Chemistry Reference
In-Depth Information
10
200
truck transport
8
160
car travel
6
120
Heavy duty trucks
4
80
Light trucks
Bus
Mopeds & motorcycles
2
40
Passenger cars
0
0
1990
1995
2000
2005
2010*
Fig. 3 NO
x
emissions by road vehicles in Europe (EU-27) versus transport demand during
the period 1990-2010. The 2010 data are still provisional [
4
,
7
]. The calculations have been
made at 5-year intervals. The changes in 1995 and 2000 are mainly due to the competing effects of
decreasing emission limits and increasing dieselisation
3NO
x
Emissions from Road Vehicles
3.1 Formation Mechanisms
Almost all road vehicles are powered by ICEs. The only notable exceptions include
vehicles powered by electricity drawn from an external grid (e.g. trolley buses),
from on-board batteries (e.g. electric cars) or, possibly in the future, from fuel cells.
In an ICE energy is derived from the burning of fuel in air, with the main
oxidation products being CO
2
and water vapour. However, some of the nitrogen
in the combustion air is also oxidised, leading mainly to the formation of NO.
NO formation is favoured by high temperatures and pressures found in the com-
bustion chamber, as well as lean (i.e. oxygen rich) fuelling conditions.
NO formation proceeds via two main mechanisms known as “thermal”
(or Zel'dovich) and “prompt” [
8
], with the former being responsible for more
than 90% of emissions [
9
]. “Fuel NO” may also be formed from nitrogen chemi-
cally bound in fuels. However, for road vehicles this is responsible for only a small
proportion of total NO due to the negligible nitrogen content of fuels.
The thermal mechanism is shown in reactions (
1
), (
2
) and (
3
)[
10
]:
N
2
þ
O
!
NO
þ
N
(1)
N
þ
O
2
!
NO
þ
O
(2)
