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HandWiki. Mercedes-Benz OM138 Engine. Encyclopedia. Available online: (accessed on 19 April 2024).
HandWiki. Mercedes-Benz OM138 Engine. Encyclopedia. Available at: Accessed April 19, 2024.
HandWiki. "Mercedes-Benz OM138 Engine" Encyclopedia, (accessed April 19, 2024).
HandWiki. (2022, October 24). Mercedes-Benz OM138 Engine. In Encyclopedia.
HandWiki. "Mercedes-Benz OM138 Engine." Encyclopedia. Web. 24 October, 2022.
Mercedes-Benz OM138 Engine

The Mercedes-Benz OM 138 is a diesel engine manufactured by Daimler-Benz: 5,719 were produced between 1935 and 1940[A 1] 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 1500[A 2] were also offered with the OM 138 as an alternative to an otto engine.

diesel engine mercedes-benz daimler-benz

1. Impact

Daimler-Benz started the mass production of the six-cylinder-inline-truck-diesel-engine OM 5 in 1928.[1] Due to technical improvements, the rated rotational speed could be increased, which made it possible to use the diesel engine as a car engine. Diesel engines have siginifantly 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 of 10 l / 100 km, while its otto-powered counterpart W 21 has a fuel consumption of 13 l / 100 km. Caused by the lower diesel fuel price compared to petrol, the W 138 was favoured especially by taxi drivers.[2]

Even though the OM 138 was designed a car engine, 3.752 out of 5.719 engines produced were used as truck engines.[3] With the OM 138, the Daimler-Benz car diesel engine production started, however, until the Volkswagen AG introduced its EA 287 in the Golf I, the diesel engine was uncommon as a car engine in Germany.[4]

2. Development

The W 138 was equipped with the OM 138.
Also equipped with the OM 138: The light trucks L 1100 – L 1500.

The truck shown on this photograph has a wood gas generator and accordingly an otto engine.

The development of a passenger car diesel engine began in the autumn of 1933. An inline-six-cylinder truck diesel engine with a displacement of 3,8 l was used. It produced 59 kW (80 PS). This engine however caused vibrations that were too strong for prototype car chassis, so that 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 22 kW (30 PS) and the OM 141, an inline-four-cylinder engine producing 26 kW (35 PS). These engines did not fulfill the requirements. Daimler-Benz decided to use the truck engine again to develop a fitting powerplant for a car.[5] In 1934,[6] the cylinder amount of the truck engine was reduced to four, bore and stroke were kept. Problems such as strong exhaust emissions and a rough engine running were solved, mass production could begin in 1935.[7]

3. Technical Description

The OM 138 is a naturally aspirated and water-cooled inline-four-cylinder diesel engine with precombustion chamber injection, wet sump lubrication and OHV valvetrain. It′s displacement is 2,54 l. The bore is 90 mm, the stroke 100 mm, this gives the OM 138 a high rotational speed of 3000 rpm as a car engine and 2800 rpm as a truck engine.[8] The rated power is 33 kW (45 PS).

3.1 Crankcase

The crankcase of the OM 138 consists of two parts, the lower part with the lower part of the crankshaft bearing and the upper part with the cylinder block and the camshaft. The lower and upper crankcase parts are connected with pin screws on the horizontal crankshaft center. The lower crankcase part is strengthened with ribs and made of a light metal alloy. The flange of the gearbox 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. On it′s 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.

3.2. 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 it′s 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 couter-weights to reduce crankshaft bearing wear. The covers of the crankshaft bearings are mounted with two pin screws each. The flywheel is flanged onto the crankshaft. On the opposite site of the crankshaft, it holds a friction- and vibration damper.

3.3. Cylinder Head

The OM 138 has one cylinder head for all four cylinders. The main part 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 glowplugs are mounted underneath the injection nozzles and are easily accessible as well. On it′s precombustion chamber side, the cylinder head also has the pushrods necessary for the OHV valvetrain. The intake and outlet are at the opposite side; the intake manifold is a part of the cylinder head and located at it′s top.

3.4. 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 center 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 arms. 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 nozzels 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.

3.5. Lubrication System and Auxiliary Devices

The oil pump is mounted in the center 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.

4. Technical Data

  OM 138
Configuration Inline-four-cylinder
Engine type Diesel
Fuel system Precombustion chamber injection
Valvetrain OHV,
1 × inlet, 1 × outlet valve
Bore × Stroke 90 × 100 mm
Displacement 2545 cc
Rated rotational speed car engines: 3000 rpm
truck engines: 2800 rpm
Rated power 33 kW
Torque at rated rotational speed car engines: 105,35 N·m
truck engines: 112,88 N·m
Average working pressure 5,2 bar
Compression ratio 20,5 : 1
Oil 5 l
Mass 300 kg
Power-displacement-ratio (kW : dm3) 13 : 1
Fuel consumption
(In the W 138)
10–11 l Diesel oil[7]


  1. 1927 – Der Vorkammer-Diesel setzt sich durch (in German)!!&rs=0
  2. Dieselmotoren in Mercedes-Benz Personenwagen
  3. null
  4. von Fersen, pp. 38 and 282
  5. Dieselmotoren in Mercedes-Benz Personenwagen – 1934 – Vierzylinder-Diesel für Pkw!&rs=2
  6. Februar 1936: Der Dieselmotor im Personenwagen feiert Premiere
  7. Mercedes-Benz 260 D Pullman-Limousine.
  8. Oswald, pp. 231.
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