EGR restriction influence on particulate matter formation

[Ross_HDTech]

Hi all,
Can someone please explain the science behind increased particulate matter formation resulting from restricted EGR flow in a diesel engine? I often see DPF failures associated with EGR cooler fouling(without coolant leaks). I know that any disruption to airflow can upset AFR's, and I see it more often with engines with intake throttle valves. So my assumption is that the valve closes more to try to increase EGR flow, causing a rich burning mixture. But shouldn't the ECM see the reduced airflow and adjust the fueling? Or because of diesel fuels ability to operate in a wide AFR range, the ECM just adds more fuel to compensate to keep the same RPM at a given load? Thanks in advance!

 

[BrianPetersen]

Don't know about others but the VW engines that have an intake throttle in order to regulate EGR flow, have a wide-band lambda sensor as a feedback device, the intent being to preclude rich operation. It is also part of the control strategy for regeneration.

 

[Lou Scannon]

You need to know the theory of operation of the particular system you're working on. Failing that, the manufacturer's troubleshooting instructions.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[Ross_HDTech]

That makes sense. Now I am curious to know why heavy duty diesel engines don't use lambda sensors. I assumed that their fuel mapping inputs would give enough data for them to identify unfavorable conditions, and adjust fueling accordingly. Or at least trigger a fault code if the ECM determines it can't compensate. But in my experience, the DPF becomes restricted, and that sets a fault code, but there are no faults for restricted EGR flow. I guess I just don't understand why an ECM would allow after treatment damaging conditions with out setting a fault.

 

[Ross_HDTech]

You need to know the theory of operation of the particular system you're working on. Failing that, the manufacturer's troubleshooting instructions.

I completely agree with you Lou. However, manufacturers don't always give detailed information. And sometimes its proprietary. Same with troubleshooting. It will tell you what to check, but not why.

 

[Lou Scannon]

Typically the owner of such a product is entitled to get an OEM service manual, with troubleshooting instructions. It may be necessary to pay for such a manual, though.
Theory of operation may be harder to come by, but the better service manuals include it.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[mc5w]

I have also seen at least 1 instance where somebody deliberately maladjusted their diesel engine ( in this case a pickup truck ) to produce soot every time they stepped on the accelerator. This can be done by backing out the screw that limits the maximum displacement of the injector pump causing the engine to have the ability to operate rich at high power. In that case the pickup truck owner probably thought that black smoke generation was cool.

Back in 1971 New Jersey passed a law that diesel engine motor vehicles could not produce visible smoke except when starting cold. In reality you do not get more power by putting excess fuel into an engine you just waste fuel. Running a diesel engine rich enough to make visible smoke might even cause maximum power to decrease. Indeed, when gasoline engines went to computer controlled fuel injection with exhaust oxygen sensors engine efficiency and maximum power went up!!!! This was because carburetors were putting too much fuel into engines which robbed them of efficiency and power.

 

[Lou Scannon]

Your explanations above are simplistic and not really correct.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

In reality you do not get more power by putting excess fuel into an engine you just waste fuel.

Diesel engines produce more power as you increase the fuel rate. This trend continues up to and beyond the point where smoke becomes visible. Returns diminish of course - and the percentage of fuel wasted increases.

je suis charlie