Why cold starts could freeze air pollution improvements

The latest analysis of the 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 functional. The resultant high levels of cold start NOx emissions, from both gasoline and diesel engines, could provide an additional challenge for urban air quality initiatives such as the proposed Clean Air Zones in the UK.

According to the Department for Transport[1] more than half of car driver trips nationally are under 5 miles. In Inner London, the average journey distance by car per-person-per-day is just 1.5 miles[2]. For the majority of vehicles tested by Emissions Analytics, it can take more than five minutes for after-treatment systems to reach operating temperature.

The table below shows the uplift in NOx for cold starts tested at 1 minute and 5 minutes from key-on, compared to when fully warm, across the combined EQUA Index cycle.

Absolute NOx emissions
Ratio to official limit
Uplift on warm Absolute NOx emissions
Ratio to official limit


Uplift on warm
1 minute
0.737 9.2 32% 0.161 2.7 422%
5 minutes
0.630 7.9 13% 0.066 1.1 113%
Warm 0.559 7.0 0.031 0.5

Gasoline 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 emissions.

By looking at the average NOx emissions of 5% of the data with lowest instantaneous exhaust temperatures from each Euro 5 and 6 passenger 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.

Absolute NOx emissions
Absolute NOx emissions
Warm 0.559 0.031
Lowest 5% exhaust temperature 0.719 0.098
Uplift 29% 217%

Gasoline engines suffer proportionally much more from cooler exhausts although produce less NOx in absolute terms, whereas the diesel engines have a 29% uplift in NOx when the exhaust temperature is lower. In terms of total emissions, the average uplift is 0.160g/km for diesels and 0.067g/km for gasoline 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 are sensitive to exhaust temperature, means that other measures will be necessary if NOx emissions are to be reduced.

Encouragingly, the EU has acknowledged the importance of cold start emissions by including their measurement in the new Real Driving Emissions regulations that start in September 2017. However, there is a danger that the effects are under-measured compared to real-world journeys of short length.

Vans weigh in on the EQUA Index

Vans weigh in on the EQUA Index

Light commercial vehicles (LCVs) travelled 48.5 billion miles last year according to the Department for Transport[1]. In its tests, Emissions Analytics has seen similar levels of variability from official fuel economy and emissions figures in vans as in cars. With van mileage growing by an average of 4.1% each year this is bad news for air quality and for van driver’s wallets. Launched today, drivers can now use the EQUA Index to check the on-road performance of light commercial vehicles.

The table below shows a sample of the vans tested. All give fewer miles per gallon than advertised. The average is 17.1% below but the range is from -5.3% to -38.8%. However, the mpg gap is smaller than for passenger cars which was 29% in 2016, perhaps because light commercial vehicles are not being hyper-optimised to the NEDC.

Similarly, all of these vehicles were homologated to either the Euro 5 or Euro 6 standard and yet there are seven Euro 5s and three Euro 6 vehicles which have been rated ‘H’ on the EQUA Aq Index, meaning they emit 12 times or more the current Euro 6 limit when they are out of laboratory conditions.

The best performing diesel van is the Euro 6 VW Transporter, scoring an B-rating on-the-road, meaning it is just 1.5 times the legal limit. This is mirrored in the passenger cars tested, where only 15 of the 131 Euro 6 diesel cars tested meet the standard, of which 10 are from the Volkswagen group.

Make Model Regulatory stage Variance to official MPG EQUA Aq rating
FIAT Doblo Euro 6 -24.8% H
FORD Ranger Euro 5 -13.2% F
FORD Transit Euro 5 -14.4% H
FORD Transit Connect Euro 5 -16.7% E
FORD Transit Custom Euro 5 -22.7% H
FORD Transit Custom Euro 6 -16.1% C
ISUZU D-Max Euro 5 -10.0% F
MITSUBISHI L200 Euro 5 -20.8% H
OPEL/VAUXHALL Combo Euro 6 -7.9% H
TOYOTA Proace Euro 6 -28.2% H
VOLKSWAGEN Amarok Euro 5 -15.0% H
VOLKSWAGEN Caddy Euro 5 -5.4% D
VOLKSWAGEN Caddy Euro 6 -38.8% A*
VOLKSWAGEN California Euro 5 -5.3% H
VOLKSWAGEN Crafter Euro 5 -8.5% H
VOLKSWAGEN Transporter Euro 5 -9.6% H
VOLKSWAGEN Transporter Euro 6 -33.2% B

The effect of load

Tested on the same EQUA cycle as passenger cars, vans additionally run parts of the route ballasted to fifty per cent of their maximum payload. The effect of load on fuel economy is an average of -11.2% for a fully loaded van. A quick calculation based on average diesel price (122.12p/l) shows that for every 100 miles driven with a fully loaded van, refuelling costs on average £1.91 more than empty. Multiply this by the average yearly mileage travelled per van [1] and this is approximately £450 per year.

With around 30 to 50 vans added yearly, the EQUA Index is available for everyone to use free of charge and enables drivers and fleets to pick the most economical as well as the least polluting vehicles. Find out more by looking at the EQUA Index online.

[1] www.gov.uk

The EQUA Mpg gap increases further in 2016

This year’s round up of EQUA fuel economy data shows a five percentage point increase in the gap between official and real-world mpg, reaching 29% in 2016.

Official mpg figures averaged 60.7mpg in the year, the highest we have seen since we started recording in 2011. This is an increase of 9% on 2015’s figures, which stood at 55.7mpg; however, real-world mpg reached just 42.3mpg. On a like-for-like basis, this represents a 3% increase on 2015.

Air conditioning

With air conditioning now widely available in cars, in 2016 Emissions Analytics updated all its mpg results to include the effect of air conditioning on fuel economy. Results are now expressed to reflect manually adjusted air conditioning (not automatic climate control) switched on at 50% of maximum throughout the test. This increases fuel consumption typically by 4%.

Incorporating the use of air conditioning, the gap between official mpg figures and real-world EQUA Index Mpg has risen to 29% on average, with the largest gaps well exceeding 40%. This increases to 75% below the official figures for hybrid vehicles that have not had their battery charged and are running purely on the ICE.

Can regulatory change reduce the gap?

From 1 September this year the World harmonised Light duty vehicle Test Procedure (WLTP) will be introduced to certify the carbon dioxide and fuel economy of cars. Work started on this around 2008 and was originally intended to be a worldwide certification standard. However, with the passing of significant time and the withdrawal of North America from the process, it has become less relevant.

It will still be an improvement on the existing type approval process, which incorporates the New European Driving Cycle (NEDC), largely due to the removal or restriction of loopholes in the procedure.  However, the test cycle itself is not much more representative of real-world driving, as it remains in the laboratory with no changes in elevation and still modest acceleration rates. Our prediction, also consistent with modelling from the International Council on Clean Transportation, is that the current mpg gap of 29% and the CO2 gap between official and real-world of around 40% will approximately halve. So, a significant gap will still remain.

Crucially, it has yet to be agreed when and in what way the WLTP results will be made available to consumers. In the meantime the EQUA Index is available for anyone wishing to find out the on-road fuel economy of both Euro 5 and Euro 6 vehicles.