Heavy diesel vehicles emit around half the nitrogen oxides (NOx) of diesel cars, per a report released by the International Council on Clean Transportation.

MODERN DIESEL CARS emit more harmful exhaust emissions of nitrogen oxide (NOx) in real-world running than do heavy diesel trucks, according to a study released by the International Council on Clean Transportation late last week.  The report comes in the wake of the #dieselgate scandal that engulfed Volkswagen last year, and revealed systematic cheating on emissions tests to meet strict Californian pollution laws. The report released by the ICCT does not discuss pollution per se, just the nitrogen oxide (NOx) emissions.

The study, by the ICCT states the “crux of the debate is that under normal operation many of these vehicles [diesel passenger cars] far exceed the limits imposed by regulation and certified by official type approval tests”. In Europe, diesel cars are under Euro VI guidelines while trucks only have to meet Euro IV guidelines. However, diesel trucks, according to the ICCT, don’t emit more NOx than their certification states because of different testing methods.

Indeed, the report reveals that trucks in Europe, where more diesel vehicles are sold than anywhere else in the world, emit around 210mg/km of NOx which is less than half of the 480-560mg/km of NOx some diesel cars have been measured as emitting under independent, real-world testing by the German government. The Euro 6 emissions guidelines call for passenger cars to emit 80mg/km of NOx.

Real-world emissions results compared with lab-based testing of 30 vehicles on-sale in Europe.
Real-world emissions results compared with lab-based testing of 30 vehicles on-sale in Europe.

In Europe, diesel trucks are measured via mobile devices while on the road which meant that there was nowhere to hide and manufacturers had to produce cleaner diesel trucks. Which they did. Not so for diesel cars which are measured in a lab (and you can read about how vehicles are tested for fuel consumption and emissions here).

But before we start jumping up and down, here’s an excerpt from the report about the difficulties faced by the diesel passenger car segment in dealing with NOx compared with trucks.

“While the chemical composition of the exhaust and the emission-control technologies are the same for light-duty and heavy-duty diesel vehicles, the technical challenges related to NOX control they present do differ in some important particulars. The relative lack of physical space in which to install emissions-control hardware is a key challenge for cars, especially small cars. In the passenger-car market diesels compete with petrol vehicles, which can control NOX emissions easily and cheaply using a three-way catalyst and do not need additional after-treatment devices. In contrast, the heavy-duty market is completely dominated by diesel; in the EU more than 99% of new heavy-duty vehicle registrations are diesels. Consequently, the incremental cost of emission controls is a far more important issue for diesel cars than trucks, and the technology required to meet Euro 6 emissions regulations imposes an appreciable cost premium on diesel cars relative to comparable gasoline vehicles.”

As we’ve written about previously, there is a new test being introduced this year in Europe called the Real Driving Emissions test protocol for light vehicles. This test will supplement the improved lab-based test also being phased in this year and will be mandated for all new vehicle registrations in September 2019. The RDE test will measure vehicle emissions during on-road driving within defined boundary conditions.

The below image shows the difference in testing methods for heavy diesel vehicles and light diesel vehicles.

Testing methods for heavy and light diesel vehicles

The following table shows the difference in conformity requirements, the most noticeable being that passenger cars are tested in a lab while heavy diesel trucks are measured via a portable emissions measurement system (PEMS).

Testing methods for heavy and light diesel vehicles

What does all this mean? Basically, that the testing of light diesel vehicles will now fall into line with that of heavy diesel vehicles with PEMS testing in the real-world to ensure NOx emissions meet mandated guidelines. However, some of the issues relating packaging of diesel passenger cars and how that affects the pollution gear fitted to reduce NOx will mean that further development is required by the car companies and could mean that the price of diesel cars increase slightly. A small price to pay for cleaner cars, though, especially when you consider the many thousands of people who die each year related to air pollution caused by diesel cars.


Plug-in hybrid Audi Q8 Concept revealed…


Why is ANCAP claiming success with 43 out of 45 cars rating 5 star?


  1. Small capacity diesels, say anything less than 2.0L, need to be progressively banned/phased out in favour of GDI. Mild hybridisation/48V architecture and PHEVs need to be promoted and incentivised. It’s about time we got serious about this chemical soup we breathe in every day.

    1. GDI ? Are you kidding ?
      Direct injection petrol engines are prone to produce way more NOx than other petrol engine types.
      In Europe is already being discussed that NOx filters will become mandatory for GDI engines.

Leave a Reply

Your email address will not be published. Required fields are marked *

Check Also