Weather conditions

Ambient weather conditions are unlikely to be stable and depend on factors like the geographical location, the local weather pattern and the seasons. The factors with the largest impact on emissions are rain, snow, temperature and wind conditions.

Rain and snow

can change the characteristics of the road surface and thus affect the rolling assistance of the vehicle. Additionally, it can be that the trip conditions of a valid RDE test cannot be met when there is excessive snow. The layer of water or ice created on the road reduce the grip of the wheels. Hence, they need more energy, which results in higher fuel consumption and emissions. In the cases of all-wheel drive cars used for a more efficient grip, again they need more energy and thus fuel consumption.

Both high and low ambient temperature

can influence a vehicle’s emission performance. On the one hand, low ambient temperatures mean increased air density and results in higher aerodynamic resistance and increased fuel consumption and emissions. Battery capacity of hybrid electric vehicles is also reduced by lower temperature levels, decreasing the electric range and thus inducing emissions through the more frequent use of the engine. On the other hand, high ambient temperatures decrease the aerodynamic resistance, and have a positive impact on the tyre condition. In particular, tyre characteristics are changed (stiffness, hysteresis of the rubber, limited air pressure) that result in lower rolling resistance and better emission performance. However, high ambient temperature increases the amount of fuel vapour generated in the fuel tank, resulting in elevated evaporative VOC emissions.

Wind conditions

can also influence vehicle emissions. Variables such as vehicle speed, wind speed, direction and even the angle between the wind direction and the vehicle speed can play a significant role to the aerodynamic performance of vehicles. Ambient winds can be present naturally or created and affected by roadside objects or other vehicles. Wind conditions can therefore have an impact on the engine load and hence on emissions as explained previously.

Altitude

Operating at high altitude means lower atmospheric pressure that leads to reduced air density and lower air drag. As a result, there is a decrease of fuel consumption and improvement in the released emissions. At 1000 m above sea level, the reduction of air density can be up to 10% compared to the conditions under which the official vehicle testing takes place, that may lead to up to 2-3% reduction in fuel consumption (Fontaras et al., 2017).

Road morphology

Road morphology contains also characteristics that can have an impact on real-world emissions. Characteristics like road grade and road surface are not reflected in the current vehicle certification tests.

In comparison to a car driven on a flat road, a car driven uphill needs more energy in order to overcome gravity. On the other hand, driving downhill requires less energy and consumes less fuel. It is measured that for a given route, which contains both uphill and downhill, the additional fuel consumed when travelling uphill is not compensated by the fuel saved when travelling downhill.

Additionally, road surface can be described in terms of roughness and texture. Roughness depends on the initial construction and deteriorates with time resulting in increased fuel consumption and increased emissions. Although roughness affects vehicle’s tyre suspensions, the relationship between road surface and tyre is influenced by road texture. This relationship is called ‘’road surface friction resistance” and the lower it is, the lower the rolling resistance and noise are. This leads to improvements in the fuel consumption and emission performance.