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Understand Marine Diesel Engines

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File:Rotterdam Ahoy Europort 2011 (49).JPG
S.J. de Waard/Wikimedia Commons

 

A diesel engine is a reciprocating internal combustion engine. Reciprocating means that pistons in the cylinder bores move up and down and turn a common crankshaft to transfer energy from the fuel to rotational motion.

Internal combustion is a common type of engine where the fuel is burned for power in an enclosed space like the cylinder bore. Other types of engines might burn fuel to heat water and produce steam used to turn the running gear of a ship. The oldest operating passenger steamship in North America uses external combustion in the form of a coal fired boiler.

The first successful compression ignition engine was operated in 1897 by the inventor Rudolf Diesel. Diesel engines don’t use a spark or heated wire to ignite the fuel and air mixture in the cylinder; instead the mixture is heated to explosion by pressure alone.

As we know from the Laws of Thermodynamics if a gas, or air and fuel mixture, is rapidly compressed it will increase in temperature. The same amount of heat is still present in the smaller compressed volume and when the heat energy reaches a critical temperature, called a flashpoint, the mixture burns in a small explosion.

 This explosion drives the piston down the cylinder bore which pushes a connecting rod which turns the crankshaft. Diesels in marine equipment can be from one to twenty or more cylinders turning a common crankshaft.

Four Stoke Diesel Marine Engines

This is a more complex and recent type of diesel engine that uses movable valves to open and close intake and exhaust ports using a camshaft and mechanical timing system. It is similar in design to gasoline fueled internal combustion engines.

Precise timing of intake and exhaust cycles helps assure complete fuel burn and reduces emissions from unburned fuel in the exhaust. Diesel engines built a few decades ago before emissions were heavily regulated would develop particulates in exhaust gasses that made many people associate diesel engines with unpleasant fumes and sooty smoke.

Valves and other components of a valve train are delicate and wear easily. This wear is increased by higher cylinder bore pressures, often referred to as compression or a compression ratio.

A compression ratio in any internal combustion engine is the volume of uncompressed gasses in the cylinder bore compared to the volume of the same gasses after full compression. Four stroke diesels operate at slightly higher pressures than gasoline internal combustion engines which allow similar valve configurations.

Two Stroke Marine Diesel Engines

A two stroke diesel engine charges each cylinder bore with fuel and air on each revolution. A four stroke engine adds fuel and air every other stroke. This action is where the types of engines get their names.

The power cycle of a four stroke engine is; intake, compression, ignition, exhaust. A two stroke only makes an intake and ignition stroke because of the way gasses enter and leave the cylinder bores.

Instead of traditional spring loaded engine valves two stroke diesel engines use intake and exhaust ports in the wall of the cylinder bore. The piston draws a fuel and air mixture from the injection device or intake port as it moves down the cylinder bore where it uncovers an exit path for exhaust gasses to exit.

The size of this exit might be up to 60 percent of the bore diameter in large ship engines. The force of the other pistons pushing on the crankshaft moves the piston back into compression position where the fuel charge explodes and the process starts again. Rotary valves which are disks with holes that rotate in passageways to fine tune the timing of intake and exhaust processes.

The ports and rotary valves are used since compression ratios are much higher than four stroke designs.

Intake air is often compressed by a supercharger or turbocharger in two stroke diesel engines. This not only pushes more fresh air into the engine but it also pushes exhaust out more efficiently and completely.

The largest reciprocating engine in the world is made by Wartsila of Finland to power the newest class of high efficiency cargo ships. It is available in a fourteen cylinder configuration that burns around 2,500 gallons of heavy fuel oil per hour while making 52,000 horsepower at half throttle and less tha 60 rpm. It is also as big as a two level house.

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