yonson

knock and pre-ignition are similar but not the exact same. Here's a decent explanation of them:
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.

psychoazn

What would you aim for, in terms of AFR, given that we generally have 10% ethanol gasoline?

BowlofRice

So this is way leaner than one of the previous posters....is there this much variation? What year do you have Mike? I've got a 2003.
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Higher the number the leaner the mixture. Lower the number the richer it is. Supposedly the sweet spot is around 13.5-12.5?

yonson

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.

speedengineer

Sure. In the cylinder, the fual-air mixture doesn't instantly burn all at once. The spark plug starts a flame front that progressively spreads outward from the spark plug, consuming the mixture of air and fuel. This process occurs over some time. This means that the air-fuel mixture at the edges of the combustion chamber (farthest from the spark plug) get consumed by the flame last. Now, it is possible for air-fuel mixture to ignite spontaneously (and burn very rapidly) just from the extreme temperature/pressure in the cylinder. Knock happens when the air-fuel mixture that is farthest from the spark plug ignites spontaneously BEFORE the flame front consumes them naturally, and results in a very fast rise in cylinder pressure. The pressure rise is fast enough that the pressure waves reverberate across the cylinder, and these pressure oscillations is what a knock sensor 'hears'. The reason more spark advance makes knock more likely is because advancing spark timing results in higher in-cylinder temperatures and pressure, making the mixture more likely to auto-ignite (knock). Knock is also called 'spark knock' since it responds to changes in spark timing.

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.

speedengineer

sheesh, you've guys' been busy, just about covered it all in the time I spent writing that book...

davidc1

Heh heh....no problem....I love to read!

davidc1

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?

davidc1

Excellent explanations, thanks.

So, 15% richer than stoic is about 12.5% AFR then?

davidc1

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?