10 Mechanical CVTs: Launch clutch Transmission Final drive Transmitting Engine torque with slip Reducing engine torque fluctuation Reducing engine torque peaks Amplifying engine torque over a larger speed coverage Reversing engine torque direction Controlling of transmission ratio Connect to PTO Distribute drive torque between the wheels Transmit drive torque to moving wheels Transmit brake torque at the wheels 10.1 Used types 10.1.1 Pulley: (Audi Multitronic, - Power transmitted frictionally through chain running between two axially adjustable taper discs - Disc adjustment diameter variation ratio variation
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Amplifying engine torque over a larger speed coverage
Reversing engine torque direction
Controlling of transmission ratio
Connect to PTO
Distribute drive torque between the wheels
Transmit drive torque to moving wheels
Transmit brake torque at the wheels
10.1 Used types
10.1.1 Pulley: (Audi Multitronic,- Power transmitted frictionally through chain running between two axially adjustable
taper discs- Disc adjustment diameter variation ratio variation
10.1.2 Cone ring:
10.1.3 Toroidal:
10.2 Geometry
10.3 Design- Tensional link chain: more efficient (less power to adapt the chain to ratio radii)- Thrust link chain: short pitch of chain elements needs more lost work. Low nois
generation-
10.4 Kinematics- Speed ratio: ispeed = np / ns
- Geometrical ratio: igeom = Rs/Rp
- Torque ratio: itorque = Ts/Tp
- Ratio coverage: iLOW / iOD
- Variogram:
10.5 ForcesCoulomb`s law of friction: Belt pulley interaction: push force build-up and release:
- Factor influencing the clamping force:o Engine torqueo Torque converter multiplication and clutch controlso Gear ratioo The lowest occurring friction coefficiento The torque transmitted by the wheels
- Belt pulley interaction and clamping force equation:
- Required minimum clamping force:
10.6 Efficiency- Typical efficiency vs ratio:
- Effect of overclamping on efficiency (66 rad/sec; Overdrive; Fsec=7.8 kN):
- Efficiency improvement:Reference CVT efficiency Reference CVT loss at 50 Nm in OD:
Pump loss: Pump displacement depends on:- CVT rated torque- Leakages- Hydraulics design- Auxiliary functionPump power loss ≈ all losses in mechanical transmission
Torque loss:- Overclamping needed to prevent slip
(safety factor Sf)- Ekĺastic deformations of pulley/shafts- Tilting play- Internal friction in V-belt- Minimum clamping force restricted by :
o Hydraulic limitations ~> 6 baro Low control band width at low
pressure due to oil aeration
Slip loss:
- Very small under normal use (<1%), especially at partial load