An Electric Assist Bicycle Drive with Automatic Continuously Variable Transmission

Peter WattersonCSIRO Materials Science and Engineering, Australia ICEMS 2008, 19 October 2008, Wuhan, China

CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

Objectives

• Provide electric assistance to a bicycle • human – electric hybrid with roughly equal power inputs

• low CO2 transportation

• ... but some exercise

• An automatic transmission – no gear changing

• Fixed ratio chain (or belt) drive from crank to rear wheel• Hence chain drive can be shrouded and kept clean

• Minimum weight

• Maximum efficiency

CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

A Prius in a Bicycle!

CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

Electric Continuously Variable Transmission,e-CVT

human input MG1

MG2

output

Ring gear

planet gears

planet CarrierSun gear

S

C

R

Planetary 1

CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

Planetary gear speed relationship

• For planetary gear constant

• Speeds obey:

s

r

N

Nk

0)1( crs kk

Ring gear number of teeth

Sun gear number of teeth

CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

Planetary gear torque relationship

• From force and torque balance on planet gears

• External torques on the gears obey:

scr TkTTk

k)( 1

1

FF/2

F/2

CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

Two hub topology

Planetary 1

spoke rims

shaft (fixed to bike forks)

MG1

chain

Planetary 2

spoke rims

shaft (fixed to bike forks)

MG2

rear hub front hub

CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

One hub topology, axial field MGs

Axial magnetic field MGs can share same stack:

Planetary 2

Planetary 1

spoke rims

shaft (fixed to bike forks)

MG2

MG1

chain

leads

CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

One hub topology, direct drive MG2

Planetary 1

spoke rims

shaft (fixed to bike forks)

MG2

MG1

chain

leads

CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

One hub topology, geared MG2

Planetary 2

Planetary 1

spoke rims

shaft (fixed to bike forks)

MG2

MG1

chain

leads

Ratio of volumes within airgaps = 3

CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

A possible specification

Maximum continuous total electric power 200 W

Maximum continuous rider power 200 W

Predominant rider cadence 75 rpm

Range of rider cadence 60 – 90 rpm

Range of bicycle speeds 15 – 36 km/h

Associated (700 mm diameter) wheel speeds 110 – 270 rpm

Maximum regenerative braking power 400 W

CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

MG ratings for draft rear hub design

• Gear ratios: Planetary 1, 7.24; Planetary 2, 5.32.• Chain ratio 2.33 gives equal Torque/Volume for MG1 and MG2

at their most demanding operating points. Ignoring all losses:

Crank speed (rpm)

Wheel speed (rpm)

MG1 MG2

speed (rpm)

torque (Nm)

power (W)

speed (rpm)

torque (Nm)

power (W)

Low cadence, high speed 60 270 -801 -1.66 139 -1437 -0.41 61

High cadence, low speed 90 110 934 -1.10 -108 -586 -5.02 308

Regenerative braking 400 W,

high speed 0 270 -1955 0.49 -100 -1437 1.99 -300

CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

Conclusions

• Trial rear hub e-CVT design • 200 W human power and 200 W electrical power

• MGs with 19 kN/m2 tangential force at air-gap – realistic.

• Higher power design (e.g. for the US market)• Two hub topology may be needed for extra volume

• Automatic control• No gear changing

• Can base desired speed on pedalling torque and/or cadence

• Can allow rider to vary level of assistance

• Other applications – petrol/electric vehicles including scooters

Contact UsPhone: 1300 363 400 or +61 3 9545 2176

Email: enquiries@csiro.au Web: www.csiro.au

Thank you

CSIRO Materials Science and EngineeringPeter WattersonPrincipal Research Scientist

Phone: 61-2-94137529Email: Peter Watterson@csiro.auWeb: www.csiro.au/science/ElectricMachines.html