Principle

Modern energy storage devices permit the storage of braking energy on-board for use in the subsequent acceleration phase. This offers the possibility of an effective brake energy recovery in diesel-electric vehicles.

Furthermore the peak demands on the diesel engines are reduced allowing for both downsizing of engine layout and better load management of diesel engines.

Energy storage in the driving cycle

Figure 1 shows the use of energy storage in a typical driving cycle consisting of the phases acceleration – constant speed – braking – standstill.

Figure 1: Driving Cycle and Storage Cycle. Line 1: Speed v, tractive effort T. Line 2: Power P, Energy E. Line 3: Voltage U, current i of a supercapacitor for energy storage

Source: Hentschel et al. 2000

During braking phases the kinetic energy of the vehicle is transformed into another form of energy (e.g. electrostatic energy in the case of a capacitor) and stored in the storage device. When the vehicle stands still the energy storage device should be fully charged to be able to deliver energy during the subsequent acceleration phase. The power supply during acceleration is supported by the stored energy. The energy management system should be designed in such a way that the external energy supply never needs to deliver the full accelerating power (important condition to downsize energy supply!). When driving at maximum speed the storage device should be completely discharged.

Table 1 shows the operating modes discussed at DB AG for trains equipped with an on-board energy storage.

Table 1: Operating modes for dieselelectric vehicles with energy storage

Source: Witthuhn, Hoerl 2001

Integration into drive system

Figure 2: Integration into drive system

Source: IZT

Choice of energy storage device

The best choice of an energy storage device heavily depends on the individual vehicle and service type. The following table shows the main characteristics to be looked at in an individual application context and the corresponding storage parameters. 

Characteristics of application context	Corresponding parameter of storage device
Braking time	Charging time / power density
Braking energy	Energy density
Drive cycles in lifetime	Product life / reliability

Source: IZT

Table 2 gives the relevant characteristics for a DMU.

Table 2: Relevant characteristics of a DMU in local service

Mass of train (brutto)	116 tons
Top speed	120 km/h
Drive power	875 kW
Maximum tractive effort	122 kN
Braking power	875 kW
Maximum braking effort	56 kN
Stored kinetic energy	18 kWh
Braking time	70 s
Drive cycles per year	5000
Drive cycles in lifetime	106

Source: Hentschel, Müller et al. 2000

The Ragone diagram plotting energy density against power density is a convenient means to compare different storage technologies and assess their suitability for different vehicles. Figure 3 shows the position of relevant storage technologies in the Ragone diagram and the corresponding charging times. They are labelled by train types with corresponding braking times. Since most diesel-electric vehicles are used in the regional or local range but hardly as LRVs, it is clear from the Ragone diagram that fly-wheels and in some cases double-layer capacitors are the first choice for energy storage.

Figure 3: Ragone diagram and charging times (corresponding to braking times of different trains)

Source: IZT, data mainly from: Hentschel, Müller et al. 2000.