Simple carburettor and its limitations

Function

The carburettor atomises petrol and mixes it with air in the correct ratio (about 15:1 for cruising) for the SI engine under all running conditions.

Construction

        air in
          |
     [ choke ]
          |
     \         /
      \ venturi/<-- fuel nozzle <-- jet <-- float chamber
       \  __  /                         (float + needle)
        |    |
     [throttle]
          |
     to inlet manifold

Main parts:

• Float chamber with float and needle valve: keeps fuel at a constant level slightly below the nozzle tip.
• Metering jet and discharge nozzle projecting into the venturi throat.
• Venturi (choke tube): narrowing that raises the air velocity and creates a depression at the throat.
• Throttle valve: butterfly valve after the venturi controlling the mixture flow.
• Choke valve before the venturi for cold starting.

Working

During suction the piston draws air through the venturi. At the throat the velocity is high and the pressure falls below atmospheric pressure. The float chamber is at atmospheric pressure, so the pressure difference forces petrol through the jet and nozzle. It is broken into fine droplets and mixed with the air. The throttle controls the quantity of mixture and therefore the power.

Flow of air and fuel (neglecting the nozzle lip):

$$\dot{m}_a = C_{da} A_t \sqrt{2\rho_a \Delta p}, \qquad \dot{m}_f = C_{df} A_f \sqrt{2\rho_f \Delta p}$$

Both flows depend on $\sqrt{\Delta p}$, but the air density changes with the throat pressure while the fuel is incompressible, and the discharge coefficients change with flow. As a result, the simple carburettor supplies a mixture which gets richer as the throttle opens.

Why it fails and the remedies