Monday 26 October 2020

How the Wehrmacht's Diesel Stalled

All mass produced German tanks in WWII were equipped with gasoline engines. This fact resulted in a lot of myths that are being repeated to this day. How did Germany end up with a Maybach monopoly on tank engine and is it true that the navy ate up all the diesel fuel? Did German designers ever manage to create a good diesel engine?

Maybach's monopoly

Early medium tanks, such as the Krupp and Rheinmetall Grosstraktors as well as the Neubaufahrzeug used 6-cylinder BMW Va aircraft engines. Its power was sufficient, but the Germans quickly decided to replace it with specialized tank engines.

BMW Va aircraft engine.

It is known that the power of an engine can be calculated by multiplying the engine's torque by its RPM. The BMW Va put out 290 hp at 1400 RPM or 320 hp at 1600 RPM: a high torque at relatively low RPM. In order for the transmission to withstand that torque, it had to be strengthened, which made it heavier. The Germans decided to develop a tank engine that produced the same 300 hp but at twice the RPM. This would allow them to make the transmission lighter. This kind of obsession with weight is not hard to understand. In those years strategic mobility was considered paramount, including the ability to cross bridges. The Neubaufahrzeug ended up too heavy: 23 tons instead of the desired 18.

Maybach rose to the challenge and developed the HL 100 engine that gave 300 hp at 3000 RPM. Later the design was developed into the HL 108 and HL 120 which was used in many German tanks. Zahnradfabrik designed the SFG 75 and SSG 76 gearboxes (the number indicates the torque in kgm). The Neubaufahrzeug used the SFG 280 gearbox. Getting rid of multiple turrets, changing the layout of the hull, and using a specialized tank engine with a lighter transmission allowed the Pz.Kpfw.IV to meet the 18 ton weight limit. 

The 300 hp Maybach HL 100 engine.

Gasoline engines best met the requirement for a compact, light, and high RPM engine. Maybach provided the Wehrmacht with a line of carburettor engines that had a wide variety of transmissions to match. The Ordnance Directorate played a key role, as they put out detailed requirements with specifications for specific engines and transmissions. The companies put together tanks from pre-approved components like a Lego set. This approach led to a Maybach monopoly, which was not broken until the end of the war.

This resulted in a lot of difficulty when moving to diesel engines. It wasn't enough to just develop a diesel engine with comparable weight and dimensions as gasoline engines, it wasn't even enough to develop a matching transmission, but one would also have to break Maybach's monopoly and change the position of the Ordnance Directorate. Indeed, decisive battles were often fought in an office and not on a battlefield.

Was there fuel?

The absence of diesel engines in German tanks is often explained by the specifics of fuel supplies. Allegedly, the navy ate up all of the diesel and the engines on land were powered by synthetic gasoline. This opinion can be heard to this day, even when fuel consumption figures are publicly available.

Let's start with the fact that the Germans synthesized not just gasoline, but also diesel, and in large amounts. For instance, at the peak of production in Q1 of 1944 they obtained 315,000 tons of gasoline, 200,000 tons of diesel, and 222,000 tons of fuel oil. Of that, 138,000 tons of gasoline and 151,000 tons of diesel were made via the Bergius process (hydrogenation). The claim that only gasoline can be synthesized from coal is just silly.

Could it be that the navy consumed all of the synthetic diesel, and so there was none left for land forces? The answer is also no. The civilian sector consumed less fuel every year, but the proportion of diesel fuel increased greatly. In 1939 the average monthly expenditure was 192,000 tons of gasoline and 105,000 tons of diesel, in 1944: 25,000 tons of gasoline and 47,000 tons of diesel. In 1941-44 Germany exported more diesel than gasoline. For instance, in 1943 21,000 tons of automotive gasoline and 24,000 tons of diesel were exported, in 1944 5000 and 9000 tons respectively. Finally, the expenditure of diesel fuel by the military in proportion to gasoline increased, with the two evening out in the end of 1944.

Despite a commonly held opinion, the Germans synthesized large amounts of both gasoline and diesel fuel. "The Effects of Strategic Bombing on the German War Economy: The United States Strategic Bombing Survey" — Overall Economic Effects Division, 1945

These numbers not only shatter the myth that diesel fuel for land vehicles was impossible to obtain, but indicates the opposite. Using diesel in military vehicles would have been preferable as the war drew to its end. There is other evidence to confirm this. After the war Gerd Stieler von Heydekampf, head of the Tank Commission 1943-1945 and tank production in Henschel was interrogated by Allied intelligence and revealed that the army standardized on gasoline engines before the war because it was told that synthetic diesel would be difficult to produce. In 1942 the situation changed, and the army was informed that synthetic diesel would be easier to obtain than gasoline. According to him, the army promoted the development of diesel engines after that revelation.

As we figured out how Maybach's monopoly on tank engines came to be and that the Germans synthesized diesel in adequate amounts, let us examine some German diesel tank engines.

Daimler-Benz diesel engines

Since the Ordnance Directorate pushed Maybach's gasoline engines, it's no wonder that the first German tank with a diesel engine was developed without its influence. In 1937-38 Daimler-Benz developed the new ZW.38 chassis for Pz.Kpfw.III Ausf.E, F, and G tanks based on requirements from the 6th Department of the Ordnance Directorate. The requirements listed a whole array of technical novelties, including, of course, a Maybach gasoline engine paired with a shaftless semiautomatic gearbox. 

The requirements were not fully met and the resulting tank was difficult to produce. When the war began the plans for production were not met and the army returned with a torrent of complaints about breakdowns. This situation was unacceptable for Daimler-Benz, and in October of 1939 it received permission to develop the VK 20.01(D) tank to replace the Pz.Kpfw.III without the Ordnance Directorate's interference.

Trials of the VK 20.01(D) in winter.

By November of 1939 Daimler-Benz had envisioned a tank with a MB 809 diesel engine and a traditional transmission. The MB 809 was developed in several variants. The bigger one 21.7 L in size put out 400 hp at 2200 RPM at a weight of 1250 kg. Another version weighed only 820 kg at a size of 17.5 L, putting out 360 hp at 2400 RPM. This variant was chosen.

The engine design was ready in June of 1940, and the first prototype went through lab trials in February. It was sent for installation into a tank in March. The experimental chassis drove for over 6000 km, but work never progressed past trials. The Germans decided to switch to development of 30 ton tanks in late 1941, and so development of the VK 20.01(D) stopped. The engine was also discarded. This was a bad decision: the MB 809 was lighter, more powerful, and consumed less fuel than the HL 120. It made sense to use it on 20 ton class chassis, which remained in production until the end of the war.

A model of the VK 30.02(D). Looks can be deceiving!

Daimler-Benz began working on the VK 30.02(D) medium tank in the winter of 1941. It is sometimes called a copy of the T-34. Allegedly, it was equipped with a diesel after a study of the V-2. In reality, the VK 30.02(D) was a further development of ideas used on the VK 20.01(D) with only some influence from the Soviet tank. After all, Daimler-Benz was pushing diesel engines, large road wheels, and bogeys on leaf springs since the end of 1939.

The VK 30.02(D) used the MB 507 diesel engine. Daimler-Benz initially promoted it as a universal engine, but the tankers took no interest in it and it was only used in the navy. This diesel was built in two variants. The smaller MB 507 had a volume of 42.3 L and put out a nominal power of 700 hp and 850 hp when pushed to its limit at 2350 RPM. The larger MB 507C at 44.5 L produced 800 hp nominally and 1000 hp at 2400 RPM for a limited time. The 44.5 L engine was installed on the VK 30.02(D), although its maximum power was excessive.

The Daimler-Benz MB 507 engine.

The MB 507C was installed on three Karl Gerat chassis, one of which had a Voith Turbo hydromechanical transmission. Later, MB 503A engines of the other SPGs of this series were replaced with the MB 507C. The BM 507 was also considered as an option for Lowe, Maus, and E-100 superheavy tanks. The second Maus prototype was equipped with the MB 517 engine, a supercharged MB 507 that put out 1200 hp at 2500 RPM. Despite its high power and plenty of experience using it on a tracked chassis, the military showed no enthusiasm for the MB 507 and its variants. As a result, the Germans never obtained a successful mass produced engine for heavy tanks, since issues with the HL 210 and HL 230 were never resolved.

Standard air-cooled diesels

Unlike Daimler-Benz, which specialized in water cooled diesels, Hitler and Ferdinand Porsche, the head of the Tank Commission, considered air cooled engines to be the best type. The Germans needed to fight in varied climate conditions, from African desert to Russian steppes. Air cooled engines had no problems with boiling or freezing water in radiators. It's no coincidence that the air cooled Tiger P was supposed to go to North Africa.

At a meeting of the Tank Commission in July of 1942 Porsche stated that Hitler once again ordered the development of air cooled diesel engines for all types of military vehicles, with production planned to start in 1943. A working committee was soon formed made up of companies including Daimler-Benz, Klöckner-Humboldt-Deutz, Krupp, Maybach, Tatra, Simmering, and Steyr. They were supposed to share their experience and progress on this topic.

An RSO/03 prime mover with a Klöckner-Humboldt-Deutz diesel engine and simplified cab.

The army needed 8 types of engines in total, from the 30 hp one for a Volkswagen to a 1200 hp colossus for superheavy tanks. The idea was to develop the lineup using standardized cylinders, which would make development, production, and repairs much easier. Two standard cylinders with a volume of 1.1 and 2.2 L were first considered, but three were decided upon:
  • 0.8 L giving 13 hp at 2800 RPM
  • 1.25 L giving 20 hp at 2400 RPM
  • 2.3 L giving 30-34 hp at 2200 RPM or 40-45 hp with a turbocharger
Of course, it was impossible to quickly produce a whole lineup of engines in wartime conditions. Existing developments had to be relied on, and thus existing engines with other cylinders were used where possible. Klöckner-Humboldt-Deutz developed the 70 hp 4-cylinder air cooled F4L 514 engine for RSO/03 prime movers. Tatra had an V-12 220 hp Typ 103 engine that was tested in the Pz.Kpfw.38 n.A. tank and also installed in Puma armoured cars. Another Tatra diesel, the 180 hp V-8 Typ 928, was used on the Jadgpanzer 38(t) Starr prototype.

Sla 16 diesel engine with turbochargers and a cooling system installed.

Porsche worked on diesel engines for tanks. A pair of 16-cylinder Typ 180/1 giving a total of 740 hp at 2000 RPM was developed for the Tiger (P2), better known as the VK 45.02(P). An alternative was an X-shaped Typ 180/2 engine with 16 standard 2.3 L cylinders. V-16 and V-18 engines for early Maus tank variants were also developed using the same cylinders.

The same cylinders could be used to develop engines of various configurations, for tall, short, long, or narrow engine compartments. It's worth mentioning that out of five different engines considered for the Maus only one was powered by gasoline, and the MB 507 and a few Porsche diesels were also considered for the Lowe.

Work on the X-16 36.8 L engine made it the furthest. This was a prospective replacement for the unreliable Maybach HL 230, as it fit into the fighting compartments of the Panther and King Tiger tanks. This engine was jointly developed by Zimmering and Porsche and is known as the Sla 16.

The Sla 16 on a test bench at Zimmering.

The Sla 16 worked at lower RPMs than the HL 230, and its driveshaft was located higher, so step-up gearing was added. It allowed the Sla 16 to be used with the regular transmission of the Tiger and Panther. Two turbochargers were powered by exhaust fumes. The cooling fans, oil tank, and radiators were installed directly on the engine. They were removed as a single unit, a technique used on many modern tanks. A heating coil was added for starting the tank in winter, which could heat the oil over the course of 1-2 minutes. The engine was then started with two electric starters. A warm engine could be started by just one.

A one-cylinder model worked on a test bench for 48 hours and put out 47 hp at 2100 RPM. Two experimental engines were then built. One had two large oil radiators and independent power for each of the four fuel pumps. This design proved poor, as it was impossible to attain even fuel intake to reach maximum power, and heavy vibrations developed due to miscalculations in weight distribution. A second prototype was built in early 1945. It had four small oil radiators and a simplified fuel pump system. It got rid of its predecessor's drawbacks and passed trials.

Diagram of the Sla 16 engine in the engine compartment of a Jagdtiger.

The second engine was installed in a Jagdtiger at Nibelungenwerke in April of 1945. The bulkheads of the existing liquid cooled compartments were removed, a new engine deck and rear plate were installed. The fuel capacity was the same as on the production vehicle, but was split between three tanks instead of seven. The diesel powered Jagdtiger went on a few test drives, after which the vehicle and documents on its engine fell into the hands of the USSR.

A pilot batch of 100 engines at the Steyr company instead of the DB 605 was discussed, but this idea went nowhere. First of all, production of the Sla 16 could only be done as a result of reducing production of Tatra Typ 103 engines, which were needed in enormous amounts (more on this later). Additionally, the Tank Commission and 6th Department of the Ordnance Directorate both lobbied for the 800 hp direct injection Maybach HL 234. The Germans didn't manage to set up lab tests with this engine, let alone install it in a tank.

Engines in a crisis

In October of 1944 the Tank Commission decided to only leave three chassis in production: the Jagdpanzer 38(t), Panther, and Tiger II. All other tanks and SPGs would be built on their chassis depending on their weight class. This rationalization came very late, but it would allow the "zoo" of many vehicles with similar characteristics to be simplified, which would also help with production and repairs in the difficult conditions Germany faced at the end of the war.

The Jagdpanzer 38(t) was a modern vehicle, its concept was good, and it was easy to produce, but it had a ton of issues. The supercharged engine was not powerful enough, the transmission was at its limit, the suspension's weight reserve ran out. The Jagdpanzer 38(t) was also poorly suited to production at German factories, even though ending production of the Pz.Kpfw.III and Pz.Kpfw.IV would free up a lot of industrial capacity.

Jagdpanzer 38D with a 75 L/70 gun and a Tatra diesel.

To solve these problems, Alkett quickly designed the new 38D chassis based on the Czech design. It used the Tatra Typ 103 220 hp diesel engine with a Zahnradfabrik AK 5-80 gearbox and a simplified hull with a new suspension. The fuel efficient diesel would stretch the 380 L of fuel to 500 km of driving on a highway or 300 off-road. The 38D chassis would be used to build the Jagdpanzer 38D tank destroyer, Aufklarer 38D reconnaissance tank, Kugelblitz 38D SPAAG, and a number of other experimental vehicles. The Germans were building experimental prototypes of the Jagdpanzer 38D in the spring of 1945, but their fate is unknown and many documents were lost. It's possible that due to a visual resemblance with the Jagdpanzer 38(t) the Allies assumed these vehicles weren't anything special and scrapped them.

The diesel engine was also an option for the Panther and King Tiger. There were other options other than the aforementioned Sla 16. Klöckner-Humboldt-Deutz was working on a 2-stroke V-8 T8 M118 water cooled 800 hp diesel. It was expected to be powerful and simple to produce. Its dimensions were about the same as the HL 230. At a post-war interrogation, Heinrich Kniepkamp stated that this engine was an alternative for the HL 234 on the E-series. MAN and Argus also jointly worked on a 700 hp H-16 LD 220 air cooled diesel. It was seen as a backup in case the Sla 16 ended up unsatisfactory.

Tatra Typ 103 diesel, the heart of the 38D program.

Therefore, at the end of the war the Germans were one step away from moving over to producing tracked vehicles exclusively with air cooled diesel engines. This would have destroyed Maybach's monopoly, and that is why Karl Maybach used all his influence to promote the HL 64 and HL 234 instead of the Tatra Typ 103 and Sla 16. He even sent a letter to the Tank Commission, wondering if the situation with gasoline was so bad that it was necessary to use diesels. The state of affairs at the end of the war managed to buckle Maybach's monopoly, and the Tatra diesel was supposed to become the Wehrmacht's most common tank engine.

Instead of a conclusion

The Germans tried to use aircraft engines on tanks in the early 1930s, but the results were unsatisfactory. They then decided to develop special tank engines with the goal of making compact, high power density, engines with low torque at high RPMs. Maybach delivered a good design with perfect timing and gained a monopoly with the support of the 6th Department of the Ordnance Directorate. This monopoly delayed the transition to diesel engines. Maybach was working on them too, but they achieved very little.

The approach to tank building where the Ordnance Directorate specified the engine and transmission that had to be used in new tanks played a key role. The result was that instead of the tried and tested MB 507 diesel the E-100 was supposed to receive a semi-mythical 1200 hp Maybach engine that was never designed, and the 140 ton experimental chassis used a regular 700 hp HL 230.

The claim that it's impossible to synthesize diesel fuel and the navy's monopoly on diesel fuel are just myths that justified Maybach's monopoly. Germany built over 150,000 diesel powered trucks for the Wehrmacht. That and the fact that there were multiple attempts to install diesel engines in tanks prove otherwise. The claims that the Germans were incapable of designing their own diesel engine or copying the V-2 are also false. As you can see, there were diesel engines of various size and power that were installed and tested in vehicles. This is not even a complete list, for instance there was also the "multi purpose tank" (Mehrzweckpanzer) with an Argus 12LD330H diesel engine, but it's impossible to address them all within one article.

To conclude, let us quote Lieutenant General Erich Schneider, an engineer who combined the experience of his field with the experience as the commander of the 4th Tank Division.
"The idea of installing a diesel engine in a tank caused many arguments in Germany, the country that first created this type of engine. The robust design, higher fuel economy, suitability for multiple kinds of fuel, and lower chance of fire were arguments in favour. The Russians proved with their T-34 tank that the diesel engine is exceptionally suitable for use in tanks. However, while military specialists and leading engine building companies openly spoke in favour of this engine, there were opponents who constantly worked to delay its introduction."

The Germans tried to urgently "dieselfy" their tank forces shortly before their defeat, but nothing came of it. Because of this, the Wehrmacht in popular culture ran on gasoline. 

1 comment:

  1. "Did German designers ever manage to create a good diesel engine?"

    There was a German designer, called Rudolf Diesel, alleged to have designed a Diesel engine.