Subsequently, electronic ignition was introduced, followed by closed-loop electronic engine control. In the latter systems, the driver still controlled the throttle directly, but the engine management system regulated the fuelling and the spark timing (Figure 2).

Figure 2: Engine control with electronic engine management

In modern engines, the accelerator is simply an input to the engine management computer and there is no direct mechanical link to the engine (Figure 3). As a consequence, it has changed from being the driver's instrument to vary engine speed to being the instrument to generate torque - a vehicle 'objective'. The driver command may be overridden or modified in pursuit of other vehicle objectives. Most drive-by-wire engine control systems are designed to 'fail safe'. If faults occur from which the control system cannot recover, the system enters a designated 'safe state', usually with the engine operating at limited power or shut-down. Redundancy in sensors and processing is introduced in this concept so that a first fault does not immobilize the vehicle.

Figure 3: Drive-by-wire engine control