You have just bought that beautiful used 1986 Labelle sundeck trawler and she’s everything you wanted in a boat. The nice sundeck, the master stateroom and the huge flybridge are perfect. And she also has twin Volvo diesel engines and a Westerbeke generator too. But you’ve been wondering, how do those engines really work? You have never owned a diesel engine.
In theory, diesel engines and gasoline engines are quite similar. They are both internal combustion engines designed to convert the chemical energy available in fuel into mechanical energy. This mechanical energy moves pistons up and down inside cylinders. The pistons are connected to a crankshaft, and the up-and-down motion of the pistons, known as linear motion, creates the rotary motion needed to turn the propeller 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 need to choose a cost effective fuel to meet our needs. Thanks to the invention of Rudolph Diesel in 1892 in Augsburg, Germany, the diesel engine has proved to be extremely efficient and cost effective. And in 1894 Rudolph Diesel was almost killed when his engine exploded. But that explosion proved 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 operate on peanut oil, vegetable oils, synthetic oils, and even hydraulic fluids. Rudolf Diesel even experimented with operating 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 discovered to be a readily available resource, a fuel we now call diesel fuel was refined to power diesel engines. Diesel fuel is priced moderately higher than gasoline but diesel has a higher energy density, i.e. more energy can be extracted from diesel as compared with the same volume 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 remember how a diesel engine works is by remembering 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 cylinder (suck). The air is then compressed by the movement of the piston, and fuel is injected as a vapor just before the piston reaches the top of the cylinder (squeeze). The compression raises the temperature of the air; this causes the fuel to combust (bang). Finally, the waste gases are forced out of the cylinder (blow) and into the exhaust system.
A diesel pump's full name is a diesel injection pump, and like the name suggests, is responsible for injecting 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. They rely entirely on the high compression pressure of the fuel in the cylinder to result in combustion. 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 necessary for the engine's functioning.
Naturally aspirated diesel engines simply draw in the air (suck) to begin the combustion cycle. These diesels produce less power than their turbocharged cousins. Turbo charging is the forcing of air into the engine allowing it to produce more power.
Turbocharged diesels refer to 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 specific power outputs, lower emissions levels, improved efficiency and higher refinement levels than their naturally aspirated counterparts.
Turbo diesels are generally considered more flexible for yacht uses than naturally-aspirated diesels, which have strong low-speed torque outputs but lack power at higher speeds. Naturally-aspirated diesels usually found in small-engine used trawlers, almost without exception, have a lower power output than a petrol engine of the same capacity whilst the same time requiring stronger (and thus heavier) internal components such as the pistons and crankshaft to withstand the greater stresses of the diesel engine's operating cycle.
Thusly, the heavier design and build of a diesel engine is built 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 still going strong.
Diesel engines can suffer damage as a result of misapplication or misuse – namely internal glazing and carbon buildup. This is a common problem in generator sets caused by failure to follow application and operating guidelines – ideally diesel engines should run at least around 75% of their maximum rated load and RPM. Short periods of low load running are permissible providing the set is brought up to full load, or close to full load on a regular basis.
