3. IDLE FUEL FLOW RATE
Since the fuel consumed when the vehicle is idling is independent
of road conditions and driving behaviour, the task of finding a
representative value should be straightforward. However, even for the
same vehicle there is considerable variation in idling fuel consumption
depending on a number of vehicle related variables such as idling speed,
combustion efficiency, ignition timing and engine temperature. The
vehicle age will therefore affect its idling fuel flow rate and although
no evidence has been obtained on this effect, it is reasonable to assume
that such rates will increase with vehicle age.
The idling fuel consumed by the vehicaes in the Leeds experiment
was recorded for a total of 158 stops. As can be seen from Figure 1
this idling fuel flow rate was observed to vary considerably for stops
of short duration. Such variation can be attributable to two main
causes:
11 Measurement error associated with very short stops (i.e. the
fuel consumed during a stop of only a few seconds may not be
recorded accurately during that time period; some may be
recorded just before and/or just after the stop durationl; and
21 The fuel consumed by an engine idling for a short period will
fluctuate until steady conditions are obtained.
Table 1 shows the mean and standard derivation of fuel flow rate
for two sets of data, namely, all stops and stops of 20 seconds duration
or more. In the latter case a mean fuel flow rate of 1.3 l/hr. was
found.