We've previously posted about an article stating that ethanol might not make sense from a financial standpoint, but what about the other definition of "economy"; that is, what do various ethanol blends due to a vehicle's mileage? Due to ethanol's lower energy per unit volume, it should take more fuel to maintain a stocheometric (perfect) air/fuel ratio, which all newer vehicles will attempt to do (closed-loop fuel control being pretty much standard for the last 20 years). For every 10% ethanol added to the fuel, a mileage loss of 3% should be expected. But what happens in the real world? The people at the American Coalition for Ethanol decided to find out. And yes, before you even start in on us, it's obvious that this source might be just a tad biased.

The test was performed with three vehicles - an Impala, Taurus, and Camry - and ethanol blends of 10%, 20%, and 30% were tried. None of the vehicles tested were specified to run on ethanol blends above 10%, so I assume they lacked a fuel-blend sensor and thus relied on closed-loop air/fuel control to maintain the proper mixture (the GM ECMs that I?m familiar with can enrich the mixture by 25%, which is more than enough to accommodate E30).
I won?t go through the results in detail, but basically the Impala and Taurus did not demonstrate the expected mileage drop-off, even with a 30% blend of ethanol (the Impala even mustered a bit better economy with E30 than with straight gasoline). The Toyota?s mileage dropped by 5% with E20 and 11% with E30 - close to what would be expected - but E10 caused no decrease in mileage. So what does this show?

Each vehicle reacted to different blends of ethanol in a different manner. That?s not surprising, considering that each manufacturer probably has a different method for dealing with a leaner-than-expected mixture. Interestingly enough, none of the vehicle set any codes during the test, but as the vehicle ages and fuel flow drops off due to pump wear or clogged filters, the fuel system may not be able to keep up. 

Why didn?t the mileage drop off consistently for each vehicle? Ethanol is both an oxygenate and an octane booster. That means that they may be able to improve power - depending on the ECM calibration - and offset the expected loss from having less energy. I wish the test would have included logging of the Impala?s timing and knock retard during the test, as the vehicle may have been able to run more advance with the higher blends of ethanol. The Toyota may not have been programmed to take advantage of the greater knock resistance of the ethanol, or the combustion chamber may be efficient enough to not make additional power with more spark advance. More data here would be nice.

There?s also a possibility that this test isn?t reproducible and repeatable, or even subject to bias by the driver (I didn?t see it specifically called out that the test was performed in a blind fashion). To determine this would require a lot of further testing. Alternatively, the test could be performed on an emissions dyno, probably at great cost. That would provide results that are much more repeatable and less subject to bias, however.

The study also delves into the consumer cost for each fuel. The ethanol fuels tend to improve the per-mile cost of operating a vehicle. It can be argued that this is due to taxpayer subsidies for ethanol, but that?s not an argument I care to get into now. Feel free to politely hash that one out in the comments.

The bottom line here is that one might be able to increase mileage with some experimentation and custom blending of fuels (for those of you that can find E85), if you know the risks. Too much ethanol might cause check engine lights, or much - much! - worse if the engine runs too lean, especially under load. I?d consider data-logging with a scan tool and a wideband O2 sensor to be mandatory, especially for older vehicles. Keep in mind that blends above 10% are going to cut into the margin of error that?s designed into a modern ECM. Also beware of fuel system corrosion and degradation due to ethanol (vehicles designed to run on E85 have ethanol-resistant components).

[Hat Tip: Adam H.]



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