appendix a - Velocidade

12
The effect of speed on noise, vibration and emissions
from vehicles
May 1998
rich setting leading to higher levels of CO and hydrocarbon emissions and lower NOx. Fuel
injection systems have been found to be more stable
In addition to the effects described above, it is extremely unlikely that two consecutive
combustion cycles, operating with the same air-fuel ratio and the same ignition point, would
produce the same pollutants, even for a well functioning engine. This would require identical
fuel and air mixing in each cycle. It is also unlikely that the fuel mix in each cylinder would
be the same. Clearly, the problem of identifying emission factors for a vehicle is not a simple
one.
2.1.2 Catalytic converters
Catalytic converters are devices which reduce levels of pollutants through oxidation of
hydrocarbons and CO to CO2 and water, and also by reduction of NOx to N2 and O2. As
their name implies catalytic converters work by a process of catalysis which requires that
they run at a high temperature (300C). They are also sensitive to the engine mixture and if
the mixture is outside the operating limits, their effectiveness is markedly reduced.
2.1.3 Other emissions
Evaporative emissions occur when volatile hydrocarbons escape from the fuel system
through evaporation. These may occur through changes in temperature between day and
night with the expansion and contraction of residual air in the fuel tank expelling
hydrocarbon vapour into the atmosphere. Evaporative emission all occur through
evaporation of fuel from hot carburettors once an engine has been switched off. There will
be a greater evaporative loss from this source per km for short journeys than for long
journeys. Watkins (1991) noted that evaporative emissions accounted for about one third of
the annual hydrocarbons emissions in Western Europe and predicted that as emission
controls became more stringent, the proportion of hydrocarbons from this source would
increase.
As well as emissions from the combustion process, dust and dirt are generated by wear of
consumable components of the vehicles, notably the tyres and brakes. These emissions are
particulates and will be deposited in the neighbourhood. As the speed of traffic increases, the
air turbulence will spread this particulate matter further.
2.1.4 Health impacts of emissions
The health effects of emissions are beyond the scope of this document. Reynolds (1996)
provides a review of each of the main pollutants and the current established view on their
health effects. Little is really known, although the general view is that people who already
suffer from asthma and respiratory problems can be seriously affected following a pollution
event. The situation is further complicated by cocktails of pollution being particularly
damaging. Some hydrocarbons and small particulates are carcinogenic. Little is known about
particulate emission and dispersion processes although much work is ongoing in the field.