How Does a Marine Diesel Engine Work?

Published: 15th February 2010
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You have just purchased that gorgeous used 1985 Labelle sundeck trawler and she is everything you wanted in a boat. The nice sundeck, the owner's quarters and the huge fly bridge are just right. She's also got twin Volvo diesels and a Onan generator too. But you've been thinking, how do those engines really work? These are the first diesels you have owned.

In theory, diesel engines and gas engines are quite similar. Both are internal combustion motors designed to change the chemical energy available in diesel or gasoline into mechanical energy. This mechanical energy moves pistons up and down inside piston chambers. The pistons are attached to a crankshaft, and the rhythmic motion of the pistons, recognized as linear motion, produces the rotary motion needed to rotate the prop on your used trawler or motor yacht.

In today's world, where diesel or gasoline costs are increasing as a consequence of increasing demand and diminishing supply, we must choose a cost effective fuel to power our vessels.

Thanks to the invention of Rudolph Diesel in 1892 in Augsburg, Germany, the diesel engine has proved to be exceedingly economical and cost effective. In1894 Rudolph Diesel was almost killed when his engine blew up. But that explosion established that fuel can be ignited without a spark.

Diesel engines were the first bio-fueled motors. Diesel's first diesel engine ran on peanut oil. In the real world, a diesel engine can function on peanut oil, vegetable oils, synthetic oils, and even hydraulic fluids. Rudolf Diesel even tried out running earlier diesel engines with gun powder. Handling and storing of the gun powder soon laid to rest that idea.

After crude oil was discovered to be a easily available resource, a product we now call diesel fuel was processed to power diesel engines. Diesel fuel is priced somewhat higher than gas but diesel has a greater energy density, i.e. more power can be extracted from diesel as compared with the identical amount of gasoline. Consequently, diesel engines provide greater power, making it an obvious choice for large used trawlers and motor yachts. Diesel weights more and is thicker compared to gasoline. Diesel fuel has a high flash point making storage aboard a boat very safe.

The simplest way to remember how a diesel engine works is by remembering the phrase "suck, squeeze, bang, and blow". This relates to a cycle of 4 strokes known as the OTTO cycle.

First of all, air is drawn into the piston chamber (suck). The air is then constricted by the upward movement of the piston, fuel is then injected as a vapor just before the piston contacts the top of the cylinder (squeeze). The compression raises the temperature of the air; which then causes the fuel to ignite (bang). Finally, the burned gases are forced out of the cylinder (blow) and into the exhaust stream.

A diesel injection pump injects fuel into the firing cylinders of diesel engines. It is important to remember that, unlike gasoline-powered engines, diesel engines do not use spark plugs to ignite the fuel in the firing cylinders. Diesel engines rely entirely on the compressing of the diesel in the piston chamber to result in ignition of the diesel. As a result, diesel injection pumps are extremely important and must be built tough to produce the compression rates of up to 15,000 psi needed for the engine's functioning.

Naturally aspirated engines pill in air without mechanical help (suck) to begin the combustion cycle. These diesels develop less power than their turbocharged first cousins. Turbocharging is the mechanical pushing of air into the engine permitting it to produce more power.

Turbocharged diesels mean any diesel engine with a turbocharger. Turbo charging is the standard rather than the exception in bigger and faster motor yachts. But with any turbocharged engine, turbo diesels can offer greater power outputs, lower emissions levels, better efficiency than their naturally aspirated counterparts.

Hi power engines means stronger (and thus heavier) internal component parts such as the pistons and crankshaft to resist the constant pounding from the diesel engine's operating cycle. Thusly, the design of a diesel engine is made to take hundreds of hours of constant use under load. I am told by the Westerbeke spokesperson of one engine, still in use today that has thirty thousand hours on her and she is still operating fine.

Diesels can be damaged as a consequence of misapplication or abuse - principally internal glazing and carbon buildup. This is a common problem in generators caused by failure to run the engines not under a load - ideally diesels should run at least about 75% of their maximum rated load and Revolutions Per Minute. Short periods of low load running are allowable providing the diesel engine is brought up to full load, or close to full load on a frequent basis.

In a different article, we'll review the primary problem with diesel engines - the quality of the fuel.

Mike Dickens, the author, is a live aboard boat owner and owner/Broker of Paradise Yachts in Florida USA.

Paradise Yachts offers used quality yachts to customers worldwide.

Visit the Paradise Yachts website to view our selection of Used Trawlers, Used Motor Yachts, and Used Sailboats for Sale

National and international sales. We ship Used Trawlers, Motor Yachts and Cruisers worldwide. Located in Florida, USA. 904/556-9431

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