Most voyaging boats have powerful diesel auxiliary engines. All it takes, however, is a teaspoon of water, dirt or the wrong type of fuel to bring a robust engine to a grinding halt. Even a small gulp of air, as most skippers of diesel-powered vessels know, will cause this venerable power plant to cough, sputter and finally stall. With the introduction of any amount of these culprits, reliability suffers. With some understanding of diesel fuel, however, you may be able to avoid all of these problems.
Why are most diesel-engine manufacturers, operators and mechanics so fanatical about obtaining clean fuel and maintaining a clean fuel system, and why is it different from the requirements for gasoline power plants? While all internal combustion engines, gas or diesel, require a clean supply of fuel, diesel engines are especially sensitive on this subject. The answer lies primarily in diesel fuel’s lubricity. This is a measure of the slipperiness of a liquid. Diesel fuel is especially slick (if you’ve ever spilled any on deck or the cabin sole and stepped on it, you’ve already learned this lesson) thanks to two attributes: hydrodynamic and boundary lubrication. The former involves the cushion created by any liquid between parts, diesel fuel and metal parts in this case. The latter is molecular in nature and it involves oxygen, nitrogen and aromatic compounds adhering to the surface of the metal, creating a film or boundary, which reduces friction.
If diesel fuel did not possess these qualities, diesel engines as we know them would not be possible. The reason for this is in order to create the extremely high fuel pressures required to enable compression ignition (all diesel engines work on the compression ignition, or CI, principle, while gasoline engines utilize spark ignition, or SI, to achieve combustion), the tolerances for the moving parts within the fuel delivery system must be extremely close, measured in ten thousandths of an inch. If diesel fuel were not as slippery as it is, a diesel engine’s fuel system would wear itself out in a matter of a few hours of running time, rather than the now customary tens of thousands of hours.
The gasoline engine, even in its fuel-injected form, relies on SI and as a result does not require the extremely high pressures found in the diesel fuel delivery system. This, in addition to vastly different cylinder compression ratios, is one of the key distinctions between the two types of engines. Additionally, because the fuel in a gasoline system is not hyper-pressurized, a small amount of air in the delivery lines, while still undesirable, is much less debilitating.Diesel fuel’s nemeses
Diesel fuel, when contaminated with dirt (more on just what this is later) or water, sacrifices much of its lubricity. As a result, the finely machined, fitted and polished components within the high-pressure side of the fuel delivery system — the injection pump and injectors — suffer from extremely rapid wear, which ultimately leads to their failure.
The primary contamination culprits for diesel fuel are water and dirt. These two troublemakers often go hand in hand because one often leads to the other. The presence of water in a fuel system is an invitation for biological organisms to set up house. Once a colony establishes itself, the byproduct is a slimy bio-ooze that quickly clogs filters and pick-up tubes.
Biological critters require certain circumstances to exist. Primarily, they must have water, which is where they live. Diesel fuel, or more specifically, the sulfur within diesel fuel, is their main source of food. This interface between the diesel fuel and water becomes the way station where most bacteria colonies grow and eventually flourish.
Additionally, warm, ambient temperatures accelerate this bio-life cycle. In fact, bio-contamination is almost unheard of in winter months. Cold weather kills off most fuel system bio-infestations. However, this die-off presents its own problems. As it occurs, season after season, the fattened carcasses of the sulfur-munching life forms sink to the bottom of the tank, creating a primordial ooze-like substance that will rear its ugly head when agitated in a seaway. Many a voyaging vessel has limped into my boatyard after having experienced the short, steep waves of the Chesapeake Bay. The increased motion homogenizes the accumulated muck at the tank’s bottom, where it quickly clogs filters. This results in a steady drop in rpm, until the engine eventually stops.
While biological contamination is common, it’s not the only source of dirt that can incapacitate your fuel system. Gum, petroleum resid and inorganic salts, sometimes collectively referred to as asphaltene, may all be found in diesel fuel as it is dispensed from the fuel pump nozzle. These contaminants may be present in the fuel as it leaves the refinery or they may be introduced during transport and storage.
Inorganic contaminants are capable of clogging filters and stopping a diesel engine, much as the bio-slime does. There is, however, one important difference. Inorganic particles, depending on the concentration, are capable of causing rapid, severe wear ¿o piston rings, injectors and injection pumps. Conversely, the sulfur-eating bugs, while a nuisance to be sure, are less destructive in their machinations. Their slimy byproduct leads to frequent filter changes2and perhaps corrosion within metallic fuel tanks (certainly the water does) and other components, but their damage is more gradual, and usually less costly, in nature. However, the destructiveness of plain old water in a diesel fuel system must not be underestimated; it will damage all metallic components with which it makes contact. Water will often flash into steam within the injector nozzles of running engines, shattering them. The resultant shrapnel and uncontrolled flow of fuel quickly destroy the affected cylinder and piston.
Installing a stripper tube, a pick-up tube that extends closer to the bottom of the tank than the engine’s pick-up, will make water removal relatively easy. If you are having a tank fabricated, specify that the stripper tube sit within a small well in the bottom of the tank. This will serve as a containment area for water and other dirt and debris that settle out of the fuel column. Look for more on this in a future article dedicated entirely to tank design and fabrication.
The best way to combat organic and inorganic contamination is to prevent it from occurring. Water emulsifiers and bio-extermination additives should only be thought of as a second line of defense; however, they are not the preferred approach. The most direct method of achieving this goal is by keeping water out of your fuel tank from the start. Without water, bio-organisms have no base and so cannot exist.
Because of their incipient and gradual accumulation, the inorganic pollutants are a bit more difficult to prevent. Purchasing high-quality fuel (more on just what this is later) from a reputable retailer is one method. In far-flung parts of the globe, this may be impractical. In these cases, sampling and prefiltering may be the only defense.
If you are faced with a contamination problem, determining just what type of contaminant you’re dealing with will help you choose the best method of correction. The first sign will probably be clogged filters, whether they affect the engine’s running characteristics or not. Filter bowls that are filled with water are sending an obvious signal: Pump the water out of the tank.
However, the diagnosis is not always that clear. If the filter elements are saturated with black or brown muck, it may be organic or inorganic in nature (organic life is sometimes, but not always, greenish in color). Make the call by spraying the removed filter cartridge with a cleanser that contains chlorine, such as Greased Lightning or Spray Nine. If the muck turns tan or white under this assault, the contaminant is organic. If it has no effect, it’s probably an inorganic asphaltene-type contaminant, but just to be sure, try spraying it with a petroleum-based solvent, such as PB Blaster or WD-40. If the black stuff dissolves and runs freely, it’s inorganic. If the filters are clogging with either of these contaminants regularly, it is safe to conclude that the tanks are contaminated. The only permanent solution is to open the tank and scoop out the debris. In order to do this, your tank must be equipped with an inspection port into each baffled chamber. If your tanks are not so equipped, fear not, these ports are available as after-market additions from Claris Technologies (www.claristechnologies.com).
The final, and most often overlooked, contaminant in the diesel fuel system is ordinary, everyday air. Diesel fuel delivery systems do not contend with air very well. Because, as previously mentioned, CI fuel systems call for very high fuel-injection pressures, the introduction of a compressible element, such as air, prevents the fuel from being properly pressurized. This problem usually makes itself known when a diesel engine exhibits decreasing rpm, difficult or impossible starting, or surging.
The problem is that air leaks are difficult to identify because they are on the vacuum side of the fuel system (between the tank and the lift pump), and as a result, they are not usually accompanied by fuel leaks. Fortunately, there are the usual suspects that the air leak detective can interrogate first. Ninety-nine percent of diesel-fuel air leaks can be traced to the primary fuel filter or its fuel line fittings. Because this filter operates under a vacuum, and because it is regularly serviced (you hope), the law of averages dictates that it will eventually suffer an air leak. O-rings and gaskets are easy to miss-seat, crush washers sometimes don’t crush or crush too much, or sometimes fittings simply aren’t tightened enough.
Other air ingress sources include any banjo, pipe-thread or pipe-to-hose fitting. When chasing an air gremlin, ensure that all of these items, especially those on the vacuum side of the system, fit properly and are securely tightened. One final caveat in this area, never install a fuel supply hose, on the pressure or vacuum side of the system, over a smooth, un-barbed tube or pipe. This scenario is a violation of good engineering principles and the American Boat and Yacht Council’s (www.abycinc.org) Standards and Recommended Practices for Small Craft, section H-33.11, Diesel Fuel Systems.Fuel school
While the importance of clean fuel has already been established, there’s more to diesel fuel than just its clarity and slipperiness.
Like gasoline, there are several different types of diesel fuel. In North America, they are designated as 1-D, 2-D and 4-D, increasing in density and viscosity numerically. Similar products include no. 2 fuel oil, 1- and 2-GT (for use in gas turbines), Jet-A and kerosene. While all of the latter products will run in a diesel engine, the side effects certainly pose a threat of internal engine damage serious enough to prohibit their use in all cases other than dire emergencies. Most marine and automotive diesel engines specify 2-D for average use and 1-D or a mixture of 1-D and 2-D for extremely cold weather operation. For the average voyager, 2-D is the norm. Beware of operating a diesel engine on 1-D when the conditions are not optimized for it (i.e., very cold weather). This will, because of its decreased lubricity, cause rapid wear. If you fill up with winterized diesel and then don’t use it until summer, you may be asking for trouble. Likewise, ordinary warm-weather diesel is more prone to gelling and waxing in extremely cold weather. This may be particularly troublesome if the fuel tank is in an unheated portion of the boat or if the engine cannot be run long enough to warm its own compartment and filters. Fuel heaters are available as options for many primary filters and are worthwhile investments if you intend to operate in very cold conditions regularly.
Like gasoline’s octane, diesel fuel has a rating index called cetane. The higher the diesel fuel’s cetane index, the easier a diesel engine will be to start, especially in cold weather. Cetane is a measure of how readily diesel fuel begins to burn or autoignite. Under CI conditions, fuel that possesses a high cetane rating begins to burn shortly after injection, exhibiting a short ignition delay period. This attribute of short ignition delay also offers the benefit of decreasing knocking in a diesel engine.
Interestingly, while higher numbers are desirable for both types of fuel, gasoline’s octane and diesel’s cetane ratings have the opposite effect. High-octane gasoline resists autoignition (it must not ignite until the spark plug fires), while high-cetane diesel autoignites more easily, a desirable state of affairs for CI engines.
Avoid using diesel fuel that is rated at anything less than a cetane index rating of 40. If available, 45 or 47 are preferred. Think of these latter numbers as health food for your diesel engine.Sulfur and dyeing
A confusing web of regulations has arisen because both the IRS and EPA require the addition of red dye to certain classes of diesel fuel. Unfortunately, each agency requires that it be added for different reasons, to different classes of fuel, at different concentrations.
The regulations are a bit complex; however, from a technical standpoint alone, dyed diesel is high in sulfur. There are a few noteworthy differences between high- and low-sulfur diesel fuel. Low-sulfur diesel is more environmentally friendly, to be sure. Burning low-sulfur diesel produces less sulfur dioxide, a harmful gas that, when mixed with water, causes acid rain. Additionally, low-sulfur diesel is more stable. This is a result of the desulfurization process, which removes not only sulfur but also the precursors of insoluble organic particles. These contaminants, as mentioned, are undesirable and cause fuel to go sour more quickly. Finally, low-sulfur fuel is less likely to cause sulfur-induced corrosive wear of parts exposed to fuel.
Low-sulfur diesel does, however, have some drawbacks. The desulfurization process, while removing sulfur and organic particles, also removes some naturally occurring antioxidants. This process, left unchecked, will lead to fuel degradation, especially during extended storage. This apparent paradox — desulfurization making fuel more and less stable simultaneously — is usually corrected by the refiner through the addition of stabilization agents during the refining process.
Several years ago, when low-sulfur diesel was first mandated, a spate of leaking fuel systems erupted, particularly around injection pumps. This was caused more by the removal of aromatic compounds, a side effect of desulfurization, than by the removal of the sulfur itself. The aromatics in high-sulfur diesel cause rubber O-rings to swell. When the aromatics were removed, the O-rings shrank, leading to fuel leaks. Most newer diesel engines are equipped with O-rings that are not susceptible to this swelling and shrinkage. However, if you have a diesel built before 1990 and it’s weeping fuel from the injection pump, it may require an O-ring upgrade.
One final note on dyed fuel: As far as the IRS is concerned, dyed diesel is exempt from the Federal excise tax of 24 cents a gallon and may not be used for taxable purposes, other than for heating, farming, boats engaged in fishing or transport, non-profit organizations, as well as some other uses. Check out the IRS website at www.irs.gov for more information on this subject.Filling the tank
˜As it leaves the refinery, domestic diesel fuel is usually quite clean. The local refinery that produces fuel for the bulk providers, which deliver to %y boatyard, considers one milligram of sediment per liter normal, but in reality, deliveries contain far less than that. Most contamination actually occurs during transport and storage. Many transport trucks, storage tanks, pipes and hoses are far from meeting the cleanliness standards required for diesel engines.
If you are purchasing fuel from a fuel dock that has a good reputation for providing clean, high-quality fuel, then you probably have little cause for worry. However, what do you do if you have to purchase fuel from an unfamiliar locale or if your only options are jerry jugs and bulk delivery? In these cases, you have two tools at your disposal. The first is, sample the fuel before pumping it into your tank. Glass mason jars are excellent for this purpose (in spite of the danger of breakage, the clarity of glass makes it preferable to plastic for this task). Fill one of these jars, cap it and let it sit for 10 minutes or so. Any water or sediment should be plainly obvious.
If you are pumping fuel from a barrel using a hose, draw your sample from the bottom. This will contain the worst of the dirt and water. If the fuel is being delivered in a transport truck, ask the driver if you can have a look inside the tank. Have a look at the fuel he will be pumping for you as well as the condition of the other compartments. If one of these compartments is empty, so much the better because it will afford the opportunity to see the bottom of the tank, the place where most of the dirt will be found. He should have no qualms about letting you do this. Most fuel delivery drivers I’ve encountered are proud about the cleanliness of their tanks and are happy to show them off if you show an interest.
The second option is to use a filtering funnel. These are available at varying levels of sophistication, complexity and expense. Simpler ones, costing about $7, will capture water as well as coarse dirt particles. More elegant filter funnels, such as the Baja Filter, are capable of a much higher degree of filtration and water separation, while maintaining a respectable flow rate of up to about 4 gallons per minute. This increased sophistication comes at a price. Depending on the size, Baja Filters retail for about $125 to $200. While neither of these funnels is designed to replace your primary and secondary onboard filters, they will provide a first line of defense against contaminated fuel.Onboard filtration
The right stuff for a primary onboard fuel filter (the first filter the fuel encounters after it leaves the tank) include: a clear sight bowl (with heat shield if installed in the engine room), an agglomeration device (this causes small water droplets to coalesce and drop to the bottom of the sight bowl), an easily replaceable filter element, provisions for a vacuum gauge and water sensor and an ABYC-approved drain valve. If you intend to voyage in colder climes, a heater option, as previously mentioned, may also be on the list. If it sounds as if I’ve described a Racor (www.parker.com) MA-series filter, that’s no accident. These filters are cheap insurance at their retail price of about $220 for the 500 MA model, which is adequate for even the largest sail auxiliaries. They require no tools for filter replacement and optional water and vacuum sensors are easily installed, and the brass drain valve need only be equipped with a pipe plug in order to meet ABYC standards.
Resist the temptation to buy the slightly less expensive automotive FG-series filter. This version lacks the brass drain valve and heat shield and is equipped with an amber-tinted, UV-resistant sight bowl. Since most marine engine compartments are not subject to sunlight, this is hardly necessary and only serves to increase the difficulty of fuel-bowl inspection.
Vacuum gauges and water alarms are particularly useful, the latter for obvious reasons — you don’t want to let water get past your filter and, if it’s in the filter, it’s in the tank, signaling the need for a clean-out. Keep in mind, however, most water sensors, Racor’s included, only sense the presence of seawater, not freshwater.
The vacuum gauge is one of the most useful fuel-system troubleshooting tools I know of. It will enable you to determine the condition of your filter, thereby eliminating unnecessary replacement. It’s also an excellent diagnostic tool. Clogged filters and pick-up tubes cause high vacuum readings, often making for quick and easy diagnoses of poor running characteristics.
Most primary filter manufacturers recommend the use of 10- or 30-micron (a micron is a millionth of a meter) filter elements for primary filtration, color-coded blue and red by Racor, respectively. This will filter out cigarette ashes and even antifreeze. Engine-mounted secondary elements are usually found in the 2- to 7-micron range, a figure that is dictated by the engine manufacturer.
There is some controversy in this arena. Many voyagers opt for the 2-micron element in their primary filter, reasoning that all contamination should be caught as soon as possible. However, this actually reduces the amount of filter media available because all contaminants, from 2 microns and up, fill this primary filter, allowing nothing to be captured by the secondary filter. Additionally, if all the contamination is captured in the primary element, the secondary element, when replaced during routine service, is probably virgin. In my experience, dividing the dirt up between two filters doubles the media surface area available, which makes more sense from engineering and economic standpoints.
If you feel the need to use the 2-micron approach, install an intermediate filter in-line after the primary filter and equip it with a 2-micron element. This will afford supreme fuel filtration at the expense of another $220 and a square foot of engineering real estate.
The deluxe version of the MA-series filter includes two filter bodies plumbed together with the appropriate valve to enable one or the other or both filters to be used simultaneously (concurrently, not consecutively). This can be particularly useful when filter replacement is inconvenient, such as in heavy weather. These are precisely the circumstances in which fuel tank sediment is likely to be stirred up and where you can least likely afford to lose power. If an element should clog, throw a single valve and you’re running on a fresh filter body.
Diesel fuel systems possess a definite cause and effect quality. Keep them clean and well maintained and they will serve the vessel’s diesel engine, and master, well. Neglect them and the results are a near certainty: clogged filters, damaged injection pumps and poor reliability.
Steve C. D’Antonio is a marine writer and photographer and the boatyard manager at Zimmerman Marine Inc. in Mathews, Va.