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FUEL INJECTION SYSTEM

This part of the report contains :

• General overview of a fuel injection system

• Classical types of fuel injection systems

• Modern fuel injection systems

GENERAL OVERVIEW

The fuel system feed your engine the gasoline/diesel it needs to run.  If anyone of the parts in the system break down your engine will not run.  Let's look at the major parts of the fuel system,

Fuel tank:  Basically a holding tank for your fuel.  When you fill up at a gas station the gas travels down the filler tube and into the tank.  In the tank there is a sending unit which tells the gas gauge how much gas is in the tank.

Fuel pump:  On newer cars the fuel pump is usually installed in the fuel tank.  Older cars have the fuel pump attached to the engine or on the frame rail between the tank and the engine.  If the pump is in the tank or on the frame rail then it is electric and is run by your cars battery.  Fuel pumps mounted to the engine use the motion of the engine to pump the fuel

Fuel filter:  Clean fuel is critical to engine life and performance.  Fuel injectors have tiny openings which clog easily so filtering the fuel is the only way to prevent this.  Filters can be before or after the fuel pump, sometimes both.

Fuel injectors:  Most domestic cars after 1986 and earlier foreign cars came from the factory with fuel injection.  Instead of a carburetor to mix the fuel and air, a computer controls when the fuel injectors open to let fuel into the engine.  This has resulted in lower emissions and better fuel economy.  The fuel injector is basically a tiny electric valve which opens and closes with an electric signal.  In the picture below you can see the injectors towards the outer part of the intake.  By injecting the fuel close to the cylinder head the fuel stays atomized ( in tiny particles ) so it will burn better when ignited by the spark plug.

Carburetors:  A carburetor take the fuel and mixes it with air without computer intervention.  While simple in operation, they tend to need frequent tuning and rebuilding.  This is why most newer cars have done away with carburetors in favor of fuel injection.

CLASSICAL TYPES

Classically, the major way of fuel injection has been with the help of a carburetor for a gasoline engine and through a fuel injection nozzle in the case of a diesel engine. Both of them are described below a bit:

Carburetors:

The carburetor is a device which mixes air and fuel for an internal-combustion engine. Carburetors are still found in small engines and in older or specialized automobiles such as those designed for stock car racing. The mixture of air and fuel, after passing through the carburetor, enters the cylinder, where after the compression, it is ignited with the help of a spark plug. The timing of the spark plug is very important and is commonly done with the help of a make and break circuit, shown below:

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INJECTION NOZZLES:

The main purpose of the Fuel Injection Nozzle is to direct and atomise the metered fuel into the combustion chamber. The combustion chamber design dictates the type of nozzle, the droplet size and the spray required to acheive complete combustion within a given time and space. Some of the Nozzle types are :-

• Single Hole Nozzle Side Spray
• Single Hole Nozzle Center Spray
• Multi Hole Nozzle
• Long Stem Nozzle - Multi Hole
• Pintle Nozzle
• Delay Type Nozzle
• Pintaux Nozzle

But how the fuel comes into the fuel injection nozzle? The admission of fuel in the injection nozzle is made possible with the help of a fuel pump

Modern fuel injection systems

Nowadays the trend of fuel injection has changed. Many new systems for more effective fuel injection have been introduced. A few of them are:

• Electronic Fuel Injection

• Multiple Point Fuel Injection

Electronic Fuel Injection:

The principle of electronic fuel injection is very simple. Injectors are opened not by the pressure of the fuel in the delivery lines, but by solenoids operated by an electronic control unit. Since the fuel has no resistance to overcome, other than insignificant friction losses, the pump pressure can be set at very low values, consistent with the limits of obtaining full atomization with the type of injectors used. The amount of fuel to be injected is determined by the control unit on the basis of information fed into it about the engine's operating conditions.

Electronic fuel injector

This information will include manifold pressure, accelerator enrichment, cold-start requirements, idling conditions, outside temperature and barometric pressure. The systems work with constant pressure and with "variable timed" or "continuous flow" injection. Compared with mechanical injection systems, the electronic fuel injection has an impressive set of advantages. It has fewer moving parts, no need for ultra-precise machining standards, quieter operation, less power loss, a low electrical requirement, no need for special pump drives, no critical fuel filtration requirements, no surges or pulsations in the fuel line and finally, the clincher for many car makers, lower cost.

The Injector

A fuel injector is nothing but an electronically controlled valve. It is supplied with pressurized fuel by the fuel pump in your car, and it is capable of opening and closing many times per second. When the injector is energized, an electromagnet moves a plunger that opens the valve, allowing the pressurized fuel to squirt out through a tiny nozzle. The nozzle is designed to atomize the fuel -- to make as fine a mist as possible so that it can burn easily.

A fuel injector firing
.

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Multiple Point Fuel Injection
 

Mpfi’ stands for 'multi point (electronic) fuel injection'. This system injects fuel into individual cylinders, based on commands from the ‘on board engine management system computer’ – popularly known as the Engine Control Unit/ECU. Mpfi Systems can either be :

      a) ‘Sequential’ i.e direct injection into individual cylinders against their suction strokes,

       b) ‘Simultaneous’ i.e together for all the four or whatever the number of cylinders

       c) ‘Group’ i.e into Cylinder-Pairs.

These techniques result not only in better ‘power balance’ amongst the cylinders but also in higher output from each one of them, along with faster throttle response.

Of these variants of Mpfi, 'Sequential' is the best from the above considerations of power balance/output.

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