Fuel economy worse with ethynol blend?

[Dominic Richens]

Car: 95 Legacy Wagon AWD 5sp

I switched to 10% ethanol blended "regular unleaded" a month ago and could
swear I'm burning about 2 litres/100km more that I used to (city driving).
I think ethanol bends are suppose to burn a little cooler and have higher
equivalent octane rating. Could the ECU be running the engine rich because
of this?

I'm about to embark on a 900km drive (all highway) and am wondering if I
need to do something simple like reset the ECU before hand?

I switched because it is suppose to burn cleaner, but if the ECU is running
the car rich all the time, that can't be good...I'd be better off running
regular fuel (not to mention saving 1$/100km).

(or is this all my imagination and I should try it for a little more than 1
month!)

cheers!

 

[Jim Stewart]

Car: 95 Legacy Wagon AWD 5sp

I switched to 10% ethanol blended "regular unleaded" a month ago and could
swear I'm burning about 2 litres/100km more that I used to (city driving).
I think ethanol bends are suppose to burn a little cooler and have higher
equivalent octane rating. Could the ECU be running the engine rich because
of this?

Ethanol has 30 percent less energy per volume than gas. At a 10
percent blend, you'd expect a 3 percent hit in milage. If it takes
the ECU time to learn how to optimize the burn (my speculation),
the mpg hit would be bigger in the beginning.

Another issue might be water carryover with the ethanol.

 

[uglymoney]

Ethanol has 30 percent less energy per volume than gas. At a 10
percent blend, you'd expect a 3 percent hit in milage

Yep, around 75,000 btu's for ethanol versus 115000 btu's for
gas in an American gallon.

nate

 

[Dominic Richens]

Yep, around 75,000 btu's for ethanol versus 115000 btu's for
gas in an American gallon.

I expected the difference in fuel economy to be negligible, not 20% worse
(2l/100km). Will the ECU "learn" to compensate? My fear is that ECU always
thinks the engine is "cold", and is programmed to ignore oxygen sensor
reading until the engine is "warm", thereby negating the benefits of ethanol
blend. The manual does say you can used ethanol blend, up to 15% ethanol so
I would assume they have taken this into account when programming the ECU.

Maybe I should just stop worrying and enjoy the drive

 

[Edward Hayes]

O2 sensor are now fitted with a heater so the O2 sensor gives the ECU info
during the warm-up cycle. ed

 

[S]

Car: 95 Legacy Wagon AWD 5sp

I switched to 10% ethanol blended "regular unleaded" a month ago and could
swear I'm burning about 2 litres/100km more that I used to (city driving).

In addition to comments others have made, consider that the optimum
A/F ratio for alcohol (methanol, anyway; I assume ethanol is similar)
is around 7:1 vs 14:1 for gasoline. Not only does the alcohol
contribute less energy to the burn intrinsically, the fact that it
isn't getting anywhere near a stoichiometric mix means that the
alcohol really isn't contributing much at all. Less power = less
mileage.
We've had ethanol gas shoved down our throats the last few years,
don't even know if you can still get non-blended fuel around here
anymore. This makes it tough to do comparisons, but expect as much as
a 10% reduction in mileage from my experience in the past with
carbureted engines running blended fuels.
How much a modern ECU can correct for this is certainly open for
debate, but they _do_ learn. It might be worthwhile to disconnect your
battery for a couple minutes to reset the ECU, and see if it "learns"
better fuel management habits for the blended gas. Let us know if this
makes a difference; inquiring minds need to know . . . ;-)

ByeBye! S.

Steve Jernigan KG0MB
Laboratory Manager
Microelectronics Research
University of Colorado
(719) 262-3101