MPG "myths"

[glennc]

Originally Posted By: Black Bart
If you are cruising at 60 mph you will have the throttle open a small amount and pumping lose will be high.
The throttle controls the pumping lose not the filter.

If the filter restricts the air and you open the throttle more then the filter becomes the pumping lose.


I think I may be starting to sound like a broken record so I'll try to confine myself to this final clarification.

You're right that the throttle is the major pumping loss at any normal throttle setting. It's not the only pumping loss, though. Pumping losses occur throughout the system, both intake and exhaust, and in the case of an air filter there will be a pressure drop regardless of whether the filter is clean or dirty. This pressure drop will increase as the filter gets dirty, very slowly at first, then more quickly.

Because there is a pressure drop prior to the throttle, the throttle is controlling flow from an already-less-than-atmospheric-pressure environment. Yes, you open it slightly more to compensate if the filter is too dirty, and the injection feedback system adjusts to put in the right amount of fuel, but there is still a loss of efficiency. You can look at it as two pressure drops in series to simplify it, but the truth is that every time a molecule of intake air encounters a resistance of any kind, it removes energy from the intake flow which must necessarily come from the engine's output power.

 

[meep]

" it removes energy from the intake flow which must necessarily come from the engine's output power."

Yes, ONLY FROM A PEAK POWER PERSPECTIVE. Less air, less fuel, less output.

You must delineate between mpg efficiency and output efficiency. They are not the same.

Remember, fuel use is reactive to the actual air flow/pressure. Less air, LESS FUEL, less power output, but fuel spent is equivalent to less air ingested. Therefore, all other factors being equal, no loss of mpg.

Also, remember that it's much easier to pump nothing (a vacuum) than it is full atm.

M

 

[glennc]

meep, go back and read my posts and try to understand the distinctions I've made between different types of efficiency losses. Your third paragraph indicates that you have not understood what I've been trying to explain. Efficiency is work done per unit of fuel burned - if you have another definition then please spell it out so we can make sure we're talking about the same thing. And it is harder, not easier, for the engine to pump against a vacuum - that is the source of the loss.

 

[brianl703]

Originally Posted By: OVERK1LL
in open-loop situations where the values the ECM are using are mostly out of base-table


The tables used in open-loop mode are modified using readings from the oxygen sensor when ECM is operating in closed-loop mode. Therefore, what happens in open-loop mode is still dependent upon what happened in closed-loop mode. If the ECM needed to add 10% fuel at a given engine speed with a given MAF or MAP reading, it can extrapolate that into how much extra fuel it needs to add during open-loop operation.

This can be seen by connecting a scantool, viewing the long-term fuel trim PID, and going WOT and observing that the long-term fuel trim PID does not drop to 0%.

 

[meep]

oK- Sure, you are right - I understand the pumping against a vac. My statement is once again misleading... But I honestly agree with 3/4 of your above statement until we get to the pumping losses. AGREED: losses in exhaust = loss.

We also agree that MAF/MAP will adjust fuel per air consumed.

Agreed: pistons work against the vacuum. Agreed that from peak output efficency standpoint, these intake losses are purely subtractive.

It's the intake where I'd like to discuss: Your implication that it's the difference between a vac imposed at the filter vs a vac imposed by the throttle that I don't get. What am I missing? So how... does it matter the ratio of vacuum provided by a filter vs the throttle plate? It's the same total restriction, right? The MAF doesn't care - it sees X CFM regardless of where the restriction comes from, and that determines fuel rate.

Example question:
for a given cruise, power to sustain motion comes from a spent A/F mixture. I need say, 12 hp. Let's say that means 1 "units" of fuel and therefore 15 "units" of air. My 15U of air needs 5% max airflow of system potential, meaning 95% restriction. Then, how does the system care if my 5% results from (5% filter restr. + 90% throttle restr.) vs (15% filter restr. and 80% throt restr.)? It's a series circuit, flow (current if you're an EE), measured by MAF remains the same through the entire system and pressure (v again if you're an EE) remains the same at the manifold (MAP). I do not understand the difference, because mathematically the vacuum and resulting pumping losses are the same.

??? Mike ???

 

[brianl703]

Originally Posted By: meep
Your implication that it's the difference between a vac imposed at the filter vs a vac imposed by the throttle that I don't get. What am I missing?


Some might say that the increased TPS (throttle position sensor) reading causes the ECM to inject more fuel into the engine. I believe that's bunk, at least on any properly-designed fuel injection system.

That is not to say that there are not some poorly-implemented fuel injection systems that do use the TPS reading as a determinant of the air/fuel ratio. These probably will start running rich if an air filter is clogged.

 

[Black Bart]

If you put a vacuum gauge on the intake manifold it will show high vacuum while cruising because the throttle is only about 10% open and the cylinder on the intake stroke is sucking the air in.
The restriction is caused by the nearly closed throttle plate not by the air filter.

The air filter is capable of delivering 100 percent of the engines needs at WOT at 10 percent throttle the filter has 90 percent reserve.

When you go WOT the vacuum gauge will show zero vacuum.
The reason it has NO vacuum is because the throttle is no longer a restriction and atmospheric pressures is pushing the air into the engine

At sea level atmospheric pressure is 14.7 PSI and it pushes the air into the engine it is not sucking and the zero vacuum proves it so it is the throttle that creates cylinder pumping lose and at WOT pumping lose is not a factor.
Water pump and exhaust lose is another subject .

 

[brianl703]

On my Saab the Scanguage shows negative vacuum when I got to WOT...wonder why that might be...

 

[rpn453]

Originally Posted By: Drew99GT
You live in a universe where your head is up your [censored]. Perhaps if you slow down and don't drive as fast, then fuel mileage will remain the same with a dirty air filter.


All this technical talk seems to be making you cranky. Time for an afternoon nap!

 

[Drew99GT]

Originally Posted By: rpn453
Originally Posted By: Drew99GT
You live in a universe where your head is up your [censored]. Perhaps if you slow down and don't drive as fast, then fuel mileage will remain the same with a dirty air filter.


All this technical talk seems to be making you cranky. Time for an afternoon nap!



I added enough technical talk from the most respected member of BITOG. End of story. zzzzzzzzzzzzzzzzzzzzzzzzz snoring... Try again...

And again, I challenge you folks who like your air filters dirty...Just don't change them! Don't change them for 100,000 miles. After all, a dirty air filter is good for both filtration and fuel mileage. Put your money where your mouth is. And while your at it, don't change your furnace filter for, well, lets say 3 years. Should be nice and loaded by that point...for best filtration and efficiency. Track your heating bill too while your at it.

Well, better make it 150,000 miles and 5 years. Don't want to let anymore dirt through and sacrifice any fuel mileage or home heating expense...

 

[Drew99GT]

Originally Posted By: rpn453
Originally Posted By: Drew99GT
You live in a universe where your head is up your [censored]. Perhaps if you slow down and don't drive as fast, then fuel mileage will remain the same with a dirty air filter.


All this technical talk seems to be making you cranky. Time for an afternoon nap!



If an air filter is to the point where it's so loaded that it's causing a restriction, a measurable restriction (like the vacuum gauges on diesel truck filters)........in order to maintain the same performance from the engine...the same acceleration, the same steady speed on the highway, your gonna have to open the throttle more. At that point, the engine will be under more load and burn more fuel.

Will a trailer cause a truck to burn more fuel in your universe?

I'm not trying to be an arse (well maybe - it's the way I am!), I've experienced things exactly as I state them. Visibly dusty air filter results in around 1 or 2 mpg drop.

 

[Audi Junkie]

afaik, the difference in total power would be the extra power needed to pull the air through a slightly restricted filter. That is, the DIFFERENCE between moving air through a clean verses dirty filter. What is that, like 1/10th to 1/20th hp?

 

[brianl703]

That loss is probably offset by the greater throttle opening.

Engine load, by the way, is determined from MAF/VAF (air flow) sensors or MAP (engine vacuum) sensors.

 

[-Clayton-]

I still believe that a severely clogged filter will reduce mileage.
Until someone can show some proof otherwise I'm not buying it.

 

[rpn453]

Originally Posted By: Drew99GT
If an air filter is to the point where it's so loaded that it's causing a restriction, a measurable restriction (like the vacuum gauges on diesel truck filters)........in order to maintain the same performance from the engine...the same acceleration, the same steady speed on the highway, your gonna have to open the throttle more. At that point, the engine will be under more load and burn more fuel.

Will a trailer cause a truck to burn more fuel in your universe?

I'm not trying to be an arse (well maybe - it's the way I am!), I've experienced things exactly as I state them. Visibly dusty air filter results in around 1 or 2 mpg drop.


The only point I would argue about with you is the issue of engine load. The engine load does not increase, but the relative engine load might (relative to the engine's maximum performance with the restricted intake), and the throttle would have to open more to compensate at a given load, but with the same amount of fuel injected. I do believe that a dirty air filter can potentially cause an engine to run richer and waste fuel that way, but I question what kind of mileage this would occur at. The computer certainly can compensate for changes in air flow rate as well as density and temperature. I'm not saying I don't believe you that your filters are dirty enough to affect your mileage. All I've said is that I don't think my dirty air filter is affecting my mileage when I change it at 30K with my clean air driving conditions. I think the engine controls are capable of making the fuel/air ratio corrections well enough at that air filter change interval. I keep a spreadsheet of my mileage and plot the data and I haven't seen anything to indicate that my mileage is suffering as I near the 30K interval. Maybe I'll run the next one longer just to see what happens since I'll be off warranty. Of course, other vehicles driven by other people with other levels of air quality may experience different results.

All this air filter talk has actually made me decide to change the filter on my girlfriend's MX-6 tomorrow. When the filter was at 30K miles I didn't bother changing it because the filter didn't look too dirty, it was surprisingly expensive for a simple flat panel filter, and it's about 50% larger than I'd expect for her engine. She also drives very conservatively. She has the 2.0L and I imagine the 2.5L uses the same filter. So I decided to let it go 60K. It's at about 50K now and I feel like I should change it. I have no mileage data because I only recently got her to start documenting it after she was complaining about poor mileage this last winter. It was pretty much all short trip city driving at an average temperature well below 0F, so I told her that her mileage was not unusual under the circumstances and to keep a spreadsheet so she'll be able to notice any trends that may warrant further investigation. We might as well start tracking her mileage with a clean air filter!

 

[Black Bart]

Comparing a mechanical injection diesel to a computer controlled car is apples and oranges
Talking about reduced power output and reduced mileage is two different things entirely.

Changing the discussion from a dirty filter to a clogged filter is another apples to oranges comparison.

It seems that some are willing to get ridiculous in order to make their point.

A mechanical injection diesel has no way to compensate for reduced air therefore it will run richer as the filter becomes dirty. This will reduce power output and fuel mileage will get worse as the filter gets more dirt in it.

Now with a computer controlled car the dirty air filter will reduce power output once it become restrictive enough that it can no longer meet the engine demands at WOT.

But the fuel mileage will remain the same until the filter becomes so restrictive that the computer can not compensate.

The computer has limits as to how much it can adjust for this but it will require that the filter be very dirty for this to happen.

Assuming that the mileage will drop because the total power output is down shows a lack of understanding of how this system works.
No one has said that you should use a clogged filter or for that matter a very dirty one.
But if you drive on paved roads all the time and at 30K you take the filter out and it isn't very dirty and it probably won't be then replacing it may make you feel good but it sure won't change your mileage.

 

[glennc]

Originally Posted By: meep
It's the intake where I'd like to discuss: Your implication that it's the difference between a vac imposed at the filter vs a vac imposed by the throttle that I don't get. What am I missing? So how... does it matter the ratio of vacuum provided by a filter vs the throttle plate? It's the same total restriction, right? The MAF doesn't care - it sees X CFM regardless of where the restriction comes from, and that determines fuel rate.

meep, first let me apologize for the tone of my last reply. Normally I try to stay away from these sorts of back-and-forth arguments, because it seems that they often tend to degenerate into non-useful discussions of unimportant points motivated mostly be egos. Usually I try to make one or two statements on a topic and then sit back, and if people don't agree or don't seem to have understood the point I've made, so be it. Maybe I should keep doing that. Obviously I was a bit cranky when I wrote that last post. Sorry.

I'll try to politely respond to your last post, which made the same good point that a few people have made here, that it should not make a difference whether you have one restriction or more than one, as long as the add up to the correct restriction to get the amount of air you need into the engine, along with the proper amount of fuel. That seems like it should be true, but physics often finds subtle ways to penalize you when you make changes to a system that has in general been well optimized to work efficiently. In this case, the thottle body is part of an intake system that is optimized in many ways. Downstream of the tb, the intake is tuned to take advantage of pulses to assist with cylinder filling at certain rpm ranges. Upstream of the tb the airbox is designed to limit noise while also having specific volume and dimension characteristics, again designed to have beneficial affects on intake flow. And somewhere in the system there is some sort of air flow meter, the electrical ouput of which determines the amount of fuel that will be injected on each cycle of the engine.

There are several subtle ways in which changing the system by adding a restriction in a place other than the tb will change the way the system operates, and since the system is pretty much balanced to operate in the most efficient way, those changes will tend strongly to work against efficiency. Tuning pulses will work differently, vacuum signals before and after the tb will have their relative strengths altered, the density of air at the air sensor will be affected, and so on.

I know that answer by itself won't satisfy you, so I will be more specific about one precise item, which I think is really the core of our disagreement. Let's say that instead of a simple, optimized intake you devised some labyrinthine series of tubes and airboxes and who-knows-what, just for the purpose of making it harder for the air to get into the engine. This would be sort of like having a dirty air filter, but imagine it since I think it will make my point easier to visualize. This would obviously affect max power at peak and also at any given rpm, as I'm sure you'd agree, but we'll design it so it will have enough flow to fill the cylinders properly at cruise, if the tb is opened to compensate.

I suspect you will begin to agree that something like this could also affect efficiency, even though there is "enough" air and the tb can compensate by being open more. Let's say it is restrictive enough that the tb has to be open 60% at a cruise speed at which a 5% opening would normally suffice - not because I'm going to be quantitative but just to get an idea of how things would be operating.

You can say anything about this system that you could say about the "slightly dirty air filter" scenario: the only differences would be of degree. Yet I'm sure that efficiency would be way down with a ridiculous system like this, and again I imagine you probably agree. So is there a point at which a extra restriction begins to make a difference? Or does it never make a difference? Or does it always make a difference?

If it always makes a difference, then the question is why? What is the mechanism by which energy is lost, when as you say the total restriction (intake + tb) hasn't changed?

The answer is that flow restriction is really nothing more than a friction source for the intake air. The energy lost goes into friction, and the friction becomes heat. The intake flow is heated, and the heat lowers efficiency. It actually lowers efficiency in several different ways.

A throttle body or carb venturi is I imagine a pretty efficient way of introducing a restriction without losing any more energy to friction than necessary. Almost anything else, intentional or not, would probably be worse. And if you think of a better one, make sure to get a patent!

I hope that is a somewhat helpful response. I don't know to what degree a slightly dirty air filter will in reality affect engine efficiency. Like I said before, probably very little, until the filter becomes fairly dirty, but more than not at all. Physics just doesn't give freebies.

 

[Black Bart]

glennc I don't recall ever reading how much the computer on most OEM systems can compensate but I have a ACCEL
GEN.-7 wide-band fuel management system on my highly modified Corvette and I can adjust it to any percent of correction up to a Maximum of 25 percent.
I doubt that the OEM would compensate that much but maybe it can I'm sure it can go at least 15 percent so the air filter would need to louse more than that amount of air flow before the car will have a loss of fuel mileage.

 

[brianl703]

OEM computer systems can compensate +/- 25% before they set a code for a lean or rich condition.

 

[Black Bart]

Originally Posted By: brianl703
OEM computer systems can compensate +/- 25% before they set a code for a lean or rich condition.
Then the OEM is the same as my after market tunable computer. I use a laptop and build my fuel and timing tables
I set the start fueling pre-start fueling idle maps the thing has about a hundred different maps or tables to be configured.