Chemistry Reference
In-Depth Information
Fig. 4 NO
x
emission factors for a Euro 5 diesel passenger car [
23
]
(i.e. the conversion of NO to NO
2
as exhaust gas flows through the filter). In some
systems the NO oxidation is intentional, as NO
2
has been shown to be an effective
agent for oxidising PM [
21
]. Again, the conversion results in higher primary NO
2
emissions and may be one of the reasons for the observed increase in the NO
2
/NO
x
ratio in ambient air at roadside locations in cities (e.g. [
22
]).
3.3 Factors Affecting On-Road NO
x
Emissions
The main technological factors which influence on-road exhaust emissions are the
vehicle type (e.g. passenger car, heavy goods vehicle), the fuel type (e.g. petrol,
diesel) and the vehicle technology. The latter usually refers to either a specific type
of engine or exhaust after-treatment or, more generally, compliance with a particular
emission standard. Other considerations include the age and condition of a vehicle's
engine and exhaust after-treatment system. High emission rates are often a result of
component ageing, component failure, or generally poor maintenance.
Important operational factors include vehicle weight, road gradient, vehicle load
and the use of auxiliary equipment such as air conditioning, the thermal state of the
engine and exhaust emission-control system, and the way in which a vehicle is
driven (e.g. speed, or the so-called “dynamics” of driving).
Much attention has focussed on driving behaviour, as this has a large impact on
the emission level. Whilst “calm” and steady-speed driving reduces pollutant
emissions, transient operation and frequent speed changes have the opposite effect.
Such differences in vehicle operation account for part of the (high) variability of
real-world vehicle emissions. For example, NO
x
emissions from a single vehicle
can vary by more than an order of magnitude, depending on the driver behaviour.
Figure
4
shows a typical graph of emission factors (in g/km) for a Euro 5 diesel
