Air Fuel Ratios for NA Engines 1

Can't see your image.

je suis charlie

I also can't see your table. But simply stating "... 12.5:1 at any RPM..." with no mention of load value is not very meaningful. At full load a ratio of less than 12:1 would be typical, and at light load stoichiometric (14.7:1) usually works well.

So sorry dropbox failure as usual.



12.5:1 was a general range I have seen for most NA applications. And I was wondering isn't it better to go richer or leaner to run more timing. Seems 12.5:1 gives me the highest flamwfeonr peopegarion speed.

Flamefront propegation *

Here's a related chart, laminar flame speed vs Fuel to Air Ratio (Not AFR):

From the general description you gave, I'm thinking this is a Subaru flat-four.

This engine was designed in the modern emission-control era, and it's designed to run stoichiometric almost everywhere, except nearing full load where it will likely need to be a little richer - as you have shown in your map.

If going richer in the mid-range induces detonation, don't do that.

If NOx emissions are not a problem for you ... try continuing the trend that you have established by leaning it out a smidge further at light load. Perhaps in the 15.0 - 15.5:1 range. It might want a little more ignition advance.

"Lean cruise" was a common strategy before NOx emission standards put a stop to it.

The part load area will deliver better fuel consumption with leaner mixtures (Lambda 1.1 - 1.2) and additional spark advanvce to suit, but NOx emissions will suffer.

The "diagonal" yellow line on your chart is normally steeper - ie mixture is more "load dependent" than "speed dependent".

je suis charlie

thank you for the replies. unfortunately 1.0 lambda is the leanest I can go. or in other words subaru ecu doesn't allow lean mixtures. rich or stoich only. I found there is quite a good gain of torque if you only run 0.2-0.5 AFR richer than stoich. that's why i never run exactly 1.0 except for cruising speeds (that's done via the closed loop table). I wrote a paper on laminar flame burning velocity which i will summarize below this.

basically what I found is the closer you are to 12.5:1 AFR or 0.85 Lambda the higher the burning velocity and hence more torque. Also loss of burning velocity and torque is non-linear the further you go from 0.85 lambda (straight gas). but at this ratio the tolerance for timing sharply decreases so you end up losing power. I found something like 13.7-13:1 AFR makes for best power since it allows you to run more timing.

---here is the summary of what i found in regards to AFR and laminar burning velocity----------

What is it
Laminar Flame Burning Velocity is the speed at which an un-stretched laminar flame will propagate through a quiescent mixture of unburned reactants (Wikipedia). This is NOT to be confused with stoichiometric fuel mixture.

Background
I have learned so much in past couple years tuning subarus so I thought to share my experience with tuning or dialing in your air fuel mixtures correctly for best performance (or at least to know what changing your AFR exactly does). Note some of these points made may already be known and I'm in a no way an expert, I just have some education in this area and personal hands on experience.

First off, the most important factor in determining your Open Loop fuel table is the fuel you are going to be using. Different fuels have different burning velocities and properties but in this thread I will be strictly talking about Straight Gasoline! Straight Gasoline experiences its fastest laminar burning velocity at roughly ~1.15 equivalence ratio which is about ~12.5:1 AFR.



In SIMPLE terms, what this means for an average tuner is that tolerance for ignition timing falls as laminar velocity increases and more timing is needed as the velocity falls. So one would in theory need the least amount of timing at ~12.5 AFR due to good burning velocity. From the graph above, going leaner or richer than this value starts to drop the velocity.

Tuning for the Perfect AFR
So as a tuner it's your responsibility to determine where your fuel ratios should be (rich of ~12.5 or lean or perhaps at the fastest flame speed itself). From my experience ratios in mid 12s always led me to retarded ignition timing. I have found ways to improve power by slowing the flamefront so i could run more timing or boost and as I said it's either going richer or leaner. Going richer such as 11.8:1 or leaner 13.4:1 allowed me to run more timing knock free. However we all know heat rises considerably as we enlean fuel mixture. In most turbo applications running a 13.4:1 AFR can lead to dangerous Exhaust Gas Tempratures (EGT) under boost. Even still most factory turbo Subaru cars run this type of ratio or leaner at midrange even under peak boost! I can see Subaru doing this for emissions and fuel economy since Gasoline/Petrol's stoich ratio is at 14.68:1. However such lean factory fuel ratios in low 14s can generate too much in cylinder heat and possibly cause cracks inside the engine and even damage the turbo*. So I think it would be best to instead go on the rich side (anything richer than 12.5:1) and still keep the same laminar flame velocity as stock lean fuel ratios.

This is how:

If you look at the laminar table graph above you will see that an eq ratio of 1.05 (14:1 AFR) has nearly same burning velocity as eq ratio of 1.22 (11.5:1 AFR). Therefore by going to a rich AFR of 11.5:1 vs. The factory 14:1 at peak boost you will keep the same laminar velocity that will net you (THIS IS ASSUMING CONSTANT TIMING AND BOOST):

- same torque and ultimately power (roughly)
- allows you to use same ignition timing and boost tables
- much cooler temperatures across the board (egt, in cylinder, turbo, catalyst)
- more reliable

However running richer has its downside too. You increase fuel consumption and hydrocarbon emissions. Your power per amount of fuel (not Power itself) also suffers. This is called Brake Specific Fuel Consumptions and Turbo engines are known to offer poor BSFC compared to NA engines in part due to rich fuel ratios.

Of course I'm not saying 11.5:1 AFR is the perfect air fuel ratio, I just picked this number since it had similar laminar properties as the stock fuel tables which runs lean usually up to 4000 RPM. For lower octane gasoline and higher boost levels than factory one might need to go even richer than this to control detonation as sudden throttle openings and boost can causes temporary leaner conditions than 11.5:1 which could put you in high laminar velocity areas causing knock. Of course in tuning there are various ways to combat this such as tip in enrichment but I'm not writing this thread to get into tuning specifics.

Conclusion
So in short I can sum up this post in to the following:

• 
  ~12.5:1 AFR gives fastest Gasoline burning velocity

• 
  Rich or Lean side of ~12.5:1 AFR slows burning velocity

• 
  Higher burning velocity requires less timing and/or boost

• 
  Therefore lower burning velocity requires more timing and/or boost

• 
  Going too rich or too lean will cost you power no matter how much timing or boost you add

• 
  Perfect Fuel mixture for turbo applications would be the one that let's you run near MBT timing and good boost without diminishing the burning velocity too much (there's a balance)

- All these conditions make finding the perfect AFR impossible but you can get real close

For starters I would start with factory fuel table and enrich mixture (reduce burning velocity) according to load (boost) and not engine speed (RPM).
At same boost and ignition timing (two of which have greater impact on power than reasonable AFR levels), if a 2.5 g/rev engine load is targeted with 11.8:1 AFR a 3.2 g/rev load must experience a slower burning velocity of 10.5:1 AFR regardless of engine speed. Of course you could do this on the lean side (14:1 to 13.4:1 AFR) as the factory tune does this (except its more RPM biased than load) but expect dangerous in-cylinder and exhaust temperatures. Personally I target 10.5:1 to 11.8:1 ratios under boost for high octane gasoline and found this to lead good results. Any richer and power starts to drop dramatically and any leaner you will experience detonation.

*note that NA engines can run leaner than 12.5:1 ratios fine under load since they develop much less pressure and heat. In NA tuning practices running relatively lean ratios like these allows for combination of good power and economy or brake specific fuel consumption (BSFC).

Just askin' - what is different about them that the Subaru would prefer to be rich (12.5) at cruise than the more usual 14.7 - 16:1 that many other current engines run?

jack vines

i'm not sure but my and factory subarus are tuned to run at 14.4-14.7 in closed loop as posted above. who does 12.5 for cruise?

Hi Pickler. Have you looked at the current F1 regulations (Fuel-Flow-Limit 100kg/hr) and thought about what mixtures they might be running? Clearly with a fixed fuel rat, best power will coincide with best BSFC.

My hypothesis is that full power AFR is somewhere between lambda 1.2 and 1.5. It depends a little on whether you calculate AFR on the trapped air mass or total air mass since such engines would have a fairly high scavenge ratio.

There is no historical precedent for such lean mixtures but aircraft engines running at best BSFC (but still with substantial boost) have run in the region of 1.2. Current F1 engines are DI so stratified charge is a certainty - allowing overall mixtures that are leaner still.

je suis charlie

@gruntguru

hey thanks for the reply. no i haven't looked at it. I would be super interested to find a F1 fuel and timing table. Of course such info is mostly secretive. My only guess for going lean is to reduce fuel consumption. otherwise rich is just the same strategy albeit more safe and also with less NOx pollution. Unfortunately with most modern cars you can't really lean the ratios that much. I have tried it and it's only suitable for cruising but I honestly didn't see any change in fuel consumption with timing locked to full advance (50* btdc).

For efficient operation significantly lean of stoich, the combustion system basically has to be designed accordingly, from the ground up.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

Depends what you mean by "significantly". Any standard combustion chamber will see best BSFC at approx 10% lean (lambda = 1.1). For most modern PFI engines it is more like 1.2

je suis charlie

I played with this on a motorcycle engine with a DOHC pentroof layout. Stock setup was obnoxiously rich (with air injection into the exhaust, plugged for testing), sometimes showing lambda = 0.75 at steady cruise, and had poor fuel consumption and range (hence, me tinkering). Leaning it out to about lambda = 1.05 to 1.08 at part load gave good driveability and fuel consumption. Any leaner than that wasn't appreciably better for consumption, and lean misfire was near 1.2. The method of adjustment (PowerCommander) doesn't have sufficient throttle position resolution at lower speed and load to accurately maintain stable lambda; I couldn't really set it up for leaner than lambda = 1.08 without having occasional excursions too lean and getting driveability problems (and running too hot).

The problem with the throttle position is that the PowerCommander mapping has map cells for 0%, 2%, 5% throttle position and so on, but with this bike (zx10r, 160-ish hp) 0% throttle according to the TPS could be idle, or no load at 5000 rpm, or 50 km/h steady cruise in higher gears puttering around town, or shut-throttle coasting, and obviously these are different conditions and the real throttle position is some differing fraction of a percent, but the coarse mapping of the PowerCommander doesn't recognize the difference.

My cruising range or any operating range in those red cells above are at near lambda 1.0. I have tried reducing lambda 1.0 to 16.5:1 AFR or this lean operation as mentioned but poor idling and cold start prompted me to go back to stoich 14.7:1 for lambda 1.0. Cruising and generally throttle response was clearly much weaker with this leaner operation despite running 50* of timing advance.

now i'm wondering for going leaner, do i need to do anything else other than add timing? will going leaner increase temperatures unnecessarily? I have a catalyst in the midpipe, will that cause an issue? I'm willing to try this lambda 1.2 under load as done on F1 engines but need to build a list of requirements and some hypothesis firsts.

i tried lean mixtures today by locking afr to 17:1 and advancing timing 3-6* accross the board. I did not suffer from any missfires. However at high RPMS i was experiencing severe timing retard due to knock. i think i'll stay with 0.85-0.9 lambda for WOT.

"..... I have tried reducing lambda 1.0 to 16.5:1 AFR or this lean operation as mentioned but poor idling and cold start prompted me to go back to stoich 14.7:1 for lambda 1.0."

In the early days of Fuel injection ( Bosch K Jetronic ) Bosch said something like this -
" Today’s standard concepts rely exclusively on stoichiometric mixtures for the operation of engines running at idle "

Are you able to assign a "special" AFR at idle ? Or are you forced to apply 1 lambda across the board?

Lean operation at high engine load will NOT be good.

It's pretty common for idle to not tolerate being as lean as the cruise setting. My zx10r won't idle smoothly leaner than around 13.5:1 air/fuel.

Don't try to apply a "blanket" lean setup, it will not work.

gruntguru said:

Depends what you mean by "significantly". Any standard combustion chamber will see best BSFC at approx 10% lean (lambda = 1.1). For most modern PFI engines it is more like 1.2

Click to expand...

No, I mean lambda 1.4 or higher.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz