PETROL ENGINE

The two-stroke cycle of an internal combustion engine differs from the more common four-stroke cycle by completing the same four processes (intake, compression, power, exhaust) in only two strokes of the piston rather than four. This is accomplished by using the space below the piston for air intake and compression, thus allowing the chamber above the piston to be used for just the power and exhaust strokes. This causes there to be a power stroke for every revolution of the crank, instead of every second revolution as in a four-stroke engine. For this reason, two-stroke engines provide high specific power, so they are valued for use in portable, lightweight applications. On the other hand large two stroke diesels have been in use in industry (i.e. locomotive engines) since the early twentieth century.
The two-stroke spark-ignition engine's invention is generally credited to Joseph Day (and Frederick Cock for the transfer-port), whereas the two-stroke valved compression-ignition engine is attributed to Dugald Clark.The smallest gasoline engines are usually two-strokes. They are popular due to their simple design (and therefore, low cost) and very high power-to-weight ratios. The biggest disadvantage is that the engine lubricant is almost always mixed in with the fuel, thus significantly increasing the emission of pollutants. For this reason, two-stroke engines are being replaced with four-stroke engines in as many applications as possible.
Two-stroke engines are still commonly used in high-power, handheld applications where light weight is essential, primarily string trimmers and chainsaws. To a lesser extent, these engines may still be used for certain small, portable, or specialized machine applications. These include outboard motors, high-performance, small-capacity motorcycles, mopeds, underbones, scooters, snowmobiles, karts, model airplanes (and other model vehicles) and lawnmowers. In the past, two-stroke cycles were experimented with for use in diesel engines, most notably with opposed piston designs, low-speed units such as large marine engines, and V8 engines for trucks and heavy machinery.
Two-stroke diesel enginesUnlike a gasoline engine, which requires a spark plug to ignite the fuel/air charge in the cylinder, a diesel engine relies solely on the heat of compression for ignition. Fuel is injected at high pressure into the superheated compressed air and instantly ignites. Therefore, scavenging is performed with air alone.
In order to allow the usage of a conventional oil-filled crankcase and pressure lubricated main and connecting rod bearings, a two-stroke diesel is scavenged by a mechanically driven blower (often a Roots positive displacement blower) or a hybrid turbo-supercharger, rather than by crankcase pumping. Generally speaking, the blower capacity is carefully matched to the engine displacement so that a slight positive pressure is present in each cylinder during the scavenging phase (that is, before the exhaust valves are closed). This feature assures full expulsion of exhaust gases from the previous power stroke, and also prevents exhaust gases from backfeeding into the blower and possibly causing damage due to contamination by particulates.
It should be noted that the scavenging blower is not a supercharger, as its purpose is to supply airflow to the cylinders in proportion to their displacement and engine speed. A two-stroke diesel supplied with air from a blower alone is considered to be naturally aspirated. In some cases, turbocharging may be added to increase mass air flow at full throttle—with a corresponding increase in power output—by directing the output of the turbocharger into the intake of the scavenging blower, an arrangement that was found on some Detroit Diesel two-stroke engines.
A conventional, exhaust-driven turbocharger cannot be used by itself to produce scavenging airflow, as it is incapable of operating unless the engine is already running. Hence it would be impossible to start the engine. The common solution to this problem is to drive the turbocharger's impeller through a gear train and overrunning clutch. In this arrangement, the impeller turns at sufficient speed during engine cranking to produce the required airflow, thus acting as a mechanical blower. At lower engine speeds, the turbocharger will continue to act as a mechanical blower. However, at higher power settings the exhaust gas pressure and volume will increase to a point where the turbine side of the turbocharger will drive the impeller and the overrunning clutch will disengage, resulting in true turbocharging.