NOx Sensor Operation
NOx sensors have a great deal in common with wideband oxygen sensors, but typically have their own module that communicates with the vehicle ECM over a CAN bus. In most applications there will be two NOx sensors used; one at the turbocharger exhaust outlet and another downstream from the SCR catalyst. These two signals are then compared to determine conversion efficiency of the catalyst. It is also possible to have a single NOx sensor at the outlet of the SCR catalyst and utilize an ECM strategy to calculate engine-out NOx (used in the 6.7 liter Ford Power Stroke).
Like oxygen sensors, NOx sensors will not work unless they are at the correct temperature, so they have an integrated heater that is also module-controlled. The power supply to the NOx sensor module is critical to proper sensor operation. In the case of the 2011 Duramax LML (VIN 8) engine, the glow plug control module (GPCM) supplies constant voltage to both of the NOx sensor modules. If system voltage is low, an internal voltage boosting circuit in the GPCM will make up the difference to ensure proper NOx sensor operation.
During a cold start, moisture in the exhaust system can interfere with NOx sensor operation. In this scenario, the Duramax ECM will not turn on the NOx sensor heaters until exhaust temperatures reach a certain threshold and condensed moisture is evaporated. This can result in a delay of up to 5 minutes before both NOx sensors are fully operational.
Diesel NOx sensors are dual-purpose, in that they are used to measure both the oxygen level in the exhaust as well as NOx (NO and NO2) content. The measurements take place in two separate chambers in the sensor, and the exhaust gases flow sequentially through one and then into the other. Gases flow from the exhaust stream through a diffusion barrier to reach the first chamber, which “pumps” the free oxygen out using a Nernst cell (simple oxygen sensor). The electric current for operating this first Nernst cell is used to measure exhaust oxygen content (lambda). In the LML Duramax, exhaust oxygen content information is provided to the ECM by NOx sensor 2 to aid in diesel particulate filter (DPF) regeneration.
With the free oxygen removed, the NOx is then left to migrate through another diffusion barrier and into the second measurement chamber. At this point, the NOx molecules encounter a catalytic element, which breaks them into nitrogen and oxygen gases. A second Nernst cell is then used to pump the newly-generated oxygen out of the chamber, and this electric current is used to calculate the NOx levels in the exhaust. The residual nitrogen gases then flow out an exit port in the second measurement chamber.
When using two NOx sensors in a urea SCR system, the upstream sensor is used to estimate what amount of DEF needs to be injected ahead of the SCR catalyst to achieve optimal NOx conversion. The downstream (post-catalyst) sensor is then used to check the results. Under ideal conditions, NOx conversion will be high and very little ammonia slip will occur.
An adaptive strategy similar to long-term fuel trim (LTFT) is used to determine the duty cycle that the DEF injector should be operated at. Since there are numerous variables (“drift” in component operation) that can affect the amount of DEF that is injected for a given duty cycle, the table is always being rewritten to maximize NOx conversion efficiency. These calculations are made based on signals from the NOx sensors, similar to the way that oxygen sensors are used in a gasoline-engine emission control system. If the correction factor (trim) of the DEF quantity changes too far in either the positive or negative direction, the ECM will set DTCs and turn on the check engine light (MIL).