Just one year after the new cab-over-engine trucks of the LP series came on the scene in 1963, they and their snub-nosed colleagues got new engines. After a good 40 years, Daimler-Benz bid farewell to the precombustion chamber principle which was introduced in 1923, introducing direct injection in 1964 in the shape of two new in-line six-cylinder engines: the 10.8 litre OM 346 and the OM 352, displacement 5.7 litres, superseded the prechamber diesels OM 326 and OM 322.
"In long years of development work we have succeeded in combining the good fuel consumption of the direct injection process with the advantages of the prechamber system." In these words, quickly followed by a reference to "long years of testing", the factory presented the new power plants, attempting at the same time to dispel misgivings about hard knock (troubled many direct-injection units in those days) and any lack of durability of the new engines.
At any rate, the 126 hp predecessor engine, the OM 322, had always kept the warranty system on its toes. In contrast, the successor, the OM 352, also with 126 hp, remained almost totally unaffected by teething troubles and, in terms of general reliability, immediately overshadowed the OM 322.
There were three main arguments in favour of a parting of the ways with the traditional prechamber engine: The new injection method cut fuel consumption by around 15 percent. True, the price of oil was still less than two dollars per barrel and would not exceed that mark until 1973.
But the tax burden on fuel climbed noticeably. From 1950 to 1960 the tax share of the price of a litre of diesel was just about four pfennigs (the equivalent of around two euro cents). As of April 1960 taxes rose almost sixfold to just under 23 pfennigs per litre (about twelve cents), increasing once again on New Year's 1964 by more than 50 percent to a good 35 pfennigs (equivalent to about 18 cents) per litre. So a 15 percent consumption reduction was music to the ears of hauliers.
Moreover, unlike the prechamber engine, the direct-injection engine made do without starting aids – at least as long as the temperatures did not fall below minus 15 degrees Celsius. Only then did the driver fall back on the so-called Start Pilot, that standard device in those days that introduced a hydrocarbon compound to the intake manifold, causing the engine to start immediately. Finally, the third major advantage of the direct-injection engine is that it generates much less soot than the prechamber engine owing to more efficient combustion.
Though the engine weight and the bore and stroke, as well as the output, of the OM 346 and OM 352 were identical with the predecessors' statistics, the details of the technology proved to be completely different: precombustion chambers and glow plugs were eliminated, the combustion chamber was now in the piston, and the intake ducts were redesigned.
Instead of entering a subdivided combustion chamber, the diesel fuel heads straight for the walls of the piston recess or bowl. Specially designed intake ducts meanwhile set the air flowing into the cylinder into rotation, giving more power to the compression stroke (owing to the smaller diameter of the bowl).
The four-hole injection nozzle drives its jets of fuel exactly into this whirl of air, providing "excellent air/fuel mixture formation", measured by the standards of the day. "Favourable thermal conditions", as the factory emphasised, "guarantee a long life".
Though both units operated according to the same principle, the layout of the injection system and cylinder heads was quite different: in the bigger engine – a four-valve-per-cylinder unit like its predecessor, the OM 326 – the injection nozzle was centred and perpendicular; in the two-valve-per-cylinder OM 352 it was arranged at an angle and slightly off-centre in relation to the piston. And whereas the OM 346 featured individual cylinder heads, the six cylinders of the OM 352 were roofed over by a single common cylinder head.
Another advantage of the new engines was that the injection pumps were connected with the engine oil circuit and thus maintenance-free. The full-flow oil filter additionally was supplemented by a bypass-type micro oil filter, raising the oil change intervals to an impressive (for those days) 9000 kilometres.
The OM 346 was available in two output variants: 180 and 202 hp, both attained at 2200 rpm. The maximum torque was 608 and 706 Newton metres, respectively, at 1300 rpm. The more powerful variant even delivered 210 hp when equipped with a thermostat-controlled viscous fan (one that goes into operation only when it is needed).
The 10.8-litre OM 346 provided the motive power for the heavy conventional and cab-over-engine trucks of the Gaggenau plant and for the O 317 and O 317 K buses.
The division of labour existing between the two output variants was interesting: for medium-distance haulage and gross combination weights up to 30 tons, Mercedes-Benz offered the relatively light LP 1418 with a GVW of 14 tons for the motortruck; its low unladen weight was the result of a lightweight five-speed transmission (combined with a light two-speed rear axle with a load-carrying capacity of 9.8 tons) and a slim chassis and small clutch.
Instead of dual-circuit air brakes, as on the 16-tonners for long-distance haulage, the factory supplied the 1418 with dual-circuit air-assisted hydraulic brakes. Put that all together and the result is a well-priced vehicle with a relatively high payload.
The LP 1620, on the other hand, a genuine long-haul flagship, was designed for gross combination weights from 32 to 38 tons.
The new OM 352 was intended for the medium-duty Mannheim trucks with the model designations 1113 and 1114 and for the O 321 H/HL and O 322 busses. The OM 352 with a displacement of 5.7 litres developed exactly 126 hp; the rated engine speed was 2800 rpm. The OM 352 attained its maximum torque of 353 Newton metres at 1600 rpm.
Both engines were in for long careers. The 5.7-litre unit, for example, was given more power as turbocharged OM 352 A with 150 hp starting in 1966; later it even was offered with 170 hp; well into the 1980s it was fitted not only in trucks and busses, but in the Unimog too.
The OM 346 lived on even longer, but incognito as it were: as early as in 1967, after its displacement was increased to 11.6 litres by enlarging the stroke from 140 to 150 millimetres, it was renamed OM 355. As such it initially offered 230 hp, from 1969 onwards 240 hp. However, in Europe it had to make way for the new V-engines of the 400 series during the 1970s.
But in the short-nosed conventional trucks that continued to be busily exported all the way up until 1994, the OM 355 was a welcome guest in faraway countries. Alternatively, as naturally aspirated engine with 240 hp, or as turbocharged unit with 280 hp, it provided reliable service in models like the 1924 or 1928.
Across the ocean, the Brazilians, for one, truly appreciated the value of the OM 355. The engine provided decades of service in Brazil, and with turbocharger and intercooler even attained a peak output of 340 hp beginning in 1988.