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dwightlooi

Dirty Air Filters WILL NOT reduce fuel economy.

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Posted (edited)

I understand basic IC operating principles just fine.

When you make an absolute claim ("ZERO effect on fuel economy") and you provide no specific data (IE; air flow of throttle body, air flow of clean filter, air flow of dirty filter, amount the driver increased the throttle plate opening to compensate), you have provided nothing to illustrate your claim, nevermind prove it. Therefore, questions about the claim are natural.
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(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.

How much? Is he limited in how much? Must it ONLY be the equivalent of the dirty filter's reduction in airflow? Can you quantify this? Can he suddenly go from a 25% throttle plate opening to 50% throttle plate opening? Why not?

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(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.

I have a feeling you did not mean to state that a filter 'causes the air mass to flow into the engine', seeing as how you were very specific about the gas pedal 'not controlling fuel flow into the engine'.

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(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.

I believe you are trying to say that when the 'gas' pedal is depressed the inverse amount more that the dirty filter reduced air flow, that the net effect is zero on the vehicle's fuel economy. Because the ONLY way that the amount of fuel used is the same is if the dirty filter + 'X' throttle plate opening = the clean filter + 'Y' throttle plate opening. That's quantifying the parallel scenarios to some degree, and such makes fine sense.

But you never stated it that way (or at least I never read it that way). And one could never expect a real world scenario like that to play out that exactly by chance alone.

Thanks for the back & forth, Dwightlooi.

Edited by balthazar
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On 4/1/2019 at 7:01 PM, balthazar said:

How much? Is he limited in how much? Must it ONLY be the equivalent of the dirty filter's reduction in airflow? Can you quantify this? Can he suddenly go from a 25% throttle plate opening to 50% throttle plate opening? Why not?

I have a feeling you did not mean to state that a filter 'causes the air mass to flow into the engine', seeing as how you were very specific about the gas pedal 'not controlling fuel flow into the engine'.

(1) As much as is necessary to produce the desired acceleration based on his arse? Nobody measure the throttle position in percentages, degrees or inches. They depress the throttle until they feel the desired acceleration that meets their driving habits. With a dirty filter more throttle will be applied. With a clean filter, less will be applied. But at the same acceleration, the net airflow to the engine and hence the net fuel flow will be identical. At WOT the engine will simply make less power and use less fuel with a dirty filter.

(2) I meant to say EXACTLY THAT. A net reduction in mass airflow to the engine of the same amount has fundamentally the same effect. Whether it is caused by a dirty filter, a smaller throttle opening or dead rat in the intake is irrelevant. The fuel use and engine output will be exactly the same, if the airflow is identical.

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6 minutes ago, dwightlooi said:

(1) As much as is necessary to produce the desired acceleration based on his arse? Nobody measure the throttle position in percentages, degrees or inches. They depress the throttle until they feel the desired acceleration that meets their driving habits. With a dirty filter more throttle will be applied. With a clean filter, less will be applied. But at the same acceleration, the net airflow to the engine and hence the net fuel flow will be identical. At WOT the engine will simply make less power and use less fuel with a dirty filter.

(2) I meant to say EXACTLY THAT. A net reduction in mass airflow to the engine of the same amount has fundamentally the same effect. Whether it is caused by a dirty filter, a smaller throttle opening or dead rat in the intake is irrelevant. The fuel use and engine output will be exactly the same, if the airflow is identical.

What about ... I dunno what else to call them... pumping losses?  There must be some additional effort by the engine to create enough vacuum to pull through a dirty filter than it is to pull through a clean one?   And wouldn't it be more dramatically noticed in a turbo-charged engine when the same amount of exhaust charge is not bringing in as much atmosphere as it did before?

I dunno about you, but it is harder to suck air through a wet rag than it is a dry one... it's that effort that I'm talking about. 

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

What about ... I dunno what else to call them... pumping losses?  There must be some additional effort by the engine to create enough vacuum to pull through a dirty filter than it is to pull through a clean one?   And wouldn't it be more dramatically noticed in a turbo-charged engine when the same amount of exhaust charge is not bringing in as much atmosphere as it did before?

I dunno about you, but it is harder to suck air through a wet rag than it is a dry one... it's that effort that I'm talking about. 

NO! The whole point of the thread to the explain that!

(1) Gasoline engines are throttled by being choked of airflow (except at Wide Open Throttle); if they are not making the maximum output at any rpm it is because the throttle plate is adding pumping losses to prevent it from doing so.

(2) Whether it is the throttle, a dirty filter or a dead rat in the intake thats doing the choking, the effect on the engine is identical if the net resulting airflow is the same.

(3) This is a main reason why gasoline engine are less efficient than Diesel Engines; Diesel Engines have no throttle plate and always run with the intake Wide Open, relying solely on fueling to control engine output. Therefore Diesel Engines have lower pumping losses at all scenarios except WOT where both are about the same.

(4) This is also why DOHC 4V heads, or any measure to make the engine breathe better, has ZERO effect on fuel efficiency. At anything but WOT (or pretty darn near WOT anyway) the throttle plate is adding all the pumping losses needed to meter engine output to the driver's needs.

Edited by dwightlooi

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