2. Disc Friction Drive CVT
GWK of Maidenhead
1910-1931, two seat cyclecars
FIRST CAR: Tenting,
France 1891
3. CVT Categories & Targets
• Successful CVT will resolve the compromises in reliability, durability,
efficiency, and controllability with low cost
• Implementation of commercially produced CVTs transmit drive through
friction
• Variable pulleys with flexible belt or chain
– sliding friction
• Traction drives with rotating surfaces
– rolling contact, shear friction
5. Metal V-Belt Transmission
• Production from 1987-
2000 about 3 million
• In the last 4 years
about
4.5 million produced
• Example shown is a ZF
–CFT23 with torque
converter input
8. CVT Benefits
• No gear shift
• Continuous transmission of torque
• Control of engine speed independently of vehicle speed
• Ability to operate engine at peak power over wider range of vehicle speeds
• Ability to operate at most fuel efficient point for required output power
9. Control Objectives
• Good fuel economy
• Good driver feel – driveability
• Easy driving as an automatic
• Comfort and smoothness for passengers
• Performance – acceleration capability
• Electronic control enables these
10. 600 Nm
2.5:1
160 mm
• Power: 600 kW
• Input speed: 15 000 rev/min
• Input torque:
• Ratio range:
• Pulley centres:
• FIA ban: 1993
Williams (FW 15C) Van Doorne LG1
12. CVT Disadvantages
• Mechanical efficiency of variator
• Parasitic efficiency of transmission system and controller
• Compromise between fuel economy and torque margin to achieve driveability
(avoid elastic band feel)
13. CVT Efficiency
• All CVT variators have losses due to the power transfer which appears as a
speed or slip loss across the variator
• CVTs are hydraulically controlled and the pump takes power as in a
conventional automatic transmission
• Efficiency is more variable for CVT than geared discrete ratio
systems
14. Engine map
WOT- wide open throttle
(maximum torque line)
IOL – ideal operating line,
gives best economy for
CVT operation
Good fuel economy
Wide ratio range needed to
achieve ideal engine operation
15. Fuel Consumption Figures
Urban 12% 22.7% 10.5% 8.1% 10.4% 6.1% -4.8% -5.0%
Extra-
urban
13.8% 12.1% 10.0% -3.9% 0.0 4.8% -1.3% 5.0%
Combined 13.2% 16.4% 8.3% 1.7% 4.9% 4.9% -2.3% 0.0
Comparison of fuel consumption figures for CVT relative to equivalent automatics shows a
5-10% improvement for 4 to 5 ratio variants.
However, the table shows comparison for equivalent manual transmission vehicles over the
European Drive Cycle (+ve in red is worse)
16. Driveability Compromise
• Driveability describes the longitudinal dynamic behaviour of a vehicle in
response to driver inputs, in a comprehensive range of driving situations, and
the related driver subjective perception of that behaviour
• Less torque available immediately with a CVT than with a gear transmission
18. Solutions for Improved Driveability
• Torque boosters
– flywheel
– integrated electrical machine (motor/generator)
• Control – modified calibration
• Match the CVT with the most appropriate engine
• Modify engine characteristics to those of the transmission by design and/or
control
20. CVT Mild Hybrid Architecture
CAN Bus
CVT or IVT
Throttle
, Fuel,
Spark
ECU TCU
Electric
Machine
Motor/Generat
or
EM
Contro
l
Differenti
al
Pressure,
Clutch
Pressure
Curren
t
+
Batter
y
-
Driver
Deman
d
DC/DC
Converte
r
Inverte
r
21. Engine Matching
• Diesel engines have higher torque and lower speed range than petrol
• Modify engine systems to develop higher torque at low speeds – interest in this
as engine downsizing for conventional transmissions too
• Use of lean burn techniques to operate engine more efficiently at higher speeds
– experimental studies have shown both improvement in steady fuel
consumption and transient response
• Development of homogeneous charge compression ignition (HCCI) and
controlled auto ignition (CAI) is also appropriate for CVTs
22. Market Share in 2004
Region Manual Automatic CVT
Europe 84% 14% 2%
N. America 9% 90% 1%
Asia 40% 52% 8%
Japan 20% 65% 15%
23. Market Predictions
• ZF – increase from 2% share of sales in 2002 to 4.4% in 2012
• ZF – CVT will occupy 10 % of market for automatic transmissions in 10 years
• Jatco – increase from 8% share of sales in 2003 to 45% by 2010
• CSM Worldwide – increase in N. America to 3% by 2009
24. Comments
• CVT has a bright future
• Market share increasing
• Research and development still required
– Improve efficiency
– Torque booster
– Engine integration
– System control
• CVT driveability is key to customer acceptance, particularly in
Europe
• CVTs share some driveability characteristics with hybrid
vehicles
• Control and calibration with new concepts and an integrated
approach to total powertrain calibration can give driveability
solutions
![Acceleration Comparisons: Porsche Boxster
Manual
Tiptronic
CVTip in D
Engine
speed
[rev/min]
Vehicle
speed
[km/h]](https://image.slidesharecdn.com/presentation1-250611021806-111016b5/85/continuestly-variable-transmission-and-transaxle-11-320.jpg)
