MAN B&W diesel engine - Basic principles and operational guideline
MAN B&W engine
The diesel engine is a type of internal combustion engine which ignites the fuel by injecting it into hot, high-pressure air in a combustion chamber.
In common with all internal combustion engines the diesel engine operates with a fixed sequence of events, which may be achieved either in four strokes or two, a stroke being the travel of the piston between its extreme points. Each stroke is accomplished in half a revolution of the crankshaft.
The L70MC is a single-acting, low-speed two-stroke reversible marine diesel engine manufactured by MAN B&W. It is one of the MC series introduced in 1982, and has a longer stroke and increased maximum pressure when compared with the earlier L-GF and L-GB designs.
Fig: MAN B&W L70MC engine
The bedplate is made of welded longitudinal girders and welded cross girders with cast-steel bearing supports . The frame box is mounted on the bedplate and may be of cast or welded design. On the exhaust side of the engine a relief valve and manhole are provided for each cylinder.
On the camshaft side a larger hinged door is provided. The cylinder frame units which comprise one or more cylinders are of cast iron and bolted together to form the requisite number of engine cylinders. Together with the cylinder liners they form the scavenge air space and the cooling water space. The double bottom in the scavenge space is water cooled. The stuffing box fitted around the piston rod has sealing rings to stop the leakage of scavenge air and scraper rings to prevent oil entering the scavenge space.
On the camshaft side, access covers are provided for inspection and cleaning of the scavenge space. The cylinder cover is a single piece of forged steel, and has bored holes for cooling water circulation. It has a central opening for the exhaust valves and appropriate pockets for the fuel valves, a relief valve, a starting air valve and the indicator cock. The exhaust valve housing is fitted into the centre of the cylinder head. It is opened hydraulkally and closed by air pressure.
During operation the exhaust valve rotates. The bedplate, frame box and cylinder frames are connected together with staybolts to form the individual units. Each staybolt is braced to prevent transverse oscillations.
The crankshaft may be solid or semi-built on a cylinder by cylinder basis. A shaft piece with a thrust collar is incorporated into the crankshaft and at the after end has a flange for the turning wheel. At the forward end a flange is fitted for the mounting of a tuning device or counterweights.
The running gear consists of a piston, a piston rod and crosshead assembly and a forged steel connecting rod. The crosshead moves in guide shoes which are fitted on the frame box ends. The camshaft has several sections, each of which consists of a shaft piece with exhaust cams, fuel cams and couplings. It is driven by a chain drive from the crankshaft.
Exhaust gas from the engine is passed into a constant-pressure receiver and then into the turbochargers. Scavenging is uniflow, and electrically driven auxiliary blowers are automatically started during low-load operation.
Lubricating oil is supplied to the various bearings and also to the pistons for cooling. Cylinder oil is supplied via lubricators from a high-level service tank. A separate lubrication system is provided for the camshaft bearings to prevent contamination of the main lubricating oil system. Fresh water cooling is provided for the cylinder jackets, cylinder covers and exhaust valves.
The engine is designed to run on diesel oil or heavy fuel oil. An electronic governor is provided as standard.
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