Two Stroke Cycle Introduction
Petrol/Gasoline Two Stroke Cycle
As the piston nears bottom dead centre (BDC), the cylinder inlet port discharges air (with entrained lubrication oil and fuel) into the combustion space. The piston then begins travelling towards top dead centre (TDC) and covers the cylinder inlet port whist also uncovering the crankcase inlet port.
The piston continues travelling towards TDC and covers the exhaust outlet port. At this point, the entrained gas within the cylinder is compressed and a spark ignites the fuel. Ignition occurs and a rapid increase in pressure and temperature forces the piston back towards BDC. At the same time, air/fuel has been drawn into the crankcase through a reed valve.
As the piston travels towards BDC, it compresses air/fuel within the crankcase (which is under the piston), then the exhaust port opens, then the cylinder inlet port opens and finally the crankcase inlet port is closed. The exhaust gas is expelled through the exhaust port and is replaced by the air/fuel mix so the cycle can begin again. Air/fuel from the crankcase will be forced into the cylinder because the piston has raised the pressure in the crankcase to a higher pressure than that within the cylinder liner.
It all sounds complicated, but it will be a lot clearer when you watch the whole process using the interactive 3D model animation.
Two Stroke Engine Advantages
Two Stroke Engine Disadvantages
Although the two stroke engine is primarily used for small engine applications, it is worth remembering that two stroke engines are also the largest engines in the world. These large two stroke engines are used on merchant navy vessels and can weigh several thousand tonnes.
Why is the two stroke light weight?
The crankcase is full of gasoline, air and oil, so there is no need for additional lubricating oil pumps, piping or filters. There is also no need for cooling water pumps because there are no coolant passages in the cylinder head (no cooling water system). The two stroke engine design also does not require push rods or exhaust valves etc., all of this leads to a large weight reduction compared to a four stroke engine.
The piston crown shown in the two stroke video/3D model may have a slightly different shape to other two stroke engines. An alternative shape allows the piston crown to be used to direct the incoming air upwards as it enters the cylinder, this leads to an increase in scavenging efficiency.
Cooling fins have a unique shape because they must have a high heat transfer rate. A large heat transfer rate allows heat to be dissipated quickly and this prevents the engine overheating.