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The Technician’s Guide to Car Transmissions

Photo by Vestman / Flickr / CC BY 2.0

The transmission of a car is an important part of the vehicle’s powertrain system, which also includes the engine. The engine produces power, but it is up to other parts, including the transmission, the drive shaft and final drive, to help turn engine power into motion.

Currently, there are two basic types of transmissions available for passenger vehicles — manual and automatic.

They both serve the same function, but each uses different methods to fulfill its purpose.

A Brief History of Car Transmissions

A 6 gear automatic gearbox with integrated, electronic all wheel allocator. Photo by Kecko / Flickr / CC BY-ND 2.0
A 6 gear automatic gearbox with integrated, electronic all wheel allocator. Photo by Kecko / Flickr / CC BY-ND 2.0

In the early days of automobiles, the only transmissions available were manual. Then and now, manual transmissions require drivers to depress a clutch while operating a gear shift to change gears, whether downshifting (switching to a lower gear) or upshifting (switching to a higher gear).

Automatic transmissions “automatically” switch gears, eliminating the need for a driver to operate a clutch or manually shift to a lower or higher gear.

Engineers began experimenting with the concept of an automatic transmission practically as soon as the automotive industry was born. Rudimentary prototypes appeared as early as 1904, with several patents issued to various inventors during the 1920s. Semi-automatic transmissions (which still required a clutch, but automated some of the shifting) were developed in the 1930s, but they did not gain widespread adoption among the U.S. automakers or their customers.

The first automatic transmission that modern drivers would easily recognize as such did not arrive on the market until General Motors offered it as optional equipment on 1940 Oldsmobile. The company named its transmission the “Hydra-Matic Drive”, and after successfully testing it on its Oldsmobiles, the company made it an option for its Cadillacs.

Photo by Joe Ross / Flickr / CC BY-SA 2.0
The Oldsmobile was the first car with automatic transmission. Photo by Joe Ross / Flickr / CC BY-SA 2.0

Over the past 75 years, there have been many engineering improvements made to the automatic transmission, as well as its manual cousin. However, the fundamental mechanics behind each remain little changed from the days of the automotive pioneers who first developed them. Modern transmissions may be smoother, more precise and less troublesome, but their function is still to connect the engine’s power to the automobile’s wheels.

People often wonder what the benefits and advantages are for each type of transmission. Before listing the pros and cons, however, an overview of each transmission type might be helpful.

Manual Transmission

Pixabay.com / CC0 1.0.
Pixabay.com / CC0 1.0.

At one time, all cars were equipped with manual transmissions as automatic transmissions had yet to be invented. Although manual transmissions have become less common in U.S. cars, there are still some drivers who prefer a manual transmission as they consider it sportier and more fun than an automatic.

A manual transmission consists of numerous components. One of the most important is its gearbox, which is a collection of gears that can be selected for different driving conditions. In relation to the speed of the engine, the wheels gripping the road turn slower at lower gears and faster at higher gears. Thus, the lower gears are best suited for accelerating from a stop or climbing a hill, while the higher gears are more suitable while moving at higher speeds, such as cruising on a highway.

Number of Gears

Most modern cars have a gear for reverse and four or five gears for forward movement although some sports cars offer six forward-movement gears. Neutral is not actually a gear; rather, it is a position in which the gears transmitting movement are disengaged. Gears are engaged when the driver selects a gear with the gearshift. Because of the distinctive way in which the gears are switched, manual transmissions are sometimes referred to as “H-pattern transmissions.”

Although the pattern could vary, the most common arrangement was that neutral was in the middle of the double H. The first gear is left and up, second gear is left and down, third is middle and up and fourth is middle and down. The fifth gear is right and up and the reverse is right and down. Some cars include an additional gear, commonly called overdrive. This is a higher gear than the standard top gear and is designed to be used while cruising at speed to improve fuel economy.

How Gears are Shifted

To shift gears, the driver depresses a clutch with his left foot, moves the gearshift into position and then releases the clutch while pressing the accelerator with his right foot. The gearshift moves a selector rod to mesh the proper gear with the shaft. The gearbox is typically attached to the rear of the engine. Selector rods are inside the gearbox and may be on the sides or at the top. The selector rods run parallel to shafts that carry the gears.

Most modern cars have what is called a constant mesh gearbox, which synchronizes the gears to eliminate “grinding” gears when the driver’s shifting technique is improper. This design incorporates three shafts besides those carrying the gears:

  • The input shaft, which is rotated by the engine. The flow of power between the engine and the input shaft is adjusted through a friction clutch.
  • The countershaft, also called the lay shaft, which is driven by the input shaft. It turns the gears on the transmission’s main shaft. These gears engage a dog clutch to lock the gear securely to the main shaft.
  • The main shaft, also called the output shaft, which is responsible for transferring the energy to the wheels.

The gears on the shafts are either fixed in place or can rotate freely:

  • The gears on the countershaft and input shaft are fixed – they can only rotate when the shaft rotates.
  • The output or main shaft gears rotate freely unless locked by a dog clutch and synchromesh device.

A synchromesh device is essentially a toothed ring that is mounted on a toothed hub. When drivers operate the gearshift, they are actually operating the synchromesh device by means of a selector rod. Once the driver chooses a gear, the cone-shaped, matching surfaces of the gear and the hub transfer power from the rotating gear to the shaft by passing through the hub. This synchronizes the speeds of the two shafts.

Automatic Transmission

Most people find that driving a car equipped with an automatic transmission is rather simple — move the shift lever into “Drive” and go. The transmission will automatically make any changes to the gear ratio that are needed. However, while it might be simple to drive a car with an automatic transmission, the underlying technology is rather complex.

The Torque Converter

The torque converter is the automatic transmission’s equivalent to the clutch. It connects to the engine by means of the input shaft, which lies next to — but does not touch — the crankshaft on the engine. The torque converter multiplies the power generated by the engine.

  • The torque converter uses hydraulic transmission fluid to help transfer energy.
  • Within the torque converter is a pump (impeller), a turbine and a guide wheel (stator). The turbine and impeller face each other, and they both have blades that spin from the pressure of the transmission fluid. These blades move the fluid to the stator, which in turn redirects it to the turbine. It is this continuous flow of fluid that multiplies the power sent from the engine.
  • The torque converter also drives the transmission fluid pump. This pump produces the pressure needed to activate the brakes and clutches in the planetary gear set.

The Planetary Gear Set

A planetary gear set. Photo by Elacy / Wikimedia Commons / CC BY-SA 3.0
A planetary gear set. Photo by Elacy / Wikimedia Commons / CC BY-SA 3.0

The planetary gear set resembles a solar system. There is a central (sun) gear, an outer ring gear with internal teeth and two or three rotating “planet” gears between the sun gear and the outer ring gear.

  • A planetary gear set adjusts the gear ratios by using brakes and clutches to control which components are rotating and which are stationary. In short, this determines which gear is selected — automatically.
  • The Ravigneaux planetary gear set incorporates two complete planetary gear sets on the same carrier. This extends the number of speeds that can be provided, yielding smoother transitions. The Ravigneaux system is also less expensive to manufacture than other options that used two separate planetary systems as the carrier is normally the most costly (and heaviest) of the components.
  • The Ravigneaux system is also less expensive to manufacture than other options that used two separate planetary systems as the carrier is normally the most costly (and heaviest) of the components.

Which is Better — Manual or Automatic?

Asking whether a manual or automatic transmission is better won’t give you a definitive answer. Both have advantages and disadvantages, but ultimately, a great deal depends on individual preference.

Manual Transmissions:

  • ✔ Give sense of greater control.
  • ✔ Are typically less expensive to repair or maintain.
  • ✔ Often (but not always) provide better fuel economy.
  • ✔ May lower the purchase price on a new car.
  • × Require more effort from the driver as they are more complicated.
  • × Require drivers to master accelerating from a stop on hills to prevent rolling back.
  • × May feature “tight” clutches that are difficult for some drivers to operate.
  • × Can be tiring in stop-and-go traffic; require frequent clutch work.
  • × May make a car harder to resell.

Automatic Transmissions:

  • ✔ Are easier to operate as the transmission handles the shifting.
  • ✔ Are more convenient in overall and congested traffic.
  • ✔ Are a better choice in hilly areas.
  • × Are more expensive to repair.
  • × May consume a little more fuel.
  • × May increase the purchase price of a new car.

In Conclusion

There is one more issue with automatic and manual transmissions: not all models of cars are available with a choice of transmissions. Some cars are sold only with automatic transmissions only, while others are only available with manual transmission.

What type of transmission do you prefer? Have you tried both? Which one did you think was better?

Since there is no universal agreement on which transmission is better, each driver must consider availability, cost and personal preferences when deciding between an automatic and a manual transmission.