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English   The birth of a legend: the 300 engine series - first unveiled in 1949 - is a major advancement
12.05.2009 von admin


Mood of optimism soon dampened

The first prototype engines for the new OM 302 design were up and running in Gaggenau by as early as mid 1941, and in 1942 the Mannheim plant even presented a schedule for series production. Vehicle testing was carried out in both the Opel Blitz and the company’s own 1.5-tonne vehicle, since the intention from the outset was to use this engine also in the Opel Blitz. However, hopes were dashed when the German army stationed in the Caucasus came into possession of petrol supplies and the freezing Russian winter greatly dampened demand for diesel engines.

At any rate, the military authorities failed to give the go-ahead for procurement of 140 engines, which would have enabled the OM 302 to be produced in series. The brand-new six-cylinder diesel engine was suddenly mothballed. Hopes of increasing the load capacity of the company’s own 1.5-tonne vehicle and equipping it with the OM 302 as competition for the Opel Blitz were also dashed when armaments minister Albert Speer decreed on 25 June 1942 that Borgward and Daimler-Benz should now build the Opel Blitz.

The Opel Blitz, on the other hand, was equipped with a 3.6-litre petrol engine and operated with irregular cylinder intervals. In the turmoil of war, however, setting up new production facilities for the Opel Blitz was not easy and production in Mannheim did not get up and running until 1944. This conicided with the moment at which the main production plant for the three-tonne Blitz, the Opel plant in Brandenburg (Havel), was razed to the ground by US air force. It was still too early to switch production to Borgward in Bremen, so the Daimler-Benz Mannheim plant was the only possible production site for the urgently needed, standard three-tonne vehicle.

Opel Blitz a godsend for Mannheim

With considerable foresight, Kissel’s successor, Dr. Wilhelm Haspel, had always insisted that the Mannheim plant should continue to manufacture vehicles. So work on the Opel Blitz came at just the right time, even if plant employees were not particularly happy about the ‘cuckoo’s egg’ that had been laid in their nest (model designation L 701). After the war, however, the only vehicle on which production could restart immediately was the Opel Blitz. A further advantage was that production facilities were paid for by the state – as if by miracle the production line had suffered relatively little damage as a result of aerial bombardment.

Joy at this state of affairs was tempered only by the fact that the contract to produce the Blitz under licence could be terminated after two years. Daimler-Benz therefore faced an impasse. At a meeting on 24 July 1945, Wilhelm Haspel explained: “If our model is to be delivered in two years’ time, then strictly speaking it needs to be ready for production today’. He went on: ‘The issue of the three-tonne vehicle is the most serious we are facing, so we need to make it our specific concern.”

When Opel did actually terminate the contract in mid 1947 it was a bitter blow. No one was sure what drawings or test engines had survived the war unscathed. Not only were the drawings found, however, the sole prototype of the L 312 with its new six-cylinder diesel engine also turned up safe and sound. In contravention of an official order it had not been scrapped, but had instead been converted to operate on wood gas and put to good use in-house.

Adapting the OM 312 takes time and effort

Suddely prospects for finally adapting the OM 302 for use in a company three-tonne vehicle were a little brighter. And by adopting the same irregular cylinder intervals as the Opel petrol engine, the OM 302 became the new OM 312 design. This modification involved equipping the machinery that bored the holes for the spark plugs (in the petrol engine) with different tools to mill the holes for the glow plugs. In addition, the diesel engine, which by definition operated at higher pressures than the petrol engine, was fitted with seven main bearings instead of the four in the petrol engine.

But since the diesel engines were relatively narrow on account of the length of the cylinder block having to extend to the same ceiling as the petrol version, they also had to cope with higher bearing loads and consequently higher oil temperatures. Because of this higher thermal load, an oil-water heat exchanger in the OM 312 was essential. At the same time, the cylinder heads were modified to allow the engine to achieve the desired increase in power rating from 80 hp in the OM 302 to 90 hp for the OM 312. What is more, the latter developed its 90 hp at just 2800/min.

This also necessitated modifications to the pre-combustion chamber and inlet ducts that entered at a steep angle through the valve cap. Although the new OM 312 had the same 90-millimetre bore as the petrol engine, the stroke was increased from 95 to 120 millimetres. This yielded a displacement of 4.6 litres instead of 3.6 litres. Despite this, the engine weighed just 355 kilograms, making it the lightest unit in its class. It was intended to serve as the powre unit in the newly designed three-tonne L 3250, which was fitted with reinforced rear axles and gearbox to accommodate the OM 312’s maximum torque of 28.2 mkg at 1600/min (equivalent to 277 Newton metres).


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