Running lean, high octane, burnt engines
#21
Registered User
Join Date: Jun 2010
Location: SoCal...
Posts: 399
Likes: 0
Received 0 Likes
on
0 Posts
Interesting...I always thought that knock and preignition were one and the same. Could you explain a little more please?
As an aside, if our engines used a MAF sensor instead, then it would compensate?
Brings up a good point...just what is "lean" vs "rich"? A stoichiometric AFR of 14.7/1 is what? Rich? Lean? Right in the middle?
As an aside, if our engines used a MAF sensor instead, then it would compensate?
Brings up a good point...just what is "lean" vs "rich"? A stoichiometric AFR of 14.7/1 is what? Rich? Lean? Right in the middle?
http://en.wikipedia.org/wiki/Engine_knocking
If our engines had a MAF it would be able to compensate for the added airflow provided it was calibrated properly for the air intake tract being used. If you change the size of the housing the calibration is skewed.
Stoichometric AFR of 100% gasoline is ~14.7:1 or lambda 1 however most of the fuels we use today are up to 10% ethanol which will lower the stoich. point to as low as ~14.1:1. Rich is anything lower than stoich. and lean is anything higher than stoich.
Assuming a stoich. point of 14.7 for maximum power on a naturally aspirated engine you want your AFR to be in the 12.7:1 - 13.3:1 range, on a boosted engine you will want to aim for ~11.5:1.
During closed loop operation the engine is programmed to run at stoich. and the oxygen sensors will tell the computer how close it is to stoich. and the computer will trim the injectors until they read stoich.
#22
Originally Posted by davidc1' timestamp='1335244101' post='21637212
Interesting...I always thought that knock and preignition were one and the same. Could you explain a little more please?
As an aside, if our engines used a MAF sensor instead, then it would compensate?
Brings up a good point...just what is "lean" vs "rich"? A stoichiometric AFR of 14.7/1 is what? Rich? Lean? Right in the middle?
http://en.wikipedia.org/wiki/Engine_knocking
If our engines had a MAF it would be able to compensate for the added airflow provided it was calibrated properly for the air intake tract being used. If you change the size of the housing the calibration is skewed.
Stoichometric AFR of 100% gasoline is ~14.7:1 or lambda 1 however most of the fuels we use today are up to 10% ethanol which will lower the stoich. point to as low as ~14.1:1. Rich is anything lower than stoich. and lean is anything higher than stoich.
Assuming a stoich. point of 14.7 for maximum power on a naturally aspirated engine you want your AFR to be in the 12.7:1 - 13.3:1 range, on a boosted engine you will want to aim for ~11.5:1.
During closed loop operation the engine is programmed to run at stoich. and the oxygen sensors will tell the computer how close it is to stoich. and the computer will trim the injectors until they read stoich.
#23
Registered User
Originally Posted by psychoazn' timestamp='1335234300' post='21636750
[quote name='yonson' timestamp='1335233508' post='21636712']
Be warned if the dyno shop uses a tail pipe sniffer they are known to read a little leaner (~.5 - 1 AFR) than the car is actually running because of reversion and the sensor being after the cat. (if you have one).
Be warned if the dyno shop uses a tail pipe sniffer they are known to read a little leaner (~.5 - 1 AFR) than the car is actually running because of reversion and the sensor being after the cat. (if you have one).
[/quote]
Higher the number the leaner the mixture. Lower the number the richer it is. Supposedly the sweet spot is around 13.5-12.5?
#24
Registered User
Join Date: Jun 2010
Location: SoCal...
Posts: 399
Likes: 0
Received 0 Likes
on
0 Posts
In an N/A road race application like we run them in I personally wouldn't run any leaner than .86 - .87 lambda, which would be ~ 12.25:1 if your sensor was calibrated for a stoich. point of 14.1:1 You can get away with running leaner, but I have always run mine on the rich side.
#25
Registered User
Join Date: Jun 2010
Location: Michigan
Posts: 285
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by speedengineer' timestamp='1335232506' post='21636658
Running lean (actually a misnomer, by 'lean' most people mean running 'less rich') by itself doesn't harm the engine any. However, a less rich mixture increases the chance of engine knock (not the same as preignition).
The S2000 ECU is a speed-density system, meaning that fueling rates are determined by engine speed and manifold pressure. If your powertrain is highly modified (say exhaust, header, intake, etc), you will have higher volumetric efficiency, which means greater airflow, but since the MAP reading is the same, the ECU doesn't know the engine is actually getting more air, and won't know to inject more fuel, so it won't run as rich.
As an aside, if our engines used a MAF sensor instead, then it would compensate?
Running lean (actually a misnomer, by 'lean' most people mean running 'less rich') by itself doesn't harm the engine any. 50 C). Heavy knock will eventually damage an engine, however, the stock ECU will use its knock sensor to retard timing and prevent knock, so I doubt that is an issue.
At part throttle, the engine runs in closed loop, meaning it uses the oxygen sensor to run at a stoichiometric AFR.
At part throttle, the engine runs in closed loop, meaning it uses the oxygen sensor to run at a stoichiometric AFR.
Brings up a good point...just what is "lean" vs "rich"? A stoichiometric AFR of 14.7/1 is what? Rich? Lean? Right in the middle?
Pre-ignition is the ignition of the air-fuel mixture BEFORE the spark plug sparks - aka 'pre spark'. This could happen in a way similar to the process of knock described above where the high temperature of the mixture causes it to auto-ignite. It can also be caused by another ignition source besides the spark, for example, if you use too hot of a spark plug and you get your ground strap glowing orange, which can be a source of ignition for the fuel. Note that pre-ignition is different from spark-knock, because changing spark timing will have no effect.
Correct, since a MAF sensor system measures the flow rate of air going to the engine, it will automatically account for any extra airflow due to modifications.
A stoichiometric ratio of air to fuel occurs when there is exactly the right amount of fuel to react completely with the oxygen. In ideal combustion, the fuel and air (O2 + N2 mostly) reacts to CO2, H20 and N2. If you are lean, you have more air than fuel, and you end up with oxygen in the exhaust too. If you are rich, you have more fuel than you can actually burn because there isn't enough oxygen present to react with it, and you end up with unburned fuels in the exhaust. Gasoline has a mass based air-fuel ratio of 14.7, although stoich for most fuels now days are in the low 14's due to 10% ethanol content. Optimal power is actually achieved when rich, typically around 15% richer than stoich.
#28
Originally Posted by davidc1' timestamp='1335244101' post='21637212
Interesting...I always thought that knock and preignition were one and the same. Could you explain a little more please?
As an aside, if our engines used a MAF sensor instead, then it would compensate?
Brings up a good point...just what is "lean" vs "rich"? A stoichiometric AFR of 14.7/1 is what? Rich? Lean? Right in the middle?
As an aside, if our engines used a MAF sensor instead, then it would compensate?
Brings up a good point...just what is "lean" vs "rich"? A stoichiometric AFR of 14.7/1 is what? Rich? Lean? Right in the middle?
http://en.wikipedia....Engine_knocking
If our engines had a MAF it would be able to compensate for the added airflow provided it was calibrated properly for the air intake tract being used. If you change the size of the housing the calibration is skewed.
Stoichometric AFR of 100% gasoline is ~14.7:1 or lambda 1 however most of the fuels we use today are up to 10% ethanol which will lower the stoich. point to as low as ~14.1:1. Rich is anything lower than stoich. and lean is anything higher than stoich.
Assuming a stoich. point of 14.7 for maximum power on a naturally aspirated engine you want your AFR to be in the 12.7:1 - 13.3:1 range, on a boosted engine you will want to aim for ~11.5:1.
During closed loop operation the engine is programmed to run at stoich. and the oxygen sensors will tell the computer how close it is to stoich. and the computer will trim the injectors until they read stoich.
So, when we talk about fuel trim numbers such as -5.0%, or +7.2% or whatever that Torque give me....% relative to what? The AFR of 14.7%? Or is it a % of the amount of fuel being used? Or what?
#29
A stoichiometric ratio of air to fuel occurs when there is exactly the right amount of fuel to react completely with the oxygen. In ideal combustion, the fuel and air (O2 + N2 mostly) reacts to CO2, H20 and N2. If you are lean, you have more air than fuel, and you end up with oxygen in the exhaust too. If you are rich, you have more fuel than you can actually burn because there isn't enough oxygen present to react with it, and you end up with unburned fuels in the exhaust. Gasoline has a mass based air-fuel ratio of 14.7, although stoich for most fuels now days are in the low 14's due to 10% ethanol content. Optimal power is actually achieved when rich, typically around 15% richer than stoich.
So, 15% richer than stoic is about 12.5% AFR then?
#30
But would a MAF sensor be able to compensate of different air pressures? Such as sea level here in SoCal vs our friends in the mile high club in Denver?