1 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. The marine diesel engine is a type of diesel engine used on ships. The principle of its operation is as follows: 2 A charge of fresh air is drawn or pumped into the engine cylinder and then compressed by the moving piston to very high pressure. 3 When the air is compressed, its temperature rises so that it ignites the fine spray of fuel injected into the cylinder. The burning of the fuel adds more heat to the air charge, causing it to expend and force the engine piston to do work on the crankshaft which in turn drives the ship's propeller. 4 The operation between two injections is called a cycle, which consists of a fixed sequence of events.This cycle may be achieved either in four strokes or two. In a four-stroke diesel engine, the cycle requires four separate strokes of the piston, i.e. suction, compression, expansion and exhaust. As its name implies a two-stroke cycle takes place in two consecutive strokes of the engine piston, or one revolution of the crankshaft. Thus each operation in the cycle is repeated during every revolution of the engine. The two strokes of the cycle may be termed: Compression stroke and power or expansion stroke. 5 Its operation is as follows(see Fig. 2-1) 1) The crankshaft is revolving clockwise and the piston is moving up the cylinder, compressing the charge of air. Because energy is being transferred into the air, its pressure and temperature increase. By the time the piston is approaching the top of the cylinder(known as top dead center or TDC) the pressure is over 100 bar and the temperature over 500 °C . 2) Just before TDC fuel is injected into the cylinder by the fuel injector. The fuel is “atomized” into tiny droplets. Because they are very small these droplets heat up very quickly and start to burn as the piston passes over TDC. The expanding gas from the fuel burning in the oxygen forces the piston down the cylinder, turning the crankshaft. It is during this stroke that work energy is being put into the engine. 3) As the piston moves down the cylinder, the useful energy from the burning fuel is expended. At about 11 0 0 after TDC the exhaust valve opens and the hot exhaust gas (consisting mostly of nitrogen, carbon dioxide, water vapour and unused oxygen) begins to leave the cylinder. 4) At about 140 0 after TDC the piston uncovers a set of ports known as scavenge ports. Pressurized air enters the cylinder via these ports and pushes the remaining exhaust gas from the cylinder in a process known as “scavenging”. The piston now goes past bottom dead center and starts moving up the cylinder, closing off the scavenge ports. The exhaust valve then closes and compression begins. 6 The two-stroke cycle can also be illustrated on a timing diagram (see Fig. 2-2). 7 The main difference between the two-stroke and the four-stroke cycles is the power developed. The two-stroke cycle engine, with one working or power stroke every revolution will, theoretically, develop twice the power of a four-stroke engine of the same swept volume. Inefficient scavenging however and other losses, reduce the power advantage to about 1.8. For a particular engine power the two-stroke engine will be considerably lighter-an important consideration for ships. Nor does the two-stroke engine require the complicated valve operating mechanism of the four-stroke. The four-stroke engine however can operate efficiently at high speeds, which offsets its power disadvantage; it also consumes less lubricating oil. 8 Each type of engine has its applications, which on board ships have resulted in the slow speed (i. e. 80-100 r/min) main propulsion diesel operating on the two-stroke cycle. At this low speed the engine requires no reduction gearbox between it and the propeller. The four-stroke engine (usually rotating at medium speed, between 250 and 750r/min) is used for auxiliaries such as alternators and sometimes for main propulsion with a gearbox to provide a propeller speed of between 80 and 100r/min.