Chainsaws run on two-stroke engines because they work upside down
A four-stroke car engine keeps its oil in a sump and depends on gravity. A two-stroke engine mixes its lubricating oil straight into the fuel, typically about one part oil to 32 of petrol, so it keeps working at any angle. That, plus a power stroke on every turn of the crankshaft, is why chainsaws and leaf blowers use them.
A two-stroke engine completes its whole cycle in one up-and-down movement of the piston. On the way up, fresh mixture is drawn in and compressed while the last exhaust clears; on the way down, the charge burns, pushes the piston and the spent gas starts to escape. Because it fires twice as often as a four-stroke, it packs more power for its weight, and with fewer moving parts it is cheaper and lighter to build. In the simplest designs the piston itself opens and closes the ports as it slides.
Those advantages come at an environmental price. The oil mixed into the fuel gets burned or spat out as droplets, and intake and exhaust ports can be open together, letting unburned fuel slip straight out. Two-strokes have historically emitted more hydrocarbons than comparable four-strokes, so strict regulations have pushed them out of cars and motorcycles in much of the world, though they survive in mopeds, outboard motors, snowmobiles and model aircraft where rules are looser.
The engine has a rich history. The Scottish engineer Dugald Clerk patented a commercial two-stroke in 1881, Karl Benz built a two-stroke gas engine at the end of 1879, and the Yorkshireman Alfred Angas Scott made the first truly practical version, producing water-cooled twin-cylinder motorcycles from 1908. Carmakers including Saab, DKW and the East German firms behind the Trabant used two-strokes, as did Suzuki and Subaru in the 1970s, before Western production ended in the 1980s.
At the other extreme, huge two-stroke diesels thrive where weight does not matter. Ships, locomotives and power stations use them because the cycle can be highly efficient, losing less heat to the cooling system and leaving more energy to drive the piston.
Source: Two-stroke engine
