When it comes to fuel flow, cold is bad news.
Diesel fuel, especially high-cetane diesel fuel, has one big advantage over gasoline: It ignites easily when mixed with hot air. But, in almost every other way, diesel puts truckers at a big disadvantage when it’s cold outside.
There are two problems. First is the paraffin, or wax, diesel contains, which is not a contaminant, but a valuable part of the product that carries a lot of its heat energy and helps fuel mileage. The wax also helps lubricate injection system parts such as pump gears and injector plungers and needles.
In warm temperatures, the wax stays in liquid form and remains mixed with the other parts of the fuel. But, as the temperature drops, everything changes. The wax begins to turn solid, forming tiny particles. The fuel first reaches the cloud point. It gets visibly cloudy, and the tiny particles that create the cloudiness may begin coating the filter element. If the temperature drops further, these wax crystals can quickly block flow through the filter. The fuel has then reached the cold filter plug point, or pour point.
Then, if the temperature drops another 10 to 20 degrees, the fuel as a whole will gel. The system can reach the point where all the fuel in it is nearly solid. The situation may become so bad that the only cure is to take the tractor into a garage and warm up everything.
The second problem is water. It can come from many sources, including condensation caused by humid air inside a cooling fuel tank; rainwater in a supplier’s tank because somebody forgot to replace the fill cap; and even an occasional trace amount left over from the refining process.
The plot thickens (and so does the fuel) when high water content works together with high wax content to create a slushy mix of wax and ice water once the temperature drops below 32 degrees.
Primary versus secondary filters
Your diesel’s fuel system has an engine-mounted fuel pump. It usually consists of two gears with teeth that have flat outer edges, rotating in a housing that fits tightly around those gear teeth. One gear is driven off the engine via a shaft; the second rotates because its teeth engage those on the first gear. The pump operates by trapping fuel in the open spaces between the teeth and forcing it from one side of the housing to the other as the gears turn.
This gear-type pump draws fuel from the fuel tank up to the engine, and then forces it under slight pressure through the injection pump, or the individual block-mounted unit injector pumps, or through passages in the cylinder head that feed unit injectors. On these systems, the pump produces a low pressure and flow – just enough of each to keep the injectors or pump(s) full of fuel, and circulate fuel back to the tank. The pressure is created by placing a small orifice in the system downstream from the pump(s) or injectors to restrict the flow.
These basic principles apply to the Cummins Celect injection system, as well as those used by Detroit Diesel, Caterpillar, Mack and Volvo. However, the traditional Cummins PT injection system, and the injection system used on the Cummins ISX/Signature engines, use the gear pump in a slightly different way. On both of these systems, this pump works in conjunction with a pressure-regulation system to create a critical low pressure. This pressure is precisely controlled (with the PT system it varies with rpm), and is used for metering or measuring the shots of fuel that go into the cylinders each time the engine fires. On these systems, all filtration must be accomplished before the fuel reaches the gear pump.
Many engines with fuel systems that don’t involve the gear pump in metering employ both a primary and secondary fuel filter. The primary filter is located on the suction, or fuel tank, side of the gear-type pump. The secondary filter is located on the pressure side, between the gear pump and injection pump, unit injector pumps or unit injectors.
The primary filter is often used to remove water, and its main job is to clean the fuel up just enough to protect the gear-type pump, whose parts fit much more loosely than the parts in the engine’s injection system. This filter is also designed to remove most of the dirt that’s in the fuel, but it filters to a much higher micron level than the secondary filter.
Why water means trouble
In the case of water, which causes problems for fuel systems even when it’s warm, the only answer is removal.
While a very small amount of water that is fully dissolved in the fuel poses no problems, free water is bad news for diesel engine fuel systems even when it’s hot outside. Here’s why:
- Low levels of water can thin out the fuel, reducing lubricity and causing scoring of the pump plungers and needles. They can also cause corrosion.
- Water boils at a much lower temperature than fuel. A significant amount of water in the fuel can lead to flash boiling or cavitation. This can cause violent flow and blow the tip right off the nozzle!
- Large drops or slugs of water do not burn, of course, so they can detract from engine performance.
- Water can soak filter media and make them swell, shortening their useful life.
- A large amount of water in the bottom of a fuel tank breeds microbes at the interface between fuel and water. This produces filter-clogging material, and may even corrode parts.
How to deal with water
The first step in handling water is to buy your fuel wisely. Bill Richards, director of sales and marketing for FPPF Chemical Co. of Buffalo, N.Y., says you should choose reputable truckstops that pump a high volume of fuel. If you run a fleet and have your own tank, have water pumped from the bottom regularly.
Ken Stirn, senior product manager of Baldwin Filters, Kearney, Neb., says it helps a great deal to fill your fuel tanks frequently in winter, to minimize the amount of condensation created inside. This works because a tank full of fuel has less outside air in it, and moisture collects on the walls of the tank that are exposed to outside air. When the walls are covered with fuel, condensation can’t occur.
FPPF makes an additive called Fuel Power that causes water to dissolve in fuel. Once this happens, the water is no longer a problem – in fact, fully dissolved water actually enhances diesel combustion and lowers emissions, as a unique fuel/water mix called PuriNox perfected by Lubrizol Corp. of Wickliffe, Ohio, proves.
Lots of companies make good additives of this type that are readily available. Another of them is Howes Lubricator of Coventry, R.I. Its product, Howes Lubricator Diesel Treat Anti-gel, “encapsulates” the water, in the same way engine oil keeps soot or other byproducts of combustion in suspension. This helps extend fuel filter life and reduces the amount of water you’ll have to drain from any fuel/water separator you use.
Long-term accumulation of a significant amount of water can create an atmosphere that supports bacterial growth right where the water meets the fuel. Bacteria grow especially fast in warm temperatures, and may double their number every 20 minutes. Racor’s “Fuel Filtration” manual says as bacteria-contaminated fuel circulates through the fuel system, the bacteria cling to most metal surfaces. Since the bacteria create acidic wastes, the result is corroded metal parts throughout the system.
Richards says Fuel Power is effective in preventing bacterial growth and the tank slime that results. FPPF also makes a biocide called Killem, which will kill bacteria in tanks that have built up a lot of water. Racor says fuel contaminated by bacteria will smell like rotten eggs, and says its Diesel Biocide will kill the bacteria.
A small amount of water is naturally dissolved in the fuel, often from the refining process, and it usually does not cause much trouble. As water accumulates, it may form tiny droplets, making it emulsified. Emulsified water may cause the fuel to appear cloudy. These tiny droplets are difficult to separate out because of their size, and almost float in the fuel. Yet they can cause the fuel to freeze in cold weather.
Larger slugs of water, or free water, are more likely to separate out because of water’s greater density when compared with diesel fuel.
Because at least some water is usually found in diesel fuel, it has become common practice to install a fuel/water separator/filter unit on modern trucks, rather than a plain filter.
How separators work
Separators use three methods to take water out of fuel: settling, coalescing and forming a barrier.
Gravity settling allows the fuel to flow slowly through a bowl so there is time for water to settle out. If the fuel were to move too fast, the motion would keep stripping the water off the bottom of the bowl. Pure settling only works on the primary side when there are low flow levels. But these units are unaffected by fuel additives and contaminants, and need little service.
Rotating the fuel with blades as it flows through the unit, or giving it a gentle turn by the design of separator passages, can give it a gentle rotating or swirling motion. This causes centrifugal force to throw the water (and heavy particles) to the outside of the unit, separating them. This works much like settling, but the unit’s action increases the available force, making the process work much better. A filter then provides the final barrier.
Coalescing often means stopping the tiny water droplets in the fuel as it flows through a fine mesh or filter. This type of separator mesh may be made of textiles like nylon and polyester that repel water. Coalescing causes the tiny droplets to accumulate and glom together to form larger ones. Once that occurs, they are heavier and settle out, ending up in the bowl underneath, from which they can be drained.
A solid material that resists the flow of even the tiniest droplets can also provide a barrier to water flow, causing droplets to coalesce and collect in a bowl at the bottom of the unit. These barriers work well even when those using a fine mesh may lose their effectiveness because of low temperature, or when contaminated.
Some filters provide a barrier, but no large space for water to accumulate below. These are the true barrier separators, and are effective only when a limited amount of water contamination can be expected.
Different types of separators are used on the suction and pressure sides of the system. The drops are large on the suction side because the gear-type fuel pump hasn’t chopped them up. So gravity, coalescer or barrier types may be used. However, on the pressure side, it’s impractical to use a gravity-type separator, meaning either a coalescer or barrier type must be used. Barrier-type coalescers need a fine micron rating (10 microns or less) to be effective on the pressure side.
Quality diesel fuel is treated for the temperature conditions at each time of year. One key to running in the cold, then, is to buy quality fuel. However, there’s a problem even when you do this: The fuel is blended for the average temperature for that time of year. If a cold snap produces lower-than-normal temperatures, which is common in winter, you get stuck. The other problem is that when you buy fuel in the South and head north, the temperature will soon drop below the temperature your fuel is treated for. That also leaves you vulnerable.
One alternative is to use a fuel that consists of a blend of No. 2 diesel and the much lighter No. 1, which is like kerosene. Bill Richards of FPPF points out, however, that No. 1 contains less wax, which is a source of tremendous energy. That means running blended fuel cuts your fuel economy quite a bit. A much better solution is to heat and/or treat the fuel and burn the much more potent No. 2 diesel.
How heaters work
The cheapest heaters to run use engine heat. A diesel taking a truck down the road at 60 mph and giving 6 mpg will give off about 420,000 BTUs per hour of heat through the coolant. It takes only 1,500 BTUs to heat 100 gallons of diesel fuel, raising its temperature 30 degrees. So there’s much more than enough engine heat to warm your fuel.
There are a number of fuel/water separator units on the market that incorporate a heat exchanger. This is a metal device that contains fuel and has water flowing through a separate circuit inside. The heat passes through the wall of a metal pipe or chamber from the coolant into the fuel. The ideal place for any heater is right before the primary filter. However, in-tank or in-line heaters can do the job if powerful enough. It’s safest to heat the entire fuel system circuit, anyway, so lines won’t clog even in extreme cold.
It’s vital to make sure any heating unit cuts off before the fuel gets too hot. This is a major consideration as you begin to drive in warmer weather. Hot fuel may cause problems with engine ECM cooling and will develop bubbles and lose its density, affecting engine operation. All engine manufacturers specify a maximum fuel operating temperature in the range of 100 degrees because of this. Heating units often contain a thermostat that gradually stops the flow of water through them when fuel temperature reaches about 100 degrees. Some will cut off just above the minimum safe temperature of about 35 degrees, saving energy.
Other forms of heat exchangers are sized appropriately for the job and then connected in series with the cab heater. So, when you turn the cab heater off because the weather is warm, the fuel heating stops.
Heating the fuel with engine coolant works fine when you’re running, but what happens when you shut down overnight? If it’s really cold, the fuel could quickly clog the filter with wax before the coolant heater has warmed it enough to melt it. The best policy is to have some means to preheat the fuel/water separator, or at least the fuel leading to it. The only way to do this is an electric heater.
Racor’s “Fuel Filtration” manual describes two kinds of electric heaters. The most widely used type is the PTC heater. This remarkable device is self-regulating: as soon as it gets hot, it cuts the current way down. Fuel that is greatly overheated can “coke,” developing pure carbon that can quickly clog a fuel passage. A PTC heater will never get so hot that it cokes the fuel because of poor flow. Even though the device regulates its output effectively, it is usually equipped with a thermostat for precise fuel temperature control. The only downside of the PTC-type heater is that it must be installed where there is plenty of fuel flow. If the flow stagnates, the unit becomes ineffective.
Resistance heaters are also used frequently. Racor, for example, makes a line of products with resistance heaters located in the plastic water collection bowl or inside the filter housing. Ratings greater than 200 watts are rarely required, as a slight reduction in flow because of waxing will quickly raise the fuel temperature high enough to keep things moving. One excellent strategy is to wire a filter/water separator-mounted heater so it can be turned on for a minute or two prior to starting, thus keeping the fuel flowing through the filter warm enough even right after a cold start.
Heaters that use normal household 100-volt current can also be attached to the outside of filter housings and plugged in when the truck is parked overnight near shore power.
Some truckers who idle all night trust engine heat and circulation to keep fuel warm. It can help in this situation to make sure fuel lines run close to the engine. Return fuel heaters that recycle fuel that’s normally returned to the tank to the inlet of the fuel filter can be used. They are thermostatically controlled and return all the fuel to the tank when it is too warm.
Of course, the cost of idling in terms of fuel and engine wear, and even engine oil deterioration, have been well established. Except for the occasional night of idling in extreme weather, you’re much smarter to equip your vehicle appropriately and then shut it down.
For more information, contact the following:
Tel. (203) 876-1484
Tel. (800) 822-5394
FPPF Chemical Co.
Tel. (800) 735-3773
Fax (716) 856-0750
Tel. (800) 523-4145
Fax (401) 294-4229
Tel. (800) 643-9089
Parker Hannifin Corp.
Tel. (800) 344-3286