HYBRID drives offer considerable potential for increasing operating efficiency and reducing emissions, particularly on lines with multiple stops. In pure electric mode especially, hybrid technology makes emission-free travel possible in urban areas, underground stations and tunnels. In addition, the combination of electric drive and diesel power aids punctuality and makes it easier to recover from delays.
In essence, the core components of a hybrid drive system are a diesel engine, an electric motor, power electronics, an energy storage system, drive control equipment and, depending on the configuration, a gearbox. As with conventional drive systems, the engine converts the energy chemically bound up in the fuel into mechanical energy to drive the vehicle. The electric motor also converts energy, but can be used both as a motor and a generator. When functioning as a motor, to accelerate the vehicle for example, it converts electrical energy from the energy storage system into mechanical energy. As a generator, when the vehicle is braking, it converts mechanical energy into electrical energy and charges the energy storage system. The energy storage system usually functions on an electro-chemical basis to store electrical energy.
The power electronics components supply auxiliary electrical equipment such as air-conditioning or they serve in depot feed systems.
A modular system was developed to produce a range of hybrid drives. This allows the drive system to be tailored to suit the vehicle and it means that operating factors such as the timetable and route can be catered for.
For example, the energy storage system, which has a 31kWh battery module composed of individual Li-ion cells, can be mounted on the roof or underfloor, or if these options are not available then within the vehicle. The capacity and power of the energy storage system can then be suitably dimensioned by linking an appropriate number of battery modules.
The drive control system manages power and energy within the drive system and is designed to implement traction and braking commands with maximum energy efficiency. The drive control system decides when the diesel engine should cut in or out, when the electric motor releases power and, for example, how braking torque is distributed between the retarder and the electric motor throughout the braking process.
The drive control system has to take account of varying route profiles, operating modes and requirements. MTU says intelligent power and energy management technology is key for the successful application of hybrid drives in rail vehicles.
Integrating a hybrid drive system presents considerable challenges, particularly for the conversion of an existing vehicle. The underfloor space available is restricted to the installation compartment for a conventional power pack and vehicle approval considerations mean that major conversion work is not an option, so underfloor installation is primarily suited to new vehicles. Nevertheless, high-power-density electric motors and sophisticated integration solutions make it possible to accommodate hybrid drives beneath the floor on existing vehicles. The interfaces with the underfloor hybrid drive in the class 642 test vehicle, for example, are compatible with those of the conventional drive system. The energy storage system can be integrated at various locations in the vehicle depending on the space available. Here too, the modular design of the energy storage system pays dividends because it is much simpler to integrate several separate battery modules in the vehicle than it is to accommodate components in a single larger unit.
With the roof-mounted design, fluctuating ambient conditions are irrelevant as the energy storage system has its own cooling system, although it is also possible to integrate the energy storage system into the vehicle’s own cooling system.
Diesel-powered rail vehicles are usually either diesel-mechanical or diesel-electric, and hybrid drives are suitable for either. Incorporating a hybrid drive in a diesel-mechanical train results in a parallel hybrid, where an electric motor is located between the diesel engine and the gearbox to produce a P1 hybrid arrangement. Adding a decoupling unit between the electric motor and the diesel engine produces a P2 configuration which makes it possible to mechanically isolate the diesel engine when the vehicle is travelling in purely electric mode to avoid drag-induced power losses and thereby increase energy efficiency.
For diesel-hydraulic trains, the drive concept is very similar to that of a diesel-mechanical train except that a hydrodynamic transmission unit is employed. Repowering diesel-hydraulic vehicles therefore involves a parallel hybrid drive system.
A diesel-electric drive system essentially consists of a diesel engine, a generator, an electric power transmission system and one or more traction motors with a dedicated traction current converter and therefore includes most of the components required for a serial hybrid system, which is created by incorporating an energy storage system, power electronics components and a drive control system.
The key elements involve the electrical integration of the energy storage system with a link to the electric power transmission system and the drive control system with power management. Ideally, an electric dc power transmission facility is a prerequisite for straightforward integration of an energy storage system.
Hybrid-drive vehicles have other benefits. A hybrid drive creates a vehicle with an autonomous diesel drive and an electric drive, and therefore opens up the potential to transition to a bi-modal vehicle as this only requires the integration of a pantograph in the intermediate electric circuit of the hybrid drive.
Such a vehicle can operate on non-electrified routes in hybrid mode using the diesel engine, the electric motor and the energy storage system, as well as on electrified lines driven purely by the electric motor powered from the overhead lines.
During three years of trials, a German Rail (DB) class 642 DMU fitted with MTU’s hybrid powerpack clocked up 15,000km. The trials were conducted on a 37km line with 13 stations near Aschaffenburg operated by DB Regio subsidiary Westfrankenbahn, and on a 23km line with nine stations operated by Staudenbahn near Augsburg.
The maximum speed on the Staudenbahn line was limited to 60km/h and the route included several open level crossings as well as speed restrictions down to 10km/h. The line has a continuous gradient rising 72m from one end to the other. These restrictions as well as the vehicle’s own limitations meant that it was not possible to fully exploit the hybrid drive system’s potential for recuperation.
The train was equipped with measuring equipment to record parameters such as current, voltage, GPS signal and diesel consumption. Acceleration sensors were also fitted to calculate and monitor the forces acting on the vehicle. A virtual driver was developed to compare the various hybrid driving modes with a diesel reference model, and included a screen to allow the train driver to monitor the system. This provided a continuous display showing the optimum speed needed to remain on schedule in a particular driving mode, while relaying control commands to the train driver.
During the trials on the Staudenbahn, which took place between January and March 2015, MTU was able to verify the class 642 hybrid train under extreme weather conditions and demonstrate the system’s operational capability, functionality and reliability. Prior to the test runs, an in-house simulation was used to run vehicle simulations.
Various operating modes, including moving off under electric power with the engine shutdown, were investigated. Numerous parameters were simulated to reduce fuel consumption and CO2 emissions which generated optimum values for the test route in conjunction with a range of different operating modes.
Despite all the restrictions on the Staudenbahn route, it was possible to verify an 18% fuel saving over pure diesel operation which had been theoretically calculated during simulations. The illustration shows vehicle speed, the energy storage
The investigations and test results show that in comparison with conventional drives, the economic efficiency of the hybrid drive can be increased even further on routes with multiple stopping points and higher speeds and that further optimisation can yield a 25% fuel saving.
Noise emission measurements were conducted in line with the Noise TSI. These involved measurements made while the vehicle was stationary and while it was passing by. With the hybrid drive, stationary noise emissions were significantly reduced. As expected, it proved possible to shut down the diesel engine when the vehicle was stationary, thereby reducing the noise level by up to 21dB. Noise levels were also lower than for conventional diesel trains when the vehicle moved off as it was driven purely electrically over the first few metres until the diesel engine cut in.
The test runs confirmed that, on local journeys in particular, hybrid technology contributes to more environmentally-friendly and efficient operation compared with conventional systems. In addition, the availability of simulation tools means that optimum hybrid drive configurations can be individually calculated for each route profile and timetable so that reliable forecasts of achievable fuel savings can be formulated in advance.
During the trials, the economic efficiency of the MTU hybrid powerpack was verified under actual working conditions. Calculated over the entire life-cycle of the unit, the procurement cost of the hybrid powerpack represents only a small part. Depending on the drive type, fuel costs for the powerpack make up between 50% and 60% of its life-cycle costs. With a hybrid drive the majority of the fuel savings are achieved through recuperation and the use of the energy recovered for acceleration. Further fuel savings are realised because hybridisation means that electrified auxiliary drive units can now be powered efficiently, and the hybrid drive is particularly efficient on lines with multiple stations.
Comparisons of different concepts should always be conducted case-by-case as the influence of technical factors implies that not all configurations can be feasibly realised. For example, when repowering an existing diesel-mechanic railcar, a hybrid powerpack in a parallel hybrid configuration achieves savings of 18% compared with a pure EU Stage IIIb drive on the route shown in Figure 6. This equates to 18 litres less for every 100km.
The comparison results are even clearer where a serial hybrid powerpack replaces a conventional diesel-electric drive. Diesel-electric drives with hybrid technology that can achieve fuel savings of up to 25% are now frequently being selected for new trains, due to the fact that conventional diesel-electric drive systems are less efficient.
Using vehicle data and the proposed route profiles, MTU can simulate the life-cycle costs for specific applications with various drive systems to identify the optimum concept.
To summarise, based on the Type 6H 1800 engine, MTU’s hybrid powerpack demonstrated its reliability throughout a programme of test runs, which also confirmed forecasts developed using simulations. As a result, MTU says it can supply customers with reliable operating efficiency, fuel savings, and reduced noise and exhaust emissions forecasts, as well as customised hybrid solutions.