treyZ28

Should timing be advanced for lean cruise since the burn is slower?

I'm a bit lost, to be honest when it comes to detonation- mainly due to misinformation. I was reading J-Rod's post on the subject and I'm confused.

If you make the spark come earlier (ie further from TDC) it is more likely to detonate? Pressure and temp are rising, so wouldn't the start of the flame earlier be better for it? It is because the "peak of the burn" comes closer to TDC? Retarding timing (making it come later) pushes it further ATDC?

Now if leaner means a slower burn, does that explain the higher tendency to knock? it seems counter intuitive. It seems like it would be the same thing as retarding timing.


Lastly, and a bit off topic: why does leaner = better gas mileage? It should have all the air it needs at 14.68 or whatever. Add a bit more air and you're at ~15:1. Why would any leaner result in more energy? or is this a case of "it just does?" I certainly cant understand why it would result in higher EGT for the life of me. It seems like the same amount of thermal energy is distributed amongst more air

My90Iroc

iTrader: (26)

"Lean Cruise" and "detonation" probably aren't 2 concepts that are going to be linked very often. Lean cruise will be a light load, light throttle condition and therefore will have a high tolerance for detonation.

That being said, both concepts need to be looked at in terms of timing and fueling. Generally speaking a leaner condition will have a faster burn rate, less timing, and higher EGT's. The faster burn rate will occur simply because there are more air molecules available for the fuel molecules to find and mix with for the burn. In a leaner condition cylinder temps will go up which in turn makes cylinder pressure increase. This will cause you to use less timing since the explosion will be more powerful naturally. Max torque will be acheived by having the max pressure (total combustion) a little after TDC and with the faster burn rate you can see how retarding the timing helps this (to a point). Retarded timing also increases EGT's since obviously there's less time for heat dissapation before the exhaust valve opens versus more advanced timing.

A richer mixture allows for more advanced timing. Obviously there's more fuel in the cylinder and the extra fuel helps cool the temp in the cylinder. This helps lower pressure and create a slower burn rate. Between that and the earlier timing EGT's will go down in a richer condition.

Therefore a "lean cruise" condition would want less timing. Slightly leaner than stoic does help fuel mileage but it's usually minimal, like maybe 2% and keeps you farther from an a/f that you'd want during tip in if you decided to open the throttle a little.

RednGold86Z

Leaner than stoich burns a little slower, so you'll need to advance (ideal peak pressure is 14-17degrees ATDC, depending on which mad scientist is measuring the pressures). -

Stock timing table is usually a little less than MBT, for emissions. But, in Lean mode, you're disregarding emissions (NOx). So, neglect the normal retard (and add a few more for the slower burn). +

Running leaner reduces the power a little, so you open the throttle a little, which reduces pumping losses. -/+

Leaner also ensures that all the fuel is burned, which means better use of the fuel injected. +

Since leaner burns slower, it increases the chance of detonation, because detonation is caused by end-gases (fuel/air farthest from spark plug) heating up to the auto-ignition point before the flame reaches them - then they go bang. If the burn was faster, it would burn those gases in normal combustion without the bang. The slower burn also allows the piston to get hotter. So, don't lean-burn too much at high loads. Light loads just don't have enough pressure to get the temperature high enough for easy end gas explosions, and don't have enough energy release to melt a piston. --

Patrick G

iTrader: (14)

Moved from Advanced Tech.