Schematic of a simple fuel injection systemIf we ignore for a minute the actual EMS, the basic component parts of an injection system are very straightforward.  Shown below is a schematic of the major parts of a multi-point injection system.

Fuel system constituent parts

  • Fuel tank:  Holds a reservoir of fuel for the engine, is normally baffled to prevent fuel sloshing around, and the resultant fuel starvation.

  • Fuel filter:  Since an injector pump is a positive displacement pump any foreign material ingested can stall the pump and kill it stone dead.  This ‘pre-filter’ prevents rubbish from entering the pump.

  • Fuel pump:  A high-pressure pump running at around 6 bar which supplies fuel to the injectors.  The fuel pressure regulator regulates to this pressure between 3 and 4 bar (43 and 58 psi).  On most modern installations the pump is housed inside the fuel tank with rudimentary filtration, and the fuel filter then follows in the fuel line.

  • Fuel line:  Fuel pipe that transports the fuel from the pump to the fuel rail.

  • Fuel rail:  A small fuel gallery from which the injectors take their fuel supply.

  • Injectors:Electric valves which when open allow fuel to be injected into the engine under high pressure.

  • Pressure regulator:  A device that keeps the fuel pressure at a constant rate and returns any excess fuel to the tank.

  • Fuel return line:  Fuel pipe which bleeds excess fuel back to the fuel tank, often now incorporated into the fuel tank itself.


Most injection systems run at quite high fuel pressure compared to a system using carburettors.  Typically an injection pump will produce around 6 bar and the system will run at around 3-4 bar (43-58 psi).  This is far in excess of the pressure supplied by a typical fuel pump from a non-injected system (3-10 psi).  The injection system relies on a constant supply of fuel at a predetermined pressure and generally the pump runs all the time with excess fuel being returned to the tank.  The map for the engine will have been derived with the fuel supply at this pressure; variations in fuel pressure will affect the quantity of fuel injected and will seriously affect the running of the engine, sometimes terminally.

Carburettors can generally cope with a short interruption to their fuel supply since they have their own reservoir of fuel in the float chamber that can be drawn from.  Injection systems on the other hand cannot cope with fuel supply interruptions so it is necessary to ensure that such interruptions don’t take place.  It is standard practice to baffle the fuel tank and use one-way valves to prevent fuel surge.  Where space allows, a surge pot can be fitted to ensure that fuel surge doesn’t rob the injection system of fuel at the wrong moment.

Most fuel injection pumps are gravity fed so they need to be mounted lower than the lowest point in the fuel tank.  An alternative to this is to mount the pump in the fuel tank itself.  Most pumps can be run completely immersed in fuel.  In practice they do this anyway since inside the pump the fuel runs up and around the armature of the pump in order to keep it cool.  The pump's operation is often controlled by the EMS to prevent the pump delivering fuel when the engine is not running, for example if the vehicle is involved in an accident.

The pump supplies fuel to the injectors via a fuel rail which is a long narrow tube with a connection for each of the injectors.  The fuel supply enters the rail at one end, while at the other is the fuel pressure regulator which ensures that the fuel pressure remains constant.  On more modern systems the fuel is only supplied to the fuel rail, the fuel pressure regulator bleeding off the excess fuel and pressure earlier in the fuel line or even inside the fuel tank itself.  Since the fuel pressure can affect the amount of fuel discharged in any given injector time, it is essential that this pressure is kept constant.

Example high pressure fuel pumpExample fuel pressure regulatorExample fuel injectors

It is not uncommon for fuel pressure regulators to be tampered with to supply extra fuel pressure.  This is a common dodge when an engine has been tuned and as a result needs more fuel.  Since the map inside the OEM EMS cannot be varied, a certain increase in fuelling can be had by upping the fuel pressure.  Rising-rate fuel pressure regulators achieve the same objective.  They increase fuel pressure when the engine's air demands are high, often increasing the fuel pressure in response to low vacuum in the inlet manifold, e.g. when the throttle is increased.  Some EMS systems are able to cope with a small increase in airflow on their own since they know when the engine is running weak due to the Lambda feedback, and will increase fuelling to compensate.  This can only be achieved during steady state running so there will still be glitches in the fuelling here and there.

The injectors themselves are connected to the fuel rail via a clip and ‘O’ ring which has to contain the high pressure within the fuel system.  An injector is simply an electric valve or solenoid.  Fuel is supplied to the injector at a known and regulated pressure, and the valve or solenoid is normally closed.  Fuel is introduced or injected to the engine by firing (opening) the injector for a predetermined period of time once per engine revolution or per engine cycle.  The longer the injector is held open the more fuel is introduced.  This injector time is known as the ‘pulse width’ and the technique of varying fuel in this manner is known as ‘pulse width modulation’ as it is the pulse width that is varied according to requirements.  Since the fuel injected is per revolution or cycle, as engine RPM is increased, so is the number of times the injectors are fired.  This has the effect of meeting the engine's requirements for fuel regardless of rpm.

Multi-point injection

Schematic of multi-point fuel injection systemMulti-point injection systems are the most common and usually have an injector per cylinder located in each individual manifold runner.  This configuration gives much better control of fuelling and better emissions since the fuel can be metered more closely, and there is less opportunity for the fuel spray to condense or drop out of the airflow since it is introduced as small streams rather than one large one.  The closer to the inlet valve the fuel injection takes place, the better the economy and transient throttle.  Most systems use one injector per cylinder but on certain engines (notably the Rover ‘A’ series) there are only two inlet ports since two cylinders share a siamesed port.  In this case multi-point would mean two injectors, one per inlet port.

With multi-point (or multi injector) systems there is scope for timing the injection of fuel to better suit the engine's duty cycle.  If the EMS knows the relative position of each cylinder within the engine's cycle (usually from a cam-phase sensor) then it can fire the injectors at the optimum time for that cylinder.  This is known as sequential injection.  Sometimes the EMS will only have knowledge of the crank position rather than the duty cycle position, and in this case it can optimise for a pair of cylinders.  This is known as semi-sequential or grouped injection.

Some EMS systems ignore the crank and cycle position when injecting fuel.  They fire all of the injectors at the same time once per revolution, and this is known as batched injection.  There is no penalty to pay power-wise when using batched injection.  However, grouped and sequential injection give a slight edge on economy and transient throttle/emissions.