The Two Stroke Cycle

Dugald Clark, the inventor of the two-stroke engine

Developed in the late 19th century by Scottish engineer Dugald Clark, the two-stroke engine has been used in a handful of production cars and several motorcycles and can be found in both spark-ignition and compression-ignition forms, although the latter is not very common. An advantage of this engine was that there were less strokes meaning that the complete cycles was complete within two revolutions of the piston and only one of those strokes was the engine not producing any power as opposed to three in a four-stroke engine. These engines run on a mixture of fuel and lubricating oil with typical ratios of fuel-oil being around 32-40:1. It would be Joseph Day's modification of this engine that would first come into use some ten years later and the fundamentals of it's workings in both petrol and diesel forms are described as follows:

In the case of a two-stroke petrol engine, beginning with the piston approximately half way up the cylinder, and all the relative ports covered, the rising piston compresses the mixture above and pressure below the piston is reduced. as the piston passes TDC, a fresh air:fuel mixture is forced into the crankcase by atmospheric pressure and the piston is now at the top of its stoke. The mixture above the piston is now ignited and the high pressure of these burnt gases forces the piston downwards, as is the case with the four-stroke engine. As the piston is nearing bottom dead centre, the fresh mixture in the crankcase is now compressed and the burnt exhaust gases are forced out of the cylinder under their own pressure. as the piston passes BDC, the transfer port is now uncovered and the compressed mixture below can now flow into the cylinder above the piston and is deflected upwards by a specially shaped deflector formed on top of the piston, preventing it from escaping across the cylinder and out of the exhaust port however, in modern two-stroke engines, the dispenser is no longer used as the transfer ports are shaped and aimed towards the top end of the cylinder and away from the exhaust port.

Although this engine has advantages of being simple in design, relatively smooth in operation and has just one idle stroke to one working stroke, it is still much less effective in its operations as some of the fresh mixture can easily be mixed with the exhaust gases and escape through the exhaust port and incomplete scavenging of burnt gases from the cylinder can occur.

Less common is the two-stroke diesel engine, which originally was used for low-speed industrial and marine applications and is now fitted to a handful of commercial vehicles. This engine has the advantages of being smooth, small and simple in its construction. Also, the loss of fuel to the exhaust when both ports are open is not an issue to this design as the cylinder contains only air.

The sequence of operations are as follows;

As air enters the inlet port, exhaust gases exit via the exhaust port and both ports are then closed by the ascending piston, providing the compression stroke. The air has now been compressed to a ratio of 12-16:1 and at this point the fuel is injected into the cylinder and ignited by the heat of the compressed air, producing the power stroke.

Most of these engines will incorporate a a blower to pressure-charge the cylinder with air to ensure the supply is adequate however, the engine can be operated by making use of pressure waves or pulses in the exhaust system to induce new air into the cylinder.

In the case of uniflow-type two stroke engines, an exhaust valve is incorporated and roots-type blower used to compress air into the cylinder improving engine output and breathing as it enables more air and fuel to be drawn in and exhaust gases to be evacuated more efficiently, potentially giving the same power of a four-stroke engine of the same displacement. By the time the air ports are uncovered, the exhaust valve has already opened, allowing the remaining gas pressure to start pumping out the exhaust gas, followed by an air charge approximately 30% greater in volume than the cylinder capacity, cooling and scavenging the cylinder more effectively.