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Oxygenated fuels have been required by law since 1992. Though they are not required in all areas of the country their use has become more wide spread since that time. The effects of oxygenated fuels are more felt with marine engines versus automotive use as marine engines are run harder and with less frequency.
Oxygenates are combustible liquids made up of carbon, hydrogen and oxygen atoms. Gasoline is made up of hydrogen and carbon atoms only (gasoline is called a hydrocarbon). Mix the two together and you effectively have a gasoline with oxygen added to it.
The components currently used as oxygenates fall into one of two groups, alcohols and ethers. As most people realize, alcohol added to gas is nothing new, it has been done since the late 1970's. Methyl tertiary butyl ether (MTBE) is the most widely used ether though there are others that may be combined with gasoline.
Reformulated gasoline (RFG) is gas designed to limit the amount of toxic emission and limit it's ability to evaporate. Evaporated gasoline is a major source of pollution. It also requires a detergent package to be added. During the winter months it is oxygenated and it might be oxygenated year-round. RFG may or may not be an oxygenated gasoline.
The basic function of an internal combustion engine is to capture the energy from expanding gases and convert it into mechanical movement. A mixture of gasoline and air is ignited in a small area and the resulting expanding gases create pressure that pushes a piston downward. Obviously if more energy is produced by the ignited fuel mixture the engine develops a higher energy output. If less energy is released by the burning fuel the engine produces less horse power.
Several factors determine the energy produced from ignition. Compression is one factor. The higher the compression the higher the energy release. Engine displacement would be another. A larger engine allows more fuel to be burnt at any given moment. A third factor, one that is affected by oxygenates, is the ratio of hydrocarbons to oxygen.
Hydrogen atoms and carbon atoms chain together in various patterns to make gasoline molecules. On it's own gasoline is useless, it needs oxygen atoms in order to burn. The oxygen allows the combustion to take place but it is the hydrocarbons that produce the energy. In order for gasoline to produce a maximum power output every hydrocarbon molecule needs to go through combustion. In order to burn every hydrocarbon molecule you need to add an exact amount of oxygen atoms.
There is an ideal ratio of air to gasoline that supports such maximum combustion and that ratio is called the stoichiometric ratio (how's that for a $10 word? - stoi'-ke-o-met-rik). It is defined as 14.7 parts air to 1 part gas by mass (weight). The ratio expressed in terms of volume, such as a cubic inch, would be 11,500 parts air to 1 part gas.
Carburetors are sloppy at best at providing an ideal ratio. Usually they are set rich to compensate for a carbureted system's shortcomings. Fuel injection engines has made it possible to come close to the ideal. But even with computer control it isn't quite 100%.
Since the stoichiometric ratio is the point where complete combustion takes place it's easy to understand that moving away from that point results in incomplete combustion. Actually it takes just a small change from that ratio to produce dramatic effects as far as burning the fuel is concerned.
Hand in hand with incomplete combustion is air pollution. One of the pollutants produced by incomplete combustion is carbon monoxide, a colorless, odorless and deadly gas. Carbon monoxide is made when one of the carbon atoms from the gas combines with one oxygen atom.
It was discovered that when more oxygen was added to the combustion process a carbon atom would combine with two oxygen atoms and form carbon dioxide. Carbon dioxide is much more life friendly. It is for this reason we have oxygenated gas. The amount of oxygen added depends on area's pollution levels. As well, there is a greater need during the winter months as carbon monoxide levels in the air tend to be higher during this period.
One of the drawbacks to oxygenated fuel is that for a given amount (a gallon for instance) there will be more oxygen atoms as compared to a non-oxygenated fuel. That means there are fewer hydrocarbons. As stated above, the power from gasoline is derived from hydrocarbons. Thus with oxygenated fuels you can expect a loss of performance and a loss of fuel economy. Another problem with oxygenated fuel is that it raises the octane rating of the fuel and for the marine engine that could be a negative factor.
Octane is a rating of gasoline's ability to resist preignition and detonation. As shown elsewhere on this site these two problems can destroy an engine. Choosing a proper fuel is essential to engine life.
The octane number you see on the pump is called the Anti Knock Index (AKI) rating of the gas. This number is derived mathematically from two other numbers, Research Octane Method (RON) and Motor Octane Method (MON).
These two numbers, RON and MON, are determined by the use of a special gasoline engine in which the compression, timing and fuel ratio can be adjusted as desired as well as the engine speed. A sample of the fuel to be tested is fed to the engine and conditions are varied until a knock is produced. The results are compared to a known reference which indicates the rating to be assigned.
Research Octane Method is determined with the test engine set to low rpm and reduced spark timing. This most duplicates an engine that is used under light conditions and low temperatures. Under these conditions the engine is less likely to produce a knock making the RON number higher than the MON number. Another way of saying that is with a light load a higher octane number can be given to the gas.
Motor Octane Method is determined with the test engine set to a higher rpm with an advanced timing. This tends to duplicate an engine under heavier loads and ends up the lower of the two octane numbers.
AKI is determined by adding RON and MON and dividing by 2. If a gasoline's RON was 94 and it's MON was 84 then it would have an AKI of 89. A RON of 97 and a MON of 81 would also produce an AKI of 89.
A marine engine, for the most part, is not run under light load conditions. It is normally used at heavy loads, high rpm and advanced timing. It becomes clear then that the MON number of gasoline is more important to boaters than the RON number.
Another specification of gasoline is Octane Sensitivity. Sensitivity is determined by the formula RON - MON. In the example above with a Ron of 94 and a MON of 84 the gasoline's sensitivity would be 10. A RON of 97 and a MON of 81 gives us a sensitivity of 16. Since the MON is important to boaters, a gas with a lower sensitivity number is much more desirable.
The sensitivity factor also answers a question many have has about gasoline - why one brand of 87 octane gas doesn't knock in the car and another brand of 87 does. Answer - the other brand could have a higher sensitivity. Depending on the car's operating conditions problems like this can arise. This is also true with marine engines.
Oxygenates tend to increase the AKI rating of gasoline It also increases the sensitivity rating of that gas. That means while the 92 rating of premium may look good for the boat it might have a MON rating of 84. An 89 AKI gas with a sensitivity of 10 would have a MON of 84 and would be just as good octane-wise. It would also contain less oxygenates resulting in better performance and better fuel economy.
Mind that the numbers above are presented for educational purposes only. They may or may not represent actual gasoline ratings. There are there just to demonstrate a problem that may arise.
A solution to this situation, it would seem, is to obtain the sensitivity numbers of the various brands of gasoline. The problem to this solution is that sensitivity is not a set standard. After having contacted representatives of several gasoline manufactures the best answer I was given was "we require a sensitivity of 10 or less for our 87 grade gas". As well, I was told the sensitivity number of a manufacturer's product can vary in different parts of the country.
Another monkey wrench thrown into the works, and it's a fact that caught me by surprise, is that gasoline is, and has been for the past several years, considered a fungible product. Per the American Heritage Dictionary the definition of fungible is "Being of such a nature or kind that one unit or part may be exchanged or substituted for another equivalent unit or part in the discharging of an obligation." That means you could go to a Chevron station and be getting gas made by Texaco or Exxon. Just how wide spread this practice is I don't know. It is a practice that gas companies don't openly advertise.
There are two basic ways used to increase the octane rating of gas. One method is by the addition of boosters, such as an oxygenate, and that was discussed above. The other method is by a process called reformulation. In this process, the hydrocarbon molecules are re-formed, converting them into a molecule type that better resists detonation and preignition. During this process the volume of product is reduced. If you put 10 barrels of gasoline through reformulation you might end up with 7 or 8 barrels. The higher the octane is raised the higher the loss in volume. Gasoline that has been reformulated produce higher MON numbers and give a lower sensitivity. Gasoline that has been reformulated costs more.
If you were going to sell gasoline at a lower price than the guy down the road you would want to buy some gas, boost it with additives and open the doors for business. This gas might work well for cars but it may not do for marine engines.
The brand name station, on the other hand, tend to rely on reformulated gasoline. They do add oxygenates as mandated by law but they are less likely to depend on them for octane ratings.
So what you can do to help in this situation is follow the rule that has been given for years, buy gas from a brand name station that has a high turn over rate. Avoid the discount stations. It is by no means the perfect solution but in light of the information available it does seem to be the best solution.
Mercury Marine, MercCruiser Division, recently issued a bulletin concerning gasoline additives. In it they state that the detergent additives used with today's gas are supplied by several different companies, none were named. They further stated that most brands of the detergent additive will not cause problems but they found that certain brands have been found to leave a sticky, rusty colored substance when the gas has evaporated. Problems that have occurred from this substance includes stuck carb floats, stuck fuel inlet needles, stuck injectors and stuck check valves in electric fuel pumps. These problems have occurred in as little as 3 to 4 weeks. It also seems to be regional in nature, not all areas of the country will see this problem. Their recommendations to all this is to either shut off the gas to the engine and run the carbs dry (not recommended on oil injected outboards) or to add stabilizer to the gas tank if the boat is going to sit for two or more weeks.
The above information doesn't contain any new great rules for dealing with gasoline, rather it reinforces the rules that have been given for the past few years. It's most of all a collection of information that I've come across over the last several months. I hope you've enjoyed it. If you have any comments, good or bad, I'd enjoy hearing them.
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