Electronic throttle control

Electronic throttle control (ETC) is an automobile technology which severs the mechanical link between the accelerator pedal and the throttle. Most automobiles already use a throttle position sensor (TPS) to provide input to traction control, antilock brakes, fuel injection, and other systems, but use a bowden cable to directly connect the pedal with the throttle. An ETC-equipped vehicle has no such cable. Instead, the electronic control unit (ECU) determines the required throttle position by calculations from data measured by other sensors such as an accelerator pedal position sensor, engine speed sensor, vehicle speed sensor etc. The electric motor within the ETC is then driven to the required position via a closed-loop control algorithm within the ECU.

The benefits of ETC are largely unnoticed by most drivers because the aim is to make the vehicle power-train characteristics seamlessly consistent irrespective of prevailing conditions, such as engine temperature, altitude, accessory loads etc. The ETC is also working 'behind the scenes' to dramatically improve the ease with which the driver can execute gear changes and deal with the dramatic torque changes associated with rapid accelerations and decelerations.

Contrary to popular belief, except in concert with other technologies such as gasoline direct injection, ETC provides only a very limited benefit in areas such as air-fuel ratio control, exhaust emissions and fuel consumption reduction. ETC however makes it much easier to integrate features to the vehicle such as cruise control, traction control, stability control and others that require torque management, since the throttle can be moved irrespective of the position of the driver's accelerator pedal. A criticism of the very early ETC implementations was that they were "overruling" driver decisions. Nowadays, the vast majority of drivers have no idea how much intervention is happening.

Much of the engineering involved with drive-by-wire technologies including ETC deals with failure and fault management. Most ETC systems have sensor and controller redundancy, even as complex as independent microprocessors with independently written software within a control module whose calculations are compared to check for possible errors and faults.

Anti-lock braking (ABS) is a similar safety critical technology, whilst not completely 'by-wire', it has the ability to electronically intervene contrary to the driver's demand. Such technology has recently been extended to other vehicle systems to include features like brake assist and electronic steering control, but these systems are much less common, also requiring careful design to ensure appropriate back-up and fail-safe modes.

As of 2005, the Toyota Prius is the most prominent example of drive-by-wire technology, featuring electronic throttle, brake and transmission control. This is largely by necessity of the Hybrid Synergy Drive system, which assigns complete engine control and regenerative/friction braking decisions to a hybrid control computer. Further extending the drive-by-wire concept, in Europe and Japan automatic parking assist is also available — the car can control the steering to guide itself backwards into a parking space.

Some fanciful theories and applications abound as to what the ultimate implications of drive-by-wire technology might be. It has been suggested that drive-by-wire might allow a car to become completely separate from its controls, meaning that a car of the future might theoretically be controlled by any number of different control systems: push buttons, joysticks, steering wheels, or even voice commands — whatever device that designers could come up with. (This would have many advantages, such as increased flexibility for handicapped or disabled drivers.) Coupled with fuel cell applications, futuristic designs for such a car have been proposed, including a car whose entire functional driving components are concentrated in its chassis — the actual 'shell' of the car being a module that can be swapped out and replaced with different models as the driver dictates. Competitors in the DARPA Grand Challenge, an automated driving competition, relied on 100% drive-by-wire systems, in some cases including a steer-by-wire system provided by the manufacturer.