Chapter 4 Principles of Engine Operation 75 Copyright Goodheart-Willcox Co., Inc. open when the intake ports are also open. This creates a strong upward flow through the chamber that pushes out the exhaust gases. Up to four exhaust valves per cylinder are used to maximize scavenging. Another major difference between two- and four-cycle engines is that two-cycle engines use a positive displacement blower assembly to supply a strong airflow to the engine, Figure 4-29. The blower supplies the airflow needed for com- bustion, exhaust scavenging, crankcase ventilation, and for internal engine cooling. The blower supplies air through an air box built into the engine block. This air box runs the length of the block and surrounds all of the cylinder liners. It is important to note that two-stroke diesel engines are no longer being manufactured and are becoming less common in the industry though they can still be found on many pieces of older equipment. Current demands in fuel efficiency and emis- sions standards seem to have led the diesel engine industry to rely on the four-stroke design for today’s modern diesel engines. Some manufacturers are still experimenting with two-stroke engines in an attempt to comply with emissions regulations but, at the current time, their use in the diesel industry is rare. Operation In a two-cycle engine, as the piston moves down and uncovers the intake port, the blower supplies a strong flow of air into the cylinder whenever the intake ports are not blocked by the piston. Air flows in one direction only—from the intake ports toward the exhaust valves at the top of the cylinder. Air intake occurs as part of the compression stroke in a two-cycle engine. At the start of the compression stroke, the piston is at bottom dead center, the intake ports are open, and the exhaust valves are open. Fresh air rushes into the cylinder. The force of the air entering the cylinder pushes any remaining exhaust gases out of the open exhaust valves, Figure 4-30A. As the piston moves upward, it covers the intake ports. The exhaust valves are timed to close at this point in the cycle. There is now a full charge of clean air in the cylinder and compression begins as the piston continues moving toward top dead center, Figure 4-30B. Slightly before TDC, fuel is injected into the cylinder and combustion begins, Figure 4-30C. As in a four-cycle engine, the piston passes TDC and moves downward on its power stroke. The power stroke continues until the piston is about halfway down the cylinder. At this point, the exhaust valves open, and the burned gases escape through the exhaust ports. See Figure 4-30D. As the piston continues downward, it uncovers the intake ports, allowing fresh air to rush into the cylinder to scavenge the remaining exhaust gases. The entire combustion cycle is completed in a single (360°) revolution of the crankshaft. Two-Cycle Engine Valve Timing A typical valve timing diagram for a two-cycle engine is illustrated in Figure 4-31. Remember that all actions occur during a single crankshaft revolution. As in four-cycle Figure 4-30. Two-stroke engine cycle. A—Intake. B—Compression. C—Power. D—Exhaust. All four cycles take place in one crankshaft revolution. (Detroit Diesel Corp.) A B C D Air Air Air Air Fuel Air Exhaust