Estimate the percentage of ethanol in pump gas, courtesy Conoco, Inc and EAA.org.
See also All About Fuel.
Most paramotor pilots use pump gas as opposed to avgas. In all likelihood, that means you’re getting alcohol, specifically ethanol, in your fuel. How much of a problem that is depends on your equipment and how much alcohol is in the fuel. Apparently, the float bowl carbs and inverted engines are particularly averse to alcohol. How much alcohol is in the gas? Here is a way to find out.
You’ll add 8 oz (or 8 of whatever units you prefer) of gasoline and 2 oz of water together in a graduated container. The 2 oz of water will sit at the bottom with the gas on top. Since water attracts alcohol, after shaking the container vigorously for a half-minute or so, the water will absorb nearly all the alcohol. Then you set the container down for 10 minutes so the water can come out of solution, complete with its suspended alcohol, and sit on the bottom again. If the fuel had alcohol init, there will appear to be more water than before. How much depends on how much more water there appears to be.
You can calculate the percent alcohol content of the fuel using the following method. F stands for the 2 oz of fuel, W is the 8 oz of water that you started with, and T is the total of both, or 10 in this case. WA is the amount of water that appears to remain after shaking and waiting. That water will now be holding most of the alcohol that was previously in the fuel. More apparent water, less apparent fuel. DIFF is the difference in water from before to after.
The percentage of alcohol in the sample fuel (%FA) is determined using the formula: %FA = DIFF/F x 100. So if you ended up with 3 oz of water and 7 oz of fuel then: %FA = 1/7 * 100 or 14% alcohol in the sample fuel.
It’s not perfect since not all of the alcohol will come out of solution but it’s pretty close.
According to the EAA’s documents, up to 5% alcohol is enough to cause problems. They recommend fuel not even remain in tanks or fuel system for more than 24 hours (this is for aircraft, mind you). Vapor lock may be a problem.
Over 5% can cause serious problems. They say not to fly airplanes with this amount. In fact, they recommend draining the fuel system, flushing all parts, then running with clean alcohol-free fuel long enough to exchange fuel in carburetor bowl.
Here is the rest of the article regarding known problems with alcohol.
Alcohol attacks some seal materials and varnishes on cork floats of fuel level indicators. This could cause leakage of seals and release particles of varnish from floats, causing blocked screens in fuel lines or blocked carburetor jets. Excessive entrained water carried by alcohol could lead to fuel line blockage or blockage at screens or values when operating at low ambient temperatures at ground level or at high altitude.
Fuel volatility is also increased with the addition of alcohol in a manner that is not detected by the Reid Vapor Pressure test, which is used to determine if a fuel meets the automotive specification. For example, a gasoline with alcohol will meet the Reid Vapor Pressure limit of 13.5 psi but it will behave as
though it has a volatility of roughly 20 psi.
Gasolines with alcohol will also phase separate. Phase separation occurs as the gasoline/alcohol blend cools, such as when a plane climbs to a higher altitude. When water that is absorbed in the fuel by alcohol comes out of solution, it takes most of the alcohol with it. The quantity that comes out of solution cannot be handled by the sediment bowl and tank sumps. Furthermore, if the alcohol is used to raise the octane of the base gasoline, the gasoline that remains will not have sufficient octane to prevent detonation.
A good reference for this phase separation problem is: Paul Corp., Laboratory Investigations into the Effects of Adding Alcohol to Turbine Fuel, DOT/FAA/CT-TN88/25 July 1988, FAA Technical Center, Atlantic City International Airport, NJ 08405.
Visit www.EAA.org for the complete article.