I've been thinking lately about gas milage. I've noticed that my lpg car seems to get about the same milage no matter how i drive. got me to thinking...

if the AFR is 14:1 needs 14 parts air and 1 part fuel ( by weight... 1 lb fuel 14 lbs air. )

the cylinder draws about 1.5 liters of this 14:1 mixture every rotation. this is true, but at a pressure less than atmospheric except at WOT. So we really draw 1.5 liters at a pressure of 15" vacuum so the density is way lower than if we mixed that AF up in a big bucket.

the amount of fuel is always the same for a given volume of air at a given pressure.

Is what i'm seeing that a tank of fuel is good for N rotations of the engine and what gear you use those in has more to do with how far you go than 'being a lead foot'

it's an air pump right? what makes it slow down is the vacuum in the manifold makes the density of the AF mix in the cylinder less dense and so the engine slows. Opening the throttle makes the mixture denser by keeping it closer to atmospheric pressure ( as long as the flow stays up, otherwise that would be 'lugging'.

That process would have an effect on fuel usage but only during transitions to and from idle.

that means there's really 3 fuel usage numbers ( compression braking, idle, acceleration ) the first is somewhat useless, it's how much fuel the engine can suck with the throttle closed and a high vacuum, the second is somewhat relevant a high idle wastes fuel ( wastes a few miles idling ), and the last is where most of the fuel would be used.

how much does the density change between 15" and 1" of vacuum?

it seems like the rpm has several effects, an engine at 3000 rpm pumps less than an engine at 5000 at the same pressure. But an engine at 5000 rpm and high vacuum pumps less than a engine at 3000 rpm at moderate vacuum.

so does fuel economy come from balancing low rpm with mild vacuum? You want the most vacuum that allows you to maintain a given RPM.

Which is why a turbo makes more power, by stuffing as much perfectly mixed air into the cylinder as possible.

let me know where i'm wrong...