The overdrive is a mechanism used in the automatic transmission system. In an automatic transmission system, “overdrive” refers to a specific gear ratio designed to reduce the engine’s RPM (revolutions per minute) and increase fuel efficiency when the vehicle travels at highway speeds. In this article, I will discuss, the components of overdrive in automatic transmission, and also the working principle of overdrive in automatic transmission.
Overdrive in Automatic Transmission
Overdrive is a device interposed between the transmission and propeller shaft to pennant the propeller shaft to turn faster than, or overdrive, the transmission main shaft. It is so called because it provides a speed ratio order that of the high-speed ratio.
The overdrive permits the engine to operate only about 70% of the propeller shaft speed, when the vehicle operates in the high-speed ranges, which in turn extends the engine life, improves fuel consumption, and reduces vibration and noise.
The overdrive is essentially suited to high-powered cars employing three-speed gearboxes, since in order to produce flexible top-gear performance a low-gear final drive may be necessary, resulting in the engine running faster at high speeds than is desired. Generally, an overdrive is fitted to the top gear only, but some sports cars have overdrive on second, third, and top gear, giving seven forward speeds. Overdrive is usually employed to supplement conventional transmission. It is bolted to the transmission’s rear between the transmission and the propeller shaft. A slightly higher rear-axle gear ratio is employed with an overdrive than without one.
The overdrive includes two essential devices, a freewheeling mechanism, and a planetary gearbox set.
Construction of Overdrive in Automatic Transmission
This Fig shows a design of an overdrive. It consists of the following parts :
- A set of planetary gear.
- A solenoid ·and pawl arrangement for locking the sun gear.
- A rail and fork assembly linked to a dash control knob for disconnecting the overdrive when not in
- A freewheel assembly or over-running clutch that drives the main shaft below the cut-in speed.
The planetary gears are used to increase speed by arranging to have the ring gear driven by the planet pinion cage when the sun gear is locked. Because the increase in speed of the main shaft decreases the power available to drive the wheels, the overdrive ratio can be used only when the engine is running fast enough to develop enough torque to offset this handicap.
The maximum speed at which the engine can do this is called cut-in speed. Below this speed, the drive is made direct by unlocking the sun gear. The ring gear is splined to the outer case of the freewheel assembly, which is a part of the overdrive main shaft. When the pawl is not engaged in the gear plate, the sun gear is unlocked and the planetary gears cannot transmit power. Then the unit is in direct drive. In this case, the power flows from the transmission main shaft to the freewheel assembly and then to the overdrive main shaft.
If the driver wants to go into overdrive, when the car is traveling above a pre-determined cut-in speed (usually 35 to 55 km/h), he momentarily releases the accelerator pedal. If the driver wants to come out of the overdrive, he merely pushes the accelerator pedal past the full throttle position. If the driver wants to lock out of the overdrive, he pulls a control knob on the car dash.
Overdrive is usually the highest gear in the transmission, often labeled as “D with an O” or simply “OD.” When overdrive is engaged, it allows the transmission to shift into a higher gear than the standard “drive” position. This results in the engine running at a lower RPM for a given vehicle speed.
Overdrive is primarily used during highway driving or when the vehicle is cruising at a constant speed. At these speeds, the engine doesn’t need to work as hard, and the lower RPM in overdrive mode can lead to better fuel efficiency. By using overdrive, the engine can run at a more efficient RPM, which can lead to fuel savings. Additionally, reduced engine wear and noise are benefits of overdrive operation.
In modern automatic transmissions, the control module, often referred to as the Transmission Control Unit (TCU), manages when to engage and disengage overdrive based on factors like vehicle speed, throttle position, and load.
Overdrive Electric Controls
The overdrive electric control serves the following purposes :
- It energizes the solenoid as the car reaches cut-in.
- It disconnects the ignition circuit momentarily.
- It opens the solenoid circuit when the Kick-down switch is closed as the driver wants to come out of
This Fig shows a wiring circuit of the electric control system used with the overdrive. When the driver wants to go into overdrive, he pushes the control knob on the dash. When the car reaches out-in speed. the governor closes its contacts to connect the overdrive relay winding to the battery.
The overdrive relay, in turn, closes its contacts to connect the solenoid to the battery. Now, the overdrive is ready to go into action. When the driver momentarily releases the accelerator pedal, the solenoid sends the pawl into a notch in the run gear control plate. This puts the transmission into overdrive.
When the driver wants to come out of overdrive, he pushes the accelerator pedal past the full throttle position. It causes the upper contacts of the kick-down switch to open and the lower contacts to close. The opening of the upper contacts causes to open the overdrive relay circuit.
The overdrive relay, therefore. opens its contacts to open the solenoid circuit. Also, closing the lower contacts in the kick-down switch causes the ignition. With this interruption of ignition system action, the engine stops delivering power and begins to slow down. With this action, the thrust on the solenoid pawl is relieved: and the. spring pressure pulls the pawl out of the notch in the sun gear control plate. It causes to underground the ignition coil and thereby permits the ignition system to function again. This series of actions takes place so quickly that no appreciable lag is noticeable in power delivery.
Benefits of Overdrive in Automatic Transmission
The use of overdrive in an automatic transmission provides several benefits, which contribute to improved fuel efficiency, reduced engine wear, and a more pleasant driving experience. Here are some benefits of using the overdrive-
- Overdrive allows the engine to operate at lower RPM while maintaining a constant vehicle speed, typically during highway driving.
- When the engine runs at lower RPM in overdrive mode, it generates less noise and vibrations.
- Overdrive also reduces the load on the transmission system.
- Overdrive gears are often used for high-speed cruising.
- Overdrive is especially useful for drivers who want to economize on fuel during long highway trips.
Disadvantages of Overdrive in Automatic Transmission
If some features in a vehicle serve several benefits then there are disadvantages too. The disadvantages are-
- Overdrive lacks the engine boost at higher speeds because they are made to increase the efficiency of the fuel.
- When the vehicle needs extra torque when climbing at higher peaks, this system lacks the torque.
- Overdrive can cause the transmission to generate more heat due to the extra gear changes.
- When you force the transmission to stay in overdrive at too low a speed, it can result in “engine lugging.”
- In some vehicles, the control over the use of overdrive might be limited, which can be frustrating for drivers who prefer more manual control over their transmission.
The overdrive system in an automatic transmission system can increase the fuel economy of the vehicle as well as the engine life. Overdrive has several advantages but comes with some sort of disadvantages also. Read the article with patience, you will get an idea of the automatic transmission system car.
Frequently Asked Questions(FAQs)
When should I use overdrive in my automatic transmission?
Overdrive is best used during highway or freeway driving when you want to conserve fuel and reduce engine noise. It’s not typically needed for city or stop-and-go driving.
Are there situations where I should avoid using overdrive?
Yes, it’s best to avoid using overdrive on steep hills, when towing heavy loads, or in situations requiring rapid acceleration, as it may not provide sufficient power in those cases.
What is the difference between “D” and “OD” on my gear shifter?
“D” stands for “Drive” and is the standard gear for everyday driving. “OD” stands for “Overdrive,” which is an additional gear used for highway cruising to improve efficiency.
Can overdrive be used in manual transmissions?
Overdrive is typically associated with automatic transmissions. In manual transmissions, you select gears manually, so there’s no separate “overdrive” mode.
Can overusing overdrive lead to transmission problems?
While overdrive is designed for highway driving, excessive use in situations like towing very heavy loads may put extra strain on the transmission and potentially lead to problems if not used wisely.