How Does Automatic Overdrive Work?

black gear shifter

Today, most passenger vehicles feature automatic transmissions with anywhere from five to eight gears, maybe more. For many years, though, automatic transmissions had only three or four gears. While simple, these transmissions would require the engine to run at high RPMs at highway speeds. This made for a lot of noise and, more importantly, wasted fuel. So, auto manufacturers began to include an alternative. Automatic overdrive is a system that allows an engine to run at lower RPMs when cruising consistently at highway speeds. At the push of a button or the position of a shift lever, a driver can direct the transmission to engage overdrive mode.

How overdrive works

The input shaft of your automatic transmission is connected to the engine by way of a hydraulic coupling mechanism (the torque converter filled with transmission fluid) that allows the engine to run freely at low RPMs while the transmission is in gear. That way, you can stop at a light without stalling. As you accelerate, the torque converter transfers power and the transmission responds in first (low) gear. You speed up and the transmission shifts from low gear to higher gears, similar to how you might manually change gears on a bicycle (or manual transmission car). Each gear is matched to the effort the engine needs to offer for a given speed and amount of resistance from the wind or steep incline. And with each gear, the ratio of the speed of your engine to the speed of the transmission output changes.

Now, every engine has an optimal internal speed (RPMs) at which it runs at maximum power. And every car meets increasing amounts of resistance as it speeds up on the road. As long as you are accelerating, there is a range of gears (gear ratios) that can automatically match the engine speed to that resistance. For maximum fuel efficiency, the best gear is the one that results in the lowest engine speed for the circumstance.

When you are on the highway driving at a high rate of speed, there is a point at which the speed of your engine operating at peak power equals the amount of resistance to your vehicle. And there is one specific gear ratio to accommodate that balance. Once you have accelerated to cruising speed (with your transmission in overdrive mode), overdrive is engaged and the input to output ratio changes. Instead of the engine and input shaft of the transmission turning faster than the output shaft (underdrive), or turning at the same speed (direct drive), overdrive causes the output shaft to turn faster than the engine. That way, you can cruise faster with your engine running slower.

Over time, and since the development of front-wheel-drive systems, the definition of “overdrive” has changed. So has the way a transmission achieves the proper gear ratio at high speeds. But in a typical overdrive system, the transmission shifts its way up to cruising speed. Once acceleration levels off, the torque converter locks into the higher overdrive gear, effectively changing the gear ratio so that your car can travel faster at lower engine RPMs. You can feel the change as your engine relaxes when you ease off of the throttle to stop accelerating. And you can feel the torque converter unlock when you press on the gas pedal again. Or when you have to climb a steep hill.

To engage overdrive (if your car is so equipped), you either press a button on the side of the shift lever or place the lever in “Drive” (a “D” enclosed in a circle represents overdrive mode). When you want to deactivate overdrive, you simply shift to the highest gear available below the overdrive setting or turn it off with the button.

When to use overdrive and when to turn it off

If your automatic transmission has a selectable overdrive function, the button (or shift lever) will need to be in the correct position for the feature to work. Without overdrive, your vehicle will operate as usual except when you reach higher speeds. You may notice that the engine sounds like it is working harder or faster. It is. You may also see that the tachometer on your dashboard display is registering higher RPMs.

Some drivers prefer to leave overdrive off when they are driving on surface streets. This is because overdrive can cycle on and off if you need to change speeds continually or if you encounter hilly terrain. They will switch overdrive on when they are on the freeway. But most drivers tend to leave overdrive engaged all the time, unless the torque converter locks and unlocks repeatedly. You can tell because the car will surge randomly when your car meets a bit of resistance. Certainly, if you are travelling consistently at highway speeds, you should make sure overdrive is engaged. Otherwise, your engine will experience unnecessary stress, rev higher than it should, and use more fuel.

If, on the other hand, you are driving at slower speeds around town – especially if you need to change speeds frequently – you might consider switching off your overdrive. Likewise, if you encounter hill country, you should disengage the feature, since the torque converter will lock and unlock repeatedly as it tries to match the right amount of engine power to the road conditions.

Another instance where overdrive is unhelpful and should be switched off is when you are towing, especially something heavy. When the torque converter locks and unlocks repeatedly under a heavy load, it tends to overheat. And excessive heat is the enemy of your transmission. (Actually, overheating of the transmission or the engine can cost thousands of dollars worth of catastrophic damage). You might get away with overdrive if you pull a light trailer, but to be on the safe side, turn it off. If you want your car to last 200K miles or more, it is better to drive in third or fourth gear at a higher engine speed than to burn up your transmission.

Alternatives to overdrive

Overdrive was common for many years as a solution for automatic and manual transmissions that had only three or four gears. Third gear might get you up to speed, but once you were driving seventy miles per hour, the engine would sing along loudly and use a lot of fuel. With the addition of overdrive, the transmission could relax and let the engine slow down.

Advances in technology have made traditional overdrive unnecessary. Not only do front-wheel-drive vehicles rely on a different system of gearing, but transmissions with a higher gear count have left overdrive behind. Now, the higher gears are engineered to alter the input to output ratio of the transmission, and because there are more gears, there is a higher likelihood that there is an appropriate gearing for every speed.

Still, the idea is the same. As with overdrive, the goal is to allow the engine to run at a slower speed when your car is travelling at a high speed. That way, your engine does less work and uses less fuel. “Overdrive” has become more of a concept than a component or system. Every transmission operates in a way that maximizes engine efficiency at high speeds.

That is even true of Continuously Variable Transmissions (CVTs) that have no gears at all but rely instead on two variable-width pulleys, one on the engine side and the other on the drive side, connected by a flexible belt. A CVT is designed to elicit maximum power from an engine while providing an infinite range of output ratios for any travel speed.

No matter what type of transmission is included in your car, truck, or SUV, its job is basically the same: transmit torque from the engine to the wheels. Each type of transmission – manual stick shift, traditional automatic, or CVT – might go about its job a little differently, but each must make for an efficient use of engine power. And that means including a method of putting more revolutions out than it takes in. Automatic overdrive is one way a transmission gets that job done.

Hogan & Sons Tire and Auto | Author: Mike Ales | Copyright 

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