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More information on homologation tests, deviations in practice and references

The data we use to calculate a car's Ecoscore originate from the homologation test procedure that these cars have to undergo before they could be offered on the European market. This procedure involves a number of tests, such as crash tests but also emission tests. These emission tests allow to ascertain whether a vehicle complies with the currently applicable emission standard, the so-called Euro standard. This standard sets maximum values ​​for the emission of about 4 pollutants: CO, NOx, HC and particulates. The standards are becoming more stringent every few years: Euro 1 was introduced in 1992, Euro 5 came into force since 2011 and Euro 6 will follow in 2015. If a car emits less than the standard values, then it may be sold on the European market. A test cycle (New European Driving Cycle or NEDC) was conceived in order to determine these emissions. So as to be homologated, cars have to cover this entire cycle on a chassis dynamometer. During this test, the emission of the above-mentioned four pollutants is measured. This cycle is identical for every car, and as a result it must be travelled by each type of car (from a small city car to a powerful sports car). This test cycle should mimic a trip in real-life traffic. However, a major problem arises in this respect. The NEDC cycle is characterized by very slow accelerations, low dynamics, and a very small proportion of high speeds. For example, this cycle includes only a few seconds of driving 120 km/h. In reality, nobody drives like the test cycle requires and emissions will thus probably not correspond. You can compare this with the fuel consumption value that the car manufacturer specifies: in practice it is very difficult to achieve this fuel consumption, and usually you're quite significantly above because official fuel consumption is also determined by the same test cycle.

The European ARTEMIS project (Assessment and Reliability of Transport Emission Models and Inventory Systems) has resulted in the design of an alternative test cycle (Common Artemis Driving Cycles or CADC), which better approximates real-life traffic. This test cycle is characterized by much stronger accelerations, much higher driving dynamics and a realistic share of high speeds (with peaks up to 130 km/h).



Figure 1: NEDC cycle                                                        Figure 2: CADC cycle

Subsequently, for a large number of cars emissions were also measured according to the CADC, in order to determine the emissions in real traffic. For most pollutants, there is no problem at all: they stay below Euro standard limits according to this cycle as well. However, there is a problem for diesel vehicles with regard to NOx emissions. According to TU Graz, a modern Euro 5 diesel vehicle emits NOx exceeding Euro 1 standard levels (which already came into force in 1992) in real-life traffic situations (read: according CADC) (Hausberger, 2010). This is to say that according to TU Graz, a Euro 5 diesel vehicle emits 5 times more NOx in real-life traffic than he should according to the Euro 5 standard. For older diesel cars as well, the tests by TU Graz have shown that NOx emissions in real-life traffic are significantly higher than the standard requires. In the figure below you can see the NOx emissions (according to CADC) compared to the limits prescribed by the Euro standards. For petrol cars introduced since 2000, this is not an issue. Petrol cars meet the Euro standard limits quite easily in real-life traffic.


Figure 3 & 4. 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]

Moreover, a study from JRC (Weiss et al., 2011) indicates that a Euro 5 diesel car has a NOx emission approximately equalling the Euro 2 standard. This is still 3.5 times more than what is allowed under Euro 5.

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. At that moment, there was sufficient scientific evidence to assume that Euro 6 diesel cars perform worse under real traffic conditions than the Euro 6 NOx standard prescribes.See for example Figure 5, from which it is clear that only a very small percentage (the green bar) of the trips with Euro 6 diesels complies with the NOx emissions standard. The majority of the real-life tests shows NOx emissions clearly higher than or even a multitude of the NOx emissions prescribes by the emission standard: the so-called “NOx conformity factor (CF)” that respresents the ratio between the actual emissions and the emission standard value, is indeed >1 for all red bars. A disadvantage is the fact that there exists no clear link between the specific NOx aftertreatment technology used (in-cylinder control, EGR, SCR, LNT) and the performance to reduce NOx. Therefore, we are forced to apply the same correction for all Euro 6 diesels: as shown by literature, the average NOx emissions from Euro 6 diesels are in reality about 7 times higher than the Euro 6 standard, this implies an emission factor of about 0.56 g/km (Franco et al., 2014). As a result, as from March 2015 we calculate the Ecoscore using a (approximate and) fixed NOx emission 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.


Figure 5. Percentage of trips as a function of the NOx conformity factor (CF): a conformity factor >1 indicates an exceeding of the Euro 6 NOx standard (Franco et al., 2014)





Franco V., Posada Sánchez F., German J. and Mock P. (2014). Real-World Exhaust Emissions from Modern Diesel Cars – A meta-analysis of PEMS emissions data from EU (EURO 6) and US (TIER 2 BIN 5/ULEV II) diesel passenger cars. ICCT White Paper October 2014. Part 1: Aggregated Results.

Hausberger S. (2010). Fuel Consumption and Emissions of Modern Passenger Cars. TUGraz, Report Nr. I-25/10 Haus-Em 07/10/676.

Land K. (2014). Potential for reducing emissions from road traffic and improving air quality. Presentation by ACEA in European Parliament – ENVI Committee – Public Hearing on Air Quality Policy. Slide 3: The issue for Real Driving Emissions.

Ntziachristos L., Galassi M.C. (2014). Emission Factors for new and upcoming technologies in road transport. European Commission – Institute for Energy and Transport. Report EUR 26952.

Weiss M., Bonnel P., Hummel R., Provenza A. and Manfredi U. (2011). On-Road Emissions of Light-Duty Vehicles in Europe. JRC. Environmental Science and Technology. 2011, 45, 8575–8581.

Weiss M., Bonnel P., Kühlwein J., Provenza A., Lambrecht U., Alessandrini S., Carriero M., Colombo R., Forni F., Lanappe G., Le Lijour P., Manfredi U., Montigny F., Sculati M. 2012. Will Euro 6 Reduce the NOx Emissions of New Diesel Cars? - Insights from On-Road Tests with Portable Emissions Measurement Systems (PEMS). Atmospheric Environment 62, 657-665.


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