We all have know that every petrol or diesel car need fuel to run the car. The quantity, quality and proper timing of the fuel injected into the engine cyclinder play a vital role into the performance of an engine.
lets understand how it affect the performance of engine.
When a cold engine is started, the fuel system delivers a very rich mixture. This has a high proportion of fuel. After the engine warms up, the fuel system lean out the mixture. Then it has a lower proportion of fuel. For acceleration and high speed the mixture is again enriched. The engine output is substantially affected if the quality of the mixture reaching the engine cylinder is irregular.
The traditional fuel injection system are slightly unable to provide acurate quantity of fuel at right timing at right cylinder. One big problem occurs with them is that when the mixture passing through the intake manifold usually contains some qualities of fuel as liquid droplets. These liquid droplets have greater inertia than the gaseous mixture and whenever there is sudden change in the direction of flow the droplets get carried away in their original flow direction and this leads to a variation in air fuel ratios of the mixture reaching a particular cylinder.
A powerful engine running under varying operating condition and having a better exhaust emissions control require a complex carburetor. As such carburetor are being replaced with fuel injection system and with this replacement, it is possible to accurately meter the quantity of the fuel supplied to each cylinder of multi cylinder engine running under varying operating condition. This result quicker warm up, better acceleration, more power and economy.
Some of the recent automotive engines are equipped with the fuel injection system, instead of a carburetor for one or more of the following reasons:-
- To improve fuel distribution in a multi cylinder engine
- To improve air capacity I.e. volumetric efficiency
- To reduce knocking and detonation
- To prevent loss of fuel during scavenging.
Fuel Injection System:
Components of injection system:
The objectives of the fuel injection system are to meter, atomize and distribute the fuel throughout the air mass in the cylinder. At the same time it must maintain the required air fuel ratio as per the load and speed demand on the engine.
To move the fuel from fuel tank to the cylinder.
To measure the supply of the fuel at the rate demanded by speed and load conditioning on the engine
To adjust the rate of the metering elements for change in load and speed of the engine.
To adjust the ratio of the fuel and air as demanded by the load and speed.
To divide the metered fuel equally among the cylinder.
To fix the start and stop of the fuel air mixing process.
The injector consists of the nozzle, nozzle valves, spring and body. The fuel is forced under pressure by the fuel injection pump. The fuel lifts the nozzle valves because of the pressure, and then the fuel is sprayed through the nozzle hole. The valve is returned to its seat by spring. Some amount of oil which is not injected, passes through the nozzle valve and reaches the tank through the leaf-off pipe.
Types of injection system
- Single point injection system
- Multipoint fuel injection system
Single point injection system
Single point injection is an electronically controlled injection system in which an electronically magnetic fuel injector injects the fuel intermittently into intake manifold at a central point ahead of the throttle valve. The various single point injection systems differ in the design of the central injection unit. All the system features an injector located above the throttle plate, they differ from multi point injection unit in that they frequently operate at low pressure. The injector is flushed continuously by the fuel flowing through it in order to inhibit the formation of the air bubble.
Multipoint fuel injection system
Multipoint injection creates the ideal precondition for satisfying the demand placed on a mixture formation system. In multipoint injection system, each cylinder is assigned a fuel injector which injects the fuel directly ahead of that cylinder intake valves. In modern multi point injection system, not only each engine cylinder assigned as electromagnetic fuel injector, but also this fuel injector is activated individually for each cylinder. In this way, both the fuel mass appropriate to each cylinder and correct start of injection are calculated by the control unit (ECU). Injection the precisely metered fuel mass directly ahead of the cylinder intake valves at the intake manifold walls with fuels, thus the advantages of multipoint injection can be fully exploited. The engine intake manifold thus carries only the combustion air and can therefore be optimally adapted to the gas dynamic requirements of the engine.
Methods of fuel ignition
There are two methods of fuel injection in compression ignition system
- Air blast injection
- Air less or solid injection
Air blast injection
This method was originally used in large stationary and marine engines. But now it is obsolete. In this method the air is first compressed to a very high pressure. A blast of this air is then injected carrying the fuel along with it into the cylinders. The rate of fuel injection is controlled by varying the pressure of the air. The high pressure air requires multi stage compressor so as to keep the air bottles charged. The fuel ignites by the high temperature of the air caused by the high compression. The compressor consumes about 10% of the power developed by the engine, decreasing the net output of the engine.
Airless or solid injector
In this method, the fuel under high pressure is directly injected into the combustion chamber. It burn due to the heat of compression of the air. This method require a fuel pump to deliver the fuel at high pressure around 300kg/cm^2. This method is used for all types of small and big diesel engines. It can be divided into two systems
- A. Individual pump system: in this system each cylinder has its own individual high pressure pump and a measuring unit.
- Common rail system: in this system the fuel is pumped by a multi cylinder pump into a common rail, the pressure in the rail is controlled by relief valve. A measured quantity of fuel is supplied to each cylinder from the common rail.
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