exhaust gas emissions tests with EGR leakage; first results
- March 31, 2014
- Marcel Romijn
BRACE Automotive is cooperating with the HAN University of Applied Science. In this project the effects of an EGR (Exhaust Gas Regeneration) leakage of a Diesel engine are measured. For BRACE this data is used to validate and further improve data analysis methods as used in customer projects on the topic of OBD. For the HAN this data is used to improve the knowledge level on emission control systems and the failure mode effects in these systems. See also the earlier blog posts on this topic.
As stated in the earlier blog post the Citroën C6 was equipped as part of this project with a system to have a controllable EGR leak to the environment. The car has a V6 Diesel engine with a separate Turbocharger and EGR system per cylinder bank. However on the inlet side the inlet air - as compressed by both Turbochargers - and the exhaust gas - as delivered by both EGR systems - are combined before being split up again to the cylinder banks. The leakage system was made into the tube between the front cylinder bank EGR valve and the combined inlet chamber. A T-split was made in the tube and a manually operated valve was inserted with some tubing after it to have a safe exit point for the gasses coming out as “leakage”.
As a result the engine was exposed to an overall lack of EGR with its origins in the EGR system of the front cylinder. Also due to the leaking to a lower pressure (the environment) additional exhaust gas is withdrawn from the system when the EGR valve is opened leading to less exhaust gas being send to the front Turbocharger. When the EGR valve is closed part of the Boost pressure in the inlet chamber is leaking out to the environment through the leakage. This shows that a simple leakage can lead to many complex disturbances.
By reading data from the Engine Electronic Control Unit (Engine ECU) some insight is available on the effects in the control loops of the Engine management. The data is read using the OBD system with a Citroën workshop tool.
The ECU data shows that both front and rear cylinder banks have the same EGR commands suggesting they are controlled by one control loop that does not distinguish between cylinder banks. Also it shows that with increasing size in leakage the EGR valves are commanded to open more widely and are even at times hitting the maximum 80% opening. When the EGR valves reach their limits the EGR control can’t achieve desired EGR rates. This will certainly increase exhaust emissions.
When looking at the Boost pressure it is visible that the leakage is impacting the Boost pressure as well. In the stronger accelerations in this section of the FTP75 the EGR system is shut-off. In these cases the Boost system does loose some pressure from the inlet chamber but it shows that the Boost pressure does not suffer by that much. When accelerating with EGR however a significant amount of energy from the exhaust gasses does not reach the front Turbocharger and in these cases the Boost pressure is lacking.
In the investigation of ECU data the operation of the Diesel throttle valve was also monitored. This valve however was not active during the FTP cycle, not even when the EGR system couldn't reach the desired EGR rates.
When looking at the actual emission data differences are visible between running normally and when running with leakages. Accuracies from the measurement are limited due to the fact that we used more “basic” emission measurement technology compared to the measurement systems that are used for example in type-approval.
The EGR system’s purpose is to reduce NOx emissions and has increased Soot or PM emissions as a drawback. The Citroën C6 is equipped with a Diesel Particulate Filter that can capture the Soot emissions. Our measurements for Soot however are done “Engine-out” meaning that we can measure how much Soot is produced before the DPF can filter it. The NOx emissions are however measured at the tailpipe.
The mid-size leakage shows the typical effect of EGR leakage - higher NOx and slightly less Soot. The large leakage however shows even higher NOx with a slight Soot increase as well. Possibly the increased loss of Boost pressure is to blame for slightly increased Soot. What comes out of the tailpipe however is only the additional NOx emissions since the DPF will capture the Soot emissions.
The purpose of OBDII and EOBD systems is to capture the failure modes, like this leakage, that give out these increased emissions at the tailpipe. More on that in a later update.