AKRON (Nov. 17, 2000)—Does your dealership offer automotive services? If so, remember that the transmission only performs as well as the engine does.
Service personnel who ignore this fact pay dearly for it.
In this column, I´ll give you a layman´s explanation of how engines transfer power to a transmission. In my next column, I´ll explain how a poor-running engine disturbs this process and creates a false sensation of either trans trouble or trans failure. Stay with me because this is much easier to understand than you realize!
Since the very late 1970s and early 1980s, vehicle manufacturers have improved fuel economy by increasing the efficiency of automatic transmissions. Engineers achieved this goal largely by eliminating or minimizing the slippage between the engine and the transmission.
Most Tire Business readers probably aren´t engineers, but you need not be one to understand the slippage issue. Most, if not all, of you have driven a manual transmission vehicle. Here, a dry clutch provides a direct mechanical link between the engine and the trans.
The upside to the dry clutch (most of us simply call it "the clutch´´) is its fuel efficiency. When the clutch is engaged (your foot is off the clutch pedal), power is flowing from the engine to the trans as efficiently as possible. Because no power´s wasted in the transfer from engine to trans, the engine delivers superior gas mileage.
The downside to the dry clutch and manual trans is that you must disengage the clutch (push in the clutch pedal) when engine output is too low to operate the vehicle smoothly. Most drivers know the experience of shifting a manual trans into high gear too soon during acceleration or leaving the trans in high gear too long during deceleration.
To grossly simplify, the vehicle bucks or shudders violently in these situations because the engine isn´t generating enough power to satisfy the powertrain. To restore smooth operation, you must mechanically sever the engine from the powertrain by pushing in the clutch pedal.
Now, let´s apply this information to the ubiquitous automatic transmission and show how it can fool your dealership´s service department. Traditionally, automatic transmissions were not linked solidly to the engine. Instead, power flowed from the engine, through a hydraulic coupling called a torque converter and then into the transmission.
If you aren´t following me here, create a hydraulic coupling in the privacy of your own home. Place two window fans on the floor facing each other. Turn on one fan. In a moment, you´ll see that air flow from the operating fan begins turning the blades of the other one—even though the fans are not mechanically connected to each other.
The fan you turned on represents the engine side of a torque converter. The other fan represents the transmission side of a torque converter. Indeed, power does flow smoothly from the first fan to the second one. But because the fans aren´t mechanically connected, the second never turns quite as fast as the first one. Because the first fan does not transmit all its energy to the second fan, we say the process is wasteful or inefficient.
In a traditional automatic trans, transmission fluid takes the place of the air. As the engine side of the torque converter turns, it hurls fluid into the transmission side of the converter. Like the second window fan in the example above, the transmission side of the torque converter never turns as fast as the engine side.
So, energy is wasted—lost as heat—as power flows from the engine to the trans in a traditional automatic transmission. Bottom line: Fuel economy never matches that of a manual transmission vehicle.
To improve efficiency, engineers began installing a clutch plate between the two sides of the torque converter. When applied, this "wet" clutch mechanically links the engine to the trans as effectively as the traditional dry clutch in a manual trans vehicle. By eliminating slippage and energy loss between the engine and trans, this clutch allows an automatic trans vehicle to deliver fuel economy rivaling that of a manual trans.
Thanks to the marvels of computerization, a transmission computer or powertrain computer (TCM or PCM) applies this wet clutch at the appropriate times. Tune in next time to find out how a poor-running engine upsets this well-planned scheme.