Skl

Four-stroke engines in a number of bore sizes—160 mm, 200 mm, 240 mm and 320 mm—are produced by SKL whose pedigree dates back to 1905 when the Magdeburg, Germany-based company's forerunner (Lokomobilenfabrik R Wolf ) took out a licence to build diesel engines. Now a subsidiary of the Laempe group, SKL Motor's medium speed engine programme includes the VDS 29/24 AL series for propulsion and genset duties which is offered in six, eight and nine in-line cylinder versions to cover an output range up to 2350 kW at 750/900 and 1000 rev/min. Its brake mean effective pressure rating is 23.9 bar.

The 240 mm bore/290 mm stroke design (Figure 28.19) is based on a rigid nodular cast iron cylinder block housing a suspended crankshaft. The block is free of cooling water thanks to a special cylinder liner design which incorporates a raised, rigid and separately cooled collar; this feature protects against corrosion and deposits, and reduces maintenance costs. The alloyed flake-graphite cast iron liner benefits from all-round nitration to enhance its wear resistance. Four cylinder head screws with long anti-fatigue shafts transmit forces via intermediate walls and ribs in the block and via the main bearing screws to the main bearing covers.

Figure 28.18 Wärtsilä Nohab R25 design

A deformation-resistant bedplate forms the bottom of the housing and can, in appropriately extended form, also serve as the mounting for an alternator. The camshaft is carried by bearings in the cylinder block and driven by gearing at the opposite end to the coupling. A hardened and tempered crankshaft, upset forged from C-steel, is supported by interchangeable thin-walled multi-layer slide bearings. A viscosity-type torsional vibration damper is arranged at the free end of the crankshaft. The control gear and vibration damper flange are attached to the crankshaft by a cone interference fitting and hydraulically mounted. Up to 60 per cent of the rated output can be taken off at the opposite end to the coupling. Also hardened and tempered and forged from alloyed C-steel, the connecting rod is divided horizontally at its large end and supported by interchangeable thin-walled multilayer slide bearings.

The composite piston consists of a steel crown and aluminium alloy skirt, bolted from the top and supplied with cooling oil through a longitudinal bore in the connecting rod. The ring pack comprises three compression and one oil scraper rings with chromium-plated surfaces.

Four necked-down bolts secure the cylinder head to the block, the head incorporating two inlet and two exhaust valves. Engines destined for heavy fuel burning are equipped with turning devices for the exhaust valves. The hard facing of the valve seat surfaces in combination with the cooling of the exhaust valve seat rings, mounted in the cylinder head, foster extended operational lifetimes for the components. The fuel injection valve is located in the centre of the cylinder head. The valve drive (lubricated by pressure oil and arranged in an oil-tight enclosure) is effected from the camshaft via roller tappets, pushrods and forked rockers.

Turbocharging is based on a three-pulse system with a claimed maximum efficiency of at least 64 per cent. A compressor washing device and, for heavy fuel service, a turbine washing device are specified to reduce maintenance demands.

Fuel is injected by individual top-mounted pumps at maximum injection pressures of 1400 bar-plus which, in conjunction with an optimized cam geometry, ensures a short combustion time. Ideal combustion with minimal noxious emissions is further pursued by short injection pipes (incorporating double walls for safety) and optimized nozzle spray angles and injection hole diameters. In heavy fuel engines the injection nozzle is continuously cooled with lubricating oil from the engine circuit and fuel pipings sheathed with heating piping are isolated. The quantity of fuel injected is controlled by a mechanical-hydraulic speed governor via a compound control rod arrangement.

Engine cooling is effected by two circuits. The cylinder liner, head and turbocharger are cooled via a high temperature circuit while a low temperature circuit serves the oil and charge air cooler as well as the heat exchanger (water). Both circuits are designed for the direct fitting of centrifugal pumps. Engine lubrication is carried out by a pressure circulation system served by a lubricating oil pump, filter and cooler mounted on the engine. The system also supplies the piston with cooling oil.

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