Oil instead of diesel
German laboratory
By the fall of 1944, Germany's fuel supply was in a state that could, without exaggeration, be described as catastrophic. The loss of the Romanian oil fields at Ploiești in August deprived the German war machine of up to a third of all oil. From May of that year, the Allied strategic "oil campaign" aviation The Germans systematically disabled a dozen and a half of the largest synthetic fuel plants—bombers visited Leuna, Böhlen, Zeitz, and other coal hydrogenation centers with enviable regularity. Gasoline and diesel fuel were rapidly becoming scarce resources in the Third Reich.
Under these conditions, any agency responsible for fuel supplies was forced to seek unconventional, almost desperate, solutions. One such idea, which at first glance seems like an engineering gamble, was to fuel combat and transport vehicles with unrefined crude oil. However, it wasn't entirely crazy: diesel engines are theoretically capable of processing a wide range of fuel fractions, from light gas oils to heavy fuel oils. This principle, in fact, was the basis of Rudolf Diesel's original design, whose first engine successfully ran on peanut oil in the late 19th century. The question was whether modern high-speed military diesel engines could withstand exposure to crude oil—a substance far more aggressive and unpredictable than vegetable oil.
The top photo shows a clogged Solex oil filter. The filler is made of viscose/cellulose wool. The photo below shows a tested Bosch filter with a felt substitute.
The tests were assigned to the Armored Vehicle and Automotive Testing Station, located at the famous Kummersdorf proving grounds near Berlin. The test material was oil from the Vienna Basin—more specifically, from fields near the Austrian town of Zistersdorf in Lower Austria. By 1944, this region remained one of the few truly accessible sources after the loss of Romanian reserves.
Bosch fuel filters clogged with paraffin. Paper filter on top, felt filter below.
The testers focused on the weakest link—the fuel filter. On a special rig, oil was forced through replaceable elements under controlled pressure, and a pressure gauge recorded the increase in pressure drop as contamination increased, allowing for a quantitative comparison of the performance of different designs. They used German models (a Bosch pleated paper filter and a felt filter made of pressed plates), as well as captured ones: from the American Sherman (a General Motors AC slot-type wash filter and mesh filter), from the T-34 and KV-1 (a Soviet two-stage system consisting of a wire spiral prefilter and a fine mesh filter in front of the pump), and even a specialized Solex filter with a viscose fiber filler, structurally more reminiscent of an oil filter.
An experimental setup for testing a filter using crude oil. The setup includes: a drive shaft (Antriebswelle) — rotates the mechanism. An angular drive (Winkeltrieb) — transmits rotation to the pump. An oil pump (Ölpumpe) — draws oil from the reservoir. A reservoir with an oil level indicator (Ölstand). A pressure regulating valve (Druck-Regulierventil).
A pressure gauge (Manometer) — for monitoring system pressure. A filter (Filter) with a bypass valve (0,85 atm for Bosch filters). A bypass line (Oberlaufleitung) — for Bosch filters. A container with filtered oil (gefiltertes Öl)
Captured fuel filters that failed as a result of testing with Austrian oil. Above - from tank "Sherman." Below are from the T-34 and KV-1.
The expected breakthrough did not materialize. All filters proved powerless against the oil. The main enemy was not mechanical impurities, but "soft paraffins"—high-molecular hydrocarbon compounds that precipitate as a semi-solid sediment on the cold surface of the filter element. In the photographs taken by technical staff for the final report, all the elements looked depressingly similar: a dense, shiny, oily, dark deposit, resembling an unnatural mixture of carbon deposits and hardened wax.
The Bosch paper filter failed the fastest, retaining up to a third of a liter of unfiltered oil per cycle. Captured Soviet filters, originally designed for low-quality fuel and harsh frontline conditions, demonstrated better resistance to clogging thanks to their separate two-stage design and larger filter surface area. The American slotted AC was also designed for repeated cleaning without element replacement and performed relatively well.
Above are failed German Filtermasse Pollopas filters. Below is residue on the filter paper with radial folds. This is the result of filtering half a liter of petroleum product—you can see how much contaminants and deposits have accumulated in the center of the filter.
Physical and chemical studies of the oil were conducted simultaneously. The pour point of Vienna crude was around -15 to -20°C, but at +10°C, the viscosity increased so significantly that the high-pressure fuel pump could not ensure uniform delivery. Short rig runs on single-cylinder engines confirmed that combustion in the diesel cycle is fundamentally possible only with meticulous pre-filtration, but this quickly leads to injector coking and intensive deposits on the valves—notably faster than with standard diesel fuel.
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Despite the fundamental difficulties identified, the Germans proceeded with a full-scale operational test. Since Wehrmacht tanks used carburetor engines, they used Mercedes-Benz L4500 and Büssing-NAG 4500 diesel trucks—workhorses of the army's rear, produced in thousands—as test subjects. The vehicles differed in their high-pressure fuel pumps (Bosch and Deckel, respectively), allowing for an objective assessment of the impact of these design differences on resistance to low-quality fuel. The trucks were driven on crude oil for several months, with a total mileage of 700 to 5500 km—modest by military standards, but sufficient to identify systemic problems.
The first problem surfaced even before the engine started: the supplied oil had to be run twice through a makeshift filter made of gauze stretched over a wire frame before each refueling. In combat, such a procedure was unthinkable. But even this didn't help: the paper filter failed every 600–700 km, and the simple protective screen of the fuel line sight glass clogged even more frequently—sometimes every 20–30 km, meaning several times a day.
Mercedes Benz L4500
The drivers showed ingenuity, sometimes bordering on outright violation of regulations. One crew installed a reserve tank of high-quality diesel fuel on the roof of the cab to warm up the engine after an overnight layover. The idea worked until it was discovered that the return line led to the main tank, and the precious diesel quickly mixed with the oil, rendering the whole idea useless. Engineers were forced to modify the fuel system on the spot, adding a separate valve to the return line and replacing the standard strainer with a larger, experimental Mann-Hummel filter, which increased the service interval to approximately 230 km.
A telling incident occurred with another Mercedes-Benz L4500: the driver, without notifying the test supervisor, removed a clogged filter and drove for approximately 1500 km without any filtration, including one long-haul trip of over 300 km. Contrary to expectations, the engine failed, and upon subsequent inspection, the injectors remained pristine, showing no signs of coking. This result puzzled the engineers rather than pleased them: it clearly demonstrated that the quality variability between different batches of supplied oil was critically large, making any general conclusions unreliable.
Büssing-NAG 4500
Things were noticeably worse on two Büssings. On the first, after 1800 km, the engine began running rough and smoking heavily. A disassembly revealed severe coking of the injectors, requiring regrinding of the needles. The second truck suffered a completely different problem: the RPMs spontaneously increased at idle, and dissection of the high-pressure pump revealed a twisted plunger element in one of the cylinders—its metering edge was incorrectly positioned for the increased fuel flow. The cause could not be definitively determined: abrasive wear from contaminants or a hidden manufacturing defect were attributed. This truck was soon withdrawn from testing and converted to a wood gas generator—another surrogate technology from wartime Germany, which Kummersdorf engineers were turning to in parallel with their crude oil experiments.
Following field testing, program management issued a final negative verdict: unrefined Vienna Basin crude oil was deemed unsuitable as a full-fledged diesel fuel replacement. The reasons for this were excessive contamination of the supplied feedstock, a significant increase in viscosity even at above-zero temperatures, and an unacceptably high failure rate of filter elements, which was incompatible with the pace of frontline operation.
The tenacity of the doomed
The Germans, being Germans, wouldn't have been so if they hadn't pursued the idea to its conclusion. When the Wehrmacht's fuel situation became desperate in early 1945, the alternative fuel program was elevated to the level of the Reich Ministry of Armaments and War Production, which had been headed by Albert Speer since 1942. Within the ministry, a specialized "Fifth Research Circle" operated, focusing on fuels, oils, and synthetic materials; its representative, engineer Bockemüller, became the key coordinator of the program, working closely with specialists from Daimler-Benz.
The list of test grades was expanded. The program included crude oil from the Nienhagen field in northern Germany, geologically close to the country's oldest oil field, the Wietze field, which had been in production since the late 19th century. Engineers hoped that the northern German oil would be less waxy than the Viennese, which would theoretically make it more suitable for direct use in diesel engines. They also tested so-called "master oil"—a product of the thermal processing of bituminous shale from the Swabian Alb in Württemberg, one of the many responses the German chemical industry had made to the fuel shortage. Unlike depleting oil wells, shale deposits were a virtually unlimited resource, although their processing remained complex and expensive, and the quality of the final product was disappointingly low. The tests were carried out on a 12-cylinder Tatra 103 diesel engine with a power of 220 hp, the filter of which consistently became clogged after 800–900 km of travel.
The Germans tried to feed the Tatra 103 diesel with oil.
There were many plans, but the result remained unchanged. After a series of failures, it was decided to shift the emphasis from chaotic field runs to strictly controlled rig tests using a uniform methodology: break-in tests on high-quality fuel, switching to oil with precise recording of the filter clogging point, plotting a full-load curve, and then two ten-hour runs at nominal conditions with controlled intake air temperature (no higher than 20°C) and oil temperature (no higher than 90°C). However, by early 1945, German specialists bitterly acknowledged that even crude oil, considered an emergency substitute, was rapidly becoming a scarce resource. A few months later, all work in Kummersdorf was halted for obvious reasons. The Third Reich was in its death throes, and engineering research gave way to the chaos of final military collapse.
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