EGR System - Theory Behing BPT Valve?

[Captain Obvious]

So... About the EGR system.

What's the theory behind the Backpressure Transducer (BPT) Valve? I know what it does... It disables the EGR system if the exhaust backpressure is too low. Question is "Why?"

What would be wrong with having the EGR enabled even when the exhaust backpressure is low? Seems to me that it would simply recirculate less gas under low exhaust pressure conditions. Is there really a need to disable it completely under those conditions?

[240260280z]

I guess if the EGR opens when the exhaust has less pressure than the intake manifold (low "rleative" back pressure) then the intake charge may flow into the exhaust rather than into the head. It probably has to do with pressure waves and pulses.

It may also be a fail safe feature if the exhaust is crushed or clogged when there is way too much back pressure and it would over fill the manifold with exhaust gasses.

[Captain Obvious]

Thanks for the thoughts Blue.

The failsafe suggestion is backwards. What you suggested makes sense, but the valve works the other way. It enables the system if there is high backpressure, not disables it. If the exhaust is clogged or crushed, the higher backpressure would open the BPT and the system would be "happy".

I'm thinking you're right about the waves and pulses. It's probably a second order phenomenon. The average exhaust pressure is way higher than the average intake pressure (duh), but there might be intake and/or exhaust waves that would reverse that situation for short burst transients.

However... I wonder what is the speed of response of the BPT and EGR diaphragm is. I thought I read somewhere that the EGR will stay open for 30 seconds after vacuum is removed. I'll see if I can find that again.

[240260280z]

Found this: Planetisuzoo.com ? View topic - egr backpressure transducer

Seems like it has a similar purpose to the manifold vacuum line to the FPR in fuel injected cars.

[Wade Nelson]

"I know what it does... It disables the EGR system if the exhaust backpressure is too low."

I don't think that's quite right Captain. I'm pretty sure it disables EGR if backpressure is too high at low-to-mid throttle openings (high to middle intake vacuum).

The Japanglish in the FSM may be a bit confusing here.

If you've got a high backpressure (load) situation you don't want EGR "flooding" the intake manifold when it's most "vulnerable," which could cause a serious stumble right when you are calling for maximum power.

EGR, in theory, when it's working correctly, should BENEFIT an engine with higher efficiency. How, you ask?

EGR SHOULD allow increased spark advance without pinging at mid throttle and load levels. It should also decrease backpressure (ever so slightly), and reduce pumping losses.

But here's the thing. I've seen hundreds of cars with inoperative EGR systems. In most cases it fails closed. And yet there's seldom any pinging, no noticeable loss of mileage, etc. Only in the few cases where it fails open and causes a rough idle or stumble on take-off does anybody ever bother to fix it.

So what does that mean, really? That many makes of cars are leaving 2-7 degrees of spark advance (meaning power and MPG's on the table?) Perhaps. On the Z's, perhaps owners are compensating by running premium fuel where they might be able to get away with regular with a properly functioning EGR???

On Hondas in particular, EGR tends to clog individual passages in the intake manifold until a single cylinder is receiving ALL the EGR gas, causing a rough running situation. On older Nissans like the Z it's usually the EGR valve that sticks (closed) and no harm, no (apparent) foul.

I've always wanted to "clean up" an EGR system on a car like a Z and carefully measure what sort of spark advance was tolerable, if any MPG improvement was discernable, etc. Never have had time to bother. Too busy driving it and having fun!

Like air pumps, catalytic converters, and other pollution control equipment, a lot of owners consider EGR a boogeyman and immediately rip it off or disable it instead of going to the time and expense of getting it working properly. Pity.

References:

EMISSIONS SUB SYSGTEMS -- EGR

Toyota Factory Documents

http://dept.sfcollege.edu/InTech/AUTO/content/courses/AER2840%20materials/Toyota%20EGR%20systems.pdf

So Why do we have EGR

by Glen Beanard

Better Breathing: EGR Improvements: Underhood Service

EGR Systems - Operations & Diagnosis

By Henry Guzman

Australian 300ZX Owners Association Technical Library: Content / EGR (Exhaust Gas Recirculation) / EGR, what does it do?

Edited August 3, 2013 by Wade Nelson

[Wade Nelson]

Here's a case study about a problematic EGR system on a newer vehicle (Chevy Tracker). It incorporates ECM control of an EGR solenoid, but the key take-away is this:

P0400 Exhaust Gas Recirculation (EGR) Low Flow

"The pressure transducer regulates the amount of vacuum to the EGR valve depending on backpressure in the exhaust (engine load). Unless at wide-open throttle, the more the load, the more the EGR valve opens."

Wide-open throttle = zero intake manifold...

Another interesting EGR read:

http://www.tomco-inc.com/Tech_Tips/ttt9.pdf

Edited August 3, 2013 by Wade Nelson

[Captain Obvious]

I don't think that's quite right Captain. I'm pretty sure it disables EGR if backpressure is too high at low-to-mid throttle openings (high to middle intake vacuum).

The Japanglish in the FSM may be a bit confusing here.

Thanks Wade, but that's not the case. I just checked the operation of the BPT and it works just like the manual says it should... At high backpressure it passes control vacuum to the EGR valve, but at low backpressure it vents that control vacuum to atmosphere thereby disabling the EGR system.

Not saying it makes sense, but I just confirmed that's really, truly what it does.

Thanks for the other info as well. I haven't dug into it, but I will when I get the chance.

[Wade Nelson]

We're BOTH right here, Captain. There is negative logic AND positive logic at play.

At sufficient load (backpressure) the BPT indeed PASSES control vacuum. But at high throttle openings there is very low manifold vacuum so the EGR valve itself doesn't get pulled open even with the BPT wide open.

If the engine is lightly loaded (low backpressure) you don't WANT EGR. EGR is for mid-to high loads, but NOT WOT.

All engines - GM, Ford, Nissan, Toyota schedule EGR under pretty much the same operating conditions. The Toyota document shows a really informative graph of this.

Edited August 3, 2013 by Wade Nelson

[Captain Obvious]

Wade, I went through all the documents you linked to and I understand the theory now. You just threw me when you said that it "disables EGR if backpressure is too high at low-to-mid throttle openings", because the BPT never disables the EGR for high backpressure, only for low backpressure. I'm sure it's just a confusing use of words.

In any event, here's the concept for the rest of the readers who haven't figured it out by now...

The 280Z EGR system uses vacuum to control the amount of EGR opening. The more the vacuum, the more the EGR valve is opened. The source of that vacuum comes from a small port (small pin sized hole) which is located up in the throttle body. It's location within the throttle body is in a position such that it provides little to no vacuum at idle or at WOT. That means that even without any other manipulation of the control vacuum, EGR will always be disabled at idle and at WOT.

However, Datsun decided that simply being disabled at idle and WOT wasn't good enough and used the BPT to modify the vacuum signal profile between those two points to better fit the engine and/or emissions requirements. The BPT uses exhaust backpressure as an indication of engine load... More backpressure, higher engine load, more EGR is desired. Less backpressure, lower engine load, less ERG is desired.

The BPT is a normally open valve that vents (bleeds off) the EGR control signal to atmosphere, thereby disabling the EGR system. The BPT valve will close as the exhaust backpressure increases and as the valve closes, the less the EGR control vacuum will be bled off, until you reach an exhaust backpressure condition such that the BPT valve is completely closed and all the available vacuum is fed to the EGR valve with none of if bleeding off to atmosphere.

Thanks again Wade for points to info. Makes perfect sense to me now.

[Captain Obvious]

Getting off a little into the theoretical...

Wouldn't AFM position be a good indication if engine load? The more air in, the more air out, right? If one were to try to come up with an electrical representation of engine load, would AFM resistance be a simple option since it's already easily available?

Just thinking that might be an alternative to exhaust back pressure as a means to determine and modulate EGR vacuum.

[Wade Nelson]

Wouldn't AFM position be a good indication if engine load?

Hey, great summary of our discussion Capt. I think that'll really help other Z owners.

As for AFM indicating load, this is how I think about it:

Imagine you're in 5th gear at 2000 rpms, at an easy cruise.

Whether you're on the gas pedal lightly or have it shoved to the floor, just about the same amount of airflow will be going past the AFM. At 2000 rpms the engine is going to pump virtually the same amount of air regardless of throttle opening.

Opening the throttle up WILL increase back pressure, however, because you've reduced pumping pressure losses past a closed throttle, even though the volume stays ABOUT the same. The engine isn't having to SUCK the air in as hard, so it can use the same pump to EXPEL it harder.

[Captain Obvious]

Imagine you're in 5th gear at 2000 rpms, at an easy cruise.

Whether you're on the gas pedal lightly or have it shoved to the floor, just about the same amount of airflow will be going past the AFM. At 2000 rpms the engine is going to pump virtually the same amount of air regardless of throttle opening.

Opening the throttle up WILL increase back pressure, however, because you've reduced pumping pressure losses past a closed throttle, even though the volume stays ABOUT the same. The engine isn't having to SUCK the air in as hard, so it can use the same pump to EXPEL it harder.

Wade,

I might be missing something, but that doesn't sound right to me.

At a fixed RPM, the air volume passing through the AFM cannot possibly be the same at easy cruise as it is at WOT. I guarantee that there is more air flowing through the AFM at WOT than there is at light cruise. That's the whole principal behind L-Jet! If it weren't for that, then the system wouldn't work at all.

At a fixed RPM... More air volume through the AFM at higher throttle openings, more gas supplied by the ECU accordingly, and more power to the wheels as a result.

L-Jet in a nutshell.

Think about it this way... Cruising along the highway at steady speed. Come to a hill and have to press the gas pedal down farther to maintain that steady speed. What happened?

The further down gas pedal allowed the engine to suck more air through the intake system (including the AFM). The increase in air through the AFM is detected by the ECU and it injects more gas accordingly. More power to the wheels as a result, and hence, steady speed even in the face of increasing load requirements.

That's why I'm thinking the AFM resistance would be a good indicator of load.