Diesel fuel doesn’t burn very easily, and in order to burn quickly, cleanly and reliably it has to be in the form of fine droplets, like an aerosol spray. You’ll remember from the previous chapter that the air in a diesel’s cylinders is made hot by being compressed to 20 or 30 times its normal atmospheric pressure, so producing an aerosol spray inside the cylinders means that the fuel has to be at an even higher pressure – in the order of 2,500 psi.
It’s also essential for the proportions of fuel and air to be exactly right, so each squirt of fuel has to be very accurately measured. If you think of a typical fourcylinder diesel developing 80 hp when it’s running flat out at 4,000 rpm, it will be burning about 4 gallons of fuel an hour. Each cylinder will be receiving 2,000 squirts of fuel every minute – making 8,000 squirts per minute, or 480,000 squirts per hour. Each squirt, then, must be less than 10 millionths of a gallon, 0.04 ml, or less than a hundredth of a teaspoon.
At low loads the amount of fuel sent to the cylinders has to be even less. It’s hardly surprising, then, that the fuel system includes some of the most sophisticated and expensive parts of the engine, responsible for achieving pressures of almost 200 atmospheres, measuring doses of fuel accurate to less than a thousandth of a millilitre, and repeating the process perhaps half a million times an hour!
Basic Fuel System of Diesel Engine
The fuel system starts, however, with the crudest component of all: the tank. It’s worth bearing in mind, though, that a full tank can be very heavy, so it needs to be well supported and secured against the boat’s motion. A big tank – anything over about 5–10 gallons – should include internal baffles to stop the fuel sloshing about, and any tank needs a vent, or ‘breather’, to let air in as the fuel is used up.
Unfortunately, the fuel received from the hose may not be perfectly clean, and the air that comes in through the breather will almost certainly be moist enough to allow condensation to form inside the tank. The end result is that the tank will include some dirt and water.
To prevent this reaching the engine, the engine installation should include a component known as a primary filter, pre-filter, separator, sedimenter or filteragglomerator, usually mounted on a bulkhead in the engine compartment rather than on the engine itself.
The lift pump is responsible for pulling the fuel out of the tank, through the primary filter, and passing it on to the rest of the system. In most cases, it’s a simple diaphragm pump, very much like a min – iature version of a manual bilge pump. It’s driven by the engine, but usually has a hand-operated priming lever so that you can pump fuel through the system without running the engine. The fuel then passes through another filter, sometimes known as the main filter or secondary filter or fine filter, whose job is to remove particles of dirt that – at less than a thousandth of a millimetre in dia – meter – may be too small to see, but that are still capable of wearing the very finely engineered surfaces of the rest of the system.
If a diesel engine has a ‘heart’, it has to be the injection pump, because this is where the fuel is measured and pressurised. Injector pipes, with very thick walls to withstand the pressure, carry the highly pressurised fuel from the injection pumps to the injectors that spray it into the cylinders.
Some of the fuel that is pumped to the injectors, however, never actually reaches the cylinder but is returned to the tank through a leak-off pipe, or return line.
Single-element Injection Pump
There are three main types of injection pump, of which the simplest is the kind found on single-cylinder engines. Even if you have a multi-cylinder engine, it’s worth knowing a bit about the single-element ‘jerk’ pump, because many multi-cylinder engines use derivatives of it. The principle is much like that of a bicycle pump or an old-fashioned bilge pump, with a piston (usually called the plunger) moving up and down inside a cylinder.
A hole called the spill port in the side of the cylinder allows fuel to flow into the cylinder when the plunger is at the bottom of its travel. As the plunger rises, however, it covers the port to shut off the flow and trap some fuel in the cylinder.
As it continues to rise, the trapped fuel has to go somewhere, so it escapes by lifting the delivery valve off its seat, and flowing out into the injector pipe.
The measuring part of the fuel pump’s job is taken care of by a spiral-shaped cutout in the side of the plunger. As the piston nears the top of its travel, the spiral cut-out eventually comes level with the spill port in the side of the cylinder, allowing fuel to flow round the spiral and out of the spill port.
Pushing or pulling on a toothed rod called the rack makes the plunger rotate, so the spiral can be made to uncover the spill port at any stage in the plunger’s stroke, varying the amount of fuel that is delivered without having to change the distance the plunger actually moves. This is significant, because the up and down movement of the plunger is achieved by the action of a cam, very similar to the cams that operate the valves in the engine’s cylinder head.
It’s worth noting that thin metal packing pieces called shims are usually fitted between the base of the pump and the cylinder block or crankcase. Increasing the number of shims raises the pump body, so the ports are higher, which means that the pump doesn’t start delivering fuel until slightly later in the cycle.
In other words, the number and thickness of the shims has a critical effect on timing – the moment at which fuel is sprayed into the cylinder – so if you remove the fuel pump for any reason, it’s essential to make sure that you retain all the shims and put them back when the pump is re-installed.
In-line Injection Pump
A few multi-cylinder engines use a separate single-element fuel pump for each cylinder, but it’s more common to find all the separate elements combined into a single component that looks rather like a miniature engine. It’s called an in-line pump because it consists of several jerk pumps in line, driven by a camshaft in the pump body instead of in the engine block.
Rotary Injection Pump
The rotary or DPA injection pump is lighter, more compact, and can cope with higher engine speeds than the in-line type, so it’s eminently suitable for small, highrevving engines.
Unfortunately, it’s also more vulnerable to dirty or contaminated fuel and – unlike an in-line pump that may fail on one or two cylinders but keep going on the others – a DPA pump that goes wrong will often pack up altogether. The reason for this ‘all-or-nothing’ operation is that a DPA pump consists of a single high pressure pump, distributing fuel to each injector in turn through a spinning rotor.
The lift pump supplies fuel to the injection pump at one end, where a vane-type transfer pump – similar in principle to the engine’s raw water pump – increases its pressure. The fuel then flows to the high pressure pump through the metering valve, which controls the amount of fuel that will be delivered to the engine’s cylinders.
The high pressure pump consists of two small plungers built into a rotor. Fuel from the metering valve flows into the space between the two plungers forcing them to move apart. As the rotor turns, however, bulges on the cam ring that surrounds it force the plungers back inwards.
Fuel, now at very high pressure, is driven out of the space between the plungers and through a drilling in the rotor, which directs it to each injector pipe in turn.
The injectors convert the tiny squirts of high pressure fuel into an atomised spray in the cylinders. They are usually cylindrical in shape, about 6 in (15 cm) long and 1 in (25mm) in diameter, but are clamped into the cylinder head so that only a couple of inches of the injector body and a couple of pipe connections are visible.
The injector body is basically a tube, almost completely filled by a needle valve, push rod, and a strong spring. Fuel from the injection pump enters the side of the injector from the injector pipe, and then flows down a narrow passage to the pressure chamber, just above the nozzle. The nozzle is sealed off by the needle valve, which is held in place by the push rod and spring.
When the injector pump delivers one of its pulses of fuel, the pressure within the pressure chamber rises sufficiently to lift the needle valve off its seat. Fuel then rushes out of the nozzle so quickly that it breaks up into a spray. Of course, this sudden escape of fuel means that the pressure in the pressure chamber drops again, allowing the needle valve to snap back into its seat to stop the flow.
Although the movements of the needle valve are very small, they happen so quickly that lubrication is essential. This is achieved by allowing some of the fuel from the pressure chamber to flow up the injector, past the needle valve and push rod, and out through the leak-off pipe at the top to return to the tank.
If too much fuel took this route, it would entirely defeat the object of the exercise: the pressure in the pressure chamber would never rise enough to lift the needle valve, so no fuel would get into the cylinder. The fact that it doesn’t is entirely due to the very high precision engineering of the injector, which keeps the clearance between the needle valve and the injector body down to something in the order of 0.001 mm (about 40 millionths of an inch).
That’s so small that if you were to strip an injector and leave the body on the bench while you held the needle valve in your hand, your body heat would expand the needle valve enough to stop it going back into its hole! There are three reasons for mentioning this, of which the first is to make the point that you should never strip an injector: it may look rugged, but it’s so finely engineered that injector servicing is definitely a job for a specialist company.
The second reason is that it goes a long way towards explaining why new injectors can cost several hundred pounds each, and the third is that it explains why all those filters are so important: the tiniest specks of dirt can be sufficient to abrade the surface of the needle valve enough to increase the leak-off to such an extent that the injector doesn’t open properly, or to wedge the valve open and allow fuel to drip out of the nozzle instead of forming a fine spray.
The same applies to injection pumps, because there is nothing an amateur mechanic can achieve by tinkering with them, other than a lot of damage. Even the apparently simple job of removing an injection pump is more complicated than it may seem, because re-fitting it involves adjusting it to make sure that the squirts of fuel are delivered to the right cylinders at the right time: it needs confidence and the right workshop manual.
High-tech Fuel Systems
The last few years of the twentieth century saw growing concern, worldwide, about the use of fossil fuels and atmospheric pollution. Customers wanted cleaner, quieter cars and lorries, and legislators wanted to be seen to be doing something.
Almost inevitably, fuel systems came under close scrutiny. The effect was that by the beginning of this century we started to see new, radically different ways of getting fuel into cylinders being introduced in cars and commercial vehicles.
It’s taking longer for these to trickle down to marine engines, and it will undoubtedly be many years before conventional fuel systems disappear altogether, but it is worth being aware of developments such as electronic control, unit injectors, and common rail injection systems.
A key part of any conventional fuel pump is the governor. At its simplest, this consists of a set of weights connected to the shaft of the pump. As the engine speed increases, the pump shaft turns faster, so the weights try to fly outwards. As they do so, they operate a mechanical linkage which reduces the amount of fuel being sent to the injectors.
This, of course, slows the engine down, allowing the governor weights to move inward again. The engine control, in the cockpit or wheelhouse, is connected to the governor by a spring. By adjusting the engine controls the helmsman adjusts the spring tension so as to increase or decrease the speed at which the shaft has to turn before the weights move outwards far enough to slow the engine down.
The aim of all this is partly to stop the engine over-revving, but it also means that when you — the user — set the throttle for a particular engine speed, the governor will keep the engine running at that speed even if the loading varies. Simple mechanical governors like this have been used to control machinery for centuries: you can see rudimentary versions in watermills, windmills, and on steam engines, but now their place is increasingly being taken by electronic versions which monitor other factors such as air temperature and inlet manifold pressure as well as shaft speed.
Unit injectors, in principle, are almost a retrograde step: they take us back to the days when each injector had its own high pressure pump. As the name suggests, however, the modern unit injector combines the pump and injector in a single unit, mounted in the cylinder head in much the same way as a conventional injector.
In some cases the pump is mechanically driven. Each cylinder has three rockers instead of the usual two. Two of the three rockers open the valves, exactly as they do in a conventional engine, while the third one operates the plunger of a small pistontype pump in the head of the injector. An alternative is to dispense with mechanical operation, and use hydraulics instead, with an electric solenoid controlling the pump plunger.
Common Rail Injection
Perhaps the most exciting development is known as ‘common rail’ or ‘reservoir’ fuel injection. The key feature of this is that metering and control functions have been taken away from the injector pump altogether: its sole job is to produce a constant supply of fuel at enormously high pressure — up to about 30,000psi (2,000bar).
From the pump, the pressurised fuel passes to a thick-walled tube (the ‘common rail’) or to an equally rugged reservoir, from which injector pipes carry it to electronically controlled injectors.
The advantages of the system are that the higher pressure means that the fuel spray from each injector is much finer, while the electronic control means that the amount of fuel, the timing and duration of each squirt, and even the number of squirts per cycle can be varied by the electronic processor to give increased fuel efficiency, less toxic exhaust gas, and lower noise levels.
The down-side of the system (apart from price!) is that it has done away with the rugged simplicity which used to be one of the advantages of a diesel engine, and has made a diesel just as dependent on electricity as a petrol engine.
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