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Eye On Electronics

Motor, Mar 2004 by Dale, Mike

The adoption of Electronic Throttle Control may mean the command to 'step on it' will have a very different meaning for future generations of drivers.

After 22 years of making S-10s, General Motors announced this fall the introduction of its new Chevy Colorado/CMC Canyon trucks. Tucked in among all of the new and improved standard features were the letters "ETC." That stands for Electronic Throttle Control, and it's been around in small volumes since BMW introduced it in 1988. For the last three years, ETC has been finding its way into higher end domestic vehicles. What s new is that ETC is now a standard feature on a mass-market, high-volume vehicle that sells for as little as 14 grand.

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GM is not the only carmaker that has added ETC; Subaru has announced that all of its 2005 models will have it, and Fords flagship vehicle, the F-150, has it as well. ETC as a technology has matured-the price is down, the value/performance is up and as a result, ETC has gone mainstream.

Engine speed has always been controlled by controlling the amount of air flowing into the engine. This throttle plate positioning has traditionally been handled by a mechanical link-age (a "Bowden" cable), in combination with springs, pulleys and other hardware.

While the Bowden cable assembly has worked pretty well for a long time, it has some disadvantages. From NVH and assembly standpoints, the throttle cable represents a hole through the firewall and a passage-way for engine noise to get into the vehicle by way of the cable itself. The assembly is a multistrand wire cable linked at each end to the gas pedal and the throttle plate. This represents quite a few parts that must be assembled and installed into cramped spaces. A third issue is that what the driver wants, and what the engine might need to produce the best fuel economy and best emissions performance, are often not the same. Engine tuners have known that software programs that intervene between the driver and the engine could result in more precise throttle control that would lead to improved engine performance.

ETC, or drive-by-wire, is a great example of how tactical can be turned to practical. Most of the work to prove that these systems would be safe in automobiles was first done to prove they were safe in military and civilian aircraft. Some of the newest planes cannot fly at all without constant computer monitoring and adjustment of the control surfaces. It was the need for ultra-reliable, remotely computer-controlled actuation in airplanes that has driven drive-by-wire technology.

A drivers intent is received by a Throttle Pedal Assembly (TPA), which may have as many as three parallel, redundant position sensors. In addition, there are idle and WOT switches that monitor minimum and maximum input from the driver. The TPA also has the secondary purpose of creating the normal "feel" of the mechanical cable and springs it replaces.

On the throttle body, throttle position is controlled by a motor that can rotate the throttle plate from 1° to 80°. In typical ETC systems, signals from the computer can command the motor to rotate the throttle plate with an accuracy of better than +.5° over most of the range, with even tighter .1° accuracy at idle. The motor itself is fitted with multiple position sensors to complete a feedback loop to the computer. Once the computer sends its instructions to the motor, it listens to the sensors to make sure the action has taken place correctly.

The motor itself works on the principle of magnetic bucking. Here, there are two opposing magnetic fields, and pulse width modulation techniques are used to determine the size of one field vs. the other By increasing the percentage of ON time, greater amounts of rotation are achieved. As a fail-safe measure, the throttle plate is spring-loaded so that if power should fail, the plate returns to the idle position.

The automakers are after a handful of the advantages ETTC has to offer. One is improved emissions performance. In the deceleration mode, for example, the driver takes his foot off the gas, closing the throttle plate. The continued rotation of the engine creates a high intake manifold vacuum that can lead to fueling errors. By putting software algorithms in between the TPA and the stepper motor, GM expects to realize as much as a 15% reduction in HC and NO^sub x^. On one model, GM was ahle to eliminate the AIR pump used to inject air into the exhaust system. From a corporate standpoint, that allows more vehicles to fit into the Low Emission Vehicle (LEV) categories.

In a paper published by SAE, Delphi points to other advantages to its version. Its system integrates unique driver performance features such as driver-selectable pedal response (Power for performance, Winter for control) altitude compensation for more normal pedal response in the mountains and vehicle and engine speed governors to protect the vehicle and the engine from "unwise" pedal positions. (You'd have to think the rental car companies asked for that feature.) There are a couple of driveability-related features, as well-tip-in bump elimination on manual transmissions and an engine timing retard feature to improve catalytic converter light-off on startup.

Automotive Industry

Eye On Electronics

Motor, Mar 2004 by Dale, Mike

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