Vehicle specifications
Certain vehicle specifications have a significant impact on vehicle emissions as well as on fuel economy. The most important ones are:
Engine power and torque - or else horsepower that describes the speed at which the energy is converted or in other words it is the rate of doing work. Torque describes how much work an engine can do. These two definitions are related i.e. the engine power is calculated as the multiplication of torque and engine speed (RPM). The higher the torque, the higher the vehicle engine power and as a result more fuel is consumed. Under typical driving conditions it is not beneficial to use high horsepower. Thanks to significant advancements in engine technology and efficiency, vehicle emissions and fuel consumption were substantially reduced while the vehicles have become more powerful.
Vehicle size, aerodynamics, and weight - are strongly correlated with the energy consumption of vehicles. The heavier, larger, and less aerodynamic a vehicle is, the higher its rolling and air resistance. The frontal area of the vehicle, that is the area where a moving car hits the air, is a significant parameter in terms of vehicle size (see also Figure 10). The bigger the frontal area of the vehicle, the higher the aerodynamic resistance. In addition to the frontal area, the overall design and shape of the vehicle also affects the aerodynamic resistance. Various equipment fixed to cars, such as roof racks, increase the frontal area, reduce the aerodynamic coefficient, and add weight. As a result, it requires more energy and greater amounts of emissions are released. Therefore, reducing vehicle size and weight is an effective way to reduce vehicle’s emissions (Mock, 2017).
Frontal area of vehicles and typical values of passenger car and truck. Source: (Société de Technologie Michelin, 2003)
Tyres – Rolling resistance accounts for 20-30% of a vehicle’s fuel consumption. A fuel-efficient tyre contributes to the reduction of fuel consumption and emissions. Tyres come in many shapes and sizes. The exact configuration in terms of dimensions, including diameter, height, width, determine the rolling resistance of the tyre. The current CO2 legislation requires the introduction of low rolling resistance tyres on new vehicles. The European Commission has introduced a tyre labelling scheme to inform consumers about the fuel efficiency, the wet grip and the external noise level of each tyre (Wilson and Sajn, 2018). It is advised to select the tyre with the lowest rolling resistance, without compromising safety and low noise levels.
Utility Factor – is an indication of the share of kilometres driven on electric motor versus kilometres driven on combustion engine. It depends on the battery size, which determines the electric range of the vehicle, and the battery charging frequency and pattern (Wu et al., 2015). Recent data has revealed that it is only around 37% for real-world driving (ICCT, 2020). Since the battery capacity of a PHEV is fixed, it is the driver’s responsibility to adopt a good recharging habit so that the vehicle operates on electricity as much as possible. Such a responsible consumer behaviour will maximise the environmental benefits from PHEVs and will also reduce usage costs.