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Name: Alisha
Status: Student
Grade: 9-12
Location: IL
Country: United States
Date: January 2009

Hi, we hope to create an E85 mixture using the ethanol we produced to power a two-stroke carburetor lawn mower engine. Do you have any specific research or information about rebuilding a lawnmower engine (specific parts, guides, etc) and the adjustments that need to be made in order for it to operate on E85 fuel?

I don't have any of the engineering considerations for converting gasoline to E85, although I'm sure you can find that on the Internet. However, I have a more "global" idea for you to consider. Despite the political pressures, ethanol has not and will not serve as a biofuel in the long term. The production of ethanol whether from corn or sugar cane depends upon the availability of water, either from rain or irrigation. No water, no ethanol. Water is not a stable resource, so neither is ethanol. Perhaps you and your class should sustainable energy resouces. I pose the challenge, I don't have an answer.

Vince Calder

Hi Alisha,

Ethanol is a rather poor fuel to use in an internal combustion engine. One problem is that ethanol contains much less energy per liter than gasoline does. So for an ethanol-powered motor to produce the same amount of power, it must use about 51% MORE ethanol than it would use if it running on gasoline. The situation is almost as bad when using E85, since E85 is 85% ethanol!

In addition, the fuel-air ratio of a motor using ethanol (or E85) must be made more "rich" in order for the motor to even run at all. A gasoline motor runs best when there is about a 14.7:1 ratio (by weight) of air-to-fuel. That means that for every gram of gasoline the motor uses, there must be 14.7 grams of air "inhaled" to burn the gasoline with. When burning E85, the air-to-fuel ratio must be 9.76:1 Since the amount of air a motor "inhales" when it runs remains unchanged (no matter what fuel it uses), the lower fuel-to-air ratio for E85 means that the motor must use a lot more E85 than gasoline.

Now to your question: If you just poured E85 into your lawnmower motor, it would most certainly run very poorly (and may not even start at all). The carburetor must be modified to allow a lot more E85 to be fed to the motor in order to get it to run properly, because you need to feed much more E85 to the motor to make the correct air-to-fuel ratio. It is unlikely that you will be able to modify the carburetor for such a large change in mixture ratio. The carburetor was designed to use gasoline, and cannot feed enough E85 to allow proper operation without making large (and very difficult) internal modifications to it.

Now a couple of observations. It seems odd that you are trying to experiment with what I assume you hope is a "cleaner" fuel, with a 2- stroke motor! Small 2-stroke motors are terrible polluters. One lawn mower motor actually produces as much pollution as 20 to 30 modern cars!

Most importantly, E85 is not particularly environmentally friendly. The production of ethanol results in large amounts of CO2. The CO2 comes from the fermentation process, and also from the fuel used to heat and distil the ethanol. In a report published in Scientific American some time back, it was stated that comparing ethanol to gasoline, and taking into account the manufacturing processes and the poor fuel economy of ethanol, using ethanol generates almost as much CO2 as using gasoline would, if natural gas was burned to provide heat for the industrial distillation process. However, if coal was used for fuel in the distillation process (as it is at many ethanol plants), using ethanol actually produces more CO2 than using gasoline! It is becoming clear that using ethanol for fuel is not, at this point, a viable solution. Sometimes, things are not as simple as they seem!


Bob Wilson

Hi, Alisha. Depending on the make of your engine, you can get very good books about them relatively inexpensively. Search or Google for 'small engine repair briggs' (for a Briggs and Stratton) or 'small engine repair Tecumseh' (for a Tecumseh), varying the text depending on which make your engine is. I have a detailed Briggs manual written by 'Clymer's' if that helps with a place to start your searches. If you need parts, you can get them from most any lawnmower shop.

Are you sure it is a two cycle (also known as a two stroke)? Most lawnmower engines I am familiar with are four cycle or four stroke. Two-strokes are usually smaller, and are used in string trimmers and such, though I don't doubt that there are older lawnmowers out there that use a two-stroke.

As far as adjustments for a gas engine to run on E85, there is usually no adjustment required, although you may determine the carburetor should be adjusted a bit leaner or richer to get that particular engine to run at its best. Lawnmower engines usually have an adjustment screw in the carburetor for this. One thing you may want to check is to determine what materials the o-rings and gaskets in the carburetor are, and check to make sure they are compatible with ethanol.

If it is a two-stroke engine, you will probably get the E85 to work, but your reduction in pollutants will be minimal, because two-stroke engines require oil to be mixed with the fuel. I know of no experiments that have been done using a two stroke, but that just means you will be doing something nobody else has. That also means more work, but possibly more fun with the experiment, too.

The oil mixed in the fuel is to lubricate the engine's moving parts, and you may need to vary the amount used in the mix to get it to a good amount for use with the E85. Determining this amount may involve repeatedly tearing down the engine to check how quickly it is wearing.

David Brandt

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