Sensors for Onboard Diagnostics and Emissions Monitoring
As emission regulations become more stringent, so the need for onboard diagnostics and monitoring systems increases. The use of diesel particulate filters has dramatically reduced harmful toxic exhaust gas emissions and similar technology is being implemented in gasoline direct injection engines via gasoline particulate filters.
Euro VI regulations, however, also include tighter onboard diagnostics standards for monitoring powertrain components and systems whose functionality has an impact on exhaust gas emission levels.
The OBD standard specifies that if there is a failure with any system or component which results in increased levels of emissions, the driver must be warned through the MIL (Malfunction Indicator Lamp). This leads to a requirement for a robust system including sensors, software and hardware to ensure malfunctions, failures and defects can be swiftly diagnosed.
OBD requirements have increased with each stage of Euro emission regulations, with the Euro VI OBD requirements (2013/16) adding a variety of improvements to previous legislation. For example, in heavy-duty vehicles, Euro VI OBD requirements include:
● More stringent OBD threshold values and type approval based on the World Harmonized Test Cycle.
● Adoption of in-use performance ratios. IUPs give an idea of how often the conditions subject to monitoring occurred and how frequently the monitoring was conducted. The regulation established a minimum IUPR of 0.1 (1 in 10 times).
● Additional monitoring requirements for EGR flow, EGR cooling system, boost (turbo and superchargers), and fuel injection systems.
During the Euro VI OBD phase-in period, manufacturers were granted several areas of flexibility, including:
● Manufacturers are allowed to define a reagent concentration for threshold monitoring different to that ultimately specified.
● Manufacturers are allowed to activate driver warning systems based on more relaxed deviation from typical urea consumption rates - 50% instead of 20% at the final date.
● During phase-in period, there are no in-use requirements for the OBD system.
● During phase-in period, manufacturers are not required to comply with minimum in-use performance ratios.
● The regulations allow the use of performance monitoring of the DPF, via delta pressure, for example, in place of monitoring against the PM OBD threshold.
For light vehicles, manufacturers have the flexibility to choose NEDC or WLTP cycle for OBD threshold part creation as well as for demonstration testing during the transition phase. After the transition phase, only the WLTP cycle is applicable.
Onboard Monitoring Functions & Communication Protocols
OBD regulations state that all systems and components related to exhaust gas emission levels must be monitored for malfunction. Monitoring is performed in the appropriate ECU by means of software functions, and the result of the monitoring function should be a report to an external diagnostic device in the format specified by the standard.
For diesel engines, OBD regulations state that the following components be monitored:
● Non-Methane Hydrocarbon (NMHC) converting catalyst monitoring
● Nitrogen Oxide (NOX) converting catalyst monitoring
● Crankcase ventilation (CV) system monitoring
● NOX adsorber monitoring
● Boost pressure control system monitoring
● Particulate matter filter monitoring
In gasoline engines, the following components should be monitored:
● Catalyst monitoring
● Heated catalyst monitoring
● Evaporative system monitoring
● Secondary air system monitoring
● Positive crankcase ventilation (PVC) system monitoring
● Direct ozone reduction system monitoring
And in the case of both diesel and gasoline engines, the following components should be monitored:
● Misfire monitoring
● Fuel system monitoring
● Exhaust gas sensor monitoring
● Exhaust gas recirculation (EGR) system monitoring
● Engine cooling system monitoring
● Cold start emission reduction strategy monitoring
● Air conditioning system component monitoring
● Variable valve timing and/or control (VVT) system monitoring
● Comprehensive component monitoring
● Other emission control or source system monitoring
In terms of communication protocols, OBD regulations are defined by a series of SAE/ISO standards that describe in detail how OBD functions should be implemented on the ECU side, and how communication between ECU and the diagnostic tool should be performed.
While vehicles are engineered to meet type approval emission limits based on NEDC or WLTC cycles, OBD emission levels are higher than type approval limits and are used to determine whether a particular component/system failure should lead to activation of the MIL.
The rules state that, if a failure occurs and the emission levels of the vehicle are above type approval limits, a diagnostic trouble code (DTC) must be stored in the ECU memory. If emission levels for the same failure also exceed OBD limits, the MIL should be activated. Manufacturers must perform several tests to measure the level of additional exhaust emissions for each failure type, and based on that analysis, the engine control module should be programmed to activate the MIL depending on the severity of the emission level failure.
If the failure disappears for any reason, such as in the case of a loose electrical contact, the diagnostic trouble code must be stored in the ECU memory for 40 engine cycles. If the MIL was activated for the same failure, it will be deactivated after three fault-free engine cycles.
Sensing equipment is therefore an essential part of the diagnostic system to monitor the various components which affect emission levels. Two of the biggest suppliers of sensors are Delphi and Bosch, and both have produced technology designed to meet OBD standards.
Delphi’s particulate matter sensor is an advanced sensing technology which helps diesel engine manufacturers meet OBD standards. The patented sensor helps detect leakage in the to meet diagnostic standards, combining fast response and high sensitivity with simplicity of design and self-diagnosis features. Delphi also says that it is an affordable technology which allows manufacturers to integrate a low-cost system.
The sensor measures PM in exhaust gas to help detect DPF leakage at required threshold levels. It uses a spin nut connection to control the direction of the sensing element for consistent performance. The heater is compatible with 12V or 24V electrical systems. It is capable of regeneration across a range of exhaust velocities and temperatures. And the sensor is offered with two shield lengths to match exhaust pipe diameter.
The variable shield length means the sensor is suitable for a wide variety of applications, from light to heavy-duty vehicles. This provides improved sensitivity by allowing exhaust gas sampling to occur close to the centre of the exhaust pipe. While in conventional systems, leakage is detected by a pressure drop across the filter, Delphi's particulate matter sensor features a resistive technique that directly measures PM levels in the exhaust stream, providing greater accuracy and reliability.
Bosch continues to develop a family of sensors designed to meet emission standards and OBD regulations, including a NOX sensor, a particulate matter sensor, a lambda sensor for diesel engines, and a differential pressure sensor.
The NOX sensor supports control of the required urea dosing in SCR systems for NOX reduction and OBD monitoring of the SCR components. The ceramic sensor element works on amperometric double chamber principle, measuring NOX content in exhaust gas. It can be used in light and heavy-duty vehicle applications and communicates via a CAN interface.
The particulate matter sensor enables diagnosis of the particulate filter. It is integrated into the exhaust tract, downstream of the filter, and the sensor function is based on resistance measurements. Adsorbed soot particles form conductive paths between electrode combs on which an electric current is flowing. The sensor element is regularly regenerated by heating, and the diagnostic software uses the measured current to evaluate DPF functionality.
A wide-band lambda sensor measures the residual oxygen content in the exhaust gas to help meet the strictest emission and OBD regulations. It uses a planer sensor element with integrated measuring cell and heater. the measured data serves to adjust the optimal air-fuel mixture via the air intake system. the latest generation sensor is specifically adapted for diesel engines.
Finally, a differential pressure sensor measures exhaust pressure difference across the particulate filter using a piezo-resistive sensor element. The measured value can be used to calculate the loading stage of the filter, serving as a precondition for demand-controlled, fuel-saving particulate filter regeneration.
Emission targets in the EU and across the world continue to become stricter, and as the onus moves to real-world driving emission limits, onboard diagnostics become more important. The general focus in recent years has been on diesel engines but new gasoline direct injection engines are also coming under closer scrutiny. The implementation of sensing equipment presents a challenge to the automotive industry because while costs must remain viable, robust diagnostic systems are required to meet OBD standards and ensure that vehicles on the road are not exceeding emission levels.