Electric drive (bicycle)

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The electric drive of a bicycle is understood to mean the essential components of the electric motor , traction battery and control unit, as well as their interaction. In Europe and Japan, the electric drive is most often used in the so-called pedelec , where it only supports the cyclist when pedaling and does not fully propel the bike himself.

General

The electric drive is basically the same for all types of electric bicycles . The only difference is the control technology as a result of the different treatment under traffic law . The legal restrictions are most severe for pedelecs. The electric drive may only work when pedaling. In addition, the power of the engine and the driving speed are regulated. The stepping on the pedals and the driving speed are to be recorded by sensors .

Engines

In the meantime , brushless DC motors with permanent magnets are almost exclusively used. This has the advantage of being able to do without carbon brushes . The magnets for generating the magnetic field are located on the rotor. The rotating field is generated in the windings of the stator. The task of the current reversing sliding contacts , which used to be necessary for the electrical generation of a rotating magnetic field , is now usually performed by an electronic commutator (EC) in the so-called brushless or EC motor. The behavior of such a motor is like that of a DC motor with a magnetic field generated in the shunt and kept constant.

In hub motors, a low-speed gear is often built in, this can be a planetary gear .

Drive concepts

The development of the so-called pedelecs is advancing rapidly. While in previous years, the hub motor was more common today (2016) a lot of wheels with central engine built. Other concepts ( friction roller , sprocket engaging in the chain, etc.) almost no longer play a role. In principle and in full, there are the following options:

Starting points of the electric drive

Concepts in which power is transmitted to the rear wheel are widespread. The concept of the two-wheeler with front-wheel drive is less common .

In the impeller:

On the impeller (mostly at the rear):

On the bike chain :

  • via pinion engaging in the chain (mid-engine, often),

On the crankshaft:

  • Pedal crankshaft with motor / gearbox in common housing on a differently designed bicycle frame (often)
  • Engine in front of / under crank bearings,
  • Motor / gearbox on the crankshaft on the side of the bottom bracket housing

Motor (and battery) on a trailer ( push trailer , rare).

Drive with hub motor

Front hub motor with integrated gear

A gearless, direct-acting bicycle hub motor (e.g. BionX ) must have a relatively large diameter, because the relative speed between the stator poles (stationary part) and the magnetic poles of the rotor ( external rotor - rotating part, transmits the torque via the spokes on the rim) of an electric motor cannot fall below a minimum value in order to ensure reliable commutation and to deliver the required torque with an acceptable level of efficiency. In motors with a larger diameter, a higher number of pole pairs and winding slots can be accommodated; Due to the larger radius of the air gap in relation to the overall diameter of the impeller, the torque is relatively higher due to the better lever ratio.

In the case of the hub motor with built-in ( epicyclic ) gears for speed reduction, the rotor runs correspondingly faster, which means it can be smaller. Because of its lower weight and mass moment of inertia , this is preferred for front-wheel drive. The steering is less influenced because the gyro torques are also smaller.

The engine works i. d. Usually via a freewheel on the hub, so that the rotor and possibly the gearbox do not have to be spun with pure pedal operation. If there is no freewheel for the motor, the magnetic fields that have built up are still noticeable for approx. 500–1000 meters braking after the motor is switched off, as well as the drag resistance of a transmission. A hub motor always needs a torque support to support the counterforce of the generated drive torque against the frame or the front fork, the usual axle clamping is not sufficient for this.

A hub motor in the rear wheel has better traction because more weight is distributed on the rear wheel. As a rule, a gear hub cannot be combined with a rear wheel hub motor. A motor in the front wheel causes an increased risk of slipping and falling due to the high forces exerted by the drive, steering and braking with a lower contact force.

A major disadvantage of the hub motor compared to the central motor, which drives the wheel via chain and gear shift, is the rigid relationship between its number of revolutions and the wheel speed. There is no variable mechanical adjustment to the optimal engine speed. At low driving speeds (e.g. on inclines) and thus a low engine speed and high torque at the same time, the efficiency is lower, and the resulting increased motor current increases the temperature rise. Because you sometimes have to drive with less efficiency, the range with the same battery capacity is smaller for certain driving profiles than with a mid-engine with a downstream mechanical circuit. As a rule, the climbing ability of bicycles with a mid-engine is therefore better than that of those with wheel hub drives, which is particularly important for electric mountain bikes (EMTB, eMTB or e-MTB) and cargo bikes .

Pedelec with gearless rear wheel hub motor from BionX

The fixed ratio between the engine speed and the pedal crank speed in the mid-engine concept makes it easier to adapt the engine power to the pedaling power of the rider. In the case of hub motors, this adaptation requires a special effort in the electrical control.

A hub motor makes retrofitting to this additional drive relatively easy, since no or only minor modifications to the frame and fork are required. The power is supplied from the side through the hollow bored axle. In addition to the installation of a new wheel with a motor, the sole replacement of the hub by lacing it into the previous wheel using now shorter spokes is an option.

Drive on the impeller

As early as 1982, Heidemann in Einbeck presented a retrofit kit with a wheel roller motor acting on the tire. This drive principle, which was used earlier in the Vélosolex - a conventional moped with a combustion engine - is currently used in retrofit kits from GP-Motion GmbH and go-e GmbH on the rear wheel of electric bikes .

The two motors of the Velospeeders, which are only roughly sized like a side-rotor dynamo and attached in pairs, act with their drive rollers on the braking surface of the rear wheel rim. The contact pressure required for this is achieved via the transmitted force through a self-reinforcing wedge effect through the positioning of the swivel axes of the motors.

In the Dolphin developed by Michael Kutter around 1990 , the speeds of the pedal and motor drive in the epicyclic gear were added up in the hub. Production was stopped in 2014.

Drive on the bicycle chain or the crankshaft

Pedelec FLYER, mid-engine from PANASONIC, bike frame adapted to the motor housing
Common housing for mid-engine (left),
2-stage spur gear (right)
and crankshaft (top).
Bike frame adapted
BAFANG motor, placed in front of the bottom bracket shell

The mid-motor drive via a pinion on the chain was introduced by a drive unit manufactured by Panasonic . Instead of a conventional bottom bracket shell, the frame must be manufactured with a specially shaped mount in the bottom bracket area so that an integrated drive unit with a high-speed motor can be attached to the slow-speed gearbox and the crankshaft (figure, left). The crankshaft itself is not involved in the transmission of engine power. Only a sensor developed by Panasonic for measuring the pedaling torque is attached to it. The motor pinion engages behind the chainring of the crank on the lower, returning area of ​​the bicycle chain (the so-called empty strand ).

The obvious further development made by Bosch was to transfer the engine power directly to the crankshaft with the chainring, which is included as standard in the housing , and to do without the separate pinion for power transmission to the chain. The special construction of the bicycle frame in the area of ​​the bottom bracket remained necessary (illustration, right).

A motor mounted in front of or under the bottom bracket shell of an unchanged standard bicycle frame was previously offered as part of a retrofit kit. Several manufacturers are now using this solution. The motor is attached to the bottom bracket housing. A new crankshaft with torque sensor is part of the mounting kit. The power transmission from the engine to the crankshaft also enables a gear ratio to slow down. The large wheel of a spur gear or belt drive required for this is usually placed inconspicuously between the chainring and the frame (additionally covered by the right sheet metal bar that connects the motor and the crank sleeve).

Motors attached to the side of the crank housing coaxially on the crankshaft and also acting directly on this use the free space between the pedal cranks and the frame tubes, which is too narrow for an integrated gearbox. The motor diameter must therefore be relatively large ( Acron , silent-E ).

Philippe Kohlbrenner , who was involved in the development of the original Flyer , has been using a further, but basically the same drive variant in the Speedped since around 2010 . This bike has a gear hub. The motor is mounted on the right side of the rear frame and drives the rear wheel via a toothed belt parallel to the pedal chain (alternatively a toothed belt). The two pinions are fastened next to one another on the drive shaft of the hub gearbox.

Since the drive torque can be reduced via the derailleur in central motors, this allows correspondingly high torques on the rear wheel in low gears. E-mountain bikes benefit considerably from this property and enable even less trained riders to cope with enormous inclines.

The traction battery

Removable Pedelec battery pack from lithium-ion battery cells in the charger

The traction battery , often a removable battery pack (colloquially often just: "the battery") is an interconnection of several battery cells . It is the storage of the drive energy and thus the component that sets the limits of the range (based on trips with motor assistance).

The battery cell types used over the years (lead gel , nickel cadmium ( NiCd ), nickel metal hydride ( NiMH ) and lithium ion batteries) show an increasing energy density in this order. It increases from type to type to about double the value. The amount of energy stored by modern lithium-ion batteries (0.12–0.2 kWh / kg) is, however, still considerably smaller in terms of weight than the energy density of liquid fuels (around 10 kWh / kg). Due to their low energy density, lead-gel batteries are hardly used any more. The sale of NiCd batteries for this purpose was banned in Germany in 2009 under implementation of the RoHS guidelines due to the environmental damage of the cadmium they contain, with a few exceptions.

The charging times of the batteries are less than one to 16 hours, depending on the type and charging technology, around 2 to 9 hours are typical.

The particularly light Li-Ion batteries are currently used by most manufacturers. They can be charged a few hundred times (counting full charge cycles). They can also trigger violent chemical reactions in the event of a short circuit or overheating. However, these dangers are almost eliminated by modern battery management . Li-ion batteries deliver significantly less power at low temperatures and usually cannot tolerate frost .

Manufacturers who equip their Pedelecs with NiCd batteries usually include a charger that fully discharges the NiCd battery before the actual charging process in order to reduce the memory effect . NiMH batteries have a significantly lower memory effect. In lithium-ion batteries, this is completely absent.

In the near future, lithium polymer batteries with an even higher energy density will be available. There are also first practical test models in which the accumulator was replaced by a fuel cell and a hydrogen tank. This construction offers the advantage that charging times and battery wear are eliminated and additional tanks can be taken on a longer tour.

Alternatively, lithium iron phosphate batteries such as. B. be installed in the model of the Saxonette Beast 250, which are significantly more durable than the currently preferred lithium-ion batteries. Their use could reduce the running costs caused by battery wear. In the German area, however, they are not yet available except at Sachs Bikes .

Engine control

The motor control (the third part of the additional electric drive on an electric bike next to the motor and battery) fulfills three different tasks:

  1. Operation of the motor (electronic commutator in the brushless motors), protection of the motor and battery against overload and overheating, switching off the drive when the battery is discharged,
  2. Ensure user-friendly and simple application in everyday operation, interface to the user (operating elements, display for charging status, speed, distance, remaining range, selected support level), protection against unauthorized use
  3. Comply with legal restrictions, these can be:
- Drive assistance on the Pedelec only if the driver moves the cranks,
- prevent the motor assistance from acting above a permitted driving speed,
- prevent the electric drive from being the sole drive above a permitted speed (starting or helping to push the bike).
Angular momentum sensor on the crankshaft
Angular momentum sensor on the wheel (rear wheel)

Sensors

Speed ​​sensor: The number of pulses per unit of time gives the speed. Measurements are taken on the crankshaft, on the impeller (the travel speed is determined by the diameter of the impeller), in the motor. The measurement principle commonly used is the Hall effect .

Torque sensor: Measurements are made on the chain (rarely, force measurement on the deflection bearings) or on the bottom bracket shaft (measuring principle: inverse magnetostriction ).

Compliance with legal restrictions

Drive aid only when the driver moves the cranks: The pedaling speed is recorded with a rotation sensor, which is either attached to the crankshaft (retrofitting with a kit) or integrated in compact mid-motors (measurement on the crankshaft). The motor is switched on when a minimum speed is reached.
Drive only below permitted driving speeds: The driving speed is recorded by a rotation sensor located on the wheel, in the hub motor or in the motor that acts on the wheel with a constant ratio. The motor is switched off when the permitted starting / pushing speed is reached, if not pedaling, or when the supported maximum driving speed is reached (pedelec: 25 km / h; fast pedelec: 45 km / h).

Speed ​​control:
The driver has the choice of setting a driving speed from several preset levels (setpoints). The motor automatic produces the selected value by comparing it with the impeller speed determined by the rotation sensor. The driver must set a lower setpoint if the motor's limit power is only achieved at a lower speed during "heavy driving" (uphill) (under such circumstances he also selects a slower gear to maintain his usual cadence). The automatic motorized system does not ensure that the required foot force (generally reduced by the motorized assistance) remains the same. The appropriate speed level, which can be found from experience, must be set, which is the better the more levels there are. This type of control is not very user-friendly and has consequently not been able to establish itself.

Torque or force control:
To ensure that there is always the same foot force when riding a pedelec , the motor must always provide the same torque (foot force times the length of the pedal arms). The prerequisite is the technically relatively complex measurement of the foot force or the torque on the crankshaft compared to speed measurement. A technically and economically favorable torque sensor was first brought onto the market by PANASONIC as an integrated component of a mid-engine. This controls the torque of a mid-engine in such a way that it is proportional to the foot force at all times. This double proportionality - the other is the one between the engine speed and the pedaling frequency in the mid-engine concept from the outset - more is achieved than keeping the pedaling frequency and the rider's foot strength as constant as possible. It is an additional benefit for usability.

Control with the help of two speed sensors
With the large number of pedelecs equipped with hub motors, the speed ratio between pedaling and motor is not constant because of the gears in between. This means that it is not possible to set a constant torque ratio between motor contribution and pedaling. The speed ratio between the impeller and thus between the hub motor and the pedal crank is only dependent on the gear set. It results from the speed values ​​of the motor and pedaling measured with two angular momentum sensors. Depending on this, the engine torque can be controlled in such a way that, converted to the pedal crank speed, it has the same constant value for each gear. In this way, with the help of simple speed sensors, dispensing with a complex torque sensor, it is achieved that the rider's cadence and foot force are always the same as possible. The additional advantage achieved in torque control as a result of the proportionalities existing at any point in time between the variables of motor torque / foot force or motor speed / pedaling frequency does not exist.

Performance levels:
The ratio between the driver's own performance and the additional performance of the engine can often be set by the driver. In this way, he can decide whether he wants to use the battery sparingly and thus drive far, or whether he wants greater support at the expense of a shorter range. Adjustment can also be made continuously using a “throttle grip” .

Starting aid:
The starting aid allows pedelecs to be assisted by the motor at the push of a button or by turning the throttle grip, even without pedaling. It is used for easier starting from a standing position and as a pushing aid for independent “driving” of the bicycle. This type of motor support is usually limited to the legally permitted maximum speed of 6 km / h. The pushing aid has the advantage that you can let the bike roll next to you with the assistance of the motor, without having to pedal or push yourself (e.g. when you are carrying a heavy load or so that you can run the bike up a slope on your own ). In any case, it allows faster (and more physically controlled) starting from a standing start at traffic lights that switch to "green" .

literature

  • Christian Smolik, Michael Bollschweiler, Verena Ziese: The electric bike: types, technology, trends. BVA , Bielefeld 2010, ISBN 978-3-87073-435-0 .
  • Teja and Eberhard Müller: E-Bike Technology: Function and Physics of Electric Bikes. Books on Demand GmbH, Norderstedt 2011, ISBN 978-3-8423-6194-2 .

Web links

  • S. Wetzel: Planetary gears on bicycles: 4th planetary gears in electric hub motors [15]
  • S. Wetzel: Motor for electric bicycle [16]

Individual evidence

  1. ↑ DOLPHIN: The motor attached to the seat tube drives the rear wheel together with the chain pinion via a speed summing gear . [1]
  2. ↑ a b SPEEDPED: The chain pinion and the toothed belt wheel driven by the motor work together on the input shaft of the hub gear. [2]
  3. ↑ PENDIX: middle motor engaging in chain by means of a pinion [3]
  4. ↑ BOSCH: The crank shaft is located in the housing (motor-gear unit, custom-made bicycle frame required). [4] (second picture)
  5. ↑ a b BAFANG: A belt drive that translates into slow speed between the bicycle frame and the chainring . [5] (first or second picture, enlargement possible)
  6. ↑ a b ACRON: motor mounted on the left between the pedal arm and the bicycle frame (symmetrical to the chainring on the right) [6]
  7. ↑ a b AEG: motor archive link mounted on the right outside the chainring ( memento of the original from March 4, 2016 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. (especially the last of the precursors) @1@ 2Template: Webachiv / IABot / ebike.velototal.de
  8. ↑ The speed of a 28-inch wheel is only about 2½ / s at a speed of 20 km / h. Electric motors usually have an order of magnitude higher speed.
  9. ↑ In a hub motor without a gear, there is basically the space required for a gearshift, but this is rarely used due to the technical complexity (example: Sparc from SRAM , hub motor with built-in 5-speed gearshift)
  10. ↑ a b Dan Popa: AFFE ™ control for pedelecs [7]
  11. ↑ VELOSPEEDER: Friction wheel drives [8]
  12. ↑ a b c PANASONIC: Torque measurement on the crankshaft [9]
  13. ↑ BOOSTY drive [10]
  14. ↑ FLYER Classic, mid-1990s [11]
  15. ↑ The battery: The power plant on the e-bike [12]
  16. ↑ In 2016, the first hydrogen bicycle comes onto the market . VDI Verlag GmbH. December 10, 2015. Accessed March 25, 2019.
  17. ↑ NCTE company information: How do the NCTE sensors work? [13]
  18. ↑ S. Wetzel: A special control of an additional electric drive [14]