From The Road Up – Engine Fuel Control
It is common knowledge that petrol and CI engines require fuel and that this fuel needs to be stored for a period of time and also 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. Please note that as the vast majority of vehicles today use fuel injection of one type or another, carburettor systems will be ignored.
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/engine management ECU providing a current supply via an electrical relay, fuel then passes up to the engine bay inside high pressure solid and/or flexible pipes where it joins the pressure regulator, this unit governs the fuel pressure available to the injector(s). The regulator usually has three connections; 1) supply from tank, 2) feed to injector(s) 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 40psi (pounds per square inch) or 2.7bar. The supply now reaches the injectors which are small electro-magnetically operated valves mounted close to the back of the inlet valves which, when energised, 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 0.15mm 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/air mixture control depending on engine operational requirements.
Sensor Inputs And Filteration
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 aspect of the system ensures that there is 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/engine management ECU takes information from the various sensors around the engine notably, engine speed, coolant temperature, inlet air temperature, engine load, etc 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
With the CI engine, many similarities with the petrol engine exist in so 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 two main types of injection 1) rotary pump, and 2) common rail; in the first an engine driven pump produces pulses of high pressure fuel delivered to the injectors through thick walled steel pipes and in 2) 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. It is this pulsed supply which builds up behind each injector until the pressure 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/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 summarize, fuel control exists to provide the engine with the correct amount of fuel at the correct time taking into account all engine operating conditions.