Envirowatch: Filters in a greener world
| March 26, 2009 |
Diesel engines are getting cleaner all the time. But the issue facing manufacturers and fleet managers is that diesel is still a dirty fuel and filtering it in this clean and green age is more of a challenge than ever before.
Diesel fuel has always been prone to contamination from water, algae, asphaltines, wax and oxidation and degradation products – not to mention microscopic particles of dirt and sediment. Until about five years ago this contamination was easily handled by a dual fuel filter system – a water separator prefilter on the suction side of the pump and a much finer final filter on the pressure side.
But over the past six years engine manufacturers ratcheted up the pressure at the fuel injector nozzles – 20,000 or more psi in some cases – to make diesel engines more fuel efficient and reduce emissions. To cut emissions further, the oil companies were forced to strip out most of the naturally occurring sulfur in diesel fuel, robbing it of its natural lubricating qualities. And in just the past two years, biodiesel, with blends of up to 20 percent plant-derived fuel (B20), became the darling of environmentalists and mandated by some states and municipalities.
High pressure needs low contamination
The relatively new high pressure fuel injectors have moving metal parts that must open and close within extremely tight tolerances several hundred times a minute. The high pressure better atomizes the fuel for more compete combustion and some engines even use “multiple injection events” (typically three bursts of fuel) per piston stroke. But in this super-precise environment, any microscopic particles and contaminants that get past the fuel filters can scratch or gall the metal injector components leading to a less than perfect seal and injector tip wear.
Once they start wearing, the injectors leak fuel into the cylinder. You don’t get as much pressure, leading to reduced atomization of the fuel and poor fuel spray patterns. When this happens, the power and performance of your engine declines, you start burning more fuel than necessary and you start getting more fuel diluting the engine oil. To make matters worse, your emissions start to increase as your in-cylinder combustion sends more and more unburned fuel out the exhaust valves. Gross contamination can lead to injector tip failure and a very expensive repair.
In the past, the lubricity of diesel fuel helped prolong the life of these injectors. But with low sulfur diesel (500 parts per million) mandated for off-road equipment and ultra low sulfur diesel (15 ppm) the law for on-road vehicles, lubricity became a problem. To meet those needs manufacturers put additives in the fuel, which bring back some of the lubricity, but the additives have the side effect of making it easier for water to mix with the diesel, thus requiring more aggressive water separation.
Dealing with water
Biodiesel also has an appetite for water, and the combination of plant-based fuel mixed with low or ultra low sulfur diesel, makes today’s biodiesel a very hydrophilic (water loving) fuel. Water promotes bacteria growth, which leads to acidic conditions. It can also rust iron parts and rust particles are extremely damaging to injector tips.
Biodiesel also acts as a fuel system cleaner, dissolving a lot of the varnish deposits and other impurities in your fuel lines, storage units, and machine and vehicle fuel tanks. In higher concentrations, biodiesel will raise the cloud point and pour point of the fuel. This can lead to faster filter plugging in cold weather.
On the suction side of the pump most heavy diesel engines have primary filters that prefilter the fuel and pull out water. There are several ways to filter out the water. A stripping filter uses a silicon treated medium, a coalescing filter uses gravity to drop water to the bottom of the filter and an absorption filter soaks up water while allowing fuel to flow freely.
The synthetic trend
In the past, fuel filters only needed to take out particles in the 8 to 15 micron range. But today’s high pressure injectors require filtration in the 2 to 5 micron range. To meet these demands, many fuel filter manufacturers have gone to synthetic filter media for the final filter on the pressure side of the fuel pump.
Cellulose (non synthetic) filter media are also capable of filtering down to the 2 to 5 micron range, but plug up sooner at this level of filtration. Synthetic filter media have a more open, web-like structure that holds more particles for longer periods of time and this helps to maintain service intervals. Some manufacturers combine cellulose filter media with nano-scale synthetic fibers in the base sheet or in overlays. Another strategy to maintain service intervals is to make the cellulose filters larger.
Filter manufacturers stress that while synthetic media have advantages, cellulose still has its place and each system has to be designed with a recipe that fits the application. That recipe includes an application specific system design from the prefilter/water separator through the final pressure side filter. The filter system for a heavy truck that spends most of its time on the highway will look very different from one designed for excavator that works in one place all day long.
Biodiesel and maintenance
For equipment managers who are switching to biodiesel, it’s important to take into account this fuel’s solvent-like properties. The junk it strips out of the system will quickly clog a fuel filter and you will have to change the first two or three fuel filters on an accelerated schedule after the switch to biodiesel. Once the dissolved contaminants are gone, your fuel filter PMs can return to normal.
In cold weather you may need to add a fuel line heater as a starting aid, to combat biodiesel’s high cloud point and pour point. And since water is more prevalent in biodiesel, the water separator will likely have to be drained more frequently.
To make sure biodiesel doesn’t cause you any more problems than those mentioned above, be sure to only use biodiesel that meets the ASTM D 6751 spec. Also note that while B20 has become the highest concentration of biodiesel allowed by most engine manufacturers, higher concentrations do exist. If you plan to use anything more than B20 make sure your filter seals and gaskets are rated for that fuel as high concentrations have been show to cause some gasket and seal leakage. Biodiesel may also cause problems for copper and other light metals. Make sure any filters or other fuel system components are free of these metals.
The Filter Manufacturers Council has some good technical bulletins on biodiesel and how it affects filter life. You can download these at the website: www.filtercouncil.org.