A lightweight diesel starts the ball rolling
The engine family causes a sensation worldwide
The design holds up until well into the 1990s
A false start that turned out well in the end. The history of the 300 engine series, launched in 1949 for light and medium-duty commercial vehicles, was certainly not without drama.
Deep in the company archives were design drawings for an engine with the model designation OM 302 that had never been built. Conceived in the late 1930s, the engine had been conceived for use with small and medium-duty commercial vehicles and intended to power both in-house trucks and the Opel Blitz, a three-tonne vehicle very popular with the German army at the time.
Resolution to lose weight
It was precisely in this truck category that Mercedes-Benz had one small problem at the time: the company’s three-tonne L 3000 was just a little too heavy. For the same ‘contingency weight’ of two L 3000s it was possible to manufacture three equivalent Opel Blitz trucks. The Opel Blitz’s operational weight was around 20 percent lighter than that of the L 3000, and the Opel Blitz was also a robust and inexpensive vehicle.
Against this background, in autumn 1938 General Manager Dr. Wilhelm Kissel called for the development of a three-tonne vehicle that would be as light and as inexpensive as the Opel Blitz. The Gaggenau plant was commissioned with the order. From 1938 onwards developers at Gaggenau worked on the design of a four-cylinder diesel engine for light trucks. It was given the model designation OM 301 and was to achieve the unusually high rated engine speed of 3000/min. By comparison, 2200/min was a more common engine speed at the time. Equally unusual features of this engine were its bore and stroke parameters of 90/120 millimetres respectively – dimensions that would feature in all light diesel engines manufactured by Mercedes-Benz for many years to come.
“OM 302” was in-house code for the corresponding six-cylinder engine that designer Adolf Wente began work on in June 1940. The specifications were clear: as low a curb weight as possible, yet with a power rating of 80 hp and thus a relatively high rated speed of 3000/min. By comparison, the standard OM 65/4 engine used in the L 3000 was relatively heavy and reached its 70 hp rated output at 2250/min.
Mood of optimism soon dampened
The first prototype engines for the new OM 302 design were up and running in Gaggenau by as early as mid 1941, and in 1942 the Mannheim plant even presented a schedule for series production. Vehicle testing was carried out in both the Opel Blitz and the company’s own 1.5-tonne vehicle, since the intention from the outset was to use this engine also in the Opel Blitz. However, hopes were dashed when the German army stationed in the Caucasus came into possession of petrol supplies and the freezing Russian winter greatly dampened demand for diesel engines.
At any rate, the military authorities failed to give the go-ahead for procurement of 140 engines, which would have enabled the OM 302 to be produced in series. The brand-new six-cylinder diesel engine was suddenly mothballed. Hopes of increasing the load capacity of the company’s own 1.5-tonne vehicle and equipping it with the OM 302 as competition for the Opel Blitz were also dashed when armaments minister Albert Speer decreed on 25 June 1942 that Borgward and Daimler-Benz should now build the Opel Blitz.
The Opel Blitz, on the other hand, was equipped with a 3.6-litre petrol engine and operated with irregular cylinder intervals. In the turmoil of war, however, setting up new production facilities for the Opel Blitz was not easy and production in Mannheim did not get up and running until 1944. This conicided with the moment at which the main production plant for the three-tonne Blitz, the Opel plant in Brandenburg (Havel), was razed to the ground by US air force. It was still too early to switch production to Borgward in Bremen, so the Daimler-Benz Mannheim plant was the only possible production site for the urgently needed, standard three-tonne vehicle.
Opel Blitz a godsend for Mannheim
With considerable foresight, Kissel’s successor, Dr. Wilhelm Haspel, had always insisted that the Mannheim plant should continue to manufacture vehicles. So work on the Opel Blitz came at just the right time, even if plant employees were not particularly happy about the ‘cuckoo’s egg’ that had been laid in their nest (model designation
L 701). After the war, however, the only vehicle on which production could restart immediately was the Opel Blitz. A further advantage was that production facilities were paid for by the state – as if by miracle the production line had suffered relatively little damage as a result of aerial bombardment.
Joy at this state of affairs was tempered only by the fact that the contract to produce the Blitz under licence could be terminated after two years. Daimler-Benz therefore faced an impasse. At a meeting on 24 July 1945, Wilhelm Haspel explained: “If our model is to be delivered in two years’ time, then strictly speaking it needs to be ready for production today’. He went on: ‘The issue of the three-tonne vehicle is the most serious we are facing, so we need to make it our specific concern.”
When Opel did actually terminate the contract in mid 1947 it was a bitter blow. No one was sure what drawings or test engines had survived the war unscathed. Not only were the drawings found, however, the sole prototype of the L 312 with its new six-cylinder diesel engine also turned up safe and sound. In contravention of an official order it had not been scrapped, but had instead been converted to operate on wood gas and put to good use in-house.
Adapting the OM 312 takes time and effort
Suddely prospects for finally adapting the OM 302 for use in a company three-tonne vehicle were a little brighter. And by adopting the same irregular cylinder intervals as the Opel petrol engine, the OM 302 became the new OM 312 design. This modification involved equipping the machinery that bored the holes for the spark plugs (in the petrol engine) with different tools to mill the holes for the glow plugs. In addition, the diesel engine, which by definition operated at higher pressures than the petrol engine, was fitted with seven main bearings instead of the four in the petrol engine.
But since the diesel engines were relatively narrow on account of the length of the cylinder block having to extend to the same ceiling as the petrol version, they also had to cope with higher bearing loads and consequently higher oil temperatures. Because of this higher thermal load, an oil-water heat exchanger in the OM 312 was essential. At the same time, the cylinder heads were modified to allow the engine to achieve the desired increase in power rating from 80 hp in the OM 302 to 90 hp for the OM 312. What is more, the latter developed its 90 hp at just 2800/min.
This also necessitated modifications to the pre-combustion chamber and inlet ducts that entered at a steep angle through the valve cap. Although the new OM 312 had the same 90-millimetre bore as the petrol engine, the stroke was increased from 95 to
120 millimetres. This yielded a displacement of 4.6 litres instead of 3.6 litres. Despite this, the engine weighed just 355 kilograms, making it the lightest unit in its class. It was intended to serve as the powre unit in the newly designed three-tonne L 3250, which was fitted with reinforced rear axles and gearbox to accommodate the OM 312’s maximum torque of 28.2 mkg at 1600/min (equivalent to 277 Newton metres).
Start of production suddenly recedes into the distance
The supply situation in 1947 for coal, electricity and gas was calamitous, however. The first test vehicle was assembled at the beginning of that year, but the start of production, originally scheduled for July 1948, now receded far into the distance. Opel extended the contract until 1949, which suited the designers just fine, because it allowed them to subject both the L 3250 and the OM 312 to more thorough testing. For one thing was clear – the new design simply had to succeed.
Wilhelm Haspel was very conscious of the risks involved, particularly with the OM 312. At the end of 1947, he warned: “… The approach to this engine in terms of testing has not been as circumstances should have dictated.” The developers used the additional time to put the engine design on a solid footing.
Enthusiastic reception for the L 3250
When the engine was first unveiled to the public in May 1949 at the export fair in Hanover, it caused a sensation in the truck sector – a diesel unit with the same power per litre as a petrol engine. Moreover, in the shape of the L 3250 they were looking at a diesel truck with virtually the power-to-weight ratio of a petrol truck. Nevertheless, the OM 312 did have certain idiosyncrasies: since the right-hand side had an open design similar to the Opel engine and was sealed only with a cap, it lacked something in terms of rigidity. Cracks in the relatively weak connecting rods were not infrequent.
In professional circles, the OM 312 was ecstatically received. In issue 9/1950 of the magazine Das Last-Auto, for example, vehicle tester Hans-Arnold König made the following comment: “What liveliness and power the engine has! This test driver has to admit that he was astonished at the incredible tractive power of this 4.6-litre engine.” The L 3250 brochure had similar praise for the engine qualities: “The newly designed six-cylinder diesel engine has a character such as was thought possible up to now only in high-speed passenger car petrol engines.”
All of this also generated international interest, which fitted well with Wilhelm Haspel’s efforts to kick-start exports. His principal focus was on the South American and Indian markets. In March 1949, for example, he wrote to his trusted friend Moritz Straus in Zurich, who had owned the Horch plants before the creation of Auto Union: “In particular, we have to tackle the South American market … and see what can be done there.”
Start of an international career
It was not long before Daimler-Benz was able to deliver 1000 CKD (completely knocked down) chassis to Brazil in 1950 for the L 3500, as the L 3250 with slightly increased load capacity was now called. This was the starting point for further orders of over 2000 truck and 500 bus chassis, and culminated in the establishment of Mercedes-Benz do Brasil in São Paolo, Brazil, on 23 October 1953.
In India, too, the OM 312 made a successful career for itself. Haspel initiated negotiations with Hindustan Motors, but following problems with implementation concluded the talks with Tata. On 1 April 1954, a technical aid agreement was signed with the Tata Engineering and Locomotive Company Ltd. (Telco), with a term of 15 years. It envisaged cooperation on sales and marketing and on the construction of a new assembly plant for Mercedes-Benz trucks weighing three tonnes and over. Daimler-Benz took a stake in Telco of around 12.5 percent.
The OM 312 and L 3500 were the foundation stones for the development of the group’s global presence in the commercial vehicles sector in later years.
In tandem with the production of the new engines, designers tried out new technology in the OM 312. As early as 1951, there were prototype versions with direct injection, although, as developer Wolf-Dietrich Körner remembers, “they suffered from terrible piston scuffing.”
Advent of turbocharged variants
Behind the scenes, developers were also already working on turbocharged engines. First, however, they gave the OM 312 a power boost using conventional methods: the engine in the L 4500 delivered 100 hp and the rated engine speed was increased from 2800 to 3000/min.
But then the first turbocharged engine arrived. In 1954, the engineers ushered in a new era with the OM 312 A. Even though the company would still baulk for several decades at turbocharging heavy-duty engines, the situation was very different with the 300 series. The first Mercedes-Benz commercial vehicle engine with turbocharger developed 115 hp with an unchanged displacement of 4.6 litres and an additional weight of just 15 kilos. It featured changes to valve timing with much greater valve overlap, a slightly reduced compression ratio and a new burner for the pre-combustion chamber. The new turbocharging technology was as well as possible assured.
The engine was principally intended for use by fire services, which although still somewhat sceptical towards diesel engines, would now – it was hoped – be attracted by the superior power rating. Freight carriers of the day, on the other hand, had reservations about turbochargers rather than about diesel engines – and with some justification as it turned out.
At the time it was considered more important to achieve a significant increase in performance in the compact engine than to add half a kilogram to the weight of the large engines. But high-level turbocharging did not suit the pre-combustion chamber, as it resulted in overheating. Moreover, as Wolf-Dietrich Körner recalled: “The turbochargers in those days were not quite right. You needed to replace them virtually with every oil change.” And oil changes in those days were much more frequent than today. Nevertheless, at high altitudes, for example, the turbocharged engine performed extremely well, and the OM 312 A became the engine of choice for use in snow clearance machinery.
Power rating for naturally aspirated engines also increases
However, the widespread distrust of turbocharging persisted in parallel with the demand for even better performance. As a result, deigners were also forced to improve the power rating of the naturally aspirated engine. Since an increase in the pressures was undesirable, the focus turned to increasing the displacement. And since the engine was already of long-stroke design, the engineers boosted the displacement by increasing the bore from 90 to 95 millimetres. This presented the foundry at the Mannheim engine plant with a challenge. Instead of casting each cylinder individually as before, they were now manufactured together in a single unit with a reduced interval of 108 millimetres.
In addition, for reasons of thermal efficiency, the enlarged engine was fitted with the burner from the OM 312 A. The introduction of the turbocharged engine completed the redesign of the pre-combustion chamber from an annular intake burner to a ‘cup burner’ with eight outlets. The new engine was given the model designation OM 321 and went into series production in 1954. It had a displacement of 5.1 litres and a power rating of 110 hp. This asembly became the standard engine in the L/LP 321, O 321 H and initially also in all variants of the L/LP 322 and L/LP 328.
At this time an engine that was to all intents and purposes a descendant of the Opel Blitz petrol version also made its appearance: the little-known carburettor variant of the OM 312 was known as the M 312, and was manufactured from 1954 to 1963. This petrol engine had a power rating of 110 hp and a maximum torque of 30 mkg at 1600/min (equivalent to 294 Newton metres). The engine was principally intended for export to the Middle East.
Sudden introduction of the ‘six hp per tonne’ formula
Meanwhile, the 110 hp developed by the diesel OM 321 was not sufficient for all applications, e.g. for use in the new O 321 bus. Once again a turbocharged engine was required, and this came in two variants. Production of the OM 321 AM began in 1956, the German abbreviation AM standing for ‘mild turbocharger’. Despite its name, this ‘mild’ version was nevertheless capable of developing 126 hp. The figure was no accident: alongside a range of strict limitations governing length and weight, German Transport Minister Seebohm also prescribed a minimum power rating that applied a formula of six hp per tonne – which equated to exactly 126 hp for a 21-tonne tractor-trailer combination such as the L/LP 322 or L/LP 327. The OM 321 A engine with
132 hp was available to meet the special performance requirements of fire services,.
Haulage companies were still wary of turbocharged engines, however. So once again, in 1959, a naturally aspirated engine with increased performance was required. The solution was given the model designation OM 322. Developers again increased the displacement, the piston stroke was increased from 120 to 128 millimetres, and the bore from 95 to 97 millimetres. This left very little room in the crankcase, so as a standard approach cylinders were always cast together in pairs. The OM 322 introduced a trend-setting displacement of exactly 5.675 litres, since its successor, the OM 352, would also retain this dimension throughout its career. Thanks to a new roller injection timer, the engineers were able to time fuel injection precisely, allowing an increase in mean pressure and thereby yielding improved performance. This also enabled a reduction in rated engine speed from 3000 to 2800/min.
The pre-combustion chamber engine reaches its limits
In the OM 322, changes such as a new pre-combustion chamber, new valve seat rings, a specific microsection for the pistons and an improved honing pattern for the cylinders paid testimony to the efforts and ingenuity of the developers. Nevertheless, there were some teething troubles with the engine: despite modifications that were promptly implemented, its reputation was damaged by cracks in the cylinder head, piston scuffing, and fractures in the connecting rods and crankshafts. The unit was one of a kind, and this was also true of its appearance: for in order to avoid confusion with the OM 321 produced in parallel, the OM 322 was painted blue rather than the traditional green. It became clear that the pre-combustion chamber engine had finally reached its limits in terms of thermal capability. In 1964 the damage frequency was 9.7 percent; by way of comparison, the corresponding value for its successor, the OM 352 (which now operated with direct injection), was just 0.8 percent. Tests with direct-injection systems acquired top priority as early as 1956.
This kept operations in Mannheim very busy from 1959 to 1964: the OM 312, OM 321 and OM 322 – three generations of the 300 series – were all produced in parallel. In addition, the first four-cylinder engine in the 300 series was built just a year after the premiere of the OM 322. Derived from the OM 321, it was given the model designation OM 324. With a bore of 95 millimetres and a stroke of 120 millimetres, it offered a displacement of 3.4 litres and had a power rating of 75 hp. However, the engine was to play only a minor role, finding use in Toyota offroaders but not in the company’s own vehicles. The engine had not yet found the appropriate light truck or large van, but this situation would change a few years later.
Further developments with durable direct injection engines
But first the 300 series reached a watershed with conversion to direct injection. “New engines with a special focus on economy” was the title of a Mercedes-Benz brochure in 1964. Following testing of direct-injection systems over a period of more than twelve years, Mercedes-Benz finally presented the OM 352 in 1964 – a little late, perhaps, but not too late. The company announced: “After thorough testing over an unprecedented period … all Mercedes-Benz trucks with a payload over 3.5 t will now be fitted with new engines featuring the Daimler-Benz direct injection system.”
This phase in the history of the 300 series played a major part in establishing the excellent reputation of the 300 series engines. Lengthy experiments on various combustion systems produced a more or less centred four-hole injection nozzle at an angle of 30 degrees, and a cylindrically configured combustion chamber in the piston. The bore and stroke, and thus also the 5.675-litre displacement of the six-cylinder engine, originated from the OM 322.
The engine was available in several performance variants, developing 100, 110 and 126 (and later 130) hp. Greater efficiency and improved combustion reduced the thermal load compared with predecessor engines. The higher ignition pressure of
80 bar instead of 60 bar presented no problems for the engines. The new units soon gained an excellent reputation and replaced the OM 312, OM 321 and OM 322
Advent of the first four-cylinder engines with direct injection
In tandem with the six-cylinder engines with which they had been developed concurrently, Daimler-Benz presented for the first time four-cylinder engines bearing the model designation OM 314. They had a displacement of 3.78 litres and a power rating of 80 hp, later 85 hp. Like their larger brothers, they were used in the new cab-over-engine vehicles of the LP series. The engines series also subsequently found use in the company’s large-capacity vans, which were christened “Düsseldorfer” after their production location, in the small O 309 bus, a derivation of the Düsseldorfer, in the Unimog and in the MB-trac agricultural vehicle.
Inevitably, the large four-cylinder engines did not possess the smooth running characteristics of the six-cylinder engines, and in this case the long stroke design proved a serious drawback. Developer Körner’s generous assessment was that “it was not a sophisticated engine”. Even though the engineers were successful in trying to tame the engine using two balancer shafts, the cost involved militated against this additional measure. “It would have been unreasonably expensive,” was Körner’s realistic appraisal of the situation. On the other hand, after brief initial problems, the four-cylinder achieved a reputation for excellent reliability – it just went on and on.
In the meantime, Daimler-Benz was also obliged to increase the power rating of the OM 352. First came the OM 352 A (the ‘A’ stood for Aufladung, turbocharging) for snow clearance machinery. This was also used in trucks from the latter half of the 1960s, with power ratings of 150 and 156 hp. The turbochargers were initially supplied by Eberspächer, then by Garrett and also KKK. During this period the power rating was increased initially to 168 hp and then to 172 hp. The OM 352 was the first turbocharged engine to win full acceptance from customers. A turbocharged version of the four-cylinder, the OM 314 A, also went into small-scale series production.
Development of turbocharged engines with intercoolers commenced in the mid 1970s. Once again it was the 300 series that led the way: the OM 362 LA, developing 192 hp, went into series production in the early 1980s with an unchanged displacement of 5.675 litres.
The OM 352 A was used in the cab-behind-engine truck, the various angular and rounded LP models and also in the New Generation (NG) vehicles. In addition, it made a name for itself as an industrial engine, both naturally aspirated and turbocharged. On the international market it has enjoyed a long career in Iran, where to date around 180,000 engines have come off the assembly lines. In 1981, production also started under licence in South Africa at Atlantis-Diesel-Engine (ADE).
Petrol-driven variants ordered by NATO
Meanwhile, a petrol variant, the OM 352, managed to hold its own for a certain period; from 1966 to 1977 this unit was to be found as a 130 hp unit mainly in NATO supply vehicles under the model designation M 352.
With the development of the OM 352 and OM 314 in the 1980s, the 300 series seemed to have exhausted all possibilities – increased displacement in conjunction with a higher load was no longer possible. After 30 years, developers were now keen to usher in a new era with a completely new engine series. New engine types were developed: a four-cylinder engine with a 4.7-litre displacement, and a six-cylinder engine with a
7.0-litre displacement instantly met demands for lower consumption and emissions, in conjunction with greater load capacity. There was also inherent potential for further refinements. Everyone was delighted. Looking back today, engine developer Körner recalls: “It was a fantastic engine. Even Prinz, the Chairman of the Board of Management, was keen on the idea.”
But before the new engines could be manufactured, the Board of Management put the brakes on: required investment was too high. The new engines were expected to cost DM 150 million, and that was considered too much. The project was shelved and the engines were mothballed in a basement of the test department, seen only by a handful of interested customers who gave them the nickname ‘basement engines’.
The 300 series begins its lap of honour
The developers did not give up, however. Even though development options for the 300 series seemed to have reached a stumbling block on account of the long-stroke design and the short cylinder interval alone, the developers were commissioned with the task of once again updating the engines. Instead of DM 150 million for the new engines, the Board of Management now allocated only DM 20 million for advanced development of the existing ones.
There was a desire to start again from scratch. The objectives were challenging: increased output, reduced fuel consumption and exhaust emissions, longer service life and reduced susceptibility to malfunctions. No stone was left unturned. The cylinder bore was again increased by 0.5 millimetres to 97.5 millimetres, and the piston stroke was extended by 5 millimetres to 133 millimetres, resulting in a 3.97-litre displacement for the four-cylinder engine and a 5.96-litre displacement for the six-cylinder engine.
All-round state-of-the-art engineering
Simply adding an extra feature such as an intercooler to the existing engines, as had been done in the case of the 192 hp OM 362 LA in 1983 (the unit that shared identical dimensions to the OM 352 A) would not have been a viable solution in the long term. The engineers therefore decided to redesign the combustion chamber, using spiral instead of swirl downdraught inlet ports. Inlets and outlets were all arranged on the left of the vehicle so as to eliminate long ducts with turbocharged engines. The housing was simplified and at the same time redesigned using ribs and thicker walls to enable it to cope with higher loads. Forged integral counterweights on the crankshafts and a modified hardening process on the crankshaft bearings and big-end bearings also provided for a high load capacity.
The pistons now featured only three rings with a barrel-shaped microsection that was highly resistant to scuffing. In addition to numerous other changes, there was also a new lubrication system and splash cooling was added for the turbocharged engines. The new OM 364/366 model series arrived in 1983/1984 to an enthusiastic reception: it was the birth of the most advanced engine series for the mid-range performance class of the day.
The engineers even paid attention to attractive packaging. To begin with, the engine specialists wanted to use the in-house designers led by Bruno Sacco, but that would have proved too expensive. Instead, the engineers tackled the job themselves, giving the engines a highly elegant exterior to go with their new inner qualities. They managed to push the power rating of the six-cylinder engine up to 200 hp, and later even to
240 hp. The four-cylinder started off as a naturally aspirated engine developing 90 hp, but from 1986 with the help of a turbocharger it developed 115 hp, meaning it could be used in the 711 D and 811 D large capacity vans.
Still able to meet Euro 2 standards
At some point, however, even the best model series in the world exhausts development possibilities. As Körner, the series developer, explained: “We could have developed the 300 model series further, but then we would have needed a new crankcase.” Any further refinement of these engines would have involved an unjustifiable expense. Furthermore, ingenious developments such as multivalve technology and constant throttle could not have been incorporated into the 300 series, and even the production machinery itself was getting old.
Nevertheless, as the OM 364 LA developing 136 hp, the four-cylinder engine unveiled at the RAI congress centre in Amsterdam in 1994 easily complied with the Euro 2 emissions standards that were to come into force from 1996. The features that made it more eco-friendly included an intercooler, a turbocharger with wastegate, a redesigned combustion chamber and five-hole instead of four-hole injection nozzles, as well as a higher injection pressure. In much the same way, the technology in the six-cylinder
OM 366 LA changed in 1991 with its adaptation to meet Euro 1 standards. Thereafter, it had a maximum engine power rating of 240 hp. Additionally, intercooling (German abbreviation ‘L’) and turbocharging (German abbreviation ‘A’) made it generally possible to reduce the rated engine speed once more: the LA engines now required only 2600/min to develop their maximum output. This not only helped reduce fuel consumption and noise, it also extended the engines’ life expectancy.
A worthy successor in sight
Despite all this, however, the introduction of the Euro 3 emissions standard sounded the death knell for the long-serving 300 model series, which had proved its worth in so many different fields, from vans, trucks and buses to forklift trucks, power generators and special-purpose vehicles such as the Unimog and MB trac. Its successor was unveiled in January 1996 (initially in the LN 2) as the new 900 engine series which, with strokes of 102 and 133 millimetres, resulted in displacements of 4.2 litres
(four-cylinder version) and 6.4 litres (six-cylinder version) respectively.
With three valves per cylinder (two inlet valves and one outlet valve), and electronically controlled pump-line-nozzle elements, they were well equipped to meet the upcoming Euro 3 standard, and even today, thanks to SCR technology, they have no problem meeting the Euro 5 standard. The 900 engine series has demonstrated its quality around the world as a truly global engine, and in this respect is a worthy successor to the 300 series engines, many of which are still engaged in active service in various parts of the world: indeed it is still possible to get hold of an OM 366 LA with 211 hp in the Brazilian cab-behind-engine L-1620 Classic.