1. Field of the Invention
The field of the invention relates to control of diesel engines having exhaust gas recirculation.
2. Background Art
Diesel engines typically operate at substantially wide-open throttle conditions. This provides high fuel economy since pumping work is substantially eliminated. Since airflow is essentially unregulated, engine output is adjusted via a fuel injection amount. Since the diesel engine also operates almost exclusively in a lean air-fuel condition, changes in fuel injection amount predominantly control changes in engine output.
Some attempts have been made to introduce exhaust gas recirculation into diesel engines. Such recirculation lowers the formation of engine NOx emissions. However, to draw the recirculated exhaust gas flow into the manifold, engine throttling is used to create a pressure difference (i.e., either a vacuum or positive pressure). Thus, a throttle is used to create some vacuum or pressure, while position of an exhaust gas recirculation valve is adjusted to provide a desired recirculated exhaust gas flow.
The inventors herein have recognized a disadvantage with such a system. In particular, such systems are costly to implement since two active control valves are utilized, one for airflow and one for exhaust gas recirculation. Further, such systems may provide degraded estimates of EGR flow since they measure pressure across a valve having a variable orifice area. Since valve area may not be precisely known, such an approach may be inaccurate under some conditions.