Fuel Injection System

Fuel Injection is a method or system for metering fuel into an internal combustion engine, where the fuel is burned in air to release energy in the form of energy, which is then converted into mechanical work by engine based on the gas laws. In modern automotive applications, the fuel metering task is the only one of several functions performed by an engine management system.

For the gasoline engines, carburetors were the predominant method to meter the fuel prior to the widespread use of Electronic Fuel Injection (EFI). However, a wide variety of injection schemes have existed since the earliest usage of the internal combustion engine.

One major distinction between the carburetor and fuel injection system is that the fuel injection atomizes the fuel by forcibly pumping it through a small nozzle under high pressure, whereas carburetor relies on the modest air pressure created by intake air rushing through it to add the fuel to the airstream.

Another notable difference is that a carburetor performs several important functions in one single component: it measures engine load, calculates the amount of fuel needed, and adds the required fuel to the airstream. With fuel injection system, these functions are performed by different subsystems and components. This means that each subsystem can be specialized and optimized for its particular role, which brings a number of important performance benefits compared to the compromise solution offered by carburetors.

Carburetors do have some advantages over fuel injection: lower cost and lower complexity. Prior to 1980s, nearly all automotive gasoline engines used carburetors. However, carburetors are simply not accurate enough to deliver the performance (particularly with respect to the emission) that is major concern today. Since 1990, almost all gasoline passengers car sold in developed markets like USA, Europe and Japan use Electronic Fuel Injection (EFI).

Objectives of Fuel Injection System (FI):

The functional objective of fuel injection system may vary according to the requirements. All share the central task of supplying the fuel to the combustion process, but it is a design decision how a particular system will be optimized. There are several competing objectives such as:

·        Power output,

·        Fuel efficiency,

·        Emission performance,

·        Ability to accommodate alternative fuels,

·        Durability and reliability,

·        Drive ability and smooth operation,

·        Initial cost,

·        Maintenance cost,

·        Diagnostic cost,

·        Range of environmental operation.

Certain combination of these goals are conflicting, and it is impractical for a single engine control system to fully optimize all criteria simultaneously. In practice, automotive engineers strive to best satisfy a customer’s need in a competitive manner. The modern digital EFI system is far more capable at optimizing these competing objectives than a regular carburetor.

Types of Fuel Injection System:

Fuel injection system can be generally divided into two general categories, as follows:

1.      Air injection,

2.      Air-less, or solid, or mechanical injection.

       I.            Air Injection

It was first developed by Rudlof Diesel. The arrangement of the system is shown in figure given below:

In this system, air and fuel both are both injected into the cylinder during the supply of the fuel. The required pressure of the air for injecting the fuel is about 70 bar or higher.

A fuel pump is driven by the engine itself. A cam shaft operates the fuel pump through a cam and the power required to rotate the cam shaft is taken from the main shaft of the engine (Crankshaft) with the help of gears and discharges a fixed amount of fuel into the injection valve. The injection valve is mechanically driven and high pressure air drives the fuel chars and some air in the combustion chamber. The amount of fuel delivered is under the control of oil pump suction valve, which is operated by governor.

The air pressure is raised to 70 bar by a three stage compressor providing intercooling. The compressor is also operated by the engine. The high pressure air projects the fuel into the combustion chamber and atomises it.

This type of system is rarely used nowadays in diesel engine. The advantages and disadvantages of this system is listed below:

Advantages:

·        It provides better automization and distribution of fuel,

·        As the combustion is more complete, the BMEP is higher than with other types of injection system,

·        It allows to use the inferior fuels.

 Disadvantages:

·        It requires complicated mechanism to run the compressor,

·        The weight of the engine increases,

·        Part of the power is used to drive the compressor and so the BHP of the engine is reduced.

   II.            Air less, or Solid, or Mechanical injection system

In this system, fuel is supplied at a very high pressure at about 150 bar from the fuel pump to the fuel injector from where it is injected to the combustion chamber with the help of injector. The main parts of this system are fuel pump and fuel injector. The fuel pump is operated with the help of cam which is mounted on the cam shaft. The power required to operate the cam is taken from the engine crankshaft. Depending upon the location of the fuel pumps and fuel injectors, and upon the method used to meter the fuel, solid injection may further be classified as follows:

a.      Common rail system

b.      Distributer system

c.      Unit injector

d.      Individual pump and nozzle system