Delving into diesels: Practicalities by Andrew Bray

PRACTICALITIES BY ANDREW BRAY

Nearly all yachts have diesel auxiliary engines, all of which depend on precisely metered amounts of fuel to be injected into each of their cylinders at very high pressure, and at just the right moment.

This is achieved by various means in different engines, but all diesel injection systems are complex mechanisms.

They rely on tiny apertures and very fine working clearances between moving parts, which is why injector and fuel pump maintenance is specialist work.

Not only are they vulnerable to physical blockage by dirt, but many of the polished components are entirely lubricated by the fuel passing around them, making them very susceptible to wear and corrosion by contaminants in the fuel.

And while air is not a contaminant, even small amounts of it can disrupt the injection system and even stop the engine.

To a large extent the reliability of your diesel is determined by measures you take (or fail to take) to prevent air and contaminants reaching the fuel injection mechanism.

Potential contaminants include dirt and algae. Water and corrosive chemicals, such as acids formed from sulphur in the fuel itself, are other contaminants.

A diesel fuel system is more than just a storage tank connected to the engine. Other components are specifically there to prevent contaminants and air from reaching the injection system and to facilitate maintenance and the removal of air that enters when filters are replaced or the engine has run out of fuel.

Diesel tanks should preferably be stainless steel or aluminium, be leak-tested before installation, and have a generous access panel in the top and internal baffles.

To avoid tank leaks, all connections (except a sight glass if fitted and perhaps the drain, both of which must have valves fitted right at the tank) should enter through the top. In case of fire the engine supply line should have a shut-off valve that can be closed without entering the engine space. Return lines should not have shut-off valves.

Filler caps should be marked ‘diesel’ (not just fuel), sited where an overfill can be prevented from flowing overboard, and if flush, not mounted where ponding water or boarding waves have the opportunity to test the seal.

Diesel and water fillers should be in different locations and be visually distinct, with no chance that overflow from one will enter the other.

The accidental addition of petrol to a diesel tank happens often enough at busy marinas and can catastrophically damage the engine. Even if detected in time, that tank can’t be used until the contaminated fuel has been extracted.

The vent line should be at least 19mm internal (25mm is better) and loop higher than the filler fitting. It should be fitted with an external mesh screen at the outlet, which should be able to be inspected, face downwards and where spray or waves never reach.

Tank drains
All unsealed rigid tanks experience internal water condensation whenever the overnight temperature falls below the dew point of the air. Some of this moisture drips into the fuel.

The rate of accumulation can be reduced by routinely keeping the tank as full as possible, so there is less air for water to condense out of.

Water tends to sink to the tank bottom, but rolling at sea stirs it up, allowing some to be drawn into the fuel system.

In sufficient quantities water will stop the engine dead, but more commonly the sediment in the water quickly blocks the primary filter, and the engine slows and may stop through fuel starvation.

Replacing the filter will only bring temporary relief unless the source of the problem - water in the tank, is first removed.

This can be impossible once it has become churned up at sea, so preventative water removal is essential, as is a good supply of filters in case the tank is accidentally contaminated at sea.

Apart from chronic condensation, relatively large amounts of water can get into a tank very quickly - in heavily contaminated fuel, through a leaking filler cap, badly located vent, or mistake with a water hose.

All tanks should have provision for removing water and sediment from the bottom while the tank remains in service. This drain should be used to release a sample from the very bottom of the tank at pretty frequent intervals, and particularly before going to sea.

If water is present, keep draining until it has all been removed. This simple precaution may avoid not only an acute overload of the filters but contributes to the long-term reliability of the injector system, and probably the tank too.

Tanks should be built so that fuel for the engine is drawn off some inches above the tank bottom, with a water drain at the very bottom, (or a small sump into which a water draw-off line protrudes from above).

The tank bottom should slope towards this drain which, if in the bottom, must be fitted with a valve, not a drain plug, and lead to some sort of collection point, a transparent container for example.

Level indicators
If the deck filler is immediately over the tank, a dipstick is an accurate way of monitoring tank contents, but if a side of the tank is visible the sight glass principle remains valid too, provided the tube is unbreakable (or guarded) and fitted with a normally closed isolating valve.

There are various electronic and pneumatic devices available that either alarm at pre-determined levels or provide an estimation of the level in the tank.

Diesel plumbing
Flexible lines, vents and filler hoses should be reinforced neoprene-lined ‘oil & fuel delivery’ type hoses.

These can be contorted into tight bends and are preferable to cheaper plastic hose because they are more durable and more fire resistant. Any hoses downstream of the lift pump are under some pressure and should be flexible metal reinforced fuel hose with properly swaged end fittings, not just hose-clamped.

Solid metal fuel lines are the most fire-resistant, but need good support to avoid fatigue cracking, and should not be used to connect a flexibly mounted engine to the rest of the system.

Extra Features: dual tanks
Seagoing yachts should preferably have two diesel tanks, each with its own filler, breather, drain, level indication, draw-off and return line.

The draw-off and return lines should be selected by linked two-way valves to prevent accidentally draining one or overfilling the other by returning fuel to the wrong tank.

Balance lines running between tanks are a mixed blessing. Periodic evening up of the tank levels may avoid a list and is a good precaution against sudden loss of one tank, especially on longer passages.

However, they can transfer contaminants, and if left open may allow a leak to drain both, or may overfill the leeward tank and its connections.

A transfer pump via a filter provides greater flexibility, but does require precautions against overfilling a tank. One may then for example routinely run the engine from tank A but add new fuel only to tank B.

This effectively becomes a large settling tank, while tank A can be kept nearly full to minimise condensation, with fuel that has already had dirt and most water removed.

Wherever possible the tank(s) should be higher than the engine’s lift pump because among other advantages (such as easier filter changing), leaks in the plumbing to the engine then betray themselves by weeping fuel, rather than secretly sucking in air until the engine stops.

Header tanks
Header or ‘day’ tanks (topped up automatically by a float operated transfer pump) can provide gravity flow if the main tank cannot be high enough.

Temporary header tanks can also be created using a siphon from a container of fuel on deck, a useful dodge if the main tank is found to be grossly contaminated, or the lift pump fails.

Removing Contaminants
The colour of diesel fuel varies (it’s even dyed in some countries) but whatever its clarity, all fuel must be considered dirty as far as injection equipment is concerned. Ideally injectors should be protected from particles larger than about three microns. One micron is 0.001mm, too small for the naked eye.

On most small craft engines filtering is carried out in two places: using a primary filter fitted between the tank and the lift pump and a secondary between the lift and injector pumps.

Both inlet and outlet :Primary filters
Most primary filters use a pleated resin impregnated paper cartridge which is itself often encased in a metal ‘spin-on’ canister.

TyPaper element filters cannot be cleaned for re-use, so a good supply must be carried, and knowing how to replace them and ‘bleed’ the system of air are essential boating skills.

Secondary Filters
Many installations utilise the factory-fitted, engine-mounted filter as the secondary. This too is often just an ordinary resin impregnated paper type, so some particles larger than 5 microns probably get through to the injectors.

Really fine filters are available (for example, 2 micron Racor) but may need to be mounted in a vibration-free location, off the engine.

In-line water removal
Most primary filter units (and some secondary) incorporate a settling chamber and a drain, taking advantage of the fact that water tends to coalesce into drops on the discharge side of the paper element. These often have clear bowls so that accumulated water can be seen, although metal bowls are safer, and just as effective if the drain is opened daily.

Water traps can only remove moderate quantities and can easily be overloaded by water stirred up from the bottom of the main tank, so the frequent use of the main tank drain remains important.

Additional Precautions
Some filter units incorporate additional water ‘agglomeration’ features and in larger boats it’s common to have a dedicated agglomerator as part of the in-line pre-filtering process.

Pre-filters
Further protection can be obtained by installing a pre-filter or sedimenter before the primary filter. A sedimenter contains no filter, but slows the fuel velocity down so that heavy particles and larger water drops tend to settle and can be drained out.

Because such systems are small and completely filled with fuel they remain fairly effective at sea, where sloshing stirs up the main tank and prevents water from settling.

Sedimenters and some agglomerators can be provided with sensors that warn when significant water has been accumulated.

Filter gauges
As a filter progressively blocks flow, the pressure drop across it increases and some are fitted with a pressure gauge.

If this is routinely checked, it may give early warning of dirty fuel, as well as indicating when a filter change is imminent, easier to do in port than at sea. Otherwise they should be replaced at intervals suggested by the engine manufacturer and at least annually.

Standby filter
A simple two-filter system can prove very inconvenient, or even dangerous, if the engine has to be stopped while a filter is replaced. A better system is to have two primary filters in parallel, plumbed with ‘Y’ valves at inlet and outlet. Only one filter is in service at any one time, with the other held as an already ‘bled’ standby that can be brought into circuit without stopping the engine, just by switching two valve levers.

Other problems: Fuel bugs
Fuel bugs are bacterial and algal life-forms that grow in the water contaminating the fuel.
While alive they usually float at the top of any accumulated water and collectively form a slime that in heavily contaminated tanks can become repetitively expensive in terms of filters and inconvenience.

Minimising condensation and routinely draining water should discourage growth within the tank but bugs may be in the fuel when you buy it so dosing with a biocide fuel ‘conditioner’ is a good precaution when adding fuel. These can add several pennies per litre to the fuel cost, but are cheap insurance. However, they do not prevent filter blockage by dead organisms.

Badly contaminated tanks may need to be tackled by circulating the contents through special portable ‘fuel polishing’ filtering systems, but even this plus physical draining and cleaning of the system may not prevent problems re-developing.

Apparently some fuel sources, particularly in warmer climates, are chronically contaminated.

Additives that claim to prevent water accumulating by mixing it with the fuel could prove troublesome, in that the water then passes through the injection system where it may cause corrosion, and perhaps convert to steam and erode the injector nozzle holes.

Dirty fuel
Really dirty fuel can be encountered anywhere, but is more likely in remote places, where there is no alternative source.

Fuel delivered in drums should always be presumed to be heavily contaminated. Funnels containing mesh filters do little more than trap gross visual evidence - a better approach is to temporarily store suspect fuel in plastic drums.

After standing for some days, the contents can be siphoned through a strainer, leaving most water and sediment behind in the dregs.

Final advice
Regular filter changes and opening of water drains will prevent or alert you to most potential fuel problems, but if an engine fails at sea you may have to do some quick detective work.

If it can be turned over in neutral gear apparently normally with the starter motor but still doesn’t fire, the problem may be a broken control cable or fuel stop solenoid.

Otherwise it’s probably not a mechanical issue and very likely due to water or air in the fuel reaching the injectors.

Air or water tends to cause erratic running before the engine stops (because it doesn’t reach all cylinders simultaneously), but if the engine just slowed progressively (without producing black smoke, which would suggest a fouled propeller), the prime suspect is fuel starvation, probably caused by a nearly blocked primary fuel filter.

However, it may also be due to a lift pump failure, forgetting to open the fuel cock, or a blocked tank vent!

If the problem was due purely to a blocked primary filter, just replacing this and bleeding that section (or switching over to a standby filter) may be all that’s required to get on the move again.

Replacing a filter inevitably lets air into the fuel system, which must be removed before the engine will run again.

Removing air is called ‘bleeding’, an important boating skill. No one should put to sea in a diesel-powered boat without spare filters and without someone on board who knows how to replace them.