Conventional vehicles are governed by the operator through the mechanical connection between the accelerator pedal and the throttle valve that controls the airflow entering the engine. When an electronically controlled throttle is used, the mechanical connection is replaced by an electrical connection. This gives the engine control system greater flexibility in delivering the operation requested by the driver while optimizing constraints related to regulated emissions and fuel economy. However, an additional constraint when using an electronically controlled throttle is that the valve typically includes a so called, "limp home" position. This limp home position allows the throttle to return to a position to allow some airflow through the valve bore, thereby allowing greater valve control under certain engine operating conditions.
One approach to providing a limp home position is to use opposing biasing springs to urge the throttle plate to an intermediate position between the maximum power position (or maximum area position, typically termed WOT) and the minimum power position (or minimum area position). The intermediate position can be selected to provide just enough airflow to idle the engine and provide the limp home mode.
Another approach to providing a limp home position is to use a biasing spring that urges the throttle plate only in one direction to a position past the normally closed throttle position. In other words, the throttle plate is able to rotate in the throttle bore through the closed position to a partially open position. This partially open position can be selected provide to just enough airflow to idle the engine and provide the limp home mode.
The inventor herein has recognized disadvantages with the above approaches. For example, when using opposing biasing springs to urge the throttle plate to an intermediate position between the maximum power position and the minimum power position, there is a discontinuity in the spring force at this intermediate position. In other words, the spring force changes direction at this intermediate position. This causes poor closed loop control performance when the desired throttle plate position is near this intermediate position. The problem is exacerbated in that this intermediate position is selected to be near the normal idling position, which is where throttle plate control is critical. Thus, the total engine control system is extremely sensitive to this discontinuous spring force during a critical engine operating mode. This may cause poor engine idle quality and low customer satisfaction.
Another disadvantage is that the intermediate limp home position can not be easily adjusted. Changing the intermediate position requires changing hardware in a complex mechanism.
When using a biasing spring that urges the throttle plate only in one direction to a position past the closed throttle position, the engine control problem near idle is reduced; however, another control problem becomes more apparent. In particular, it is sometimes necessary to completely restrict the throttle airflow to control the engine due to very low airflow requirements and leaks caused by other air sources, such as, for example, fuel purging and vacuum actuators. Thus, because this prior art does not have a "No Flow" position, the minimum flow position must be adaptively learned as the components wear, expand and contract due to temperature variations, and move do to manufacturing tolerances. In addition, decreasing the flow at the minimum flow position requires increasingly complex and expensive manufacturing processes because the throttle plate must be a perfect circle at the edge with, ideally, infinitesimally small thickness. Indeed, because the throttle plate must rotate through the closed position, it is impossible to completely seal the throttle plate relative to the throttle bore.
Yet another disadvantage is that while the limp home position may be easily adjusted, the minimum flow position can not be easily adjusted. Changing the minimum flow position requires changing hardware and manufacturing processes.