Advanced Automotive Fault Diagnosis

3.4.2 Exhaust gas measurement
It has now become standard to measure four of
the main exhaust gases namely:
●
carbon monoxide (CO);
●
carbon dioxide (CO
2
);
●
hydrocarbons (HC);
●
oxygen (O
2
).
The emission test module is often self-contained
with its own display but can be linked to the main
analyser display. Often lambda value and the air
fuel ratio are displayed in addition to the four
gasses. The Greek symbol lambda (␭) is used to
represent the ideal air fuel ratio (AFR) of 14.7:1
by mass. In other words just the right amount of
air to burn up all the fuel. Typical gas, lambda
and AFR readings are given in Table 3.1 for a
closed loop lambda control system, before (or
without) and after the catalytic converter. These
are for a modern engine in excellent condition
(examples only – always check current data).
The composition of exhaust gas is now a crit-
ical measurement and hence a certain degree of
accuracy is required. To this end the infrared
measurement technique has become the most
suitable for CO, CO
2
and HC. Each individual
gas absorbs infrared radiation at a specific rate.
Oxygen is measured by electro-chemical
means in much the same way as the on vehicle
lambda sensor. CO is measured using a beam of
infrared light. A similar technique is used for the
measurement of CO
2
and HC. At present it is not
possible to measure NOx without more sophisti-
cated laboratory equipment.
Good four-gas emission analysers often have
the following features.
●
Stand alone unit not dependent on other
equipment.
●
Graph screen simultaneously displays up to
four values as graphs and the graph display
order is user selectable. Select from HC, CO,
CO
2
, O
2
and rev/min for graph display.
●
User can create personalised letterhead for
screen printouts.
●
Uses the non-dispersive infrared (NDIR)
method of detection (each individual gas
absorbs infrared light at a specific rate).
●
Display screens may be frozen or stored in
memory for future retrieval.
●
Recalibrate at the touch of a button (if calibra-
tion gas and a regulator are used).
●
Display exhaust gas concentrations in real
time numerics or create live exhaust gas data
graphs in selectable ranges.
●
Calculate and display lambda (␭) (the ideal air
fuel ratio of about 14.7:1).
●
Display engine rev/min in numeric or graph
form and display oil temperature along with
current time and date.
●
Display engine diagnostic data from a scanner.
●
Operate from mains supply or a 12 V battery.
Accurate measurement of exhaust gas is not
only required for annual tests but is essential to
ensure an engine is correctly tuned. Table 3.1 lists
typical values measured from a typical exhaust.
Note the toxic emissions are small, but none-
the-less dangerous.

tải về 7.07 Mb.

Chia sẻ với bạn bè của bạn: