February 10, 2009

A Closer Look At Ethanol And Fuel Mileage



Ever since I wrote about ethanol and fuel mileage for the first time I have received emails from readers about how they have experienced larger losses than what can be explained by the difference in the BTU content of ethanol blended gasoline and regular gasoline. A couple of days ago another email came in and I asked permission to republish it here. Below is the email I received on February 4, 2009.

Don't know if you care but I track our two vehicles pretty accurately and
both have now seen 10-12% milage reductions from ethanol inclusion.

RAV 4 pre-ethanol: 25-26mpg month after month in mixed driving. Now 23mpg+-

Ford Focus pre-ethanol: 36-38mpg month after month. Now 33 mpg.

Both cars are standards, the RAV has 194k miles, the Focus 138k miles. Both
maintained consistently, tires rotated every 7,500, neither uses oil, etc.

Both show loss of power as well.

How can this be a positive step forward given the adverse economic and
environmental impacts of ethanol; corn diversion? runoff of pesticides into
water resources, etc.


This email is fairly typical of the emails I have received concerning ethanol and fuel mileage. The first thing I would note is that they always contain a range of mileage on gasoline such as the 25-26 miles per gallon on the Rav 4. We all understand that variables in driving style, weather, mix of city and highway driving and others can effect fuel mileage and that is why people always give a range of mileage. But there is one variable that you don't hear talked about much and that is the BTU content of gasoline.

Most people think that all gasoline is the same. But that just isn't the case. Gasoline varies quite a bit in it's BTU content.

According to the EPA summer gasoline varies from 113,000 BTU per gallon to 117,000 BTU per gallon. That is a difference of 3.4%. Winter gasoline varies from 108,500 BTU to 114,000 BTU. That is a difference of 4.8%.

Unlike gasoline which is mixture of many different hydrocarbons, ethanol is a consistent product. It has a BTU content of 76,100 BTU per gallon. That considerably less than gasoline but the difference between gasoline and gasoline blended with 10% ethanol is generally around 3.5% less BTU. Generally you would expect fuel mileage to mirror the BTU content of fuel.

Since we know that adding 10% ethanol to gasoline raises it's octane level by about 2 points we know that if we are comparing 87 octane unleaded with 87 octane E10 that the base fuel is different. To get 87 octane E10 the fuel supplier would have started with a base fuel that had 85 octane.

So the question is how much of the fuel mileage loss was due to ethanol's lower BTU content and differences in the base gasoline's BTU content?

As I have always said there are variables that the average user cannot control when trying to compare the fuel mileage of one fuel to another. That is why I put more faith in formal studies and less in user testimonials. Formal studies attempt to control as many variables as possible. Studies generally buy ethanol and gasoline separate and blend their own so there is no difference in the base gasoline BTU content. And the formal studies like this one from the state of California have consistently shown between a 1% and 3.5% loss in fuel mileage.

Fleet average fuel consumption increased by 1.4% when ethanol content was increased from the zero to the high level.


And as far as your cars having less power on ethanol. If you are experiencing a decrease in power on ethanol you should have your car looked at by a qualified mechanic. Any fuel that contains oxygen such as ethanol, methanol, nitromethane, and others should result in increased power.

3 comments:

Certified ASE Mechanic said...

"Any fuel that contains oxygen such as ethanol" will lower your fuel mileage due a vehicle's oxygen sensors reading this increased oxygen as a lean burn condition. How much fuel mileage your vehicle loses depends on how much a vehicle's computer (ECM) increases the fuel mixture to offset this "lean burn condition". Now there's one variable that you don't hear talked about much.

ASE Certified Mechanic said...

"Any fuel that contains oxygen such as ethanol, methanol, nitromethane, and others should result in increased power." That would be a misleading statement. Yes, these fuels can help produce more power in higher compression engines that are designed exclusively to run these fuels. However; vehicles designed not to run on these fuels will see a loss in power. Do not waste your time and money on getting your car "tuned up" with a qualified mechanic.

mus302 said...

Just so everyone knows, even though the 2 comments above were signed different, the logs show that both comments were left by the same person.

There is another variable that you don't hear talked about when it comes to oxygen content and fuel mileage and that is before ethanol was widely used to boost oxygen content of gasoline MTBE was used. MTBE had less BTU than gasoline as well and since it has less oxygen than ethanol was used in higher percentages, up to 15% MTBE. Yet you never heard any issues with mileage losses. MTBE was a product of the oil industry and there really wasn't anybody to tell people that they should expect a loss in fuel mileage with it's use so nobody noticed a loss in mileage. But it also suggests that areas that were using MTBE before and are now using ethanol should not see much of a difference in fuel mileage.

Is the statement that any fuel that contains oxygen should increase power misleading? Only it it's absoluteness. I should have said that fuel that contains a higher percentage of oxygen than the outside air will cause an increase in power and mentioned that this only applies to spark ignited engines. You need to refer back to engine design to see that engines are limited in the amount of power they can produce by the amount of air that can be packed into the combustion chamber. Yes an engine can be limited by the amount of fuel it can add but if that is the case you have bigger issues than fuel choice. So any fuel that has more oxygen than the outside air will increase power. And that has nothing to do with compression ratio or design because as you note the ECM will add the appropriate amount of additional fuel required.

And although I am not an ACE certified mechanic, a few years back I owned a performance shop and an ASE certified mechanic was my partner in that venture.

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