How A Transmission Works

September 12th, 2016 by


Kia of Cerritos has heard this question many times. The Car Connection has a lengthy piece and we’ve shared the nuts and bolts of it here.

The transmission is one of the most important, as without it, the power from the engine could never reach the wheels, and your car wouldn’t be much use.

There are two major types of transmissions in most cars on the road today: automatic and manual. We’ll go into more detail on how those work in a moment, but it’s important to note that several other types of transmission have also been gaining ground in recent years. The dual-clutch transmission functions on principles similar to that of the manual transmission, but with computer control, bridging the gap between manual and automatic. The continuously variable transmission (CVT) does away with individual gears altogether, ramping up its power transmission ratio according to a set of rules. And electric cars often don’t have a transmission at all, strictly speaking, having just a single fixed ratio to transmit power to the wheels.

But before we delve into the way the manual and automatic transmissions work, we should define some key terms:

gear: in this context, a gear is a set of toothed wheels that work together to determine or alter the relation between the speed of a vehicle’s engine and the speed of its wheels; it is also the term used to describe each of the gears a driver can select, which is in fact a ratio of the selected gears on the input and output shafts.

gear ratio: the ratio between the rates at which the input and output of a set of gears rotate

clutch: a mechanism for connecting and disconnecting a vehicle’s engine from its transmission system

transmission: a mechanism for transmitting power from a vehicle’s engine (or motor) to its wheels

shift lever: a control lever which the driver uses to manage the current gear (or gear range) of a vehicle’s transmission

H pattern: an arrangement of gears, usually designated on the shift lever knob, where in the gears are arranged in a series of parallel rows, reminiscent of an “H” with extra legs

Now on to how the two most common types of transmissions work.

The automatic transmission is far and away the most popular kind on the road, and the most popular in new-vehicle sales. But the manual transmission is simpler in its construction and function, so we’ll start with it in the descriptive process. Much of the function of the manual transmission can be carried over to, or analogized to the study of automatic transmissions.

In its most basic form, a manual transmission consists of a set of gears along a pair of shafts, the input shafts and output shafts. The gears on one shaft engage with those on the other shaft. The resulting ratio between the gear selected on the input shaft and the gear engaged on the output shaft determines the overall gear ratio for that “gear.”

The driver selects gears in a manual transmission by moving a shift lever, which engages a linkage that controls the movement of the gears along the input shaft. Moving the lever forward or rearward chooses between the two gears available on a given linkage; cars with four gears, or speeds, use two linkages; cars with five or six speeds use three linkages. The driver changes between linkages by moving the shift lever left and right.

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