How Engine Fuel Control Works

Four things decide whether the burn is efficient: pressure, flow, when injection starts, and how long it lasts.

It is common knowledge that petrol and CI engines require fuel, that this fuel needs to be stored for a period of time, and that it needs to be introduced into the combustion chamber in the correct form for the engine to run properly. For the most efficient burn, four main requirements need to be met: pressure, flow, timing of injection and period of injection. Since the vast majority of vehicles today use fuel injection of one type or another, carburettor systems are ignored here.

Fuel pressure and engine management

In the average petrol engine car, fuel from the tank is fed under pressure through a fuel filter by the fuel pump. This pump can be mounted externally onto the body or internally within the tank. The pump unit itself is controlled by the fuel and engine management ECU, providing a current supply via an electrical relay.

Fuel then passes up to the engine bay inside high pressure solid and flexible pipes, where it joins the pressure regulator. This unit governs the fuel pressure available to the injectors. The regulator usually has three connections: supply from the tank, feed to the injectors, and a vacuum line from the inlet manifold which allows changes in fuel pressure depending on engine load. Under full load conditions the regulated fuel pressure can reach 40 psi, or 2.7 bar.

The supply now reaches the injectors, which are small electromagnetically operated valves mounted close to the back of the inlet valves. When energised, they allow the pressurised petrol to flow through a nozzle, producing a mist — atomisation — which is directed into the inlet air stream. For those interested, the injector valve moves approximately 0.15 mm, and this occurs over a time period of between 1.5 and 10 milliseconds, completely governed by the ECU. This time variation allows for a very high degree of fuel and air mixture control depending on engine operational requirements.

Sensor inputs and filtration

As the quantity of fuel delivered by the pump is far in excess of that required by the engine, a return line is provided to allow this excess to return to the tank. This ensures a constant supply of continually filtered fuel at a constant supply pressure.

The next factor to be decided is the amount of fuel delivered, via adjustment of injection duration. Simply, longer injector open time equals more fuel injected. The fuel and engine management ECU takes information from the various sensors around the engine — notably engine speed, coolant temperature, inlet air temperature and engine load — and calculates the fuel requirement.

For example, a morning cold start will involve a longer injection period than when starting from warm, and the main sensor input will be from the engine temperature sensor, as the engine load will be very low, as will the engine speed until the vehicle is driven off. As the engine temperature increases, the requirement for cold start enrichment decreases with the shortening of the period of injection. The injection duration only then increases with the increase in engine load.

Pumps and pressure on the diesel side

With the CI engine, many similarities with the petrol engine exist, in as much as the fuel must reach the combustion chamber in the same form. From the fuel tank, fuel is drawn by a primer pump through a filter and delivered to the injection pump.

In the modern motor vehicle there are two main types of injection: rotary pump and common rail. In the first, an engine driven pump produces pulses of high pressure fuel delivered to the injectors through thick walled steel pipes. In the second, an engine driven pump supplies a fuel rail in turn connected to every injector — hence the term common rail. This is slightly different to the rotary system in that the pressure at the injectors is not pulsed, it is kept constant.

With the pulsed supply, pressure builds up behind each injector until it is sufficient to lift a needle arrangement clear of its seal against spring pressure; fuel then flows through the injector nozzles producing a fine mist. As the pressure at the injectors is constant in the common rail system, the injectors are opened electrically under the control of the engine and fuel ECU. The overriding advantage with the common rail system is that, as the injection process is under computer control, far more accurate fuelling is available throughout the engine speed range.

To summarise, fuel control exists to provide the engine with the correct amount of fuel at the correct time, taking into account all engine operating conditions.