Torque Converters – The Clutches of Automatic Transmissions

We all know almost by instinct that manual transmissions have a clutch to operate correctly, a device that allows to engage or disengage gears according to the vehicle speed.

The engine is a component that when the vehicle is in use is spinning most of the time, however, we may not want to be spinning the vehicle driveline at the same speed of the engine particularly on pull-away. This said, the clutch can provide a smooth engagement (dependent on acutation!) between a spinning engine and a non-spinning driveline by disconnecting the engine from the gearbox input shaft.

Now comes the important question that resumes the topic of this blog post, but what about automatic transmissions, do they use clutches as well? These types of transmission use a totally different device called “Torque Converter” although the same concept is implemented, which is to decouple or allow relative angular velocity between the engine and the gearbox.

STRUCTURE

The torque converter consists of a turbine, a pump or impeller, a stator and a lock-up clutch (included on modern torque converters only) as can be seen in Figure 1.

OPERATION

Phase 1 – Stall

• The impeller or pump receives the mechanical energy produced by the engine, but the turbine does not rotate because the brakes are still applied.

Phase 2 – Acceleration

• The brakes are no longer applied and the acceleration pedal is pressed, as a result, the impeller rotates faster and produces torque multiplication working in conjunction with the turbine.

Phase 3 – Coupling

• At this stage, the vehicle speed has increased, hence, the turbine reaches approximately 90% of the speed of the impeller and the torque multiplication ceases.
• Modern torque converters use a lock-up clutch to reduce the energy losses within the coupling fluid by mechanically locking the turbine to the impeller.

TYPES

Table based – K-factor (C and MPC formulations also exist).

Torque converter that works based on a K-factor input table.

K-factor = rpm/sqrt{torque}

Dynamic

Torque converter which models the transmission fluid behaviour based on fluid mechanics.

COMPARISON

In this blog post a comparison between a table based and a dynamic torque converter was carried out with the objective to highlight their differences.

Two simulations were run employing the same experiment (TCRig within VeSyMA – Powertrain), the only difference resided on the torque converter settings.

Figure 3 displays the results obtained. For the first plot, the torque converter was set to the K-factor characteristics, and for the second one, it was set to the dynamic characteristics.

K-factor characteristics (top)

It can be seen from Figure 3 that there is not a delay between the input and output torques, equally, the longitude among the signals is nearly the same (the slight difference is due to the torque multiplication). This means that we are talking about an ideal case.

Dynamic characteristics (bottom)

Meanwhile, from the second plot, it can be appreciated that the dynamic characteristics bring up a clear delay between the input and output signals and a damped torque on the output shaft compared to the K-factor results.

The torque delay and amplitude reduction is related to the fluid inertia and friction that is modelled within the dynamic torque converter.

CONCLUSIONS

After carrying out the comparison between both torque converter models it can be concluded that the dynamic characteristics recreate a more realistic model as it clearly shows the delay for the transmission fluid to travel from the impeller to the turbine (phase offset) and the energy losses within the system (output torque amplitude reduction).

In the opposite case, if the desired outputs are required to be conservative, the K-factor model can be implemented. Within VeSyMA – Powertrain a calibration function exists to be able to calibrate the Dynamic torque converter.

A RECENT INTERESTING APPLICATION

The Swedish brand Koenigsegg is popular for designing in-house components such as the 7-clutch and 9-speed automatic transmission denominated LST or Light Speed Transmission (covered in a previous blog post – Synchronisers in Dymola), this time it was not the exception.

One of its latest creations is called “Regera”. A plug-in hypercar with a twin-turbo 5.0-liter V8 and three electric motors, making this a 1500+ hp machine.

However, here we are not entirely interested in the power source, this is a vehicle like no other due to lack of a transmission. The main device connecting the drivetrain to the wheels is a torque converter capable of transferring a torque of 1475 lb.-ft to the rear wheels.

Let’s put it this way, as there are not actual reduction gears the engine is in “variable-drive” mode all the time. Here is the area where the torque converter shines, as it has the target to instantly translate the power to the road without engagement and disengament actuation required other than a lock-up clutch where fitted.

Written by: Jose Miguel Ortiz Sanchez, Project Engineer

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