Impact of driving conditions and driving behaviour
Driving behaviour
Driving behaviour refers to the personal style of a driver in terms of speed, acceleration, gear selection and has a significant impact on vehicle’s emissions performance. Additionally, the decisions taken by the driver under different driving conditions occurred in real-life, also contribute to the emissions of the vehicle. Factors such as personal character, age, gender or even current psychological situation, all play role in the driver’s choices and behaviour. Some useful recommendations concerning the driving behaviour and driving conditions include:
Driving style
Making small changes to the driving style, adopting a more eco-driving style, can have a significant positive impact on the level of emissions. For example, changing gear earlier (i.e. not exploring the upper reaches of the gear range), maintaining steady vehicle speeds, smooth acceleration, deceleration and stopping, will minimise exhaust emissions, will reduce wear and tear, while maintaining good efficiency. On the contrary, steep accelerations and decelerations, high maximum vehicle speeds characterize a dynamic – or even aggressive in some cases – driving and may lead to substantially increased emission rates and fuel penalties of up to 24% (Zacharof and Fontaras, 2016). In particular, the choice of the driving speed can play a significant role in the total fuel consumption and pollutant emissions. It is estimated that trips with low average speed (less than 30 km/h) produce the highest pollutant emissions, at least as high as – or even higher than – trips above 100 km/h. In most of the times, the optimal average driving speed in terms of CO2 emissions is in the range of 50-90 km/h. The following figure illustrates an example of how pollutant emission levels for NOx are affected by the average travelling speed of the vehicle.
NOx emissions of Euro 6 diesel passenger cars as a function of speed[1]
Driving mode
Some cars offer technologies of built-in driving modes that – when used – optimize vehicle’s functions providing better efficiency. For example, “eco-driving” primarily targeting fuel economy, may also reduce pollutant emissions. “Sport driving” mode provides a better performance in terms of vehicle responsiveness and power output, but it is also the least fuel efficient and may increase pollutant emissions due to late gear shifting. “Normal driving” mode is suitable for baseline operations of the car (Fontaras et al., 2017). Always choosing the appropriate driving mode for the expected driving conditions contributes to lower emissions and better fuel economy.
Unnecessary driving
It is very common nowadays that people use their cars even for short trips. Because many of these short trips take place in urban – mostly densely populated – areas this adds up to the existing environmental pressures caused by road vehicles. Extra emissions due to cold start associated with short trips are of particular concern. However, in case of no other option than driving, drivers can combine two trips into one journey or even consider the choice of car sharing. Not driving at all, if possible, is the most environmentally friendly solution, with no impact on emissions. Leaving the car at home and walking, or even cycling are much better options in that respect.
Open windows
Driving while windows are open, affect the aerodynamic resistance of the vehicle, not allowing the normal flow of air around it. This has an effect on fuel consumption and on pollutant emissions. When not necessary, closed windows are suggested for driving. The car ventilation can be used instead.
Driving conditions
Cold start
It happens when the car engine and emission control systems are below normal operating temperatures. In practice, this normally accounts for the first 0 to 5 kilometres of a trip, depending on ambient temperature (see the following figure). During the cold start phase, extra amounts of emissions – above the normal emissions levels – are observed. The cold start affects vehicle emissions mainly in the following two ways:
- Most emissions control devices – such as the SCR and the TWC – do not work efficiently at low temperatures. This results in increased pollutant emissions during the cold start period. The emission levels immediately after a cold engine is turned on may reach several times above those of a thermally stabilised engine.
- Low engine temperature makes the oil more viscous; hence more energy is required to circulate the oil. This affects fuel consumption and CO2
Cold start emissions of a typical petrol and a diesel car
In general, the impact of cold start depends on the pollutant and the vehicle type. For example, CO and NOx emissions are mostly affected during the cold start phase. To mitigate the low temperature effects of cold start, the use of parking heater in the winter could be a good solution as well as the avoidance of unnecessary short trips with a cold engine.
Idling period
When the vehicle engine is left running when the vehicle stands still, such as when stuck in traffic or parked. Extensive idling is not part of RDE legislation, although it may have a substantial impact on NOx emissions of modern diesel vehicles. This figure illustrates, the emission performance of a modern diesel passenger car during extensive idling. For a nearly two-hour trip, 15 minutes of idling, may cause almost 45% of the total NOx produced during the whole trip, given that the start-stop system is not activated by the driver. In the case that vehicle is equipped with a start-stop system, it is recommended that these systems are activated when the vehicle is stationary. Otherwise, if drivers know that they are likely to be waiting for longer than 10 seconds, but certainty for more than a minute, it is necessary to switch off the vehicle engine manually. Additionally, idling leads to increased fuel consumption and CO2 emissions. Ten seconds of idling wastes more fuel than if the vehicle were switched off and switched on again.
NOx emission performance during extensive idling for a diesel passenger car for a nearly two-hour trip
Occupancy rates
In real world emissions, the factor causing the greatest variations in vehicle mass is the number of passengers, also referred to as occupancy rate. Increased vehicle mass increases the fuel consumption of the vehicle and deteriorates emissions. However, a high occupancy rate means that the ratio of emissions per passenger-kilometre is lower. Hence, despite the fact that additional passengers may increase the emissions of that vehicle, this is actually favourable from an environmental perspective as this increase is much lower than the additional emissions produced by the passengers if they had taken their own vehicles.
Congestion
It is the result of increased traffic, which means lower speeds and longer trip durations. Extensive congestion is not covered by RDE legislation. Congestion affects fuel consumption and pollutant emissions in several ways. It alters the speed profile of the vehicles through the whole trip, resulting in limited steady speed driving. Moreover, driving in congested conditions leads to more transient engine operations such as accelerations and decelerations. It increases engine idling time, and it may have an impact on the driver’s behaviour and psychological situation causing stress and confusion or even leading to an increase in aggressive driving. Drivers in an effort to overcome congested roads and reduce driving time, often make use of traffic re-routing strategies. These systems collect real-time traffic data from vehicles via satellite systems, providing re-routing guidance when signs of congestion are observed. Often, the use of re-routing strategies may lead to mis-arrangement of the vehicle fleet, creating again increased traffic in a different area.
[1] Data taken from the COPERT model, developed by EMISIA with support from the EEA and the JRC. https://www.emisia.com/utilities/copert/