Electronic throttle control is a drive-by-wire system that replaces a conventional cable-actuated throttle body with an electronic set-up.
In a conventional throttle configuration, such as that found in an older car, a cable would run from the pedal to the throttle body. In an ETC-equipped car, the driver's inputs are instead monitored by a sensor on the pedal assembly and relayed to an electric motor that actuates the throttle plate.
The primary advantage of such a system is that the car can automatically regulate the engine speed, by reducing the throttle opening, without any driver input. This permits a wide range of safety and comfort-related capabilities, including cruise control and traction control, in a neat and easily packaged assembly.
Using a 'throttle-by-wire' system also has other benefits, including removing the need for convoluted cable assemblies - which can wear and bind over time, causing driveability and safety issues.
How do electronic throttle controls work?
In a conventional cable-actuated set-up, a cable runs from the pedal in the footwell to the throttle body. When the driver steps on the pedal, the cable transmits the motion of the pedal to the throttle blade within the throttle body, opening the throttle. When the driver lifts off, springs pull the throttle closed again.
An electronic throttle system removes the cable from the equation. Instead, a sensor on the accelerator pedal assembly monitors the position of the pedal. When the driver accelerates, the sensor transmits data regarding the pedal position to the car's electronic control unit. After processing the data from several other inputs, the ECU will then generate suitable signals which are sent to an electronically controlled throttle body.
This, instead of being opened by the motion of a cable, features a form of electronic actuation - typically a servomotor or stepper motor - which is continuously monitored and positioned by the ECU.
When a change in the throttle opening is required, appropriate signals will be sent from the ECU to the electric motor in the throttle body. The motor will then reposition the throttle blade to suit, increasing or decreasing airflow into the engine as required. An integral spring can help the throttle blade return to the idle position but, in any case, one will be present to ensure the throttle closes if there is a problem with the control system.
Are there any advantages or disadvantages to this throttle-by-wire approach?
Prior to the adoption of electronically controlled throttles, systems existed that allowed the car to override the driver's input. Bosch, for example, developed an 'Anti-skid Regulation' unit that enabled traction control in cars with cable throttle assemblies.
However, these set-ups were often complicated and bulky. Electronically controlled throttles, on the other hand, are far lighter and more compact, making them easier to package. They are also far more flexible in terms of what they can achieve.
The relationship between the motion of the pedal and the throttle opening can also be changed on the fly to suit the conditions or driving mode using ETC set-ups; a sport mode could grant very quick and substantial throttle openings when the driver tips in on the accelerator pedal - resulting in an artificially lively, punchy-feeling engine and experience.
Alternatively, the engine's response could be very gently and gradually metered out over the entire length of travel of the accelerator. This kind of response is ideal for urban driving or in snowy conditions, as it reduces the chance of wheelspin or jerky acceleration.
Electronically controlled throttles can also be used to regulate the engine's idle speed without any additional hardware, helping keep cost and complexity down - although many do retain standalone idle air control systems.
There are otherwise no disadvantages to using an electronically controlled throttle, provided it is calibrated well, as these configurations allow for lightning-quick changes of throttle position when commanded by either the ECU or driver themselves.
A brief history of electronically operated throttles
Electronically actuated throttle bodies were reportedly first prototyped by Bosch in the 1980s for use in motorsports applications. They were easier to package, requiring no convoluted cable routing, and permitted a wide range of additional features without needing any extra hardware. For example, a team could easily instate traction control without needing a bulky motor assembly to actuate the existing cabled throttle set-up.
BMW then adopted a development of Bosch's electronically controlled throttle bodies for its new V12-engined 7 Series flagship, which was unveiled in 1987. Doing so allowed BMW to equip the luxury saloon with the likes of cruise control, traction control and a speed limiter without resorting to an overly complicated cable and servomotor-equipped set-up.
The company's 5.0-litre 'M70' V12, which initially went into the 750i and 750iL, used twin Bosch Motronic engine management systems and a pair of electronically controlled Bosch throttle bodies. BMW's own 'Elektronische Motorleistungsregelung' - electronic power regulation system - monitored all the required inputs, such as accelerator pedal position and wheel slip, and would then send signals to the throttle bodies to open them to the optimum position.
Other manufacturers, such as General Motors, soon followed suit. As costs fell and the demand for improved safety systems and creature comforts continued to rise, electronically controlled throttle systems quickly became commonplace - with their applications ranging from basic city cars through to the likes of the Ferrari 458 Speciale and beyond.