Page 210 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

2
194 Automobile mechanical and electrical systems

Figure 2.230 Main jet in the fuel feed to the venturi

that point. Inside the fl oat chamber, atmospheric pressure is applied to the top of
the petrol held there. A vent in the top of the fl oat chamber allows a free passage
of air and atmospheric pressure.
A pressure differential exists at each end of the fuel supply tube between the
fl oat chamber and the venturi supply beak, when there is suffi cient air fl ow to
create a vacuum in the venturi. It is this pressure differential that is used to lift
petrol up to the beak. From here, it passes into the air stream through the venturi
and into the engine cylinders.
Although there is an increase in fuel delivery with an increase in air fl ow, these
do not match suffi ciently to maintain the correct air and fuel ratio over the full
operating range. Other devices are needed to adjust the metering of petrol to
the correct ratios. These are explained later in this section. The venturi can
be positioned vertically with the air fl ow being downward or upward, or it can
be positioned horizontally. This gives the expressions downdraft, updraft and

sidedraft for descriptions of carburettors.
There are six clearly identifi able engine and vehicle use conditions, known as the
Key fact
stages of carburation. These are outlined in Table 2.1 .
There are six clearly identifi able The development of twin-choke progressive carburettors was a way by which
engine and vehicle use conditions,
known as the stages of carburation. designers tackled the problems of maintaining correct mixture strengths, over
the full range of engine operating conditions. The primary venturi works at the
low throttle positions and a secondary venturi is added at the higher throttle
positions. However, carburettors have had their chance, so fuel injection it is
from now on!

2.5.2 Petrol fuel injection systems

Electronic fuel injection (EFi) systems ( Fig. 2.237 ) have been in use now for many
Key fact years, fi rst on expensive and sports vehicles and now as standard equipment on
petrol (gasoline)-engined vehicles. The tougher standards of exhaust emission
The tougher standards of exhaust
emission regulations have made regulations have made the use of microelectronic control systems for fuel
the use of microelectronic control delivery a virtual necessity. There are many different manufacturers of electronic
systems for fuel delivery a virtual fuel, so this section concentrates on fundamentals as well as looking at one of
necessity. the key OEMs, Bosch.

EFi systems are named by the position and operation of the fuel injectors. There
is a range of throttle body injection (TBI) systems. They are also known as single