- Original Paper
- Open Access
Reducing DMU fuel consumption by means of hybrid energy storage
© The Author(s) 2011
- Received: 3 September 2010
- Accepted: 16 June 2011
- Published: 30 June 2011
This paper discusses a hybrid energy storage concept and its control strategy for hydro-mechanical DMUs. The hybrid energy storage consists of double layer capacitors and batteries. The new concept aims on reducing fuel consumption and avoiding unhealthy emissions during idling in station area.
Development of a hybrid propulsion concept and control strategy requires adequate methods for modeling system behavior. The simulation environment Dymola is used to develop and evaluate the control strategy and for dimensioning the system components.
Simulations indicated fuel savings of up to 13% depending on track characteristics. Measurements of a scaled storage system in a HiL-environment showed good accordance of the simulated and measured behavior of battery and double layer capacitor.
The newly developed storage system and its control strategy enable reduction of fuel consumption and unhealthy emissions in station areas.
- Diesel multiple unit
- Control strategy
- Hybrid energy storage
- Hardware in the loop
- Fuel consumption
increasing energy costs
growing importance of life cycle costs compared to investment costs
challenging emission regulations for diesel powered vehicles
competition with other modes of transport.
As part of the Next Generation Train (NGT) project presented at InnoTrans 2008 scientists of German Aerospace Center (DLR) are developing intelligent concepts to meet future demands of vehicle requirements. In Germany diesel multiple units (DMU) are widely used in regional traffic on non-electrified lines. In 2003 Deutsche Bahn operated about 1300 DMUs  and in 2004 more than 75% of diesel traction train kilometres of Deutsche Bahn were accomplished by DMUs. This is a share of 18.1% of Deutsche Bahn’s overall train kilometres in passenger transport . DMUs are essential to provide passenger service in regional traffic on lines with low utilization. They are either single coaches or fixed coupled coaches consisting of 2 or max. 3 power cars. Normally each power car is driven by one medium power diesel engine (250 kW–560 kW) which is often a derivate of truck or industrial engines. Basically three variants of propulsion systems are used in DMUs: diesel-electric, diesel-hydraulic and diesel-hydro-mechanical. In terms of efficiency the hydro-mechanical propulsion system is superior to the other ones, reaching a maximum efficiency of 95% compared to 85% for electric and hydraulic power trains [3, 4].
The diesel engine also powers auxiliary systems such as air compressors, engine cooling, air conditioning and electric onboard system. In literatures [4–7] the share of energy used for the auxiliary systems ranges between 5% and more than 20% of total fuel consumption depending on source of information and use case. Typical engine load profiles of DMUs point out that the engine is operated at low load over a significant amount of time, for example during station stops when the engine is only powering auxiliary loads. According to  a typical DMU in Germany is stopped for about 30% of time. Measurements performed by the Danish public transport provider DSB show that the maximum engine power is requested only for about 21% of time while for more than 41% of time the engine is idling with low or no load . As a result of low load the overall efficiency of a diesel engine is far away from its optimum resulting in increased fuel consumption as well as noise and pollutant emissions during station stops.
Improve fuel economy by recuperation of braking energy.
Increase overall engine efficiency by avoiding low load engine operation.
Avoid noise and emissions within stations by switching off diesel engines and powering auxiliary systems with stored energy.
Provide additional traction power while accelerating (boost-effect).
This paper discusses a new hybrid propulsion concept for hydro-mechanical DMUs. For the hybrid propulsion system an energy storage system consisting of double layer capacitors and batteries is developed. It is modelled as part of the DMU propulsion system to compare performance and fuel consumption of the hybrid against a conventional DMU. In the second part of the paper the integration of the simulation model in hardware in the loop environment is described. The DMU model is used to simulate train environment and drive train while a scaled energy storage including converter and auxiliary load is built up in hardware on a test bench. Through this approach both the thermal and electrical behaviour of this energy storage system in the given application are evaluated.
2.1 Energy storage system
discharge power for auxiliaries in stand still 35 kW, during acceleration 55 kW
traction support power ~ 100 kW, charge power in braking phase 150 kW
required energy during stand still 2.9 kWh (35 kW for 5 min)
high operational availability, robustness, redundancy, lifetime
additional mass of the energy storage components is limited to 1 t per power pack
For the given application double layer capacitors and batteries are taken into account. Flywheel storage systems are not considered because they are still under development, mechanically complex and some safety issues are still open . The main advantages of double layer capacitors are high power density and a great number of load cycles. The disadvantages are limited energy density and therefore high costs for high energy applications. Batteries on the other hand can store more energy but are more expensive concerning power. The main disadvantage of batteries is lifetime in terms of duty cycles. While double layer capacitors provide up to 1 million duty cycles batteries only last for up to 5000 duty cycles when the full capacity is used [20, 21]. Therefore only a limited range of 10 to 20% of the available energy is used during charge and discharge cycles.
When the DMU is stopped, discharge power of the storage is low but the amount of energy required sums up over time. This discharge profile would perfectly fit to battery storage.
During braking energy is available in a short period of time requiring high power capability. Double layer capacitors would fit very well to this load profile.
In the hybrid DMU with HES the battery is used to power the auxiliaries in stand still and during acceleration. The power demanded from the battery is limited by the maximum power of the auxiliaries keeping mass and costs of the battery system low. Because of the low charge power it is not possible to store the energy requested for long station stops during the braking phase. Therefore the battery is also charged during cruising and coasting phase. Due to safety reasons a Lithium-Iron-Phosphate battery system is chosen. Compared to other Lithium battery systems this material combination provides very good thermal stability and high performance in terms of power and lifetime at relatively low costs . The double layer capacitors are mainly used to recuperate braking energy with high power and to support traction.
2.2 Hybrid energy storage topology
Basically three topologies are possible for the hybrid storage. Each concept has its advantages and drawbacks which are discussed in the following.
For the hybrid DMU concept the third topology (active HES with variable terminal voltage) is chosen because it offers the most advantages in the given application.
Vehicle and propulsion systems parameters of conventional and hybrid DMU
Davis coefficient A
Davis coefficient B
Davis coefficient C
Max. acceleration/avg. deceleration
1.0 m/s²/0.6 m/s²
Rated power/rated torque
560 kW (2000 rpm)/3200 Nm (1200 rpm)
Fuel consumption (best point)
Gear ratio range/axle gear ratio
Electric drive parameters
Generator power elec./max. torque
60 kW/770 Nm
200 kW/2000 Nm
Motor power elec./max. torque
90 kW/1200 Nm
3.1 Auxiliary load profile and driving cycles
Simulated drive cycles and fuel consumption compared to conventional DMU
Maximum Velocity [km/h]
Station Distance [km]
Coasting Position [m]
Average Station Stop Time [s]
Fuel consumption [%]
3.2 Simulation results
Mainly two factors contribute to fuel savings and CO2-reduction of the hybrid DMU. The first contributor is the additional power of the electric drive which leads to increased tractive effort. Therefore the hybrid DMU accelerates faster which results in longer coast phases where the diesel engine is switched off. This effect is apparent when comparing simulation setups 1, 2 and 3 or 6, 7 and 8. The sooner the hybrid DMU starts coasting, the lower is the resulting fuel consumption compared to the conventional DMU.
The second contributor to fuel savings is the possibility to switch off the diesel engine during station stops. Inside stations the diesel engine is operated at low load because the power demanded by the auxiliaries is small. This operation regime results in low engine efficiency and therefore high specific fuel consumption. When comparing the simulation setups with identical maximum velocity and coasting position but different station stop times (i.e. setup 1 and 4 or 6 and 9) the velocity profile and therefore fuel consumption during driving is the same, but the fuel savings are higher when station stop time is increased.
The comparison of the different simulation setups with a station distance of 7.5 km indicates, that the fuel consumption reduction achieved with the hybrid system increases with decreasing velocity. In these driving cycles the average fuel consumption simulated with a maximum speed of 140 km/h is 91.9% compared to 89.8% with a maximum speed of 120 km/h. Another important result is the correlation of fuel consumption reduction and station distance. Simulation setups 11, 12 and 13 refer to a station distance of 5 km. All simulated cases with the shorter station distance show a significant reduction of fuel consumption compared to the setups with corresponding maximum speed and 7.5 km station distance.
Comparison of real sized system and test bench model
Double layer capacitor
A step-down converter with a maximum output current of 250 A connects the different voltage levels of battery and double layer capacitor. The power limit of the step-down converter at battery voltage level is approximately 3 kW corresponding to 96 kW at 410 V for the real sized system. Taking into account the scaling factors between real system and test bench components, the K2 Energy battery  and the Maxwell double layer capacitor stack  on the test bench each behave like one pack in the real sized system. The main difference between real system and test bench system is the voltage ratio between double layer capacitor and battery. In the real sized system battery voltage is ½, whereas on the test bench the voltage is ¼ of maximum double layer capacitor voltage. Nevertheless the increased losses of the DC/DC converter due to its lower efficiency are expected to be small.
Except of electric drive, energy storages, DC/DC-converter and auxiliary load all drive train components are simulated by the Modelica hybrid DMU model introduced before. The signals needed to control the system are fed to the simulation model from measured values of the corresponding test bench components.
In general, system models must be strongly simplified to make them run on a hard real time system like dSPACE or xPC. Especially the usage of highly efficient variable step solvers like DASSL is not possible in hard real time mode. To avoid any loss of modelling precision a client/server architecture is chosen. The system simulation runs on a conventional PC and acts as a simulation server for a lean hard real time task which implements the control and safety system. The latter is implemented in MATLAB/Simulink and runs on an xPC real time system. The communication and synchronization of both systems is realized via a dedicated point-to-point Ethernet connection using the User Data Protocol (UDP).
In addition to the test bench control, all major measurands are collected by analogue signal/CAN converters and a CAN data logger manufactured by IMC. The voltage measurement of double layer capacitor and battery is done by the analogue signal/CAN converter directly. For current measurements a current transducer is used which allows a galvanic isolated and lossless measurement. For validating the thermal behaviour, resistance thermometers (PT100) are placed at the hotspot of each component. Main focus is the temperature of double layer capacitor and LiFePO battery.
The measurements show additional effects which are not modelled in the storage models used in the simulation model. For example the simulated double layer capacitor voltage remains constant when the lower voltage limit of 440 V is reached. In contrast the measurements show a rising voltage level although only a minimal current is fed to the double layer capacitor. The differences in simulated and measured voltages are due to an offset in the DC/DC converter control.
Although an active cooling of the storages was not necessary on the test bench the results of the measured driving cycle setup do not allow the evaluation of long term thermal behaviour. To clarify which kind of cooling (air cooling, forced air cooling or fluid cooling) is required for a real size hybrid energy storage additional long time measurements under defined boundary conditions have to be performed.
The presented paper describes a new concept for a hybrid hydro-mechanical DMU. This new concept allows fuel savings ranging from 6 up to more than 13% depending on operational parameters such as maximum speed and station distance. Corresponding to the fuel savings CO2-emissions are also reduced by 6 to 13%. In addition the new concept also enables switching off the diesel engine while the DMU is stopped in stations leading to reduced noise and pollutant emissions in the station area.
For the new concept a hybrid energy storage system consisting of battery and double layer capacitors was developed. Compared to battery or double layer capacitor storage systems the hybrid storage is better suited for the requirements and saves roughly 20% of weight.
A scaled model of the hybrid storage was built on a hardware-in-the-loop test bench where a simulation model of the train propulsion system controlled power flows in the hybrid storage system. The measurements of battery and double layer capacitor voltages and currents have shown good accordance to the simulated values. As a major result a tool chain was developed which allows to use the same Dymola software model for the theoretical simulations and to control hardware devices on the HiL test bench.
On the test bench temperatures of battery and double layer capacitor storage could be measured. The HiL-test bench enables validation of models but additional measurements have to be performed to evaluate thermal behaviour of the storages and to develop an appropriate cooling concept for the hybrid storage.
In the future also the simulation models of the storages have to be improved concerning modelling of thermal behaviour. Also a reliable method to detect the actual state of charge of the storage while they are in operation has to be developed.
- “Unsere Schienenfahrzeuge im Regional- und Stadtverkehr”, Publication by Deutsche Bahn AG, 12/2003, available at http://www.deutschebahn.com/site/shared/en/file__attachements/publications__broschures/regional__urban__rolling__stock.pdf, Date: 20th of December 2010
- Allianz pro Schiene (2006) Bestandszahlen der Diesel- und Elektrotriebfahrzeuge (Anteil der Dieseltraktion am Gesamtbestand 2004)Google Scholar
- Tretow H-J, Weclas M (2002) Fahrzeugantriebe—Stand der Technik und Perspektiven, Sonderdruck Schriftenreihe der Georg-Simon-Ohm-Fachhochschule Nürnberg Nr. 17, ISSN 1616–0762, Nürnberg, GermanyGoogle Scholar
- Nolte R (2003) EVENT—Evaluation of energy efficiency technologies for rolling stock and train operation of railways, Final Report, Commissioned by Union Internationale des Chemins de fer (UIC), Paris, FranceGoogle Scholar
- Oettich S (2005) Die flexible S-Bahn: Energiesparende und anschlussoptimierende Flexibilisierung der Fahrweisen und Fahrzeiten, PhD Thesis, Technische Universität DresdenGoogle Scholar
- Meyer M, Lerjen M et al (2008) Das Energiesparprogramm der SSB, SER—Schweizer Eisenbahn Revue(07/2008)Google Scholar
- Peckham C (2007) T618—Improving the efficiency of Traction Energy Use, Final Report, Rail Safety and Standards Board, London, UKGoogle Scholar
- Bartosch S (2009) Umweltfreundlich auf der Schiene—Voith Power Pack mit Eco-Komponenten, 4. ÖPNV-Innovationskongress BaWü, March 9th–11th 2009, Freiburg, GermanyGoogle Scholar
- Knörr W, Borken J (2003) Erarbeitung von Basisemissionsdaten des dieselbetriebenen Schienenverkehrs unter Einbeziehung möglicher Schadstoffminderungstechnologien, Final Report, Förderkennzeichen 299 43 111. Umweltbundesamt, GermanyGoogle Scholar
- Nick M (2002) Hybridantrieb mit Nutzung der Bremsenergie bei Dieseltriebwagen, EI—Eisenbahningenieur 53(09/2002)Google Scholar
- Dittus H (2009) Reducing Diesel Railcar CO2-Emissions by means of Electric Energy Storages, ECTRI Young Researchers Seminar 2009, June 3rd–5th 2009, Torino, ItalyGoogle Scholar
- Pehnt M (2001) Ökologische Nachhaltigkeitspotenziale von Verkehrsmitteln und Kraftstoffen, STB-Bericht Nr. 24, Deutsches Zentrum für Luft und Raumfahrt—Institut für Technische Thermodynamik, Stuttgart, GermanyGoogle Scholar
- Behmann U (2009) Rückspeisen von Bremsenergie bei der DB, eb—Elektrische Bahnen 2009(1–2)Google Scholar
- Hillmansen S, Roberts C et al (2009) DMU Hybrid Concept Evaluation—Follow on Work DfTRG/0078/2007 funded by Department for Transport. University of Birmingham, UKGoogle Scholar
- Furuta R, Kawasaki J et al (2010) Hybrid traction technologies with energy storage devices for nonelectrified railway lines. IEEJ Trans 2010(5):291–297Google Scholar
- Ogasa M (2010) Application of energy storage technologies for electric railway vehicles—examples with hybrid electric railway vehicles. IEEJ Trans 2010(5):304–311Google Scholar
- Kondo K (2010) Recent energy saving technologies on railway traction systems. IEEJ Trans 2010(5):298–303Google Scholar
- Söffker C, Tutzauer R (2007) Bahn-Antriebstechnik für sensible Streckenabschnitte und zur Energierückgewinnung, eb—Elektrische Bahnen 2007(7)Google Scholar
- Steiner M, Klohr M et al (2007) Energy storage system with ultra-caps on board of railway vehicles, EPE 2007–12th European Conference on Power Electronics and Applications, September 2nd–5th 2007, Aalborg, DenmarkGoogle Scholar
- Lunz B, Sinhuber P et al (2009) Potenziale von Energiespeichern zur Elektrifizierung des Nahverkehrs. Der Nahverkehr 2009(7–8)Google Scholar
- Sauer D (2007) Speichertechnologien für Hybrid- und Elektrofahrzeug, Internationaler ETG-Kongress 2007, October 23rd–24th 2007, Karlsruhe, GermanyGoogle Scholar
- Uhlenhut A (2009) Oberleitungsfreier Straßenbahnbetrieb mit Sitras MES und Sitras HES. Verkehr und Technik 2009(6)Google Scholar
- “LFP300HPS”, Manufacturer Datasheet, K2 Energy, Henderson, Nevada, USA, Datasheet available on request at http://www.peakbattery.com/index.html
- “BMOD0165 P048”, Manufacturer Datasheet, Maxwell Technologies, San Diego, USA http://www.maxwell.com/docs/DATASHEET_48V_SERIES_1009365.PDF, Date: 20th of December 2010
- Riegel B (2009) Energiespeicher: Schlüsseltechnologie für die Elektromobilität, Der Nahverkehr (7–8)Google Scholar
- Fritzson P (2004)) Principles of Object-oriented Modeling and Simulation with Modelica 2.1. Wiley-IEEE, PiscatawayGoogle Scholar
- Hülsebusch D, Ungethüm J et al (2009) Multidiszilinäre Simulation von Fahrzeugen. ATZ—Automobiltechnische Zeitschrift 2009(10)Google Scholar
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