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Car Transmissions Explained

Niel Stender

A modern car transmission comes in three flavors – manual, automatic or continuously variable (CVT). Let’s look at the basics of each one.

Three Options for a Car Transmission

A modern car transmission typically comes in one of three flavors – manual, automatic or continuously variable (CVT). Regardless of configuration, the transmission’s purpose is to transfer engine power to the wheels. Specifically, a car transmission ensures the proper amount of power is delivered to the wheels in relation to how much power the engine is producing. Let’s look at the basics of each one.

Manual Transmission

Manual Transmission shift lever
Manual Transmission shift lever

Like its name implies, a manual transmission requires the driver to manually select gears via the shifter, which is often situated in the center console or on the floor. Inside the transmission, there are three shafts– input, intermediate and output – with intermeshed gears of varying sizes. The input shaft connects with the engine, the output shaft connects with the shifter and the intermediate shaft is in between to facilitate gear changes between the two.

When the driver selects a gear, a cable transmits this movement by locking the selected gear on the transmission output shaft. That output shaft has a series of gears affixed to it. The selected gear hooks up with the appropriate intermediate shaft gear, which in turn meshes into the input shaft gear. The input shaft connects to the clutch plate, which can be engaged or disengaged from the flywheel, via friction, using the clutch pedal.

Clutch and transmission - Learn Engineering on YouTube.com
Clutch and transmission - Learn Engineering on YouTube.com

That flywheel is attached to the crankshaft and therefore rotating at engine speed. When the driver depresses the clutch pedal, the clutch plate is released from its connection with the flywheel. At this point, the shifter is used to select the next gear. As the clutch pedal is released, the clutch plate re-engages with the flywheel to continue transmitting engine power to the wheels via a different gear ratio.

A lower gear ratio, like first, offers more torque and is used to get the vehicle moving from a stand still. As the vehicle picks up speed, the driver moves through the shift pattern to select the appropriate gear based on current speed.

Honda CR-Z 6-Speed Manual Transmission - hondanews.com
Honda CR-Z 6-Speed Manual Transmission - hondanews.com

Modern cars use synchronizer rings on forward gears to help smooth shifts and avoid hearing the drivers favorite phrase, “If you can’t find it, grind it!” The reverse gear, however, usually would not have this setup and requires that the car be stopped, so that the transmission shafts are not moving as the driver shifts into reverse. To avoid this inadvertently occurring at speed, a lockout mechanism will be employed, requiring the driver to slide a collar up the gear shifter or press down on the shifter before selecting reverse.

Proponents of manuals feel they have a closer connection to the car’s performance, especially in spirited driving, as you can more finely modulate the throttle depending on road conditions and weight transfer. The downside, to that connection, comes into play with bumper-to-bumper traffic requiring constant shifting. A big upside to driving a stick is the ability to start the motor, if the battery dies, by pushing it forward and then dropping the clutch. Moreover, on the car-transmission-option-box, a manual is less costly compared to an automatic. While learning to drive a stick is one of its main challenges, it ultimately becomes one of the joys of a manual transmission.

Automatic Transmission

Automatic Transmission shift lever
Automatic Transmission shift lever

Not to beat the dead horse of “like its name implies” but if your car transmission is automatic, it will change gears automatically! The shifter is normally in the center console or on the steering column and allows the driver to select Park, Reverse, Neutral, Drive and Low Gear. Unlike a manual transmission, all of the forward gear ratios are encapsulated in Drive and Low Gear so that the driver can select one and be off, without manually shifting gear as speed changes.

While manual transmissions are mechanically actuated, automatics incorporate hydraulic means to change gears via pressurized automatic transmission fluid (ATF). When a driver moves the shifter from Park to Drive, the engine control unit, or a dedicated transmission control unit, signals a solenoid to operate internal transmission clutches and lock into the gears requested.

Another key difference here is the use of a “planetary gear set” where the gears are arranged around one another in the way planets orbit the sun. There is a central “sun gear” surrounded by multiple “planetary gears”, all sitting inside the outer “ring gear”. The computer-controlled transmission will change gear ratios by determining how to engage the gears relative to one another, as efficiently as possible.

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Planetary Gear Set - Learn Engineering on YouTube.com
Planetary Gear Set - Learn Engineering on YouTube.com

Much like the clutch plate in a manual provides coupling and decoupling of the transmission and engine, an automatic utilizes a torque converter. But instead of using to friction to transmit energy between the car transmission and engine, a torque converter is filled with ATF. There are two sets of fan blades within the converter, an impeller connected to the crankshaft and a turbine connected to the transmission input shaft. The simplified version is that as the engine turns, that internal ATF starts spinning in the direction of the impeller. This moving fluid, in turn, rotates the turbine blades to provide torque for rotating the transmission input shaft.

The obvious benefit to driving an automatic is the simplicity of operation, just select what you’d like to do and move along. With the drive mode selection feature on modern cars, you can even turn a knob to change the power delivery characteristics from fuel-sipping Eco mode to high-revving Performance mode. A historical downside to automatics was their tendency to up- or down-shift when the driver is least expecting it, when looking to overtake a slower vehicle for instance. However, as today’s 9- and 10-speed automatics replace older 5- and 6-speed versions, that issue will fade away.

Dual Clutch Transmission - TREMECtv on YouTube.com
Dual Clutch Transmission - TREMECtv on YouTube.com

A variation on the traditional automatic is the dual-clutch automatic transmission (DCT) which blends the features of a manual and automatic. Like a manual, the DCT uses a clutch to transmit power, albeit more than one. There are two clutch packs, one that handles even-numbered gears and the other for odd-numbered gears. When the driver is using an even-numbered gear, the DCT can pre-select an odd-numbered gear – and vice versa – allowing for much faster shifting times. Like an automatic, there is no clutch pedal, the driver simply selects Drive and internal computers take over control of the transmission work.

Initially, the DCT was used in supercars like the new Ford GT or the McLaren lineup due to their super-fast shift speed. But this performance has trickled down to more affordable vehicles like the Kia Stinger and VW Golf R. Typically, the DCT is shifted using a console shift lever or steering wheel mounted paddle-shifters allowing anyone to channel their inner racecar driver.

Continuously Variable Transmission

Continuously Variable Transmission - Nissan USA on YouTube.com
Continuously Variable Transmission - Nissan USA on YouTube.com

The CVT technically functions as an automatic but due to its unique mechanics, can be considered a separate style of transmission. Like an automatic, the driver slides the shift lever into gear or reverse to move the car and then the computer controlled electronic systems take over. But while any of the other types of car transmissions use a fixed number of gear ratios and involve mechanical gears being pushed together, the CVT can run through any number of effective gear ratios without the noise, vibration and harshness of actual gearshifts.

The CVT has two variable-diameter pulleys, one connected to the engine and the other sending power to the wheels. Each pulley is made of two cone-shaped halves that can move towards and away from each other to change the effective diameter of the pulley. The engine and drive pulleys are connected by a v-shaped steel belt. As the cones move in and out, the v-belt moves higher on one pulley and lower on the other, which is in effect changing the gear ratio.

Honda CR-Z Continuously Variable Transmission - hondanews.com
Honda CR-Z Continuously Variable Transmission - hondanews.com

CVTs have the benefit of always being ready to deliver power since the pulleys are always running in the optimal power band. In addition, since there are no gears to grind, like in a manual, or slam together in a hard-shifting automatic, the CVT is far smoother as you increase and decrease speed. Unfortunately, early CVTs had the tendency to drone and feel “rubbery” for those unfamiliar with them. However, much of that is being improved with newer models that are programmed with simulated stepped gearing.

The car transmission is a critical system in making any vehicle run, and each type discussed – manual, automatic and CVT – comes with its own sets of pros and cons. Keep in mind, there is a lot more engineering that goes into what makes these transmissions work but this overview should give a good idea on where to start. Ultimately, the best way to find out what works best is to get behind the wheel and go for a ride!

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Niel Stender
Niel Stender

Niel got his start in the car world as a vehicle dynamics engineer with SVT at Ford. He still has dreams of running figure eights on the wet pad in an S197 GT500. These days, when not fending off karate kicks from the 3 tiny humans that run his household, he can be found on St. Louis backroads pretending to be Ken Block in his 6-speed Mazda 6.

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