Fig. 2: Overview of resistance factors Overview of resistance factors
Fwi Total resistance
Density of atmosphere cw Aerodynamic resistance
coefficient A Front area
v Speed kr Rolling resistance
coeffcient m Mass g Gravitation constant
Gradient angle km Acceleration resistance
Source: [VW AG 2002]
Fig. 2 exemplifies the resistance factors for a passenger car. With the exception of aerodynamic resistance, all resistance factors are linear dependent on the mass of the vehicle (encircled in Fig. 2). The aerodynamic resistance, however, depends on the dimensions of the vehicle and the square of speed. Therefore, besides the mass, speed and acceleration determine the energy consumption as well. They are highly de- pendent on the driving situation and driving behaviour for the vehicles. With the same driving situation and behaviour assumed, the correlation between energy consumption and vehicle weight is linear ([EHINGER et al. 2000], [EBERLE & FRANZE 1998]) for ve- hicles with the same dimensions. Therefore, the absolute weight induced basic en- ergy savings for a 100 kg weight reduction in the same driving cycle and with the same technical specifications are independent of the vehicles’ absolute weight level.
Fast vehicles with a steady speed (e.g. high speed trains or cars on highways) will therefore have a high aerodynamic resistance and low acceleration resistance and thus low specific energy savings by weight reduction. Slow vehicles with frequent stops and accelerations (e. g. city buses or subways/ urban trains) will have a high accumulated acceleration resistance and a low aerodynamic resistance, thus high en- ergy savings by weight reduction. Rail vehicles, however, tend to have a lower share of aerodynamic resistance at the same speed than road vehicles, because of their small front compared to the length and the weight of the train.