Diesel fuel is the lifeblood of a diesel engine. If left unmanaged, diesel fuel quality can degrade rather quickly rendering it harmful, rather than helpful to an engine. Understanding the sources and nature of diesel fuel contamination is the first step in maintaining optimal fuel quality aboard a yacht.
All forms of contamination have their own unique properties that can have an effect on the efficiency and reliability of an engine and its fuel system. Water is the most damaging contaminant found in diesel fuel, and it is the primary cause of additional fuel breakdown. Water can exist in three different states within diesel fuel: Dissolved, free and emulsified.
Dissolved water, sometimes called entrained water, is the result of diesel fuel being hygroscopic. That means that fuel has the ability to attract and hold water from the environment, whether it is from humid air or condensation on the wall of a fuel tank. And as the fuel temperature in the tank increases, so does the amount of water that can be dissolved and held in solution in the fuel.
Free water is water that gathers at the bottom of the fuel tank due to its higher density relative to diesel fuel. This can only occur once diesel fuel has become saturated with dissolved water and the fuel temperature is not high enough to hold any additional water. For example, since a diesel engine does not burn all the fuel that reaches the injectors, the remainder is returned to the fuel tank. This unburned fuel has passed through tubing next to a very hot engine and picked up some of this residual heat. Once the returned fuel mixes with the fuel already in the tank, all the fuel in the tank is warmed up, which allows it to absorb more water. By the end of a day’s run, the fuel has more water entrained in it.
While at the dock or at anchor overnight, as the fuel in the tank cools down, the saturated fuel releases this moisture as free water and it falls to the bottom of the tank. The simplest way to detect this is by coating the bottom six inches of a fuel tank dipstick with water finding paste. Then stick the tank. If the paste changes its color, then free water is present.
Emulsified water is the result of mixing free water with diesel fuel. Microscopic water droplets become bound within the fuel for an extended amount of time. A useful analogy is a bottle of salad dressing. The olive oil floats on top of the balsamic vinegar (which is mostly water). Shake the bottle, and voila, vinaigrette is made as the vinegar and oil are emulsified. After a few minutes both separate back out. On a yacht, emulsified water occurs when bunkering fuel with a high-speed delivery pump. This churns the fuel in the tank with the free water at the tank’s bottom. Also, on a very rough day at sea, when a yacht pitches and rolls severely, the fuel in the tank gets a good shaking and mixing.
There is one basic rule when it comes to water in diesel fuel, if it is present, remove it. If not, microbial growth will occur. The water at the bottom of the tank serves as a perfect medium for
these microorganisms to live in. And the hydrocarbons in the fuel are a tasty food source for many species of microbes. The result is a proliferation of bacteria and fungi feeding at the fuel/water interface and large colonies floating in the free water below that interface. In addition, microbes can adhere to the tanks walls and can grow fast enough to quickly coat those walls with a slime of organisms.
The first step in removing water and microbes from a yacht’s fuel is developing a three-phase fuel preventive maintenance program: sampling, testing and polishing. Implementing and strictly adhering to this maintenance program is a simple and inexpensive alternative to the cost of repairing or replacing damaged high-pressure fuel pumps, injectors and pistons.
Capt. Jeff Werner has been in the yachting industry for 25 years, and is the owner of Diesel Doctor (MyDieselDoctor.com). All Triton readers receive a 10% discount on online orders. Contact him at Jeff@MyDieselDoctor.com.