| Mercedes-Benz OM 138 | |
|---|---|
 | |
| Overview | |
| Manufacturer | Daimler Benz AG |
| Production | 1935 (1935)–1940 (1940) |
| Layout | |
| Configuration | Naturally aspiratedStraight-4 |
| Displacement | 2.5 L; 155.3 cu in (2,545 cc) |
| Cylinder bore | 90 mm (3.54 in) |
| Piston stroke | 100 mm (3.94 in) |
| Cylinder block material | Grey cast iron |
| Valvetrain | OHV 2 valves per cylinder |
| Compression ratio | 20.5:1 |
| Combustion | |
| Operating principle | Diesel |
| Fuel system | Precombustion chamber injection |
| Fuel type | Diesel oil |
| Oil system | Wet sump |
| Cooling system | Water-cooled |
| Output | |
| Power output | 33 kW (45 PS) |
| Torque output | car engines: 105.35 N⋅m (77.70 lb⋅ft) truck engines: 112.88 N⋅m (83.26 lb⋅ft) |
| Dimensions | |
| Dry weight | 300 kg (661 lb) |
| Chronology | |
| Predecessor | None |
| Successor | OM 636 |
TheMercedes-Benz OM 138 is adiesel engine manufactured by Daimler-Benz. In total, 5,719 units were produced between 1935 and 1940.[1][2][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 theMercedes-Benz W 138.[1] 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 the standardOtto engine.[3]
Daimler-Benz started mass production of the six-cylinder inline truck diesel engine OM 5 in 1928.[4] 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 thanOtto engines; this was the motivation for the adaption of the diesel engine as a car engine. TheW 138 powered by the OM 138 has afuel consumption of 10 L/100 km (28 mpg‑imp; 24 mpg‑US), whereas its Otto-powered counterpartW 21 has a fuel consumption of 13 L/100 km (22 mpg‑imp; 18 mpg‑US). Caused by the lower diesel fuel price compared to petrol, the W 138 was favoured especially by taxi drivers.[5]
Even though the OM 138 was designed as a car engine, 3,752 out of 5,719 engines produced were used in trucks.[A 1] The OM 138 marked the beginning of Daimler-Benz car diesel engine production; however, until Volkswagen AG introduced itsEA 827 in theGolf I in the 1970s, the diesel engine was uncommon as a passenger car engine in Germany.[6]
The development of a passenger car diesel engine began in the autumn of 1933. Albert Heeß, designer of theSilberpfeil racecar,[7] was the head of development.[8] For the first tests, a straight-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 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.[9] In 1934,[3] 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.[1]
The OM 138 is anaturally aspirated andwater-cooled[10] inline-four-cylinderdiesel engine withprecombustion chamber injection,[11]wet sump lubrication[12] andOHVvalvetrain.[11] Its displacement is 2.54 L (155 in3) The bore and the stroke is 90 mm × 100 mm (3.54 in × 3.94 in), this gives the OM 138 a high rotational speed of 3000 rpm as a car engine[13] and 2800 rpm as a truck engine.[14] The rated power is 33 kW (45 PS; 44 hp).
Thecrankcase of the OM 138 consists of two parts, alower part with the lower part of thecrankshaft bearing[12] and anupper part with the cylinder block and thecamshaft.[10] 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 thegearbox is cast onto the lower crankcase part.[12] The upper crankcase part is made ofgrey cast iron, it reaches from the crankshaft to thecylinder head. On its front, the upper crankcase part has a bulge that holds the camshaft,[10] so that the camshaft can be driven by the crankshaft using only two gears.[12] 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.[10]
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. Theflywheel is flanged onto the crankshaft. On the opposite side of the crankshaft, it holds a friction- and vibration damper.[12]
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 thepushrods necessary for the OHV valve train. The intake and outlet are at the opposite side; theintake manifold is a part of the cylinder head and located at its top.[11]
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 theoil pump.[12] 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 androcker 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.[15] The fuel is pumped to the injection nozzles by aBoschsize Ainjection pump, that is driven by the gear between the flange and camshaft gear on the camshaft. The injection pump has aHele-Shaw clutch and a pneumaticgovernor.[12]
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.[12] It is driven by a belt that also drives the governor.[16]
| OM 138 | |
|---|---|
| Configuration | Inline four-cylinder |
| Engine type | Diesel |
| Fuel system | Precombustion chamber injection |
| Valvetrain | OHV: 1 × inlet, 1 × outlet valve |
| Bore × Stroke | 90 mm × 100 mm (3.54 in × 3.94 in) |
| Displacement | 2.5 L; 155.3 cu in (2,545 cc) |
| Rated rotational speed | Car engines: 3000 rpm Truck engines: 2800 rpm |
| Rated power | 33 kW (45 PS) |
| BMEP | 5.2 bar (75 psi) |
| Compression ratio | 20.5:1 |
| Oil | 5 L (5.3 US qt; 4.4 imp qt) |
| Mass | 300 kg (661 lb) |
| Power–displacement ratio | 13 kW/L |