Page 48 - Piston Engine-Based Power Plants

40 Piston Engine-Based Power Plants

Until the 1980s the main device for mixing fuel with air before feed-
ing it into the cylinders of the engine was called a carburettor. This
was developed and patented by Karl Benz in 1888. It is a mechanical
device that uses the Venturi effect to draw fuel into the air as shown in
Fig. 4.3. Air for the engine flows through a tube in which there is a
constriction called a Venturi. As it passes through this constriction, the
velocity of gas increases but its pressure drops. A small jet in the
side of the Venturi allows fuel to be drawn into the air flowing past it:
the fuel is sucked from this small jet as a consequence of the pressure
drop as air flows through the Venturi. The fuel is fed from a small
float chamber which operates a little like a toilet cistern. Meanwhile
there are two valves in the air-flow tube. One before the Venturi is
called a choke because closing it chokes-off the air flow. This leads to
relatively more fuel mixing with the air than if it were fully open, lead-
ing to a richer fuel air mixture. The second valve, after the Venturi, is

Air cleaner

Choke valve

Venturi
Float valve

Float arm

Float chamber
Jet

Throttle valve

Figure 4.3 Cross section of a carburettor. Source: Wikimedia.