Mercedes-Benz OM 138

The Mercedes-Benz OM 138 is a diesel engine produced by Daimler-Benz from 1935 to 1940. It is the first diesel engine especially developed and made for a passenger car. The first vehicle that makes use of the OM 138 is the Mercedes-Benz W 138, which was produced 1967 times.^[1] Light Mercedes-Benz trucks were also offered with the OM 138 instead of an otto engine.^[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.^[3] In 1934,^[2] 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.^[1]

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 min⁻¹. The rated power of 33 kW (45 PS) is reached at this speed.

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 strenghened with ribs and made of a light metal alloy. The flange of the gearbox is casted 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 casted 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 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.

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.

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.

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.

Technical data

References

1. 1 2 3 Mercedes-Benz 260 D Pullman-Limousine.
2. 1 2 Februar 1936: Der Dieselmotor im Personenwagen feiert Premiere
3. ↑ Dieselmotoren in Mercedes-Benz Personenwagen – 1934 – Vierzylinder-Diesel für Pkw

Bibliography

• H. Kremser: Der Aufbau schnellaufender Verbrennungskraftmaschinen für Kraftfahrzeuge und Triebwagen, Springer-Verlag, Wien 1942, ISBN 978-3-7091-5016-0, pp. 125–130.
• Werner Oswald: Mercedes-Benz Personenwagen 1886-1945. Band 1. Motorbuch Verlag Stuttgart, 2001, ISBN 3-613-02167-6.

External links

• Photograph of the engine
• Photograph of the engine
• Engineering drawing of the OM 138