Marine Engine Systems

Introduction. Diesel propulsion plant. Two-Stroke Diesel Engine. Four-Stroke Diesel Engine. Pumps and pumping systems. Pumps. Ejectors. Piping systems. Pipes. Valves. Cock. Propulsion plant service systems. Cooling system. Fuel system. Lubricating system. Starting air system. Heating systems. Distillation systems. Oil/water separators. Fuel oils, lubricating oils and their treatment. Machinery Automation Systems. Literature.

In two-stroke cycle, as the name implies, there are two strokes of the piston for each cycle of the engine, resulting in one complete revolution of the crankshaft. Air intake, compression and ignition occur in one stroke, while combustion, expansion and exhaust take place in the next. Thus, two consecutive strokes of the piston – they may be termed as compression stroke and expansion or power stroke – complete one revolution of the engine. The various operations like air intake, compression, ignition, etc repeat in the same sequential order for each revolution. They occur when the piston reaches a corresponding position during its stroke and can be represented by circles in a timing diagram – in terms of angles of crank position measured from the top dead centre (TDC) or the bottom dead centre (BDC). Timings differ to some extent for different engines depending on variables like stroke to bore ratio, connecting rod length to crank length ratio, engine power and speed.

The cylinder liner, made of alloy cast iron, sits inside a cooling water jacket. This enables efficient cooling and allows easy replacement of a worn liner. The annular space between the liner and the engine block – formed by cast iron jackets bolted together – is sealed by silicone O-rings that withstand high temperatures. Scavenge and exhaust ports in the bottom half of the line facilitate air intake and expulsion of exhaust gases.

The two-piece piston (alloy steel piston crown and cast iron skirt bolted together) has number of annular chrome-plated grooves on the outside that carry piston rings ensuring an efficient seal between the high pressure combustion gases and the under-piston scavenging spaces. The long piston skirt controls the exhaust timing by uncovering the exhaust ports in the liner. Exhaust ports are placed slightly higher than the scavenge ports facilitating efficient loop scavenging. The piston is cooled by circulating water (or oil) that is led through telescopic tubes into the hollow piston crown. The vertically reciprocating piston rod is sealed at the diaphragm at the base of the entablature (enclosed scavenge air spaces) by a piston rod gland called the stuffing box. This prevents air from escaping into the crankcase. A turbocharger (driven by exhaust gases, and in turn driving an air blower) supplies compressed air into the scavenging spaces under the piston. The downward movement of the piston compresses the air further and pumps it into the cylinder liner during the air intake stroke.