Fuel Injection System (basic)

Fuel Injection System

Introduction:

     Fuel Injection system is vital to the working and performance of CI engines. This system serves the purpose of initiating and controlling the combustion to meet the demand requirements.

     Fuel is injected into the combustion chamber towards the end of compression. It is atomized as it enters under high velocity and the droplets get vaporized to form a fuel-air mixture. Due to continued heat transfer from hot air to fuel, the fuel reaches to its self-ignition temperature to ignite spontaneously initiating combustion.

     Depending upon the demand requirements, the fuel injection system continues to deliver the fuel during the initial part of combustion.

Functional requirements of an Injection System:

     For proper engine operation and satisfactory performance, the following requirements must be met by fuel Injection system:

Accurate metering of fuel injected per cycle to meet changing demand of speed and load.

The Precise timing of fuel injection in the cycle to ensure performance, power, fuel economy, emission.

Proper control rate of injection to achieve desired heat release during combustion without knocking.

Proper automization of fuel into fine droplets.

Proper spray pattern to ensure rapid mixing of fuel and air.

Uniform distribution of fuel droplets throughout the combustion chamber.

To supply equal quantities of metered fuel to all cylinders in case of multi-cylinder engines.

No lag during beginning and end of injection to eliminate dribbling(fall slowly in drops) of fuel droplets into the cylinder.

Classification of Injection Systems:

Injection system was classified as follows:

Air Injection system.

Solid Injection system.

Air injection system:

     Fuel is forced by means of compressed air. It has good mixing with higher rep(mean effective pressure).

It requires a compressor.

It has the ability to use high viscosity fuel.

It is not much in use.

Solid Injection:

Liquid fuel is injected directly into the combustion chamber.

Solid injection system can be classified as;

Individual pump and nozzle system.

Unit Injector System.

Common Rail system.

Distributor system.

Typical comments on the quantity of fuel injected:

Fuel injector should develop a pressure higher than the highest desired pressure in the engine.

Also, make additional pressure available to impart high-velocity head and adequate automization.

Excessive automization is not always conducive to proper mixture formation.

Fuel has compact and denser core, surrounded by a cone of fuel droplets of various sizes and vaporized liquid.

Larger droplets provide higher penetration.

Smaller droplets required for quick mixing and evaporation of fuel.

Droplet size (5 microns) depends on

Decrease with an in Injection pressure and air density, Increase with an increase in fuel viscosity and size of the orifice.