?More-electric? technology developments are not only facilitating a wide range of advanced concepts related to engines and propulsion systems but are also helping to improve safety, comfort and driveability.
The need to reduce emissions, improve fuel economy and use cleaner fuels are just few of the drivers for further development and wider adoption of ‘more-electric’ technologies, which are not only facilitating a wide range of advanced engine and propulsion system concepts but are also helping improve safety, comfort and driveability. Clearly, these are very important in an increasingly competitive market. However, many of the new and emerging applications pose huge challenges for electrical machines/actuators, in terms of their operating environment, speed and the available space envelope, whilst others are particularly challenging in terms of being highly cost-sensitive or safety-critical.
The Electrical Machines and Drives Group at the University of Sheffield is researching various ‘more-electric’ technologies in collaboration with a number of European companies. These include active vehicle suspension, automatic-manual transmission systems and electromechanical valve actuation. However, the adoption of such technologies will significantly increase the electrical load, which may soon exceed the capability of conventional claw-pole alternators. By incorporating a turbogenerator in the exhaust gas stream, though, a significant proportion of the energy, which would otherwise go waste as heat, can be recovered as electrical energy. Indeed, this has the potential to eliminate the need for an alternator altogether and the load which it imposes on the engine. Because of the high rotational speed (up to 80krpm) and temperature (~300C at machine rotor), the exhaust gas energy recovery system which is being developed employs a switched reluctance machine. Further, rather than being sized for the maximum engine exhaust gas flow rate, the turbogenerator has been optimised to operation at the highest engine residency operating point. Therefore, the system incorporates a waste-gate exhaust throttle to regulate the exhaust gas mass flow rate through the turbine, as well as to protect the system in the event of a fault situation. The available energy at the turbine is a highly non-linear function of the exhaust gas conditions, in terms of temperature and mass flow rate, the pressure ratio across the turbine and the turbine speed. Increasing the generated power, by increasing either the mass flow rate or the turbine speed, increases the exhaust back pressure on the engine. However, the energy management strategy optimises the turbine speed and throttle valve position for a given electrical power requirement so as to minimise the back pressure.
An effective means of reducing fuel consumption and emissions is by engine down-sizing. However, whilst acceptable performance can be achieved at high engine speeds, a down-sized engine has a lower torque capability at low engine speeds, which compromises the drive-away and acceleration performance. To overcome this torque deficit, an electrical machine can be incorporated alongside the engine to provide transient torque-boosting, as well as to facilitate regenerative braking. The torque-boost system which has been developed employs a permanent magnet brushless AC machine, the primary energy source being a bank of super capacitors. Because of the annular space envelope, the machine has a relatively large number of poles. Further, since the engine cranking torque is similar to the torque-boost requirement, it can also be used to crank the engine. Hence, a separate cranking motor is not required. By combining a smaller displacement IC-engine with an electrical torque-boost machine, the fuel consumption can be reduced by ~30 percent, which will lead to significant environmental benefits.
The use of high-speed flywheel technology for energy storage, and to act as a peak power buffer during acceleration/deceleration, has recently become quite topical, particularly since kinetic energy recovery will be permitted on Formula 1 racing cars from 2009. A major benefit of employing a flywheel to handle transient power demands is that the primary energy source only has to provide the average power. The energy storage capability is determined by the flywheel material properties and geometry, an annular fibre composite rim offering a significantly higher specific energy capability than a solid, high strength steel disc. An annular fibre composite rim has, therefore, been used in a demonstrator flywheel system having a stored energy capacity of 1.5MJ at 60krpm, around 300Wh being recoverable in slowing down to half speed. In the system, the inner bore of the flywheel was clad with hard and soft magnetic materials so that the fibre composite rim could also serve as the external rotor of an integral permanent magnet motor/generator, and to enable it to be supported on active/passive magnetic bearings, whose stiffness and damping could be controlled to enable the flywheel to run through various critical speeds. The whole assembly was housed in a high vacuum containment unit which incorporated appropriate safety features, with the central hub comprising the stators of the motor/generator and magnetic bearings being water-cooled.
Free-piston energy converter technology has the potential to provide a power-dense, energy efficient power source for series hybrid vehicles, which arguably offer the greatest prospects for modularity and, therefore, low cost. Freeing a piston from the crankshaft and enabling the geometric cycle of the piston movement to be fully controlled, offers unique possibilities for realising advanced combustion strategies, such as HCCI and CAI. Of the various linear machine technologies which can be used to convert the kinetic energy of a free-piston into electrical energy, a tubular permanent magnet machine is most compatible with the packaging requirements, has zero net radial force and no end-windings and is volumetrically efficient. Thus, in the 2-stroke free-piston energy converter which is under development, the piston is clad in rare-earth magnets which are magnetised in such a way that back-iron is not required. The magnets can, therefore, simply be mounted on a high strength titanium tube without compromising the thrust force capability. Further, the low moving mass is conducive to maximising the generated electrical power for a given combustion volume.
In summary, ‘more-electric’ technologies will continue to feature prominently in the quest to develop more environmentally friendly vehicles. The few examples which have been mentioned serve to highlight some of the challenges which are pushing electrical machines and actuators to the limits of their capabilities in terms of power density, operating environment and dynamic performance.
The author acknowledges the support of colleagues in the Electrical Machines and Drives Group at The University of Sheffield, as well as the numerous companies with which the Group has collaborated in developing ‘more-electric’ technologies.
