Vehicle cold starts could freeze air pollution improvements
The latest analysis of Emission Analytics’ EQUA Index data
shows that the average daily distance driven in passenger cars is not
sufficient for a vehicle’s pollution control system to warm up and become fully
The resultant high levels of cold start NOx
emissions, from both petrol and diesel engines, could provide an additional
challenge for urban air quality initiatives such as the government’s proposed
Clean Air Zones in the UK, it is claimed.
According to the Department for Transport more than half of
car driver trips nationally are under five miles. In Inner London, the average
journey distance by car per-person-per-day is just 1.5 miles.
However, for the majority of vehicles tested by Emissions
Analytics, it can take more than five minutes for after-treatment systems
to reach operating temperature.
Petrol has lower NOx in absolute terms but
proportionally has much higher NOx in the first minute, but
which then falls more rapidly than for diesel cars. This is typically as
the three-way catalyst reaches effective operating temperature.
More generally, the thermal management of exhaust systems
for engines where the exhaust frequently cools, such as with stop-start
technology or hybridisation, is of growing importance in limiting NOx
By looking at the average NOx emissions of
5% of the data with lowest instantaneous exhaust temperatures from each
Euro5 and 6 car test - excluding data points where the engine is off)
-and comparing it to the average NOx emissions when the engine is
warm, it shows a very similar picture to the cold start data.
Petrol engines suffered proportionally much more from cooler
exhausts although produce less NOx in absolute terms, whereas the
diesel engines had a 29% uplift in NOx when the exhaust temperature
was lower. In terms of total emissions, the average uplift is
0.160g/km for diesels and 0.067g/km for petrol vehicles.
The potential introduction of Clean Air Zones in UK cities
is a cornerstone of the government’s strategy to reduce air pollution.
However, driver behaviour in cities - short trips, the use
of stop-start technology and/or choice of hybrid vehicles - when combined with
exhaust after-treatment technologies which were sensitive to exhaust
temperature, meant that other measures would be necessary if NOx emissions
were to be reduced, according to Emission Analytics.
Encouragingly, it said, the European Union, had
acknowledged the importance of cold start emissions by including their
measurement in the new Real Driving Emissions regulations that start in
September. However, Emission Analytics added, that there was a danger that the
effects were under-measured compared to real-world journeys of short length.