Selective Catalyst Reduction (SCR) Operation

WARNING: This page is about a different variant/trim than selected.

While diesel engines are more fuel efficient and produce less HC and CO than gasoline engines, as a rule they generate much higher levels of NOx. In order to meet today's tighter NOx limits, an SCR catalyst, along with reductant, is used to convert NOx into N2, CO2 and H2O.

NOx Sensor

The ECM uses two smart NOx sensors to control exhaust NOx levels. The first NOx sensor is located at the turbocharger outlet and monitors the engine out NOx. The second NOx sensor is located in the exhaust pipe downstream of the SCR and monitors NOx levels exiting the aftertreatment system. The smart NOx sensors communicate with the ECM over the serial data line.

The NOx sensors incorporate an electric heater to quickly bring the sensors to operating temperature. As moisture remaining in the exhaust pipe could interfere with sensor operation, the ECM delays turning on the heaters until the exhaust temperature exceeds a calibrated value. This allows any moisture remaining in the exhaust pipe to boil off before it can effect NOx sensor operation. Depending on engine temperature at start up, the delay can be less than a minute or as long as two minutes. Typically, NOx sensor 1 will reach operating temperature faster than NOx sensor 2 as it's closer to the engine's hot exhaust. At idle or low engine speeds, NOx sensor 2 may require up to 5 minutes to reach operating temperature. The sensors must be hot before accurate exhaust NOx readings are available to the ECM.

Heated Oxygen Sensor (HO2S)

The Heated Oxygen Sensor is used to determine the proportion of oxygen in the exhaust gas and works in conjunction with other vehicle and engine information sources to help determine an ideal air/fuel ratio (about 0.5% lean of stoichiemetric ratio of 14:1) in closed loop feedback mode. An ideal air/fuel ratio improves fuel burning efficiency and also help to reduce the amount of generated hydrocarbons (HC) caused by unburned fuel due to too rich ratios, carbon monoxide (CO) caused by a slightly rich ratio and nitrogen oxides (NOx), caused by excessive combustion temps due to too lean ratios.

The sensor does not measure the actual amount of oxygen in the exhaust gas, but determines the amount of oxygen needed to oxidize remaining combustibles in the gas.

Reductant (Diesel Exhaust Fluid or DEF)

Reductant is a mixture of deionized water and urea. Within the SCR, exhaust heat converts the urea into ammonia (NH3) that reacts with NOx to form nitrogen, CO2 and water vapor. Optimum NOx reduction occurs at SCR temperatures above 250°C (480°F). At temperatures below 250°C, the incomplete conversion of urea forms sulfates that can poison the catalyst. To prevent this poisoning, the ECM suspends reductant injection when exhaust temperature falls below a calibrated limit.

Exhaust Gas Temperature (EGT) sensor

The 2.0L (LUZ) engine uses exhaust gas temperature management to maintain the SCR catalyst within the optimum NOx conversion temperature range of 200-400°C (390-750°F). The ECM monitors EGT sensors located upstream (EGT 2) and downstream (EGT 3) of the SCR in order to determine if the SCR catalyst is within the temperature range where maximum NOx conversion occurs.