21 Motoren

The German designer MaK, formerly a member of the Krupp group, came under the umbrella of Caterpillar in 1997 as MaK Motoren, adding a valuable medium speed portfolio to the US-based high speed engine specialist's programme. The company was renamed Caterpillar Motoren in the year 2000. At the time of the takeover, the MaK range embraced five designs with bore sizes from 200 mm to 580 mm offering outputs up to 10 000 kW: the 'new generation' M20, M25 and M32 and the longer-established M552C and M601C models. The latter pair of designs have since been phased out and a fourth new generation series, the M43, added in 1998. V-cylinder versions of this 430 mm bore design were launched in 2002, extending the power range of the MaK portfolio to 16 200 kW.

The new generation of long stroke medium speed engines was headed in 1992 by the 200 mm bore/300 mm stroke M20 design for small ship propulsion and genset drive duties. A power range from 1020 to 1710 kW at 900/1000 rev/min is covered by six-, eight- and nine-cylinder models. The M20 was joined in 1994 by the 320 mm bore/480 mm stroke M32 (detailed below to typify the new generation engines). The power gap between the M20 and M32 models was bridged in 1996 by a 255 mm bore/400 mm stroke M25 derivative (Figure 21.1) which covers an output band from 1800 to 2700 kW with six-, eight- and nine-cylinder models running at 720/750 rev/min. All three engines shared common development goals: robustness, high reliability, low overall operating costs, simple maintenance and extended intervals between overhauls.

Operational reliability was sought by avoiding corrosion, reducing mechanical wear and tear, increasing component strengths, and reducing the number of interfaces. Low operating costs were addressed by low fuel and lube oil consumptions, and low grade heavy fuel-burning capability. Modest maintenance costs were sought from a reduced number of components, a long service life from components, simple regulation and adjustment procedures, and plug-in connections.

Reduced installation work was achieved by providing simple and easily accessible interfaces for the shipbuilder. All connection points

Figure 21.1 The M25 design typifies the new generation of engines from MaK (contrast with Figure 21.8)

for fuel, lube oil and cooling water systems are arranged for convenient access at the free end of the engine; and the fuel and lube oil filters are already mounted on the engine. The turbocharger mounting arrangement is variable, either at the flywheel end or free end of the engine.

High functional integration of components achieved engines with

40 per cent fewer components than their predecessors. A reduced amount of piping work in particular (and hence fewer connection points) yielded benefits in assembly, installation and maintenance. The remaining connections, where possible, are simple plug-in arrangements. Air, water, lube oil and fuel are guided through bores and plug-in connections inside the individual components, leaving minimal external piping in evidence. The charge air duct, for example, is integrated in the crankcase. The air flows through channels in the crankcase and water guide ring into the cylinder head to the inlet valves. Thus, when maintenance work on the head is necessary, no charge air pipe has to be removed and any danger of leakage or working loose is eliminated. The camshaft is also integrated into the crankcase. Another example is the integration of a slide valve gear into the fuel injection pump, eliminating starting air distributor and control air pipes.

Summarizing the common design features, MaK highlights:

• Underslung crankshaft.

• Stiff engine block with integrated charge air and lubricating oil ducts.

• Dry cylinder block; cooling water only where necessary.

• Long stroke (stroke/bore ratio of 1.5 in the case of the M32 inline cylinder engine).

• Up to 40 per cent fewer components than earlier designs.

A high stroke/bore ratio fosters good fuel injection and combustion in a large combustion space; additionally, the high compression ratio underwrites low fuel consumption and emission figures.

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