The fuel tank stores fuel in a convenient location, away from the engine. It is commonly made of steel or aluminum. Baffles ensure the pickup tube is always submerged in fuel. This stops air entering the system. The inside of the tank can be treated to prevent rust. Galvanizing must never be used, because diesel fuel, reacts with zinc to produce powdery flakes that can block fuel filters.
A diesel fuel tank should be kept full, to prevent water condensing on tank surfaces and contaminating the fuel.
In light commercial diesel engines, two fuel lines are used. One carries fuel from the tank to the filters, and then to the fuel injection pump. The other is the return line. It carries back to the tank the fuel that is used for lubricating and cooling the injectors, the injector pump, and for bleeding the filters.
They are usually made of seamless steel tubing, coated with tin to prevent rust. Sometimes cadmium is used instead of tin. A fuel line must be large enough to provide enough fuel flow for maximum power.
It’s supported under the vehicle by nylon insulators in brackets. Reinforced synthetic rubber hoses allow for movement, and vibration of components. The reinforcement is needed because the fuel line is subject to variations in pressure.
Injector lines are made of cold-drawn, annealed, seamless steel tubing. The bore of the pipe is kept to the smallest diameter possible, and all of the pipes are the same length. If pipes of different lengths were used, it would affect injection timing.
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.
The lift pump transfers fuel from the tank to the fuel injection system. In modern vehicles, the tank is mounted below the engine, and the fuel has to be lifted up to the level of the engine. 3 types of lift pump are common on light vehicle diesel engines - the diaphragm-type pump, the plunger pump, and the vane pump.
The diaphragm-type pump can be mounted on the engine, or on the injection pump. It is fitted with inlet and outlet valves, and an eccentric on a camshaft acts on a 2-piece rocker arm connected to a diaphragm. Rotating the eccentric causes the rocker arm to pivot on its pin, and pull the diaphragm down. This compresses the diaphragm return spring, and increases the volume in the pumping chamber above the diaphragm.
Atmospheric pressure at the fuel tank forces fuel along the fuel line, to open the inlet valve. Fuel flows into the pumping chamber. The eccentric keeps rotating, and the rocker arm is released. The spring exerts force on the diaphragm, to pressurize the fuel in the chamber. This pressure closes the inlet valve, and opens the outlet valve, letting fuel be delivered to the injection system.
If the system doesn’t need all of the fuel delivered, the pressure in the outlet fuel line rises to the same level as in the pumping chamber. That holds down the diaphragm, and keeps the diaphragm return spring compressed. When this occurs, the split-linkage in the rocker arm, allows the lever to maintain contact with the eccentric, without acting on the diaphragm pull-rod.
A second type of lift pump in light vehicle applications is the plunger pump. It is mounted on the in-line injection pump, and it’s driven by a cam inside the in-line injection pump housing.
Internally, a spring-loaded cam-follower converts the rotary motion of the camshaft into reciprocating motion. The reciprocating motion is transferred to a spring-loaded plunger, fitted with close tolerance in a cylindrical bore. It has 2 spring-loaded check valves - an inlet valve, and an outlet valve.
As the engine drives the injection pump, the lobe of the camshaft pushes the cam follower into the plunger pump. The cam follower acts directly on the plunger, pushing it towards the end of the cylinder bore. Fuel is displaced from one side of the plunger, through the outlet check valve, to the other side of the plunger.
When the cam follower retracts, spring force on the plunger moves the plunger out of the cylindrical bore. Fuel from the fuel tank enters behind the plunger through the inlet check valve. Fuel in front of the plunger is displaced out of the pump to the fuel injection system.
If the quantity of fuel required is reduced, so is the movement of the plunger.
A vane pump is used in distributor type injection pumps. It is also known as a transfer pump. It is mounted on the input shaft in distributor type injection, and pumps fuel whenever the distributor pump is driven by the engine.
It consists of a rotor, mounted off-centre in a pump housing. Slots are machined in the rotor to carry vanes. As the rotor rotates, the vanes can move into, and out of, the slots. The vanes seal on the edges of the rotor slots and the pump housing.
As the pump rotor rotates, trapped fuel is carried around by the action of the rotor, until the leading vane uncovers the outlet port. Since the rotor is offset, as it turns further, the volume between the vanes reduces, and fuel is squeezed out of the pump. A pressure relief valve controls the pump’s operating pressure.
All diesel engines in light vehicle applications have a priming lever on the lift pump, or a separate priming pump to allow for removing air from the fuel system. This is called bleeding, or priming.
Air can enter the system during filter replacement, or when a fuel line is disconnected.
Without a priming facility, the start motor would have to crank the engine over, to bleed and prime the system. Excessive use of the starter motor for this purpose would damage it, and it would soon discharge the battery.
This diaphragm lift pump has a lever that acts on the diaphragm rocker arm. Moving the priming lever moves the diaphragm down. Releasing the lever allows the diaphragm return spring to force the diaphragm up. The action of the diaphragm and valves during bleeding is the same for normal operation of the pump.
Since distributor-type injection pumps use an internal vane pump, their fuel supply system incorporates a diaphragm type priming pump, usually located on top of the fuel filter.
This filter housing includes a diaphragm-type priming pump. The diaphragm is connected to an actuating button, and it’s held in its uppermost position by a diaphragm spring. Reed valves connect the priming pump housing to the filter.
Pressing down on the actuating button reduces the volume in the pumping chamber, which forces fuel into the filter element.
Releasing the button lets the spring lift the diaphragm, which increases the volume in the pumping chamber. Air pressure in the fuel tank then forces fuel into the pumping chamber.
A plunger-type priming pump is often used with plunger lift pumps that are mounted on in-line injector pumps.
It can also be mounted on top of fuel filter housings for distributor-type injector pump systems.
The plunger-type priming pump consists of a plunger, and a barrel assembly, mounted on the side of the plunger pump, or on a filter housing.
This plunger priming pump uses the valves of the plunger lift pump to direct fuel flow.
Plunger pumps mounted on the top of filter housings contain inlet and outlet check valves. Pulling the primer plunger increases chamber volume, which decreases the pressure in the chamber below that of atmospheric. Fuel in the tank is forced along the fuel lines, through the inlet check valve, and into the pumping chamber. Pushing the plunger into the barrel decreases the volume in the chamber, which forces fuel out through the outlet check valve.
When the priming pump is not in use, the hand knob must be screwed closed. This stops fuel leaking out past the plunger, and also to stop air leaking into the priming plunger barrel.
Some diesel engines use in-line injection pumps to meter, and raise the pressure of the fuel. The basic principle is for a plunger to act on a column of fuel, to lift an injector needle off its seat.
Inside the pump is a pumping element, and a delivery valve for each cylinder of the engine. The element has a barrel, and a plunger that fits inside it. Their accurate fit and highly-polished finish ensures only minimal fuel leakage past them, without needing positive seals. The barrel usually has 2 holes, or ports, called the inlet port, and the spill port. They connect the inside of the barrel with the gallery. The gallery contains filtered fuel from the low-pressure system. At the top of the barrel is a delivery valve, delivery valve holder, and the pipe to carry fuel to each cylinder.
The upper end of the plunger has a vertical groove, extending from its top to an annular groove. The top edge of this annular groove is cut in a helix, also called the control edge. Some pumps have a helix cut on top of the plunger.
A camshaft, cam follower and spring, move the plunger in a reciprocating motion.
When the plunger is below the ports, fuel from the gallery enters the barrel above the plunger. This ensures the barrel is full of fuel. As the camshaft rotates, the plunger is pushed past the ports. The highly polished surfaces cause a sealing effect, trapping the fuel above the plunger. Moving the plunger further raises the pressure of the fuel. This forces the fuel out past the delivery valve, along the fuel line to the injector.
Fuel flows to the injector until the control edge uncovers the spill port. The pressurized fuel above the plunger then moves down the vertical groove, to the annular groove, and into the spill port. The delivery valve stops fuel leaking from the pipe back into the element. It reduces pressure in the fuel line to ensure there is no dribbling by the injector.
The delivery valve has a relief plunger, and a conical face which is held against its matching seat by the delivery valve spring. The relief plunger on the valve is a close fit inside the bore of the delivery valve seat.
When the fuel pressure rises, the delivery valve is lifted off its seat. When the plunger is clear of its bore, fuel flows to the injector. When injection ceases, the pressure below the delivery valve drops to gallery pressure.
Fuel pressure above the delivery valve forces the valve towards its seat. The relief valve enters the seat bore, sealing the volumes above and below the delivery valve. Further movement of the delivery valve towards its seat, increases the volume in the injector pipe, and reduces the pressure in there. This drop in pressure causes the injector needle to snap shut, helping to prevent fuel dribble from the injector. The conical face of the delivery valve then contacts the seat, further sealing the plunger from the injector pipe.
Rotating the plunger controls the length of the stroke for which the spill port is covered. This is called the effective stroke. It influences how much fuel is delivered to the injector. A short effective stroke means a small amount of fuel is injected. A longer effective stroke lets more fuel be delivered. To stop the engine, the vertical groove on the plunger is aligned with the spill port, which stops pressure in the barrel rising.
The plunger is rotated by a control sleeve, a rack, and a pinion. Moving the rack rotates the pinion, the control sleeve, and then the plunger. The rack’s movement is controlled by the governor.
For light automotive use, governors on in-line pumps are usually mechanical or pneumatic.
A mechanical governor uses rotating fly weights to control movement of the fuel control rack against a spring. Removing the load from the engine lets its speed rise. Centrifugal force pushes out the weights, which push a sleeve against the spring. The force from the spring tries to push the rack to the maximum fuel position. The force on the sleeve from the fly weights acts against the spring to try to push the rack to the minimum fuel position.
For any governor position, the fuel control rack determines the volume of fuel delivered, and therefore engine speed. During idling, the governor prevents the engine from stalling. It also stops it from over-speeding.
Mechanical governors in automotive use are called idling and maximum speed governors, because idling speed and maximum speed is all they control. They can also be called limiting speed governors. For other throttle positions, the operator determines the rack position, by moving the position of the floating link.
A pneumatic governor has a manifold-mounted venturi unit, linked by tubing to a sealed, diaphragm assembly on the in-line injection pump housing. This venturi unit has a main venturi, and an auxiliary one. A throttle butterfly controls airflow through the venturi and into the engine.
The venturi is narrow, and shaped so the air speeds up as it passes through.
A similar effect occurs around aircraft wings. The shape of the wing section speeds up the airflow over the top of the wing, and creates a low-pressure area there, lower than the atmospheric pressure below. The result is an upward force that provides lift for the aircraft.
The shape of the venturi is designed to apply the same principle.
When the engine is not running, the diaphragm spring pushes the diaphragm and fuel control rack, towards the full-fuel position.
With the engine running, at idle, the throttle butterfly is almost closing the intake, and air flows through the auxiliary venturi at high velocity. This produces low pressure there, which is transferred through the connecting hose to the sealed chamber on the spring side of the diaphragm.
Atmospheric pressure on the pump side now forces the diaphragm and rack towards the no-fuel position. This reduces the effective pump stroke, and the amount of fuel injected.
Depressing the accelerator allows more air to enter the engine, but decreases the air velocity through the auxiliary venturi. Pressure in the sealed chamber rises, and allows the spring to move the diaphragm, and control rack, against atmospheric pressure, to increase the fuel delivered. The diaphragm position at any given time is determined by the air velocity through the auxiliary venturi, in accordance with engine speed, and load. This provides a rack setting which allows the correct quantity of fuel to be injected, to match the operating condition.
Distributor-type injection pump
The distributor-type pump uses a vane-type transfer pump to fill the single pumping element. This then raises fuel pressure to injection pressure.
A distribution system then distributes fuel to each cylinder, in the firing order of the engine.
The most common type in light automotive use is the Bosch VE pump.
A drive shaft driven from the engine, rotates a plunger, and a cam disc. Cams on the face of the disc have as many lobes as cylinders in the engine. A plunger spring holds the cam disc against rollers that rotate on their shafts.
The lobes move the plunger to-and-fro in its barrel, making it rotate, and reciprocate, at the same time. Its rotation operates the fuel inlet port to the pumping chamber, and at the same time distributes pressurized fuel to the correct injector. The reciprocating motion pressurizes the fuel in the pumping chamber.
The plunger’s pumping action forces fuel through a delivery valve, to the injector. This pump is for a 3-cylinder engine, so it has 3 delivery valves.
The barrel has 1 intake port and 3 distribution ports. The plunger has a central passage, a connecting passage to the distributing slit, and a cross-drilling to a control sleeve. As the plunger rotates, each intake slit aligns with the intake port, and the distributing slit with the distributing port.
As the plunger rotates, the intake slit moves away from the intake port. At the same time, the plunger is acted on by the cams, causing it to move axially along the barrel, pressurizing the fuel in the pumping chamber.
The distributing slit now uncovers the distribution port, and the pressurized fuel passes through delivery valve to the injector. Further rotation of the plunger closes off the distribution port, and opens the intake port. At the same time, the plunger spring moves the plunger back along the barrel for the next pumping stroke.
For intake, fuel from the feed pump reaches the open intake port in the barrel. The intake slit aligns with the intake port, and fuel fills the pumping chamber and passages in the plunger.
For injection, the plunger rotates to close off the intake port, and moves along the barrel, to pressurize fuel in the pumping chamber. The distributing slit aligns with the distribution port, and the pressurized fuel forces the delivery valve off its seat, and reaches the injector. In this phase, a cut-off port in the plunger is covered by the control sleeve.
To end fuel delivery, the plunger’s cut-off port moves out of the control sleeve, and lets pressurized fuel spill back into the pump housing. This relieves pressure in the pumping chamber, the delivery valve closes, and injection ceases.
Metering the fuel is controlled by effective stroke of the control sleeve, and that’s determined by the action of the governor sliding the control sleeve along the plunger. Sliding it one way opens the cut-off port earlier, and reduces effective stroke. Sliding it this way delays its opening, and increases effective stroke.
The governor changes the position of the control sleeve to vary the quantity of fuel delivered, according to throttle position and load.
When the ignition is switched off, an electrical solenoid closes off the intake port, and stops fuel delivery.
Diesel injectors
Most diesel fuel injectors use the same basic design, made from heat-treated alloy steel. The actual shape will vary according to the application.
The injector assembly has several main parts. The nozzle assembly is made up of a needle and body. A pressure spring and spindle hold the needle on the seat in the nozzle body. A nozzle holder, sometimes called the injector body, may allow for mounting the injector on the engine, and some method of adjusting the spring force applied to the needle valve. A cover keeps out dirt and water.
The injection pump delivers fuel to the injector. The fuel passes through a drilling in the nozzle body, to a chamber above where the needle-valve seats in the nozzle assembly. As fuel pressure in the injector gallery rises, it acts on the tapered shoulder of the needle valve, increasing the pressure until it overcomes the force from the spring, and lifts the needle valve from its seat. The highly pressurized fuel enters the engine at a high velocity, in an atomized spray.
As soon as delivery from the pump stops, pressure under the needle tapered-shoulder drops, and the spring force pushes the needle down on the seat, cutting off the fuel supply to the engine.
Some of the fuel is allowed to leak between the nozzle needle and the body, to cool and lubricate the injector. This fuel is collected by the leak- off line, and returned to the fuel tank for later use.
There are 2 main types of injector nozzle, hole and pintle.
Hole-type nozzles are commonly used in direct injection engines. They can be single-hole, or multi-hole, and they operate at very high pressures, up to 200 atmospheres. They give a hard spray, which is necessary to penetrate the highly compressed air. The fuel has a high velocity and good atomization which is desirable in open combustion chamber engines.
In pintle-type nozzles, a pin, or pintle, protrudes through a spray hole. The shape of the pintle determines the shape of the spray, and the atomization of the spray pattern. Pintle nozzles open at lower pressures than hole-type nozzles.
They are used in indirect injection engines, where the fuel has a comparatively short distance to travel and the air is not as compressed as in the main chamber.
Glow plugs
Glow plugs are used to heat the combustion chambers of diesel engines in cold conditions to help ignition at coldstart. In the tip of the glow plug is a coil of a resistive wire or a filament which heats up when electricity is connected.
Glow plugs are required because diesel engines produce the heat needed to ignite their fuel by the compression of air in the cylinder and combustion chamber. Petrol engines use an electric spark plug. In cold weather, and when the engine block, engine oil and cooling water are cold, the heat generated during the first revolutions of the engine is conducted away by the cold surroundings, preventing ignition. The glow plugs are switched on prior to turning over the engine to provide heat to the combustion chamber, and remain on as the engine is turned over to ignite the first charges of fuel. Once the engine is running, the glow plugs are no longer needed.
Indirect-injection diesel engines are less thermally efficient due to the greater surface area of their combustion chambers and so suffer more from cold-start problems. They require longer pre-heating times than direct-injection engines, which often do not need glow plugs at all in temperate or hot climates even for a cold start.
In a typical diesel engine, the glow plugs are switched on for between 10 and 20 seconds prior to starting. Older, less efficient or worn engines may need as much as a minute (60 seconds) of pre-heating.
Large diesel engines as used in heavy construction equipment, ships and locomotives do not need glow plugs. Their cylinders are large enough so that the air in the middle of the cylinder is not in contact with the cold walls of the cylinder, and retains enough heat to allow ignition.
Modern automotive diesel engines with electronic injection systems use various methods of altering the timing and style of the injection process to ensure reliable cold-starting. Glow plugs are fitted, but are rarely used for more than a few seconds.
Glow plug filaments must be made of materials such as platinum and iridium that are resistant both to heat and to oxidation and reduction by the burning mixture. These particular materials also have the advantage of catalytic activity, due to the relative ease with which molecules adsorbed on their surfaces can react with each other. This aids or even replaces electrical heating.
Cummins & Detroit Diesel injection
Unit injectors have been available for many years in heavy commercial diesel engines. These include the 'Cummins Pressure Time System' and the Detroit Diesel System.
Diesel fuel injectors are all very similar in construction. For both systems actuation is a result of an extra lobe on the engine camshaft for each cylinder. This extra lobe actuates the injector at the appropriate point in the engine cycle. Each cylinder has an injector which is fed by a common fuel and return line.
In the Cummins Pressure Time system, the fuel pressure in the fuel lines is very high, and the cam lobe has a low point which allows the injector needle to be lifted by the fuel pressure and enter the cylinder at a given time which is determined by the cam lobe on the cam shaft.
The amount of fuel that is injected at any particular time is controlled by adjusting the fuel pressure in the system.
In the Detroit Diesel System, each injector is like an individual jerk pump and fuel injector in one assembly. The injection cam on the camshaft has a “high point” on it and rapidly increases the fuel pressure in the unit injector, forcing the fuel down into the nozzle which the lifts the needle and allows the fuel into the cylinder. The amount of fuel that is injected is controlled by a small control rack in the unit injector, which is connected to the throttle system via a common rod that links all the unit injectors together.
