Editing Two-stroke engine (section)

== Emissions ==
Crankcase-compression two-stroke engines, such as common small gasoline-powered engines, are lubricated by a [[petroil]] mixture in a [[total-loss oiling system|total-loss system]]. Oil is mixed in with their petrol fuel beforehand, in a fuel-to-oil ratio of around 32:1. This oil then forms emissions, either by being burned in the engine or as droplets in the exhaust, historically resulting in more exhaust emissions, particularly hydrocarbons, than four-stroke engines of comparable power output. The combined opening time of the intake and exhaust ports in some two-stroke designs can also allow some amount of unburned fuel vapors to exit in the exhaust stream. The high combustion temperatures of small, air-cooled engines may also produce [[NOx|NO<sub>x</sub>]] emissions.