Torque Converter
Torque Converter
The torque converter is located inside the torque converter housing which is on the engine side of the gearbox casing.
The torque converter acts as the coupling element between the engine and gearbox. The driven power from the engine is transmitted hydraulically and mechanically in certain gears and operating conditions, through the torque converter lock-up clutch to the gearbox. The torque converter is connected to the engine by a drive plate.
The torque converter consists of an impeller, stator and turbine. The engine drives the impeller, while the turbine drives the gearbox. The stator is situated between the impeller and turbine on a one-way clutch. The impeller picks up fluid and throws it out into the turbine, thereby causing it also to rotate and transmit power.
The stator redirects the fluid thrown back by the turbine, so that it re-enters the impeller in the same direction of rotation as the impeller and at the most efficient angle.
The one-way clutch prevents the stator from moving backwards, so that this accurate redirection of fluid can be achieved. When the engine is idling the impeller throws out very little fluid. The turbine is not forced to turn, and the power is not transmitted to the gearbox.
As engine speed increases the impeller throws out more fluid. The turbine begins to turn and picks up speed as the engine speed rises. As the speed of the turbine increases the fluid is thrown against the back of the stator, causing it to turn in the same direction.
When turbine speed approaches impeller speed, centrifugal force in both units is almost equal and all three components move at nearly the same rate. This is called the 'coupling point'.
The torque multiplication or drive ratio varies until a one to one coupling point is reached.
To achieve the power required to climb a hill, the driver depresses the accelerator pedal and the torque converter reacts by increasing the torque multiplication.
When driving on a flat road at cruising speed, the power required is not as great and therefore, the torque converter stays at one to one.
Torque Converter Lock-Up Mechanism
In a torque converter there is always a certain amount of slip between the impeller and turbine. This will contribute to a reduction in fuel economy especially during high speed cruising.
The lock-up mechanism comprises a lock-up solenoid valve, a lock-up control valve and a lock-up clutch.
The lock-up control is provided by the EAT ECU which operates the lock-up solenoid valve. The EAT ECU controls lock-up clutch engagement and release according to the lock-up schedule programmed into the ECU and the vehicle speed and throttle angle.
The lock-up mechanism operates with the gearbox in 'D' (normal mode 4th and 5th gears) and in manual 4th and 5th gears. In an emergency condition when high fluid temperatures are reached, the EAT ECU can also operate the lockup mechanism in 2nd and 3rd gears to help reduce fluid temperatures.
In addition to the lock and unlock conditions, the lock-up control can also initiate smooth lock-up, coast lock-up and lock-up prohibition control. Smooth lock-up minimizes lock-up shock by smoothly and slowly engaging the lock-up clutch.
Coast lock-up control maintains the lock-up condition after the throttle pedal has been released in the lock-up range at certain high speed driving. This prevents the lock-up control switching between the locked and unlocked condition caused by repeated on-off use of the throttle pedal.
Lock-up prohibition control prevents clutch lock-up within the range if the fluid temperature is below 40°C (104°F). This promotes faster warm-up of the gearbox fluid. This strategy is also used by the EAT ECU to prevent lock-up in 1st gear, park, reverse and neutral ranges.
Unlock Condition
The unlock release pressure is supplied via the control valve to the lock-up clutch. The pressure forces the clutch mechanism away from the torque converter and moves the lock-up mechanism into the unlock condition. The torque converter pressure is decayed to the drain port, removing the applied pressure from the torque converter, allowing the clutch mechanism to move.
Lock-Up Condition
