How Does a Diesel Engine Work?

You have just bought that stunning used 1986 Vista sundeck trawler and she is everything you needed in a yacht. The good sundeck, the master stateroom and the big fly bridge are just right. She also has twin Volvo diesels and a Onan gen set too. But you’ve been thinking, how do diesel engines function? You have never owned a diesel before.

In Crankshaft Repair theory, diesels and gasoline engines are rather similar. They are both 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 connected to a crankshaft, and the up-and-down motion of the pistons, recognized as linear motion, creates the rotary motion needed to rotate the prop on your used trawler or motor yacht.

Today, where diesel or gasoline prices are increasing as a aftermath of spiraling demand and decreasing supply, we need to choose a cost effective fuel to power our vessels. After the invention of the diesel motor in 1892 by Rudolph Diesel in Augsburg Germany, the diesel engine has proved to be extremely efficient and cost effective. In1894 Rudolph Diesel was nearly killed when his prototype engine blew up. But that explosion established that diesel can be ignited without a spark plug.

A diesel motor is truly a bio-fuel engine. Diesel’s first engine ran on peanut oil. In practice, a diesel can run on peanut oil, vegetable oils, synthetic oils, and even hydraulic fluids. Rudolf Diesel even experimented operating earlier diesels with gun powder. Handling and storing of the gun powder soon laid to rest that idea.

After oil was discovered to be a easily available resource, a fuel we now call diesel fuel was processed to power diesel engines. Diesel fuel is priced somewhat higher than gas but diesel has a higher energy density, i.e. more power can be withdrawn from diesel as compared with the same amount of gasoline. Therefore, diesel engines provide greater power, making it an unmistakable choice for big used trawlers and motor yachts. Diesel is heavier and oilier compared with gasoline. Diesel fuel has a high flash point making storage aboard a boat very safe.

The easiest 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 recognized as the OTTO cycle.

First of all, air is drawn into the cylinder (suck). The air is then compressed by the movement of the piston, and fuel is injected as a vapor just before the piston contacts the top of the cylinder (squeeze). The compression raises the temperature of the air; which causes the diesel fuel to combust (bang). Eventually, the waste gases are blown from the piston chamber (blow) and into the exhaust system.

A diesel injection pump is responsible for injecting fuel into the firing cylinders of diesel engines. It is important to remember that, different from gasoline-powered engines, spark plugs are not used to ignite the fuel. They rely entirely on the compressing of the diesel in the piston chamber to result in combustion. Therefore, diesel injection pumps are extremely important and must be built tough to create the compression values of up to 15,000 psi necessary for the engine’s operation.

Naturally aspirated diesel engines just pull in the air (suck) to begin the combustion cycle. These diesels produce less power than their turbocharged cousins. Turbocharging is the mechanical forcing of air into the engine permitting it to produce more power.

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

Hi power engines requires stronger (and thus heavier) internal component parts such as the pistons and crankshaft to withstand the continuous hammering from the diesel engine’s operating cycle. Therefore, the design of a diesel engine is made to take hundreds of hours of constant use under load. I am told by the Westerbeke representative of one engine, still in use today that has 30,000 hours on her, and she is still operating fine.

Diesel engines can sustain damage as a result of misapplication or misuse – principally internal glazing and carbon buildup. This is often found in generators caused by failure to run the engines not under a load – ideally diesel engines should run at least about 75% of their maximum rated load and Revolutions Per Minute. Short-term times 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.

 

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