Understanding Gasoline
Spring 2012

Gasoline has been used for many years to power internal combustion engines. The fuel stores energy and the engine converts as much of this energy as it can, and turns it into rotary motion of the crankshaft. Gasoline, over the years, has changed significantly and some of the information in the marketplace is not highly accurate.

Gasoline is made up of several groups of compounds. These are specific types or families of compounds needed to ensure that the fuel will behave in certain ways and deliver the energy needed to power the engine. The focus of the presentation is not to make you a chemist, but rather to explain in simple detail what is in fuels, how to properly handle and store fuel, and to give you an understanding of the new fuels of today.

Let’s start at the beginning. Crude oil is taken from the earth and refined. This process heats the crude oil until components of it vaporize from the added heat. Remember back in chemistry or physics class, what you had to do to a liquid to make it vaporize? Add heat. Plug in the kettle or place a pot on the stove with water in it, add heat and at around 100°C, the water begins to change state and steams water into vapour. The crude oil behaves similar as we add heat. Different compounds change state at different temperatures and the compounds are captured and cooled, leaving the base for various fuels like gasoline, diesel fuel, naphtha and jet fuels. Many other compounds are also captured and used in the polymers, plastics and other chemicals used in industry and other parts of our lives.

This is a very basic explanation of the refining process. After refining, fuel is further developed to give us the properties required for engines built today.

Gasoline is made up of up to 350 compounds all blended together. Octane is a specific compound in gasoline and should not be confused with the octane number or rating. The octane number quantifies or rates the ability of the fuel or a fuel component to resist auto ignition or, in simple words, its ability to resist burning. Now it seems rather counter productive that we may want fuel not to burn too easily; however think about this. Gasoline must ignite at precisely the correct time in the engine combustion chamber so that the engine can extract the maximum amount of energy from the gasoline. In our two-cycle engines running at 12,000 rpm, the spark plug must ignite a fuel charge about 200 times per second. If fuel and its ability to burn is not controlled, this fire or burning of the fuel may begin because the fuel is too volatile or burns too easily. These conditions are referred to as detonation or pre-ignition; two distinctly different conditions that can destroy a two-cycle engine. Compounds with a high octane rating like octane itself are responsible for not allowing that to occur.

A higher octane rating does not give you more horsepower, or give you cleaner running engines, or somehow make your fuel better, as it is not a measurement of the energy level or the quality of the fuel. Higher octane fuel may contain other additives that may enhance the fuels’ stability, and fuel system cleanliness, particularly in automobiles. Remember, octane rating is a measurement of a fuel’s ability to resist auto ignition or uncontrolled burning of the fuel.

STIHL recommends that customers use a minimum of 89 octane fuel. If a customer chooses to use a 91 or 93 octane fuel, no damage to the engine will result. However, using an 87 octane fuel could lead to serious engine damage.

In recent years in different parts of the world and in this country, ethanol has been added to the fuel. Ethanol is an alcohol that may be derived from corn and grains. This too is changing and some ethanol is now produced from the stock of the plant, and is referred to as cellulosic ethanol. However, ethanol behaves the same, independent of the source from which it has been produced. Ethanol has some interesting properties, some are benefits and some can cause problems for the end user. For instance, ethanol is very high in octane and is used to increase the octane rating of a fuel. Ethanol also has solvent properties. This can become an issue when introducing ethanol based fuels into older fuel systems. Any debris built up in the system may be cleaned off the walls of the tank’s fuel lines, and may end up plugging ports or jets inside carburetors. This solvent property also tends to erode the elastomers that the fuel lines are made of, leading to shortened service life of some components in the system. Ethanol fuel also contains less energy by volume than gasoline. This means if you could run a machine on 100% ethanol and the same machine on 100% gasoline, the gasoline machine would do more work on the same volume of fuel. The optimum air-fuel ratio of gasoline is 14.7:1, while for pure ethanol it is 9:1 in an internal combustion engine at sea level.

There have been many articles written about the great problems that occur when using ethanol fuels. Some of these are correct and others contain elements of truth. Some dealers call the help desk with a diagnosis that the ethanol fuel caused the machine to fail. This is highly unlikely, since our machines are engineered and able to consume up to 10% ethanol. If the machine is maintained and set up properly, then the ethanol will not be detrimental to the machine. So why all the confusion? Most of it is because of the lack of understanding of ethanol based fuels and their use in STIHL products. Here are some key points to consider when using ethanol based fuels.

Ethanol fuel has a tendency to attract water. In fact, it will bind with water at a molecular level. Therefore, it is imperative that a fuel storage container be sealed, except when refueling. The moisture in the atmosphere on a humid day can cause phase separation to occur in the fuel. This is when the ethanol combines with water, binds together and will settle in the bottom of the container.

This mixture of ethanol and water contains no oil and will contaminate the machine the fuel is being dispensed into. Since all our fuel systems draw fuel off the bottom of the fuel tanks, you can quickly see how this mixture is sure to cause running issues, and maybe even severe damage to the fuel system and possibly the engine. This damage may be in the form of corrosion from the water combined in the fuel, usually in the carburetor, or in extreme cases, no oil entering the engine while running can cause seizure.

As our laboratory suggests, mix ethanol based fuels in small quantities and store for no more than 30 days to ensure a good mix without any moisture contamination.

With E10 only in very extreme cases such as running in deep cold, -10°C or colder, would a carburetor need to be readjusted because of the ethanol content.

The other property of fuel that can get customers into trouble is when they purchased the fuel. Gasoline is blended specifically for winter and summer usage. Winter fuel will vaporize at a lower temperature as compared to summer time fuel. What this means is that if a customer purchases fuel in April and then uses it in May when temperatures could rise to 28°C, the fuel can turn to vapour in fuel lines or in fuel pumps, and can cause running conditions, stalling or, in extreme cases, engine seizure. If the customer uses a summer fuel in the winter time, they may encounter hard start, poor run and stalling because the temperature required to vaporize the fuel is higher than the outside temperature and temperature the machine can give to the fuel. Winter fuel will vaporize at approximately 34°C and summer fuel will vaporize at approximately 45°C. Therefore, on a warm day of 28°C and some added heat from the running engine, winter fuel may become active or percolate in the fuel tank and could vapour lock in the fuel lines or carburetor.

As you can see, when a customer purchases the fuel is also important.

At STIHL, our engineers are constantly looking for new materials and different blends of material to withstand the ever-changing properties of fuel. This, along with good fuel handling practices, should ensure years of reliable machine use.