The effect of diesel engines in road emissions

Recently, the diesel engine and its reputation have drop dramatically as the interest in the environment was growing. The current Spanish vice president and Ecological Transition Minister Teresa Ribera said in 2018 that it is a public health matter, and that diesel will have no future. Cities like Madrid or Barcelona, as other European Countries started to implement measures and restrictions to old vehicles to decrease the pollution levels. Scandals like the Dieselgate, where a car manufacturer had intentionally falsify the results of diesel controls, only evidence the importance of the current problems and the reliability crisis in the industry.

In this post, differences between the diesel and petrol emissions are addressed. Experts from the European Environment Agency published a method to estimate the emissions inventories of countries named the EMEP/EEA air pollutant emission inventory guidebook, that Meteosim includes in their own emissions model AEMM. A small study has been done by applying this procedure, comparing the diesel and petrol exhaust emissions for different categories of cars and light commercial vehicles. For this analysis, the best available method in the document has been chosen (the tier #3 method) to compute the emissions due to the internal combustion in the engine at their normal operational temperature.
In next section, results for 2 main pollutants are commented: particles and NOx. For each specie 2 plots are shown: One of them represents the emissions rate vs. speed, and the other the emissions rate depending on the Euro Standard (Euro 1 to Euro 6) at 50 km/h. In each plot, some information is shown:

  • Passengers cars in 4 hues of red depending on the car size: mini, small, medium and SUV.
  • Light commercial vehicles in 4 hues of blue depending on their weight: N1-I, N1-II y N1-III.
  • With dashed line and ‘P’: petrol vehicles. With solid line and ‘D’: gasoil vehicles.
  • 2 shadows of green to show reference values for diesel and petrol (current Euro 6 limits)

The current Euro Standard is split in 3 groups: Euro 6c for vehicles up to 2016, Euro 6d-temp for the 2017-2019 period and Euro 6d from 2020 on.

Primary PM2.5

One of the main problems of diesel has been the emissions of particles. They are compound of elemental carbon, organic carbon and inorganic components as metallic ashes and ions (nitrates or sulphates).
Only primary emissions are considered. It is excluded all particles generated in atmosphere because of chemical reactions between emitted species. These particles are smaller than 2.5 µg and very dangerous in consequence: their small size make them able to penetrate into the alveolus and thus the blood, transporting harmful substances into damaged organs, worsen diseases and increasing the mortality from respiratory or cardiovascular causes.

Until the Euro 5 Standard, diesel emissions appear several times higher than petrol. Currently, due to the development of technology as common-rail systems, DPFs (diesel particle filters) of the refinement of fuel (regarding sulphur content) this emissions have decreased to comparable levels and below the current standard (0.0045 g/km) for most of the range of speeds. It is important to know that these results do not consider neither secondary particles nor other origins different than the engine (as tyres, brakes, and resuspension of road dust). For this reason, it is expected higher emissions that a more complete study would show.


The main problem of diesel vehicles is, without any doubt, the emissions of NOx. This group is composed of NO and NO2 emissions. The problem is so important that, according with data from MITECO, in the urban areas normally more of the 75% of NO2 is emitted by the traffic. Moreover, the exposition of the population in cities is much higher than in other zones.
In atmosphere, NO is quickly oxidized to NO2, a toxic and irritant compound that causes irritation in eyes and the respiratory system and also chronic cerebrovascular and respiratory diseases. Furthermore, it enhances the generation of smog and ozone and contributes to the acid rain.

According to these results, it is estimated that, even in the Euro 5 Standard, the emissions of NOx would be very high for diesel engines. After that, it is shown a reduction in the emissions rates due to the development of SCR (Selective Catalytic Reduction) or EGR (Exhaust Gas Recirculation. Nevertheless, according to the chosen estimation method, diesel vehicles could release an amount of NOx rate much higher than the standard at every speed (0.080 g/km). This is not happening with petrol emissions in average (by the way, having a more restrictive limit of 0.060 g/km).
This issue explains the environmental interest in these engines, the transcendence of the unveiled scandals and the planning of new measures (or, simply banning) to control their impact in locations and periods where pollution events could happen (namely, in zones with dense traffic under very atmospheric stable conditions.