90 years of medium-duty commercial vehicle diesel engines from Mercedes-Benz means 90 years of innovations of engines in trucks, buses and special vehicles. The new OM 934/936 engine series has an impressive gallery of ancestors to look back on.

Benz and Daimler vie with each other over the diesel engine

90 years ago, in the autumn of 1922, the first of three prototypes of the OB2 diesel truck engine with prechamber injection ran on a test rig at Benz in Mannheim. That October, Benz installed this four-cylinder engine in a truck chassis with a payload of five tonnes. Tests against a petrol-engine model of similar size produced a clear result: fuel costs fell by 86 percent. Approval for series production was given in the spring of 1923: the world's very first diesel truck engine had an output of 33 kW (45 hp) at 1000 rpm.

In that same year, the Daimler Motor Company (Daimler-Motoren-Gesellschaft, DMG) would develop a diesel truck engine with secondary air injection system at its truck plant in Berlin-Marienfelde. It had an output of 29 kW (40 hp).

Breakthrough for the oil engine

Following the merger between Benz and Daimler in 1926, the prechamber concept was the one to prevail. The first diesel engine to emanate from the joint development effort was the six-cylinder OM 5 engine of 1927, with an output of 55 kW (75 hp) out of a displacement of 8.6 litres. The German term "Ölmotor", or "oil engine", commonly used at that time for a diesel engine, has lived on ever since in the form of the "OM" prefix to the model designation, used in the nomenclature of Mercedes-Benz diesel engines.

Initially diesel engines continued to be offered alongside petrol engines. The breakthrough for the diesel came with the engine in the Mercedes-Benz Lo 2000 of 1932: this was the first light-duty truck to feature a diesel engine as standard. The new OM 59 six-cylinder engine had an output of 40 kW (55 hp) from a displacement of 3.8 litres.

The career of the 300 series

From 1949 on, the 300 series would go on to become a legend among diesel engines. Its story actually dates back to 1940, when development of the OM 301/302 series with four and six cylinders began. However work was stopped by the authorities, and only recommenced in the second half of the 1940s, after the end of the war.

New terms dictated that the engines would have to be built on the machines that had been used by Daimler-Benz AG in Mannheim from 1944 onwards to produce the Mercedes-Benz L 701/Opel Blitz that was manufactured there under licence. The new OM 312 of 1949 therefore had the same unequal cylinder spacing of that engine and different dimensions for crankcase and cylinder head. The bore of 90 mm corresponded to that of the old petrol engine, while the stroke had grown from 95 to 120 mm. The OM 312 six-cylinder engine extracted a full 66 kW (90 hp) from a new displacement of 4.6 rather than 3.6 litres.

1954: first turbodiesel to bear the three-pointed star

This is where its unique career took off. In 1954 Mercedes-Benz launched its first turbocharged engine, in the form of the OM 312 A. Conceived originally for the fire services, it had an output of 85 kW (115 hp). The turbo engine did not, however, achieve popularity. Especially since the brand at that point launched the further developed OM 321, the bore of which had grown to 95 mm, giving it a displacement of 5.1 litres and an output of 81 kW (110 hp). Another petrol model also replaced the diesel for export purposes – this time the M 312, also with 81 kW (110 hp).
In 1956, in the shape of the OM 321 AM ("Aufladung mild" = mild turbocharger) came the next generation of turbo engines with 93 kW (126 hp), later 97 kW (132 hp). In 1959 the naturally aspirated engine expanded to become the OM 322, with a displacement of precisely 5.675 l, joined in 1960 by the four-cylinder OM 324.

The next major step for the engines came in 1964 when the four- and six-cylinder engines, as OM 314 and OM 352/OM 352 A, were converted to direct injection. The output rose initially to 124 kW (168 hp). A new petrol variant joined the range for use in military vehicles, followed in 1985 by the M 353 G natural gas version in Brazil.

The development of a successor model, which had already begun, was halted for financial reasons in the early 1980s. In its place, the 300 series would go on to enjoy something of a renaissance from 1983/84 as the OM 364/366. The engines were comprehensively revised. The displacement of the four- and six-cylinder engines rose to 3.97 and 5.96 litres respectively and the output to up to 147 kW (200 hp), later even to 177 kW (240 hp) – more than two and a half times that of the original UM 312 power unit.

After more than two million units had been built, the career of the 300 series finally came to an end in Europe with the introduction of the Euro III emissions standard in 1996, but it would continue to go from strength to strength in other parts of the world. Even in 2009, 60 years after the birth of the OM 312, it was still possible in Brazil to buy a conventional, bonneted L-1620 Classic with the OM 366 LA engine and 155 kW (211 hp). The cylinder spacing and basic structure remained unchanged since 1949.

Milestone in engine technology: the 900 series

The year 1996 was a time of radical change for the truck engines from Mercedes-Benz. The revolution began in the spring of that year with the series 900. Its fully electronic engine management system, and a direct injection system that had individual unit pumps for each cylinder, made it truly impressive. Both features represented a premiere: this was the first time that it had been possible to control the injection for each cylinder, and also that each cylinder could be individually monitored.

The new series offered four- and six-cylinder units from launch. The OM 904 LA, with a displacement of 4.3 l, delivered an output of up to 125 kW (170 hp), while the six-cylinder OM 906 LA, with 6.4 l of displacement, delivered up to 205 kW (279 hp). Turbocharging and charge-air cooling were obligatory on both. A constant maximum torque of 1200 to 1600 rpm was typical – but was just as unexpected on an engine of this size as its three-valve technology.

The 900 series represented a milestone. Over the course of its career it also proved its true potential. While the output of the original engines rose only modestly to a maximum of 130 kW (177 hp) and 210 kW (286 hp), an increase in the bore from 102 to 106 mm and in the stroke from 130 to 136 mm took the engines themselves into new realms. As the OM 924 LA, the enlarged four-cylinder unit delivered 160 kW (218 hp) from a displacement of 4.8 litres. The six-cylinder engine even managed 240 kW (326 hp) from a displacement of 7.2 litres.

In the next stage of their evolution, the engines benefited from the BlueTec technology introduced by Mercedes-Benz in 2004. SCR technology (SCR = Selective Catalytic Reduction) with AdBlue injection made the engines fit for the new Euro IV/V emissions standards. This reduced the level of pollutants in the exhaust gas by up to 80 percent, without any negative impact on power delivery. At the same time operators were able to benefit from noticeably lower fuel consumption.

Right from the beginning, the economic efficiency of the 900 series made it the benchmark in its class. Later this year the one millionth engine from this series will leave the production line – testimony to the pioneering nature of its technology. Now it is time to move on – for the better can always beat the good. With its completely new OM 934/936 series, designed to meet Euro VI, Mercedes-Benz is once again setting a milestone with its medium-duty diesel engines.