CVT and other variable transmissions

Category: Dyno usage

CVTs normally consist of a set of two pulleys that can change their width and a belt that can travel from a closer position in the drive pulley (and a more open position in the driven pulley) which works as a first gear, to a wider position in the drive pulley as the drive pulley becomes compressed (by centrifugal rollers or other mechanisms) which work as a high gear. An important feature of CVTs is that it exists infinite positions between the “low” position to the “high” position, which gives also multiple combinations for the ratio value which change during the test, thus it is NOT possible to perform a standard ratio calibration for CVTs.

There are two ways to test a vehicle with a CVT (Continuous Variable Transmission)

Testing the CVT on its normal way of operation.

  • Set Fixed Ratio = 1.000 (as the ratio changes during the whole acceleration phase) and
  • use the starting and ending points referenced to speed. Note that it is also possible to start from a standstill either using manual start or setting the starting speed to 1 km/h
  • The resulting test should be displayed as HP (wheel) vs Speed.
  • Aux Torque can be used to get the Linear Force (thrust), but not ETQ
  • Remarks:
    • It is not possible to get the engine HP and the graphs vs actual the RPM (and makes not much sense). As the clutch is centrifugal in most cases the only way to disengage it is LIFTING the wheel and blocking the wheel, but this is a complicated and risky maneouver
    • Also, it is not possible to get the actual engine TQ, it is better to use the second method (blocked CVT)

Testing the CVT with the pulley blocked in the “high gear”

This is the method to get the most information from the Engine, almost the same as a vehicle with a manual gearbox (with the limitation of the centrifugal clutch)

  • Depending on the type of CVT, the pulley with the rollers may be set in a fixed position using washers, then the CVT will act as a fixed gear (high gear)
  • When this modification has been performed, it is necessary to start the engine carefully as the clutch will start the vehicle in the high gear. The recording at full throttle shouldn’t start until the clutch is totally engaged. For instance if the vehicle normally gets the clutch totally engaged at 15 km/h, it is possible that with this modification this happens at 30 km/h
  • When a test is recorded in this way, WHP and ETQ (uncorrected) channels are available vs actual Engine RPM, but EHP and corrected TQ are still not available due to the fact that the cetrifugal clutch is quite complex to be disengaged at top speed.

Displaying graphs vs actual Engine RPM

The X-Y graph can be used to display the graphs vs actual RPM (if the Engine RPM channel has enough quality), but in our opinion the resulting graph can be very confusing compared with the graphs-vs-speed, because indeed the purpose of the CVT is to keep the RPM almost constant during the widest (speed) range as possible

Press CTRL+F3 to enter the X-Y graph, and choose the 0x31-Engine RPM for the X axis

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