This document discusses continuous variable transmissions (CVTs). It begins by explaining the basic function of a transmission to change the speed ratio between an engine and wheels. It then discusses problems with manual and automatic transmissions. CVTs are introduced as having benefits like providing optimal engine torque, no shift clunk, continuous ratio adjustment, better acceleration, and improved fuel efficiency. The main types of CVTs are then outlined as pulley-based, toroidal, and hydrostatic. Details are provided on the mechanisms and operation of each type. Control and modeling of CVTs are also briefly covered. Advantages are said to include decreased engine fatigue and improved efficiency over automatic transmissions, while disadvantages include limited torque and higher cost
2. Basics of Transmission
• The job of the transmission is to change the
speed ratio between the engine and the wheels of
an automobile
• The transmission uses a range of gears -- from
low to high -- to make more effective use of the
engine's torque as driving conditions change. The
gears can be engaged manually or automatically.
4. Benefits of CVT
Provide engine torque in optimum condition (road
loads / drive shaft torque)
No shift clonk
Transmission ratio can be adjusted continuously
Better acceleration
Improved fuel efficiency
6. CVT MAIN TYPES
Pulley based CVT
Toroidal CVT
Hydrostatic CVT
7. Pulley based CVT
The gear ratio is changed by moving the two sheaves
of one pulley closer together and the two sheaves of
the other pulley farther apart.
The distance between the pulleys does not change
and neither does the length of the belt
so changing the gear ratio means both pulleys must
be adjusted (one bigger, the other smaller)
simultaneously in order to maintain the proper amount
of tension on the belt.
11. Hydrostatic CVT
Power transmitted by hydraulic fluid
variable displacment pump
& hydraulic motor
• Rotational
motion
ENGINE
• Fluid flow
HYDROSTATIC
PUMP
• Rotational
motion
HYDROSTATIC
MOTOR
12. Hydrostatic CVT
often,hydrostatic transmission is combined with a
planetary gearset and clutches to create a hybrid system
known as a hydromechanical transmission.
Hydromechanical transmissions transfer power from the
engine to the wheels in three different modes
At a low speed power is transmitted hydraulically
at a high speed, power is transmitted mechanically
Between these extremes, the transmission uses both
hydraulic and mechanical means to transfer power.
Hydromechanical transmissions are ideal for heavy-duty
applications, which is why they are common in agricultural
tractors
14. Mathematical Model of CVT
Ie :engine inertia
Is :active cvt pulley inertia
Iq :passive cvt pulley inertia
Be,Bs,Bq:damping coefficient
i : speed ratio of cvt
n : transmission efficiency
Tr : resistance torque of
car weight & load converted
to cvt output
15. ADVANTAGES OF CVT
Decreases engine fatigue
Maintains the engine for an optimum power
output
Best acceleration
More mechanical efficient than the automatic
transmission
Improved fuel efficiency
16. DISADVANTAGES OF CVT
Limited torque when compared with manual
Larger and more costly than automatic or manual
transmission
Slipping in the drive belt or pulleys