monsta

Heres what a local tuner thinks about leaning out afr due to increased octane.
Note that the octane is in RON.

http://www.bebo.com/Profile.jsp?MemberId=5848347626

Whay do you guys think?

I think his idea is fundementally flawed- The (simplified) interpetration of octane rating is that the scale of 0 to 100 was originally devised by assigning a value of 0 to n-heptane (a fuel prone to knock), and a value of 100 to iso-octane (a fuel resistant to knock) Other fuels are then compared to that standard (and its test engine) to give their octane result
Although octane rating may affect stoich it cant be simplified as a percentage of octane (iso-octane) as he assumes.

For example Methanol has a RON of 113 octane but has a stoich of @6.5:1 and Ethanol has a RON of 116 octane and has a stoich of @9:1. Both these fuels have alot more oxygen in them hence the lower stoic values.

Also "octane" is incorrect discribed as a chemical compound in this instance, because we are talking about RON "octane rating" of gasoline fuel

He has confused iso-octane with gasoline and if we used iso-octane as a fuel and it "happened" to have a stoich of 14.7 then I guess he is on the right track. But iso-octane (100% iso-octane) has a stoich @15.13 and gasoline has a number of other chemicals (Toluene @11.5:1) and oxygenates which drive the stoic value DOWN

ACTUAL STOICH VALUES FOR TYPICAL FUELS (AA Burluka et al, SAE 2004-01-2998)

95RON Shell gasoline- 14.49
98RON Shell gasoline- 14.35
100% Iso-octane- 15.13


Typical compositions both 90.5RON (not including all the additives)

type 1: typical of what the tuner is using for his calc
iso-octane 90.5%
n-hepatne 9.5%

type 2: what gas stations typically use
iso-octane 54%
n-heptane 13%
toluene 25%
cyclohexane 8%





AND if you have fuel higher than 100 RON thats still gasoline- it doesnt mean that there is different oxygen content (based off percentages) as we dont exactly know what has been used to increase the octane rating. Also there are alot of high octane unleaded fuels which use quite a bit of oxygen enhancing chemicals.


I think that although he has tried to be exacting in his calculations, his basis is too in-exact to make me run 16.37:1 on the 98oct fuel I use.
We do however, through experimentation, know we can run mid 15s for cruise/light load without issues. We still have to allow for real-life issues, like changes in temperature etc, so we dont do permanent damage over long periods of time.



Below is taken from Tuners Bebo site


What is stoichiometric?
For the best fuel economy your air fuel ratios should be at the most efficient which is where all the fuel is being burnt and there is little or no excess fuel which will produce carbon monoxide (CO) emissions.

For a long time the accepted stoichiometric air fuel ratio has been 14.7:1. I'm guessing that was calculated using a different fuel to what we now put in our cars so it's time to re-calculate the CORRECT stoichiometric AFR.

First to gather the info needed.

IMPORTANT POINTS

There is 20.95% Oxygen (02) in the air we breath. (This is the same at sea level or at the top of Mt Everest).
98 Octane fuel is (supposedly) 98% Octane (95 = 95% etc)
Octane is a hydrocarbon written as C8H18


ATOMIC MASSES

Carbon = 12.011
Oxygen = 15.994
Hydrogen = 1.00794
(So C8H18 = 114.23092 and O2 = 31.988)

The stoichiometric chemical equation for the combustion of hydrocarbons is,
CxHy + (x+(y/4))O2 =} xCO2 + (y/2)H2O

Therefore the equation for octane is,
C8H18 + 12.5O2 =} 8CO2 +9H2O

If there was too much fuel or not enough O2 the Carbon monoxide would be produced. For example if the O2 is reduced from 12.5 to 11 the equation would look like this,
C8H16 + 11O2 =} 5CO2 + 9H2O + 3CO

And if there is even less O2 again there would be carbon emissions (black smoke)
C8H16 + 8O2 =} 9H2O + 7CO + C (10.7:1 AFR)

This means that for every 114.23092kg of C8H18 there needs to be 399.85kg of O2 or 1908.59kg of air.

1908.59/114.23092 = 16.71 parts air to 1 part octane.

So 16.7:1 is the stoichiometric AFR for 100 octane but since we don't get 100 octane here we should work it out for 98, 95 and 91 octane.

98 = 1908.59/(114.23092/0.98) = 16.37 AFR
95 = 1908.59/(114.23092/0.95) = 15.87 AFR
91 = 1908.59/(114.23092/0.91) = 15.20 AFR

Working backwards we can also work out for what octane rating the 14.7 stoichiometric value would be correct and comes out at 88% octane.

How about C16 (or C16H34 in it's full name).
C16H34 + 24.5O2 =} 16CO2 + 17H2O

Therefore for every 226.44596kg of C16 there needs 783.706kg of O2 or 3740.84kg of air.

3740.84 / 226.44596 = 16.52 AFR for 100% C16.

So I guess that's why cars seem to use more fuel than they need to. Cars are tuned to run at 14.7 AFR which means on average they're using 7.5% too much fuel.

PS. This only applies for cruise and light load situations. Running 16:1 AFR at wide open throttle (WOT) would generate too much heat especially on a forced induction engine and pistons would melt.

steadly2004

iTrader: (19)

I think you have to run it a hair richer than would be in a "perfectly" balanced equation because the fuel is never perfectly spread out, or atomized and in a uniform distribution throughout the cylinder. Extra fuel makes sure there isn't any places where it could potentially create more heat than the overall production, I mean no hot spots.

Thats just a guess on my part though, maybe injectors are more efficient than I realize, but I figure there is always a little amount of unburned fuel (not necessarily because there is too much fuel, but maybe its not perfectly combusted), creating the necessity for catalytic converters and such, right? On the other hand, if it were leaned out, maybe there would be a more complete combustion, and less emissions. I dunno...

JohnBradley

iTrader: (30)

Interesting post. One thing I noticed is you have the MON values for Methanol and Ethanol listed as RON.

Cars typically cruise at 14.7 because thats ideal AFR for the catalytic converter to work most efficiently. We have found on petrol tunes we can run the AFR out to 15.5 or 16 (roughly your 7.5%) and increase mileage 2-3mpg. We have had 3 seperate Evos using this method achieve almost 30mpg. One was 29.9 average on flat ground cruising at 70mph, the other two were mid 29s. The best I personally have seen was 27.6, though that was the 6 speed but also on 275s.

steadly2004

iTrader: (19)

I've seen over 30mpg on my 2.4L (no cruise adjustments), but my car is lighter and has less drivetrain drag

03whitegsr

iTrader: (8)

Permanent damage to what? The engine will actually run COOLER when you go leaner then the stoichiometric AFR. A leaner AFR means less energy content per air mass which will lead to lower peak temperatures. The chart below shows the burn temeprature with respect to lambda AFRs of various fuels.

There are a couple reasons why we run at stoichiometric AFRs under cruise, but preventing damage to the engine is not one of them.
1. Maximum burn temperature = maximum Carnot cycle efficiency
2. Stoichiometric AFR = best balance between CO, hydrocarbons, and NOx emissions...
http://www.autoshop101.com/forms/h55.pdf

3. Catalytic Converter Efficiency, leaner AFRs produce lower temperatures in the exhaust and higher levels of NOx and the cat has trouble burning off NOx normally and has trouble burning anything off if it's not up to temp.


Stoichiometric AFR has nothing to do with fuel economy but emissions.

I would imagine 14.7:1 comes from some basic assumptions about the ratios of typical gasoline. Each fuel will vary.

It's kind of irrelevant though, because cars have at the very least narrowband O2 sensors and operate in closed loop. It doesn't matter is 14.7:1 or 15.3:1 is stoichiometric, the O2 sensor feedback system will minimize the oxygen content in the exhaust by running the true stoichiometric AFR. An O2 sensor is after all is an oxygen sensor that reads the amount of oxygen in the exhaust, not an air fuel meter that reads the actual air-fuel ratio.

As mentioned earlier, a car running 16:1 actually runs a cooler flame temperature then 14.7:1. You'll also make more power at 14.7:1 then 16:1 because there is more energy available for combustion at 14.7:1.

JohnBradley

iTrader: (30)

White,

Interesting comment about the O2 sensor. Obviously on the cars I mentioned (including pretty much all of the ones we tune) we automatically include the "lean cruise" (open loop defined AFR tuning) as part of the function. I knew the NOx went up, but I havent ever seen a graph until now.

The other thing your comment was making think about how is I am not sure if most people understand that when we are talking about Ethanol AFRs we are using numbers generated by the voltage in the WB converted to a gas figure and not an actual Ethanol AFR (which would be in the low 7s or something vs the 12 we are at WOT).

nothere

iTrader: (23)

as a side note. I believe modern gasoline stoich is really around 14.4 give or take. After all gas isn't what it used to be.

monsta

Please note- everything in bold in the first post is not wriiten by me- it is the text from the tuners Bebo site that Ive copied any pasted.



I run my ecu in open loop and run @15.5 for fuel economy in very very light load areas and idle- but have found significant knock (9-15) in the 60-100 load points when trying to run any leaner than 14.7 to the point where you can hear it pinging. I appreciate that the combustion chamber may be cooler but det is det and damage is happening with knock levels that high.
It was easily remedied by keeping it @14.7.

Mattjin

Did anyone really look at the temperature vs lambda graph. You need to be running lambda so lean that I doubt the motor would run, before you see a temperature drop. If you do see a temp drop you are probably lean misfiring anyway, of which having the small plug gap we use in the evo's is not helping.

Monsta, did you edit about the octane vs gasoline? I didnt remember reading that bit earlier and was going to comment.

03whitegsr

iTrader: (8)

Temps start dropping around 16:1 which is very obtainable under cruise from what I have seen. You are right though, you probably wouldn't SEE that it was dropping temps until you got quite a bit leaner.

Something worth mentioning in all of this, leaning out the AFR WILL NOT directly improve your fuel economy.

The car takes XXX energy to go YYmph under cruise to offset frictional/drag losses. If you lean the car out, you are reducing the amount of energy converted in the engine. Thus, all else being equal, you have to increase throttle angle to increase airflow, thereby increasing fuel flow to get back to the same amount of energy to keep the energy consumption constant.

The only way you will improve fuel economy is to improve the efficiency of the car under those conditions. Running lean MAY improve combustion efficiency, but the act of merely running leaner WILL NOT improve fuel economy in its self. There is something secondary going on that allows the gains. I believe that understanding the secondary affects is the key to all of it.

I'd really like to play with something like direct injection and a turbo. Hell, while I'm dreaming, add on a throttle-less variable lift and timing control like BMW uses. Maybe even toss in some Saab variable compression. Add on Toyotas hybrid electric drive... You'd have a car capable of 500+ HP on 91 octane and still get 60mpg on the freeway. And yet we are still driving 300 HP 20mpg EVOs when we have the technology to do so much more.

monsta

Nup- All I did was add the line about the bold been an excerpt

Fox_IX

Leaner air fuel ratios lead to higher cylinder temps. In practice, as we've seen from another user (and personal experience) leaner air fuel ratios can easily lead to detonation due to rising cylinder temps. Also if you take a look at an EGT gauge and lean out your idle, you can watch your gas temps rise. As you richen the mixture these will begin to lower.

Also, cats like leaner mixtures because cats work better when they are hotter. This is also why many vehicles have cat air injection systems.

I don't know where you got your information, but it seems you've misread or something. Seriously, plug in an EGT gauge and see what happens when you play with your mix. Try it for yourself.

As for the topic, leaner mixtures will obviously increase gas mileage. Generally for cruising on economy tunes I like to find an even balance between EGT and AFR. Shoot as lean as I can until I start to see the EGTs rise a little too high, then richen it up a hair and call it a day. This works well for me, I'm getting about 29-30 MPG with mild EGT increase.

cerevo

iTrader: (1)

Dude, this thread is 7 years old...