Internal Combustion Engines – 2 stroke engines

In this type of engine the piston periodically covers and uncovers openings - known as ports - in the cylinder wall (the two-stroke engine is seldom equipped with valves).

The operation of the two-stroke engine:

 At the start of the first stroke, the piston is in its highest position. When the compressed petrol-and-air over the piston is ignited, the latter is thrust downwards and, in so doing, releases the exhaust port. The burned gases in the cylinder, which are still under high pressure, can thus escape through this port. When the piston descends further, its upper edge releases the inlet port, which admits fresh petrol-and-air mixture (delivered by the fan) into the cylinder, so that the remaining burned gases are flushed out.

When the piston rises again (2nd stroke), all the ports are closed for a time, and during this period the petrol-and-air mixture is compressed, so that a fresh cycle can commence.

The crankcase-scavenged two-stroke engine has no scavenging fan. Instead, the crankcase is hermetically sealed, so that it can function as a pump in conjunction with the piston. When the piston ascends, a partial vacuum is produced in the crankcase, until the lower edge of the piston releases the inlet port and thus opens the way to the fresh petrol-and-air mixture into the crankcase.

When the piston descends, the mixture in the crankcase is compressed a little so that, as soon as the top of the piston releases the transfer port and overflow duct (connecting the crankcase to the cylinder), it can enter the cylinder. Meanwhile, what happens above the piston is the same as in the fan-scavenged engine.

In the latter type of two-stroke engine the fan adds to the cost. However, as the overflow duct between the cylinder and crankcase is eliminated, the crankshaft can be provided with forced-oil lubrication without involving a risk that the oil in the crankcase can find its way into the cylinder.

In the cheaper crankcase-scavenged engine the lubricating oil is mixed with the petrol (‘petroil’ lubrication) or is, alternatively, supplied to the points of lubrication drop-wise by small lubricating oil pumps.

The oil which enters the crankcase is liable to be carried through the overflow duct and transfer port into the cylinder, whence it passes through the exhaust port and into the exhaust system, where it may manifest itself as blue smoke in the exhaust.