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More on fuel;

More on fuel;

 

Gasoline is the most common fuel used in small engines today. Gasoline is derived from the fossil fuel we know as crude oil, and was originally an undesirable by-product created during the refining process of lamp oil, also known as coal oil (kerosene and currently also diesel).  Gasoline was too flammable and volatile to use in wick fed lamps but did find some use as a solvent (Early dry cleaning processes used gasoline which gave dry cleaned clothing a rather interesting aroma.). It wasn’t until the advent of spark ignition internal combustion engines that gasoline was perceived to have much value at all. The energy in gasoline is released thru burning in the combustion process and this is where it excels. There are few fuels used in spark ignition engines that are as capable in the energy department as gasoline.

 

When gasoline burns, it releases energy in the form of heat. A chemically correct oxidation of hydrocarbons yields H2O, CO2, Nitrogen and heat. In other words, if gasoline is combined with air in the proper volumes in an environment where it can burn completely, the results will be water vapor, carbon dioxide, nitrogen and heat. The perfect ratio of atmospheric air to fuel at which the most efficient combustion occurs is referred to as the Stoichiometric Ratio. Although the stoichiometric ratio is theoretically the most efficient air fuel-ratio, it does not produce maximum engine power. A charge with an air-fuel ratio slightly richer in fuel is easier to ignite and generates energy that is more useful. The most efficient and complete combustion occurs when there are 14.7 parts of atmospheric air for every part of fuel. The stoichiometric ratio for gasoline, then, is 14.7:1.

 

Besides price and brand name , the Octane rating also known as the Antiknock index (AKI) is the only characteristic of gasoline that, as consumers, we have any choice over. The Antiknock Index is the numerical value assigned to gasoline that indicates the ability to eliminate knocking and/or pinging in an operating engine caused by pre ignition. Pre ignition is a condition where fuel ignites in the cylinder prematurely due to the heat of compression and can cause serious damage to cylinder, head, and or valve components. Additives are compounded with gasoline to increase the stability of the fuel-air mixture charge during  combustion, the higher the AKI the more resistance to pre ignition is achieved. All gasoline is tested and assigned an AKI number using the research octane number and the motor octane number. The research octane number (RON) is the octane number that affects engine knock at low to medium speed. The motor octane number (MON) is the octane number that affects engine knock at high speed and performance in severe operating conditions and under load. Octane should not be mistaken as a gauge of gasoline quality as a high-octane fuel may cause an engine to run poorly if it only requires low octane. A point to understand is that octane rating is affected by altitude. (Example; At sea level the recommended octane rating for most engines is 87. At 5000 ft above sea level the octane rating of 85 meets the standard.).

 

Volatility and vaporization  also play a significant role in assuring that an engine will run properly, particularly when it needs to do so under varying climactic conditions. Vaporization is the process in which a liquid is sufficiently heated to change a liquid to a vapor. Liquids will vaporize due to evaporation or boiling. Volatility is a measure of the propensity for a liquid to become a vapor. Gasoline is by nature volatile and is blended to take advantage of its propensity to vaporize based on the environment in which it is used. If the ambient temperature is low, the gasoline must be blended to make it vaporize more easily. If the ambient temperature is high, the gasoline must be blended to prevent it from vaporizing to easily.

 

At the heart of adjusting the vaporization rate of gasoline is the fact that gasoline in a liquid form is not flammable.