|It is very difficult to estimate the energy consumption of vehicles and a production system that are only in the concept stage. For aerodynamic reasons, it is however obvious that X self-propelled cars consume more energy than a loco-hauled freight train with X wagons carrying the same freight.
Study by Rauschenberg
There is a theoretical study (Rauschenberg (no year)) examining the external effects of the new system. A very rough assumption is made about running resistance of self-propelled freight cars:
- rolling resistance is assumed to be equal to conventional freight trains
- air resistance is assumed to be equal to road trucks
These assumptions, which seem reasonable for a first approach, lead to the running resistance shown in Figure 2. For an average speed of 80 km/h, this yields the comparison given in Table 1. It shows that the running resistance of self-propelled freight cars is roughly 3 times that of a freight car in a loco hauled train
Figure 2: Comparison of running resistance of conventional rail transportation, self-propelled rail vehicles and road transportation
Source: Rauschenberg 2000
Table 1: Comparison of running resistance at 80 km/h for conventional rail transportation, self-propelled rail vehicles and road transportation (figures give load-specific resistance force, i.e. resistance force per unit of weight force of load)
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Source: IZT, data from Rauschenberg 2000
Estimate using data from Vollmer 1989
Since a self-propelled freight car will aerodynamically behave similar to the first car in a train the data from Vollmer 1989 can be used to make an estimate on the additional air resistance met by self-propelled freight cars. Since the first car in a train configuration roughly causes 4 - 10 times as much air drag as the following ones, this is also a good estimate for the difference between self-propelled freight cars and loco-hauled stock. This is in reasonable accordance with the factor 10 assumed by Rauschenberg (cf. second line in Table 1)
Assuming that the (load-)specific air drag of self-propelled cars is about 4 - 10 times higher than that of loco-hauled stock, and taking into consideration that air drag accounts for about 50 % of total energy consumption in freight transport, one derives a total energy consumption increased by a factor of 2 to 5.
The additional energy demand will be reduced due to the fact that self-propelled cars travel shorter distances owing to point-to-point service and do not need additional freight handling along the route.
Energy efficiency of self-propelled could be improved by two factors:
- Running resistance can be substantially reduced by automatic coupling of self-propelled units to form long trains on major routes. This way self-propelled freight cars could exploit the aerodynamic advantages of long freight trains without sharing their drawbacks.
- The same holds for the concept of Virtually coupled trains with much higher requirements for train formation.
- If self-propelled freight cars are equipped with electric traction, they can use regenerative braking, which would cut the effect of mass acceleration. This is of course a cost issue, since electric propulsion will be more expensive than diesel propulsion.
The environmental assessment of self-propelled freight cars requires a new perspective. It is hardly reasonable to compare energy efficiency of the new train concept with conventional freight trains since self-propelled freight cars compete for a market segment presently dominated by road trucks rather than rail transport.
Therefore the concept of self-propelled freight cars can be seen as an energy-efficient option even though the comparison with conventional freight trains does not favor them.
Assuming that the system of self-propelled freight cars can be realised which uses automatic train formation on main routes, optimised point-to-point logistics and load management, the overall energy consumption could be comparable or even beat conventional railway freight systems.