Overdrive Mechanism for Cars - An Overview

Since the health of our planet became a significant societal focus decades ago, automotive engineers have been on a mission to find a balance between performance and efficiency. In the past, these two characteristics were typically at odds with each other. However, technological advancements have blurred those lines, and today's transmissions are less clunky, more efficient, and snappier than ever.

When the automatic transmission first debuted in the early 1920s, it had only a few gears, which were easy to count on one hand. However, as transmission technology has advanced, more gears have been added to improve efficiency and performance. Honda has already developed a triple-clutch transmission with 11 gears, demonstrating how far we have come. One of the critical advancements in transmission technology is the introduction of overdrive gearing.

Overdrive is a transmission gearing that lowers the engine's revolutions per minute (RPM) at select times to bring numerous beneficial effects. To understand the working of overdrive mechanisms and their benefits further, read below.

Overdrive Mechanism: What Exactly Is It?

Overdrive may sound like a term related to a vehicle's high-performance attributes, but in reality, it is quite the opposite. This feature can be activated or deactivated with a simple button and is designed to reduce the engine's revolutions per minute during sustained high-speed driving, resulting in improved fuel efficiency. In addition, overdrive minimizes the strain on the engine, resulting in a quieter, smoother operation and less overall engine wear. Over time, consistent use of overdrive can reduce engine maintenance issues and extend the life of a vehicle's powertrain.

Overdrive serves as a high-gear mode for a vehicle, adding an extra gear that expands the transmission's shifting range at its top end. By shifting to a higher gear, the powertrain reaches lower gear ratios (the higher the gear, the lower the ratio), enabling the engine to operate at lower RPMs during highway cruising of 50 miles per hour or more.

While a vehicle typically operates at lower gears with greater output and torque, overdrive inhibits acceleration and maximum power. It means the vehicle is over-geared or overdriven, sacrificing top speed and performance for better fuel economy and a more effortless driving experience.

Today, there are many cars that use overdrive mechanisms. High-end vehicles like the Mercedes E-Class and Audi A6 prioritize luxury and a smooth riding experience above all else. To accomplish this, these vehicles are equipped with automatic transmissions that feature overdrive, which enhances their luxurious feel.

Benefits of Overdrive

• Improving Efficiency and Performance on Long Drives

When embarking on a long drive, drivers typically shift our car's transmission to the top gear, a direct drive that causes the final output shaft to rotate at the same RPM as the engine. However, this can increase the load on the engine's shaft, leading to decreased fuel efficiency and potential damage to the engine over time.

To address this issue, car manufacturers have integrated overdrive into their transmission systems, which can help reduce the engine's load and provide higher speeds than the engine's RPM during extended drives.

• Fuel Efficiency

In countries like India, where fuel economy is a primary concern for middle-class people, overdrive can be an effective solution. As discussed earlier, overdrive reduces the engine's load, improving fuel efficiency. By minimizing the strain on the engine, the vehicle can operate more smoothly and efficiently, resulting in cost savings for the driver. 

• Vehicle Maintenance

Using overdrive can also have a positive impact on the maintenance of a vehicle. By reducing the engine and transmission box load, overdrive can help decrease wear and tear on these critical components. The vehicle's overall maintenance needs can be reduced, saving drivers money and time in the long run. Additionally, by minimizing stress on the engine and transmission, the vehicle may experience fewer breakdowns or mechanical issues.

How Does Overdrive Mechanism Work?

The principle behind car overdrive is based on the same concept as any other sun and planetary or epicyclic gear arrangement in an automatic gearbox. Changing the position of any of the sun, planetary, or annulus gears alters the power output of the drive.

Overdrives can be activated or deactivated in a car using various actuation methods such as electrical, magnetic, or pneumatic systems through a button or knob. Let's first consider both the enable and disable cases to understand better how overdrive works:

Overdrive Enabled

When a driver enables overdrive, the sun gear is fixed in place, causing the planetary gears to rotate the annulus gear. It means that the input from the input shaft is transferred through the planetary gears to the annulus gear, resulting in overdrive. As a result, the output shaft rotates at a higher RPM than the input shaft due to the higher reduction ratio of the planetary gears and annulus.

Overdrive Disabled

When the driver disables overdrive, the input shaft passes through the sun gear, causing it to rotate along with the constantly meshed planetary gears. These planetary gears then rotate the annulus gear, resulting in a direct drive where the output shaft rotates at the same RPM as the input shaft.

Overdrive in Manual Vs. Automatic Cars

Both manual and automatic cars can offer overdrive functionality, which allows the vehicle to achieve its highest gear ratio for more efficient operation at highway speeds. In automatic transmissions, overdrive is typically an intuitive function initiated by the vehicle's ECU (electronic control unit) when the vehicle reaches a certain speed. In manual gearboxes, on the other hand, overdrive is the highest gear but must be engaged by the driver.

For instance, overdrive is typically the fifth gear in a five-speed transmission, while in a six-speed transmission, it is the sixth gear. In a manual transmission, the driver actively engages overdrive by physically transporting into the highest gear. Engaging overdrive in manual transmission increases fuel efficiency and lowers engine RPMs while cruising at high speeds.

We hope that you now have a better understanding of the overdrive mechanism and its advantages. To learn more about the technologies and techniques deployed in teh automotive industry, enroll in Skill-Lync’s Post Graduate Program in Complete Passenger Car Design & Product Development. Talk to our experts to get your free demo.