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Engine Room

Diesel Digest: Fuel properties drive diesel engine’s performance

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Diesel Digest: by Capt. Jeff Werner

Whether diesel engines are powering a yacht, a jet drive tender, an onboard generator or a crew car, their performance is directly related to a variety of operational characteristics:

Starting ease
Low emissions
Sufficient power
Low wear
Long filter life

Unquestionably, engine design has the most impact on these characteristics, and modern diesel engines are engineered to assure the best possible performance. Diesel fuel, the lifeblood of the engine, must also be reliable to maximize engine performance. The fuel properties – whether the type of crude oil used, how it is refined and blended, its stability and how it conforms to standards – all affect performance.

Starting ease
Diesel engines use the heat developed by compressing the fuel/air mixture in the cylinder to spark ignition and run the engine. Therefore a cold engine (one that has cooled overnight) can be more difficult to start than a warm engine (one that has been stopped a short time). The easier it is for diesel fuel to ignite and burn, the easier a cold engine will start. That ignition quality is called the cetane number or index. “A fuel with a high cetane number starts to burn shortly after it is injected into the cylinder; therefore, it has a short ignition delay period. Conversely, a fuel with a low cetane number resists autoignition and has a longer ignition delay period,” according to Chevron Diesel Fuels Technical Review.

Low emissions
At the refinery, petroleum is distilled to make diesel and other fuels. A chemical process called upgrading is used to remove undesirable emission components found in fuel through use of a catalyst. The most common catalyst used in refining diesel fuel is hydrogen. This technique of mixing diesel with hydrogen removes sulfur from the fuel. It is an important step, and necessary to meet the stringent emissions requirements imposed by the International Maritime Organization on new yacht engines that require the use of ultra low-sulfur diesel fuel.

Sufficient power
The power of a diesel engine is measured in units of brake horsepower (bhp). Brake horsepower is the power output at the drive shaft and is always less than that at the piston due to friction and other losses. Once a diesel engine reaches its rated brake horsepower, any increase in the engine load will cause the amount of smoke in the exhaust to reach an unacceptable level. This is known as the smoke limit and it is related to the viscosity of the fuel. During the refining process, diesel fuel that is produced to the proper viscosity standards will always provide more power at peak torque and full load than fuel that is out of specifications for thickness.

Low wear
The ability to reduce friction between solid objects in relative motion is called lubricity. For example, engine oil has a very high lubricity and it keeps the moving parts in an engine from wearing down prematurely. Diesel fuel must have a certain level of lubricity to protect fuel pumps and injectors from wear as the fuel moves through those components at high pressure. Sulfur found in varying quantities in crude oil is a natural lubricant in refined diesel and it is also the base element of sulfur oxides that are a major pollutant caused by combustion. In order to reduce harmful emissions, sulfur has been removed from diesel fuel to a maximum level of 15 parts per million. In order to restore the lubricity that was lost, additives are introduced at the refinery and with aftermarket products.

Long filter life
Filters are not intelligent devices. They remove contaminants in the fuel as they pass through the filter media. Reducing the level of contaminants before the fuel reaches the engine filter will increase its service life and dramatically reduce the amount of dollars spent on filters every year. The stability, or shelf life, of fuel and its cleanliness are directly related to having a fuel preventive maintenance program. Installing a diesel dialysis system on board to clean fuel will always assure low counts of particulate matter to reduce the risk of damage to and the costly repair of engine components.

Capt. Jeff Werner is a 25-year veteran of the yachting industry as a captain and as a certified instructor for the RYA, MCA, USCG and US Sailing. He also owns Diesel Doctor (MyDieselDoctor.com). Comments are welcome below.

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