Thursday, March 24, 2011

Air Filters: Does Air Flow Impact Mileage?

Best Air Filters Review: Air Filter Facts
Air Filter Facts Part 5: Air flow and mileage

Can air flow improve your vehicle's fuel economy? Many users report improved mileage upon the adoption of an air filter with measurably increased airflow, such as when you change from a stock OEM filter to a K&N filter. They are (mostly) wrong.

We already proved that different air filters can increase (or worsen) air flow (+/- 0.5kPa), and that clogged air filters  significantly impact air flow (up to +6.25 kPa, much higher impact than air filter swap). How does air flow impact mileage?  is a federal government site focused on - guess what - fuel economy.  Under their aegis, the famed Oak Ridge National Laboratory published, in February 2009, a investigating the effect of air filter condition (i.e. air flow restriction) on fuel economy. They were able to show, once again, that aftermarket (high flow) filters were able to provide more air flow than OEM filters, measuring a lower pressure loss across aftermarket filters than OEM filters:

The February 2009 Oak Ridge National Laboratory research report on fuel economy measured the difference in mileage between a clean, new air filter, and the severely clogged air filters we showed in our last post. The researchers experimented with both modern fuel-injected engines, and with older carbureted engines. Despite the difficulty at getting very low airflow, the difference in fuel efficiency for all newer vehicles was statistically nonexistent, as shown below for one of the experiments (for this illustrative graph, we picked the Dodge Charger, which showed, in our previous test, the worst clogging):

Oak Ridge National Laboratory's conclusion: "Results show that clogging the air filter has no significant effect on the fuel economy of the newer vehicles (all fuel injected with closed-loop control and one equipped with MDS). The engine control systems were able to maintain the desired AFR [air fuel ratio] regardless of intake restrictions, and therefore fuel consumption was not increased. The carbureted engine did show a decrease in fuel economy with increasing restriction. However, the level of restriction required to cause a substantial (10–15%) decrease in fuel economy [...] was so severe that the vehicle was almost undrivable."

If clogging the air filter - which increases restriction to air flow by 6.25 kPa -  does not have an effect on mileage,  then clearly, swapping air filters - which varies air flow restriction by +/- 0.5kPa -  certainly does not impact mileage either.

There is an important note to add to the Oak Ridge's results: the vehicles tested were all gasoline engines. Normal diesel engines are unthrottled in normal use after warm-up, which means that, even under light loads, they operate at high air flow, and throttle cannot be used to adjust fuel fix. It is possible that air filters could impact fuel consumption for modern diesel engines.  

We found multiple confirmations for the conclusions of the Oakridge National Laboratory's tests. For example, Consumer Reports, in 2011, published a post on fuel economy, and listed dirty air filters under myth busters (...): "Our tests show that driving with a dirty air filter no longer has any impact on fuel economy, as it did with older engines. 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 causes the engine to automatically reduce the amount of fuel being used. Fuel economy didn't change [...]." Offroad Adventures tested a high flow drop-in filter replacement and found no impact to mileage: "Flow capacity is 15-20 percent higher than stock, so it gives you a little over-capacity for mods. We ran the panel in the stock housing for several months and noted no change in fuel economy."

All the scientific evidence clearly points at the same conclusion: air flow in modern vehicles does not impact mileage. Everything else being equal, high flow filters with higher air flow do not improve a vehicle's flow efficiency. In fact, several studies (e.g. Fodor & Ling, "Friction reduction in an I.C. engine through improved filtration and a new lubricant additive", Lubrication engineering, 1985, vol. 41, no10, pp. 614-618; Andrews, Li, Jones, Hall, Rahman & Saydali, "The Influence of an Oil Recycler on Lubricating Oil Quality with Oil Age for a Bus Using In-Service Testing,"SAE 2000 World Congress, Paper 2000-01-0234) have shown better mileage resulting from better oil filtration. As a result, is possible that filters with higher air flow but worse filtration (these two traits often go hand in hand), which contribute to more wear and more particles in the oil, may ultimately result in worse fuel economy.

  • The choice of an air filter can materially impact air flow
  • Air flow does not affect fuel economy in gasoline engines
  • Drop-in high flow air filters have no effect on mileage in gasoline engines

But does air flow impact performance? Next we investigate the effect of clogged air filters on vehicle performance... So come back soon!

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