I think I know why I am seeing a momentary ATF temperature rise when doing a “coasting downshift” in both vehicles. Coasting downshift is defined as a transmission downshift (gear change to a lower gear) and a subsequent decrease in fuel flow. In a situation in which the Torque Converter Lockup Clutch is disengaged during a coasting only with no downshift, the temperature spike is very short.
System Energy Considerations:
When moving on a flat surface or up a hill, the engine is transferring mechanical energy to the vehicle, via the Torque Converter (TC), and either maintaining or increasing its kinetic energy, respectively. Engine coolant temperature increases on the hills, and reaches equilibrium when on level roads.
When the vehicle is coasting or going downhill during deceleration, the potential energy of the vehicle is being transformed as reverse Torque, via the TC, back to the engine in the form of mechanical energy to increase engine RPM for drag. Engine coolant temperature decreases because little fuel is being burned with the majority of the heat coming from the internal friction of the engine.
Detailed Mechanical Considerations:
Review:
Let’s consider a four-element torque converter (TC). The engine is on the left, TC and AT is toward the right. The flywheel is on the crankshaft of course, and the flexplate is connected to both the flywheel AND TC Impeller Pump. The Impeller pump forces fluid forward (to the left) through the stator and into the Turbine. The Turbine, splined to the transmission input shaft, reacts and drives the input shaft to the transmission gearing. Thus mechanical energy in the form of Torque is transmitted via the fluid coupling of the TC assembly.
In addition, the stator has a “one-way” clutch free wheeling mechanism. The free wheeling one-way clutch prevents stator backward rotation and stator blades redirect the oil with little energy loss so the fluid enters the impeller in the same direction of the rotation as that of the impeller.
The extra (fourth) element is the Torque Converter Lockup Clutch which is also splined to the transmission input shaft and controlled by actuator valves. In most transmissions, the lockup is only programmed to activate in 3rd or 4th gear.
In certain transmissions such as my PathFinder, the Torque Converter Lockup Clutch can be manually forced into a “lockout” condition at any time by a switch on the console shifter. I find this feature to be especially useful in ice and snow conditions in four-wheel drive, since extra drag is induced when decelerating on slippery surfaces.
During a coasting downshift transition, the turbine accelerates the oil flow towards its outside and into the impeller where the oil’s energy is absorbed in trying to increase engine speed, due to which the stator is also forced to overrun. This type of operation, although not efficient, helps to slow the vehicle by transferring some of the vehicle’s potential energy to the engine.
The fluid in an ATF provides these primary functions:
1. Lubrication,
2. Cooling,
3. Friction control at the friction surfaces
4. Wear control in the planetary gearing system and bearings.
In addition to the oil flow description given in the previous paragraph, gear translation is further impacted by a rise in the interface temperature of the on-coming band and drum and/or the interface temperature rise of the on-coming clutch and plate, which increases the likelihood of a torque reversal during the gear shift. If interface temperatures rise, so does the ATF temperature.
When a transmission’s Torque Converter Clutch is locked up to the corresponding Impeller friction surface, power flows directly from the engine to the transmission gearing. TC fluid temperature drops.
When a transmission’s Torque Converter Clutch is unlocked, the energy coupling is provided by the TC’s fluid coupling. Hence TC fluid temperature rises momentarily until equilibrium is reached.
When a “coasting downshift” is initiated, both the TC fluid temperature rises and the internal fluid temperature from the clutch packs rise.
Because the engine coolant temperature is decreasing, the ATF fluid temperature (lagging behind the coolant temperature) will be decreasing as well.
