The trucks of the Mercedes-Benz series Schwere Klasse (SK, for "heavy class") were produced from 1989 through 1998, when they were succeeded by the new Mercedes-Benz Actros truck. The Mercedes Benz SK, in essence is basically a revamped Mercedes-Benz NG chassis with a few added driver comforts and electronic assistance and diagnosis. Therefore, it is a much more comfortable ride than its predecessor. There are at least two versions of Mercedes Benz SK that got licensed manufacturing by other companies.
1989
The SK series is basically a mere evolution of the Neue Generation, which was built up to the year 1989. There were few changes to the cab: radiator grill and wind leading plates were combined optically, and the lower edges of the side windows were made with a progressive decline towards the front. This design feature had already been realized in 1977 with the T1 and in 1984 with the LN series.
1994
In 1994 the SK series, which apart from the upgraded engines was still based on the NG series from 1973, was facelifted for the last time with a new front design. The focus was on better aerodynamics which would yield better fuel economy in testing.
Engines
The Mercedes Benz SK were available with the ever popular OM44X series engines, which were an evolution from the highly respected OM42X series used in the predecessor NG chassis. As a result, the OM4XX series share many similar (not to be confused with identical) components which date as far back to the very early Mercedes Benz OM401 and 402 diesel engines.
With the introduction of Euro 1 in 1992, designers and engineers had to find a way to reduce diesel motor emissions over its predecessor. One way this was achieved was by effectively 'de-tuning' the old motor by retarding the ignition timing by several degrees and thus reducing combustion temperatures and NOx emissions. Such de-tuning led to an 10% increase in fuel consumption over earlier model engines.
These new generation OM44X engines, with their robust cast iron blocks, much like their predecessor engines, were specifically very popular in harsh hot desert climates due to their large displacements and low operating temperatures.
The OM44X series engines had various upgrades from the OM42X and OM40X engines. The emphasis on upgrades were to improve cooling of engine components, engine pressure tolerances and tolerate higher engine loads.
Improvements such as: Further reinforced cast iron blocks with strengthening at the intermediate walls, housing top and outer walls. Overall an increase of approximately 6 kg of reinforcement material over the predecessor block. Further improved cylinder heads developed to meet increased operating pressures and temperatures. The cylinder heads have been strengthened at the top sections and have improved exhaust passages, valve seating and nozzle section to match higher pressures. Intake and exhaust valves as well as seat rings have also been redesigned for better gas flow, cooling properties and valve springs improved to deal with higher engine operating pressures. Finned air compressor to assist air flow around cylinder liner used in conjunction with coolant of the cylinder and piston. Motor oil cooler core with reinforcement to limit thermal expansion and movement.
These engines were initially released as two variants:
(Released from 1989 to 1992) OM441 Naturally aspirated V6 engine which yielded approximately 224 hp OM442 Naturally aspirated V8 engine which yielded 290 hp (approx. 1100 N m of torque) OM442LA twin turbo V8 which yielded between 350 hp (approx. 1650 N m of torque) (without intercooler) and 500 hp.
The later variants (Released from 1992 to 1998) were classed as EURO 1 Emission standard engines. OM441 LA Single Turbo V6 engine which yielded 220 hp and up to 340 hp when fitted with an intercooler. OM442 LA 14.6 Litre Twin Turbo V8 engine which yielded between 350 hp and up to 530 hp when fitted with an intercooler.
Fuel consumption of the OM442 LA twin turbo V8 is said to be tested at approximately 40 L/(100 km). On comparison, its successor the OM501La of the Mercedes Actros yields fuel economy of 37 L/(100 km).
Transmissions
With the introduction of Euro 1 class emission control regulation, it is only understandable that the engines gained a mass disadvantage compared with the earlier models. As the engines were now getting heavier due to Euro 1 class emissions fitting turbos, intercoolers, piping etc. As a result, Mercedes Benz moved away from the ZF branded 16s series Ecosplit cast iron transmissions used in the late model NG chassis and towards a Getrag manufactured aluminium 16 speed synchromesh transmission of models G155 and G180.