Two Stroke Cycle
The two stroke engine is named so as it completes a full cycle with every two strokes of the piston, meaning two movements of the piston. One stroke down and one stroke up, meaning there is a power stroke every two strokes. Half that of the four stroke engine. Now as I am Scottish I would like to make note that the invention of the two-stroke cycle is attributed to Scottish engineer Dugeld Clerk who patented his design in 1881.
The two stroke cycle is achieved by commencing the compression stroke at the end of the power stroke. Exhausting and induction are done at the same time. So lets look at the two stroke cycle. From TDC the piston starts its down stroke. This is the power stroke. Fuel injection ceases at this point. All engines are slightly different but for this example I will say fuel injection ceases approximately 10 degrees of crank angle after TDC. Combustion continues until all the fuel is burnt. The gasses produced by the combustion expand and exert force on top of the piston driving it down the cylinder. Before BDC is reached, exhaust ports are uncovered in the lower section of the cylinder liner. Some of the exhaust gasses will escape as they continue to expand but to properly be expelled from the cylinder they must be forced out. This is done by forcing air into the cylinder, a process known as the gas exchange process or scavenging. As there is no induction stroke like there is on the four stroke engine, the engine cannot be naturally aspirated, therefore the air must be forced into the cylinder by an external means. This air is known as charge air. Charge air is produced by superchargers, turbochargers and at low revs when these two are ineffective, electrical blowers provide the air.
There are three types of scavenging, each have their own advantages and disadvantages:
- Loop scavenging
- Cross-flow scavenging
- Uni-flow scavenging
Loop Scavenging : This is where there is a set of shaped induction ports below the exhaust ports at the bottom of the liner. As the piston nears the bottom of its stroke the piston uncovers the exhaust ports and the gasses start to leave. Just after this the induction ports (also known as scavenge ports) are uncovered allowing charge air to enter the cylinder. The shape of the scavenge ports cause the air to be directed up to the top of the cylinder, across the cylinder head and down the other side and out of the exhaust ports. The exhaust gasses are driven out by the charge air. The piston travels through BDC and begins its upwards stroke, known as the compression stroke. As the piston moves up it first covers the scavenge ports and then the exhaust ports and compression begins. With this type of scavenging a long piston skirt is required to keep both sets of ports covered as the piston is at the top of its stroke.
Cross-flow Scavenging : This is where the exhaust and induction ports are on opposite sides of the cylinder liner. Cross scavenging is not very common these days as it was superseded by the loop scavenge design. The reason for this is that the piston fitted to a cross scavenged engine requires a deflector to direct the air upwards to the top of the cylinder to scavenge the gasses. This deflector not only adds weight to the piston but reduces the combustion cycle efficiency.
Uni-flow Scavenging : This is the most efficient form of scavenging and is the method now incorporated into all modern marine two stroke designs. There is still a set of scavenge ports at the bottom of the cylinder but the exhausting is done through a valve fitted into the cylinder head. The most common method on modern engines of this valve being operated is by hydraulics. The charge air enters via the bottom of the cylinder and exits at the top travelling in one direction.
Continuing on with our cycle. After scavenging is completed, compression commences. The piston travels up the cylinder on its compression stroke. Reducing the volume in the cylinder thus causing the charge air to become highly pressurised and very hot. Before the piston reaches TDC fuel is injected. Again the position at which this happens will vary engine to engine and could be anything from 10 degrees to 20 degrees of crank angle before TDC. The fuel meets the hot air and combustion begins.
All of the above can be shown on a two stroke cycle diagram. It is a common and easy question to be asked to draw the cycle on both written exams and in an oral exam, even at 2nd engineer and chief engineers level. I myself was asked to draw cycle diagrams on my chief engineers oral exam with the MCA.
