ORNL researchers quantify the effect of increasing highway speed on fuel economy

[redneck]

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Green Car Congress: ORNL researchers quantify the effect of increasing highway speed on fuel economy


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[Ryland]

This is an interesting part.

An ORNL tested vehicle was observed to transition from stoichiometric operation to protective enrichment between 75 and 80 mph, and this change caused an unusually steep drop in fuel economy. Protective enrichment occurs when a vehicle’s engine is at a high load point such that the exhaust temperatures may become great enough to damage the catalyst or other components: damage is avoided by injecting extra fuel (rich fueling) which produces lower temperature exhaust.

[RobertISaar]

really depends on the vehicle....

i can maintain 90MPH on flatish sections of road and not run into either power enrichment or cat overtemp enrichment.

and STILL maintain close to 30MPG.

[redneck]

It would have been better had they listed a individual break down of the vehicles tested.

However...

Data has been collected for 23 light-duty vehicles at ORNL’s vehicle research laboratory and a valuable data set for 51 vehicles was loaned to ORNL by Chrysler LLC, under a non-disclosure agreement.

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[basjoos]

Since these test were run on a dynamometer, they didn't take aero effects into account. At Cd 0.16, at 90 mph I'm still running in lean burn. Lean burn would drop out around 105 mph and I would have to go a bit faster than that before the engine would start running in protective enrichment mode.

[Frank Lee]

The article I read said that the dynos had the appropriate road loads applied for the speeds tested. I presume they are aware of aero road load and programmed that in.

[some_other_dave]

In one of the footnotes, it says that the road loads were calculated from coast-down tests -- one of our favorites! So, yes, they did take aero into account.

Remember, the figures they are releasing are averages from a lot of different vehicles.

-soD