The fuel filter removes abrasive particles, and water, that could damage the accurately-sized, and polished injection equipment.
The most efficient filtering system uses the first filter to remove larger particles, and subsequent filters to remove smaller particles.
Water traps and sedimenters trap water, and larger dirt particles. They can be separate units, or combined with an impregnated paper element filter.
Separate units pass the incoming fuel over an inverted funnel. At the edge of the funnel, the fuel changes direction very quickly. Water and dirt are heavier than fuel, so they are trapped, away from the funnel edge. They fall under gravity, and settle at the base of the sedimenter. The lower housing is usually clear for easy inspection, and it can include a drain plug so sediment can be drained off daily.
The most common type of filter material in light diesel vehicles is resin-impregnated paper, pleated to offer a large surface area to the fuel. These filters are also considered the most efficient.
In some of the filters that use this paper, fuel flows from outside to inside. In others, it flows from the base to the top. In others, from top to bottom.
A fuel filter with the resin-impregnated, paper element can be combined with a water sedimenter. When this combined unit is used, the method of fuel flow is determined by the manufacturer.
One method has fuel pass through the paper element first, to trap abrasive particles. Any water in the fuel is usually in the form of small droplets, which are forced through the paper element. Once through the filter, the small droplets combine into larger droplets, and form a sediment layer in the base of the filter. The bowl may be transparent, and it contains a drain plug.
Other types pass fuel into the sedimenter first, and force the larger particles out of suspension by a change in direction. The fuel is then forced through a filtering medium, ready for use. The base usually has a drain plug for daily draining of the water from the filter. This type is usually contained in a disposable element. A water level switch can activate a light on the dash, to warn the operator, the sedimenter chamber may need draining.
The switch has a float that is lighter than water, but heavier than fuel. In the float is a magnet. As the float rises on the water level in the fuel, the magnet closes a reed switch, which turns on a warning light in the instrument cluster. The operator can then remove the drain plug to drain the water.