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This posting is part of a technical paper by the German transmission firm, LuK, concerning how they control the lock up clutch function of a torque converter. This article was not specifically about the CVT in the Pathfinder. However, it should be noted (see other posting) that LuK developed the chain that is used in the CVT8HT, which is in the Pathfinder. Any text in italics was noted by me, and was not marked that way in the original article.

The Control Strategy of the Lock-Up Clutch by LuK

In order to ensure sufficient noise reduction in this low rpm-range, at high loading condition, or to avoid engine lugging, a large amount of slip is necessary during lock-up. With the use of a high performance damper, the noise level can be reduced further or the amount of slip can be lowered. The loss due to the high amount of slip can generate excessive heat and temperature build-up at the friction surface of the lock-up clutch. The high temperature causes ATF-oil and friction material degradation. The result is falling friction coefficient with increasing slip speed, which leads to shudder and possible transmission failure. As a counter measure, the lock-up clutch must have a high cooling capacity.

Cooling Capacity
Sufficient cooling capacity of the lock-up clutch is very important. The power losses that are generated at the friction surface can cause high peak temperatures, which can destroy the oil additives and, in interaction with the mechanical shear load, split the molecule chains. This causes a negative friction gradient, which can lead to shudder and finally can cause failure of the transmission.

The thermal loading of the high performance lock-up clutch is mainly dependent on the lock-up strategy. The strategy will determine the maximum power loss and the magnitude of the total energy, to be absorbed at the friction surface. The following is valid:

>The lower the rpm at which the lock-up engages (lugging limits), the higher is the needed slip to avoid noise and, therefore, the higher the power losses.

>The lower the gear, the more critical are tip-in reactions. Immediately after the tip-in the lock-up clutch is slipping, which is meant to eliminate the usual and annoying surging vibration. However, this leads to high power losses.

>The lower the gear and the lower the lugging limits, the more comfortable, and with this longer, the engagement of the clutch needs to be. The longer the engagement, the higher is the total energy absorbed. Figure 12 shows a comfortable engagement. The maximum power losses are 7 kW during this non-noticeable engagement.

Things to consider:
Inadequate cooling of the torque converter and its clutches may lead to shudder, degradation of the clutch friction material, and possible transmission failure. The graphs along with this article show the relative heat buildup in a torque converter clutch as a function of time (the red lines, marked as TCC). Note how the heat builds up rapidly as the throttle is activated. So starting off can be hard on a torque converter and its clutch, and the transmission system must be designed and built to remove this heat buildup.


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