Chapter 4 Principles of Engine Operation 73 Copyright Goodheart-Willcox Co., Inc. changes direction at BDC and moves up in the cylinder, displacing all exhaust gases through the exhaust ports. As the piston approaches TDC, the intake valves open and the process repeats itself. Figure 4-27 illustrates the duration of the various piston strokes, the opening and closing points of intake and exhaust valves, and the duration of the positive valve overlap for a specific engine. Piston Position All engines are timed so their pistons will be at different stages in their firing cycles anytime it is running. As men- tioned earlier, each firing cycle equals 720° of crankshaft rotation. A six-cylinder, four-cycle engine will have its pis- tons set 120° (720° divided by 6) apart in the firing cycle. Figure 4-28 illustrates the relative piston firing positions in a six-cylinder, inline, four-stroke engine with a firing order of 1-5-3-6-2-4. As you can see, all six cylinders are in different stages of the combustion cycle. Since a 4-stroke engine requires two full revolutions of the crankshaft to complete a cycle, most diesel engines will have two cylinders with their pistons at exactly the same position but on opposite strokes. The cylinders that share the same piston position are known as companion cylinders. For instance, in the example of the 6 cylinder engine with the firing order of 1-5-3-6-2-4, the piston for cylinder 1 will be at TDC of the compression stroke at the exact same time as cylinder 6 will be at TDC of the exhaust stroke. Cylinders 1 and 6 are companion cylinders. Cylinders 2 and 5, and cylinders 3 and 4 are also companion cylinders. TDC BDC Figure 4-27. Duration of the various piston strokes and valve positions in a typical four-cycle engine. (Detroit Diesel Corp.)