All modern automatic transmissions utilize a torque converter which is located between the transmission and the engine. The torque converter is a device that looks similar to a large bagel which makes the ‘clutch-less’ smooth-shifting found in automatic transmissions possible. Let’s take a look at how it works.
The basic function of the torque converter is to multiply the turning force of an engine while at the same time replacing the clutch found in manual transmissions. The engine is connected to the torque converter by means of the crankshaft. The crankshaft ends in the torque converter, which is also where the main input or transmission shaft ends as well. However, these two shafts are not connected to each other so there isn’t any physical connection between the engine and transmission in any way. The two shafts have the slightest gap between them and are immersed in transmission fluid.
The transmission shaft is turned by the crankshaft by means of a process called hydraulic coupling. As all fluids are not easily compressed, this property is utilized in the automatic transmission. The hydraulic fluid used is the automatic transmission fluid (ATF) which fills the entire transmission and torque converter.
The one downside to utilizing hydraulic coupling is that it causes automatic transmissions to be slightly more inefficient than their manual counterparts. The fluid connection between the crankshaft and transmission shaft allows some slippage to occur. As a result, there is a two to eight percent loss in energy transferred from one shaft to the other. This is the reason why automatic cars generally get slightly worse fuel economy than the manual version of the same car model.
Inside the torque converter casing, there are three main components: the impeller (pump), stator (guide wheel), and turbine. Hydraulic coupling is also at work within the torque converter itself, and to improve the efficiency of energy transfer, both the impeller and turbine have blades that are designed to increase the surface area that the fluid can act on.
Inside the transmission fluid-filled torque converter, the impeller is located on the engine side while the turbine is situated on the transmission side. When the impeller starts to spin, the turbine wheel spins in tandem with it through the hydraulic coupling. As this happens, centrifugal forces cause the fluid to move towards the outer edges of the blades, where the stator redirects the fluid towards the turbine side of the torque converter. This continuous flow of transmission fluid is what causes the torque converter to multiply the turning power applied by the engine.
The majority of modern torque converters will also include a lock-up feature through the use of a torque converter clutch. The highly pressurized fluid flowing through the transmission is channeled through the transmission shaft as the speed of the vehicle nears forty miles an hour. When this happens, the torque converter clutch or a metal pin, physically ‘locks’ or connects the turbine to the impeller.
The pin remains connected until either the vehicle slows down below forty miles an hour or a gear-shift occurs. This feature was developed and incorporated into the modern torque converter to improve the highway fuel economy.
The pin remains