74 Diesel Technology Copyright Goodheart-Willcox Co., Inc. It is important that the diesel technician understand what one cylinder is doing in relation to the others at any given position of the crankshaft. Knowing the firing order of the engine and the position of each piston at any point in the cycle is important when performing timing and valve adjustments. Two-Stroke Cycle Engines As described previously, a four-cycle engine requires two full turns of the crankshaft and four separate piston movements to complete the intake, compression, power, and exhaust strokes of one combustion cycle. A two-stroke cycle engine completes this sequence of events in only one complete revolution of the crankshaft (360°) and two sepa- rate piston movements. This is done by eliminating the separate intake and exhaust strokes. Two-cycle engines differ from four-cycle engines in a number of ways. Two-cycle diesel engines do not have intake valves. Instead, each cylinder liner is designed with a series of intake ports or holes in its wall. These ports are located about halfway down the length of the cylinder and are open when the piston is at the bottom of its stroke, Figure 4-29. Air is free to flow into the cylinder whenever the piston is not blocking the intake ports. In this way, the piston of each cylinder acts as its own intake valve. Cylinders 1&6 TDC 60 60 60 60 60 ATDC BTDC ABDC Cylinders 2&5 BBDC Cylinders 3&4 BDC Comp. TDC Exhaust ABDC Intake BBDC Power BBDC Comp. ABDC Exhaust TDC 1 2 3 4 5 6 Firing order 1-5-3-6-2-4 60 Figure 4-28. Relative piston firing positions in a six-cylinder, inline, four-stroke diesel engine. (Detroit Diesel Corp.) Exhaust valves Injector Exhaust gases Air Blower Intake port Intake port Figure 4-29. The blower action in a two-stroke engine. The additional compressed air supplies the combustion chamber with fresh air and scavenges any remaining burned gases to the exhaust.