Some Carburetor Theory – Part 1

All gasoline engines need to burn fuel in order to operate. Contrary to popular belief, liquid gasoline does not burn – only vapor burns – so the liquid must be converted to vapor before it enters the combustion chamber. Enter the carburetor. Gas-powered engines need to run with an air-to-fuel ratio somewhere between 9:1 and 16:1, depending upon temperature, speed and load. In new cars, fuel injection systems do this work but for the first 75 years (or so) of the last century, the carburetor was the device that provided fuel vapor to the cylinders.

So how does this happen?

Carburetors Type

Carburetors are either:

Fixed Venturi: the varying air velocity in the venturi alters the fuel flow; this architecture is employed in most downdraft carburetors found on American and some Japanese cars

Variable Venturi: (Constant Depression) the fuel jet opening is varied by the air flow to alter the fuel flow. This is done by a vacuum operated piston connected to a tapered needle which slides inside the fuel jet. The most common variable venturi (constant depression) type carburetor is the sidedraft SU carburetor and similar models from Hitachi, Zenith-Stromberg and other makers. The UK location of the SU and Zenith-Stromberg companies helped these carburettors rise to a position of domination in the UK car market, though such carburetors were also very widely used on Volvos and other non-UK makes. Other similar designs are used on some European and a few Japanese automobiles.

Conditions and Functions

The carburetor must under all engine operating conditions:

  • Measure the airflow of the engine
  • Deliver the correct amount of fuel to keep the fuel/air mixture in the proper range (adjusting for factors such as temperature)
  • Mix the two finely and evenly

This job would be simple if air and petrol (gasoline) were ideal fluids; in practice, however, their deviations from ideal behavior due to viscosity, fluid drag, inertia, etc. require a great deal of complexity to compensate at exceptionally high or low engine speeds. A carburetor must provide the proper fuel/air mixture across a wide range of ambient temperatures, atmospheric pressures, engine speeds and loads, and centrifugal forces:

  • Cold start
  • Hot start
  • Idling or slow-running
  • Acceleration
  • High speed / high power at full throttle
  • Cruising at part throttle (light load)
  • In addition, modern carburetors are required to do this while maintaining low rates of exhaust emissions.

To function correctly under all these conditions, most carburetors contain a complex set of mechanisms to support several different operating modes, called circuits.

Some great references can also be found in the Books Reference page.

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