discrepencies air fuel ratios with high and low comp

trinydex · May 11, 2006, 05:28 PM

i posted this question in socalevo and i'd like to pose the same question and subsequent discussion here.

it seems in my limited research on the subject that higher compression na vehicles like a higher afr (leaner). high 12s and even 13s are common in na tuned vehicles.

our low comp boosted engines like low 11s to be safe and high 11s to be daring.

boosted s2ks like around mid 12. and they run crazy compression plus boost... and still so lean...

short question, i don't get it.

trinydex · May 11, 2006, 05:29 PM

rhyzin posted:

QUOTE if we're talking about a turbo car on-boost, cylinder pressure is higher than that of a high compression NA engine. a turbo car will run a hotter combustion temp due to compressing so much air into a tiny space. the more compression, the more heat. less compression, less heat so you can run a leaner AFR on a NA engine. a turbo car will run a richer AFR to bring temps back down to prevent preignition. if we say VE is 100%, a car running a comp ratio of 9.0:1 on 28psi would have an effective comp ratio of 27:1, where an NA engine only sees what it's rated it, 12.0:1?

if we're talking high compression and high boost in a s2k, it's low stroke would help make this happen. the flame front is distributed along a higher area of piston than higher stroke/higher torque engines. the heat and pressure is distributed over a larger area. the lower stroke also gives less load on an engine overall since it's not producing as much torque. it depends on the high revs to produce it's power.

looking at VE on a high rev engine as well, if it's VE is low over it's last few k of rpm, higher boost will strain the valve train instead as boost is a measurement between the turbo and when the valves are closed. boost isn't that much of an indication of how much air is in the combustion chamber, although flow rate is a better indicator. high compression + high boost doesn't mean much towards destruction if the VE isn't there. END QUOTE

Flyin EVO 2 · Jul 4, 2006, 03:54 PM

You have to also take timing into consideration. You can run ideal timing, boost, and a/f ratio for the amount of octane you have in the fuel. If you start raising the boost which raises the intake charge, then you need to lower the knock threshold by either reducing timing, richening the mixture, both, or raise the octane.

ez76 · Jul 5, 2006, 12:20 AM

To some extent, running rich is a hedge against the air/fuel mixture not atomizing completely evenly, right?

S2000 head is pent-roof where ours is hemispherical - maybe this plays a part.

SaabTuner · Jul 9, 2006, 04:11 PM

Quote:

Originally Posted by trinydex

Quote:

if we're talking about a turbo car on-boost, cylinder pressure is higher than that of a high compression NA engine. a turbo car will run a hotter combustion temp due to compressing so much air into a tiny space. the more compression, the more heat. less compression, less heat so you can run a leaner AFR on a NA engine. a turbo car will run a richer AFR to bring temps back down to prevent preignition. if we say VE is 100%, a car running a comp ratio of 9.0:1 on 28psi would have an effective comp ratio of 27:1, where an NA engine only sees what it's rated it, 12.0:1?

Turbocharging, especially with an intercooler, is not an adiabatic process. 3x the manifold pressure isn't the equivalent of 3x the compression ratio; before combustion starts, the turbocharged engine will have less cyllinder pressure, and less cyllinder temperature than the high compression engine. But after combustion starts, because the turbocharged engine has a higher airmass/combustion, it will have higher cyllinder pressure and temperature. (all other things equal)

Notwithstanding, because the turbocharged engine will have a much larger mass of air in the cyllinder each combustion, it will need a larger surplus quantity of fuel to keep it all cool.

Makes sense, right?

trinydex · Jul 9, 2006, 05:01 PM

wait so having more air does not give rise to a higher overall cylinder pressure in squish cycle (turbo compared to na)? i guess i'm just lookin' for some hypothetical figures.

i can see how it might have less cylinder temp as the heat is well taken by the increased air mass.

and your reference to the adiabatic process just throws out the ideal gas equation right? so how about are you calculating?

SaabTuner · Jul 9, 2006, 06:06 PM

Quote:

Originally Posted by trinydex

wait so having more air does not give rise to a higher overall cylinder pressure in squish cycle (turbo compared to na)? i guess i'm just lookin' for some hypothetical figures.

i can see how it might have less cylinder temp as the heat is well taken by the increased air mass.

and your reference to the adiabatic process just throws out the ideal gas equation right? so how about are you calculating?

Adiabatic formula and explanation here: http://www.stealth316.com/2-adiabat1.htm

In terms of peak pressure ...

--- a 3x higher compression ratio results in more than 3x the cyllinder pressure and exactly 3x the density.

--- 3x the intake pressure doesn't necessarily result in 3x the density; the temperature has to be the same if it is a total pressure measurement and near the same if it is a static pressure measurement.

Net result is that twice the compression ratio results in more peak pressure and peak temperature than twice the manifold pressure. However, the average pressure will be higher with the boosted engine because there will be twice, or close to twice, the total mass in the cyllinder.

If one keeps that in mind, it makes sense why an 13.5:1 compression N/A engine may sometimes reach detonation quicker than a 9:1 compression engine running 0.5 bar of boost even if both are at MBT and on the same octane. The peak pressure in the N/A engine will be higher.

-Adrian

trinydex · Jul 10, 2006, 06:00 PM

so there IS a slight overalp where more boost is still less cylinder pressure than more compresstion ratio. cool.