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How Automatic Transmissions Work

admin | Mar 19, 2012 | Comments 2 9,243 views

Automatic Transmission Overview

When driving a vehicle, the driver operates the steering wheel, the gearshift lever, the accelerator pedal, the clutch pedal, the brake pedal and other similar controllers in response to the condition of the road and to the type of driving which is being performed. In this, the clutch pedal and the gearshift lever are required to be operated with great frequency, placing a considerable burden on the vehicle’s driver. Devised as a means of reducing this burden, the automatic transmission (or AT) allows the vehicle to automatically perform gearshift operations in response to load, speed, and other relevant conditions.

2. Basic Construction

The principle components of an automatic transmission are the torque converter, the gear unit, and the hydraulic control unit.

Automatic Transmission

The function of the torque converter mechanism is to transfer the engine’s output power to the gear unit through the action of a special fluid. However, it also operates in the complete reverse also. Consider an example where the vehicle is stopped — it will be important for the torque converter’s fluid to absorb any variations in speed and to “not transfer” engine power in such a situation.

Furthermore, the torque converter has another important role to fulfil in the operation of the vehicle; specifically, it must function to increase the engine torque in a set output range.

However, it will not be possible to deal adequately with all possible situations using torque increases generated by the torque converter alone, and for this reason, further changes to the driving force are made to the gear unit in accordance with the current driving condition. In addition, the gear unit is also called upon to implement the necessary gear changes when the vehicle is to be driven in reverse.

The hydraulic control unit converts engine load, speed, and other similar variables into hydraulic signals and controls the shifting of the gear unit in accordance with the corresponding hydraulic pressures. This component also performs control of the lock-up clutch, which will be discussed later in this article.

3. Torque Converter

Learn More: Torque Converter

4. Gear Unit

Generally speaking, the gear units in automatic transmissions are essentially common with those used in manual transmissions. At present, 4-speed transmissions are most widely used.

1 2nd clutch
2 4th clutch
3 Mainshaft 4th gear
4 Mainshaft 1st gear
5 1st clutch
6 Countershaft reverse gear
7 Reverse selector hub
8 Reverse selector
9 Countershaft 2nd gear
10 3rd clutch
11 Mainshaft 3rd gear

The most fundamental difference between the gear units of automatic transmissions and of manual transmissions is the fact that the hydraulically-controlled multiple-disc clutches are implemented in the former instead of synchronizers. These clutches are used to connect gears to the shafts so that gearshifts may be implemented.

1 Clutch discs
2 Snap ring
3 Clutch end plate
4 Clutch plates
5 Snap ring
6 Spring retainer
7 Return spring
8 Disc spring
9 Clutch piston
10 O-rings
11 4th clutch drum
12 3rd clutch drum

However, in the case of reverse gear, hydraulic pressure is used to actuate a shift fork so that the counter shaft and counter reverse gear can be engaged.

5. Gearshift Mechanism

In an automatic transmission, the shifting of gears within the range selected by the driver is performed in an automatic fashion. Specifically, the motion of the gearshift lever is directed via a control cable to the hydraulic control unit’s manual valve. This action allows fluid passages within the control unit to be switched over so that the correct range can be selected. Note that in the D4 range, automatic selection is performed from 1st gear to 4th gear; in the D3 range, from 1st gear to 3rd gear; and in the 2 range, shifting is fixed in 2nd gear. In addition, reverse gear is selected in the R range, and the countershaft is mechanically immobilized in the P range.

Automatic Transmission Shift lever

6. Hydraulic Control Unit

Hydraulic pressure is used in a number of situations inside the automatic transmission for the purpose of control. This hydraulic pressure is supplied by an ATF pump which is driven by the torque converter housing’s pump drive gear.

The hydraulic control unit receives signals relating to the driver’s desired operation, the current condition of the vehicle, and other similar factors — these signals having been converted into hydraulic form. Valves inside the control unit operate in response to the hydraulic signals, switching the channels along which flow will occur. For example, by applying the pressure in the 2nd clutch channel while releasing pressure in the 1st clutch channel, it is possible to switch from 1st gear to 2nd gear smoothly. All other gear-shifting operations can be performed in a similar manner.

The hydraulic control unit contains the following valve bodies.
• Servo valve body
• Governor valve body
• Secondary valve body
• Regulator valve body
• Main valve body
• Lock-up valve body

Main valve body

1 Servo valve body
2 Governor valve body
3 Secondary valve body
4 Regulator valve body
5 Main valve body
6 Lock-up valve body

7. Differential Valve

One of the most frequently used components in a hydraulic control unit is a type of valve known as a differential valve. When hydraulic pressure is applied to a differential-type valve, differences in the areas to which this pressure is applied will cause the valve to actuate in a specific direction.

For example, if a uniform pressure was applied to the valve from the following diagram, the valve would move to the left due to the fact that the exposed area is greater on the right than on the left.

Alternatively, the valve would move to the right if the difference in areas was similar to that
shown in the following diagram.

By assembling a number of these valves together, it is possible to perform switching within
hydraulic circuits.

8. Automatic Transmission Shift Change Method

By these two pressures, the unit moves the shift valves to change the gear clutch to which the hydraulic pressure is applied to engage a gear.

As mentioned previously, the hydraulic control unit changes the vehicle’s gearing in response to various vehicle conditions. To do this, the hydraulic control unit uses the hydraulic pressures which represent the engine load and vehicle speed.

Valve Body

Detection of Engine Load
The detection of engine load is carried out using throttle valve A, which moves in synchrony with the opening and closing of the throttle. The hydraulic pressure corresponding to the current degree of throttle opening (i.e., the throttle-A pressure) is generated.

Detection of Vehicle Speed
The governor valve regulates the line pressure in accordance with the vehicle speed, when the vehicle speed is low, the governor valve opens slightly. When the vehicle speed is high, it opens widely. In this way, the line pressure is modified to the governor pressure that represents the current vehicle speed.

Looking at the following schematic diagram, the connections between the 1-2, 2-3, and 3-4 shift valves can be identified. Each of these valves is of a different diameter, and furthermore, the strength of the integrated spring varies from valve to valve. Also, because the current situation corresponds to times when the accelerator pedal is depressed, the left side of each of the valves is exposed to the throttle pressure; the right side, to the governor pressure.

In such a case, the line pressure which exists at the top of the diagram is primarily applied to the 1st clutch, causing the1st gear to engage.

Then, because the condition of balance between [throttle pressure + spring force] and [governor pressure] within the 1-2 shift valve then causes the passage to be opened, hydraulic fluid can flow into the 2-3 shift valve.

Within the 2-3 shift valve, the passage to the second clutch is closed and the passage to the 3-4 shift valve is open; consequently, hydraulic fluid flows to the 3-4 shift valve without operating the 2nd clutch.

In addition, the 3-4 shift valve’s passage to the 4th clutch is closed while that to the 3rd clutch is open. As a result, the line pressure will be applied to the 3rd clutch and the 3rd gear will be engaged.

In accordance with this situation, although both the 1st and 3rd gear will be engaged, torque will only be transferred by the latter due to the fact that the counter 1st gear is set to freewheeling at this time by the one-way clutch.

If the accelerator pedal were to be suddenly depressed while in this condition, the accompanying increase in the throttle-A pressure would cause valves to move to the right so that either 1st or 2nd gear would be selected.

All other gears are selected in a similar manner to that detailed above.

9. Automatic Transmission Fluid (ATF)

A fluid known as “automatic transmission fluid” (or ATF) is used within an automatic transmission. This fluid functions as a hydraulic fluid inside both the torque converter and the hydraulic control unit and as a lubricant inside the gear unit. In addition, ATF is directed to all parts of the automatic transmission by an ATF pump.

10. Electronically-controlled Automatic Transmission

As its name suggests, an electronically-controlled automatic transmission (or E-AT) combines the operation of an automatic transmission with electronic-type control. In such a set-up, throttle opening and vehicle speed signals — which are conventionally communicated by hydraulic pressures — are detected in an electrical fashion. These and other signals are then used by the transmission control module (or TCM) to determine how the hydraulic control unit’s solenoid valves should be controlled. In this, it is possible to perform much more precise control of the transmission.

Furthermore, more advanced articles can determine the type of road on which the vehicle is currently travelling (i.e., whether it is level or inclined, etc.) and can control speed accordingly.