Types of Fuel Injection Systems in Gasoline and Diesel Engines
Posted: Nov 30, 2017
The purpose of the fuel injection system is to deliver right amount of fuel for combustion process at proper timing. The way how fuel is delivered to the engine cylinders depends upon the type of the fuel injection system used in the vehicle. The injectors are fed by the fuel pumps mounted inside or outside the fuel tank. Before we go into the main topic of diesel and petrol fuel injectors, let us understand how combustion process takes place in their engines.
Combustion Process In Gasoline And Diesel EnginesSince gasoline/petrol is more volatile than diesel, it easily gets mixed with air to form a combustible mixture. The petrol and air mixture is then compressed with a piston and is ignited with a spark plug. However, the mechanism is different in diesel engines. Diesel is low volatile and cannot form desired air fuel mixture easily. First the air needs to be compressed to high temperatures and then fuel is sprayed into the cylinders by electronic fuel injection systems. Diesel engines, unlike petrol can operate at much higher compression ratios. Moreover, diesel has low self-ignition temperatures than petrol. So, the temperature produced by compressing the air to higher pressures is enough to ignite the diesel when it is sprayed into combustion chambers by fuel injectors. But the fuel air mixture in petrol engines cannot be compressed beyond certain range because it can lead to pre-ignition and engine knocking. That is why petrol engines need an external heat source like spark plug to ignite the combustion mixture. Hence depending on the system, different types of fuel injectors are used in petrol/gasoline engines and diesel engines.
Types Of Fuel Injection SystemsFuel injection system is basically of two types – indirect and direct. Earlier, it first started with indirect injection replacing the carburetors and then the design has been developed further to different types to achieve more fuel economy & engine performance. The type of injection mechanism used in petrol and diesel engines differ in design and operation because of the difference in the process of fuel combustion.
Indirect InjectionIndirect injection works on the mechanism of pumping fuel into the intake manifold of the engine where the initial combustion happens before it is introduced into the main combustion chamber. The throttle body and multi-port are common types in indirect injection system.
Throttle Body InjectionThrottle body is an air intake mechanism which regulates the amount of intake air to produce required combustion pressure in cylinders. This system is not suitable for diesel engines because diesel engines regulate the amount of fuel to produce the required amount of combustion pressure but do not change the amount of air intake. The design is similar to carburetor type but differs in that it has one or more downward facing injectors located in the central throttle body to shoot the fuel into the engine via intake manifold. Instead of allowing fuel to be drawn by the engine vacuum, the fuel is pressurized and sprayed into pre-combustion chambers through injectors.
Multi-Port Fuel InjectionThis mechanism uses separate fuel injectors for each cylinder which are fed by single or multiple fuel pumps to inject the fuel. The fuel is sprayed into fuel intake valves through intake port of the manifold. Here the injection system fires the injectors all at a time and the fuel remains in the intake until it is needed. Earlier multi-port fuel injection systems were mechanical but today they are run electronically controlled by computers. Today, more advanced electronic injectors like sequential fuel injectors came into use with even better performance and fuel economy. Like throttle body, multi-port injectors are also not suitable for compression ignition engines i.e diesel engines.
Direct Fuel InjectionIn direct injection system, the fuel is directly injected into the combustion chamber where it mixes with hot mass of compressed air forming a suitable combustion mixture for ignition. It is a normal fuel delivery mechanism for diesel engines. This system is more fuel efficient than indirect injection because no fuel is wasted passing through intake manifold or intake valves. However, the efficiency depends on the type of the fuel pump used for feeding the injectors. The pump should be capable of developing high and stable pressures as per the demands of fuel injectors.
Common Rail InjectorsIt is one of the common fuel delivery methods found in modern diesel engines in which fuel is supplied to all injectors through a common fuel rail or reservoir. The mechanism is similar to the combination of multi-port with direct injection. Here, injectors work in parallel with fuel pumps and computer control units to control the timing and amount of fuel. The fuel is stored at variable pressures in cylinders connected through a common rail, and gets directly injected into the combustion chamber controlled electronically using a programmable control module.
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