What is a 2 stroke engine?  How do they Work?

The terms "two-cycle" and "two-stroke" are often inter-changed when speaking about two-cycle engines. These engines derive their name from the amount of directional changes that the pistons make during each power stroke. Internal combustion engines are used to produce mechanical power from the chemical energy contained in fuels. The power-producing part of the motor's operating cycle starts inside the motor's cylinders with a compression process. Following this compression, the burning of the fuel-air mixture then releases the fuel's chemical energy and produces high-temperature, high-pressure combustion. These gases then expand within the cylinder and transfer power to the piston. Thus, as the engine is operated continuously, mechanical power is produced. Each upward or downward movement of the piston is called a stroke.

Two-cycle engines deliver one power stroke for each revolution of the crankshaft.

Where are two-cycle engines used? Two-cycle engines are inexpensive to build and operate when compared to four-cycle engines. They are lighter in weight and have a higher power-to-weight ratio than a 4 stroke engine. For these reasons, two-cycle engines are very useful in applications such as lightweight scooters. Two-cycle engines are also easier to start in cold temperatures. Part of this may be due to their design and the lack of an oil sump. This is a reason why these engines are also commonly used in snowmobiles and snow blowers. Because two-cycle engines can effectively double the number of power strokes per unit time when compared to four-cycle engines, power output is increased. However, it does not increase by a factor of two. The outputs of two-cycle engines range from only 20 to 60 percent above those of equivalent-size four-cycle units. This lower than expected increase is a result of the poorer than ideal charging efficiency, or in other words, incomplete filling of the cylinder volume with fresh fuel and air.

How are two-cycle engines different from four-cycle engines? The fundamental difference between two-cycle engines and four-cycle engines is in their gas exchange process, or more simply, the removal of the burned gases at the end of each expansion process and the induction of a fresh mixture for the next cycle. The two-cycle engine has an expansion, or power stroke, in each cylinder during each revolution of the crankshaft. The exhaust and the charging processes occur simultaneously as the piston moves through its lowest or bottom center position.
In a four-cycle engine, the burned gases are first displaced by the piston during an upward stroke, and then a fresh charge enters the cylinder during the following downward stroke. This means that four-cycle engines require two complete turns of the crankshaft to make a power stroke, versus the single turn necessary in a two-cycle engine. In other words, two-cycle engines operate on 360 degrees of crankshaft rotation, whereas four-cycle engines operate on 720 degrees of crankshaft rotation.

How are two-cycle engines lubricated?
If you have ever used a two-stroke engine, you know that you have to mix special two-stroke oil in with the gasoline.  In a four-stroke engine, like your car uses the crankcase is completely separate from the combustion chamber, so you can fill the crankcase with heavy oil to lubricate the crankshaft bearings, the bearings on either end of the piston's connecting rod and the cylinder wall. In a two-stroke engine, on the other hand, the crankcase is serving as a pressurization chamber to force air/fuel into the cylinder, so it can't hold a thick oil. Instead, you mix oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls. If you forget to mix in the oil, the engine isn't going to last very long!
Two-cycle motors are considered total-loss type lubricating systems. Because the crankcase is part of the intake process, it cannot act as an oil sump as is found on four-cycle engines. Lubricating traditional two-cycle engines is done by mixing the oil with the fuel. Internal engine parts are lubricated by this mixture of oil in the fuel during the intake portion of the power stroke.


parts of the above were excerpted from AmsOil news article.