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Hi guys I have some questions that came to mind that I haven’t really found answers for online.
1. I know the ideal air/fuel ratio is 14.7:1. What I’m wondering is, is a car constantly trying to achieve this ratio throughout different speeds/rpms? Like is a car tuned for this base ratio and then tries to maintain it with different engine loads?
My confusion stems from looking at charts and noticing the ratio changes with speed/rpm but my understanding is (maybe I’m wrong) is its supposed to be as close to that ratio as possible for emissions/efficiency.
2. I’m a bit stumped how retarding ignition timing helps with pre ignition. If it’s preigniting causing detonation due to whatever factor (low octane, carbon build up etc) how exactly does retarding the ignition so it fires at a later time help?
Does it just delay the amount of pressure going on inside the chamber? If it’s pre-igniting that means there’s already combustion happening so does the spark going on later just create less combustion?
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alright, so as the car accelerates your air fuel mixture will actually get rich and usually go above 14.7/1, in fact most performance vehicles will run 12.7-1 or even a 10.7-1 air fuel mixture to Richen the mixture….
you see the leaner you run the air fuel mixture, the hotter the fuel will burn, and the quicker… leading to pre ignition, also with forced induction or high compression the combustion chamber pressure during the compression stroke is much higher than a normal engine, therefore the more pressure you have the more temp right? so what happens is it gets so hot a regular 87 octane gas will explode before the spark plug ignites it. so you require a higher octane…
now what happens when you run a richer air fuel mixture is the fuel does not burn completely, but more of it evaporates during the compression stroke, and…. like most fluids that evaporate when they evaporate they draw the heat energy from the area, so running it rich keeps the combustion chamber temps down, this allows more boost or higher compression without pre ignition without needing a higher octane fuel or just because you can’t get any higher octane.
now your ignition system, if your timing is too late what happens is the expanding gasses during the power stroke don’t push down on the piston in time to create a power stroke, this causes a miss… now… if it ignites the air fuel mixture too early what happens is it starts pushing down on the piston as its still coming up for the compression stroke, causing knock.
it is common practice also with the 3800 supercharged engines to put in a 180 degree or 160 degree thermostat in the engine when you do serious mods, because when you start adding turbos or smaller supercharger pullies you are adding more boost, you are increasing combustion chamber compression pressures, so as a result the compression stroke temp goes up, and spark knock happens, the ignition system tries to retard the ignition to compencate, but there is only so far retarding the igniton will go.
so reducing engine temp and tuning drops combustion chamber temps make it so you can run more boost and not have to run it on race gas.
Thanks for that. Still a bit confused about air/fuel ratio.. So an engine isn’t trying to maintain the proper ratio throughout rpms? You mention the ratio will get rich as a car accelerates.
The oxygen sensor comes into my mind, I figured it’s job would be to make sure the ratio is always around 14.7/1 if that’s what it’s tuned for. Or am I missing something here? Thanks
your Oxygen sensors job is to detect the air fuel ratio buy comparing the oxygen on the outside of the exhaust to the inside of the exhaust.
it does this by producing voltage, the difference in oxygen between the inside and the outside of the exhaust produces the voltage, the voltage the 02 sensor emits should be about .5 volts or half a volt, but in reality it fuxuates between .300 volts and .800 volts, the upstream 02 sensor measures the air fuel mixture. with that the bigger difference between the inside and outside air the more voltage the 02 sensor puts out, so simply, the higher the voltage during fluctuation the richer your engine is running, the lower the voltage the leaner.
the PCM takes this voltage and takes it as a measure of the air fuel mixture, and adds more or less fuel depending on what readings it gets setting the “Fuel trim”, what also contributes to the ammount of fuel goes into the system is the engine temperature, and the throttle position. if your throttle is wide open of coarse the PCM will tell the fuel injectors to inject more gas, if you are at a really low temp (Cold start.) the PCM will set the air fuel mixture rich for cold start until the PCM hits closed loop operation and the engine is at a high enough temp it doesn’t need to have a rich mix to run (A job a choke would do on a cold start with older carburreted engines.)
a bad oxygen sensor would get no readings or poor readings, a good 02 sensor upstream measuring your air fuel mixture should be reading a fuxuating reading between .300 and .800 volts, give or take a little, if it measures higher than that you are running rich, if it measures below that you are running lean! a result of a bad 02 sensor would be a engine that is running like crap…
bad temp sensors can also have a massive effect on air fuel mixture as well as bad MAP (manifold air pressure) sensors. so in the instance you suspect a issue that a 02 sensor may be causing, before you replace the 02 sensor you may want to plug a scan tool into your carn and see what kind of codes come up.
Great thanks for that in depth explanation.. Though maybe my question isn’t too clear.
Is the air/fuel ratio a base setting that every other sensor references to?
Like for example at wide open throttle, yes more gas is needed because more air is being drawn in.. But is it in the ratio of 14.7/1 still? If that makes sense
Not always, sometimes at WOT the air fuel mixture is Richer than 14.7-1
also during the instance lets say your fuel mixture doesn’t richen up during acceleration on a normal car you will get stumbling and missing like the car is being starved of gas… then as you let off the throttle the missing and trailer hitching what ever you want to call it settles, this situation of leaning out during acceleration can be caused buy a MALTITUDE of issues not just a bad o2 sensor, and if you are having these issues you should have a engine light, and in this case i would plug a scan tool in and see what kind of codes pop up to narrow things down.
in a high performance engine that has high compression or low amounts of boost this can cause knock, running lean on a engine with really high compression or really high boost is VERY BAD! as stated before in the least this can cause spark knock! which can “Chip a piston.” or burn up exhaust valves and valve seats! in extreme situations with really high boost with turbocharged or crazy supercharged engines literally melt the pistons!
these issues can be caused buy a maltitude of things… things that give the PCM readings and then the PCM makes adjustments to the engine.
a faulty MAP sensor. (Manifold air pressure sensor) that detects the amount of vacuum or boost.) if this is faulty it can cause a lean or condition or a rich condition because manifold air pressure is used to tell how much fuel to put into the engine
a faulty MAF sensor. (A faulty mass airflow sensor) this is used to detect the amount of air flow going into the engine, this is used to tell the pcm how much fuel to go into the engine, if this is bad it can cause a lean condition or a rich condition
a faulty TPS. (A faulty throttle position sensor) can cause fuel system issues if you put the car at WOT and it doesn’t detect that it can cause the engine to lean out and die, but can also cause shifting issues with the transmission because this also tells the transmission to hold higher RPMs between shifts if the throttle is wide open with a automatic transmission
a falty O2 sensor. as described, the o2 sensor reads air fuel mixture by varing voltages and sending it to the PCM, but in this case the engine would just run horribly
low fuel pressure. Commonly caused by a clogged fuel filter or wearing out fuel pump, as the engines demands for fuel go up the fuel pump cannot supply enough pressure or flow, it leans out, the engine is starved of fuel.
you will notice that during acceleration the air fuel ration hits 12-1 at one point and the system doesn’t consider it rich, sometimes during acceleration the engine likes to have a little richer mixture, and as stated before with boosted cars a rich mixture sometimes is essential to prevent pre detonation or knock, which can damage the engine.
if you have ever been behind a car that accelerates really hard you will notice a rotten egg smell, this is caused by the engine running rich during acceleration! so most engines actually run on a little bit of the rich side during harder acceleration… a air fuel ration of 14.7-1 is more ideal for steady cruising in fuel economy cars, some cars will actually run the engine lean to save gas.
So the ratio isn’t a standard every car shoots for then? The ecu alters it depending on driving conditions? I was curious why that ratio was even determined if it’s being altered
under ideal conditions most all engines shoot for air fuel ratios of 14.7-1… but not under all conditions.
like i said during acceleration it will run a bit on the rich side because the engine naturally wants more fuel when its under load and accelerating, more gas makes it go faster in essence.
and like i also said under some conditions some engines perilously run lean to save gas at steady cruse, some engines run rich for the horsepower with high boost or compression, and during start up you will run rich because a cold engine needs more gas.
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