While the majority of us will know a certain amount about cars - at least enough to keep them running well - there are a variety of things that many of us will not know. This is especially true when it comes to the differences between a clutch and an automatic transmission.
While there are a few slight differences between the two, especially in how they operate, the primary difference is the addition of a hydraulic torque converter, which replaces many of the functions of a clutch.
In manual cars, the clutch is responsible for the connection between the transmission and the engine; this means that it's chief function is to ensure that your wheels move when you want it to, as well as the appropriate speed. This clutch assembly also prevents a car from stalling through the use of a throttle stop, which is subsequently driven by the air/fuel mixture that it allows into the cylinders.
With that in mind, a clutch is responsible for the disconnection and eventual reconnection that's needed when slowing down at the likes of traffic lights and then speeding back up. This is naturally vital to keep an engine running smoothly and avoiding the car needing to be restarted regularly.
However, this changes completely with an automatic car, which doesn't have a clutch system installed, although it still has systems installed that achieve many of the same results seen in a manual car. This is primarily done through the installation of a hydraulic torque converter, although it's done through a much different process.
This has led to many people wondering how the converter achieves this. The device uses what's known as a fluid coupling, which transfers the rotational energy from the engine to another device, which then extends it; this bypasses wheels and allows you to slow down.
However, it should be noted that the engine doesn't fully disconnect during this process; instead, it simply reduces the amount of torque that's delivered to your wheels and then ramps it back up when needed. This is something that many of us can feel when driving an automatic car; when our foot is taken off the break, there may be a slight 'creep' when you're coming from a standstill.
This overall process is controlled through the passing of fluid throughout the torque converter and its connected devices, hence the 'fluid coupling' name.
This is done through a pump that sends an appropriate amount of fluid throughout the engine, which is heavily dependant on the crankshaft's rotation. When connected to a turbine, this liquid is passed through the turbine’s vanes, which then dictates the amount of torque that's passed through the input shaft and into your car's transmission.
Throughout this process, your hydraulic torque converter will be attached to your flywheel through its casing, which makes it spin at the same rate as the crankshaft. The converter houses a variety of tools that allow it to act in much the same way as a clutch. This includes a fluid centrifugal pump and stator, both of which are contained within the turbine.
Both of these have a drastic effect on the torque converter's efficiency, especially the stator, which acts as a barrier between the turbine and transmission. This means that the fluid will be passed through to the turbine instead of the pump, which has various benefits in terms of how much torque it will be able to generate.
When your car is idle, this torque will naturally be quite low, as you'll need none to remain still; once the engine is engaged, the crankshaft will increase its rotation speed, thus improving the overall torque that's sent to your wheels.