Can someone explain this to me?
01-10-2006, 10:43 PM
#1
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Can someone explain this to me? http://www.misterfixit.com/deton.htm
Item number 2 under "A Dozen Ways to Prevent Detonation"
My brain really hurts now. I knew the primary use of the EGR system was to reduce emissions but reducing knock is a new one to me. I always thought that the reason people run richer than stoich for maximum power is because the added fuel reduces combustion temps and therefore allows more timing (which usually equals more power). In theory 14.63:1 would allow the maximum power, but you can't run that high under load becuase the combustion temps are too high and you'll burn your engine up.
With this in mind, how then does leaning out the AFR then help reduce detonation (which is in essence unwanted ignition from excessive tempuratures)? I are confused
Item number 2 under "A Dozen Ways to Prevent Detonation"
2. Check for loss of EGR. The Exhaust Gas Recirculation (EGR) system is one of the engine's primary emission controls. Its purpose is to reduce oxides of nitrogen (NOX) pollution in the exhaust. It does this by "leaking" (recirculating) small amounts of exhaust into the intake manifold through the EGR valve. Though the gases are hot, they actually have a cooling effect on combustion temperatures by diluting the air/fuel mixture slightly. Lowering the combustion temperature reduces the formation of NOX as well as the octane requirements of the engine.
If the EGR valve is not opening, either because the valve itself is defective or because its vacuum supply is blocked (loose, plugged or misrouted vacuum hose connections, or a defective vacuum control valve or solenoid), the cooling effect is lost. The result will be higher combustion temperatures under load and an increased chance of detonation.
If the EGR valve is not opening, either because the valve itself is defective or because its vacuum supply is blocked (loose, plugged or misrouted vacuum hose connections, or a defective vacuum control valve or solenoid), the cooling effect is lost. The result will be higher combustion temperatures under load and an increased chance of detonation.
With this in mind, how then does leaning out the AFR then help reduce detonation (which is in essence unwanted ignition from excessive tempuratures)? I are confused
01-10-2006, 11:22 PM
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I dont think the EGR 'leans out' the air/fuel mixture. It adds in more dead-air (non-combustible, like nitrogen) that can't be used for combustion, this dead air helps flush the heat out of the combustion chamber and reduce combustion temps. Really the air/fuel mixture stays relatively the same or actually gets richer. You end up with the same amount of fuel with a less oxygen rich air.
Same thing goes for nitrous injection. It would seem to make more sense to just inject pure oxygen and fuel, but it would burn WAY too hot / fast even at super low a/f ratios. To solve that problem, you inject nitrous oxide, two parts nitrogen / one part oxygen. The nitrogen helps keep the heat down.
If you have tunercat or lt1edit (ls1edit and ls1 tuning programs should have this too) you will find that when the EGR comes on, the timing will actually advance 2-5 degrees as well. Why? HC and NOX values on a sniffer.
In a basic car without AIR / EGR the only way to change these two values is to change a/f ratio and change timing. Lean out the a/f mixture and add timing, you get a hotter more efficient combustion. The result is low HC and high NOX. Richen up your a/f mixture and reduce timing and you get lower temp, less efficient combustion, but your HC will shoot up because of the unburned fuel.
The EGR allows you to get a complete combustion (low HC levels), and keep combustion temp and inherently knock and NOX values low.
Also EGR is only effective to around 2000 rpm. So it wont help reduce knock at any decent rpm.
Same thing goes for nitrous injection. It would seem to make more sense to just inject pure oxygen and fuel, but it would burn WAY too hot / fast even at super low a/f ratios. To solve that problem, you inject nitrous oxide, two parts nitrogen / one part oxygen. The nitrogen helps keep the heat down.
If you have tunercat or lt1edit (ls1edit and ls1 tuning programs should have this too) you will find that when the EGR comes on, the timing will actually advance 2-5 degrees as well. Why? HC and NOX values on a sniffer.
In a basic car without AIR / EGR the only way to change these two values is to change a/f ratio and change timing. Lean out the a/f mixture and add timing, you get a hotter more efficient combustion. The result is low HC and high NOX. Richen up your a/f mixture and reduce timing and you get lower temp, less efficient combustion, but your HC will shoot up because of the unburned fuel.
The EGR allows you to get a complete combustion (low HC levels), and keep combustion temp and inherently knock and NOX values low.
Also EGR is only effective to around 2000 rpm. So it wont help reduce knock at any decent rpm.
01-11-2006, 07:01 AM
#3
some modern engines also dump the oil vapour into the intake aswell! this reduses the octane of the fuel air mix increasing the chance of det!!
as for the other stuff, i think it has been exsplaned perfectly well above!
Chris.
as for the other stuff, i think it has been exsplaned perfectly well above!
Chris.
01-11-2006, 11:14 AM
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When it says "under load" it doesn't mean WOT.
EGR doesn't burn with the fuel, but is just a filler that dilutes and slows the burn rate.
Factory spark advance programming takes EGR into account and adds extra spark advance to make up for the slower burn rate. If your EGR isn't functioning, this extra spark advance can be too much and you get spark knock and possibly detonation from the extra heat.
From my experience this is actually a rare happening and most often seen on high mileage cars with less than adequate cooling systems.
Bottom line is that your spark advance has to match your burn-rate and the ability of your cooling system to take away heat.
The EGR is only a real factor on a pretty much unmodified engine.
EGR doesn't burn with the fuel, but is just a filler that dilutes and slows the burn rate.
Factory spark advance programming takes EGR into account and adds extra spark advance to make up for the slower burn rate. If your EGR isn't functioning, this extra spark advance can be too much and you get spark knock and possibly detonation from the extra heat.
From my experience this is actually a rare happening and most often seen on high mileage cars with less than adequate cooling systems.
Bottom line is that your spark advance has to match your burn-rate and the ability of your cooling system to take away heat.
The EGR is only a real factor on a pretty much unmodified engine.