AVTC Model Based Design Curriculum Development Project

Automatic Transmission Module 3

Automatic Transmission Request Vehicle Speed Mode Selector (P, D, R) Engine Power Transmission Transmission Controller Shift Maps Load

Why Do We Need an Automatic Transmission? Driving a vehicle with a manual transmission is oftentimes challenging for a beginner Improper operation of the clutch with a manual transmission can result in the engine stalling or a jerky motion for passengers It is considered inconvenient by many drivers that they are frequently having to change gears For those with disabilities may impose a difficult challenge

History of Automatic Transmission Chrysler, in 1941, introduced the first automatic transmission with a torque converter that replaced the manual clutch but had the same gear set as a manual transmission A hydraulic cylinder was used to perform the gear shifting The automatic transmission for current automobiles uses a planetary gear set Source:

Basic Principles Drive wheels receive engine power through the torque converter and transmission Hydraulic pressure with a fluid coupling allows power to flow through the torque converter to the transmission’s input shaft Input shaft drives planetary gear sets which provides various gear ratios including forward, reverse and neutral position Source:

Operation of Torque Converter Torque converter automatically engages and disengages power from the engine to the transmission somewhat similar to the manual clutch A torque converter consists of – Impeller – Turbine – Stator The hydraulic fluid transfers torque through kinetic energy of the fluid as it is forced from the impeller to the turbine Stator increases torque Source:

Planetary Gear Set Planetary gear sets used instead of traditional transmission gear sets Planetary gear assembly set consists of: – Sun Gear – Planet Gears – Planet Carrier – Ring Gear By locking the position of one of the rotating components, the selected rotational direction and gear ratio is obtained Source:

Source: HeldPower Input Power Output Rotational Rotational Direction SpeedTorque Ring Gear Sun GearCarrierReducedIncreased Same direction as drive member CarrierSun GearIncreasedReduced Sun Gear Ring GearCarrierReducedIncreased Same direction as drive member CarrierRing BearIncreasedReduced Carrier Sun GearRing BearReducedIncreased Opposite direction from the drive Ring GearSun GearIncreasedReduced Planetary Gear Set One of three components of the planetary gear set is held stationary and the other two components are attached to either the input or output The relative speed, torque, and rotational direction are shown for each possible combination in the table The shaded configuration designate four of the six possible arrangements that are used in this example

Devices to Hold Planetary Gear Set Three types of holding devices used with planetary gear set – Multiplate Clutch: Clamps friction discs attached to splined shaft and stationary steel discs – Roller or Sprag One-way Clutch: Holds planetary component in one rotational direction and freewheels in the other direction – Multiplate Brake and Brake Band: Holds planetary components to the transmission case with band as illustrated in figure Illustrated by: Shasank Pramode

Devices to Hold Planetary Gear Set (cont’d) Multiplate Brake and Brake Bands are commonly used Source:

Power Flow From Input Shaft to Driving Gear

Methods to Reduce Losses in Automatic Transmission Losses in automatic transmission are reduced by improving efficiencies of – Fluid recirculation – Improved volumetric efficiency by reducing rotating part clearances – Lock torque converter

Disadvantages of Automatic Transmission Complex and expensive Significant shift time Reduced efficiency because of torque converter

Specifications for a Popular Automatic Transmission (GM model 6T45) Source:

Principle Shift Map for an Automatic Transmission

Highway Drive Cycle for Audi A6 Time (s)

Fast Acceleration Data for Audi A6

Simulink Model of an Automatic Transmission

Output Response from Simulink Model

Simscape Model of an Automatic Transmission

Output Response of Simscape Model

Animation video

Review and Test Why is an automatic transmission “better” than a manual transmission? a.Driving a vehicle with a manual transmission are often times challenging for a beginner b.Improper operation of the clutch can result in the engine stalling or a jerky motion for passengers c.For those with disabilities may impose difficult challenges d.All the above Which one of the following is not a component in a planetary gear set? a.Sun Gear b.Planet Gear c.Saturn Carrier d.Ring Gear The torque converter is very efficient – True – False

References J. Erjavec, “TechOne: Automatic Transmissions,” New York, Thomson: Delmar Learning, P. Samanuhut, “Modeling and Control of Automatic Transmission with Planetary Gears for Shift Quality,” Ph.D. dissertation, Dept. of Mechanical Engineering, Univ. of Texas, Arlington, Texas, S. Enocksson, “Modeling in MathWorks Simscape by Building a Model of an Automatic Gearbox,” Uppsala Universtet, 2011, UPTEC STS “6T45 Specifications,”