Injector cross-section diagram

How Does A Fuel Injector Work? Petrol vs Diesel


 | 5 minutes read

The purpose of a Fuel Injector:

Basically, the purpose of a fuel injector is to spray the fuel in atomized or mist form so that it burns completely & uniformly. Fuel Injection Pump (FIP) supplies pressurized diesel fuel thru’ high-pressure lines to each injector's inlet port. However, the conventional or first-generation injectors open with hydro-mechanical pressure. Inside the conventional injector, a spring holds the needle valve in a ‘closed’ position until the pressure in the high-pressure lines reaches the specific value. Earlier generation DI & IDI diesel engines used conventional injectors, as seen in the diagram below.

Cross sectional view of conventional Diesel Injector diagram
A cross-sectional diagram of a conventional Diesel Injector

Working principle of a conventional fuel injector:

The needle valve is controlled precisely by a pressure-sensitive spring. It lifts off its seat while spraying the diesel into the cylinder in highly atomized or mist form. The moment the pressure drops, the needle valve comes back to its seat, which causes the injection to stop. The injection nozzle has extremely critical tolerances. The clearance between its moving parts is barely 0.002 mm or 2 microns.

Modern-day injector unit forces the diesel through a small opening in the nozzle of merely 0.25 mm² size. The injected fuel quantity can vary from 1 mm³ to 350 mm³. Conventional injectors open and close hydro-mechanically. They have average nozzle opening pressures ranging from 140 to 210 kg/cm2. A modern-day Bosch-unit sprays the diesel at the speed of up to 2,000 km/h. Bosch and Lucas are the world's leading manufacturers of diesel nozzles.

Working principle a Petrol Injector:

Newer generation petrol injectors are quite different in construction and size than conventional diesel injectors. The Gasoline Direct Injection (GDI) engine generates the air-fuel mixture inside the combustion chamber. The opening of the inlet valve allows only the fresh air to come in. While high pressure-injectors inject the petrol into the combustion chamber, it improves combustion chamber cooling. This enables higher engine efficiency due to higher compression, which, in turn, increases fuel efficiency and torque.

Petrol GDI-Injector (Photo courtesy: Bosch)
Petrol GDI-type (Photo courtesy: Bosch)

A high-pressure pump feeds the fuel to a high-pressure fuel-rail (aka common rail). Besides, the Bosch HDEV5 High-pressure solenoid-injector has a nominal system pressure of up to 20 MPa and a Droplet size/SMD (Sauter Mean Diameter) only 15 μm. The injectors are fitted onto the fuel-rail/common rail. Moreover, the injectors meter and atomize the fuel at high pressure and extremely rapidly. Additionally, the injectors provide an optimum mixture and spray petrol into the combustion chamber.

For more information, read about GDI.

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