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 ship. The principle of its operation is as follow: A charge of fresh air is drawn or pumped into the engine cylinder and then compressed by the moving piston to very high pressure.
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 beat to the air charge, causing it to expand and force the engine piston to do work on the crankshaft which in turn drive the ship’s propeller.
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. If we combine the suction and exhaust operations with the compression and expansion stroke, the four-stroke engine will be turned into a two-stroke one.
The two-stroke cycle begins with the piston coming up from the bottom of its stroke, i.e. bottom dead center (BDT), with the air inlet ports or scavenge ports in the sides of the cylinder being opened. The exhaust ports are uncovered also. Pressurized fresh air charges into the cylinder, blowing out any residual exhaust gases from the last stroke through the exhaust ports. As the piston moves about one-fifth of the way up, it closes the inlet ports and the exhaust ports. The air is then compressed as the piston moves up.
When the piston reaches the top of its stroke, i.e. the top dead center (TDC), both the pressure and the temperature of the air rise to very high values. The fuel injector injects a fine spray of fuel oil into the hot air and combustion takes place, producing much higher pressure in the gases.
The piston is forced downward as the high-pressure gases expand until it uncovers the exhaust ports. The burnt gases begin to exhaust and the piston continues down until it opens the inlet ports. Then another cycle begins.
In the two-stroke engine, each revolution of the crankshaft makes one power or working stroke, while in the four-stroke cycle engine, it takes two revolutions to make one power stroke, that is why a two-stroke cycle engine will theoretically develop twice the power of a four-stroke engine of the same size. Inefficient scavenging and other losses, however, reduce the power advantage to about 1.8.
Each type of engine has its application on board ship. The low speed (i.e. 90 to 120 r/min) main propulsion diesel operates on the two-stroke cycle. At this low speed the engine requires no reducing gearbox between it and propeller. The four-stroke engine (usually rotating at medium speed, between 250 to 750 r/min) is used for alternators and sometimes for main propulsion with a gearbox to provide a propeller speed of between 90 to 120 r/min.



Related Terms

GEARBOX

A system of gears which alter the ratio between the revolution of the engine and the propeller shaft so the propeller operates in a relatively efficient speed range. By using a gearbox the engine and the propeller shaft will revolve at different speeds.

PISTON VALVE

A cylindrical type of steam engine slide valve for admission and exhaust of steam.

PISTON ENGINE

A type of engine characterized by reciprocating motion of pistons in a cylinder. Also known as displacement engine; reciprocating engine.

PERFORMANCE NUMBER

One of a series of numbers (constituting the PN, or performancenumber, scale) used to convert fuel antiknock values in terms of a reference fuel into an index which is an indication of relative engine performance; used mostly to rate aviation gasolines with octane values greater than 100.

OVERRUNNING CLUTCH

A clutch that allows the driven shaft to turn freely only under certain conditions; for example, a clutch in an engine starter that allows the crank to turn freely when the engine attempts to run.

OVERSQUARE ENGINE

An engine with bore diameter greater than the stroke length.

OVERDRIVE

An automobile engine device that lowers the gear ratio, thereby reducing fuel consumption.

OVERHEAD-VALVE ENGINE

A fourstroke-cycle internal combustion engine having its valves located in the cylinder head, operated by pushrods that actuate rocker arms. Abbreviated OHV engine. Also known as valve-in-head engine.

OVERGEAR

A gear train in which the angular velocity ratio of the driven shaft to driving shaft is greater than unity, as when the propelling shaft of an automobile revolves faster than the engine shaft.

OPTICAL INDICATOR

An instrument which makes a plot of pressure in the cylinder of an engine as a function of piston (or volume) displacement, making use of magnification by optical systems and photographic recording; for example, the small motion of a pressure diaphragm may be transmitted to a mirror to deflect a beam of light.

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