What Accelerates a Vehicle With a CVT?

Question

Back story This has been on the back of my mind for a while. I once had a long-term rental (5 months) with a CVT transmission, and after a couple of days, I figured out the best way to drive it, and, once I got used to the engine sound not matching the vehicle speed in a certain gear, I started to appreciate the CVT a lot more. I could accelerate aggressively and still get excellent fuel mileage.

Premise With a manual or a conventional automatic, there is a linkage between the engine's crankshaft and the gears. Accelerating involves getting the engine to increase it's revolutions (RPMs), and through the linkage, the gears and wheels will increase their revolutions. When the engine revolutions are outside the desirable range (too high or too low), changing gears will provide a more suitable linkage, but again, accelerating involves getting the engine to increase it's revolutions. The engine is providing the power for the acceleration, gears are only converting the engine speed into wheel speed.

With a CVT, high throttle demand will have the engine rev to it's optimal powerband, and then the vehicle speeds up by having the CVT continuously change the ratio between the crankshaft and driveshaft. Since the engine remains at the same revolutions during the acceleration, it's the CVT that is providing the actual acceleration, not the increase in revolutions as with a conventional transmission.

The Question During hard acceleration with a CVT:

• is the CVT simply fighting an increase in the engine's revolutions by increasing the effective gear ratio, making a CVT an automatic with an infinite number of gears and better linkage?
• would the CVT changing the effective ratio more quickly make the vehicle accelerate faster, or would it simply bog down the engine with too much of a load?

Answer 1

It is not strictly the increase in engine RPM that causes a vehicle to accelerate.

Newton's 2nd law of motion states:

In an inertial reference frame, the sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F = ma.

In other words, the engine, via the transmission puts a force (in the form of torque) on the vehicle, which causes it to accelerate.

In a manual transmission vehicle, there is a proportional link between engine RPM and speed. Thus as you accelerate, the engine RPM will correspondingly increase.

However in CVT, due to its continuously variable nature, there is no such direct proportionality. This allows the engine to operate much more of the time at its peak efficiency RPM, thus giving better performance and fuel economy over a range of speeds.

Answer 2

Acceleration is primarily due to the changing ratio of the CVT (continuously variable transmission). Think of it as the inverse of the usually set up where you have a transmission with a number of fixed ratios and continuously variable input speed (RPM).

With the CVT the engine speed can be held (relatively) constant at an optimal value - best economy, best torque, etc. - while the transmission adjusts its ratio in response to the throttle inputs. To respond to changing power requirements the engine controller will increase fuel as load increases (for example when accelerating or climbing a hill at constant speed) and vice versa.

Answer 3

As it has been said before, the (continuous) changing of the (infinite) "gears" in CVT is what makes it accelerate.

Here's a video that explains visually the CVT.

The control unit will request a certain torque from the engine (for speed or economy) while the acceleration is achieved with the variation of the ratios in the actual CVT. The two pulleys will change their distance thus changing their effective 'diameter'.

Answer 4

Most videos that explain CVT explain very well WHAT they do, but are always more than foggy on HOW a CVT changes ratios. I've found this video that drops a few keywords relevant to HOW the CVT moves clutches and sheaves, such as "flyweights," "helix" and "spring tension."

Basically, the input and output shafts use these flyweights, springs and helices to respond to their rotation speed and the belt tension between them. It isn't a motor that moves the sheaves or clutches, it's just the laws of physics affecting the flyweights, helices and springs.

To finally answer the question, the engine is still what provides the power of acceleration. Acceleration is not something that "gets made," power and torque are. Acceleration is the outcome when you apply power and torque to a mechanical device like a transmission.

So, the engine provides power and torque, the CVT flops about to change the effective gear ratio. The engine is kept at the optimum power or torque by the ECU in response to the throttle, the laws of physics simply let the CVT keep up with the engine.