K, I'm trying to understand fuel requirements based on rpm as such.

We have a 2 L engine, so after 2 revolutions(1 compression, ignition, exhaust, intake cycle), our engines should sweep out 2L of air. (Actually 1996cc, and ignoring EGR). So if our car idles at say 1000rpm, we are pumping 1000L of air (that sounds so high!?)

So how many cc of fuel did we add to that 1996cc of air, if the air was at 1 standard ATM of pressure?

The "perfect" a/f ratio in terms of efficiency is 14.7:1 (right?) but this isn't what we normally see in our engines. Engines run richer (If you could tell me our real operating ratio at idle, that would help too). The ratio I see is unitless, and I understood that to be really in units of volume.

So 14.7cc of air uses 1cc of fuel???? Well, I'm obviously missing a big idea here, b/c I calculated we are using 68L of fuel/min at idle, if we are idling at 1000rpm, and thus sweeping 1000L of air! That's just wrong!

Help me out guys so I can go "oh yea, duh..." when I hear the answer.

>K, I'm trying to understand fuel requirements based on rpm
>as such.
>
>We have a 2 L engine, so after 2 revolutions(1 compression,
>ignition, exhaust, intake cycle), our engines should sweep
>out 2L of air. (Actually 1996cc, and ignoring EGR). So if
>our car idles at say 1000rpm, we are pumping 1000L of air
>(that sounds so high!?)

AHH NOO! Remember the otto cycle, only one out of the four stokes is actually taking in any air. So for a 2L 4 cyl engine, only one cylinder is actually sucking in air at a time. It should be 0.5L

Also, it's more complicated than that. That woud be at WOT at 1000 rpms. At lower throttle positions, the engine is taking in less air. Yes the SWEPT volume is the same, but less air is filling that cylinder. This makes sense, since at part throttle for the same rpm, less air is getting into the engine than compared at full throttle.

Try the calculations now. I commend you for asking and trying to learn more about the otto cycle and air fuel ratios. Too many people just gloss it over when you have to pay close attention to what is going on with your engine.

1. intake
2. compression
3. power
4. exhaust

4 stroke engine cycle...

a/f, i dunno

u wanna be at about 0.88 a/f ratio

And isnt the 14.7 the pressure of air at see level, not the air/fuel ratio.

>And isnt the 14.7 the pressure of air at see level, not the
>air/fuel ratio.

Both.

14.7:1 air/fuel ius the ratio at which all of the fuel and available oxygen are burned during the power stroke. it's not the best for power, it is IS the best for emissions and effeciecy.

I'm trying to remember, isn't 12.5:1 a better power ratio?

And .88 is the voltage reading on a digital O2 meter for a good a/f ratio, not the actual ratio. If you go dyno tune it is represented in air fuel ratio

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>>>AHH NOO! Remember the otto cycle, only one out of the four stokes is actually taking in any air. So for a 2L 4 cyl engine, only one cylinder is actually sucking in air at a time. It should be 0.5L

Calm down :P That's exactly what I meant, though. Perhaps I wrote that in a difficult way to understand, but I understand the otto cycle and I know the intake stroke is the one bringing in air. When I said to revolutions sweeps 2 liters, I meant for all 4 pisons in total during those 2 revolutions.

Yes, thanks for pointing out the throttle body point. I know there is a vacuum, so in fact we aren't drawing a full .5L into a cylinder during the intake stroke. However the lower intake pressure still isn't significant enough to account for the crazy figures I'm getting.

I guess it all lies in the one question I'm trying to answer - How much air goes into the cylinder during the intake stroke? It can't be .5L at 1ATM of pressure, obviously. If we standardize the volume of that air brought in to 1 ATM, what will the volume of that air be?????
(lets say we are doing this at idle, make it 1000 rpm for easy calculations)

Thanks for replies guys