Engine operation can be adjusted to improve engine emissions or fuel economy. However, engine adjustments (e.g., EGR amount and fuel injection timing) that increase fuel economy can also increase engine emissions. One way to improve tailpipe emissions without extremely increasing fuel consumption is to adjust an exhaust gas recirculation (EGR) amount provided to the engine and process engine exhaust with a selective catalyst reduction system (SCR). Increasing the EGR amount supplied to the engine may reduce engine feedgas NOx without extremely increasing fuel consumption. Further, a SCR positioned in an engine exhaust system processing exhaust gas can reduce feedgas NOx to provide lower vehicle tail-pipe NOx emissions. In this way, tailpipe NOx can be decreased via a combination of EGR and exhaust after treatment devices. However, engine fuel economy may decrease somewhat when EGR is increased. As a result, engine fuel economy may be less than is desired.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for operating an engine, comprising: adjusting an EGR amount supplied to an engine in response to an amount of NH3 stored within a SCR catalyst and an amount of urea stored in a tank.
By decreasing an EGR amount supplied to an engine when efficiency of a SCR catalyst is high, it may be possible to improve vehicle fuel economy while meeting a desired level of emissions. For example, efficiency of a SCR catalyst may be high when an amount of NH3 stored within the SCR catalyst is greater than a threshold level. As such, the engine may be operated at more fuel efficient conditions via decreasing EGR while tailpipe emissions are maintained below a threshold level by the efficiently operating SCR catalyst. Further, the EGR amount supplied to the engine may be adjusted in response to an amount of urea remaining in a tank when the tank is not substantially empty. Such operation may allow the system to improve engine fuel economy while limiting use of urea to provide a desired interval between urea tank refills.
The present description may provide several advantages. Specifically, the approach may reduce engine emissions and improve engine fuel economy. In addition, the approach may also help to provide a desired interval between urea tank refills. Further, the approach may reduce urea injection during periods when SCR catalyst efficiency is high to extend a urea tank refill interval.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.