By Rosie Ogden
IN response to the recent VW Group emission scandal and renewed focus on air quality, Next Green Car, the UK’s No.1 green car website, has updated its unique NGC Rating to account for known discrepancies between official and real-world CO2 and NOx emissions.
The updated methodology also powers a new Car Emissions Calculator which enables car buyers to calculate total carbon dioxide (CO2), nitrogen oxides (NOx) and particulate (PM10) emissions for any new or used UK car or van. Official or real-world driving emissions can be calculated according to distance travelled and driving style.
The need for an update to Next Green Car’s analysis is due to the known gap between official and real-world test data, a difference which appears to be increasing year on year. While ‘rolling-road’ tests were originally sufficient as a way of estimating on-road emissions, well documented practices employed by manufacturers have now undermined the validity of many of the test results.
To more accurately estimate real-world emissions, a series of correction factors is applied to the official test figures. Real-world CO2 emissions are estimated by applying ‘mpg factors’ published by the ICCT1 for petrol and diesel cars. These factors vary by year from 1.07 in 2000 to 1.39 in 2015. This means, for example, that tailpipe CO2 emissions for a petrol or diesel car registered in 2015 are typically 39 per cent higher than the official test figures suggest.
For tailpipe NOx emissions, real-world emissions are estimated by applying ‘conformity factors’ to official figures. These factors, which vary by Euro standard: Euro 2 (1.0), Euro 3 (1.5), Euro 4 (2.4), Euro 5 (3.6), Euro 6 (5.5 est.), are published by COPERT 42 and the ICCT for diesel cars.
This means for example that the NOx emissions for a Euro 6 diesel car are typically 5.5 times higher than the official test figures (assuming that ‘defeat devices’ are not used).
Incorporating the new correction factors, the NGC Rating more accurately represents the environmental impact of different fuel and vehicle types. As compared to the original score, the updated NGC Rating penalises diesel powered vehicles by around 4 points due to the higher estimates of NOx emissions. Ratings for electric vehicles remain largely unchanged.
To help car owners better understand the most important emissions, the results presented by the Emissions Calculator include CO2, NOx and PM10 for selected vehicles. The results are broken down into direct emissions (tailpipe) and indirect emissions which are generated during fuel and vehicle production.
Car buyers can calculate the level of emissions for a given journey distance and driving style.
A model-specific Emissions Calculator is also now available on all the individual model pages, the same methodology being used for all power-trains including pure electric and plug-in hybrid vehicles as well as conventional petrol and diesel cars.
Dr Ben Lane, Managing Editor of Next Green Car explains: “In part a response to the VW Group emissions scandal, we have updated the NGC Rating to account for the higher CO2 and NOx emissions associated with real-world driving. In particular, the new NGC Rating better reflects the higher tailpipe NOx emissions from Euro 4, 5 and 6 diesel vehicles.
“We have also launched a new Emissions Calculator for CO2, NOx and particulates as part of our mission to provide motorists with powerful yet easy-to-use online tools. The calculator is the latest in a set of cost, emissions and car buying tools developed by Next Green Car.”