch, 9-cylinder radial that made 450 horsepower at 2000 rpm and a maximum of 1470 lbs-ft at 1200 rpm. That’s more than the gas Continental radial made but not as much as the 500 horsepower Ford GAA gas V8 that became known as the best of the Sherman powerplants.

One of the 75 M4A6 models under test by the Ordnance board. It’s one of the extended hulls of composite construction. The nose is a cast piece similar to cast hull Shermans mated the a welded hull common on other models. Even though heavier, the diesel M4A6 unit essentially matched the performance of the gasoline powered units with a bit better fuel economy. There are probably test reports on these tanks somewhere in the bowels of the National Archives. Just remember the idea wasn’t abandoned for lack of merit, just for the sake of practicality and standardization.

The engine was good enough that they designated a special Sharman variant for it, as they did with the Jimmy-powered M4A2. The RD-1820 powered Shermans were designated M4A6. Chrysler built most of the hulls. Because the engine was larger and heavier than the radials or the GAA V8, it was installed into the hull of the M4A4, which was a longer Sherman powered by the monstrous Chrysler A57 multibank engine… a surprisingly successful 30 cylinder radial made from five Chrysler six cylinder flatheads around a central crankcase.

The original order was for 775 M4A6 to be built starting in late 1943. Before many had been built, Ordnance changed their minds. With the war going well overall, the success of the Ford GAA V8, the availability of the M4A2 when diesels were needed and a “KISS” (Keep It Simple Stupid) attitude, it was decided the M4A6 model was redundant. Even after Cat set up a special factory in Decatur, Illinois, to build them, only 120 engines were actually built and put into only 75 tanks.

The tanks were relegated to training and soon faded away. There are no known surviving engines or M4A6 tanks. It’s an idea that could have worked. Multifuel engines became “The Thing” in the Cold War American military of the ‘50s and ‘60s, so the RD-1820 was a sneak peek of that. It wasn’t until the mid-1950s that most American tanks went to diesel power. It took that long to get package size down, the power output up, and the cost low enough to make it practical. Such is progress.

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