Flight Mechanic

Aircraft Mechanic School Study Supplement for Future Aviation Maintenance Technicians




You are here: Home / Powerplant / Engine Fuel and Fuel Metering Systems / Pressure Injection Carburetors – Typical Injection Carburetor
Regretfully, Flight-Mechanic will be turning out the lights after fifteen years. Google, in its infinite wisdom, has chosen to remove Flight-Mechanic from its search results (the claim is that the content on this site is spam). We appealed their decision to shut us down, to no avail.

Unfortunately, since Google has a monopoly over internet search, this means that traffic levels on the site will drop to a level that makes maintaining it uneconomic. As time progresses, we will no longer be able to maintain the server space and will remove the site from the internet. Thanks to all who have supported us over the years.

To fight back against the monopolistic practices of companies like Google, we recommend using a competing search engine such as Brave (you get the added benefit of not being spied on by Big Tech) and advocating for anti-trust legislation from your representatives.

Pressure Injection Carburetors – Typical Injection Carburetor

Filed Under: Engine Fuel and Fuel Metering Systems

Pressure injection carburetors are distinctly different from float-type carburetors as they do not incorporate a vented float chamber or suction pickup from a discharge nozzle located in the venturi tube. Instead, they provide a pressurized fuel system that is closed from the engine fuel pump to the discharge nozzle. The venturi serves only to create pressure differentials for controlling the quantity of fuel to the metering jet in proportion to airflow to the engine.

Typical Injection Carburetor

The injection carburetor is a hydromechanical device employing a closed feed system from the fuel pump to the discharge nozzle. It meters fuel through fixed jets according to the mass airflow through the throttle body and discharges it under a positive pressure.

The illustration in Figure 2-23 represents a pressure-type carburetor simplified so that only the basic parts are shown. Note the two small passages, one leading from the carburetor air inlet to the left side of the flexible diaphragm and the other from the venturi throat to the right side of the diaphragm.

Figure 2-23. Pressure-type carburetor.
Figure 2-23. Pressure-type carburetor.

When air passes through the carburetor to the engine, the pressure on the right of the diaphragm is lowered because of the drop in pressure at the venturi throat. As a result, the diaphragm moves to the right, opening the fuel valve. Pressure from the engine-driven pump then forces fuel through the open valve to the discharge nozzle, where it sprays into the airstream. The distance the fuel valve opens is determined by the difference between the two pressures acting on the diaphragm. This difference in pressure is proportional to the airflow through the carburetor. Thus, the volume of airflow determines the rate of fuel discharge.

The pressure injection carburetor is an assembly of the following units:

  1. Throttle body
  2. Automatic mixture control
  3. Regulator unit
  4. Fuel control unit (some are equipped with an adapter)

Flight Mechanic Recommends

Rod Machado's Private Pilot Handbook -Flight Literacy recommends Rod Machado's products because he takes what is normally dry and tedious and transforms it with his characteristic humor, helping to keep you engaged and to retain the information longer. (see all of Rod Machado's Products).
   
-->