Today we will discuss about continuous variable transmission. We also discuss about advantages and disadvantages of CVT transmission. Till now we know that we use gear box to change the torque according
to the driving requirement. For this we have four to five gear ratio which we
shift and change the gear. But CVT is an automatic transmission that can select
any desired drive ratio within its operating range. Unlike a conventional four
or five-speed transmission, CVT is an infinite speed ratio transmission.
In the manual gearbox one problem facing that the one speed
ratio is not fully efficient in the desire range. So the manufactures have to
work on a new system which can obtain infinite speed ratio and there is one
unique speed ratio for one speed. This is the basic idea of CVT.
ALSO READ : AUTOMATIC VS MANUAL TRANSMISSION
Principle of CVT Transmission:
As an ordinary gear box CVT also works on the same gearing principle
that when we attached a big driving gear to one small driven gear we obtain
high torque and vice versa. But the CVT does not used gear to obtain speed
ratio of transfer power. The other power transmission system belt drive used in
CVT can change its diameters.
There are three main component of CVT which make is able to
obtain infinite speed ratio transmission. These are a variable input driving
pulley, a variable output driven pulley and a metallic belt.
Apart from these, there are also various microprocessors and
sensors and even epicyclic gearing and clutch. Both the driving and driven
pulleys are of variable diameter type. Each pulley is made of two 20 degree
cones facing each other. A belt is there in the groove between the two cones. The
belts are specially constructed to provide for the required flexibility.
The driving pulley is connected to the engine crankshaft
while the driven pulley transfers motion to the drive shaft. When the two cones
of the pulley are close together, the belt rides higher in the groove and the
pulley diameter apparently increases, whereas the belt rides lower in the
groove, making the effective diameter decrease, when the two cones of the
pulley are far apart. These cones are moved closer or far apart using hydraulic
pressure, centrifugal force or spring tension.
If diameter of
driving pulley increases, the diameter of driven pulley decreases, and vice
versa. This is done to keep the belt always tight. As the pulley change their diameter
relative to each other, infinite number of gear ratio are obtained. For low
gearing the diameter of small on the driving pulley and large on the driven
pulley and for high gearing vice versa.
A conventional CVT uses the information from engine
management ECU to determine the torque being delivered by the engine and
accordingly the pressure exerted on the belt to avoid slipping. Further the
drive is transmitted separately to forward gears and to reverse, through two
multiple clutches. There are also two small hydraulic chambers for moving the
variator pulleys instead of one larger chamber in case of conventional CVT,
this reduces the power consumed.
Advantages and Disadvantages of CVT Transmission:
Advantages:
1. It provides the smoother ride compare to conventional
gearing system.
2. It increases the fuel economy of vehicle by deliver optimum power
range under all conditions.
3. It eliminates gear hunting while moving up and incline.
4. It decreases emissions due to better engine control under
all conditions.
Disadvantages:
1. Torque handling capacity of a CVT is limited by the strength
of the belt.
2. CVT has much shorter life compare to conventional gearing
system.
3. CVT need large central distance which cannot be provided in
inline configuration.
The CVT are already used by some manufactures like Nissan to
obtain high efficiency. We can say that this technology continues growing and manufactures
are looking forward with it. User loves them because of its efficiency and
smooth riding. This system is also ecofriendly
so the manufactures and government promoting it and soon it will adopted by
most of manufactures. Audi’s Multitronic is also example of it.
We have discussed how CVT works and what are benefits and disadvantages of CVT. If you got any query against what is CVT kindly ask by commenting.