You don't need to be a technological genius to understand why a car needs an engine. It provides the motive force to move the vehicle. And it is also pretty evident that it is critical to get the motive force from the engine to the wheels to drive the car. The system that does that is called the transmission. But there's more to it than a simple mechanical hookup between the engine and drive wheels.
An internal combustion engine creates power and torque in a very narrow operating range. This necessitates that the car has gears that enable the engine to operate in its range as the vehicle accelerates, reaches a constant cruising speed, and decelerates. Without that, a car's speed potential would be extremely limited.
In an internal combustion engine car with a manual transmission, the driver changes the gears. A reasonably straightforward mechanical arrangement enables the driver to use a shift lever to choose the individual gear from among a selection of gears appropriate for the immediate driving situation. Manual shifting between gears requires the frequent disconnection of the engine from and reconnection to the driving mechanism—the propellor shaft and half-shafts that go to the drive wheels—during any drive. The mechanical clutch is the means of disconnection and reconnection. It is foot-operated using the car's clutch pedal. Changing gears requires dexterity and footwork. Though doing it well can be fun, it also takes effort.
