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Model Transmissions Using Gear Ratios and Clutch Schedules

In a real drivetrain, you couple an input or drive shaft to one of many output or driven shafts, or to one driven shaft with a choice of several gear ratios. The drivetrain then requires several clutches to switch between gears. You couple one of the driven shafts or one of the gear sets by engaging one of the clutches. You then switch to another output shaft or another gear ratio by disengaging one clutch and engaging another.

You can also engage more than one clutch at a time to use multiple gear sets simultaneously. Transmissions engage multiple gear sets at the same time to produce a single effective gear ratio, or drive ratio. Changing gears requires disengaging one set of clutches and engaging another set. You can specify the set of clutches to engage and disengage for each gear ratio in a clutch schedule. Designing a clutch schedule and shaping and sequencing the clutch pressure signals frequently constitute the most difficult part of transmission design. A realistic transmission model must also include losses due to friction and imperfect gear meshing.

To learn how to model transmissions by creating a transmission model from gears and clutches, see Model a Two-Speed Transmission with Braking and Model a CR-CR 4-Speed Transmission Driveline with Braking.

Some Important Transmission Design Principles

When creating or modifying a transmission model:

  • Connect inertia blocks with nonzero inertia values to gear shafts for realistic simulation and preventing acceleration singularities when torques are applied.

  • Set up the clutch schedule for your transmission to specify the engaged and free clutches for proper transmission gearing. Set all clutch pressures to 0 only if you want to disengage the transmission completely (place it in neutral).

    Do not engage any more or fewer clutches than necessary, at any time during simulation.

  • If you redesign a transmission by adding or removing gears, you might also have to:

    • Add or remove clutches.

    • Redesign the clutch schedule.

    • Add or remove gear shaft inertias.

On the relationship clutch pressure signals to solver choices and settings, see Driveline Simulation Performance.

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