How Many Gears Before The Gear Head Spins?

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How Many Gears Before The Gear Head Spins?

Jeff Bogue • May 14, 2021

so shall we try to demystify the automatic transmission/transaxle...

Ok, I have to admit, my understanding of the inner workings of a modern automatic transmission is akin to a frog watching TV. I see what it does. Understand it? Not really. The sheer number of moving and rotating parts is almost incomprehensible. As they say, “The journey of a thousand miles begins with a single step”, so shall we try to demystify the automatic transmission/transaxle. The transaxle generally refers to front wheel drive vehicles, with the transmission and axle built into one housing, while transmission generally refers to front engine rear wheel drive vehicles. For simplicity I will use the term transmission for both.

In the modern drivetrain that pushes our vehicles down the road, the engine and transmission work together as one, but they are two very different things. Your engine connects to the transmission inside the “Bell Housing”. Inside the bell housing is the torque converter. In a manual transmission the clutch is used to couple the engine to the transmission. In the modern automatic transmission, the torque converter is used to couple the engine and transmission together.

The torque converter is a type of fluid coupling that connects the two main components, the engine and the transmission. An easy way to describe this is by placing two fans facing each other. Turn on one fan at low speed and leave the other turned off. After a few seconds the fan that is turned off will start to spin. Turn the running fan on high and the second fan will pick up speed matching the wind velocity of the first. A torque converter does this except with transmission fluid. As the engine runs, it turns a set of blades, or impellers, inside the torque converter. A second set of blades, called the Turbine, attached to the transmission, is driven by the first without ever actually coming into contact. The system uses only liquid to drive the transmission from the engine. A torque converter differs from a standard fluid coupling in that it provides a variable amount of torque depending on driving needs and engine speeds.

In my search for education on the matter, I found a video on fluid coupling. It was made by the U.S. Army in 1953 and is the best video regarding fluid coupling I have ever seen. This video is for education and has a Creative Commons Attribution license, so you can show this in your classroom.

The torque converter’s operation can be simplified into three stages: Stall, Acceleration, and Coupled. In the stall mode, the transmission is in gear, but the applied torque from the torque converter is low enough that it can easily be overcome by applying the brake. During acceleration, the engine speeds up and the torque converter follows suit slowly allowing the transmission to accelerate to the point that it almost matches the engine. As the vehicle reaches highway speeds and the engine and transmission get closer to matching speeds, the torque converter Clutch is applied and the two components are then coupled to work as one direct drive from the engine to the transmission. The key to the torque converter’s ability to do this is the Stator. The Stator sits between the Impeller and the Turbine. Its primary function is to minimize losses due to churning and increase torque output by redirecting fluid as it is needed.

Following the torque converter is the main body of the transmission and it houses all of those spinning gears. The gears in the automatic transmission are never disengaged from one gear to the next like in a manual transmission. They are always working in conjunction with each other. These gear sets are called Planetary Gears.

The modern automatic transmission will employ several sets of Planetary Gears to get all of the gear ratios that it needs. Two Planetary Gear sets will give you up to four forward gear ratios. Add another and you are talking about six to eight forward gears. There are three main parts of all Planetary Gear sets. These are the Sun gear, the Planet gears and their carrier, and the Ring gear. These Planetary Gear sets are meshed together into one single housing about the size of a soccer ball. Each of these gear sets can be utilized as the input, the output, or held stationary. This is how you can get so many different gear ratios out of a single set of gears without ever having to engage or disengage any of them. How does it do that?

If you look inside a transmission you will find several sets of bands and clutches. These bands and clutches grab the individual gear sets of the Planetary Gears to make them either the input or the output. These bands and clutches are controlled by the valve body of the transmission.

The valve body and its maze of oil journals, tiny springs and valves control those bands and clutches via hydraulic solenoids. Their purpose is to either couple things together, decouple them, or hold them stationary. Bands can grab the outside Ring gears on the multiple Planetary Gear sets to let the Sun gears and Planet gears drive the vehicle, and the clutches can grab the Sun gears and Planet gears to either couple them together or hold them stationary. The solenoids are controlled by numerous inputs including the PRNDL switch, the gas pedal, etc. A computer takes all of these inputs and activates the appropriate band and/or clutch to set the transmission into the proper gear ratio for the gear your automobile needs to do its job efficiently.

If you watch this video, you will have a better understanding of how the multiple planetary gear sets work together. It too is from 1953, has the same license, and is equally informative.

Now that we have a simple understanding of how the modern transmission works to give us all of those gear ratios, we can look at what is coming up around the corner. The commonality of the eight speed transmission has become old hat. The ten speed transmission has been out for a few years now and has proven itself reliable. The first vehicle to receive the ten speed transmission was the 400 horsepower Ford Raptor back in 2017, and it has trickled down to other light duty trucks and high end sports cars. This new transmission was developed as a joint venture of both Ford and GM engineers. The ten speed transmission has six forward under drive ratios (the input turns faster that the output), a direct drive ratio and three over drive ratios (the output turns faster than the input). Inside the ten speed transmission there are four planetary gear sets and six shift elements. Of these six shift elements, two are brakes (bands for the ring gears) and four are clutches to couple and decouple the Suns and Planet gears. All of this technological wizardry allows high horsepower engines to stay at their optimal RPM, or power band, for both fuel economy and power (or incredible acceleration and ridiculous top speed). To go beyond the ten speed transmission would require yet another planetary gear set. Both Ford and Honda have registered patents for an eleven speed transmission and are expected to release details in the coming years. The advantages of an extra gear are being weighed against costs and reliability. When will it end? Not really sure. Is it all worth it? Not really sure. Are they going to be reliable? Only time will tell. The ten speed has already proven itself as a fuel economy partner and a marvel at keeping engines at that perfect RPM for every task in both trucks and cars. How many gears does a truck really need? That is a point of discussion.

Jeff Bogue

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I have been dreaming about and studying engines and engine performance of cars, trucks, and motorcycles for the better part of 45 years. It’s a hobby. It keeps me busy on cold winter evenings. It’s something that has become a large part of my life and, like playing with dogs, is something I love to do. I have written about engines and the engineering that is involved in them for the last 15 years, and I have always been forward-thinking. When a new engine is being lauded by the manufacturers as the next big thing, I read all about it and report those findings here. In the past, I have written about every one of the engines in this article at one time or another, and each one was glowingly revealed by the engineers with the statement “We can expect these engines in our cars in the next few years”. Well, a few years have passed and now most of these engines ARE in our cars and trucks and should be reaching repair shops and dealerships now. We just haven’t heard that much about them because people, as a general rule, really don’t care that much about which engine is in their car if it gets good gas mileage, is reasonably reliable, and has a little umph to get on the highway. Well, that is not us. We care. We study it. It’s what we do and why you are where you are reading this article. Without further ado, let us look at the “Engines of the Future” that you can buy (and service) now.