412 Mercedes M112 V6 engines

Early in 1997, Mercedes introduced the first two M112, 90° V6 engines, Fig.

4.13, of a range of V6 and V8 units from 2.8 to 5 litre piston swept volume ultimately to be produced. These are of outstanding interest for many reasons, so they will be described here in some detail. They have aluminium alloy cylinder liners, two sparking plugs per cylinder, three valves per head and the open-top cylinder block construction has been adopted. This type of construction was used for the Standard Vanguard 4-cylinder in-line engine of the late 1940s, last described in the 10th edition of The Motor Vehicle but, because of its inherently low torsional and bending stiffness, has since not been employed except, for convenience in production, where diecasting has been the preferred method of production. For the Vanguard, the attraction was ease of coring for sand casting.

That the open-top layout can now be used satisfactorily is possible by virtue of the application of modern techniques of design, such as CAD. Moreover, the wide 90° V layout, with its short crankshaft, is inherently stiffer than that of the in-line engine. Also, whereas the Vanguard engine had removable cylinder liners, the liners of the Mercedes engines are cast into the cylinder block and crankcase, Fig. 4.14. Furthermore, the cylinder heads, Fig. 4.15, are exceptionally stiff in both torsion and bending, and specially tough gaskets, mainly of the liquid metal type, are employed. Also, a cast aluminium sump and the transmission casing contribute to the overall stiffness.

The first two versions of this Mercedes engine were produced with cylinder axes 106 mm apart. This spacing was decided upon because it is adequate for accommodating cylinders large enough for the engines of up to 5 litres swept volume planned for subsequent introduction. The power output of the smaller, 2799 cm3, engine is 150 kW at 5700 rev/min and, of the larger 3199 cm3 unit, 165 kW at 5600 rev/min. Their maximum torques, both developed at about 3000 rev/min, are 270 Nm and 315 Nm, and are maintained virtually constant from about 2700 up to approximately 5000 rev/min, Fig. 4.16.

A common compression ratio of 10 :1 has been adopted. Both engines have bores of 89.9 mm diameter, but the strokes are 73.5 mm and 84 mm

Mercedes Benz M112 Cylinder Layout
Fig. 4.13 Basic layout of the Mercedes V6 and V8 engines ranging from 2.8. to 5 litres piston swept volume
Aluminum Block Silicone Cylinders
Fig. 4.14 The aluminium liners are fused into the cylinder block
Manson Cycle Engine
Fig. 4.15 Except for the head-block gasket, which is of steel, the other joints on the head assembly are sealed with liquid gasket material
Fig. 4.16 An interesting feature of these engines is that the torque is virtually constant between about 2700 and 5000 rev/min

respectively for the smaller and larger versions. Sintered steel connecting rods 154 and 148.5 mm long respectively, with cracked big ends, are employed, Section 3.14. These rods are sintered with a V-notch around the joint face between the bearing cup and cap; then, after sintering, the two parts are separated by shock loading. A major advantage of this method of production is that the rough fractured faces match perfectly, so machining them is unnecessary and there is no need for any other location device. The diameters of the four main and six big end bearings of both versions are 64 and 52 mm respectively.

Unusually, the angle between the banks of cylinders is 90°. This is because, from the outset, it was intended to produce also 4, 3 and 5 litre V8 engines the cylinder blocks of which were to be machined on the same production line. To obtain good balance with the V6 arrangement despite the 90° cylinder block angle, the two crankpins for each of the three pairs of opposed cylinders are angularly offset 30° to produce equal firing angles, and a Lanchester type balancer rotating at crankshaft speed compensates for the first order inertia forces not countered by the crankshaft balance weights. With only 60° overlap between the pairs of adjacent crankpins, the crankshaft had to be of forged steel.

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