Technology: How fuel filters and water separators prevent damage to the engine

Text by Peter Rutke

The refuelling process on boats is often unpleasant. In many cases, the tank vent is located below the filler neck in the side of the boat, which means that fuel often escapes when refuelling. Diesel fuel in particular, which foams up easily, tends to gush out of the filler neck before the nozzle stops automatically. However, there are overflow systems on the market that reliably prevent fuel from escaping when refuelling.

It is also important to note that potential equalisation between the filler neck and the fuel tank and between the hull and the water is often missing or no longer present. This is particularly relevant for petrol systems, as the significantly lower flash point of petrol (-25 °C) compared to diesel (+55-70 °C) can cause sparks if the nozzle comes into contact with the filler neck, which can have catastrophic consequences for the boat and the crew.

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It goes without saying that all supply lines should be checked regularly for leaks. If you can smell petrol or diesel, you should be on high alert. Petrol vapours are heavier than air and collect in the bilge. For this reason, the exhaust fan should always be switched on for a few minutes before starting petrol engines as a precaution.

Openings in injection nozzles are often no bigger than a human hair

With modern injection technology, water, rust or bacteria in the tank can inevitably lead to serious problems in the engine system. If you look at the technical development of injection systems, from in-line injection pumps to distributor injection pumps and common-rail systems with increasingly sophisticated technology, this may be obvious to anyone.

The result of modern fuel treatment, regulated by electronic controls, are low-consumption, comfortable, smoke-free and environmentally friendly engine systems in our boats that nobody would want to do without. The openings in the injection nozzles, through which the diesel fuel is injected into the combustion chamber at over 1,800 bar, are barely larger than a human hair, with a diameter of 0.15 millimetres. This makes it clear what effects even the smallest impurities in the fuel can have. At road petrol stations, the fuel is stored underground and is therefore protected from temperature fluctuations. Brisk sales ensure a constant supply of fresh fuel.

This is often different at boat refuelling stations. We sometimes get dirt or water in the fuel practically "free boat" into our tank when bunkering. If a steel tank is installed in our boat or, in the case of a steel boat, the tank is even integrated into the hull, condensation will also form on the inside walls of the tank due to rapid temperature fluctuations. It therefore makes sense to either empty the tank completely during winter storage or to fill it completely.

A diesel engine delivers more fuel from the tank than required

A diesel engine pumps considerably more diesel from the tank than is required for the actual combustion process. A not insignificant proportion is used for lubrication and cooling of the injection pump and injection nozzles and then flows back into the tank via the return lines, significantly warmed up. Some engine manufacturers install a fuel cooler in the return line to slow down this heating process. Otherwise, the temperature in the tank is naturally much higher than the outside temperature after a few hours of operation. Condensation forms on the inner walls and, as it is heavier than fuel, collects on the bottom of the tank.

The often encountered feeding of the returning diesel into the prefilter (instead of back into the tank) prevents the fuel in the tank from gradually heating up, but ensures that the fuel for lubrication and combustion becomes hotter and hotter. Due to the lower density, the power output is then lower. However, this is only noticeable if the engine is operated at full load for a longer period of time.

The movements of the boat cause dirt particles and water to enter the suction pipe

Another reason for water in the fuel tank is the frequent carelessness when installing the tank ventilation system. The venting hose should be routed in as high a curve as possible from the onboard outlet to the tank so that no water (splash water) can penetrate from the outside. Older steel fuel tanks in particular, which were still elaborately manufactured, have welded-on recesses with a drain tap. This offers the opportunity to drain the sump occasionally.

If you don't have this option, you hopefully won't feel anything for a long time, as the intake nozzle usually ends a few centimetres above the bottom of the tank and therefore above the sump. Problems are known to occur when you least need them, namely in rough seas. Dirt and water are stirred up by the boat's movements and then enter the intake pipe to the engine.

This can be remedied by fitting a piece of hose to the intake manifold after cleaning the tank. This allows the fuel to be taken directly from the bottom of the tank in future.

The next component in the intake line is the fuel shut-off valve. To enable shut-off in an emergency, it should be possible to operate this from outside the engine compartment. Before reaching the machine, a fuel pre-filter - ideally in combination with a water separator - is installed if the installation is carried out properly. Unfortunately, however, savings are often made in the wrong place. Often there is no pre-filter at all.

A second filter can also be useful

The ideal case is also the most expensive variant and consists of so-called circular filters in combination with water separators which, according to the manufacturer, filter out 99 per cent of water. Here, the fuel is set in circular motion by guide vanes. Because water is heavier than diesel, the water particles are thrown to the inside of the bowl and run off downwards. If the filter has a glass bowl, the separated water can be recognised visually. It is more convenient to use an electronic sensor that signals water in the punch bowl on the cockpit. The sensor is indispensable if a filter with a metal bowl (CE version) is installed. A visual check is naturally not possible here. In the second stage, the fuel freed from water then passes through the replaceable filter element. There is a drain tap at the bottom of the bowl.

If you want to "go one better" in terms of safety, you can also install such a circular filter/water separator as a switchable dual version. If water is signalled in the fuel, you can switch to the second filter and drain the water from the first filter without having to stop the machine - a safety gain in any case. Filters of this type are offered by the manufacturers SEPAR and RACOR. To extend the service life of the filter element, the SEPAR filters can be flushed by opening a ventilation screw in the cover and opening the drain tap at the same time.

Regardless of the operating hours, both fuel pre-filters and fine filters should be changed at least once a year. After all, shipping is all about preventive safety.