Air Fuel Ratios for NA Engines 1

[pickler]

I'm used to seeing naturally aspirated cars run rich. by rich I mean around 12.5 parts air vs fuel ratio at any given RPM almost. So i'm wondering for the engine I built, is the following table suitable?

Engine is a street flat 4, 4 valve, high tumble/swirl NA with 10:1 static and 8.5:1 dynamic compression ratio on midgrade gasoline. Peak power is at 6200 RPM and peak torque at 3800-4600 RPM.

x-axis is engine load vs y-axis being engine RPM.

My observations running the engine with this fuel table is:
- NO knocking or detonation on midgrade gas
- allows near MBT ignition timing advance
- good fuel consumption
- good emissions
- moderate EGT
- going richer in the midrange area induces knocking/pinging.

 

[Tmoose]

Hi Pickler,

" timing was advanced until best torque.."
How was best torque determined?
Was it some kind of a "flash" reading?
Or, was the load maintained for at least several seconds?


What is the make model and year of this car?
What modifications have you done?

 

[BrianPetersen]

Higher load = higher intake manifold pressure = the residual exhaust in the cylinder at the end of the exhaust stroke means less, and there is less "wrong way" dilution of the flow which contaminates intake charge with exhaust during the overlap period. Low load on the engine means more "uncontrolled EGR".

 

[Deividas]

Higher load = higher intake manifold pressure = the residual exhaust in the cylinder at the end of the exhaust stroke means less, and there is less "wrong way" dilution of the flow which contaminates intake charge with exhaust during the overlap period. Low load on the engine means more "uncontrolled EGR".

So at high load cylinders have more "fresh" mixture and then why it requires less advance and less "fresh" mixture requires more advance?

 

[BrianPetersen]

Full load ... it is almost all "fresh" mixture in there. The amount of exhaust residual should be only a few percent.

Light load ... it is a mixture of "fresh" mixture and recirculated exhaust, which can be a significant fraction of the charge.

A mixture of "fresh" mixture and recirculated exhaust will slow down the burn rate and require more ignition advance.

There are some other temperature, density, and turbulence related effects, too. Lots of mixture inflow means lots of swirl and/or tumble in the cylinder. Very little mixture inflow will give less charge motion. How much that is meaningful, will depend on the combustion chamber design.

 

[pickler]

For lean burn I added 10% timing in leaner cells. Before lean burn I added timing until knock and backed off a degree or two.

 

[Tmoose]

"So at high load cylinders have more "fresh" mixture and then why it requires less advance and less "fresh" mixture requires more advance?"

Detonation is one pretty strict limit of ignition advance.
Best torque may, or may not, be produced by ignition timing set just before detonation occurs.

There are a dozen or so factors affecting whether detonation will occur.
As far as I know it boils down to whether the end gasses are hot enough to form radicals in a series of pre-flame reactions that lead to autoignition.
Maybe 900 - 1000F.

http://www.factorypipe.com/t_deto.php

I don't think ( the right mount of ) extra advance being best when cruising is just due to lean-ness. Other wise the ignition maps ( and simple vacuum advance) would not dip as the load changes but mixture A/F remains ~ constant.
there is the matter of "compressing" a rarified mixture, so the fuel molecules are 4 times farher apart, gas temperature at TDC being a lot cooler, the exhaust valve running less red hot, the piston dome and cylinder head being cooler, and they all

 

[pickler]

I started getting knock advancing low load cells to make up for lean mixture. I had to back up to previous timing. Which is still well advanced of stock settings.

I think 16.5 AFR with 40* timing is most efficient it can get.

 

[pickler]

Also could someone help me understand FUEL air ratio? Not Air Fuel Ratio (AFR). For example from this chart what is 0.07 FAR in AFR for gasoline?

 

[djhurayt]

just takeing a guess here as most of this is way over my head but I do enjoy "trying" to understand, wouldn't is be the inverse of the air/fuel ration, i.e. 14.7:1 is .068
???

 

[gruntguru]

Looking at your fuel table it appears the cells you have chose to lean out are medium load. One cell is in the 0.6 g/rev column which is almost 50% of the full load figure. Is this proportional?

Ignition advance is not something that lends itself to "percentage" increments. e.g. if timing is at TDC how much is a 20% increase.

je suis charlie

 

[pickler]

yes i believe lambda 1.0 would be around 0.07 FAR if i'm not mistaken.

Yes 50% relative engine load is correct. This is the engine load at which my car cruises on the highway. Could be due to the all wheel drive. This is why I don't think lean burn can work for me. At slower speeds my RPMs are too low ~around 1300-1500 and on the highway load is too high.

I'm having difficulty fighting knock with leanburn and making the same amount of power. The lean conditions at low loads 'make their way' to higher loads with sudden opening of throttle cause detonation. I think I'll stick with stoich/rich map for now which allows me to advance timing to MBT everywhere in the powerband excluding at low RPMs (1000-2000 RPM):

VE:

also on a separate note, my engine experiences higher loads than defined in this table. It's not unusual to see 1.7 g/rev at higher RPMs specially with colder temperatures. MODs are mostly on the exhaust and intake side, like headers and higher flowing piping. MAF has been re-scaled as well. I'm just happy I managed to kill detonation with an unusual way of leaning (closer to stoich). As said I experience knock near the VE peaks 4000 and 5500 RPM. Currently I'm not getting any knock.

 

[pickler]

correct timing table

 

[Tmoose]

volumetric efficiency over 100% with a modified street vehicle at 5300 rpm?
How are you calculating that?

============

I guess the load scale units are grams/per revolution passing thru the mass air flow sensor.
Are the values for the whole engine?

regards,

Dan T

 

[pickler]

I'm not calculating it. The ecu is estimating VE based on airflow from Maf and other sensors. Stock with no mods it was around 95%

MAF = Engine Load * RPM / 60
Example:
at 3534rpm * 2.91g engine load / 60 = 171g/s maf

 

[Deividas]

I don't wanna create new topic, so i decide to ask here: if in cylinder are more air/fuel mixture(when i press accelerator pedal more), peak cylinder pressure also will be higher?

 

[BrianPetersen]

In the absence of complicating factors - Yes. (Why would you think otherwise?)

 

[Deividas]

I don't think otherwise, i just wanna make sure that i thinking correctly