Air filter and mpg.

I just heard on a local car show that a dirty air filter only affects
performance. It does not lower miles per gallon.

 

You can hear all kinds of funny jokes and fairy tales on car-related shows
on the radio...both National and local.

 

Doesn't lower performance usually equal lower MPRG?

-- Christian

 

Only in the real world, not in the make-believe world of idiot car-talk
radio show hosts who care more about laughing at their own dumb jokes than
they do about providing good information.

 

Are you sure about this? Performance is usually measured as the maximum
output from the engine. A dirty filter will clearly reduce max output.

But MPG? On a car equipped with MAF and oxygen sensors? The only
difference with a dirty filter would be a small power loss due to the
extra energy required to pull the air through the filter -- but would this
be measurable?

 

Naw, ya caught me, I'm just makin' shit up as I go along. Yes, I'm sure of
it. It's basic and very easy to understand.

Yes, MPG. Reduced airflow through the filter --> reduced airflow through
the engine --> reduced power --> you push your foot down further to
accelerate --> TPS tells ECM "More coal on the fire!" --> lower MPG.

Even on a car equipped with MAF and O2S.

 

And I call BS. If the car has a MAF sensor, the amount of fuel that is
injected is a function of the mass of air that is inducted into the
engine. Result: engine runs at same A/F ratio, economy is unchanged.

You seem to be assuming that the amount of fuel injected is a direct
function of throttle position. I don't think it is: I think it is a
function of the air mass flow as measured by the MAF sensor.

Think about what you are suggesting: tell me, do they adjust cars to run
in Denver these days? I don't think so with modern cars.

But what do I know? I only used to work on the design of MAF sensors, fuel
injection design systems, ignition timing controllers, knock detection.

 

Are you suggesting that the TPS has no authority over fuel
delivery?

Of course "they" adjust cars to run in Denver these days, it's
just done in the engineering department by the OEM instead of a
service shop. Just because it's handled by the PCM doesn't mean
it isn't being done.

Then you'd know that Ford vehicles (for example) outdate BARO via
the MAF reading at wide open throttle and that BARO is
extrapolated based of stored values in the PCM which equate to
what the volumetric efficiency should be at WOT, when things
don't match because of a restricted air filter, you get a skewed
BARO reading and incorrect fuel control.

You'd also know the many GM vehicles use both a MAF and a MAP
sensor and that the MAP sensor updates BARO at wide open throttle
and again, based upon calculations stored, will result in a
skewed BARO reading due to the pressure drop across the
restricted air filter.

But what do I know? I -still- fix IM-240 CO and CO/HC failures
on MAF equipped vehicles that are caused by dirty air filters.

There was an interesting number of posts a week or so ago by a
Ford dealership tech on i-ATN regarding Fram air filters causing
all manner of driveability problems because the element size is
much smaller than what the system was designed for.
(hmmm... wonder why?)

Of course since this is a Chrysler newsgroup and since (so far)
most Chrysler vehicles use MAP sensors only and that the MAP
sensor again is used at wide open throttle to up-date the BARO
reading, a restricted air filter will cause incorrect fuel
delivery.

 

well if this stupid statement was correct, then a vehicles miles per gallon
consumption should be the same regardless of load and speed! So then MPG in
the city compared to Highway MPG is a false reading as it should be
specifically 23 MPG regardless of driving type, WOT or otherwise. Or are you
saying the MAF has ESP and can tell the injectors to inject the exact amount
of increased fuel before the throttle plate is opened to introduced the
increased airflow? I guess we should really not even have a throttle peddle
in the car as the MAF seems to do everything for us.

 

Not necessarily. Most ECMs try to run near their factory default
cailibration with the throttle position sensor as the primary input, and
use the O2 sensors and MAF or MAP sensors to apply correction factors.
Often, they tend to be a little less efficient as bigger and bigger
correction factors are needed. The effect may be small, but not absent.

 

No, not true. I agree with you that the statement isn't correct, but not
for this reason. His argument is that the throttle itself is nothing but
a restriction to air flow, so if you add restriction to the air FILTER
and compensate by opening the throttle further, then the MAF and O2
sensors will correct everything back to normal and mileage should be the
same as if there were no air filter restriction and more at the
throttle. If the A/F ratio was strictly computed from sensor readings,
he would definitely be correct. But A/F ratios are started from a fixed
baseline depending on RPM and throttle position, and then biased away
from the pre-set table by O2 and MAF sensor readings, thus there is
still some effect from a dirty air filter, because the bias offset has
to be larger. Its much the same with a carbureted car- the carburetor
*ideally* only cares about the speed of air flowing thru the venturi,
not the absolute pressure above the throttle plates because the float
bowl is also exposed to the same pressure. But we all know it doesn't
*quite* work that way.

 

No, I neither said nor implied that. Engine (and car) efficiencies depend
on how they are operated. Load and speed clearly have an impact.

The MAF sensor measures air flow. The provide an inaccurate measure of
air flow. Your statement is nonsenical -- according to you a TPS can measure
throttle opening before it is opened!

There are plenty of reasons to have a TPS, irrespective of measuring air
flow into the engine.

 

So, what you are saying is that the PCM can handle reduced pressure due to
reduced atmosperic pressure, but not reduced pressure due to a dirty air
filter?

What the heck do you think a closed loop system with O2 sensors, is
designed to do? It is designed to keep air/fuel ratios at the optimum
point -- and it adjusts for external factors such as atmospheric pressure
and dirty air filters

We are talking economy, not performance.

 

....and what the O2S says. And what the TPS says.

Nope, I'm making no such assumption. You, on the other hand, are positing
that injector pulsewidth is a direct and sole function of MAF readings.

Think about what *you* are suggesting: Tell me, is the MAF the only sensor
on the engine?

Good question.

 

Yes, especially since measuring airflow into the engine is not one of the
reasons to have a TPS.

 

No. I ti sabsolutley different to a carburetted car -- a carburetter is an
open-loop device. There is no compensation for changes in pressure, and
other factors.

You describe the MAF and O2 sensors compensating for the differences. Just
because the bias offset may be bigger, why would that make the engine any
less efficient? It's just an adjustment to the amount of fuel injected to
keep the A/F ratio correct.

Essentially, what you and other are arguing is that, with a dirty filter,
the A/F ratio will be wrong, despite a closed-loop system and despite air
flow being measured by a MAF sensor.

 

No. I did not say that.

Ah, that's right, don't respond to my question.

Clearly, the MAF sensor is not the only sensor. Did I say it was? Did I
say the fuel injected is solely a function of the MAF sensor reading? If
you think so, then I suggest you take some lessons in comprehension.

Anyway, please explain why the O2 sensor readings are not used to adjust
the A/F ratio to take account of variations such as dirty filters?

There is one other factor, which is that the difference in pressure drop
between a brand new filter and one that has been used for a 1000 miles is
probably far greater than the difference than that filter after 1000 miles
and after it has passed its proper life. But I will be frank and tell you
that I am guessing here.

 

The two are inexorably linked, despite your handwaving. Let's put it in
real-world terms: Mr. Green and Mr. Black live on the same block in
Englewood, a suburb of Denver. They both work at Beau-Jo's in Idaho
Springs, 44 miles away, and they drive identical cars. Mr. Green changes
his air filter on schedule, while Mr. Black tends to let it slide until
the car is running noticeably poorly.

Both Mr. Green and Mr. Black take the same route to and from work every
day. They take Quincy Ave. East to I-25 Northbound (an uphill onramp),
I-25 North (a gentle uphill climb) to 6 West (an uphill flyover) to 70
West, at which point the remaining 19 miles of the trip are a fairly steep
climb.

Because Mr. Black neglects his air filter, airflow through his engine --
and therefore power -- at any given throttle opening is less, so he must
push the accelerator further down to accelerate and climb the hills. On
the onramps that require rapid merging, and on the uphill climbs, he
frequently pushes the accelerator down both to the point where the ECM
looks at the TPS and says "Oh! Rapid acceleration! OK, let's get that
acceleration enrichment subroutine online. More pulse width!", and to the
point where the ECM looks at the TPS and says "Oh! We're at WFO!" and goes
into open-loop operation.

Because Mr. Green maintains his air filter, airflow through his engine --
and therefore power -- is not reduced, so he needn't push the accelerator
down as far as Mr. Black. Therefore, less acceleration enrichment and
less open-loop operation.

Now, the $64,000 question:

WHO BURNS MORE FUEL?

 

Really? Are you SURE you want to stick with that statement? For all
carbs? For all factors? ;-) I claim that carbs (at least the 4 I own)
are actually pretty WELL compensated for the case of a dirty air filter,
because the float bowl sees the same lowered pressure (caused by the
dirty filter) as does the venturi, and therefore its compensated.... but
not necessarily PERFECTLY, which is the same as for software operating
highly skewed away from the basic lookup tables.

You're right, and it stays CLOSE. But, depending on the programming of
the PCM, there will be a skewing of the A/F ratio.

Depending on the programming of the PCM, that is true to varying degrees.

 

Because the driver's foot has crammed the accelerator to the floor to
accelerate because his air filter is plugged up, so the engine's in open
loop. Next dumb question?

And I will be Daniel and tell you that you're full of it.