The Mercedes-Benz OM 138 is a

diesel engine The diesel engine, named after the German engineer Rudolf Diesel, is an internal combustion engine in which Combustion, ignition of diesel fuel is caused by the elevated temperature of the air in the cylinder due to Mechanics, mechanical Compr ...

manufactured by Daimler-Benz. In total, 5,719 units were produced between 1935 and 1940.According to Oswald, 1,082 engines were made for the L 1100 and 2,670 were made for the L 1500. Daimler says that 1,967 diesel-powered W 138s were produced. It was the first diesel engine especially developed and made for a passenger car. The first vehicle powered by the OM 138 was the Mercedes-Benz W 138. The light Mercedes-Benz trucks L 1100 and L 1500 as well as the bus O 1500The O 1500 is a variant of the L 1500, however, it has its own model number. were also offered with the OM 138 as an alternative to the standard

Otto engine The Otto engine is a large stationary single-cylinder internal combustion engine, internal combustion four-stroke engine, designed by the German Nicolaus Otto. It was a low-RPM machine, and only fired every other stroke due to the Otto cycle, a ...

Impact

Daimler-Benz started mass production of the six-cylinder inline truck diesel engine OM 5 in 1928. Technical improvements allowed an increase in rated rotational speed, thus allowing more power with lower displacement, which made it possible to use the diesel engine as a car engine. Diesel engines have significantly lower running costs than Otto engines; this was the motivation for the adaption of the diesel engine as a car engine. The W 138 powered by the OM 138 has a

fuel consumption A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but ...

of , whereas its Otto-powered counterpart W 21 has a fuel consumption of . Caused by the lower diesel fuel price compared to petrol, the W 138 was favoured especially by taxi drivers. Even though the OM 138 was designed as a car engine, 3,752 out of 5,719 engines produced were used in trucks. The OM 138 marked the beginning of Daimler-Benz car diesel engine production; however, until Volkswagen AG introduced its EA 827 in the Golf I in the 1970s, the diesel engine was uncommon as a passenger car engine in Germany.

Development

Mercedes-Benz-Museum 2015-01 by-RaBoe 116

The development of a passenger car diesel engine began in the autumn of 1933. Albert Heeß, designer of the Silberpfeil racecar, was the head of development. For the first tests, a straight-six-cylinder truck diesel engine with a displacement of 3.8 L was used. It produced . This engine, however, caused vibrations that were too strong for prototype car chassis, so Daimler-Benz tried to develop a less powerful and smaller diesel engine. Two prototype engines were developed from scratch: The OM 134, a water-cooled inline-three-cylinder engine with a rated power of , and the OM 141, an inline-four-cylinder engine producing . These engines did not fulfill the requirements. Daimler-Benz decided to use the truck engine again to develop a fitting powerplant for a car.Dieselmotoren in Mercedes-Benz Personenwagen1934Vierzylinder-Diesel für Pkw
/ref> In 1934, the cylinder count of the truck engine was reduced to four, and the bore and stroke dimensions were kept. Problems such as heavy exhaust emissions and rough engine running were solved, and mass production could begin in 1935.

Technical description

The OM 138 is a

naturally aspirated A naturally aspirated engine, also known as a normally aspirated engine, and abbreviated to N/A or NA, is an internal combustion engine in which air intake depends solely on atmospheric pressure and does not have forced induction through a turboc ...

and

water-cooled Cooling tower and water discharge of a nuclear power plant Water cooling is a method of heat removal from components and industrial equipment. Evaporative cooling using water is often more efficient than air cooling. Water is inexpensive and no ...

inline-four-cylinder

with precombustion chamber injection,

wet sump Within piston engines, a wet sump is part of a lubrication system whereby the crankcase sump is used as an integral oil reservoir. An alternative system is the dry sump, whereby oil is pumped from a shallow sump into an external reservoir.Wet su ...

lubrication and OHV

valvetrain A valvetrain is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture (or air alone for direct-injected engines) into the combu ...

. Its displacement is 2.54 L. The bore and the stroke is , this gives the OM 138 a high rotational speed of 3000 rpm as a car engine and 2800 rpm as a truck engine. The rated power is .

Crankcase

The

crankcase A crankcase is the housing in a reciprocating engine, piston engine that surrounds the crankshaft. In most modern engines, the crankcase is integrated into the engine block. Two-stroke engines typically use a crankcase-compression design, res ...

of the OM 138 consists of two parts, a ''lower part'' with the lower part of the

crankshaft A crankshaft is a mechanical component used in a reciprocating engine, piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating Shaft (mechanical engineering), shaft containing one or more crankpins, ...

bearing and an ''upper part'' with the cylinder block and the

camshaft A camshaft is a shaft that contains a row of pointed cams in order to convert rotational motion to reciprocating motion. Camshafts are used in piston engines (to operate the intake and exhaust valves), mechanically controlled ignition syst ...

. The lower and upper crankcase parts are connected with pin screws on the horizontal crankshaft centre. The lower crankcase part is strengthened with ribs and made of a light metal alloy. The flange of the

gearbox A transmission (also called a gearbox) is a mechanical device invented by Louis Renault (who founded Renault) which uses a gear set—two or more gears working together—to change the speed, direction of rotation, or torque multiplication/r ...

is cast onto the lower crankcase part. The upper crankcase part is made of grey cast iron, it reaches from the crankshaft to the

cylinder head In a piston engine, the cylinder head sits above the cylinders, forming the roof of the combustion chamber. In sidevalve engines the head is a simple plate of metal containing the spark plugs and possibly heat dissipation fins. In more modern ...

. On its front, the upper crankcase part has a bulge that holds the camshaft, so that the camshaft can be driven by the crankshaft using only two gears. The camshaft is supported in five bearings. For camshaft maintenance, the bulge on the crankcase front has a removable cover. Mountings for the starter motor, the alternator and the injection pump are also cast onto the crankcase. For the lubrication of the crank- and camshaft, the crankcase has an oil pipe drilled into.

Pistons and power transmission

The pistons are made of light metal alloy and have three compression rings as well as one oil ring. They are connected to the crankshaft with I-shape connecting rods made of heat treatable steel. The bearings of the connecting rods are made of a lead bronze alloy and are fixed with a pin. Each connecting rod has a small oil pipe for the lubrication of its bearings. For weight reduction, the connecting rod pins are hollow-drilled. The crankshaft with hardened pins is supported in five bearings and is equipped with counterweights to reduce crankshaft bearing wear. The covers of the crankshaft bearings are mounted with two pin screws each. The

flywheel A flywheel is a mechanical device that uses the conservation of angular momentum to store rotational energy, a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, a ...

is flanged onto the crankshaft. On the opposite side of the crankshaft, it holds a friction- and vibration damper.

Cylinder head

The OM 138 has one cylinder head for all four cylinders. The key element in the cylinder head are the precombustion chambers. They are located in a 45° angle above the combustion chamber and placed in a bulge in the cylinder head. Like other early OM diesel engines, the OM 138 has a sieve for fuel spraying purposes between the main combustion chamber and precombustion chamber. The injection nozzles inject fuel into the precombustion chambers, they are mounted on the cylinder head and can be maintained with ease. The glow plugs are mounted underneath the injection nozzles and are easily accessible as well. On its precombustion chamber side, the cylinder head also has the

pushrod A valvetrain is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture (or air alone for direct-injected engines) into the combu ...

s necessary for the OHV valve train. The intake and outlet are at the opposite side; the

intake manifold An inlet manifold or intake manifold (in American English) is the part of an internal combustion engine that supplies the fuel/air mixture to the cylinder (engine), cylinders. The word ''manifold (engineering), manifold'' comes from the Old Eng ...

is a part of the cylinder head and located at its top.

Valvetrain and fuel system

The camshaft in the crankcase has a flange to hold the camshaft gear. Between this flange and the camshaft gear, the camshaft has another gear that drives the injection pump. In the centre of the camshaft, a third gear drives the oil pump. The overhead valves have double valve springs; each cylinder has one inlet and one outlet valve of the same size. The valves are pushed by tappets, pushrods and

rocker arm A rocker arm is a valvetrain component that typically transfers the motion of a pushrod in an overhead valve engine, overhead valve internal combustion engine to the corresponding intake/exhaust poppet valve, valve. Rocker arms in automobiles are ...

s. The rocker arms, which are supported in bronze bearings, are lubricated by the wet sump lubrication system. They are secured with a horizontal screw each. The fuel is pumped to the injection nozzles by a Bosch ''size A'' injection pump, that is driven by the gear between the flange and camshaft gear on the camshaft. The injection pump has a Hele-Shaw clutch and a pneumatic

governor A governor is an politician, administrative leader and head of a polity or Region#Political regions, political region, in some cases, such as governor-general, governors-general, as the head of a state's official representative. Depending on the ...

Lubrication system and auxiliary devices

The oil pump is mounted in the centre of the engine in the oil sump and flanged onto the crankcase. It has a small oil pipe with a sieve and a funnel to pump the oil from the sump through the oil filter into the main lubrication oil pipe. The governing valve for setting the oil pressure is easily accessible. The water pump, which also holds the fan, is mounted on the cylinder head on the front of the engine. It is driven by a belt that also drives the governor.

Technical data

References

Mercedes-Benz 260 D Pullman-Limousine.
/ref> Februar 1936: Der Dieselmotor im Personenwagen feiert Premiere
/ref> Kremser, p. 125 Kremser, p. 126 Kremser, p. 127 Kremser, p. 128 Kremser, p. 129 Kremser, p. 130 Oswald: ''Mercedes Benz - Lastwagen und Omnibusse 1896-1986.'' pp. 231

Notes

Bibliography

* H. Kremser: ''Der Aufbau schnellaufender Verbrennungskraftmaschinen für Kraftfahrzeuge und Triebwagen'', Springer-Verlag,

Wien Vienna ( ; ; ) is the capital city, capital, List of largest cities in Austria, most populous city, and one of Federal states of Austria, nine federal states of Austria. It is Austria's primate city, with just over two million inhabitants. ...

1942, , pp. 125–130. * Werner Oswald: ''Mercedes-Benz Personenwagen 1886-1945''. Band 1. Motorbuch Verlag, Stuttgart, 2001, . *Werner Oswald: ''Mercedes Benz - Lastwagen und Omnibusse 1896-1986.'' Motorbuch Verlag, Stuttgart 2008, ., pp. 139, 231. * Olaf von Fersen: ''Ein Jahrhundert Automobiltechnik. Personenwagen.'' VDI-Verlag, Düsseldorf 1986, {{ISBN, 3-18-400620-4., pp. 38, 282.

External links

Photograph of the engine

Photograph of the engine

Engineering drawing of the OM 138
Diesel engines by model OM 138 Straight-four engines