EGR System - Theory Behing BPT Valve?

[superlen]

Wade

I 100% agree with Captain.

1) At 2000rpm and a light throttle position (light load), L-Jet is going to meter out X amount of fuel (small pulse width) to match the incoming air flow.

2) At 2000rpm and WOT (heavy load, going up steep hill, trailer full of beer in tow), your engine is going to flow more air, & L-Jet is going to meter out X + Y amount of fuel to match.

Your statement has the phrase "just about the same amount of air". That's subjective and I'm assuming your "just about" may be a lot more than Captain & mines. I think the two conditions described will have greatly different air flows. This is assuming operating at steady state. If one is cruising at 2000rpm and then steps on it, for an instant it will still be the same, as it will take a bit for the air flow to increase, but it surely will.

Lenny

[Captain Obvious]

Lenny, I think you hit the nail on the head...

I got a big arse beer trailer, and apparently you do too.

I don't pull light beer. :cheeky:

[Zed Head]

One more angle to think about that might help - the pistons are pulling against the throttle blade. If the throttle was completely closed, no air would flow through the AFM, each stroke would produce a vacuum, except for any remaining exhaust gases. The more closed the throttle, the less "charge" (oxygen/fuel blend ready to burn) there is for each combustion event, because the air that entered is at lower pressure (fewer gas molecules). Opening the throttle increases intake manifold pressure, more molecules flowing through the AFM.

I've always thought of the EGR as a way to produce an effectively smaller engine also. The gas that's added to the intake system is essentially inert, it's oxygen has been burned. So the more EGR gas that's pulled in, the smaller the "charge" is, just like in a smaller engine, or like at low intake pressures (high vacuum/closed throttle). The fuel/oxygen ratio is still correct though, so proper combustion can still occur.

Beer.

[LeonV]

Since we're being pedantic, I'll make a few comments about performance effects of EGR...

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.

EGR does not "allow" increased spark advance, it DEMANDS it. EGR dilutes the fresh air/fuel mixture which slows the burn rate of the entire charge. Because of the slower burn rate, spark timing MUST be advanced in order to maintain performance. Thus, spark timing is increased to sustain performance, not increase it.

On the pumping loss point, it's not the decrease in exhaust pressure that reduces pumping losses, or at least that's not the main driving factor. The increase in intake pressure (holding load constant) is what boosts performance, as the intake has a throttle valve to deal with.

There's also another important factor in the performance bump and that's reduced heat loss to the cylinder and combustion chamber walls because of the lower flame temperature dictated by EGR. This means more of the fuel's energy is transferred to the piston and not lost as heat.

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.

Agreed.

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.

You're right. If you're talking about a diesel engine.

A spark-ignited internal combustion engine has a throttle, which throttles the amount of air that enters the cylinder (where's Captain Obvious? )! Volumetric efficiency is the key word here, and it's drastically different from easy cruise and WOT. This is a very basic, yet important fundamental concept of SI (spark-ignition) engines.

This is the whole reason for multiple-circuit carburetors!

Edited August 5, 2013 by LeonV

[Wade Nelson]

The original question was "Why use back pressure instead of the AFM signal" to determine load, for purposes of enabling or disabling EGR.

Can we agree that the AFM might output a similar signal for an engine at 2000 rpm, with the pedal floored, and, perhaps 4000 rpms with the engine loafing?

So a "raw" AFM signal gives no direct indication of load. You'd have to compare RPM's versus AFM signal in some sort of a look-up table and go "Hey, this is a very large AFM signal relative to RPM's, so that PROBABLY indicates a high load.

Except the L-Jetronic is analog, so a "lookup table" is beyond it's ken. I suppose some sort of slope comparator circuit COULD be devised to do it, but ..

So for purposes of MECHANICALLY enabling/disabling EGR, exhaust back pressure gives a simple, direct indication of load with no "processing" required. And it's inexpensive.

Maybe I'm EGR-dyslexic or something; sometimes my initial arguments are just wrong, but the feedback leads me to where I'm trying to go.

Ain't that great? Love this place!

Edited August 5, 2013 by Wade Nelson

[Captain Obvious]

I'm no expert on the subject, but my thinking behind the concept is:

The position of the AFM vain is directly proportional to the volume of air in.

The volume of air in is directly proportional to the volume of air out.

The volume of air out is directly proportional to the pressure in the exhaust manifold.

Therefore:

The position of the AFM vain is directly proportional to the pressure in the exhaust manifold.

That's why I was thinking that the AFM position could be used as an indicator of engine load. Seems like it should be proportional to backpressure.

I know the backpressure measurement is available and doesn't require any translation, but the thing is... What about exhaust system changes? What happens when you put on a free flowing low backpressure exhaust system? The air in doesn't change, but the pressure in the manifold does.

My PO put on a huge diameter exhaust and a free flow muffler. I don't know if I even generate enough backpressure to close off the BPT.

[superlen]

Can we agree that the AFM might output a similar signal for an engine at 2000 rpm, with the pedal floored, and, perhaps 4000 rpms with the engine loafing?

Wade,

I can agree with that, but I think the reason the two AFM readings are similar are because the loads are similar. I do think the raw AFM reading is proportional to load. (with a small temperature dependent caveat - see below)

Consider this argument:

1) In order to maintain a steady speed, the instantaneous power generated by an engine must equal the current load required of it.

2) If the load increases & power stays same, you will slow down...(You're driving your beer trailer @ 20mph flat and level though town & 40 of your closest drinking buddies jump on the trailer. You will slow down unless you step on the gas to shake those pesky drunkards, friendly as they are)

3) Once you punch the gas & they fly off into the ditch, you now have an excess of power than your load requires and you speed up.

If we agree on the above, now consider:

4) Power generated is a direct function of the mass of air flowing through the engine. All other things being equal, more air=more fuel = more power.

5) The stock AFM measures the mass of air flowing into the engine. Technically it measures volume of air & incoming air temp (IAT) & then uses (Boyles law) pv=nRt to calculate mass. ( A side note for others following along as I everyone discussing knows this) -- Our LJet ECMs don't calculate anything but just implement the final transfer function of the math via analog circuits to create a pulse width that is proportional to the mass of air entering engine.-- Regardless of how the ECM does it's task, what it does is meter FUEL mass to match AIR mass.

Having said all that, now back to the original question: Is the AFM proportional to Load? I say yes. We have shown that

1) load drives power requirement.

2) power generated is a function of MASS air flow through engine

3) MASS air flow is proportional to AFM volume air flow (albeit modified with Incoming Air Temp)

Therefore the position of the AFM vane & resultant signal is proportional to the load. Again with the caveat that IAT modifies it slightly.

Now, don't ask me about the EGR. I blocked mine off a long time ago & haven't thought about it since. Although after this discussion, I'm now rethinking that I have placed it on my list of things to investigate.

Lenny

[kacrow76]

Ok after doing some searching i came across this thread and wow. way more information then i needed. my question is can i remove the EGR? i ask this because i seen thishttp://datsunspirit.com/?wpsc-product=egr-block-off-plate-kit-for-280z280zx not sure i posted that right but anyway. my EGR is a eye sour and i thought that this would clean it up but now i am thinking that i am missing something.

but the question is still the same can i remove the EGR valve and what will it do?

[240260280z]

Now that i have re-read this old thread:

WRT the AFM following load, it is spot on. The throttle valve controls how much air is desired and the AFM measures how much actually flows. It is similar to the piston in an SU and that is why SU's work when a car is out of tune.... they follow the actual air flow, not the throttle valve.

[zKars]

Kacrow76, I believe C.O. and Wade will thank you for this post as it justifies all the effort and thought that went into this excellent discussion. The answers to your questions are, in fact, all explained in the text above. I suggest you re-read and see if additional insight can be gained. You are indeed missing something. One clue; Beauty is in the eye of the beholder....

[Captain Obvious]

my question is can i remove the EGR?

Could you? Sure. No problem.

Should you? Probably not.

Beauty is in the eye of the beholder.

[FastWoman]

I disagree: CAN you? No, at least in most states, according to the law, if you're going to drive the car on public roads. I don't know whether Kansas is such a state.

BTW, there's at least one reason the EGR wouldn't be controlled by the AFM output. That would require some sort of electrical servo as an output from the ECU, meaning more complexity required of Bosch's EFI. The EGR system is designed as a purely mechanical system, and that's something Nissan could do on their own, without requiring design changes from another company.