Most diesel engine failures start in the fuel tank. If all the mechanical parts are in good condition and the cooling, air intake and lubrication systems are all working properly, then a diesel engine could run almost forever. The only limiting factors are fuel quality and keeping the fuel system clean. Many yacht captains and engineers understand this, so they frequently ask three questions about diesel fuel:
<bold>What is “bad fuel”?
The American Society of Testing and Materials (ASTM) has developed 2,400 standards for diesel fuel. In the petroleum industry, if fuel doesn’t comply with those specifications, they say it is “out of spec” and is considered “bad fuel”. Working aboard a yacht, we tend to be a bit more empirical, and judge fuel quality by our observations.
We say that diesel is bad, when we notice:
Dark, hazy fuel
Fuel filter clogging
Sludge build up the in fuel tanks
Poor engine performance
Excessive smoke and emissions
<bold>What tests are recommended?
Out of the 2,400 ASTM specs for diesel, there are only seven standards that an engineer or
captain should be concerned about for everyday use, and they should be tested for on a quarterly basis:
<bold>Distillation</bold> gives information on the behavior of fuel during storage and use.
Microbial growth determines the presence of bacteria, mold and fungi and the size of the colonies.
<bold>Flash point</bold> indicates contamination of diesel fuel by other fuels such as gasoline.
<bold>Water determination by Karl Fischer titration</bold> measures the concentration of water entrained in the fuel.
<bold>Water and sediment</bold> determines the volume of free water and particulate matter in the fuel.
<bold>Appearance</bold> using a visual inspection procedure to determine the clarity of fuel.
<bold>Stability and accelerated aging</bold> measures the oxidation of fuel which affects its shelf life.
These seven tests are typically performed for a package price by a dedicated fuel testing laboratory using a fuel sample provided by the yacht. Any crew member can be taught to draw the sample needed from each tank. If you suspect you have bad fuel, or conditions which accelerate the natural loss of fuel quality, then these tests should be run more frequently than four times per year.
<bold>Is “dark fuel” the same as “bad fuel”?
Diesel fuel ranges from clear to amber in color, depending on the type of crude oil started with and how it is refined. A number of countries like the United States, Canada, the United Kingdom and France add light-colored dyes to differentiate marine diesel, which tends to have more sulfur, from diesel used for roadway purposes.
Dark fuel means that oxidation is taking place and that fuel degradation is at an advanced stage. Hazy fuel is an indicator of water emulsified in the fuel. Emulsifying is similar in concept to shaking up an oil and vinegar salad dressing. When the vinegar mixes with the oil, both lose their transparency and the blend is more opaque. In general, dark, hazy fuel will not damage your engine. It will burn, but the poor fuel quality will only offer mediocre engine performance. This low level of efficiency will also accelerate the aging process of the engine. Using questionable quality fuel can shorten the life of the very expensive marine diesel engines used aboard superyachts.
The University of Idaho conducted tests on the life expectancy of diesel fuel stored in a tank. If your yacht sits idle for just one month during the year, then expect a significant loss in quality of the fuel in its tanks.
<bold>Aren’t the filters supposed to keep the fuel system clean?
The external or primary filter, and the engine mounted or secondary filter cannot do that. Those filters are designed to remove inorganic debris from the fluid stream. Fuel degradation also clogs those filters with organic contaminants suspended in the fuel.
Since filtration alone has no effect on the fuel breakdown process, the filters continue to get clogged by the ever increasing amount solids forming in the fuel. At the same time maintenance costs increase, as a steady stream of plugged filters need to be replaced. Filtration will also not remove the sludge coating the walls and baffles of your yacht’s fuel tank.
Every diesel fuel system must have water separator built in to its filtration components to function properly. The water separator is designed to remove free water from fuel. However, a water separator or centrifuge cannot remove emulsified water.
Stopping, preventing or reversing the process of fuel breakdown is a completely different matter that can only be addressed by a proper fuel preventive maintenance program. This program solves the problem through fuel testing, separation, filtration, conditioning and restoration.
Capt. Jeff Werner has been in yachting for more than 20 years on private and charter yachts, both sail and power. He is an instructor for RYA, MCA, USCG and US Sailing courses and owns Diesel Doctor (MyDieselDoctor.com). Comments on this column are welcome at firstname.lastname@example.org.