What does an automatic transmission use instead of a clutch?
Since its invention in 1886, cars have been running only sticks for over 25 years until the invention of automatic transmission in 1921 by Alfred Horner Munro. So, automatic transmission has been around for ages, 95 years to be exact, yet have you ever pondered, what makes an automatic tick? Why are there only 2 pedals in an auto transmission instead of the traditional 3? What does an auto use instead of a clutch? It’s all witchcraft and wizardry, not the traditional sleight of hand, mumbling incantation, casting spell kind of wizardry but a magic nonetheless, the magic of mechanical ingenuity fluid dynamics in the form of a Torque Converter.
With thousands of intricate moving parts that needs to be synchronized, delivering power from any powertrain could a legitimately complicated process. All the components need to be meticulously engineered to allow them to smoothly keep your vehicle on the move when you just shift a gear and step on the gas. In a manual transmission, the clutch acts like a bridge between the engine and the transmission, connecting and disconnecting the drive to the wheels. There is an ideal limit for the engine to work, below which there isn’t enough air/fuel mixture to actually keep the engine turning so it starts to stall. A clutch essentially applies a stopper over the throttle allowing the car to idle without shutting down the engine. When slowing down to a stop, an engine would stall as the feedback from the transmission would take it below its workable rev limit in the absence of a clutch. There needs to be a disconnection between the transmission and the engine to keep the engine running and then connect again along with some throttle to keep the vehicle up and running.
However, in an automatic transmission there is no clutch, well not a traditional one anyway. Instead what it has is a torque converter. Essentially it functions in the same way as a regular clutch, it ascertains that the engine keeps running while the transmission slows down to a stop. What it does in not as spectacular as the how; the torque converter implements the concept of fluid coupling, allowing the rotational energy of one moving component to be transferred to another.
The torque produced in the powertrain and delivered to the transmission is subdued immensely by the torque converter, hence the engine can rotate freely even when the car creeps to a halt. There is never any disconnection however, this can be observed in an auto transmission because it will slowly inch forward from a standstill if the driver takes his foot off the brake pedal.
The torque is controlled via a pump that sends fluid around the torque converter, the force of which is determined by the revolution of the crankshaft. Inside the converter, there is a small turbine that rotates on account of the pumped transmission fluid hence it acts as a measure for the amount of torque that makes its way to the transmission.
The Exterior of the torque converter is connected to the flywheel (used to store up rotational energy) hence it spins at the same speed as the crankshaft. Contained by the housing is the turbine, the centrifugal fluid pump and the stator which surrounds the rotator. The pump throws back the fluid into the turbine which generates torque which is transferred to the transmission. The stator prevents the fluid from flowing back to the pump thus greatly increasingly efficiency of the whole system. When idling, the rate of fluid pumped to the turbines is quite slow hence only a meager amount of torque is being transferred to the transmission. With increased throttle, the rate of fluid dissipation increases, the crankshaft rotates faster which results in the flywheel rotating faster and propelling fluid at a higher rate into the turbine. The turbine, in turn, spins faster which allows more torque to be transferred to the transmission.
As with all systems, there is bound to be energy loss, and the automatic transmission is no different. The loss in energy is amplified since the engine torque has to transfer through the gearbox as well. This loss might be very little but it puts a large impact on the transmission system. The inefficiency results in the turbines spinning at a lower rate than the pump, this is primarily the reason automatic transmissions have been fuel inefficient compared to their manual equivalents. Recent developments in the torque converter technology have recessed the gap a little though, now they are designed to “lock up” at certain speeds allowing the pump and the turbine to spin together and the loss of energy is minimized to the point of being almost nonexistent.
However simple it appears from being behind the wheel, the engineering and ideation that went into developing an automatic transmission is quite complex. It’s a spectacular feat of technological innovation worthy of anyone’s respect. Connecting and controlling the wheels to the driver and seamless connection between the engine and the drivetrain is incredibly effective, so much so that most people take it for granted. With so many vehicles trotting out of the production line with fully automatic gearbox, the pedal driven clutch mechanism is becoming quite redundant. That’s not to say people doesn’t enjoy it, quite the contrary, petrol-heads prefer the full control of the vehicle, shifting gears and feeling the grinding of the metal sliding into place but for the ordinary folks going from A to B, torque converts are a god send.