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Old 03-22-2004
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Tom Wood is on a distinguished road
Diesel Fuel Essentials


The auxiliary diesel engine is much more than just a source for propulsion, so keeping your boat's powerplant up and running is a critical aspect of any maintenance schedule.
Marine diesel engines are generally robust and remarkably dependable products and you'll find sailors the world round that will swear by them. But keeping auxiliary engines up and running isn't something that happens on its own; it takes diligent maintenance and monitoring on the part of the boat owner. That's why it's good to know that the majority of problems that do occur with auxiliary engines can usually be traced to the fuel system. Since common contaminants such as water or dirt particles can stop an auxiliary cold, sailors have traditionally gone to great pains to maintain clean fuel.

In an attempt to reduce toxic emissions from diesels, recent federal legislation in the US requires fuel to have reduced levels of sulfur. This Low Sulfur Diesel (LSD) is undoubtedly good for the environment, but troublesome for owners of older diesels. Let me explain.

It's not the sulfur itself in diesel fuel that provides the benefits. In fact, high levels of sulfur when mixed in the combustion process form sulfuric acid, which can be highly corrosive to many internal engine and exhaust parts. But the diesel oil with the highest cetane level and and best lubrication qualities unfortunately comes with sulfur as a byproduct.

Cetane is a diesel fuel rating roughly equivalent to octane in gasoline. Fuels with a higher cetane number "burn" better and deliver more power. Older engines were intended for high-sulfur fuel with its elevated cetane level, and may not perform well without adding cetane. Some newer diesels have been designed around this problem. This is especially true for many of the lightweight, high-RPM diesels.


All engine parts are important, but keeping the fuel clean may just be the most important thing you can do for the longevity or your engine.

Most engines also use the diesel fuel itself to lubricate close tolerance parts by pumping vastly more fuel through the injectors than is needed for combustion and returning the overage to the tank. The lubricity in the excess fuel cools delicate moving parts, lubricates them, and keeps seals moist, hence a move to the lower lubrication values of LSD may cause long-term problems. Failure to replace this fuel-borne lubrication will ultimately result in increased engine wear, higher maintenance bills, and a premature need to re-power.

Fuel Additives    Most sailors should consider additives to compensate for the loss of high-sulfur fuel's lubricity. Single-purpose fuel additives are available just to increase cetane ratings while others only add lubricity. Some do both. When considering additives, however, sailors should consider attacking several other ailments of diesel fuel simultaneously.

Moisture gets into the vessel's fuel system in several waysnot counting the blunder many of us have made of mistaking the on-deck fuel cap for the water fill. Marina fueling stations often have underground tanks that weep moisture. Moisture also condenses directly into the boat's fuel if tanks are left partially full for long periods of time. Since water is heavier than diesel oil, the droplets eventually find their way to the bottom of the tank. Thus, while shopping for a fuel additive, it makes sense to find one that contains water dispersants at the same time.

"Diesel is also the favorite food of certain types of bacteria, algae, fungi, and yeast that can turn a tank full of fuel into a gooey mess quickly."
Diesel fuel is also the favorite food of certain types of bacteria, fungi, and yeast that can turn a tank full of fuel into a gooey gelatin in less than a week. While most prevalent in the tropics, these potential problems are not immune to colder climates. Many single and multipurpose fuel additives contain biocides that kill these critters and some will even help re-liquefy fuel that has become moderately gelled.

Additives may also be necessary when operating diesels in temperatures below freezing, as fuel has a propensity to thicken under those conditions.

Additives used to combat Low Sulfur Diesel's lower cetane and lubricity levels, water intrusion, micro organisms, and cold weather are available from most marine stores, truck stops, and farm stores. But be sure to read the information carefully and follow the manufacturer's directions to the letter. And be careful in handling and storing some of these additives as many are caustic or hazardous.


When it comes to your primary fuel filters, redundancy is a good idea.
Filtration    The smallest piece of dirt, rust, or lint carried in the fuel will damage the highly polished inner workings of a diesel. Fuel purity is just as important as keeping clean lubricating oil in the crankcase and is required to prevent both poor performance and long-term wear and tear.

The struggle against dirty fuel is best conducted by keeping dirt out of the boat's tanks in the first place. Where fuel comes from an unknown source or is stored in rusty drums, the use of a "Baja filter" or funnel packed with cheesecloth is advised. Even with this precaution, however, the boat's tanks will eventually require cleaning to remove a sludge of dirt, scale, and the bodies of millions of dead microbes.

Most engines come from the manufacturer with a primary fuel filter, but most are rather coarse, being around 20 microns (20/millionths of a meter). Several famous brands of diesel fuel filters are available with particulate sizes as low as two microns, which remove the finest contamination before it enters the engine. Many of these proprietary filters incorporate a water separation feature and fancier models have an alarm that warns of water or restricted fuel flow.

Follow the filter manufacturer's recommendations; too small a filter will most certainly cause problems while too large a filter wastes money and may hinder the filter from completely separating water from the fuel. In this same vein, it is possible to add too many filters in series, restricting the free flow of fuel just like a clogged filter. And it's highly recommended that owners change filters on a regular schedule and keep a log of the engine hours. Carrying spare filter elements is also important since they seem to have a penchant for fouling in the dark or in the most remote anchorages. If the engine "misses," has reduced power, or has especially black, sooty exhaust, change all the fuel filters before proceeding.


One tank with two parallel filters is a better arrangement than just one filter.

What appears to be a serious mechanical problem is often no more than clogged filters. Having two filters arranged in a parallel fashion in the line, with valves to allow a quick change, may save the boat if the one in use becomes fouled while transiting a narrow entrance or a crowded harbor. An even better solution is to have multiple tanks, each with its own filter. By carrying a five gallon jerry jug of clean fuel, a boat with even multiple tanks and filters that are fouled may be brought home safely by simply inserting the pick-up and return hoses directly into the spare fuel jug.

Spin-on filters are popular because the elements are convenient to change, but these require bleeding the entire fuel system each time a filter is replaced. Filters that have elements that are removed from the top generally allow fuel to be added after the new element is inserted and may need no bleeding at all.


Two separate fuel sources each with their own filter is the best approach to filtration.

Having a 12-volt pump in the fuel line can make bleeding the system far easier than using the little thumb primer found on most diesel fuel lift pumps, especially if a large volume spin-on filter, or series of filters, must be re-filled. A squeeze-bulb primer from an outboard motor is a good alternative on smaller engines. These rubber bulbs have a check valve built in to stop fuel back flow and can serve the same purpose as an electric fuel pump with far less complexity and cost.

Magnetic Biocides    Liquid biocides are the most hazardous of diesel fuel additives. True, they kill all the life forms in the tank, but they kill everything else they touch as well. The carcasses left behind sink to the bottom of the tank and will eventually clog filters. Sludge at the bottom of the tank should be handled as special toxic waste, which can cause difficult disposal problems.


Most cruisers use their boat's auxiliary engine often, and if so, they probably won't need to install a recirculation pump. For those whose boats sit unused for prolonged periods, having such a device is a good idea.

Several companies have developed magnetic "bug zappers" to address these problems. Bacteriologists found that single-celled organisms have an electrical potential and that disruption of their delicate electrical balance causes the cell to sicken and die, sometimes even bursting the cell wall. By routing the fuel next to a strong, permanent magnet, or series of magnets, cell reproduction can generally be halted, the existing micro organisms killed, and the cells themselves may even be shredded.

It is clear that the fuel must be circulated through these units and over the magnetic field to be effective. For work boat or charter vessel engines that are run often, this presents no difficulty. Diesels sitting unused for long periods of time would need to install a recirculation pump with diverter valves to occasionally pass all the fuel through the magnets and other filtersnot such a bad idea in any event.

The Following User Says Thank You to Tom Wood For This Useful Post:
Weg3 (07-20-2013)
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