EGR Valves... Confused.
Discussion
Hi guys,
Just wondering if someone can explain to me how exactly the EGR valve works.
It came up at work yesterday that a colleagues Discovery had problems and it was suspected that it could be MAF or EGR valves.
I was able to explain to him what the MAF sensor was, where it was but I hadn't a clue about EGR valves. I googled for it and was rather confused by the general explanation of them:
The valve recirculates exhaust gases back into the cylinder to aid in cooling.
and that's where I got confused, how can hot exhaust gases help cool the engine.
If anyone cares to enlighten me that'd be just dandy!
Just wondering if someone can explain to me how exactly the EGR valve works.
It came up at work yesterday that a colleagues Discovery had problems and it was suspected that it could be MAF or EGR valves.
I was able to explain to him what the MAF sensor was, where it was but I hadn't a clue about EGR valves. I googled for it and was rather confused by the general explanation of them:
The valve recirculates exhaust gases back into the cylinder to aid in cooling.
and that's where I got confused, how can hot exhaust gases help cool the engine.
If anyone cares to enlighten me that'd be just dandy!
The explanation you found was wrong, would be the best answer.
EGR is about emissions and economy. Having already burnt almost completely, the exhasut gas is effectively an inert gas like halon (for the sake of this argument).
The engine has been designed to deliver a certain amount of energy/power from each stroke - great on full throttle, but inefficient at part-throttle/cruising/tickover.In the traditional model you are restricting power by the throttle (causing lower cylinder pressureand choking the engine) or restricting fuel flow (but there are limits to this where the mixture fails to ignite or the flame fails partway through combustion). Both reduce fuel efficiency, and can increase certain emissions.
By pumping some back into the next cycle of the cylinder, some of the combustion volume is fuel/air and some inert gas (EG). You can get the ignition and full burn using less fuel per stroke, but still leaner than if you had fresh air rather than inert EG (that one being explained in the chemistry). You still need to control power with either a throttle or fuel injection (e.g. with EGR the throttle would need to be slightly more open, but still providing less fuel), and any half-decent ecu system can control this.
So:
EGR is about emissions and economy. Having already burnt almost completely, the exhasut gas is effectively an inert gas like halon (for the sake of this argument).
The engine has been designed to deliver a certain amount of energy/power from each stroke - great on full throttle, but inefficient at part-throttle/cruising/tickover.In the traditional model you are restricting power by the throttle (causing lower cylinder pressureand choking the engine) or restricting fuel flow (but there are limits to this where the mixture fails to ignite or the flame fails partway through combustion). Both reduce fuel efficiency, and can increase certain emissions.
By pumping some back into the next cycle of the cylinder, some of the combustion volume is fuel/air and some inert gas (EG). You can get the ignition and full burn using less fuel per stroke, but still leaner than if you had fresh air rather than inert EG (that one being explained in the chemistry). You still need to control power with either a throttle or fuel injection (e.g. with EGR the throttle would need to be slightly more open, but still providing less fuel), and any half-decent ecu system can control this.
So:
- On full throttle, no EGR
- On part throttle, some EGR which takes up space in the cylinder, this means less fuel going into each combustion cycle (so like a leaner mixture, or smaller cylinder capacity). For the same, less-than-maximum, power requirment, less fuel, better efficiency and reduced emissions.
- Your ECU balances power demand, how much EGR and how much fuel delivered.
Ahhh excellent description Rich, thank you!
That makes a lot of sense, unlike the original explanation I'd found!
(just to make sure I've got this right)
So the exhaust gases just serve as a filler to ensure highest possible combustion ratio? Whereby pumping excess oxygen in would actually smother the spark->flame?
Safe to say that that is therefore almost an active (for lack of a better word) reduction in the cc of the engine when at <full throttle?
Still... sounds rather risky. I guess EGRs aren't used on performance vehicles, just commerical and shopping cars?
Thank you for answering Rich!
That makes a lot of sense, unlike the original explanation I'd found!
(just to make sure I've got this right)
So the exhaust gases just serve as a filler to ensure highest possible combustion ratio? Whereby pumping excess oxygen in would actually smother the spark->flame?
Safe to say that that is therefore almost an active (for lack of a better word) reduction in the cc of the engine when at <full throttle?
Still... sounds rather risky. I guess EGRs aren't used on performance vehicles, just commerical and shopping cars?
Thank you for answering Rich!
GT Kodiak said:
So the exhaust gases just serve as a filler to ensure highest possible combustion ratio? Whereby pumping excess oxygen in would actually smother the spark->flame?
Sort of. Ideally, you need one air/fuel ratio to initiate a flame, and another, leaner one can maintain the flame. Those are your bottom limits. Running very lean can reduce all the noxious emissions (until misfire) and reduce fuel consumption. But misfire restricts the lean limt. More importantly, the three-way catalyst requires exhaust gasses at the stoichiometric (chemically correct) ratio - that can mean a lot of unnecessary fuel. The EG has less effect on the combustion than fresh air, although there is still a limit how much you can add. Then the (chemically correct) EG recirculated goes with the (chemically correct) new combustion down the exhaust to the cat as a chemically correct mixture. Combustion process is happy and the cat is happy.GT Kodiak said:
Safe to say that that is therefore almost an active (for lack of a better word) reduction in the cc of the engine when at <full throttle?
Yes, that is a good analogy - with EGR is like a smaller cc engine running at closer to full throttle (a more effiicient condition), with the ECU balancing the EGR and more open throttle.GT Kodiak said:
Still... sounds rather risky. I guess EGRs aren't used on performance vehicles, just commerical and shopping cars?
The risks are of a misfire, or damaging the cat. With modern engine management systems and a handful of sensors, these are easy to control so the risk is very limited. The concept was developed more than 30 years ago and (on Japanese cars more than SU-carbed MGBs) worked OK back then. Snesors and mapping have advanced immensely since then.Would the EGR Valve be affected by a replacement ECU or is it completely mechanical?
In either guise is if affected by backpressure?
Colleagues LR Discovery V8 Diesel Auto feeling bogged when cold starting, after it's warmed up everything seems fine.
If the EGR is mechanical and affected by backpressure seems reasonable to me that it's getting trapped open until sufficent pressure has built up.
In either guise is if affected by backpressure?
Colleagues LR Discovery V8 Diesel Auto feeling bogged when cold starting, after it's warmed up everything seems fine.
If the EGR is mechanical and affected by backpressure seems reasonable to me that it's getting trapped open until sufficent pressure has built up.
HiRich said:
By pumping some back into the next cycle of the cylinder, some of the combustion volume is fuel/air and some inert gas (EG). You can get the ignition and full burn using less fuel per stroke, but still leaner than if you had fresh air rather than inert EG (that one being explained in the chemistry).
The primary purpose of EGR is to reduce the formation of nitrogen oxides, by slowing combustion and lowering peak temperatures. Reduced throttling losses are a useful side effect in petrol engines only.GT Kodiak said:
Would the EGR Valve be affected by a replacement ECU or is it completely mechanical?
In either guise is if affected by backpressure?
Colleagues LR Discovery V8 Diesel Auto feeling bogged when cold starting, after it's warmed up everything seems fine.
If the EGR is mechanical and affected by backpressure seems reasonable to me that it's getting trapped open until sufficent pressure has built up.
The EGR valve is usually driven by a vacuum, which is admitted by an ECU controlled solenoid valve. If the replacement ECU is identical to the original it's unlikely the EGR function would be changed.In either guise is if affected by backpressure?
Colleagues LR Discovery V8 Diesel Auto feeling bogged when cold starting, after it's warmed up everything seems fine.
If the EGR is mechanical and affected by backpressure seems reasonable to me that it's getting trapped open until sufficent pressure has built up.
EGR valves tend to get gummed up with sticky black tar from the exhaust gasses and can stick open causing rough running. It's not beyond the bounds of plausibility that heat could free things of sufficiently to restore operation.
Edited by Mr2Mike on Wednesday 16th June 22:30
Edited by Mr2Mike on Wednesday 16th June 22:31
Mr2Mike said:
EGR valves tend to get gummed up with sticky black tar from the exhaust gasses and can stick open causing rough running. It's not beyond the bounds of plausibility that heat could free things of sufficiently to restore operation.
Is there a safe/effective way to clean these out and reuse them?Edited by Mr2Mike on Wednesday 16th June 22:31
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