Consider a case when you are given a fuel which is kept in a vacuum environment. By providing a spark just above the fuel pan what will be the results? Will it burn? The answer remains NO. Every fuel requires a certain volume of air(oxygen) to get burned

A/F RATIO: it is the ratio of air to the solid, liquid, gaseous fuel. it determines whether the system will ignite or not.

A/F ratio for diesel= 14.5:1, this means 14.5 grams of air is required to burn 1 gram of fuel effectively

A/F ratio of petrol: 14.7:1

History of fuel injection: 

In very early times the fuel is fed to the engine by fuel dripping system. This system was very inefficient and was having higher fuel consumption. In the year 1888 carburetor was introduced by KARL BENZ ( founder of Mercedes) it was a phenomenal discovery of those times. But later it was replaced with direct injection systems.



The carburetor is an arrangement of a VENTURI TUBE. The basic working of a carburetor is based on the pressure difference principle. High pressure and low-velocity air enter from the intake port. When this air reaches at B, due to the reduction in the area the pressure decreases, and velocity decreases as stated by the flow equation. By the virtue of this reduction in pressure head the fuel from the fuel, sump starts raising to the tube because the fuel sump is under higher pressure that is atmospheric pressure. When a spray of fuel is encountered with high velocity air it gets mixed up with air and goes to the cylinder for combustion.

Piston type throttle valve carburetor:

In this type of carburetor venturi size is controlled by a piston. The piston is connecting to the throttle wire.

Piston Type Carburetor
Piston Type Carburetor

Butterfly slide-type throttle valve carburetor: 

In this type of carburetor both piston and butterfly valve are used to control the area of venturi as per the requirements (idling, medium speed, high speed).

Butterfly Type Carburetor
Butterfly Type Carburetor


Diesel engines are always equipped with direct injection fuel systems. For gasoline engines, direct injection is a new technology as it has to shift from carburetor type injections. Injection systems like Gasoline direct injection GDI are installed in a good number for petrol cars.

Port Injection System:

Port Fuel Injection

In port injection, gasoline is injected using an electronically controlled injector. Fuel comes to the injector after a series of pumps and oil filters. Fuel is injected at a pressure range of 3 to 6 bar. Gasoline is injected just above the intake valve so that the air-fuel mixture can be achieved. This system is capable of increasing the air density as gasoline when injected gets evaporated and exhibits a cooling effect. Due to port injection, valves can be cleaned off any carbon deposits.

Direct Injection Systems:

Direct Injection

In direct injection system, an electronically controlled fuel injector is placed just above the cylinder head. The fuel is injected in a fine mist of atomized fuel with a pressure range of 100 to 500 bar. The direct injection sprays fuel all over the cylinder and its evaporation causes a cooling effect and reducing the knocking of engines. Controlling the temperature range is very important for high-speed engines.

To achieve better power and better air to fuel ratios direct injection systems are the most viable option. For high-speed racing car combination of port and direct injection is used. At lower RPM port injection works and shift to direct injection in higher RPM. The major advantage of a carburetor type engine is its simple design and cheap components. Direct injection systems are very complex and costly.

Authored By:- Cdt. Hardik Vats, TMI

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