1923: Go-ahead for the OB 2 prechamber diesel

Mar 24, 2003
  • Prechamber combustion principle replaces complicated air injection
  • Thorough testing of concept in farm tractors and motorized ploughs
  • Enormous improvement in efficiency over benzene engines
Rudolf Diesel did not live to see the advent of his invention in the motor vehicle, but he did foresee the use of the diesel engine in vehicles. “I am firmly convinced,” he wrote in 1913, the year of his death, “that the automotive engine will come, and I will then consider my lifework to be complete.” Though the brilliant design engineer Prosper L’O range had already developed the prechamber combustion principle for Benz & Cie. by that time, it was to take more than ten additional years until a truck diesel based on it would reach the stage where it was ready for production.
Rated speed one thousand revs
The decision was taken on April 14, 1923, and called into existence a first series, exactly ten units, of the OB 2 automotive diesel engine. The dry technical data of the first Benz automotive diesel hardly gives an inkling of the epoch-making revolution which took its starting point here: four cylinders, each with a bore of 125 millimeters and a stroke of 180 millimeters, and 45 to 50 hp (33 to 37 kW) at a rated speed of 1,000 rpm. In those days that more than sufficed to be called “high-speed”, and it was an achievement that had called for a great deal of tricky work on the originally rather phlegmatic diesel engine.
The spates of relevant patents filed by Benz & Cie. over the long years until the start of production indicate the great effort invested in the pursuit of the automotive diesel engine by the shops in Mannheim from the outset of work in 1909. But there were also interruptions. The First World War, for example, completely shelved development for four years.
Brown coal tar oil sloshing in the tank
On the other hand, the decision to build the OB 2 was quickly translated into action. It then took no more than four months until these units were ready for installation around late August 1923. And once again the Benz people did not dither about but swiftly fitted the engines into five-ton chain-drive chassis. They set out from Gaggenau on September 10, 1923, for a first test drive in hilly country, with brown coal tar oil splashing against the walls of the tanks. Afterwards, they were able to sum up with satisfaction that fuel consumption was “25 percent less than in benzene engines”.
This was music to the ear in that period, as fuels like brown coal tar oil cost far less than highly taxed benzene. The new OB 2 engine was not picky in the choice of its food anyway. “Gas oil, kerosene, Texas oil and yellow or brown paraffin oil” were listed as possible alternatives in a publication sent out by the management of the Mannheim factory already in mid-August of 1923 to put the sales outlets in the picture about the new engine.
Fuel costs cut by 86 percent
But for the time being the factory wisely and coolly kept the sensational news for itself. Testing manager Kurt Eltze waited until December 20, 1923, before presenting the following impressive results to the Mannheim chapter of the Association of German Engineers: “A road test with five fully laden five-ton trucks over 103 kilometers of demanding, hilly terrain under absolutely identical conditions showed a total consumption of 40.66 kilograms for the benzene-powered vehicle and 29.95 kilograms for the diesel truck, i.e., savings of 32 percent in terms of fuel weight and savings of 86 percent of fuel cost.” The response of the experts was resounding.
But originally the motivation for this breakthrough was entirely different. As early as in 1909 the Mannheim works decided to take up diesel engine design once Diesel’s basic patent of 1892 had expired in 1907 and after the stationary diesel engines of that period had shown themselves to be stiff competition for the Benz-made stationary suction-gas engines. Apart from the classic diesel engine, Prosper L’Orange, head of engine design, immediately set his sights on small, lightweight diesel engines which, with speeds of around 400 rpm, would be suitable for small-scale industry and transportable machinery. A diesel suitable for vehicles appeared to be still far off, particularly since a lot of people, including Rudolf Diesel himself, had found this too hard a nut to crack. The necessary substitute for the unwieldy air injection method simply did not want to be discovered yet.
Prechamber combustion principle as key to success
Mainly two patents smoothed the way for the diesel which in the end was to take it into the Gaggenau five-ton 5 K 3 chassis. Back in 1909 Prosper L’Orange hit upon the electrifying idea of equipping the engine not with air injection but with a specially shaped combustion chamber featuring a hemispheric head – the so-called prechamber was born. It took up about one fifth of the compression space and was designed to ensure gradual combustion instead of explosive combustion (as, for example, in the afterchamber engine favored by Deutz). In the summer of 1909 the method already passed the first crucial test: for eight days and nights the completely newly designed unit (160 millimeter bore, 240 millimeter stroke) readily ran on the test stand and achieved the remarkably low consumption of 245 grams of fuel per horsepower and hour.
For several reasons, work on the new concept ceased until Benz obtained the second basic patent for the coming diesel vehicle engine ten years later. First, it had to take second place to the reorganization of the engine manufacturing department, a task soon entrusted to Prosper L’Orange. Beginning in 1912 L’O range even took a seat on the company’s management board, and to top things off, had to report for military duty on August 4, 1914.
Getting it right
But immediately after war’s end the brilliant designer again got down to developing the prechamber engine. The most important innovation issuing from the work was the so-called funnel patent, which allowed L’Orange to redesign the transition from precombustion chamber to main combustion chamber as well as to eliminate the coking of the fuel that had been common until then and was just as dangerous as it was harmful. The ignition element was now inserted in the water-cooled cylinder head so that only its inside came in contact with combustion gases; it thus served solely to atomize the fuel. Ignition itself was brought about solely by compression to 35 or 40 atmospheres. The grand result: it made the engine operate “with reliable ignition and good combustion at all loads”, according to the inventor.
Parting of the ways
In diesel engine construction the trend now went in two directions. At the one end we have the classic branch, with diesel units for industrial drives of all types, in the output range of approximately 15 hp per cylinder and normally restricted to speeds around 400 rpm. At the other end, the manufacture of high-speed diesel engines for tractors and trucks began to emerge.
Benz concentrated on advancing the lightweight, high-speed prechamber diesel and sold off the stationary engine manufacturing department in April 1922. Its new name was “Motorenwerke vormals Benz, Abteilung stationärer Motorenbau AG, Mannheim” (engine factory, formerly Benz, stationary engine production department) – MWM for short. A special paragraph in the contract limited the activities of this new company to “ ready-to-run engines with weights of over 25 kilograms per horsepower”, meaning that automotive diesels were taboo to MWM: these units were too heavy to come under consideration for vehicle use.
But together with the stationary engines, Benz & Cie. lost, of all people, the inventor and persistent refiner of the prechamber combustion principle: Prosper L’Orange left Benz and pursued a career in the management of MWM. Benz senior engineer Kurt Eltze took the further fate of the prechamber engine into his energetic hands. The circumstances from which he had to start out were in good order, because shortly after the funnel patent had been issued L’Orange had suggested, for the present, putting the new engine on the market as a cheap auxiliary engine for small-scale industry and agriculture. Manufacture of the new two-cylinder prechamber engine as stationary and marine engine commenced immediately.
A two-cylinder in vehicle trim
Just after hiving off the stationary engine division, in June 1922 Benz put out a first trial series of the said two-cylinder for farm tractors and undertook extensive tests not only in tractors but also in trucks. 800 rpm was its rated speed, the bore and stroke dimensions were 135 and 200 millimeters, respectively. The two-cylinder enjoyed a brief but remarkable career: Benz deemed it ready for production in March 1923 and launched a series of 100 units for farm tractors. By the end of 1924, 36 of these two-cylinders had been installed in tractors and motorized ploughs and sold. It continued to be built until 1931, and Benz & Cie. sold a total of 1,188.
Four cylinders for five tons
But in a way this success was denied the proper recognition by the public because the new truck diesel of 1923 simply outshone everything. As soon as he had registered the trial tractor-engine series in June 1922, Benz set about the great task, designing a new four-cylinder based on a bore of 125 millimeters and stroke of 180 millimeters and intended to develop an output of 45 to 50 hp (33 to 37 kW) at 1,000 revs. As early as in September 1922 the first of the total three prototypes constructed was running on the test stand.
In late October of the same year, all three units found their intended place as driving force under the hood of the Gaggenau 5 K 3 chassis, which was designed for a payload of five tons and now was used for the “thorough in-service testing” of the new diesel heart. The new OB 2 engine gave such a good account of itself that the final decision to produce it could already be made six months later.
That Benz & Cie. had scored a great success with these prechamber engines was respectfully attested to them later by no less an authority than Gasmotorenfabrik Deutz, who knew possibly more about the complicated subject than any other enterprise: “The great simplification made possible by the compressorless diesel method, that is to say, the diesel with airless injection, opened the way to crude oil use in vehicle operation,” was the verdict of a 1928 publication.
Robert Bosch enters the picture
But it would take a while until the new concept of 1923 prevailed on a broad scale. A major role was played by the very person who functioned as first buyer of a diesel-powered five-tonner from Benz: Robert Bosch in Stuttgart pricked up his ears and, right away, on July 7, 1924, ordered the new five-ton diesel truck, which Benz then delivered in September.
Bosch’s curiosity was no accident. In late 1922 he had decided to begin producing injection pumps. The ready-to-drive delivery from Gaggenau opened the design engineer’s eyes in more than one respect: The Benz diesel truck, Bosch recalled later, showed “how justified our work up to then had been, and the sources of error which could crop up.” And further: “But above all we were encouraged to continue our endeavor.” From which, ultimately, the diesel engine itself in turn profited. For the big diesel boom did not begin until Bosch had systematically improved the Acro-Lang injection pump and, in 1927, finally got ready to conquer the world market with his new Bosch injection pump.
Competition from the partner
But from the beginning, the prechamber diesel from Mannheim faced sharp competition from the air-injection diesel of Daimler-Motoren-Gesellschaft (DMG) developed at Berlin-Marienfelde. Its fathers were just as convinced of their construction as the Benz people were of their prechamber engine. Early on, DMG had managed to win high acclaim with submarine diesels featuring air injection and had set about transferring the compression ignition process to vehicle engines as well. For instance, in 1913 the factory in Marienfelde built its first FM diesel engines which employed a pressure reservoir technique which was roughly equivalent to the direct injection of later years.
After 1918, work continued on the automotive diesel engine in Marienfelde, but then concentrated on the original air injection principle. A four-cylinder 40 hp (29 kW) engine emerged which likewise had a speed of 1,000 rpm and already in 1923 was able to impressively demonstrate its practicality in numerous test drives. For example, between September 20 and 30, 1923, a Daimler truck powered by this engine covered the distance from Berlin to Stuttgart and back without complaint.
Sharp competition until 1925
But it was clear that only one of the two systems would be accepted in the long run: In preparation for the subsequent merger, on May 1, 1924, Benz & Cie. and DMG concluded a combine agreement which greatly restricted the latitude for separate design work by Mannheim and Berlin in favor of a central engineering office. What direction the work there should take was the subject of intense controversy well into 1925 and slowed down development. Until 1926 the new two- and four-cylinder automotive diesel engines from Benz were unable to get beyond the double-digit sales figure range. But all together, these early automotive diesel engines, produced until 1928, did achieve the considerable volume of 631 sold units before they finally had to give way to the six-cylinder OM 5, which the freshly merged Daimler-Benz AG had newly developed beginning in 1926.
Decades of dominance
This 75 hp (55 kW) truck engine with a displacement of 8.6 liters premiered in May 1927 at the motor show in Cologne. As another compressorless diesel, it left no doubt as to which design principle had prevailed at Daimler-Benz: For plausible reasons the choice had fallen on the prechamber engine developed at Benz & Cie. Its biggest advantages over the air-injection design fielded by Berlin were: simpler design and better combustion characteristics.
Furnished with these assets the prechamber combustion principle asserted itself for decades in commercial vehicle diesel engines. The first direct-injection units which then gradually supplanted the prechamber engine in heavy-duty trucks at Daimler-Benz did not make their appearance until the 1960s. And in the van, the prechamber combustion concept developed by Prosper L'Orange even survived almost to the end of the twentieth century.