A method and a control unit for operating an internal combustion engine in overrun condition are described in published German patent document DE 33 01 742.
An overrun condition of an internal combustion engine refers to an operation in which the internal combustion engine gives off no torque, but is rather driven itself by external influences. A overrun condition comes about, for example, during braking or during downhill travel of a motor vehicle, when the driver does not call for torque. The transition into overrun condition may be detected, for instance, by a driver command sensor such as an accelerator sensor.
As power actuators, both air metering actuators, such as a throttle valve or a variable valve control, and fuel metering actuators, typically an injection system, come into consideration. As an error response, for example, the output of the power actuator may be deactivated.
Published German patent document DE 33 01 742, which was mentioned above, relates to an electronic Diesel control system for a Diesel engine (EDC). The present invention also relates to an EDC system, but is not limited to that. It may rather also be used in the case of Otto engines having E-gas (electronically controlled throttle valve) or in the case of variable valve control which is being used as a power actuator.
To the extent that mention is made below of an actuating signal, this concept is supposed to include both a pulse width with which a fuel injector is activated in the opening position and a control signal of an air metering power actuator.
In Diesel engines, the torque that is generated is determined largely by the fuel injection quantity. This applies analogously to Otto engines having direct injection in stratified charge operation. In these cases, errors in fuel metering are able to lead to undesired torque generation. An engine torque generated especially in overrun condition may be particularly critical to safety, because it may lead to an insufficient engine braking effect, or even to an undesired acceleration.
In this connection, it is known that one should monitor the duration of activation of the fuel injectors. In the transition to overrun condition, in this context, by letting up on the accelerator, it is monitored whether, above the maximum intervention speed of an idling regulator, injections still occur because of inadmissibly great activation durations.
In the error case, that is, in the case of undesired injections, an error response is initiated. To do this, the activation signal duration is compared to a fixed threshold value. The quantity of the threshold value determines the sensitivity of the error detection. If the threshold value is small, the sensitivity with which genuine errors are detected is great. However, a high sensitivity also leads to the danger that genuine torque requests of an idling speed control may be valued as errors.
The engine torque at which no release for the overrun condition monitoring takes place, that is the release speed, is rather high, specifically to each internal combustion engine, and, as a rule, corresponds to the upper limiting speed of the idling speed control. A typical value of the limiting speed is approximately 2500 min−1. The result for the monitoring is that, if there is an undesired presence of torque, the vehicle accelerates using the currently active transmission ratio in the gear box until the release speed is reached.
In view of this background, it is an object of the present invention to provide a method that makes possible an increase in the quality of the monitoring by broadening the monitored operating range of the internal combustion engine without sacrifice in the quality of differentiating between an erroneous and a non-erroneous provision of torque.