5.1.2 Specific energy savings by light-weighting
Specific energy savings by weight reduction have been determined by tests and simu- lations ([EBERLE 2000] & [WALLENTOWITZ et al. 1996]) as well as estimated in the relevant literature ([BUXMANN & GEDIGA 1998], [VW AG 2002], [REPPE et al.1998], [KIEFER et al. 1998] & [PETERSEN 2000]). [EBERLE 2000] found a wide range of es- timates stated for weight induced fuel savings from 0,15 to 1 l/ (100 km*100 kg). Most of these estimates, however, lack a scientific chargeable and practical approved basis and depend on the specific interests involved ([EBERLE & FRANZE 1998]).
The results are stated in l fuel savings per 100 km for a 100 kg weight reduction [l/ (100 km*100 kg)] mainly for the tests and simulations and in % energy savings per 10 % weight reduction [%/ 10 %] for most estimates. The two in-depth studies of [EBERLE 2000] and [WALLENTOWITZ et al.] will be discussed here in detail, while other studies are cited in a tabular overview.
Eberle ([EBERLE & FRANZE 1998], [EBERLE 1999], [EBERLE 2000]) studied the weight induced fuel savings for BMW vehicles in the NEDC with the simulation pro- gramme FALKE (FAhrLeistung, Kraftstoffverbrauch, Emission) and validated the e- sults in several tests for the BMW 528i. In order to show differences in fuel savings for different driving behaviours, an economical and a sporting driving cycle have been recorded in road tests and simulated as well. The linear correlation between fuel con- sumption and vehicle weight described in section 4 has been verified by Eberle’s tests for the BMW528i [EBERLE 2000]. The simulations also show that the fuel savings are independent from the absolute vehicle weight (Chart 7): “There couldn’t be established neither a correlation between the weight nor the power or the specific power-to-weight ra- tio and the fuel reduction coefficients of various vehicles” [EBERLE & FRANZE 1998].
Specific fuel savings have been tested and simulated with and without adjustment in the rear axle transmissions. Adjustments in the rear axle transmission maintain the performance of the original vehicle (a secondary effect of the weight reduction). While “a reduction in weight leads to an improvement of performance … an extension of the rear axle transmission ratio tends by and large to reduce the standard of performance. Adjusting the rear axle transmission ratio of a lighter vehicle in order to achieve the per- formance of the original vehicle, one can indeed achieve a further reduction of fuel con- sumption…” [EBERLE 1998].
Weight induced fuel savings with adjusted rear axle transmission are up to three times higher as for vehicles without adjustments. The savings potential is highest for the sporting driving cycle, because of the importance of acceleration and braking procedures. While the fuel savings in the sporting cycle are up to 75 % higher com- pared to the NEDC, smaller differences are found between the NEDC and the eco- nomical cycle. Fuel savings for the economical driving cycle are between 16 % lower and 5% higher than for the NEDC. Overall fuel savings with adjustments in the rear axle transmission range from 0,346 l in the economical to 0,510 l in the sporting driving cycle (Tab. 2).