EGR and Detonation

[NICAT]

Hi Friends.
I know the Exhaust Gas Recirculation (EGR) system was designed mainly to reduce harmful emissions, yet many articles claim it also helps suppress detonation (knock). I still don’t fully understand how this works.
The usual explanation is that EGR displaces some of the fresh air with inert exhaust gas, so the ECU injects less fuel. Less fuel plus more inert mass lowers peak combustion temperature, reducing the chance of knock. However, if EGR is active and we want the same power, we naturally open the throttle farther, which increases total fuel flow. How can 50 % throttle with EGR consume less fuel—or run cooler—than, say, 40 % throttle without it? If the engine is doing the same work, shouldn’t it burn roughly the same amount of fuel unless the system is somehow more thermodynamically efficient?

Does EGR actually improve thermal efficiency, and if so, how? And finally, aren’t exhaust gases much hotter than fresh intake air to begin with?

Regards.

 

[Cujet]

In gas engines at typical flow rates, EGR affects BSFC slightly in both positive and negative ways. Your description above hits on it, when at "partial throttle" EGR can reduce pumping losses, this is a slight help as the engine does less work drawing in air.

In the past EGR could allow the use of higher compression and therefore better thermal efficiency. Today, direct injection allows the use of higher compression, coupled with variable valve timing to mimic EGR, we often don't use an actual EGR system anymore.

In a past life, I was involved in camshaft testing and dyno runs (cam installation, dyno installation etc) for a production engine (as a tech, not an engineer). EGR was used and IIRC did not help BSFC numbers in our case. The situation was always better off without it. Put another way, we were always able to tune the engine for zero EGR flow and get a better result. Using high rates of EGR required more ign advance.

EGR coolers are a thing.

 

[eljefino]

However, if EGR is active and we want the same power, we naturally open the throttle farther, which increases total fuel flow

I think this hypothesis is flawed. Hopefully EGR opening is seamless to the driver so the car doesn't feel like it's surging or nonlinear in throttle application. If the engine can be more efficient, it'll take the same throttle input or less. With modern controls like drive-by-wire throttles they can adjust curves in the computer as well to give a more pleasing driving experience.

Another way of thinking about it, is EGR is essentially "nothing". You could achieve the same effect with an argon bottle plumbed in, but of course it would be much more expensive. If you have 20% EGR flow and 50% throttle vs 40% throttle, you have the equivalent of a 20% smaller displacement engine in that given moment! And you've got a stoichometric mix of gas and air doing its thing, there's just an extra party invited that's taking up space and not making heat, so your peak temps are cooler.

Remember, kids, regular leaded gas was 89 octane. Tech that came out in the 70s like EGR and ESC let regular unleaded be a couple points lower.

 

[JRay’07]

I don’t believe that a wise engine designer would allow an event, like detonation, to occur just because an EGR system malfunctions. When I had an EGR valve go bad, it resulted in some light spark knock under acceleration. Detonation is catastrophic and can destroy an engine in short order.

EGR systems have been phased out in many gasoline engines. Exhaust valve overlap has been implemented into the camshaft profiles for the same end result.

 

[SwampSurvivor]

I've had a few Fords with EGR (and no knock sensor) and they have each pinged.

 

[50yr Street Mech]

In 1988, I bought a 1986 Buick with that shaky 6 cylinder for $ 50 bucks. Went to the local junkyard and bought a 262 Olds V-8. for $75 bucks. Gutless little V-8. Ripped out all the computer stuff in it, and dropped in the V-8. Came with a 2 bbl quadrajet carb. Then came time to run vacuum hoses. Plummed in the EGR valve, the engine ran fine, until you gave it gas. Knocking and pingin all over the place. Bought a replacement EGR valve, did the same thing. Scratched my young head for awhile. Capped off the EGR lead from the carburetor, and plugged the EGR hose with a bolt. That was that. Ran great. But, that don't apply to today's world. Computers got in the way of all the fun.

 

[Hohn]

EGR never improves BSFC. Full stop. Better BSFC means the fastest, hottest combustion you can achieve. EGR both slows and cools, both of which are worse for BSFC.

 

[Astro14]

In 1988, I bought a 1986 Buick with that shaky 6 cylinder for $ 50 bucks. Went to the local junkyard and bought a 262 Olds V-8. for $75 bucks. Gutless little V-8. Ripped out all the computer stuff in it, and dropped in the V-8. Came with a 2 bbl quadrajet carb. Then came time to run vacuum hoses. Plummed in the EGR valve, the engine ran fine, until you gave it gas. Knocking and pingin all over the place. Bought a replacement EGR valve, did the same thing. Scratched my young head for awhile. Capped off the EGR lead from the carburetor, and plugged the EGR hose with a bolt. That was that. Ran great. But, that don't apply to today's world. Computers got in the way of all the fun.

Your EGR wasn't working, then. The spark advance in the distributor was predicated on EGR - you would get more advance on an EGR engine (different weights in the dissy). If it was all knocking, then, the advance was there, but the EGR wasn't.

Now, which vacuum port did you use? Because there wasn't a 2 bbl Q-jet - A Q-jet was, by definition, a 4 bbl.

Now, it may not have been working if it the vacuum wasn't plumbed correctly. If you plumbed the EGR to a manifold source, or even a ported source (as opposed to the correct source that EGR needed), then the EGR would go closed at full throttle when it should have been open. That would cause crazy detonation and pinging. Your EGR wasn't working because of the wrong vacuum signal.

There were several ports on a Q-jet - only one of them worked for the EGR.

When you "ripped out all the computer stuff" - did you leave the HEI in place? They were all spring for EGR.

Block off the EGR, and put in a pre-EGR dissy with the non-EGR advance weights, and you're back in business.

I have been down this road with a small block Olds. Put a 1970 350 Olds in my 1977 engine. Because of wiring, I installed the 1977 HEI (which had the weights and springs for EGR) on a non-EGR 2bbl manifold. Horrible pinging.

So, I retrofitted the 1977 Q-jet and 4 bbl manifold with EGR onto the 1970 engine with the HEI.

Post in thread 'Oldsmobile 350 Rocket Motor'

Feb 26, 2017

I put a 350 Olds motor from a 1970 Cutlass in my 1977 Olds 98, to replace the 350 Olds engine that was in the car. It was a 2bbl motor with a points ignition.

Years later, I rebuilt the '70 engine. Among many things, installed a Competition Cams 252H cam with new lifters and I retrofitted the cast iron manifold, Q-jet and HEI distributor. It ran great.

Still miss that car. Your post brings back memories of fixed post aluminum rocker mounts, of fine tuning ring end gaps, of plastigauges in each bearing, of fabricating an oil pump drive to spin the pump with my electric drill before...

• Astro14

Ran perfectly.

 

[Exhaustgases]

EGR ? Lets see you put exhaust gases (me) into the combustion chamber instead of a full dose of air with its O2, its doing the opposite of what say a turbo or NOS does.

 

[eljefino]

EGR ? Lets see you put exhaust gases (me) into the combustion chamber instead of a full dose of air with its O2, its doing the opposite of what say a turbo or NOS does.

Exactly. It doesn't work at WOT, it's just something there for cruise conditions to improve emissions and efficiency.

 

[Cujet]

EGR never improves BSFC. Full stop. Better BSFC means the fastest, hottest combustion you can achieve. EGR both slows and cools, both of which are worse for BSFC.

When I was involved in engine testing, EGR had no upside. None what so ever. We were testing turbocharged, conventionally fuel injected 4 cylinder automotive engines with the lower, 8 to 1 compression pistons. In those conditions, EGR, when it was tested was quite simply, awful.

However, it does seem there are a specific set of operating conditions where EGR can provide as much as a 3% boost in BSFC. Mostly at part throttle, and as a way to reduce pumping losses. There are other situations where EGR can enable a higher compression ratio to be used. But good God, I'm not defending EGR. I despise it.

 

[50yr Street Mech]

Hmmmm....then it wasn't a q-jet....It was a 2bbl with the same name as a q-jet...I forget the name....I was just 17. Yeh, I plumbed it wrong, no doubt. Stupid kid. No books. I went back to points and condenser, because that's what was on the engine when I got it from the junkyard. Used to enjoy dwelling and using the timing light. One little click on that starter, and it started right up. Have no idea what year that 262 V-8 was, but it bolted right up to the existing motor mounts, and that P.O.S. TH200 tranny. Anyway, had my fun with it, and thought who I was for dumping a V-8 into a V-6 car. Sold it a few months later, made enough $$$$$ off it to cover my costs. Can't put a price on labor when it's something you love doing ....Thanks for all you're input guys !!!!!!

 

[iCurious]

If we are just talking EGR System in general then the purpose is to reduce NOx emissions and improve fuel economy. By reducing NOx - you are also reducing the temperature in the combustion chamber. Therefore, it alleviates the chance of it pinging or to detonate. It will not eliminate it completely due to other factors.

 

[oil pan 4]

In 1988, I bought a 1986 Buick with that shaky 6 cylinder for $ 50 bucks. Went to the local junkyard and bought a 262 Olds V-8. for $75 bucks. Gutless little V-8. Ripped out all the computer stuff in it, and dropped in the V-8. Came with a 2 bbl quadrajet carb. Then came time to run vacuum hoses. Plummed in the EGR valve, the engine ran fine, until you gave it gas. Knocking and pingin all over the place. Bought a replacement EGR valve, did the same thing. Scratched my young head for awhile. Capped off the EGR lead from the carburetor, and plugged the EGR hose with a bolt. That was that. Ran great. But, that don't apply to today's world. Computers got in the way of all the fun.

Usually the vacuum advance is limited around 10 or 12 degrees when using egr.
Too much vacuum advance with egr definitely can do that.

 

[50yr Street Mech]

It did eat plenty of gas for a 262, but gas was cheap back then. I remember taking all the little marbles out of the cat, then putting back the freeze plug. That gave me a bit more power. Like I said, I was just a dumb kid, 17, playing grease monkey. That was 42 years ago. Simpler days, having greasy fun. My parents thought I was abnormal. " You should be out playing baseball or stick ball ! " they would scream at me. Meanwhile, when I was like 10-11, I bought a Revell model. It was a working plastic V-8. Took me days to put that together. Had little lights for the spark plugs. Maybe I am abnormal ? It's something in you're blood, you can't get it out. I'm still playing with cars today, but I found a forum for all of us " abnormal" people. That makes me happy Thanks guys for making me feel better

 

[NICAT]

EGR ? Lets see you put exhaust gases (me) into the combustion chamber instead of a full dose of air with its O2, its doing the opposite of what say a turbo or NOS does.

"LOL, you gave me a laughing fit with the word ‘me’! "

 

[kschachn]

Does EGR actually improve thermal efficiency, and if so, how? And finally, aren’t exhaust gases much hotter than fresh intake air to begin with?

Exhaust gas (especially after it reaches the EGR valve) is cooler than the peak combustion temperature. All it does is dilute the combustion mixture and reduce the temperature so oxides of nitrogen are less likely to form. Everything else is a detriment not a benefit.

 

[Exhaustgases]

All ICE can be referred to as "heat" engines. Its the Heat that makes the push. If you delete some of the O2 and lowering the temp your losing power. Maybe Top A fuel should start using EGR, someone said it doesn't work at WOT, it does on a diesel there is no throttle or air restriction, at least on the industrial ones.

 

[Hohn]

All ICE can be referred to as "heat" engines. Its the Heat that makes the push. If you delete some of the O2 and lowering the temp your losing power. Maybe Top A fuel should start using EGR, someone said it doesn't work at WOT, it does on a diesel there is no throttle or air restriction, at least on the industrial ones.

We also don't use EGR on our large industrial units. SCR is sufficient for NOx reduction and has superior reliability vs EGR.

We were able, in testing, to create conditions where it was condensing acid in the intake manifold. Turns out that the moisture in EGR plus NOx is a pretty bad combination for hot ferrous bits.

SCR has a lot to recommend for large industrial engines where long life is still expected.

 

[kschachn]

We also don't use EGR on our large industrial units. SCR is sufficient for NOx reduction and has superior reliability vs EGR.

We were able, in testing, to create conditions where it was condensing acid in the intake manifold. Turns out that the moisture in EGR plus NOx is a pretty bad combination for hot ferrous bits.

Ya think? Mixed oxides (including sulfur), heat and a bunch of water vapor. What could go wrong?