Now lets analyze the sequence of events for a 2-stroke cycle engine. Refer to figure 2-2 at the end of this chapter. First of all, you should be aware of one of the primary physical differences between 4-stroke and 2-stroke cycle engines: 2-stroke engines do not use intake valves. To work properly, a 2-stroke engine must have some means of forcing air into the cylinder for the scavenging event. Two-stroke diesel engines may use an external blower or turbo-charger (or both) to perform this function.

Finally, notice the location of the ports in the cylinder through which air must flow. They are located near the bottom of the cylinder. Now, lets take a look at the 2-stroke cycle of events. Locate the engine labeled SCAVENGING (view 1 and the corresponding arrow in the circle). The scavenging event (which is very similar to the air intake event of a 4-stroke cycle engine) begins almost 45 degrees BEFORE the piston reaches BDC. During this time, the exhaust valve or valves are open. As the piston moves down to BDC, it uncovers the ports near the bottom of the cylinder. (This arrangement serves the same function as the intake valves in a 4-stroke cycle engine). The blower on the side of the engine forces air to flow into the bottom of the cylinder and out through the exhaust valves at the top of the cylinder. This action serves to scavenge any exhaust gases that might remain in the cylinder and to fill the cylinder with a fresh charge of air. The first dashed line to the left of BDC shows that the exhaust valves are shut at this point. The piston continues to move up and the next dashed line shows the point at which the intake ports are blocked by the piston (view B). At this point, the compression stroke begins. The piston compresses the air within the cylinder, causing its pressure and temperature to increase rapidly. At a point several degrees before the piston reaches TDC, fuel is in-jected into the cylinder. The heat of the com-pressed air ignites the fuel and the process of combustion causes a drastic increase in tempera-ture and pressure within the cylinder (view C). The rapid expansion of combustion gases forces the piston to move down for the power stroke (view D). The power stroke continues until the exhaust valves are open (view E). This part of the cycle indicates the beginning of the exhaust event. In summary, a 2-stroke engine cycle will have one power event per cylinder for every 360 degrees of rotation of the crankshaft.