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air filter vs fuel economy (tricky question?)
11

air filter vs fuel economy (tricky question?)

air filter vs fuel economy (tricky question?)

(OP)
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?
 

RE: air filter vs fuel economy (tricky question?)

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).

RE: air filter vs fuel economy (tricky question?)

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

RE: air filter vs fuel economy (tricky question?)

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.

RE: air filter vs fuel economy (tricky question?)

"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

RE: air filter vs fuel economy (tricky question?)

3
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

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: air filter vs fuel economy (tricky question?)

Thanks for the info.  Very informative.

RE: air filter vs fuel economy (tricky question?)

(OP)
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?  

 

RE: air filter vs fuel economy (tricky question?)

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.

RE: air filter vs fuel economy (tricky question?)

"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.

RE: air filter vs fuel economy (tricky question?)

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.

RE: air filter vs fuel economy (tricky question?)

(OP)
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?
 

RE: air filter vs fuel economy (tricky question?)

(OP)
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.
 

RE: air filter vs fuel economy (tricky question?)

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.

RE: air filter vs fuel economy (tricky question?)

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

RE: air filter vs fuel economy (tricky question?)

(OP)
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?"

 

RE: air filter vs fuel economy (tricky question?)

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.

RE: air filter vs fuel economy (tricky question?)

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

- Steve

RE: air filter vs fuel economy (tricky question?)

(OP)
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  

RE: air filter vs fuel economy (tricky question?)

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.

RE: air filter vs fuel economy (tricky question?)

(OP)
I don't think you've quite got it.  If X1+S < X0+S', then he hasn't pushed the pedal far enough and manifold density is less (so the power output is less and fuel consumption is less).  If X1+S > X0+S', then the pedal has been pushed too far, and power output is too high and fuel consumption is higher.

 

RE: air filter vs fuel economy (tricky question?)

If I flow 10 lbm/min through either system, then I have the same fuel mass per unit time (based on air mass) in either engine and therefore the same power. What I cannot guarantee is that I have the same flow restriction through the inlet because the flow v. pressure curve of a valve (throttle) looks different than that of a filter. More work may be coming off the crankshaft in one case v. the other to achieve that flow rate.

RE: air filter vs fuel economy (tricky question?)

(OP)
Can't guarantee it?  Sure you can.  At a given rpm, you can't get 10 lbm/min through both systems unless the total pressure drop (and thus density) are the same:

Atmospheric pressure P1 stays the same...

Pressure after the filter is lower by some amount, deltaP_F, depending on filter restriction.
Pressure after the throttle is lower by some amount, deltaP_T, depending on throttle opening

Pressure (and thus density) at the intake manifold inlet, P_M, is what determines mass flow into the engine for a given rpm if everything downstream is held constant.  

Pressure at the intake manifold inlet (P_M again) is what will affect pumping work if everything else is held constant.

P_M = P1 - DeltaP_F - DeltaP_T
If you want to get the same power out of the engine at a given rpm, then P_M must be held constant.  P1 is constant, so any case where (deltaP_F)+(deltaP_T) is changed will give a different P_M, a different mass flow rate, and a different power.




 

RE: air filter vs fuel economy (tricky question?)

At the same flow rate, I agree that power is fixed. I don't agree that RPM is fixed or that "everything downstream is held constant." Those two conditions assume the answer.

If the piston pulls harder (i.e. takes more work off the crankshaft) in one case than the other, then your solution is not correct. Whether the piston pulls harder is in fact the question, so your solution just assumes the answer.

RE: air filter vs fuel economy (tricky question?)

The question is clearly answered several times.

The only way a fuel economy decrease could result is if there is a throttle position sensor for a WOT fuel enrichment system and the driver was driving in the range where restricted filter was at WOT, but the clean filter was at not quite WOT to get the same power level. This is a very unlikely situation for any significant time as mostly if a driver is over half throttle, they are at WOT.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: air filter vs fuel economy (tricky question?)

I'm not an expert but this is my opinion on the matter:

The mass of air that is flowing thru the air intake system is metered so that the computer can calculate the amount of fuel that has to be injected.
So if a dirty filter is used, then less mass of air will flow thru, so less mass of air will be metered and less fuel would be injected.
So fuel economy wont be affected by this, only performance.
The driver will notice that with the same pedal travel he or she used to have more power, so if X amount of pedal travel was needed to cruise at 60mph, now X+Y pedal tarvel will be required to cruise at the same speed, so the driver's economy will be affected but not the engine's.

RE: air filter vs fuel economy (tricky question?)

I notice I wrote that the driver's economy will be afected, that's wrong, I'm sorry, only engine performance.

RE: air filter vs fuel economy (tricky question?)

(OP)
jsteve, is there something other than pressure that tips off the piston to the fact that something is afoot?  Does the transmission change so that rpm @ 65mph is different?

for everyone else, a grand conspiracy:

http://www.fueleconomy.gov/feg/maintain.shtml
"Check & Replace Air Filters Regularly
Replacing a clogged air filter can improve your car's gas mileage by as much as 10 percent. Your car's air filter keeps impurities from damaging the inside of your engine. Not only will replacing a dirty air filter save gas, it will protect your engine."

http://www.knfilters.com/ads/linegas.aspx
"According to the U.S. Government, replacing a clogged air filter can significantly improve gas mileage*."

http://www.aempower.com/ViewTopic.aspx?TopicID=9
"According to the U.S. Government, replacing your car's dirty air filter can net an improvement in fuel mileage of up to 10%"

http://www.carcare.org/Fuel_AirIntake/air_filter.shtml
"Driving with a dirty filter restricts the air entering the engine, and if severe, can impact fuel economy and performance."

http://www.dailyfueleconomytip.com/uncategorized/fuel-economy-tip-change-your-dirty-air-filter/
"This past week, I conducted my own, very unscientific study after I changed my air filter.  My gas mileage for this past week was 33.77, up from 32.84 the previous week. "

But wait!  Not everybody is onboard with this conspiracy!
http://www.consumerreports.org/cro/cars/tires-auto-parts/car-maintenance/get-the-most-mileage-for-your-fuel-dollars-406/index.htm

"Keep your air filter clean. According to our tests, driving with a dirty air filter in modern engines doesn't have a significant impact on fuel economy, as it did with older engines. While fuel economy didn't change, however, power output did. Both cars accelerated much more slowly with a dirty air cleaner. We drove both vehicles with their air cleaners restricted and found little difference in gas mileage with either engine. That's because modern engines use computers to precisely control the air/fuel ratio, depending on the amount of air coming in through the filter. Reducing airflow, therefore, caused the engines to automatically reduce the amount of fuel being used."

 

RE: air filter vs fuel economy (tricky question?)

From a theoretical standpoint I'm in the "doesn't affect part throttle fuel economy, does affect max hp" camp.

From a real world standpoint it depends on the driving habits - if the driver demands near maximum power frequently fuel mileage would go down. (though it would be bad to start with) An excessively plugged filter would also cause fuel mileage to go down - though the filter restriction would have to be high enough to cause a fully open throttle at cruise.

I was thinking a little deeper, like would the piston need to do more work to draw the charge in - which is no. Take the driver out of the equation - Set the cruise control to 55mph and only vary the restriction in the air filter - within reason. The ECU will adjust the throttle to achieve the same intake manifold vacuum. Essentially you need X amount of fuel to create Y amount of HP to maintain 55mph. The ECU will let in Z amount of air to properly combust X amount of fuel by adjusting the throttle.

ISZ

RE: air filter vs fuel economy (tricky question?)

Just to confuse the hand-wavers more, a clogged filter would affect economy for a diesel, because it would introduce additional pumping losses.

- Steve

RE: air filter vs fuel economy (tricky question?)

I can pick a mass flow and filter dirtiness level, short of WOT, where the intake system has more pressure drop than a similar mass flow with just the throttle and a clean filter. Until someone tells me who pays for that extra pressure drop, I will continue to protest. Quietly after this, unless someone has something new to present.

Are air-fuel corrections by the computer enough so that moderately clogged filters in normal driving conditions possibly below the level of an unscientific consumer reports measurement? Probably.

However, it's nonsense that regardless of the means the air takes to get to the engine, it has no effect on fuel economy. Replace the filter with a tiny orifice - no effect. Put a banana (only 90% blockage, though) in the intake, no effect. Make the intake 12 miles long in a pile of spaghetti configuration - no effect. The entire intake-design industry is a scam, and you guys have exposed it.

Sorry, I'm not buying. Even under consumer reports evaluation, a dirty filter gives a severely degraded engine. Did they try to pull anything with that engine or even attempt to get similar acceleration out of it (but something less than WOT)? We don't know.

 

RE: air filter vs fuel economy (tricky question?)

(OP)
In your opinion, how is mass flow of air determined for a particular engine operating condition?  Please do just one example calculation!  Don't even post it, just write it down and think about it on your own!

So far you don't seem to be grasping the fact that the user can (and does) select the throttle restriction to compensate for filter restriction up to the point where throttle restriction reaches a minimum.  There is, therefore, no "extra" pressure drop whatsoever until you reach WOT.  For any "before the dirt" throttle position & resultant pressure drop, there exists an "after the dirt" position to give the same pressure drop, up to the point where you reach "after the dirt" WOT.  Any pair corresponding (equal pressure drop) conditions will give identical engine performance.  

Make the intake 12 miles long, as you suggest, and (assuming the total restriction is similar to idle position throttle restriction) you'll be stuck driving around with the throttle at WOT to achieve what used to be idle performance.  You'll get the same fuel economy that you would have gotten if you froze the banana and wedged it under the pedal to prevent pushing it down.  Whether the filter or the throttle restricts the flow makes no difference to the engine.
 

 

RE: air filter vs fuel economy (tricky question?)

"unscientific consumer reports measurement"

They are usually much more scientific than the average car-nut magazine, in that they take multiple samples from multiple cars purchased "off the lot" at dealerships...vs. single tests of mfgr-supplied cars, or worse, simple regurgitation of mfgr-supplied data...

And they are the only reliable source of reliability data for cars, or other major purchase appliances, etc.

RE: air filter vs fuel economy (tricky question?)

ivymike,
I'm just offering my honest opinion, not trying to get anyone spun up.

The boundary conditions we've placed are that the operator applies the throttle until the mass air flow is the same in the two systems. Just because the mass air flow is the same, how can this enforce identical intake manifold pressure where I have a piston downstream capable of performing variable work on the system? Let's assume one system had a lower intake manifold pressure but the same mass flow rate - the piston pulls harder (not a violation of physics) or the volumetric efficiency is decreased (also not a violation of physics). Either of those conditions reduce the fuel economy of the engine, and I do not see how they can be excluded.

In your proposed system, the two configurations have the same pressure drop to each other at 1 lbm/min, at 5 lbm/min, all the way up to the point where the valve saturates. Given that valves/throttles and filters have different flow versus pressure drop characteristics, I find it extremely unlikely that comparing two systems where the bulk of the pressure drop is placed in the valve or the filter that they could have the same pressure drop for all flow rates until the valve saturates. I also do not see how that result is required by the physics.

RE: air filter vs fuel economy (tricky question?)

(OP)
Let's assume one system had a lower intake manifold pressure but the same mass flow rate

That won't happen.  You're implying that you can get the same mass of air into each cylinder regardless of density.  If you could do that, then there'd be no market for devices to increase density.
 

RE: air filter vs fuel economy (tricky question?)

JSteve, please do some scientific analysis, or better still, instrument an engine and run some tests, then report back.

RE: air filter vs fuel economy (tricky question?)

"That won't happen.  You're implying that you can get the same mass of air into each cylinder regardless of density.  If you could do that, then there'd be no market for devices to increase density."
You are assuming the engine speed must be the same, and that the volumetric efficiency is fixed.

"JSteve, please do some scientific analysis, or better still, instrument an engine and run some tests, then report back."
Would that I had the time, because I think this is a really cool problem. Where does the burden of proof lie, and what has been incorrect in the current discussion?

RE: air filter vs fuel economy (tricky question?)

JSteve2 maybe you are looking at this the wrong way.

How will the engine management system operating in closed-loop tell the difference between a partially blocked filter and a throttle valve?

Assuming changes like a lower manifold pressure and the same mass flow rate assumes other changes (different engine or gearing) that invalidate the initial question.

I think the initial post was based on certain assumptions; filter isn't blocked to the point most people would have gone to see a mechanic, driver isn't a 16 year old with F1 ambitions, etc.

  

RE: air filter vs fuel economy (tricky question?)

Woow this poor old dead hoss is getting a flogging.

OK, here's one that got me beaten up on another forum.

How does variable valve lift improve economy? It changes the effective timing so the scavenging will change, but that is a second order effect I suggest.

Again, how can the engine 'see' any difference in the restrictions in the intake duct? Why is an obstruction caused by a valve closing 'better' than a butterfly valve (or a dirty air filter)?

Or are the second order effects the reason?
 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: air filter vs fuel economy (tricky question?)

(OP)
'spose it depends on the events in question.  If you're closing an intake valve early, then lemme see...  

(for ease of handwaving, I'll go with some improbable intake valve events)

let's say our hyp. engine has bore and stroke of 100mm, TDC volume of 9.5E-5m3, BDC volume of 8.8E-4m3.

let's say that 50% vol eff. is desired, and ignore pumping losses past partly-open valves (so the valves work as though the lift events were square waves, and flow extremely well when open)

(let me know if I get too far from reality for useful dicussion)

case1: restriction achieved via throttle
crazy intake valve event 1 (opening at TDC)
pressure in intake manifold 50,000 Pa
pressure in crankcase 100,000 Pa
gas up-force on piston during downstroke: 393N
gas work done on downstroke: -39.3J  (pumping work)
crazy intake valve event 2 (closing at BDC)
pressure at start of upstroke:  50kPa
pressure at end of upstroke: 1129kPa (math error? no time)
compression work done (polytr): -142.8J

case2: restriction achieved via inlet valve closing
crazy intake valve event 1 (opening at TDC)
pressure in intake manifold 100,000 Pa
pressure in crankcase 100,000 Pa
gas up-force on piston during first half of downstroke: 0N
gas work on first half of downstroke = 0N
crazy intake valve event 2 (IVC @ half stroke)
polytropic expansion second half stroke, pressure at BDC =  49.8kPa
work done on downstroke = -73.1J
poly compression, pressure at TDC = 1125kPa
work done on upstroke = -142J

the crazy valve events version seems to come out behind in this case, since it seems to have greater pumping losses on the intake stroke for a given amount of air intake... so not better the way I set it up, but clearly not the same.

 

RE: air filter vs fuel economy (tricky question?)

(OP)
messed up the last calc a bit, and I'm too tired to go fix it.  I think the downstroke work is more like -21.3J for the crazy events version, which makes it slightly more efficient than the throttle version.
 

RE: air filter vs fuel economy (tricky question?)

i want some of what ever it is jsteve is on

RE: air filter vs fuel economy (tricky question?)

meetoo!

RE: air filter vs fuel economy (tricky question?)

Sorry guys. Can't ship it across state lines.

RE: air filter vs fuel economy (tricky question?)

Jsteve

Most of the guys you are arguing with are degree qualified professional engineers who design engines for a living. They are unanimous in their disagreement with you.

What do you do for a livening.

What have you done to qualify you to continue to disagree with such an overwhelming opposition.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: air filter vs fuel economy (tricky question?)

(OP)
so anyway, greg, I think the text version is that in one case all the expansion work is disippated via friction at the throttle and other inlet restrictions, where in the other case some amount of the expansion work is recovered by the piston.  I'll do a PV diagram if I get a chance at the airport this morning (not sure whether they'll have free wifi).
 

RE: air filter vs fuel economy (tricky question?)

I always thought that moving the throttle to the valve was a publicity stunt. A throttle is a throttle is a throttle.

The real advantage I saw was the increased effective compression ratio due to earlier intake closing and the extra power stroke from later exhaust opening and reduced overlap at TDC at low "throttle" opening. I personally prefer the way Honda does it with the Ivtec where they run 2 valve, small cam 4 valve then big cam 4 valve

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: air filter vs fuel economy (tricky question?)

Patprimmer,
Also a degreed engineer working on engine controls. I spend my days challenging really smart people with tough questions like this. Really smart people - including myself (if I'm in that category) - are wrong a lot or they aren't doing anything difficult.

I'm not sure anyone has described the current system well enough to claim victory here, and we have no data of any value (see ivymike's post citing largely anecdotal evidence on both sides).

I don't see consensus as conclusion - merely that is the direction to move forward until better evidence or models are available. Ivymike's own post of external "evidence" is 5-1 on the fuel economy side, although the fuel economy fans from that post are just as worthless as the one consumer reports test with undisclosed conditions.

So if you're looking for me to concede in the face of overwhelming opposition by smart people - fine I concede. But please don't expect me to write a white paper declaring that we've shown dirty filters just degrade available power but don't affect fuel economy.

RE: air filter vs fuel economy (tricky question?)

JSteve,
Can you show or explain to us on a PV diagram (including pumping loop) what you're trying to say.

Pat, if the intake is closed earlier (vs being used as a throttle) then you avoid some of the pumping loss, since leakage aside, you have an air (vacuum) spring helping the piston back up at the beginning of the compression stroke.  I believe this type of intake valve timing is referred to as a modified Atkinson cycle, no?

RE: air filter vs fuel economy (tricky question?)

hemi,
I would expect a restriction to be reflected as a lower cylinder pressure on the intake stroke of the pumping loop, effectively increasing the PV area of the pumping loop and thereby requiring more work to complete the intake stroke.

RE: air filter vs fuel economy (tricky question?)

(OP)
Okay, sketched a PV diagram for just intake and part of compression for throttle vs intake valve comparison (Greg's comment).  Pink shaded area is "extra" work captured by the piston if the inlet restriction happens due to early IVC.  The valve events are, of course, idealized, and again I didn't bother putting any kind of static restriction in the intake system.  Red line (starts at higher pressure) is for the intake valve case, blue line (starts at lower pressure) is for the throttle restriction case.
 

RE: air filter vs fuel economy (tricky question?)

Will the dirty filter heat up any due to increased friction of the air flowing faster through it?  That could thermodynamicly account for the reduced engine efficiency.

RE: air filter vs fuel economy (tricky question?)

drwebb,
"the reduced engine efficiency" is of course a point of dispute. I assume you're going along a bit trying to figure out how it could occur if at all.

I would expect the effect you suggest to be extremely minor and easily measured. But again we're freewheeling without data. :)

My current thought is that if there is lower efficiency at all, it is dominated by the PV area effects due to overall higher intake restriction at higher air flow rates.

RE: air filter vs fuel economy (tricky question?)

I am not an engineer and may have no buisness here. I come here to learn not teach.As hard has as I try I cannot see where jsteve is coming from.The original post was for modern gas engines.Older carburated engines with poorly enginered, adjusted or malfunctioning vents were sensitive too dirty air filters, modern speed density fuel inj. engs are not.Power output would only be affected if restriction was in excess of other factors such as valve timing intake design etc. Correct me if I,m wrong

RE: air filter vs fuel economy (tricky question?)

Correct me if I'm wrong: the only one who thinks you are wrong is JSteve2.

JSteve2, how is the "restriction to be reflected as a lower cylinder pressure on the intake stroke of the pumping loop" any different whether it comes from the throttle or the air filter?

RE: air filter vs fuel economy (tricky question?)

hemi, this will just summarize what I've already posted. First, if the IMP is the same, there can be no difference as far as I can tell.

This is how I'm thinking right now:
We're fixing MAF. Filters and throttles have different MAF v. pressure drop curves. At some value, those curves will cross - say at 3 lbm/min in a given engine. At points away from that MAF (most interestingly above it), one of them (filter or throttle) will have a higher pressure drop value, and therefore a lower IMP. Since the piston is capable of applying variable work, there is nothing in the system that can enforce a uniform IMP, as the driver is closing the loop with the throttle on MAF, not on IMP.

Whichever case has the lower IMP will see a lower pressure in the cylinder during intake, and therefore the area of the pumping loop on the PV will be larger (this area is work input by the crankshaft, as opposed to the combustion loop which is work output to the crankshaft).

The idea that IMP must be equal at equal MAF is understood, but I don't see how it can be correct. Again - the proposed restrictions have different curves. If the driver keeps opening the throttle to achieve the same MAF (ie power), the engine can achieve that through an increase in engine speed just as well as equalizing pressure drop. We have two levers with different response relationships, so although MAF-IMP may match for some range of points, they cannot match for all operating points.

I understand that no one has been convinced. However, neither has anyone expressed an understanding of the above view while refuting it.

RE: air filter vs fuel economy (tricky question?)

I strongly suspect the last sentence is very true.

Thought experiment, written in English:

Imagine an intake duct with no air filter, but two butterfly valves in series. Set the first butterfly valve to the same pressure drop as the dirty air filter, use the second one as a throttle. For a given power at an engine speed the pressure after the throttle body will be "P"

Now adjust the first butterfly valve to the same pressure drop as clean filter, and set the throttle to give the same power at the same speed.

What is the pressure after the throttle body?

 

Cheers

Greg Locock

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RE: air filter vs fuel economy (tricky question?)

5
Ohh this is just too good to let alone. I've been really busy in the shop lately and even today had to fly through most of the responses gaining just enough information to attempt to catch the highlights and make some comments.

Remember this, "I'm not an engineer, just a technician that fixes the things automotive engineers create"

A DIRTY AIR FILTER WILL NOT AFFECT NORMAL DRIVING FUEL ECONOMY ON TODAY'S MAF (Mass Air Flow) sensored vehicles. There was a lot of disscussion, but little spelled out for Jsteve to go and test to prove the result. One of the tests that techs like myself use regularly when in the process of diagnosing a vehicle performance issue is to calculate the "reported VE" (Volumetric Efficiency) of the engine at several WOT engine speeds. We use this calculation combined with fuel trim data, (long and short term) to help analyze MAF sensor performance, which is easily confirmed to be accurate or not based on VE variance, combined with an associated fuel trim correction. In short, if VE is low say 60-70%, and fuel trim is making a correction to add fuel, that indicates unmeasured air is entering the engine. The air could be pirate air getting in through a torn air inlet tube after the MAF, or could be a faulty MAF. If VE calculates low, and there is NO fuel trim correction, now we look for a mechanical reason that the engine isn't breathing correctly. That could be a restricted exhaust, engine mechanical issue, or in fact an air filter issue!

In all of my years as a technician (30+) the number of times that I diagnosed a bad air filter as a performance problem accounts for only a handful. Most of them were related to rodents storing food in what appeared (to them) to be prime real estate. VBG. Almost every one of the rest were related to an exhaust manifold or gasket failure, where the thermostatic air cleaner system was picking up the leaking exhaust gasses and coated the air filter with them.

The fact that an air filter that is truly dirty is a diagnoseable condition without lifting the air filter lid and looking is important. That also means that no matter how dirty an air filter looks when inspected, proper testing can prove beyond any doubt as to whether it actually needs replacement.

From this point here is your test Jsteve. If you don't have access to this formula e-mail me and I can send you a version of it. You will need a good scan tool that will give you data as fast as possible. The faster that the data scrolls, the more accurate that the RPM, VS the Airflow reading in grams per second will be.

Go ahead and take your first VE measurement with whatever air filter you have in the car. Take note of long term and short term fuel trims under normal driving conditions at varying vehicle speeds, and throttle openings. Also take note of the calculated spark timing, and if there is spark retard occuring from a knock sensor input. (MAF vehicles use the MAF to assist in not only fuel mapping but spark timing mapping as well) Then take the air filter out for testing purposes and repeat the test! Make sure that your air inlet assembly is clean of any deris etc.

Now the fun starts. Take a can of spray paint and a cheap replacement air filter. Paint 1/4 of the filter to completely block it of airflow. Install the filter after it dries and re-test. Use an enamel paint to make sure it seals the filter. If you have access to an O.E. level scan tool such as an IDS on a ford you could go ahead and drive some type of a course and measure actual fuel economy. In fact backing up you could have started out this test that way!

Now paint another 25% of the filter. You now know without question that you have a filter that is 50% restricted, right? See how this measures out now.

Now go another 25%, so your filter is definately 75% restricted. Do you think you have ever driven a car with a filter that was THAT dirty?

You now have a filter that only 25% of it has airflow. Paint half of what is left and repeat the test, with only 12.5% of the filter allowing airflow. Depending on circumstances I'll wager this to be the first time you see a significant difference in MAX VE. However, I'll also wager you will not yet see a loss of fuel economy, and you could in fact lend this car to another person and they could drive it and not notice anything wrong!

The reason for all of this is gasoline engines are of course an air pump. The speed of the engine is set by the airflow that it recieves, and it does NOT matter where the restriction to that airflow is. The greatest restriction will always be the limiting factor. If you would take my Ford Explorer and restrict the filter to about 12.5% area, you would still have 80% of the cross-sectional area of the throttle body and therefore almost no noted restriction.

Now one of the other responses was asking about variable cam timing. We should start a thread specifically about that, instead of doing it here. Suffice it to say at the moment that scavenging of the cylinder gasses is only the tip of the iceberg. The engines actually effectively control displacement, eliminate the need for EGR, since the cam can do that job, and much more!

RE: air filter vs fuel economy (tricky question?)

Thecardoc,
If you had put this up a while ago, you could have saved me a beating. :)

That's good enough for me - I'm going to accept that and yield.

RE: air filter vs fuel economy (tricky question?)

A virtual beer for JSteve and the rest of the crew for sticking with it!

Cheers,

Geoff.

P.S.  And a virtual star for you, cardoc!

RE: air filter vs fuel economy (tricky question?)

Oops!  Forgot which forum I was in!blush
There's a real star for you cardoc.

RE: air filter vs fuel economy (tricky question?)

Here here

Regards

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RE: air filter vs fuel economy (tricky question?)

Rats, Cardoc!  Just as I was composing my comments about mass air flow systems, my computer re-boots (Windows security installation) and when I check back in, your excellent response is there.

I agree totally as well.  Good job.

Franz

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RE: air filter vs fuel economy (tricky question?)

. . . assuming the frictional heat doesn't melt the paint off the air filter.

RE: air filter vs fuel economy (tricky question?)

winky smile

RE: air filter vs fuel economy (tricky question?)

Now that this conversation has finished, time to add my 2.3 cents... :D

One thing I haven't seen (maybe I missed it?) is the effect the increased right-foot pressure has on the vehicle's fuel maps.  In addition, that increased demand from the driver, combined with an automatic transmission, may result in later shifts.

I know that when tuning a racecar, the higher the voltage read by the TPS (throttle position sensor), the "safer" you go with on your fuel tables.  In addition, the closer you get to torque peak, the safer you go on your tables.  From what I have seen of OEM fuel maps, the manufacturers are doing the same.

Of course, the question is whether or not the increased right foot pressure is enough to jump it into the next step on the map, or even if the target A:F ratio changes based on O2 sensor readings.

But, I'm just an amatuer weekend warrior when it comes to this kind of thing.  Great read either way.

RE: air filter vs fuel economy (tricky question?)

Ok now I'm going to re-ignite the flames a bit for the sake of clarity...

I agree that a modern car will not lose fuel economy due to a dirty filter since the engine always uses the correct amount of fuel for the air coming in to keep the mixture where it needs to be.

In the cited example with the blocked off air filter, the sensors still work just fine so economy doesn't suffer, but as you restrict more and more of the filter area, just how much manifold pressure do you lose and at what point do you start to notice serious power loss? Was the part about someone not noticing a difference even with only 25% or less of the air flow directed only at fuel economy? it seems counterintuitive that the same person wouldn't notice a power loss from that restriction (then again if you lend your car to someone, they wouldn't know what the actual base power feels like in the first place)

RE: air filter vs fuel economy (tricky question?)

Rockwood

See my post 09/04/08 about 4 or 5 down from top.

I may be out of date, but as far as I know, TPS on OEM systems normally only have 3 readings, closed for idle/overun, cruise and WOT.

Idle/overun position or closed throttle adjusts to cut fuel on overun, but cuts fuel in at certain rpm to maintain idle.

Part throttle or cruise gives normal economical drivable mixture.

WOT gives extra fuel for performance and/or durability.

You point re A/T change point is valid and was missed.

Regards

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RE: air filter vs fuel economy (tricky question?)

"See my post 09/04/08 about 4 or 5 down from top."

Figures I should miss one of the key posts that started a lot of these discussion points.

"I may be out of date, but as far as I know, TPS on OEM systems normally only have 3 readings, closed for idle/overun, cruise and WOT."

Yeah, this is something outside my area of experience as I have never been in the unfortunate circumstance to tune an OEM system - I don't envy those who need to iron out every single part-throttle-hiccup while maintaining proper emissions/economy.  When we tune, we just look for a target mixture that is safe and creates the most power.  If there are part throttle hiccups, we address them as they come up, but these are relatively unimportant.  In addition, if the car is turbocharged, you override the TPS reading at certain MAF sensor voltage readings because a turbo car can create large amounts of positive manifold pressure in the "cruise" range since it's just jamming air past the throttle body.  In those situations, the cylinder pressure is much too high to safely attempt a stoich target mixture.

"Idle/overun position or closed throttle adjusts to cut fuel on overun, but cuts fuel in at certain rpm to maintain idle."

Given.

"Part throttle or cruise gives normal economical drivable mixture."

That's what I figured, but with the complexity of some of the newer systems, they may have specific targets based on TPS voltage.  Even with the older systems, I wonder that if you have a TPS that outputs 0-4v, if 3.5+v is considered WOT, or just 4v, and is a dirty filter enough to get into that range (doubtful, the more I think of it).

Another wrench in the works would be electronic throttle bodies where the gas pedal is simply a torque demand pedal and has no direct affect on the throttle body.

RE: air filter vs fuel economy (tricky question?)

dalcazar,
I don't believe power loss to be part of the original controversy. If your air flow is restricted, you cannot pull as much air per unit time, which directly correlates to power loss in a stoichiometric engine.

RE: air filter vs fuel economy (tricky question?)

One thing for sure there are as many different ways to create driver command inputs to the PCM (powertrain control module) as "you engineers" (VBG) can think of, and as I a tech I have to understand and react to every one of them.

Most of the cars in the last ten years use a fairly linear TPS sensor. While inside the PCM it may well work in steps to a degree, as a tech with a scan tool I see both throttle position sensor voltage as well as throttle opening percentages. Most of the cars learn a closed throttle position on key on, and unless there is an issue that causes a TPS voltage that is lower than the original closed throttle, the closed throttle position will always report a 0-1%. (99.9 percent of the time) The one thing the the PCM does with the TPS more than anything else is respond to sudden movement. That's why a poor connection, or a sensor that is wearing out and having voltage drops/spikes drives the system crazy. The biggest thing about OBDII is that it isn't a diagnostic system for technicians, it's an emissions program. The vehicle by design is supposed to adjust itself to get the lowest emissions possible, and then test to make sure it's working. To achieve the lowest possible emissions, it needs to keep air fuel ratios in tight check. (Long term trim, short term trim, and rear trim) and then continue the adjustments to reduce fuel consumption and increase fuel mileage as much as possible, VCT (variable cam timing), EST (electronic spark timing).

Now spark timing maps, they are all over the place as far as what input has more authority over another between engine platforms. The thing is, no-one else even considered "fly by wire", or computer controlled throttle actuators. Between the computer having complete direct throttle control, MAF (mass air flow) sensor inputs, RPM, CTS (coolant temperature) , VVCT (variable cam timing), KS, (knock sensor), it only goes to figure that the PCM will learn, and then continue to re-learn as the filter gets dirtier. What makes this neat is you could have positive feedback for any actual change in the required throttle position for any type of a cruise demand as you played around with different air filter restrictions.

So can you guess where this is going?

It is not at all beyond comprehension that the PCM's programming could be written to alert a technician about whether an air filter actually required replacement or not. Everything is already there, it's just a matter of writing the code.  

RE: air filter vs fuel economy (tricky question?)

Excellent thread.  Here's my low-tech anecdote that supports the conclusion reached by the group.  I've a Toyota Landcruiser that I use to haul the family to our summer house on a lake over 200 miles away.  It's stored over the winter.  I always fill up at the same two gas stations at either end.  I'm particular with logging MPG, which has averaged 13.4 MPG over a couple of years.  Before putting it away two winters ago, I changed oil & all filters (including air)  Last summer, brought it out, and it seemed a bit sluggish on it's first trip of the summer.  Checked MPG; showed 13.6 on the way down, 13.5 on the way back.  Same thing on the next trip.  Finally got around to sniffing out the source of the sluggish operation.  Opened up air filter box, and found it virtually plugged with a huge rodent nest.  From the density of the nest, I struggled to understand how the engine produced as much power as it did.  However, I rechecked my fuel log, and could not discern any noticeable change in fuel economy, with an almost totally plugged filter.  Luckily, the mice did not chew through the filter, so nothing nasty made it's way into the engine.

Jeff

RE: air filter vs fuel economy (tricky question?)

With Carburetor yes. With EFI don't think so.
At first I thought TPS reports throttle position, but it is still the O2 sensor that is the final judge and along with the MAF sensor. The HP will suffer. This situation is just like driving your EFI car to the top of a mountain say 12,000 foot altitude. Its going to adjust for the lack of air.

RE: air filter vs fuel economy (tricky question?)

Hi Dicer.

When we first started dealing with feedback carburetor systems and the early fuel injection systems we were taught that the O2 sensor makes the computer change the air fuel ratio. The people that told us that is how it works either didn't really understand the system themselves, or else lied.  VBG..

The computer uses either a MAF system or a MAP system (speed/density) to calculate a base fuel pulse width. Then it looks to the O2 sensor (Air/Fuel) to see if it is reporting rich or lean. The computer then adds or subtracts fuel from is base calculation (Short term fuel trim) and watches for the O2 sensor (Air/Fuel) to react. This process continues until there is sufficient time for a long term trim number to be learned, where it causes the PCM to make the fuel correction right as it makes it's initial base calculation. Watching a scan tool like the Ford IDS really puts it into perspective how fast short term trim moves, and then you see the individual O2 sensors react to the short term changes.

I won't get into rear fuel trim right now, it really gets complicated, let alone alcohol compensation (E85 systems). Just remember the O2 sensor does not make the computer change the pulse width. The computer changes the pulse width and watches for the O2 sensor (or Air/Fuel) to react.  

RE: air filter vs fuel economy (tricky question?)

Thecardoc,
In the controls universe, the O2 sensor is just operating as a "feedback" controller rather than as a "feedforward" controller. You can't have perfect control with feedback because you must have a disturbance before you can adjust. However, even though it works as you describe, that would be considered as the O2 sensor controlling it in controls lingo. Therefore, I think that the people who told you how it works understood it, but just didn't want to give an entire controls lecture.

RE: air filter vs fuel economy (tricky question?)

I think a partially clogged air filter will certainly effect maximum torque through the rev range on any engine, including modern, ECU managed engines.

The rest of the time the ECU will probably just open the throttle plate more or not need to do anything on a turbodiesel. I don't see the O2 sensor innate feedback loop delay as a significant cause of mpg drop with a clogged air filter. Certainly not in high driving, for example.

The engine still has to work against the same pressure gradient (sump versus cyl. above piston crown and intake manifold). Only it's due more to a restriction in the air filter than before and less in the throttle plate.

RE: air filter vs fuel economy (tricky question?)

Sony

I think wealready said that.

Can't you sleep.

Regards

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