High Efficiency Bicycle Propulsion System Using …

1 EVS24 Stavanger, Norway, May 13-16, 2009 high Efficiency Bicycle Propulsion System Using TwoMotors and Epicyclic GearingPer H. S rensen1, Miles Hellon21 Forsandveien 13, 4307 Sandnes, Norway, Winforton Street, London SE10 8UR, , paper describes a novel Propulsion unit for cycles and other ultralight vehicles Using two brushless DC(BLDC) motors feeding sun and ring gear of an epicyclic gear unit all within a single enclosure. The twoBLDC motors having different operating characteristics are being actively managed Using a microprocessorconnected to two electronic commutation controllers, one for each motor . By optimizing the RPM for eachmotor according to vehicle speed and torque the main Propulsion motor and its controller is kept workingwithin its maximum Efficiency band. Most of the electric energy is used by this motor and controller pairmaintaining total Efficiency at a high level.

2 Keywords: Bicycle , brushless motor , Efficiency , planetary gear, powertrain1 IntroductionThe single biggest obstacle for electric vehicles isthe high cost of energy storage, both in terms ofinvestment and in added weight to the vehicle [1].For shorter distances electric bicycles are a smartchoice as their energy requirement is much lessthan a car-sized electric vehicle [2][3]. They arenow used in large numbers but mostly on flatterrain [4]. As a commuter vehicle for hilly terrain electricbicycles need refinement. This paper discuss apropulsion unit for electric bicycles and otherultralight electric vehicles that increases theefficiency and versatility of the powertrain byadding a electromechanical variable Electric Bicycle Motors Currently, most commercial electric bicyclemotors are hub motors with or without a fixedgearing.

3 Alternatively, some motors are part ofthe pedal arrangement and their power istransferred to the wheel Using the same bicyclechain as the cyclist. Sometimes friction drive,direct to the tyre or rim, is used. All the existingsolutions involves some form of compromise ascan be seen from table of GearingOf the commercially used motor solutions, onlythose which drive via the Bicycle 's chain can begeared. The other solutions use a single gear ordrive the wheel is important to match the RPM bandwhere the motor is efficient with the speed of thebicycle which depends on the available motorpower and the current road condition and limited power available due to legal reasonsand battery range issues, the lack of gearing is animportant limitation, especially in hilly terrain [5].

4 EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium1 Table1: motor comparisonMotor type:Major issues:Hub motor , directLow power to weight ratio, low torque, motor drag whencoasting, special wheel requiredHub motor , geared no regeneration due to integral freewheel, special wheelrequiredFriction motorLow torque, slippery when wet, wear on tyre,no regenerationdue to integral freewheelMotor Using existing chainno regeneration, special freewheel required for pedals, rapidchain wear, difficult with derailleur gearMotor with own chain/beltExtra pulley normally require hub for disc brakes, moreawkward wheel removal 2 New Design Based on DualMotor PropulsionAvoiding the limitations of commerciallyavailable e-bike Propulsion systems requiredsome form of speed adaptation. Using a form ofdifferential, or epicyclic gearing, similar to thatused by the Toyota Prius, it became clear that asuitable matching of motor speed to vehicle speedwas possible without manual gearing, by adding asmaller extra motor [6][13].

5 Using motor and gear designs optimized for thisapplication a compact unit has been designed andmodelled in 3D for simulation and prototyping asillustrated in figure 1. GearingThe use of epicyclic gears, often called planetarygears, is common on bicycles where they havebeen used inside hub gears for years [7].When used as a differential in a car, the twooutput shafts supply the motor power to eachwheel. If one wheel is stationary, the other wheelspins twice as fast. Here it is used the opposite way, having twomotors driving two input shafts. This results inoutput speed being controlled by the sum of thetwo motor designing the gearing ratio of the epicyclicgearing the torque ratio between the motors canbe adapted to suit the selected motors.

6 Gearing RatioFinal gearing is by belt drive to the Bicycle wheel,or alternatively, a second epicyclic gear for a hubmotor version. Ratio is selected to matchpropulsion unit RPM range to Bicycle and Electronic ControlThe main Propulsion motor is designed to supplyhigh torque and run at a moderate speed. Thegearing motor has much lower torque but can runat a much higher speed. The speed of each motoris controlled by a microcontroller. Themicrocontroller strives to maintain the propulsionmotor at its most efficient RPM and uses thegearing motor to adapt the Propulsion unit'soutput speed to the Bicycle speed. 3 Detailed Description of DesignA prototype design has been made based on theprinciples of the suggested Propulsion unit. Thisprototype design will be described in International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium2 Figure 1: Cross Section of Complete Propulsion LocationFor the prototype design, a non-hub propulsionunit Using a separate belt was selected.

7 Thismakes the design adaptable to velomobiles orother ultralight vehicles where hub motors don'tfit. It also reduces the unsprung weight onsuspension bicycles, improving road grip. Finally,hub motors are mechanically more complex dueto space limitations and dynamic load 2: motor LocationA hub motor version is possible, but wouldrequire a second stage epicyclic gear for finalgearing. This would also require a strongerhousing to support wheel and spoke loads,increasing the weight. If used on a rear wheel itwould also need to support the chain RPM Limitations When linking the motor to the wheel without afreewheeling mechanism, regenerative brakingbecomes available, but at the same time it alsoallows over-revving of the motor has an absolute maximum RPM limitand will be destroyed if this speed is driven hub motors usually have an RPMlimit higher than wheel RPM possible on abicycle, unfortunately this is not the case withgeared motors.

8 Therefore geared hub motors andmotors Using the Bicycle chain have a built-infreewheel mechanism that prevents the motorfrom being turned by the wheel. But a freewheelalso prevents the regenerative braking used toimprove battery range and prolong lifetime of thecycle this new design the maximum allowable wheelspeed is increased by dividing the wheel speed ontwo motors via the epicyclic gear. It becomespossible to extend available speed range beyondwhat a single motor can handle alone and still doregenerative FanBy adding a small fan to the high revving gearingmotor axle it is possible to do forced cooling onthe motor housing and attached components toreduce temperature and increase power output. Byaccurate controlling the speed of both motors andby making them counter rotate the fan can be usedeven when the Bicycle is stationary.

9 This fan canbe used for the motor and/or cooling MotorsTwo different types of brushless DC motorcommutation exists, with or without sensors. Most Bicycle motors uses hall sensors forcommutation control but some do not and arereferred to as having sensorless commutation [8].Unfortunately sensor based commutation requiresmore parts inside the motor and more connectionsbetween motor and controller. That is the reasonwhy sensorless design is preferred. Sensorless commutation uses induced voltages tosense rotor position. In order to get inducedvoltages, the rotor must move. For pedal assistedelectric bicycles it is possible to use sensorlesscommutation by having the user start moving thecycle before the motor is energized. For otherapplications the motor start-up will makedisturbing erratic movements forcing motormanufacturers to use the dual motor principle, sensorlesscommutation can be used for both motors.

10 Byenergizing both motors at the same time butcounter rotating, they will not move the way sensorless commutation is possible andthe problems normally associated with sensorlesscommutation are MotorFor coasting, a motor able to rotate at high RPMwith negligible losses is required for the gearingmotor. To achieve this, a disc type motor with rotatingaxial field, having two parallel PM rotors with aircore coils is International Battery, Hybrid and Fuel Cell Electric Vehicle of LossWhen a permanent magnet motor is forced torotate it will induce voltage in the will also cause eddy current losses in allconductors exposed to moving magnetic fields. By removing the laminated metal stator thelargest cause of eddy losses is removed. In addition some eddy current losses occur insidethe stator coil.