Turbochargers : Energizing The Engines

In a marine diesel engine, a fine combustion is a result of an adequate supply of air. The total output power of the whole engine can be drastically improved by increasing the density of air entering the engine. This is accomplished with the help of a device known as turbocharger and in this article we are going to see how turbochargers work.

In an engine without a turbocharger, such as the naturally aspirated automobile engines, air is sucked inside the engine by a low pressure area created by the downward stroke of the piston. But this system works on a constant air inlet pressure which can neither be increased or decreased nor is sufficient for a complete combustion. (you can check out different working cycles here)

In order to resolve this problem, turbo chargers are used to provide a higher density of air to the engine. Thus turbo-charger is a mechanism for providing forced-induction to marine diesel engines. This forced induced system compresses the air and squeezes it into the engine's cylinder, allowing a large quantity of fuel to enter the engine. This not only helps in procuring more power but also improves the power-to-weight ratio of the engines.

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The process of providing engine cylinders with pressurized fresh air using turbocharger or supercharger, is known as charging.

• Super-charging is a process by which pressurized forced air is provided with the help of an external charging pump.
• Turbo-charging is providing pressurized air by using engine's exhaust.

Nowadays, both 2-stroke and 4-stroke are provided with external charging systems. A 4-stroke engine is generally provided with a turbocharger whereas in a 2-stroke engine, in addition to a turbocharger an electrically driven auxiliary blower is also provided, as the turbocharger alone cannot provide enough air for the low speed engines.

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Both turbocharger and supercharger are forced induced systems used for providing more air to the engine's cylinder. The difference between them is that a supercharger is driven mechanically with the help of belts and gears attached to the engine's crankshaft. Whereas the turbocharger uses the exhaust air energy from the engine. Rest of the mechanism is same for both.

A turbocharger consists of two main parts - a turbine and a compressor, which are both mounted on the same shaft. The exhaust gases from the engine rotates the turbine which in turn rotates the compressor. The compressor takes air from the surrounding, compresses it and sends it to the intake air manifold.

A supercharger also runs on the same principle, with the only difference that instead of using exhaust gases it uses the crankshaft of the engine to drive it. The advantage of using a supercharger is that, as it is directly linked to the engine it provides a better throttle response and instantaneous full boost pressure. Problem of change in speed due to fluctuation of exhaust air pressure is also avoided. Whereas, using a turbocharger increases the overall efficiency of the engine, as it uses energy from the exhaust gases which is usually wasted which also increases the power of the overall unit.

In the next article we will learn about the working and construction of turbochargers, followed by turbocharger surging phenomenon.

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Introduction to marine engineering -2nd Edition by D.A Taylor

Image Credits

http://www.ecy.wa.gov/programs/spills/prevention/eom/eom10/eom10a.jpg

http://product-image.tradeindia.com/00008057/b/0/Turbocharger.jpg

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