Used Equipment

How Does A Diesel Engine Work?

You just bought that beautiful used 1987 Vista sundeck trawler and she is everything you wanted in a boat.  The good sundeck, the master stateroom and the big flybridge are just right.  She also has twin Lehman diesel engines and a Onan genset also.  But you’ve been thinking, how do diesel engines function?  These are the first diesels you have owned.

On paper, diesel engines and gas motors are quite alike. Both are internal combustion engines fashioned to change the chemical energy available in fuel into mechanical energy. This mechanized energy pushes pistons up and down inside cylinders. The pistons are connected to a crankshaft, and the rythmic motion of the pistons, known as linear motion, produces the rotary motion required to rotate the prop on your used trawler or motor yacht.

In today’s world, where fuel prices are increasing as a consequence of spiraling demand and diminishing supply, we must select a cost efficient fuel to meet our needs. After the invention of the diesel engine in 1892 by Rudolph Diesel in Augsburg Germany, the diesel engine has proved to be exceedingly efficient 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.

A diesel engine is truly a bio-fuel engine.  Diesel’s first engine ran on peanut oil.  In practice, a diesel engine can function on peanut oil, vegetable oils, synthetic oils, and even hydraulic fluids.  Rudolf Diesel even experimented running earlier diesel engines with gun powder.  But the handling of the fuel posed a significant problem and that fuel source was abandoned.

After oil was found to be a readily obtainable resource, a product we now call diesel fuel was refined to power diesel engines.  Diesel fuel is priced somewhat higher than gasoline but diesel has a greater energy density, i.e. more energy can be withdrawn from diesel as compared with the same amount of gasoline. Therefore, diesel engines provide greater power, making it an obvious choice for large used trawlers and motor yachts. Diesel is heavier and oilier compared with gasoline.  Its flash point is higher than that of gasoline making it very safe to have aboard a boat.

The easiest way to think of how a diesel engine works is by memorizing the phrase “suck, squeeze, bang, and blow”. This refers 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 movement of the piston, and fuel is injected as a vapor just before the piston reaches the top of the cylinder (squeeze). This compression raises the temperature of the air; which causes the diesel fuel to combust (bang). Finally, the burned gases are blown out of  the cylinder (blow) and into the exhaust stream.

A diesel engine injection pump is responsible for injecting fuel into the firing cylinders of diesel engines. It is important to remember that, unlike gasoline-powered engines, spark plugs are not used to ignite the fuel. Diesel engines rely completely on the compressing of the fuel in the cylinder to result in combustion. As a result, diesel injection pumps are exceedingly important and are bult rugged to create the compression values of up to 15,000 psi necessary for the engine’s operation.

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

Turbocharged diesels mean any diesel engine with a turbocharger. Turbo charging is the norm rather than the exception in larger and faster motor yachts. As with any turbocharged engine, turbo diesels can offer higher power outputs, lower emissions levels, improved efficiency than their naturally aspirated counterparts.

Hi power engines requires stronger and heavier internal components such as the pistons and crankshaft to withstand the constant lumbering from the diesel engine’s operating cycle. Thusly, the design of a diesel engine is built to take hundreds of hours of sustained use under load.  I am told by the Westerbeke representative 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 result of misapplication or misuse – pricipally internal glazing and carbon buildup. This is a common problem in generator sets 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 permissible providing the diesel engine is brought up to full load, or close to full load on a regular basis.

In another article, we’ll examine the principle problem with diesel engines – the quality of the fuel.

About the Author

Mike Dickens, the author, is a boat owner and owner/Broker of Paradise Yachts. Paradise Yachts is a Yacht Brokerage offering used yachts to customers worldwide. Visit our website to view our selection of Used Trawlers, Used Motor Yachts, and Used Sailboats . National and international sales. We ship Used Yachts and Used Boats worldwide. Located in Florida, USA. 904/556-9431

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ONAN PISTON MILITARY SURPLUS 112-0109-40 WITH RETAINER CLIPS AND PIN $15.00

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