Fuel injector is a mechanical device which controls the injection of fuel into the combustion chamber of engine. Modern fuel injectors controlled electronically with the help of ECU (Engine control unit). Injector responsible for the proper mixing of air and fuel and it also controls the pressure and timing of fuel injection. The requirement of injection of fuel into the engine after compression stroke is completed or just before the expansion stroke is start. We can easily understand by taking an example of diesel engine in which air is drawn in the combustion chamber in the suction stroke after that compress to a high pressure to raise the temperature the of the air at a value which is required to ignite the injected fuel into the combustion chamber. Injector controls the quantity of fuel injected into the cylinder. Following are the functions and requirements of injector:
For proper atomization of fuel before entering into the combustion chamber.
Proper measured quantity of fuel should be delivered for best economy.
Injection of fuel into the combustion chamber at particular interval of time as per requirements.
Injector controls the quantity of fuel at different speeds and at various load conditions.
For proper mixing of air and fuel before entering into the combustion chamber.
In case of multi cylinder engines for proper distribution of fuel in all the cylinders.
In automobiles various types of injectors are used having almost same construction and working because basic principle of all types of injector is same. A general type of injector is discussed below.
As discussed above injectors are used for proper supply of fuel into the combustion chamber by using some pumping force. Fuel injector is a small device but performs most important job in automobiles. It consists of a barrel in which a plunger is reciprocating by means of camshaft. Camshaft rotates with crankshaft. The plunger used has single stroke and single acting. Plunger and barrel has very low clearance. This small clearance provides proper sealing and provides high pressure. At the top of the barrel a spring loaded delivery valve is attached. Delivery valve is connected to the delivery pipe through one side and with the nozzle on the other side. A rack and pinion is used to rotate the piston which controls the quantity of fuel supply.
Barrel has two radial opposite holes one is for inlet port and other for controlling port for fuel supply. For varying the quantity of fuel in different stroke and at different speed a vertical channel attached which extending from top face of the plunger to an annular groove that is on the upper side of the plunger. Generally the top edge is machined in the form of helix. This upper end also called as control edge. During delivery of fuel cam rotates due to which piston goes up and supply high pressurized fuel forward this time spring is at its bottom position. Cam raises the plunger up during the delivery stork and plunger returns he spring back to the bottom position.
In fuel injection fuel is drawn from the fuel tank using fuel pump then pressure is checked by fuel regulators. Before entering into the combustion chamber fuel is goes through injectors. In case of multi cylinder engines fuel is divided using fuel rails in order to supply equal amount of fuel in all the cylinders of the engine. Working of injector is very simple like a reciprocating pump when the plunger is at rest position i.e. at the bottom the fuel goes through inlet port into the barrel and fills the whole empty space into the cylinder. After that when cam rotates; plunger starts moving up and close the inlet port and pushed the fuel up and injects the fuel into the combustion chamber of engine through nozzle. Nozzle injects the fuel at sufficient pressure with high velocity because at high velocity smaller droplets will forms or better atomization is done. Smaller droplets have less momentum so no chances of the penetration in the body of engine.
The injection process continues till the end of the upward stroke of the plunger and the lower end of helix uncovers the spill port. After the uncovering of spill port the pressure of the fuel in the barrel is drops down when fuel travels back to the suction chamber through the vertical slots on the plunger. At that time both spring loaded injector and spring loaded delivery vales are closed and stops injection process. This closing process is necessary to avoid further addition of fuel into the combustion chamber. This process continually repeated while working of the engine.
Types of injectors:
The main types of injectors which are used in automobiles are as follow
Automatic injectors: Now day’s automatic injectors are used worldwide in automobiles because of their fast and accurate working. Almost all the automatic injectors are same in construction these are consists of a spring loaded needle valve which is hydraulically operated by the fuel pressure. This pressure lifts the spring loaded needle; spring force close the needle when pressure drops below a certain limit or required limit.
Blast injectors: This is old type of fuel injection used in automobiles. The quantity of fuel is measured by the fuel pump or a special mechanism assembled for it after measurement fuel is pumped to the fuel valve by a camshaft which driven the fuel pump. A special type of mechanism used for opening the fuel valve which controls the timing of injection. This mechanism is run by cam shaft which open and close the fuel valves. Secondly the fuel valves is also connected with pressurized air line through compressor which supply air at high pressure generally it is up to 65 bar. When camshaft rotates the fuel valves is open and the blast air mix with fuel and atomize it after that a fine spray of fuel goes into the combustion chamber.
Mechanical injectors: This is the example of advancement in technology in the field of fuel injection system. It is almost similar to the blast injectors as above discussed but the only difference in some mechanisms which are installed in mechanically operated injectors. These are run by camshaft, gears and some special types of cams. All these combine work and controls the time of fuel injection by opening and closing of valve.
Injectors are used in almost every automobile for example in cars, bus, trucks and tractors. Injector delivers the fuel into the injection system of automobiles and it is located on the head of the engine or sometimes threaded into the combustion chamber with nozzle inside.
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