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dwightlooi

Dirty Air Filters WILL NOT reduce fuel economy.

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Posted (edited)

A DIRTY AIR FILTER WILL NOT REDUCE FUEL ECONOMY! Sounds counter intuitive, but it's true! Here's why...

In a gasoline engine, the engine's output is controlled by choking it with the throttle body. It restricts the airflow to the engine at all times except when you put the gas pedal to the floor. Whatever the airflow happens to be, it is measured by the Air Flow Meter which then tells the engine how much fuel to inject. A dirty filter reduces airflow to the engine just like the throttle! The filter and the throttle restrictions combined results in the net airflow to the engine. If you have a clean filter, you'll end up using a slightly smaller throttle opening (gas pedal position) to accelerate at the same rate or maintain the same speed. If you have a dirty filter, you use a deeper throttle opening (gas pedal position). But in both cases, it is the net airflow that determines the amount of fuel you burn! The engine doesn't care what is restricting its breathing be it the throttle or the filter or both!

Now, a dirty filter will limit the maximum airflow you can get to your engine. It will reduce maximum horsepower and torque. So, it will make your car slower when you are really trying to go fast. With a really dirty filter, wide open throttle may be like 70% throttle. However, that WOT application will have exactly the same power and fuel burn as 70% throttle with a clean filter. It may take you 40% throttle to maintain 70mph in top gear instead of 25% throttle.But, again, that 40% throttle will have exactly the same output and fuel use as 25% with a clean filter. To get this bad you need to not change your air filter for 300,000 miles or something.ridiculous. Even then -- when driving "normally" around town or cruising down the freeway -- it'll have NO EFFECT on your fuel economy at all! A dirty filter is like putting a limit on how far you can depress your gas pedal! A slightly dirty filter may allow you 99% of your throttle. An unchanged filter with 100,000 miles on it may limit you to 90% throttle or something. But, it'll be no different from putting a brick under your gas pedal such that you cannot depress it fully. ZERO EFFECT on your fuel economy!

Don't believe me? Do a little experiment. Next time you do an oil change, don;t change the air filter (yet). Drive 10 miles down your favorite stretch of not so busy freeway at a fixed speed and measure your fuel economy with your trip computer. Now change the filter and drive down the same exact stretch at the same exact speed. You will notice ZERO difference in your mileage before and after.

Edited by dwightlooi
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I would agree that the amount of difference is too small to measure. However, I would disagree on a few points as stated.

There have been a number of tests pitting different air filters against each other on a number of criteria. Where the said differences are much more measurable were found with oiled fiber (cotton) air filters (such as K&N units). The cornerstone of oiled filter's advertising has been 'increased airflow', and therefore increased speed at a given RPM, but these comparative tests showed repeatedly that oiled filters airflow rates decreased almost from Mile 1, falling steadily and quickly dipping below traditional dry filters. IOW; they only had higher airflow rates when completely clean. This is one reason OEMs don't ever use them (maintenance is another). If one is using an aftermarket filter, this may be a measurable factor.

A short stretch of highway driving is not the ideal methodology to test MPG, as at-cruise operation uses very little fuel/airflow. A much better (but harder to duplicate exactly) would be an 'around town' loop where from-stop accelerations were repeated- much more typical of real world driving and encompassing higher airflow draws. This is where drivers would be better familiar with how their vehicle was performing relative to what they are accustomed to. This is where increased throttle position to match expected/desired/usual performance has a strong potential to indeed be measurable. But again; how to accurately reproduce this loop on repeated cycles.

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I understand the point Dwight is making from an engineering point of view, but then you start off with a restricted intake of air and the filter as it clogs up will still end up starving the engine to a point that eventually you keep the engine from working at it's maximum efficiency.

To me, never changing the air filter is a person being cheap, much like a person never changing their oil ever and just trading in the auto at 80k miles.

I get that the sensors will adjust for air coming in through the dirty filter, yet that ignores external environments. 

Being one that lived through a volcanic explosion, Mt. St. Helen's here in Washington and how fast the ash clogged filters and starved the auto of air to where it would not run. I would NEVER say ignore changing a dirty air filter.

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Posted (edited)
5 hours ago, balthazar said:

I would agree that the amount of difference is too small to measure. However, I would disagree on a few points as stated.

There have been a number of tests pitting different air filters against each other on a number of criteria. Where the said differences are much more measurable were found with oiled fiber (cotton) air filters (such as K&N units). The cornerstone of oiled filter's advertising has been 'increased airflow', and therefore increased speed at a given RPM, but these comparative tests showed repeatedly that oiled filters airflow rates decreased almost from Mile 1, falling steadily and quickly dipping below traditional dry filters. IOW; they only had higher airflow rates when completely clean. This is one reason OEMs don't ever use them (maintenance is another). If one is using an aftermarket filter, this may be a measurable factor.

A short stretch of highway driving is not the ideal methodology to test MPG, as at-cruise operation uses very little fuel/airflow. A much better (but harder to duplicate exactly) would be an 'around town' loop where from-stop accelerations were repeated- much more typical of real world driving and encompassing higher airflow draws. This is where drivers would be better familiar with how their vehicle was performing relative to what they are accustomed to. This is where increased throttle position to match expected/desired/usual performance has a strong potential to indeed be measurable. But again; how to accurately reproduce this loop on repeated cycles.

No. The point isn't that the effects of a clogged filter is too small to measure. The effects can be very significant. The point is that it DOES NOT reduce fuel economy because there is ZERO DIFFERENCE between restricting flow with the throttle body or restricting flow with a clogged filter!

Restricting flow is how you throttle a gaosline engine. If you need 12 CFM of airflow to maintain 65 mph, it doesn't matter if that is coming from a big throttle opening and a clogged filter or a small throttle opening an a clogged filter. 12 CFM is 12 CFM. And, 12 CFM will be burned with the same amount of fuel.

A clogged filter simply means that you cannot reach the engine's designed airflow with the throttle wide open. Imagine that your throttle cannot open past 80% or 90%. Does that affect your fuel economy? No. Not at all!

Edited by dwightlooi

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Posted (edited)
16 minutes ago, Drew Dowdell said:

What about on old cars without mass airflow sensors and such?

Well, if you don't have a mass airflow meter, you have one of two things...

(1) A Manifold Absolute Pressure (MAP) sensor. Restrictions create vacuum. The combined effects of the throttle and filter will collectively result in the pressure in the manifold being whatever it is (that's lower than atmospheric pressure). The engine meters fuel by knowing the engine rpm, the pressure & temperature (hence density) of the air in the manifold. And, it injects fuel based on what the programmed fuel map says. The effect is theoretically no different (if practically less accurate) than with a MAF meter. The filter has no effect on fuel economy only in the throttle position need to create the particular manifold pressure needed to achieve the same acceleration or maintain the same cruising speed.

(2) A Carburettor. Here the air flows past a venturi sucking fuel in for it to mix with. The amount of fuel sucked in will depend on the pressure of the air flowing past it vs the atmospheric pressure. Again, the filter restriction reducing the pressure of the air going through the cabs will result in less fuel being metered by the carb(s). It'll be like driving the car at a higher altitude. Again, no difference.

Edited by dwightlooi

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I have a carb, that's why I was wondering.  But my carb has electronically controlled metering. It is said that this carb is sensitive to dirty air filters.

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Posted (edited)

Let me put it another way that is easy for everyone to understand...

Driving your car with a DIRTY FILTER is like driving your car in DENVER instead driving it in SAN DIEGO -- or some place at sea level. Your car will be less powerful and you'll need more throttle to accelerate or maintain any given speed. But, at any given rate of acceleration or cruising speed your fuel consumption is exactly the same.spacer.png

The air filter causes a drop in pressure after the air passes through it. The throttle causes a further drop in pressure. The sum of the two drops is what the engine ingests. Whether this is from a small throttle opening and no air filter, or a restrictive air filter and a big throttle opening makes no difference. Fuel is metered based on mass of air molecues the engine ingests per unit time, not what causes the restriction in airflow. If this isn't the case, then your engine will be running too rich or too lean. It'll fail SMOG, stumble or stall.

Edited by dwightlooi
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While I agree that the throttle body & air filter combined will place X amount of airflow restriction on a given engine, the general consensus would be a given driver will drive in a style that they are accustomed to. While steady-cruise is a irrelevant factor WRT fuel consumption, repeated acceleration/deacceleration runs (IE; around town/city) will show higher fuel consumption, as it is then that the vacuum is highest or W.O.. In order to go up 'Hill St' and make the green light at the top, a local driver may give their car 65% throttle. But decreasing airflow will have the driver give it more throttle/vacuum to make the light, which will increase the fuel flow accordingly.

If an air filter had zero effect, racers looking for the last .100th wouldn't remove them on acceleration runs and show an improvement. Only way that happens is with increased airflow... which means increased fuel flow.

Look at it this way: run the intake air thru 10 layers of dirty air filter- see if there's a measurable drop in FE.

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Posted (edited)

I got question.

When someone presses on the accelerator, regardless of restrictive airflow, or free flowing airflow from dirty air filter/clean air filter, longer throttle body cables, shorter throttle body cables/drive-by-wire electronic throttle body calculating how much peddle is depressed...10%,15%, etc...what happens exactly?

Fuel is sent to the pistons along with oxygen, right?   

(the proper mixture is done with carburetors or the different types of fuel injection like sequential, port, direct....) (computers calculate elevation and density of air flow, type of fuel etc on fuel injection and on a carburetor, this is done manually through trial and error for lack of a better term, right?)

OK...if Im on the right track,  let me continue with more question.

 On a carburetor vehicle, when one wants to start it, we have to press down on the accelerator to get some fuel into the combustion chamber so when we crank, fuel is present already to ignite to start the pistons going up and down. right?

OK...when we press on the accelerator too much, we flood the engine with too much fuel. On older fuel injected cars, we instantly flooded the engine by pressing the accelerator, right?

OK...by doing that, we immediately waste fuel, right? Therefore we reduce fuel economy, right?

But...on modern fuel injected cars, there is a computer that calculates the perfect(?) amount of fuel and air mixture at all times(?), therefore, if there is a restricted airflow somewhere, the computer will only allow that certain amount of fuel necessary, right?

THIS is essentially what @dwightlooi is saying, or indicating or referring to, or hinting at, because that is what Im understanding because I kinda got too caught up with the technical jargon, so I reverted back to what I understand about basic engine knowledge and Im assuming that is what this is all about...obviously with more precise engineering technicalities...

Edited by oldshurst442

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Posted (edited)

You are basically correct WRT carbureted engines; the pump of the gas pedal squirts some fuel into the intake (low pressure), while the starter rotates the pistons and starts (slowly) drawing air (and the fuel) into the combustion chamber. FI cars start squirting the fuel (high pressure) directly into the C chamber, eliminating the need to pump the pedal.

MoPar carb'd vehicles didn't need to have the pedal pumped, since it took their engines a good 10 secs of starter cranking to fire when tuned/operating correctly. :P

Edited by balthazar
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The simple way to put it is that a carburetor has a finite range of airflow under which it can operate and meter fuel. It's like an airbrush or a paint sprayer in principle; try breathing over the tip and see if any paint comes out (I don;t think so). When you are starting the engine, it is cranking at a speed which does not provide enough airflow for the carb to work. The whole pumping the pedal nonsense is not necessary at all if you can crank the engine at 1000 rpm or relatively fast speeds. Unfortunately, that isn't the case with the typical starter.

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Posted (edited)
3 hours ago, balthazar said:

While I agree that the throttle body & air filter combined will place X amount of airflow restriction on a given engine, the general consensus would be a given driver will drive in a style that they are accustomed to. While steady-cruise is a irrelevant factor WRT fuel consumption, repeated acceleration/deacceleration runs (IE; around town/city) will show higher fuel consumption, as it is then that the vacuum is highest or W.O.. In order to go up 'Hill St' and make the green light at the top, a local driver may give their car 65% throttle. But decreasing airflow will have the driver give it more throttle/vacuum to make the light, which will increase the fuel flow accordingly.

If an air filter had zero effect, racers looking for the last .100th wouldn't remove them on acceleration runs and show an improvement. Only way that happens is with increased airflow... which means increased fuel flow.

Look at it this way: run the intake air thru 10 layers of dirty air filter- see if there's a measurable drop in FE.

Vacuum and flow restriction is one and the same. The former is what you experience when the latter is present. How deeply you step on the gas pedal DOES NOT determine the amount of fuel the engine receives. The amount of throttle induced vacuum (by itself) DOES NOT determine the amount of fuel injected either. Your gas pedal DOES NOT determine fuel flow! If it did, your engine will be injecting fuel when you floor the pedal while the engine is not running. If it did, it'll inject the same amount when the engine is at 800 rpm and when it is at 8000 rpm as long as your foot is depressing the pedal the same amount! The ONLY thing that determines fuel flow is Mass Air Flow across the MAF sensor (or in a Speed Density engine a mapped or calculated function of the vacuum in the intake manifold and the engine RPM). The intake displacement of the piston will normally draw that amount of air. Vacuum created by the throttle plate and the filter restrictions collectively reduce the pressure of the air going into the engine. Mass airflow = Volume x pressure (of air). Mass air flow is measured and used to determine fuel flow.

--

As I said previously, a 50% throttle opening and a clean filter is no different from a 65% throttle opening and a dirty filter if both generate the same mass airflow into the engine. To accelerate at the same rate you will burn the same amount of fuel in both cases. If you have 10 layers of dirty filter (which causes a 90% restriction), it'll be like driving around with no filter but with the gas pedal restricted to 10% of its normal travel. Your car will be VERY SLOW, but it'll be just as economical as a car with no air filter but driven at no more than 10% throttle all the time. Get it?

--

Of course racers remove air filters. Who wants to race with their throttle limited to 90% or even 99%?

Edited by dwightlooi

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Posted (edited)
Quote

Your gas pedal DOES NOT determine fuel flow!

This is only true AT A GIVEN INSTANT in time. There will be a lag as fuel flow ramps up in accordance with engine speed, but if there was NO correlation, no engine could ever turn above idle speed and there would be no gas pedals.

Quote

If it did, your engine will be injecting fuel when you floor the pedal while the engine is not running.

Carbs have this ability, readily.

Quote

...a 50% throttle opening and a clean filter is no different from a 65% throttle opening and a dirty filter if both generate the same mass airflow into the engine.

But would your scenario be achievable in actuality? If a given air filter (with many many times more surface area), either clean or dirty, can both pass more air than the throttle plate can, they both are rendered minimal factors. The result is the core variable is a throttle open 50% and one open 65%.

I think this would be a real, demonstrable experiment if the air filter had the same area as the throttle plate (while wide open). IE; and 85mm throttle plate with an 85mm air filter.

Edited by balthazar

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Posted (edited)
9 hours ago, balthazar said:

This is only true AT A GIVEN INSTANT in time. There will be a lag as fuel flow ramps up in accordance with engine speed, but if there was NO correlation, no engine could ever turn above idle speed and there would be no gas pedals.

Let me put it in a simple way so it's easy to understand. There is NO CONNECTION between the fuel injection system and the "gas pedal". All you are doing when you step on the gas is reduce the breathing resistance to the engine. This in turn increases mass airflow, which is measured and used to determine the amount of fuel the engine receives. Where, or how, this restriction comes about is irrelevant. A Gasoline engine is NOT throttled by fuel supply; fuel is measured to match air supply. A Diesel engine is throttled by fuel, but that is a different story for a different thread.

The air filter, when clean, has a much larger porous area than the throttle body and should have a negligible effect on airflow. When it gets clogged enough though, it will start to act like a second, partially open, throttle plate upstream of the throttle. Or, like an intake restrictor on a race car if you will. This will absolutely limit maximum horsepower, but it will have no effect on fuel economy. If the filter is totally occluded -- if you dip it in cement and let it dry -- it'll be like a throttle body that is totally closed with no idle air duct or valve. The engine will not run at all.

A carburetor's ability to squirt fuel on a cranking or stationary engine is not an extension of it's normal operation and purpose. It is a work around for the fat that it won;t work at cranking speeds and too low an airflow level. The fuel metered by "squirting" is invariably the wrong amount -- either too rich or lean. Sometimes it is so wrong that the engine still won't start. This is why carburetors have a "Choke" to limit airflow to match! This is also why no carbureted engine will ever pass modern start up emissions standards.

Edited by dwightlooi

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Posted (edited)
2 hours ago, dwightlooi said:

A carburetor's ability to squirt fuel on a cranking or stationary engine is not an extension of it's normal operation and purpose. It is a work around for the fat that it won;t work at cranking speeds and too low an airflow level. The fuel metered by "squirting" is invariably the wrong amount -- either too rich or lean. Sometimes it is so wrong that the engine still won't start. This is why carburetors have a "Choke" to limit airflow to match! This is also why no carbureted engine will ever pass modern start up emissions standards.


The primary function of a carburetor's accelerator pump is to provide transient enrichment to cover a sudden change in airflow demand; electronic controls mirror this behaviour in response to TPS rate of change.  The accelerator pump shot also provides supplemental start-up enrichment (reduces cranking duration mainly below -10 C), but most of the start-up fuel comes from pull-over enrichment while the engine is cranked and the choke valve is in a fully closed position.  When you see someone frantically pumping an accelerator pedal while cranking, you know they're used to dealing with a non-functioning or improperly adjusted choke.  :)  Accelerator pump shot volume is proportional to the pedal application rate and travel, so to test this, depress the accelerator pedal at an extremely slow rate, just enough to engage the choke and compare the cranking times.


 

Edited by KevinW

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I feel like we're dancing round semantics. I'll agree that technically speaking, the gas pedal opens the throttle plate and fuel flow reacts to that. It -technically speaking- is somewhat different with carb'd vehicles as there is an initial physical 'pushing' of fuel via the accelerator pump. I don't see this as a relevant point however, because altered fuel flow is an inevitably linked result of opening the throttle plate, and -with a small lag- increases in step with air flow & engine speed. Gas pedal may not be physically connected with fuel flow in an FI engine, but you WILL get increased fuel flow when pressing the 'gas' pedal.
 

A Gasoline engine is NOT throttled by fuel supply; fuel is measured to match air supply.


It is not "throttled" by fuel supply, but it is it operated on BOTH. If we're talking here about fuel economy, we're talking about operating a vehicle in the real world, no?
Watch a modern vehicle's instant mileage readout. Step on the 'gas' pedal from a stop and see a modern 4-cyl car return instant numbers of 3, 5, 11 MPG. This is a comparison of vehicle speed to fuel metered, IE; fuel economy. Same vehicle at level, highway speeds can return MPG numbers FAR above ratings- in the 50s and 60s. Lift off the pedal and see it go to 99 MPG.

It is possible to place a lock on the throttle at -say- 30% and accelerate from a stop to 75 MPH. MPG will vary tremendously over that run as a byproduct of fuel metering (steady here) and speed (variable). But people in the real world won't accept accelerating from a stop @ 30% 'throttle plate opening'.

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2 hours ago, balthazar said:

Watch a modern vehicle's instant mileage readout. Step on the 'gas' pedal from a stop and see a modern 4-cyl car return instant numbers of 3, 5, 11 MPG. This is a comparison of vehicle speed to fuel metered, IE; fuel economy. Same vehicle at level, highway speeds can return MPG numbers FAR above ratings- in the 50s and 60s. Lift off the pedal and see it go to 99 MPG.

Right, and that is 100% due to the change in the mass air flow measured subsequent to the motion of the gas pedal, not due to reading the position of the gas pedal. Fueling is determined by net airflow measured. How much of that comes from the throttle plate, or other restrictions in the intake system, makes ZERO DIFFERENCE to the determination of fueling or fuel economy. ZERO.

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2 hours ago, dwightlooi said:

Right, and that is 100% due to the change in the mass air flow measured subsequent to the motion of the gas pedal, not due to reading the position of the gas pedal. Fueling is determined by net airflow measured. How much of that comes from the throttle plate, or other restrictions in the intake system, makes ZERO DIFFERENCE to the determination of fueling or fuel economy. ZERO.

So you are no longer talking about fuel economy as realized by a vehicle owner, operating their vehicle in the real world on public streets and contemplating when to change their air filter; the way you opened this thread ("when driving "normally" around town or cruising down the freeway").  The only way these statements make sense to me is that we're now talking about an engine on a test stand in an controlled atmosphere with minimal to almost no variables, and certainly no vehicle/ drivetrain/ road speed/ variable terrain involved.

Because: not 100%.  'Fuel economy' is expressed as 'MPG'; a calculation that incorporates distance covered.  One can achieve the same mass air flow in park and get 0 MPG.  A car's instant MPG reading is NOT solely based on mass air flow.

Does a 30% throttle plate opening at 15 MPH result in the same MPG as 30% throttle opening at 75 MPH?
Does a 30% locked-open throttle plate give the same MPG traveling UP a 25% incline, as DOWN it?
 

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25 minutes ago, balthazar said:

So you are no longer talking about fuel economy as realized by a vehicle owner, operating their vehicle in the real world on public streets and contemplating when to change their air filter; the way you opened this thread ("when driving "normally" around town or cruising down the freeway").  The only way these statements make sense to me is that we're now talking about an engine on a test stand in an controlled atmosphere with minimal to almost no variables, and certainly no vehicle/ drivetrain/ road speed/ variable terrain involved.

Because: not 100%.  'Fuel economy' is expressed as 'MPG'; a calculation that incorporates distance covered.  One can achieve the same mass air flow in park and get 0 MPG.  A car's instant MPG reading is NOT solely based on mass air flow.

Does a 30% throttle plate opening at 15 MPH result in the same MPG as 30% throttle opening at 75 MPH?
Does a 30% locked-open throttle plate give the same MPG traveling UP a 25% incline, as DOWN it?
 

I am talking about EXACTLY THAT -- dirty air filter(s) will not affect fuel economy when you drive your car in the real world (or on a test stand for that matter). Because, while it increases how much throttle you need to achieve the same acceleration, or to maintain the same cruising speed, that increase in throttle opening DOES NOT change the amount of fuel the engine uses. This is so because net Mass Air Flow ALONE determines fuel supply. How that net Mass Air Flow came about is irrelevant.

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Posted (edited)

An engine on a test stand gets ZERO MPG.
- - - - -

Quote

that increase in throttle opening DOES NOT change the amount of fuel the engine uses.

• By what unit of measure is air flow into an IC engine quantified?
• Is that measurement, in real world vehicle operation, ever a static constant?
• Is air flow volume dependent solely on throttle bore size?

Edited by balthazar

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1 hour ago, balthazar said:

An engine on a test stand gets ZERO MPG.
- - - - -

• By what unit of measure is air flow into an IC engine quantified?
• Is that measurement, in real world vehicle operation, ever a static constant?
• Is air flow volume dependent solely on throttle bore size?

An engine on a test stand has zero mpg, but it still has measurable fuel economy in form of Brake Specific Fuel Consumption (BSFC).

The MAF sensor generally reads out as a voltage. The ECU converts that to the appropriate mass air flow value in mass per unit time. This can be pounds per minute, kilograms per second or whatever unit of measure the programming prefers to work with.

The instantaneous reading of the MAF sensor while the vehicle is being driven is what determines the fuel injector cycle and hence fuel flow. The gas pedal itself has no direct bearing on fuel flow, it merely regulates the intake restriction via the throttle body with a mechanical cable or with electrical signals to a actuator motor. A dirty filter is no different from an intake restrictor and no different from a small throttle opening. Saying that a dirty filter reduces fuel economy is like saying driving your car gently like Grandma does reduces fuel economy. It's bogus.

Airflow to a gasoline engine governed by the throttle body, the idle air duct, the air filter and every geometric restriction in the intake plumbing. These things collectively limit the air flow to the engine, collectively affects the reading of the MAF sensor and consequently fuel supply. The important thing you need to understand is that NONE of these things affect fuel economy. All they do is throttle the engine no differently that the throttle body does. A gasoline engine, by design and operational principle, is ALWAYS running choked up. This is one of the priniple reason why it is less efficient than a Diesel Engine which is always running at the equivalent of Wide Open Throttle (because it doesn't have a throttle body at all).

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Posted (edited)
Quote

BSFC

I was wondering if/when you would introduce that. I believe now I've been proven correct; we are square dancing with semantic partners.
- - - - -
By your postings, air flow is the prime event of IC, not fuel flow (despite the fact that aside from a small variable range, they are intrinsically linked).

Quote

• By what unit of measure is air flow into an IC engine quantified?

Air flow is measured by CFM (cubic feet per minute).

Quote

• Is that measurement, in real world vehicle operation, ever a static constant?

In an internal combustion consumer motor vehicle as introduced in this thread; no, because

Quote

• Is air flow volume dependent solely on throttle bore size?

no; volume is dependent on both throttle plate bore (a constant), throttle plate opening (a variable) and flow RATE (a variable).

In this example there is always an element of time involved (CFM). An instantaneous snapshot of air flow (or fuel flow) is only a data point in a chain of data used to compute fuel economy. And that snapshot changes from click to click to click; constantly, even at level, highway speeds using cruise. Every time air flow changes, fuel flow changes pretty much in lockstep. This is why a vehicle's average fuel economy reading sometimes covers a moving window of the last 450 miles.
- - - - -
I'm going to go back to your original statement :

Quote

The air filter, when clean, has a much larger porous area than the throttle body and should have a negligible effect on airflow. When it gets clogged enough though, it will start to act like a second, partially open, throttle plate upstream of the throttle. Or, like an intake restrictor on a race car if you will. This will absolutely limit maximum horsepower, but it will have no effect on fuel economy.

This is ONLY true if the exact same throttle opening map is repeated, EXACTLY, in a controlled environment, between the clean & dirty filter. In the real world, the dirty filter vehicle's driver must never open the throttle plate 1 iota more to compensate for a sensation his vehicle 'feels slower' because 'Geez- when's the last time I changed my air filter??'. AS SOON AS the driver opens the throttle bore a teeny bit more, he'll burn more fuel. That's a variable that is easily eliminated in the laboratory, but impossible to do so in the real world.

Edited by balthazar

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Posted (edited)
13 hours ago, balthazar said:

This is ONLY true if the exact same throttle opening map is repeated, EXACTLY, in a controlled environment, between the clean & dirty filter. In the real world, the dirty filter vehicle's driver must never open the throttle plate 1 iota more to compensate for a sensation his vehicle 'feels slower' because 'Geez- when's the last time I changed my air filter??'. AS SOON AS the driver opens the throttle bore a teeny bit more, he'll burn more fuel. That's a variable that is easily eliminated in the laboratory, but impossible to do so in the real world.

You seem to be having a hard time understanding basic operating principles. So, let's try again...

(1) In the REAL WORLD, the driver will open the throttle more to compensate for the dirty filter because his car feels slower to get it to  accelerate at his desired pace.

(2) But, despite him opening the throttle more, the dirty filter causes in the same amount of air mass actually flow into the engine as would have if the filter is clean and he opened the throttle less.

(3) Because fuel is metered based on the measurement of Mass Air Flow, the amount of fuel used is the same, as is engine power and acceleration.

(4) The added restriction of the filter is no different from placing a limiter on the throttle opening; it absolutely reduces maximum power of the engine but has no effect on fuel economy.

BTW, Mass Air Flow is NEVER measured in CFM. CFM (Cubic Feet per Minute) is a measurement of Volume per unit time. Mass Air Flow -- by definition -- is always measured as Mass per unit time. Saying 40 cu-ft of air entered the engine does not tell you how many air molecules are in the cylinders, hence how much fuel should be burned in them. 40 cu-ft at 14.7 psi (sea level pressure) has more air molecues than 40 cu-ft at 12 psi (Denver; 1 mile altitude) and less than 40 cu-ft at 30 psi (under turbo or supercharger boost). That is why Mass Air Flow is measured in kg/sec or lbs/min or some ther unit of mass over time.

The alternative is a Speed-Density estimation of airflow whereby the CFM (Flow Speed) is read off a table based on the known corresponding volumetric efficiency of the engine at a particular RPM. An Manifold Absolute Pressure (MAP) sensor is then used to accurately determine the Pressure of the air actually entering the engine, whilst a Temperature also read off another sensor. Density is calculated as a function of Pressure and Density. Remember pV=nRT? The throttle does not affect the CFM. Neither a dirty air filter nor turbo boost affects the CFM either. However, these ALL affect the pressure of the air in the manifold. Mass Air Flow is then calculated by the ECU as Flow x Density.

I hope you get it now, because I am trying VERY HARD to explain it to you...

Edited by dwightlooi

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