Why is mileage lost when filters get dirty?

[Kestas]

This doesn't make sense to me. I keep hearing people say that air filters need to be changed regularly to prevent loss of gas mileage.

Doesn't a dirty air filter simply restrict the amount of air going into an engine as a throttle would? It's not as though the same amount of gas goes through the engine. Whether carburated or fuel-injected, the throttling system should maintain the same air/fuel ratio as with a clean new air filter. The only thing that should suffer is available horsepower with a dirty air filter.

I'm not suggesting everybody keep their dirty air filters forever, but I think losing gas mileage with dirty air filters is a myth.

 

[XS650]

Back in the olden daze of carbruetors, a dirty air cleaner acted like a partially on choke and richened the mixture.

A dirty air cleanr still acts like a partially on choke, but modern fuel injection systems compensate for the restriction and keep the mixture where it should be.

Most reported loses of fuel economy on modern engines with dirty air cleaners are from the placebo effect.

 

[pitzel]

A filter that can't flow as well = more drag, and more perturbed airflow, which means increased pumping losses.

Lower fuel efficiency definitely is not a myth.

 

[Gene K]

quote:

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Originally posted by pitzel:
A filter that can't flow as well = more drag, and more perturbed airflow, which means increased pumping losses.

Lower fuel efficiency definitely is not a myth.

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Pitzel you missed the point. On a spark ignition engine to maintain the same speed you have to close the throttle blade more with a clean air filter. The restriction is exactly the same. Otherwise the engine would make less or more power making the car go faster or slower than the desired speed. This means pumping losses would be identical.

On a Compression Ignition Engine that is Unthrottled (Diesel) a Dirty Air Filter will definately hurt fuel mileage as it does increase pumping losses.

Gene

 

[RavenTai]

quote:

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Originally posted by XS650:
Back in the olden daze of carbruetors, a dirty air cleaner acted like a partially on choke and richened the mixture.

A dirty air cleanr still acts like a partially on choke, but modern fuel injection systems compensate for the restriction and keep the mixture where it should be.

Most reported loses of fuel economy on modern engines with dirty air cleaners are from the placebo effect.

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I have been wondering the same thing. and came to the same conclusions,

But on later carbs the bowl pressure vent should have been post filter not atmospheric, should not have had an effect then also.

Only way to relieve pumping losses from the vacuum is to let more air in the motor this will certainly not help fuel economy, it would have quite the opposite effect. Every 14.7 pounds of air requires 1 pound of fuel to go with it.

 

[The_Jeff]

If the trans is controlled by a TV cable (attached to throttle) the throttle would be wider open for a given airflow, making the trans upshift later and waste fuel.

If the ECU uses TPS for spark mapping or some other stuff, and is inflexible in that regard (no block learn etc) one might end up with more spark retard for a given air input, worsening economy.

In general the OP is right, nowadays, I think.