Introduction to engine fuelling systems
Gasoline engines fuelling
In Spark Ignition engines, a homogeneous and combustible air/fuel mixture must be formed before the start of combustion.
The simplest way to achieve such a result is by means of a carburetor, where the intaken air passes through a Venturi throat, which is connected to a fuel reservoir. The highest the air mass flow, the highest will be the pressure drop in the Venturi throat, and thus the fuel mass flow drawn.
However, such a system does not allow a precise control of the air/fuel ratio, as it is mandatory to obtain a high efficiency of the exhaust catalyst, and thus has been replaced by electronically controlled fuel injection systems. In those systems, the amount of fuel injected in the air stream can be more precisely controlled by acting on the opening duration of the injectors (which can be considered as electromagnetic on/off valves).
Fuel can be injected into the intake manifold (Single point Port Fuel Injection), into each intake port (Multiple Port Fuel Injection) or directly into the cylinder (Direct Injection). In the last case, the injection must take place with a large advance before the start of combustion, in order to allow the formation of a proper air/fuel mixture.
Moreover, Direct Injection can also enable stratified charge operation, in which a stoichiometric mixture is obtained by injecting the fuel close to the spark plug, just before the start of combustion, in order to be locally rich and globally poor. Stratified charge operation allows the control of air/fuel ratio to be done only thanks to injection system and thus avoid the need of throttling the engine at part load to get a better efficiency.
Diesel engines fuelling
In Compressed Ignition engines, the fuel is usually injected directly into the cylinder towards the end of the compression stroke (Case of Direct Injection).
However, for small displacement engines (below 0.5 liters), where small fuel quantities must be injected, proper fuel jet atomization in small droplets and air/fuel mixing can hardly be obtained, due to limitations in the minimum nozzle hole diameter (0.1 mm minimum). For this engine category and also for old diesel engines, Indirect injection in a prechamber, connected to the combustion chamber by a narrow passage, is/was used, allowing a better and faster air/fuel mixing, but causing remarkable efficiency losses (Thermal losses).
To accomplish direct injection without common rail technology, fuel is usually sent under pressure by an injection pump to the nozzle pipes which carry fuel to the injectors’ nozzles in each cylinder head. Distributor-type fuel-injection pumps are normally used for automobile engines as the maximum injection pressure is only 750 bars whereas with in-line pumps, which are used for large engines, are able of 1300 bars.
However, the development of electronically controlled Common Rail injection systems, allowing the split of the injection event into 2 or more injection per cycle, has solved after-treatment or thermal losses issues, thus remarkably reducing the spread of Direct injection engines using unit injectors or Indirect injection engines.
The particularity of the Common Rail system compared to other injection systems is that all the injectors are fed in permanence by a high-pressure pump through an accumulator called Common Rail. The interest of this system is that the main functions of the injection system are operated closer to the combustion chamber. In previous systems, the fuel metering, the timing and the pressure generation were performed by the pump, whereas with common rail, the actuation of the injector is directly representing the timing and the fuel metering which allows to have a better accuracy.
Moreover, the high-pressure pump that is in charge of the pressure generation is no more dependent of the engine speed and injection quantity. It is then easier to calibrate the injection pressure according to a given operating point. Indeed, the high pressure can be generated even at low engine speed because of the independence to torque (injection quantity) and engine rotation speed.
Regarding the injectors, as they are electronically piloted, it is possible to manage several injections (up to eight per stroke) and a fine tuning of the quantity and timing. The injectors are always under pressure as they are connected to the rail.
Fuelling systems are getting more and more complex, both for gasoline and diesel engines. For example, in diesel engines, in order to have increased fuel pressure and then better atomization, systems combining unit injectors and common rail are emerging and are able to reach 3000 bars. When do you think injection systems will reach physical limits? Will the fuel pressure raise that much in gasoline engines?