Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants are composed of gaseous compounds such as, for example, the oxides of nitrogen (NOX). Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amount of NOX emitted from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. In order to ensure compliance with the regulation of these compounds, some engine manufacturers have implemented a strategy called Selective Catalytic Reduction (SCR).
SCR is a process where gaseous or liquid reductant (most commonly a solution of urea solid and water) is added to the exhaust gas stream of an engine and is adsorbed onto a catalyst. The reductant reacts with NOX in the exhaust gas to form H2O and N2, which can be safely released to the atmosphere. Although SCR can be an effective method for reducing NOX, it can be difficult to ensure that enough reductant has been injected to adequately reduce the amount of NOX present within the exhaust gas stream, without unnecessarily wasting reductant and releasing unused reductant or byproducts thereof to the atmosphere.
To help control the amount of NOX emitted to the atmosphere, most exhaust systems are equipped with a sensor that detects the level of NOX in the tailpipe of an engine and generates a corresponding signal. Based on this signal, the amount of reductant injected into an exhaust flow can then be trimmed to precisely reduce a desired amount of NOX.
Although relatively successful at helping to control the amount of NOX emitted to the atmosphere, most NOX sensors currently available are cross-sensitive to the reductant used to reduce NOX. That is, NOX sensors on the market today generate signals indicative of the presence of both NOX and reductant. As such, when either NOX or reductant levels in an exhaust flow go up, the signal value from the corresponding NOX sensor also goes up, and vice versa. Thus, it can be difficult to determine which constituent is currently being detected by the sensor, and over-injecting of reductant can occur in some situations when the injections are unknowingly based on a detected high level of reductant instead of NOX.
One attempt to help ensure that reductant injections are correctly based on detected levels of NOX is described in WO Publication Number 2006/000877 of Crane et al. that published on Jan. 5, 2006 (the '877 publication). The '877 publication discloses an exhaust system that utilizes a nonselective NOX/NH3 sensor in conjunction with feedforward and feedback control. The sensor is periodically interrogated by introducing a positive pulse in an ammonia feed rate. A positive response from the sensor to the positive pulse indicates ammonia slip is occurring (i.e., that the sensor is currently detecting NH3). A negative response from the sensor indicates NOX breakthrough (i.e., that the sensor is currently detecting NOX). Based on the positive responses, a controller inhibits feedforward control for a time and reduces reductant injections. Based on the negative response, the controller adjusts feedforward control to increase reductant injections. Feedback control is utilized between sensor interrogations to trim feedforward commands for injection based on sensor output under normal conditions.
Although perhaps somewhat effective at determining if a sensor is detecting NOX or NH3, the exhaust system of the '877 publication may be unresponsive and wasteful. That is, the exhaust system of the '877 publication requires dedicated time for the sensor to be interrogated, this time causing delays in normal operations of the system. In addition, the exhaust system of the '877 publication requires extra pulses of reductant that are used only for sensor interrogation (i.e., the pulses do not help to reduce NOX), which can be expensive and decrease a time between servicing because of the additional exposure to reductant.
The exhaust system of the present disclosure solves one or more of the problems set forth above and/or other problems of the prior art.