In traditional vehicle designs, stepping on the accelerator pedal moves a cable that is mechanically connected to a butterfly valve in the engine's throttle. The position of this valve directly controls the amount of air supplied to the cylinders and consequently determines the engine speed and torque. Most vehicles on the road today have electronic throttle controls. In these vehicles, stepping on the accelerator pedal sends an electrical signal to the engine control module (ECM). The ECM uses this information to send a control signal to an electric motor mounted to the throttle body that adjusts the position of the butterfly valve accordingly. A throttle position sensor is used to create a closed-loop control system to ensure that the throttle is open to the correct position.
A major advantage of an electronic throttle is that it can be easily linked to other systems such as engine control, traction control, electronic stability control and cruise control. These other systems are able to take control of the throttle when necessary to help improve vehicle safety, convenience and fuel economy. For example, the National Highway traffic Safety Administration has proposed a rule that all cars be required to have a brake-throttle override (BTO) system in place by September of 2014. The BTO system would give priority to brake inputs when the brake pedal and the accelerator pedal were engaged at the same time.
Like all of the electronic systems controlling safety critical functions in a vehicle, electronic throttle controls are designed with certain fail-safe features, including redundant sensors and self-diagnostic capabilities.
It's important that ETC reacts to fault detection with the correct countermeasure. Depending on the severity of the fault, the system may revert to one of the following four operating modes.
1. Limit Performance Mode
This safety mode will operate "when a reduction in the reliability of determining driver intent has been detected or when the ability to create high levels of engine power is impaired". Most typically it is caused by loss of accelerator pedal redundancy (meaning there's only one operational sensor signal) or there's a processor fault. Once tripped, Limited Performance Mode gives dulled throttle response, reduced engine torque at all throttle positions and brake pedal applications sets the engine to idle. It is still possible to continue driving.
2. Forced Idle Mode
This mode will engage when the system cannot sense reliable information on driver intent. A complete failure of all pedal sensors or a processor failure will cause this condition. As you might've guessed, all that the engine can do in Forced Idle Mode is idle. It will not respond to accelerator applications. The primary aim of this mode is simply to maintain power for vehicle heating, cooling and lighting during adverse weather.
3. Power Management Mode
The only time this mode will activate is when the system is unable to control engine power via the throttle. In this instance throttle actuation is disabled, allowing the butterfly to spring back to its default position. It is still possible to continue driving (albeit slowly) thanks to the main processor's on-going control over fuel and ignition timing.
4. Engine Shutdown Mode
This is the most severe action that the main processor can take. If unable to control algorithms or engine power, the Delphi system disables fuel, ignition and throttle outputs. The engine is totally shut down. The most likely cause of this extreme condition is a processor fault or the inability of the throttle and/or intake system to control airflow.
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