The hybridisation of propulsion systems in hybrid electric vehicles (HEVs), in both railway and automotive applications, involve the implementation of the most efficient combination between power sources and energy storage devices. The choice of power sources available for HEVs is wide and varied. Salmasi (2007) suggests that, in terms of the power sources, the term HEV is commonly associated with an internal combustion engine and an electric motor that also serves the role of an electrical generator.
With regards to the energy storage devices in HEVs, the laws of Thermodynamics dictate that useful energy obtained from an energy store is always less than the quantifiable energy originally supplied into it. Further to this, as discussed by Henning, et al. (2005), technology currently does not provide an energy store boasting the best in energy density (kWh/kg) and power density (kW/kg) – in terms of per unit mass, in this context.
Hybrid power trains have been adapted in railway vehicles since they can greatly improve the energy consumption by utilising the brake energy as a result of most rail services making many stops (Dittus & Ungethüm, 2006). Railway vehicles tend to implement diesel engines as the choice of the internal combustion engine, and Henning, et al. (2005) writes that ‘These engines run at a relatively low speed (up to 2500 rpm) and are heavy (low power-density)’.
The energy storage options in railway vehicles are predominantly Electric double-layer capacitors (EDLCs) as they boast high power densities whilst being able to withstand a large number of charge/discharge cycles, and the use of flywheel technology due to their inherently high energy density (Henning, et al., 2005) (Dittus & Ungethüm, 2006). However, Fujii, et al. (2003) argue the high power density of Lithium-Ion (Li-Ion) batteries make them a highly attractive alternative and were implemented in their “New Energy” train as the energy store. Li-Ion batteries boasting 192 kW of power and a capacity of 7.5 kWh were used in the Innovative Technology Train (ITT) developed by Ihara, et al. (2008). Dittus and Ungethüm (2006) discuss that railway vehicles are typically developed with a service lifespan of up to 30 years, with the time between the two overhauls ~10 years, and therefore the extremely large number of charge/discharge cycles within this period would result in very high maintenance costs.
The means of interfacing the power sources with the energy are typically categorised as series, parallel, and series-parallel. The series-parallel and parallel configurations are inherently complicated in design, whilst yielding similar performance as the simpler series hybrid system.
The mechanical energy produced by the diesel internal combustion engine in the series hybrid system is supplied to the traction motor that drives the vehicle as electricity via an inverter. The “New Energy” train developed by Fujii, et al. (2003) utilised a series hybrid system that was able to achieve a “regeneration ratio” of ~20% between the regenerative energy and running energy. This efficiency was much higher between stations with a shorter distance while worsening on routes with inclinations and long slopes.
A unique design of hybrid propulsion is the Motor-Assisted Hybrid Traction System developed by Ihara, et al. (2008) that incorporates a diesel engine and an active-transmission that comprises of a traction electric motor, mechanical shift transmission, static converter, and controller – this system utilised in the ITT. The motor-assisted hybrid system is more efficient than the series hybrid system as it does not suffer the ~10% experienced by the diesel engine in the series hybrid system as all of its mechanical energy is converted into electricity.
The Ultra Low Emission Vehicle – Transport Advanced Propulsion (ULEV-TAP) 2 project by Henning, et al. (2005) was concentrated on the development of a series hybrid drive utilising the angular kinetic (rotational) energy stored in flywheel technology. The Prime Mover Unit (PMU), consisting of the diesel engine, generator, inverter, and the PMU controller, managed the process of generating electricity. The Prime Power Unit (PPU) comprised of the flywheel attached to a motor/generator, converter, and PPU controller. This ULEV-TAP 2 was able to achieve a reduction of over 40% in fuel consumption in comparison with that of a typical diesel electric drive train.
The algorithms implemented in the controllers of automobile HEVs plays a far more crucial role than in railway vehicles, as their paths are far less deterministic. The control strategies in automobiles could be tailored, as per Salmasi (2007), to consider vehicle dynamics and information regarding the current roadway based on Global Positioning System (GPS) fixes. The research conducted by Salmasi (2007) is focussed on the means of control strategy most suitable for operation in a parallel hybrid structure for automobile HEVs.
The viability of hydrogen fuel cells in automobile HEVs was the research focus by Schofield, et al. (2005), where two high peak power ZEBRA batteries and a 6kW hydrogen Proton Exchange Membrane Fuel Cell (PEMFC) were the energy sources for driving a zero emission London Taxi. The viability of fuel cells in automobiles is questioned due to their extremely poor regulation with load and inability to benefit from braking energies. Restricting the physical dimensions, namely mass and volume, of the fuel cell within reasonable limits as determined by the power requirements of the vehicles drive train thereby imposes the buffering of the dynamic peak power requirements of the vehicle by a secondary energy source. The implemented energy management system was therefore required to ensure all dynamic demands of power were taken from an on-board battery.
While the utilisation of flywheel technology was easily achieved in railway applications, their implementation in automobiles is much more complicated as they tend to operate as gyroscopes with an angular momentum acting inline with the forces acting on the automobile in question and typically of a similar order of magnitude. The solution to this problem requires the mounting of the flywheel within a number of gimbals so as to decouple the vehicles dynamic translation from that imposed by the angular momentum of the flywheel.