air filter vs fuel economy (tricky question?) 11

[ivymike]

Perhaps this is a tricky question...

Does having a dirty air filter hurt the fuel economy of a modern gasoline-fueled car? (Obviously the parts stores would have you believe that it does)

If so, how exactly?

If it's inlet restriction, then would you say the effect is the same as modifying the throttle actuator such that the throttle opens slightly less for any given pedal input than it would otherwise have opened? If this is equivalent, would you expect to get better or worse fuel economy as a result of such a modification?

 

[JSteve2]

A dirty air filter will lower your fuel economy. You have greater inlet restriction, and the driver (you) will drive with the same acceleration expectation. Your suggested modification would also give you lower fuel economy for the same reason. If the driver modified his behavior, by reducing acceleration rates, for example, then it is possible to correct for this issue. However, the engine is incapable of giving identical performance for the same fuel economy with the restriction (however imposed).

 

[jetmaker]

So what about those high perfomance filters such as K&N?

Do they increase performance by reducing inlet restriction, and if so, is it at a cost of reduced filtering ability?

Are they worth the cost compared to OEM filters?

jetmaker

 

[JSteve2]

It depends upon what you mean by "worth the cost".

In dollars of gasoline saved versus cost, they are probably not worth it. If you need the incremental performance gain (which is small) it could be worth it. As I recall, K&N filters can be cleaned and reused, so if the environmental benefit of that is valuable to you, or you will be keeping the filter a really long time, again it could be worth it.

But for the man on the street just worried about how much money he's going to put into his car over the next 50,000 miles, then just the scheduled changes with a cheap filter are the lowest cost even accounting for fuel economy.

 

[SomptingGuy]

"High performance filters" will give less restriction to flow, allowing higher flow at WOT. I can't see how part throttle economy can be compromised by moving the restriction from throttle to filter.

- Steve

 

[patprimmer]

At part throttle (the vast majority of the time), as fuel is metered electronically to match airflow, it does not matter where the restriction is to control power output.

The same power output will use the same airflow and fuel, but with more restriction at the filter and less at the throttle plate.

At WOT the restricted filter will reduce airflow and power and fuel consumption in direct proportion.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[jetmaker]

Thanks for the info. Very informative.

 

[ivymike]

Jsteve, do you still believe your first answer, given the subsequent posts?

If so, then I have a question about your first answer - if you have the same "acceleration expectation" as a driver, does that mean that you're going to push the gas pedal to 105% of maximum travel (to extra-wide open throttle) to get the same performance with the added restriction? If not, then won't you be forced to accept reduced acceleration and improved fuel economy?

 

[JSteve2]

I wasn't thinking about the throttle modification correctly - so to answer your question I don't believe my first answer.

The throttle modification is really just a recalibrated throttle, as 5 mm open is 5 mm open, regardless of how much pedal you have to use to get there. For your example - the engine performance would actually just be the same since the engine doesn't care why the throttle is set where it is. The driver would notice that he used more pedal, and as you note, that he runs out of pedal so the engine would have a different (lower) maximum torque available.

For some reason when I read the throttle modification, I was thinking of an intentional restriction, not a throttle recalibration. For the dirty filter, still fuel economy loss.

 

[Lou Scannon]

"For the dirty filter, still fuel economy loss"
JSteve, do you want to try to explain your thinking on this?
It has already been refuted above.

 

[JSteve2]

No, I don't think so.

For a given engine torque X (air flow at a given speed) there is a pressure drop Y in the inlet air path. The Y is the same regardless of the throttle modification above, as the system is physically configured the same.

However, with an additional restriction (dirty filter, whatever), for the given engine torque X there is a pressure drop Y' that must be greater than Y.

That extra pressure drop must be paid for in fuel economy.

 

[ivymike]

so why is the throttle different from any other restriction in the intake path? alternatively, why is added restriction not the same as simply using less throttle?

 

[ivymike]

uh, that should be "less throttle opening"

also, why would you have a given torque output with two different intake restrictions? I would guess you'd have less torque output with restriction y', and your fuel-air ratio (for closed-loop) would still be controlled as before.

 

[JSteve2]

The throttle is not different than other restrictions. In the suggested throttle modification, the throttle position to pedal relationship was changed, but not the throttle restriction to air flow relationship. If you change the relationship of the throttle restriction to the air flow, you will in fact affect the fuel economy relationship (which is exactly how I mis-read the OP). Likewise, if you add a restriction, you change the total restriction to air flow relationship and affect fuel economy.

 

[SomptingGuy]

Absolutely. With a loaded air filter you'll have less power in reserve (as RR describe it). Doesn't affect part-load performance or economy though. Unless there is some highly tuned gas dynamics in the intake that depends on the restriction being in a certain place (unlikely).

- Steve

 

[ivymike]

Now wait a minute here, air flow is a function of total restriction, right? Why would the ratio of throttle restriction to total restriction make a difference? Isn't total restriction the only thing the engine "sees?"

 

[JSteve2]

I know the temptation of what you're saying here, but I am unconvinced. Imagine essentially a plugged filter with a huge pressure drop across it. If you flow 10 lbm/min through it with a 10 psi drop instead of 10 lbm/min with a 1 psi drop in a clean filter, you will lose fuel economy. It holds true even when the drop is 1.5 psi versus 1.0 psi, it's just not as intuitive.

If you're saying - I have a 5% restriction, and the driver will accept 5% lower torque and achieve the same mpg, then that may be true. However, that is a performance degraded engine, and you would be required to report (in a technical report) that there is a fuel economy loss because to generate the same power/torque you will use more fuel.

 

[SomptingGuy]

Hmm, didn't make it clear with who I was agreeing. Not JSteve2 (no beef, just think you're wrong).

- Steve

 

[ivymike]

I'm suggesting that the driver might accept 5% less torque and get BETTER mpg if he often uses full throttle, or unchanged mpg if he doesn't.

Say that you have a 1.5l engine, and you normally cruise at 65mph, at 2200rpm with the throttle at 30% opening, giving X throttle restriction and X+S total restriction, giving an air density at the inlet manifold entrance (for a particular set of ambient conditions) of 0.4 kg/m3... then your air mass rate would be about 0.66 kg/min and your fuel rate would be about 40g/min.

Add filter restriction to give S', now you need to open the throttle a bit more (say 35%) to give (X'+S')=(X+S), and you now cruise at 65mph at 2200rpm with an air density at the inlet manifold entrance (for a particular set of ambient conditions) of 0.4 kg/m3 and your air mass rate would be about 0.66kg/min and your fuel rate would be about 40g/min.

The only thing the driver would notice is that he's pushing the pedal farther... the engine would be none the wiser.

Take the same two cars, but try to accelerate from 65mph to some other speed as quickly as you can without shifting (put the pedal to the floor). Now your throttle position is 100%, your total intake restriction is X0+S, your density at the inlet manifold entrance is 1.0kg/m3 (for those same ambient conditions), your air mass flow rate is 1.65kg/min, and your fuel rate is about 110g/min.

Add intake restriction, same attempt to accelerate. Throttle position is 100%, intake restriction is X0+S' (S'>S), your density at the inlet manifold is 0.90kg/m3 (for the same ambient conditions), your air mass flow rate is 1.49kg/min, and your fuel rate is about 99g/min. The power output is lower and you accelerate more slowly. Bummer.

Now say that the guy next to you in an identical car wants to keep pace with you. He doesn't have a plugged air filter, however, so to keep from blowing past he must push the pedal a little less. Throttle position is 90% (perhaps), intake restriction is X1+S (which equals X0+S'), his density at the inlet manifold is is 0.90kg/m3 (for the same ambient conditions), his air mass flow rate is 1.49kg/min, and your fuel rate is about 99g/min, and his acceleration matches yours.

...so my conclusion is that having a plugged air filter is like having a mechanical device to make you into less of a leadfoot, whether you like it or not, and if you accelerate often at max throttle, it might improve fuel economy.


Now you have 100% throttle opening, you're at 65mph for the moment, 2200rpm for the moment, inlet restriction is

 

[JSteve2]

I understand what you're saying. However, we don't know that X1+S is the same as X0+S' - throttle restrictions don't have the same flow curve and packed filter restrictions.

However, since we're all accepting that the driver might be willing to accept less torque (compare that assumption to my first post) then we are at an impasse. Given that we can accept less torque, you may well be right that mpg could improve.