Storing energy in motorised road vehicles has exercised the minds of engineers since the dawn of such transport. Petrol or diesel fuel, with its high energy density and speed of replenishment, has been the most successful until this century, when environmental considerations have brought batteries or compressed hydrogen into the mix. But another form of stored energy has been used successfully in some situations: the flywheel.
An arrangement for using the energy of a flywheel, charged at stops by a steam engine or electric motor, was patented by Lanchester in 1905. Motor-generator sets incorporating flywheels were used in Southern Railways electric locomotives to move them when they stopped between sections of the third rail. But in 1945 the Oerlikon Engineering Company of Zurich investigated the possibility of a novel type that would offer the many advantages of electric traction but would be capable of operation without continuous overhead supply. The story of the development of this idea into a working system, known as the Electrogyro, is told here.
Problems to be overcome
Preliminary calculations indicated that the system would be feasible, but the main problems were the windage losses of the flywheel (frictional losses against the surrounding air), the effects of the gyroscopic action of the flywheel upon the riding characteristics of the vehicle, and the provision of cheap, simple and reliable electrical transmission between the flywheel and the road wheels.
To store as much energy as possible, most of the weight of the flywheel needs to be in the rim. At the same time, however, stress must be uniform throughout the flywheel disc, to avoid disastrous collapse of the flywheel.
The flywheel has to rotate in a housing – with the inevitable consequence of energy loss through friction or windage. This energy is converted into heat, which can considerably increase the temperature of the surrounding fluid. The actual friction of the fluid on the disc creates relatively little heating; however, the main loss is through a pumping action, as can be seen in Fig. 1: the fluid in contact with the disc is thrown outwards by centrifugal force and circulates back towards the shaft.
Fig. 1 – How pumping action of a flywheel causes windage losses
The solution in the Electrogyro was to minimise the gap between the flywheel and its housing and to replace air with hydrogen, which has a much lower specific weight. Hydrogen gave another advantage: it increased the life of the windings of the electric motor used to spin up the flywheel, by reducing the oxidation of the cellulose-based insulation. Further reduction in energy loss was achieved by using hydrogen at a pressure below atmospheric. This could be achieved since all the rotating parts were enclosed in a common housing. Finally, hydrogen gave more effective heat dispersal, since its heat transfer coefficient is 1.7 times that of air. The stator of the motor-generator was water-cooled. Figs. 2 and 3 show the Electrogyro unit.
Fig. 2 – The Electrogyro unit, comprising a 1.5-ton flywheel and a squirrel-cage motor-generator with a water-cooled stator
Fig. 3 – An Electrogyro unit ready for installation in a bus
The effect on vehicle ride
Considerable work was necessary to tune the suspension of the buses to cope with the gyroscopic effects of the flywheel. To achieve an acceptable degree of comfort, the front springs were made stiffer than those at the rear, and the flywheel assembly was installed on flexible mountings in the chassis; this prevented oscillations of the vehicle causing loads on the gyro bearings as a result of gyroscopic action. The mountings also isolated the gyro from shock loads when the vehicle was traversing rough terrain.
Cost-effective electrical equipment
The system was to be fed from the city’s power service, which carried three-phase alternating current at 50 Hertz. To allow a significant number of supply points for recharging the flywheel, the cost of each one needed to be kept to a minimum. It was therefore decided to use alternating current motors on the buses, both to power the flywheel and to drive the bus traction motors. This avoided the cost of rectifiers and control equipment to convert AC to DC. In this respect, Oerlikon could call upon considerable experience in designing AC electrification for the Swiss and French railways.
The Electrogyro buses
From September 1953 two Electrogyro buses were placed in regular service over a 2.8-mile route at Yverdon in Switzerland. Soon after, twelve buses started their service at Leopoldville in the Belgian Congo (now Kinshasa in the DRC). By 1955, a further three buses were under construction for a 4.8-mile route from Ghent to Merelbeke in Belgium.
Fig. 4 – Electrogyro buses for Leopoldville in the course of construction
An Electrogyro bus in more detail
Fig. 5 – Layout of the Electrogyro bus for the Ghent suburban service
A layout drawing of the Ghent-Merelbeke Electrogyro bus is shown in Fig. 5. It weighed 11 tons and carried 35 seated and 35 standing passengers. The 5 ft 4 in diameter flywheel, weighing 1.5 tons, was mounted with its axis vertical, under back-to-back seats near the centre of the vehicle. The flywheel was directly coupled to an AC squirrel-cage motor-generator, and both this and the flywheel were housed in a sealed casing filled with hydrogen at 0.7 atmospheres. When running at maximum speed the flywheel stress was only 30 percent of the ultimate tensile stress of the material. When the bus was being recharged at supply points, the motor-generator acted as a motor, taking AC current at 500 volts and spinning the flywheel up to around 3,000 rpm. The motor could also be fed from the normal three-phase supply throughout the night so that the vehicles was ready for operation in the morning.
During bus operation between supply points the motor-generator acted as a generator, driven by the rotating flywheel, to power the bus traction motor. For this, the motor-generator needed to be supplied with an excitation current; this came from static capacitors.
Fig. 6 – The three-motor propulsion unit
The bus traction motor consisted of three squirrel-cage induction motors assembled in a common housing, as seen in Fig. 6. An AC induction motor cannot be smoothly increased in speed by using resistors, as can a DC motor. Instead, the speed of the motor is determined by the number of poles in the motor and by the frequency of the alternating current supplied by the generator (in this case, driven by the flywheel, which will be at a relatively fixed speed, although it will steadily slow down as its power is used up). Acceleration of the bus is therefore achieved by switching in and out the three motors and the number of poles in each motor. The three motors are also geared to each other: motor 2 is geared at 31:25 to run 1.24 times faster than motor 1, and motor 3 is geared to run faster than motor 2 by the same amount.
More poles in action in any particular motor will give higher torque for starting but slower speed: for example, 8 poles in action will give a rotational speed of 2/8 of the speed with only 2 poles in action. The output speed from the motor assembly can be varied by two pedals that operate a switch to vary the number of poles in action and to switch between motors. The driver thus presses a right-hand pedal down once to effect a pole change, then again for a motor change, and so on, a total of six depressions being required to bring the vehicle up to full speed. Similarly, the driver then presses a left-hand pedal six times to come to a stop from full speed.
Fig. 6 shows which motors are in action with which number of poles, at each of the six stages. Therefore, the calculations of output speed in rpm and therefore road speed in mph, for a flywheel speed of 3,000 rpm, are as follows:
This layout permits regenerative braking, the current being fed back to the Electrogyro to increase the flywheel’s speed and thus its stored energy.
The charging point
Fig. 7 – Electrogyro buses in operation in Yverdon. The nearer bus is taking current from the main outrigger on the charging pole; the auxiliary contacts are also touching the small outrigger half way up the charging pole to switch on charging
Fig. 7 shows a charging point, consisting of a pole and an outrigger carrying the 3-phase feeds. When the bus comes in for a charge, the overhead contacts (similar to short trolley poles on a trolleybus) are lifted by an air-operated piston to touch the contacts on the outrigger. Then, two auxiliary contacts on the bus are swung sideways to touch a small outrigger about half way up the pole (just visible in Fig. 7). These earth the vehicle and actuate a main switch in the pole to activate the charging current to the Electrogyro.
Each charge point costs only about one-eighth of the price of one mile of trolleybus overhead wiring, poles and equipment.
Early results from operation in Yverdon and Leopoldville indicated that when negotiating gradients the driver needed to change down more frequently than with diesel-engined buses. However, with the vehicles being mainly intended for operation over level routes, this was not expected to be a problematic.
With four 10-second stops per mile, average speeds of 14 mph were obtained – about 10 percent lower than obtained with trolleybuses. This was mainly as a result of the time required for recharging which, in the case of the Yverdon route, was carried out at three charging points on the 2.8-mile route: one at each terminus and one intermediate charging point on the route. About 20 seconds were required to accelerate the flywheel to increase the energy stored by 1 KWhr, meaning that about 2 minutes were required to accelerate it from 2,000 to 2,950 rpm. In practice, charging at the intermediate point on the Yverdon route only required around 40 seconds to enable the bus to reach the other terminus.
The fate of the Electrogyro
The optimism expressed in 1955, when the Yverdon and Leopoldville routes were in operation and the Ghent route was under construction, was not to last. The Yverdon route had limited traffic potential, and although technically successful it was not commercially viable. Services ended in late October 1960, and neither of the two vehicles (nor the demonstrator) survived.
The Leopoldville system, with 12 buses, operated over four routes. There were major problems related to excessive “wear and tear”, put down to drivers taking shortcuts across unpaved roads, which after rains became quagmires. Other problems included breakage of gyro ball bearings, and high humidity resulting in traction motor overload. The system ultimately failed, however, because of high energy consumption. It was closed in the summer of 1959 with the buses being abandoned and replaced with diesel buses.
In Ghent, three buses started operation in late summer 1956. They stayed in service for only three years, being withdrawn in late autumn 1959. The operator considered them unreliable, and that their weight damaged road surfaces. They were also energy hungry, consuming 2.9 kWh/km – compared with between 2.0 kWh/km and 2.4 kWh/km for trams with much greater capacity.
One of Ghent’s Electrogyro buses has been preserved and restored, and is displayed at the VLATAM-museum in Antwerp.
Images courtesy of The Richard Roberts Archive: www.richardrobertsarchive.org.uk