CVT- Continuously Variable Transmission
|It is advantageous to use a CVT, instead of a manual transmission. This is mainly because the engine will operate always on the optimum regimes and throttle-positions, adapted to the varying road conditions and power demands.|
|But is this advantage enough to overcome the inherent limitations
and dissipations of the common friction CVTs?
In order to quantify the advantage of using a CVT, we will ignore the inherent limitations and dissipations of the common friction CVTs. This will be a simplified approach.
To compare performances, we calculate how much time [seconds] is necessary to accelerate a car from rest to 100 km/h using Manual Transmission (MT). Then we will calculate it with the same car, but using a CVT. In both cases, we will neglect all energy losses such as clutch transitions, aerodynamics, etc, and we consider the road is horizontal.
|1st case: Manual Transmission (MT):
Consider a utility vehicle:
The transmission ratios (output speed/input speed) include the differential ratio:
|During each gear, the torque will be almost constant, and so will be
the car's acceleration. Force=Power/Velocity. Also,
Acceleration = Power / ( Velocity * Mass ).
Consider the ultimate car speed during the 1st gear, that is 43km/h.(@5700rpm). The power is 55kW, so the constant acceleration value can be calculated by:
Acceleration1st=55kw / ( 43km/h * Mass ) * k2 = Acc1=3.7 m/s˛
Similarly for the other gears: Acc2=2.6 m/s˛ ; Acc3=1.8 m/s˛ ; Acc4=1.3
The time required to attain 100km/h is the sum of the time spent on
Final_Velocity = Initial_Velocity + Acceleration * Time <=>
Time = (Final_Velocity - Initial_Velocity) / Acceleration
Let's calculate the sime during the 1st gear:
Time1 = ( 43km/h - 0Km/h ) / ( Acc1 * k2 ) = 3,2s
Similarly for the other gears and summing up, we conclude that the theoretical required time to accelerate from rest to 100km/h, using a manual transmission (MT), will be:
|2nd case: Continuously Variable Transmission (CVT):
Now we will calculate the required time to accelerate from rest to 100km/h, using a Continuously Variable Transmission (CVT). To simplify calculations, we will consider the IVT case, because it allows continuous ratio variation from rest.
To maximize acceleration, power must be kept on it's greatest value:
While accelerating, Force = Power / Velocity ,
According to Newtons Law, Force = Mass * Acceleration ,
Equating, becomes, Power / Velocity = Mass * Acceleration ,
Acceleration is the derivate of Velocity in order to time: dv/dt.
Thus, separating variables:
Velocity * Mass * dv/dt / Power = 1 ,
Integrating (§) this differential equation on both sides, becomes,
§ ( Velocity * Mass / Power ) dv = § (1) dt
Considering null constants, ( Velocity˛/2 ) * (Mass / Power) = Time
Substituting Velocity=100km/h, M=1250kg, Power=75cV, results on:
You may compute the CVT advantage regarding other car specifications. Please use the CVT vs MT Calculator.
The Continuously Variable Transmission (CVT) is 35% more performant than the Manual Transmission (MT). With same car and engine, the CVT takes only 75% of the time to accelerate to 100km/h, compared to the MT.
|This means that:
-Although the known CVTs (ex: "V"belt, Toroidal) have greater inherent dissipations than MT, they may be still advantageous.
-To take full advantage of the whole 35% improvement of CVT, it would be necessary to invent something as a "Geared CVT", (without the unavoidable limitations of friction CVTs).
-If it is difficult to eliminate significantly the energy losses of some friction drives, then these CVT will hardly improve performances up to 35%. Power-split and similar techniques may help here.
Perhaps, the most
remarkable practical proof of the CVT performance, was the CVT use in the
800cV Formula One Canon-Williams-Renault, in 1993. Without so much
development as the MT version, the experimental CVT Formula One was 1
second faster per lap.
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