Dirty air filter kills fuel economy! - Page 4 - Mercedes-Benz Forum
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post #31 of 86 (permalink) Old 01-26-2015, 05:31 PM
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i'm a simple thinker, a byproduct of seattle public schools.

it's an internal cumbustion engine. each cylinder requires X+ cc of air per
intake stroke. add to that the amount of fuel needed to meet the
predetermined AF ratio.

so obviously air is needed and in enough volume to meet the critical minimum
amount to feed all cylinders.

any air flow restriction begins to limit that which is needed to feed all cylinders

what do you think?

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post #32 of 86 (permalink) Old 01-26-2015, 05:47 PM
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Logically

Stands to reason
If with a dirty filter the throttle MUST be INCREASED to make the SAME power (relative to a clean one)
And if the filter were clean it would make MORE power at the increased throttle position
SO
There MUST be MORE losses with the dirty filter
AND
More losses means lower efficiency
Lower efficiency means lower mpg

Last edited by Ingenieur; 01-26-2015 at 05:51 PM.
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post #33 of 86 (permalink) Old 01-26-2015, 05:59 PM
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The more throttle would apply to carburated engines, although even there it is the air flow that sucks fuel from the nozzles.
On computerized engine, pushing the gas pedal is just a wish you are giving to computer. Computer analyze how much air comes to the engine and with restricted air will restrict fuel.
Therefore you come to the point when it will take 2 minutes to reach 60 mph.
All wit top fuel efficiency.
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post #34 of 86 (permalink) Old 01-26-2015, 06:11 PM
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Sir K had been on at the very beginning.

For carbureted engine, it was true that a partially clogged air filter affects both fuel economy and performance. That was the conventional wisdom so people always think a modern (injected and computer controlled) engine works the same way.

Fortunately or unfortunately, that was really not the case in a modern engine. There had been several tests to prove that there is very little, if any, difference in terms of fuel economy between new and partially clogged air filters. (Otherwise, K&N sale would go over the roof.)

I know many in the forum do not trust Consumers Report so I will spare their study. However, let's try one done by real scientists.

The most authoritative study was done by scientists from Oak Ridge National Labs. I guess the government (think DOD) must own a lot vehicles so this is a significant things. They tested a lot of different engines and found no real fuel economy difference between new and partially clogged air filters in modern cars. There is, however, a performance difference.

I saved that info and here is their conclusion: (the 1972 reference was to a carbureted engine)

<<
Results reveal insignificant fuel economy and emissions sensitivity of modern vehicles to air filter condition, but measureable effects on the 1972 vehicle. All vehicles experienced a measured acceleration performance penalty with clogged intake air filters.
>>
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post #35 of 86 (permalink) Old 01-26-2015, 06:26 PM
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Quote:
Originally Posted by Kajtek1 View Post
The more throttle would apply to carburated engines, although even there it is the air flow that sucks fuel from the nozzles.
On computerized engine, pushing the gas pedal is just a wish you are giving to computer. Computer analyze how much air comes to the engine and with restricted air will restrict fuel.
Therefore you come to the point when it will take 2 minutes to reach 60 mph.
All wit top fuel efficiency.
Not accurate
In your example the loss percentage of power would be lower than steady state
Why? Because the ratio of filter loss to tb loss decreases
Part throttle 2/5 vs 2/ much less than 5 = better cyl filling
It would not take much more time but top speed would be decreased for the same fuel consumption
Also if it takes longer to accel that means less torque
Less torque means less power since torque = 5252/rpm x power
So you do LESS work force x distance using more fuel

Last edited by Ingenieur; 01-26-2015 at 06:34 PM.
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post #36 of 86 (permalink) Old 01-26-2015, 06:33 PM
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Originally Posted by loubapache View Post
Sir K had been on at the very beginning.

For carbureted engine, it was true that a partially clogged air filter affects both fuel economy and performance. That was the conventional wisdom so people always think a modern (injected and computer controlled) engine works the same way.

Fortunately or unfortunately, that was really not the case in a modern engine. There had been several tests to prove that there is very little, if any, difference in terms of fuel economy between new and partially clogged air filters. (Otherwise, K&N sale would go over the roof.)

I know many in the forum do not trust Consumers Report so I will spare their study. However, let's try one done by real scientists.

The most authoritative study was done by scientists from Oak Ridge National Labs. I guess the government (think DOD) must own a lot vehicles so this is a significant things. They tested a lot of different engines and found no real fuel economy difference between new and partially clogged air filters in modern cars. There is, however, a performance difference.

I saved that info and here is their conclusion: (the 1972 reference was to a carbureted engine)

<<
Results reveal insignificant fuel economy and emissions sensitivity of modern vehicles to air filter condition, but measureable effects on the 1972 vehicle. All vehicles experienced a measured acceleration performance penalty with clogged intake air filters.
>>
Works the same for any otto cycle
Fuel delivery method does not matter
k&n does increase mileage
Not sure that is good though since that means less effective filtration
The differences are on the order of ~10-20% of 10% pumping losses
a few percent at best
But it all depends on how fouled the filter is
I do not belive you can double mileage
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post #37 of 86 (permalink) Old 01-26-2015, 06:36 PM
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It is ANY intake restriction
Not only the filter
That is why mfgs of even economy cars try to minimize pressure drop thru the intake track
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post #38 of 86 (permalink) Old 01-26-2015, 06:41 PM
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Another thing to consider
Engines run open loop ie tb position tps
And closed loop sigma trim and the maf
The tps system uses a map to get fuel rate close and the other 2 inpus which have a lag to trim it
If the filter is blocked the tps will over fuel until trimmed
If bad enough the trim may run up against its limits

In a car that is constantly changing load it adds up
That is why the percent increase is greater for city start/stop than steady state highway
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post #39 of 86 (permalink) Old 01-26-2015, 08:24 PM
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Originally Posted by Ingenieur View Post
Another thing to consider
Engines run open loop ie tb position tps
For the starters, my engines do not have tps, nor do they have throttle
With my Superduties I could spend $500 on fuel a day, so I was observing all the variations pretty carefully.
Partially clogged filter plays no role in normal driving.
My filter minder would go to read when I pulled 25,000 lb set up on 12 miles grade, what translate to gas pedal firmly on the floor for 20 minutes or more, but then after resetting I could drive it for another 2000 miles with no problems.
As usually in such things it is very fine balance between what actually happen, what we I can measure and what do we believe.
I said it before and I am going to say it again, that from all the maintenance things I can do to my car - vacuuming the carpets brings the highest power gain and the best fuel economy

Last edited by Kajtek1; 01-26-2015 at 08:27 PM.
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post #40 of 86 (permalink) Old 01-27-2015, 08:13 AM
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"Our cars have a complicated fuel injection system that is designed to maximize fuel economy and reduce emissions. That being said, if you have a dirty air filter, less air is physically able to enter the engine, the MAF is smart enough to know this and thus reduces the fuel injector duty cycle to keep the air/fuel ratio happy. So it is impossible for the engine to run rich and ruin fuel economy until the filter is completely clogged in which case the engine wouldn't even idle properly if at all."

"Ball-zie" Statement! You must be pissed at a recent expense for some expensive fuel system parts? So now you are saying the car needs to deal with an old stuffed up filter, deal with it! I can't stop laughing!! I know your pulling our leg, but what an unexpected twist of facts! This sounds just like "economic forecast reporting" the facts are there, but just way out of plan....LOL!
Just yesterday my son and I were working on his MR2 we had to pressure test for a suspected leak in the cold start injector, so I took the opportunity to explain the simple 1990 fuel system, and of course we start with air! The very 1st thing I said "if you want to bust power 1st you need to know the potential volume of air you can get into the cylinder, the adding gas part is easy to do; it's all about the air and the compression!"
J99XJ, that is very cool thinking! But all we end up with, is a very confused system what!
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