Chainsaws typically operate with a 2-stroke engine rather than a 4-stroke engine common on most automobiles. The 2-stroke engine has the advantage of being lightweight and portable, has a simple design, and generates significantly higher specific power than their 4-stroke counterparts. 2-stroke engines on chainsaws don't have an internal lubrication system requiring oil to be mixed with the gasoline in order to lubricate the moving parts on the engine.
2-stroke engines, the kind normally found on chainsaws, use only 2 strokes during the firing cycle rather than the 4 strokes used by conventional 4-stroke engines. The process allows for the combustion stroke to function simultaneously with the exhaust and the compression stroke to perform the intake function. This process creates double the power of a 4-stroke engine making the 2-stroke engine ideal for chainsaw applications. By adding oil to the fuel in a prescribed ratio, the fuel acts as a lubricating agent for the moving internal parts of the engine.
Most chainsaws in the United States operate on a 50:1 oil to unleaded gasoline ratio. Some manufacturers recommend a 40:1 mixture. Still others operate on diesel fuel. Some older chainsaw models that were manufactured prior to the mid 1960s required a 25:1 ratio of oil to leaded gasoline. The only way to know for sure is to check the owner's manual of your specific model. Using the proper fuel mixture will help keep the chainsaw operating at peak performance and will prevent the engine from seizing.
The 2-cycle engine on most chainsaws will emit a small amount of smoke in its exhaust as a result of the oil in the fuel mixture not being completely consumed during the combustion process. Older model chainsaws are perceived as creating significantly more pollution than their 4-stroke counterparts. However, the newer generations of chainsaw have been engineered to burn the oil-gasoline fuel mixture as efficiently as a 4-stroke resulting near equivalent pollution emission levels.
Many manufacturers of 2-cycle engine oil package their product in pre-measured containers that can be added to a gallon of unleaded gasoline to produce the proper ratio of oil to gas. Alternately, the fuel can be mixed manually by adding 29.6ml of 2-cycle engine oil to 1418gr of unleaded gasoline to create a 50:1 mixture. Using the proper fuel mixture allows the chainsaw to operate a peak performance, keeps the internal moving parts of the engine lubricated and produces the least amount of pollutants from the engine exhaust.
2-cycle chainsaw engines require the proper mixture of oil and unleaded gasoline to operate properly. If the fuel mixture does not contain enough oil the internal moving parts will not be properly lubricated and you run the risk of the engine seizing up. Too much oil in the fuel mixture can cause the engine to smoke heavily. It can also foul your muffler and air cleaner with a thick, oily gunk and cause the engine to bog down.