The Evolution and Control of NO Emissions from Road Transport in Europe - Urban Air Quality in Europe - page 36

N þ OH ! NO þ H (3)

Reaction ( 1 ) is the rate-determining step, influencing the amount of NO which
is formed, and is highly dependent on combustion temperature due to the high
activation energy of the reaction (320 kJ/mol). Increasing the temperature from
1,200 C to 2,000 C increases the rate of this reaction by a factor of 10,000 [ 11 ].
The prompt-NO mechanism forms NO earlier in the flame than the thermal
mechanism and is initiated by reaction ( 4 )[ 12 ]. Both N and HCN react rapidly
with oxidant to form NO in the flame:

CH þ N 2 ! HCN þ N (4)

Whilst NO is the dominant NO x species formed during engine combustion,
significant amounts of NO 2 can also be produced under certain conditions. The
NO 2 which is emitted directly from vehicle exhaust is commonly referred to as
“primary NO 2 ”. As will be seen later, the amount of NO 2 emitted from the tailpipe
is dependent upon the type of exhaust after-treatment used.

3.2 Emission-Control Technologies

Various engine and after-treatment technologies have been developed for
controlling emissions. These technologies, which are summarised below, are often
used in combination to ensure compliance with increasingly stringent legislation.
For this reason emission-control systems have become increasingly complex and
expensive. Whilst after-treatment technologies are generally fitted during manufac-
ture, retrofitting to older vehicles is also a common pollution-reduction strategy
[e.g. in low-emission zones (LEZs)].

3.2.1 Control of Combustion

The quantity of NO formed in a petrol engine or diesel engine depends on the
combustion parameters and, to a certain extent, can be controlled by adjustment of
the engine operation. For example, NO formation may be limited by retarding the
spark timing in petrol engines and the fuel injection in diesel engines [ 8 ].
For diesel vehicles reductions in “engine-out” NO x emissions are harder to
achieve, mainly due to the high combustion temperatures and oxygen-rich opera-
tional regime of the engine. Moreover, the effects of engine adjustments are limited
due to a trade-off between NOx and particulate matter (PM); combustion-related
measures which aim to reduce emissions of one pollutant (e.g. NO x ) lead to an
increase in emissions of the other (PM), and vice versa. Engine calibration is
usually determined by the need to balance emissions of the two pollutants and to
maximise fuel efficiency. Alternatively, emissions of either NO x or PM can be