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. Its displacement is 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 of the OM 138 consists of two parts, a
lower part with the lower part of the
crankshaft bearing and an
upper part with the cylinder block and the
camshaft. 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 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 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 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
pushrods necessary for the OHV valve train. The intake and outlet are at the opposite side; the
intake manifold 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 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 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.
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 ==