Mercedes-Benz OM 300 (small)

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Mercedes Benz
Cutaway model OM 352

Cutaway model OM 352
OM 300 series
Production period: 1949-1995
Manufacturer: Mercedes Benz
Working principle: diesel
Motor design: In-line engine
Displacement: 3052-5955 cm 3
Mixture preparation: Pre-chamber injection
and direct injection
Engine charging: Partly turbocharged
Power: 59-176 kW
Dimensions: 385-410 kg
Previous model: none
Successor: OM 900 series

The Mercedes-Benz OM 300 (small) series comprises several generations of four- and six-cylinder diesel in-line engines (OM = oil engine ), which were mainly produced for driving light trucks. The origins go back to around the Second World War; the engines were replaced by the new 900 series in the mid-1990s .

History and Development

Even before the war, Mercedes-Benz was represented in all truck market segments, but its own 3-ton L 3000 was far too expensive and too heavy. Reason for the high weight was considerably the built-up diesel engine OM 65/4 75 bhp at 2250 min -1 . In the class of light trucks with three tons of payload, the Opel Blitz was by far the market leader; it had a six-cylinder gasoline engine with a displacement of 3.6 l, was light and inexpensive to manufacture, and was also reliable and robust. For a competitive light truck, Mercedes-Benz needed a small and light diesel engine. The engine should therefore at 3000 min -1 an unusually high nominal speed have, this time was in many ways a gamble.

Development began around 1938 and was completed around 1941. Due to the war, the production facilities could no longer be procured, and in 1942 Mercedes-Benz was obliged to manufacture the Opel Blitz under license. At the end of the war, the production facilities for the Opel Blitz engine were complete and largely undamaged. The problem was the same as before the war: a light truck and a suitable engine were required. Under the deficiency conditions of the early post-war period, the engine and the existing production facility had to be adapted to one another.

The Blitz engine only had four main crankshaft bearings; two cylinders were closer together than the cylinders with an intermediate bearing due to the lack of a main bearing. The pre-war R6 engine OM 302 was further developed: the seven main crankshaft bearings remained, the cylinder spacing was adapted to the production facilities and was therefore uneven. The OM 312 was finished. Appropriately thorough testing was only possible because the L 3250 vehicle was only finished a year later than planned.

In 1954 a turbo-charged version of the OM 312 for special vehicles was added. Due to the poor reliability of the turbochargers at the time and the low level of customer acceptance, the more powerful OM 321, essentially based on the OM 312, was introduced around the same time for normal freight traffic, and two turbo-charged versions followed two years later.

The OM 322 was later the last version of these engines with pre-chamber injection , a variant of the OM 321 with a modified bore and a modified stroke. The cramped space conditions in connection with the prechamber caused considerable thermal problems in the engine, so that at the end of its production time the damage rate was still more than ten times as high as that of the successor with direct injection in the first year of production.

Conversion to direct injection

In 1964, the OM 352 heralded the era of direct injection for this engine series, and attempts to do this had been going on for over ten years. It replaced all of its predecessors, which is why it was also available with reduced performance. After the development work on this series had begun with a four-cylinder a quarter of a century earlier, the OM 314 was the first four-cylinder diesel engine in the sales range in 1965. Except for the number of cylinders, this was identical in construction to the OM 352 and slightly intended for the new light trucks with cubic cabs . The four-cylinder engines ran very uncultivated, but the engines were very reliable and for the first time this also applied to the turbo engines that followed.

In 1983, OM 314 / OM 352 were thoroughly revised again and launched on the market as OM 364 / OM 366. This was made necessary by the stricter environmental legislation, which promoted the change towards smaller cylinder numbers with turbocharging and charge air cooling . In the 1990s, further stricter exhaust gas limits finally made a new development necessary, the 900 series.

Technical description OM 312

The OM 312 is a standing rows - six cylinder - four cycle - Two valve - diesel engine with indirect injection and water cooling . The engine's cylinder block is cast with the split crankcase and made of nickel-alloyed cast iron . The cylinder liners are cast into the block. With a cylinder bore of 90 mm and a piston stroke of 120 mm, the engine has a total displacement of 4580 cm³. Forged pistons made of light metal from Mahle with four compression rings and two oil control rings run in the cylinders . The force is transmitted via diagonally split connecting rods with lead bronze slide bearings with steel support shells to a forged crankshaft, hardened at all bearing points, which is supported seven times in lead bronze slide bearings, also with steel support shells in the crankcase. It is provided with six counterweights and a vibration damper.

In the crankcase, a camshaft located four times in plain bearings runs , which is driven via helical spur gears. It operates the vertically suspended valves via tappets , push rods and rocker arms . Each cylinder has an inlet and an outlet valve. The valves hang in a removable cast iron cylinder head that covers all cylinders. An asbestos-containing cylinder head gasket is installed towards the cylinder block.

For lubrication, pressure circulating lubrication is used, which is maintained by a gear oil pump. The oil filter is installed in the main flow. The oil is cooled by an oil cooler that has a temperature controller. The fuel is pumped out of the tank with a fuel pump and cleaned with a felt tube filter. The fuel is then fed to the Bosch PES 6 A70 B 410 RS 64/7 injection pump, which injects it into the prechamber via Bosch DNO SD 211 injection nozzles . A centrifugal governor limits the maximum speed. The air is sucked in in front of the radiator and cleaned with an oil bath air filter, which also dampens the intake noise of the engine. The motor is cooled with a tube cooler, the warm exhaust air of which is blown away by a fan.

The engine's output was initially 66 kW, but was increased to 74 kW from 1956 onwards by increasing compression, speed and other timing.

Technical description OM 352

The OM 352 is a series - six cylinder - four cycle - Diesel engine having two valves , and water cooling , substantial difference from the OM 322 and 321 is the direct injection into the piston recess. The engine's cylinder block is cast from molybdenum-chromium alloyed special cast iron, the crankcase is made from alloyed special gray cast iron. With a cylinder bore of 97 mm and a piston stroke of 128 mm, the engine has a total displacement of 5675 cm³. The pistons are forged pistons made of light metal from Mahle , each with three compression rings and two oil control rings . The force is transmitted via diagonally split connecting rods made of heat-treated steel with a double-T cross-section to a forged crankshaft with seven bearings, hardened at the bearings. The connecting rod and crankshaft have three-component bearings with steel support shells. The crankshaft is equipped with counterweights and vibration dampers.

A camshaft made of hardened heat-treatable steel with four bearings in plain bearings runs in the crankcase and is driven by the crankshaft via helical spur gears. It operates the hanging valves via bumpers and rocker arms . Each cylinder has an inlet and an outlet valve. The cylinder head made of molybdenum-chromium alloy special cast iron, which covers all cylinders, is sealed off from the cylinder block with an asbestos-containing cylinder head gasket.

The pressure circulation lubrication works with a gear oil pump and an oil filter in the main flow and an oil filter in the bypass flow. The fuel is cleaned through a felt tube filter and injected into the piston bowls with a Bosch PES 6 A70 C 410 RS 2085 injection pump through Bosch DLLA 150 S 187 injection nozzles . The injection pump has a centrifugal governor. The air is cleaned with an oil bath air filter. The engine is cooled with a tube cooler, the warm exhaust air of which is blown away by a fan.

This engine develops 126 hp / 93 kW at 2800 min −1 and delivers a torque of 36 kpm / 353 Nm at 1600 min −1 . From 1966, the engine could also be provided with a turbocharger as OM 352 A, its output increased by the charging of 150 hp / 110 kW at 2800 min -1 , a further revised version OM 352 X provides 168 hp / 124 kW at 2800 rpm - 1

Petrol engines

Some carburetor engines have been derived from diesel engines over the years. These were gasoline engines - not gas engines. They were intended (with the exception of the M 324) for export or for military vehicles in accordance with NATO requirements.

Technical specifications

Engine type built from Design Bore × stroke Displacement Combustion process Performance and remarks
OM 301 - R4 90 mm × 120 mm 3,052 cc Antechamber Prototype around 1940
OM 302 - R6 90 mm × 120 mm 4,578 cm³ Antechamber Prototype around 1941, 80 hp at 3000 min -1
lightning - R6 90 mm × 95 mm 3,626 cc Carburetor 75 hp, L701
OM 312 1949 R6 90 mm × 120 mm 4,578 cm³ Antechamber 90 hp at 2800 min -1 , later, 100 hp at 3000 min -1 , 65 PS in Unimog U406
OM 312A 1954 R6 90 mm × 120 mm 4,578 cm³ Antechamber 115 hp at 3000 min -1
M 312 1954 R6 90 mm × 120 mm 4,578 cm³ Carburetor 110 hp at 3000 min -1
OM 321 1954 R6 95 mm × 120 mm 5,101 cc Antechamber 110 hp at 3000 min -1
OM 321 Am 1956 R6 95 mm × 120 mm 5,101 cc Antechamber 126 hp at 3000 min -1
OM 321A 1956 R6 95 mm × 120 mm 5,101 cc Antechamber 132 hp at 3000 min -1
OM 322 1959 R6 97 mm × 128 mm 5,672 cm³ Antechamber 126 hp at 2800 min -1
M 324 1960 R4 95 mm × 120 mm 3,401 cc Carburetor 75 hp, OEM engine for Toyota off-road vehicles
OM 314 1965 R4 97 mm × 128 mm 3,758 cc Directly 80 hp, later 85 hp
OM 314A 1981 R4 97 mm × 128 mm 3,758 cc Directly 85 HP, only installed in the MB-Trac 900 & MB-Trac 900 Turbo (until 1987), therefore very small quantities
OM 352 1964 R6 97 mm × 128 mm 5,638 cc Directly 126 hp, later 130 hp, reduced to 100 hp or 110 hp, in the Unimog also with 65, 70, 84 or 90 hp
OM 352A > 1965 R6 97 mm × 128 mm 5,638 cc Directly 150 hp, later 156 hp, 168 hp, 172 hp
M 352 1966 R6 97 mm × 128 mm 5,672 cm³ Carburetor 130 hp at 2800 min -1
OM 353 ? R6 97 mm × 128 mm 5,672 cm³ Directly technically identical to the OM352, the renumbering was due to the large number of variants, otherwise the model numbers would have been assigned twice
OM 362LA 1983 R6 97 mm × 128 mm 5,672 cm³ Directly 192 hp at 2600 min -1 , development of OM352A as a precursor for OM366 (L (A))
OM 360 - - - - - OM 360 (new) collective name for OM 364/366, OM 360 (old), on the other hand, is an 8.4-liter engine
OM 364 1984 R4 97.5 mm × 133 mm 3,970 cc Directly 90 hp at 2800 min -1
OM 364A 1986 R4 97.5 mm × 133 mm 3,970 cc Directly 115 hp at 2800 min -1
OM 364LA 1994 R4 97.5 mm × 133 mm 3,970 cc Directly 136 hp at 2600 min -1
OM 362 - - - - - shortened for OM 362LA, see above
OM 366 1984 R6 97.5 mm × 133 mm 5,955 cm³ Directly 136 hp at 2800 min -1
OM 366A 1984 R6 97.5 mm × 133 mm 5,955 cm³ Directly 170 hp at 2600 rpm -1 , 136 hp at 2400 min -1 in the Unimog U 1300 L / truck 2t gl tmil
OM 366LA 1991 R6 97.5 mm × 133 mm 5,955 cm³ Directly 192 to 200 hp at 2600 rpm -1 , later 240 hp at 2600 rpm -1

Nomenclature:
OM = oil engine = diesel engine
M = engine, d. H. Petrol engine
h = horizontal / lying for buses
A = turbo-charged
Am = turbo-charged mild, i.e. H. Reduced boost pressure
LA = turbo-charged with charge air cooling

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