i replaced the air filter in my daily driver (not a camaro, a more modern truck) and noticed an increase in gas mileage as a result.
this got me thinking, which is never a good thing (lol) and i have stumbled into a bit of a conundrum which i need help solving.


assume for a second that driving at a constant speed, it takes 15 hp to maintain speed against friction, wind, ect.

as the engine in a car is capable of oh, somewhere around 20 times that much, the throttle is used to regulate the power output to 15 hp.

now lets say that you have a clean air filter which poses no restriction on the air entering the engine, so the throttle does all the "restricting".
lets say for arguemnts sake that you have a 6% throttle opening at this constant speed.
the computer measures the air going in, and supplies the necessary fuel for a 14.7:1 stoichiometric ratio.

now lets assume you have a clogged air filter. so much so that the throttle opening required to maintain the same speed is now 12% (for arguments sake bear with me)
the thing is, the total air going in remains the same so that the power ouptut remains 15 hp.

my question is this:
why does the fuel mileage improve?
the total air going in doesnt change so the total fuel used remains the same.

if you think about it in terms of a circuit, the air filter is like one resistor and the throttle is like a second resistor in series with the first.
if you change the first resistor you have to change the second resistor to keep the same overall resistance of the circuit.


so now, im driving myself trying to figure this out.
i understand that an air filter will noticeably increase all out max power because it becomes the only restriction once the throttle is all the way open...


if anyone has an answer, let me know, because a house full of engineers is stumped.




edit:
the truck is in the same gear, at the same rpm, at the same speed in the first and second situations.
also remember that a constant power is required for a given speed on flat level road. regardless of what this power is, no matter what, driving this speed will always require the same power. this is part of the theory that cruise control works on.

 

OK - how about this ...

When you open the throttle to allow the more air flow, you also increased the amount of fuel consumption because the linkage opens the valves inside the carb more (or, the TPS changes resistance and the injectors fire more). Just because the fuel isn't necessarily getting burned doesn't mean it's not getting used - it's getting used to foul the plugs and make the driveway black

 

yes, but no.
firstly, its an 03, so no carbs or throttle enrichment are used.
in a car with a carb, a certain throttle opening corresponds to a certain fuel flow rate.
in the beginning of fuel injection, it worked much the same way. but in modern fuel injection system, no PE enrichment is needed, so the mixture always stays right at stoich.

 

OK - not that I doubt you (because I really don't have a clue) - but I now wonder - if the fuel injectors in a later model EFI car don't 'squirt' more given an increase in throttle, then what process does make more gas get added to the increased air flow? Either the injectors flow more gas quantity per cycle, or they cycle more times per minute, or something ... there HAS to be more gas flow per increased cfm of air flow.

 

How sure are you of that?

 

apeiron:
well, its an assumption given that it takes a given power to drive a certain speed, and that a volume of air = a volume of gas, which = a certain power level.
if the air is different then it would very well explain how the efficiency went up but then we are left with a motor that is running lean (fuel = power no matter how you look at it)


camaronewbie:
the injectors spray more fuel when more throttle is used.
however, they do not spray an extra shot when you give it more throttle.
in a carb, you have an acceleration pump that squirts a shot of gas in when the throttle is pressed, in addition to the gas that would normally go in.
so what you end up with is the more throttle you use, the more rich the engine runs.

at least in the engine i am speaking of, there is no PE enrichment to speak of except under very certain situations (towing ect.) or so i have been told.
this means that the fuel is always perfectly proportional to air.


btw, i am not dismissing anybodies theories on the matter.
i am simply typing as i think.
i dont mean to dissuade anyone from thinking that they may be correct

 

One of the simplest ways to look at it would be that a dirty air filter is harder to draw air through than a clean one. Then even though it takes the same 15 HP to move the vehicle forward, the losses in the system have increased. With a dirty filter the engine might need to make 18 HP to have 15 HP available at the wheels, where with a clean filter it might only have to make 17 HP. It'll need to consume more air and fuel to produce the additional power.

 

but this draws you back to the throttle.
if you have a 315 hp engine and only need 15 at the time, then the throttle is a whopping 100 hp restriction with zero from the air filter.
if the filter is a 3 hp restiction, and you still only require 15, then the throttle only has to be a 297 hp restriction.
thats how i understand it.

 

You can't measure throttle restriction in terms of a fixed amount of power.

Even if you did, your example doesn't negate a change in mileage. If 15 net HP with 0 HP loss in the filter means a 300 HP "throttle restriction", then 15 net HP with 3 HP loss in the filter means a 297 HP "throttle restriction". Less throttle restriction means more throttle opening, means more air into the engine, means more fuel consumed.

 

but the extra air getting past the throttle is negated by the air that is trapped by the filter.
or is this wrong?

my theory is that since the MAF lies between the filter and the throttle, if you have a clogged filter, then the MAF sees a slight vacuum vs a clean filter.
the computer then thinks that it is driving at high altitude and compensates.
im still not sure how the air works out, but i have a feeling that it has to do with the reduced density of the air at the MAF, if that makes sense.

do you see where i am getting stuck in a circle?

 

The MAF sees mass air flow, not vacuum, but the MAF is irrelevant anyway. So is fuel injection itself.

Yes, less air may go into the engine for a given throttle opening due to the restriction in the filter, but that doesn't mean that enough air can't go in.

 

the maf doesnt directly measure vacuum, but it is inferred.
if there is a vacuum present, then there will be less mass of air flowing per unit time.


exactly my point... i think...
if only enough air for 15 hp is required then thats all you will get.
maybe we are saying the same thing but differently.

i am assuming a box which when fed a certain input produces a certain output.
the output is 15 hp and the input therefore has to be a certain volume of gasoline per minute and 14.7 times as much volume of air per minute by mass.

if you keep the 15 hp constant then the air has to be constant. no matter what, the air getting past the combination of filter and throttle (in series) has to be proportional to 15 hp... in other words, (x + y) = total air
if you increase x, y must decrease.

again, i may be totally wrong on this.