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.

 

[pickler]

Yes I did enriched the idle. However lean mixture just doesn't work under load. Even with more timing than with rich mixture it suffers to produce any power. Lean under light load works but torque falls dramatically as well there. I think 15.5 Afr is best in terms of balance between economy and power for cruising. Since my car is equipped with EGR it does already dilute the mixture under light load and decrease combustion efficiency. Adding lean burn to this seems to 'suffocate' the motor.

 

[Lou Scannon]

Yes I did enriched the idle. However lean mixture just doesn't work under load. Even with more timing than with rich mixture it suffers to produce any power. Lean under light load works but torque falls dramatically as well there. I think 15.5 Afr is best in terms of balance between economy and power for cruising. Since my car is equipped with EGR it does already dilute the mixture under light load and decrease combustion efficiency. Adding lean burn to this seems to 'suffocate' the motor.

See my post at 02:54am earlier today.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

Significant leaning will require significant additional spark advance. More than 50 degrees is not unusual. Most engines will reach the misfire limit at about lambda 1.2. Many will have detonation problems at these extremes. Engines designed for lean burn will have higher ignition energy, additional turbulence and sometimes stratified charge.

je suis charlie

 

[gruntguru]

Increased Charge Air Temperature, fuel temperature and smaller fuel droplet size can improve ignitability and combustion stability.

je suis charlie

 

[Tmoose]

When you added ignition advance did you add it in stages, like 2 degrees, between tests?
Did you add it to the stock or pre-existing curve, or just in a section of the rev range?

Did you end up with a load/rpm/timing map as varied as this?

http://www.911chips.com/USignst.gif

 

[pickler]

Yes I added timing in stages and it was load vs rpm based. I added more in lighter loads and slightly less as loads increased. I gave up after first stage due to loss of torque and detonation. I dont think the risk is justifiable.

I think I will stick with stoich or richer AFR tuning...14.5-14.2 AFR seems to give best torque without loss of BSFC for part loads. Above 3000 rpm at full MAP I start to enrich slightly to 14-13.7 AFR and keep it until peak torque. This gave me best BSFC rich of stoich partly because I could advance timing further than before. Also I can run 3-4° more timing than factory under cruising loads without detonation. I just wasn't sure about my method since it's leaner than factory and common practices for NA.

Total Ignition Timing

 

[gruntguru]

Factory maps are often richer than optimum at full load. This provides a safety margin for things like - flow variation between injectors, airflow variation between cylinders, variation in operating environment and engine condition over the life of the vehicle, fuel quality variation etc etc.

Some of these conditions can be detected during closed loop operation and compensated by the Engine Management System - some cannot.

je suis charlie

 

[inline6]

laminar flame speed probably has little to do with actual burn speed in a real engine as there is alot of turbulence in the chamber.

If you look at how long it would take flame traveling at 350mm/s to go from plug to farthest point in the chamber say 40mm in an 80mm bore this would be 0.11 seconds.

Then work out how long it takes for an engine to do a full revolution at 6000rpm (0.01sec) and clearly laminar flame means little

 

[pickler]

laminar flame speed probably has little to do with actual burn speed in a real engine as there is alot of turbulence in the chamber.

If you look at how long it would take flame traveling at 350mm/s to go from plug to farthest point in the chamber say 40mm in an 80mm bore this would be 0.11 seconds.

Then work out how long it takes for an engine to do a full revolution at 6000rpm (0.01sec) and clearly laminar flame means little

well if it was like that then I could get away with richer AFR and more timing without knock wouldn't I? I'm definitely sure the knock i get approaching 12.5-12.7:1 area is due to higher flame speed. the stock ecu completely avoids this ratio suggesting the engineers who mapped this experienced this knock phenomenon as well.

 

[BrianPetersen]

The stock programming/mapping avoids anything other than stoichiometric everywhere they possibly can, because a 3-way catalyst can only function effectively at that specific condition. Emissions, emissions, emissions.

High load at higher engine RPM will have to be richer, and it can be, because those conditions are not encountered anywhere in any of the official test procedures.

 

[pmrobert]

The US RX-8 Mazda rotary is factory mapped to go rather richer than stoich under certain operating conditions that are not high load (such as higher speed cruise) in order to keep the cat cool enough to last 80K miles. My point is that there are sometimes reasons other than optimum engine performance and fuel economy that account for manufacturer's mapping strategies.

 

[gruntguru]

Rapid combustion and knock do not normally go hand in hand in fact usually the opposite. Slow combustion increases the time available to heat the end-gas. Rich mixtures suppress detonation by a reduction in the temperature of the mixture and an increase in the auto-ignition temperature, not by a reduction in flame speed.

How did the knock you experienced at 12.5 AFR respond to retarding the timing? 12.5 should really be a sweet spot for power and knock resistance.

Turbulent flame velocity in an engine is 20 to 40 times higher than the laminar flame velocity.

je suis charlie

 

[Tmoose]

The best timing at high load at 1000 rpm is 11 degrees AFTER TDC?

And, at 4000 rpm under heavy load it has to be limited to 22 BDC?

Did I miss that this is a turbo engine?

 

[pickler]

The best timing at high load at 1000 rpm is 11 degrees AFTER TDC?

And, at 4000 rpm under heavy load it has to be limited to 22 BDC?

Did I miss that this is a turbo engine?

No its NA. But its compact pentroof chamber with high swirl and fast burn plus its 10:1 compression head requires much less advance than your regular big block v8.the closest V8 I think of to this engine is the ls7. Mid 20s in knock free timing makes peak torque with this engine.

How did the knock you experienced at 12.5 AFR respond to retarding the timing? 12.5 should really be a sweet spot for power and knock resistance.

I experience 90% of my knock problem at peak torque 3800-4200 rpm and at peak VE ~5300 RPM. At peak torque going any richer than 13.5 AFR gives me knock. The closer I get to 12.5 it seems the worst it gets. You can retard timing as much as 4° and it will still knock. At peak VE its not too sensitive to fuel and more so to timing. So anything higher than 25-26° at peak VE causes knock whatever your fuel ratios.

On the dyno approaching 24° starts to flatten the torque curve and any more timing the torque falls. I can run MBT on pump gas with the AFR table I posted above. With stock furl map which is richer (NOT stoich) I can Not come close to such timings. It's strange but it is what it is.

 

[gruntguru]

I said above that 12.5 should be a power "sweet spot". I should have said "13.5" - approx lambda = 0.9 which makes your results a little less surprising. Also unsurprising - this coincides with the highest flame velocity for gasoline.

je suis charlie

 

[pickler]

getting back on to the cruising and lean burn topic, how much timing should be added if you go leaner from stoich and what AFRs are typical or best? I remember reading somewhere that 15.6:1 AFR gave best BSFC for a PFI engine.

Ie. If I run at 15.6:1 vs 14.7:1 for cruising, how much more timing should be added?

 

[gruntguru]

Best BSFC mixture varies enormously from engine to engine. The 1986 Bosch handbook says "approximately 1.1". The 2011 edition says "1.2 - 1.5" (PFI) and "up to 4.0" (DI). The variables involved include engine design, load, air temp, fuel temp, droplet size . . .

MBT ign advance can only be determined on the dyno but leaning by 10% (say from 14.7 -> 16.2) at light load would typically require at least 5 - 10 deg extra advance.

je suis charlie

 

[pickler]

so it wouldn't be typical to add 10% more timing for 10% leaner mixtures?

I'm currently targeting 16.8:1 AFR which is 14% leaner than before. Adding 14% more timing would roughly equal to ~5* more timing.

 

[pickler]

New AFR Table:

Timing

note timing may seem to be over advanced at cruising ranges, this is because I performed part load tuning before this lean burn and timing was advanced until best torque. Actual timing added after lean burn was around 3-5* on top of previous tune or 14%.

 

[Deividas]

Sorry if question is dumb, but why higher engine loads needs less timing advance?