Europe's buses can be cleaner

Emissions from city buses, primarily in the form of nitrogen oxides (NOx), can play a crucial role as precursors to the formation of particulate matter (PM2.5) and ground-level ozone.?In other words, they impact air quality.

To address this, the European Commission recently released a proposal to set new Euro 7 pollutant emission standards for trucks and buses. The standards, which would be implemented in 2027, set lower emissions limits than the current regulation, Euro VI, while expanding the pollutants that are regulated, and the driving conditions that are evaluated.

Thanks to the Euro regulations, buses do not spew visible, dirty black smoke in the air. But that does not mean that the exhaust of modern buses is harmless. So, we at The International Council on Clean Transportation sought to answer the question.

How cleaner can city buses be under Euro 7?

For that, we partnered with VTT in Finland and tested two modern transit buses, certified to the Euro VI-D standard. Three types of on-road tests were performed on each bus. The tests covered different operating conditions that occur in real-world driving. The drive cycles are shown below.

We analyzed a wealth of data produced during testing using the moving average window (MAW) method. Under Euro VI a vehicle is deemed compliant if the emissions of at least 90% of all valid windows—that is, the 90th percentile MAW emissions (hereafter MAW-90%)—are below the limit. Windows can be invalid if the engine is too cold, or the engine torque was too low. Under Euro 7, the MAW-90% represents a stricter evaluation method since all windows are valid. Furthermore, the 100th percentile (hereafter MAW-100%), corresponding to the highest emitting window by definition, is also used to assess compliance.

The figure below shows the main results, as ?boxplots of the MAW emissions for each pollutant considered.

Both buses exhibited emissions performance in accordance with the Euro VI-D type-approval requirements. However, Bus 2 showed abnormal behavior over one in-service conformity test, leading to steep NOx emissions; the on-board diagnostic system reported no error and no DPF regeneration took place.

The proposed Euro 7 limits are expected to improve the NOx performance of transit buses substantially. The NOx emissions over the highest emitting windows were three to five times higher than the proposed Euro 7 cold limit. NOx emissions in hot operation ranged between one and four times the proposed Euro 7 hot limit.

The PN requirements under Euro 7 would demand higher filtration efficiency than what current technologies offer. The number of particles in the 10 to 23 nm size range, which would also now be regulated under Euro 7, is substantial and would increase the PN count between 60% and 100%. To meet the proposed Euro 7 requirements, particle filters must improve their filtration efficiency up to an order of magnitude.

Ammonia limits proposed under Euro 7 limits would prevent backsliding from current performance as NOx emissions are reduced. Bus 2 exhibited an emissions performance 40% below the proposed limit, and Bus 1 40% above it. The proposed Euro 7 limits would prevent an increase in ammonia emissions from current levels as NOx emissions are reduced. Under Euro 7, ammonia and NOx are expected to contribute similarly to the emissions of fixed nitrogen.

Nitrous oxide emissions contributed 7% to 9% of the greenhouse gas emissions of the buses tested. Euro 7 limits would prevent backsliding from current N2O performance as NOx emissions are reduced. The proposed limits are close to what can be achieved with current emissions control technologies. Like ammonia, the proposed limits would prevent an increase in nitrous oxide emissions from current levels as NOx emissions are reduced.

You can find the full report here and the VTT technical report here.