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Why and how do we take into account NOx emissions from diesel cars under real-life traffic conditions?

The ecoscore makes use of the emissions measured during the homologation tests of a vehicle. At the time of approval it is considered whether a vehicle or vehicle type meets all relevant technical requirements. Regarding emissions this implies to verify whether the emissions are lower than what the Euro standard of the vehicle imposes (see explanation on Euro standards). This is done in a standardized way in which each vehicle needs to cover a certain driving cycle on a dynamometer while emissions are being measured.

In practice, of course, no one travels exactly like this standard driving cycle, because each driver has its own driving style. Just look at your actual fuel consumption and compare this with what is in your vehicle manual (official consumption). However, it is true that a car with a lower official fuel consumption in reality usually consumes less than a car with a higher official consumption. If we want to compare cars with each other, we can only do it in a standardized way, regardless of which driver will ultimately drive it. Therefore, it is best to use homologation data.

Unlike other emission classes, there is a problem with NOx emissions from diesel cars. Recent tests show that in practice, any diesel car emits more NOx emissions than he should according to its Euro standard. The relationship between the Euro standard and the actual emissions is no longer existing. As a result, unlike with eg CO2, it is not the case that the more recent the emission standard, the lower the NOx emissions will be in real traffic. This is clear from the figure below (left). This problem is not existing for petrol cars (see right figure).

NOx emissions for cars according to the Euro standard limits vs real-life traffic (CADC) for diesel (left) and petrol (right) (Hausberger, 2010; Franco et al., 2014) [* Euro 6 based on PEMS measurements]

Consequently, using NOx homologation values for diesel cars will give a biased image since a recent diesel car will not be able to emit less NOx than an old diesel car. That's why we now adopt a fixed NOx emission level (NOx = 0.63 g/km, like Euro 2) in the Ecoscore calculation for all diesel cars with emission standard Euro 0-5. Indeed, in the above figure (left) we see that the measured (i.e. more realistic) NOx emissions from diesels remain roughly the same over the Euro standard. This modification has been implemented on our website since the beginning of October 2012. The result is that since then, the Ecoscore for diesel cars is significantly lower than before: from a couple of points for Euro 3 diesels to up to more than 10 points for Euro 5 diesels.

As from September 2015, every new diesel car sold must meet Euro 6. Since March 2015, a similar adjustment for actual NOx emissions was applied to Euro 6 diesels. Although Euro 6 diesels perform on average slightly better than their predecessors (Euro 0-5) in terms of actual NOx emissions, yet this is on average still much worse than what is required by the Euro 6 standard (Franco et al., 2014). Since March 2015, we therefore calculate the Ecoscore with an approximate and fixed NOx emission factor of 0.50 g/km (versus 0.63 g/km for Euro 0-5 diesels). Most probably starting from 2017, real traffic emission measurements on the moving vehicle itself (PEMS = portable emissions measurement system) will be performed for the purpose of homologation. If it appears at that time that certain Euro 6 diesels yet comply with Euro 6 according to PEMS, their Ecoscore will be revised upwards. However, currently we do not yet have official PEMS measurements.

For petrol cars on the other hand, no correction is needed and as such, the Ecoscore calculation method remains exactly as it used to be.

For more information on homologation tests, deviations in practice and references, click here

last database update: 12/05/2022 - next database update planned for: 15/07/2022
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