64 Diesel Technology Copyright Goodheart-Willcox Co., Inc. The piston acts as a movable seal that forms the bottom of the combustion chamber. Grooves cut into the side surface of the piston are fitted with piston rings that seal in the com- pression and combustion pressure, and also control the lubri- cating oil film on the wall of the cylinder liner, Figure 4-6. The top piston ring is a compression ring, which forms a seal with the cylinder wall. The middle or intermediate piston rings control about 10% of the compression and combustion pressures, but their main job is to reduce oil consumption inside the cylinder. The bottom piston ring is an oil control ring, which distributes the lubricating oil over the wall of the cylinder and prevents the oil from entering the combustion chamber. Crankshaft The crankshaft converts the reciprocal (up and down) motion of the pistons into rotary (turning) motion. This rotary motion is transferred to the flywheel and eventually to the transmission or driveline. Since the crankshaft must transmit all of the power produced by the engine, it is the engine’s strongest part, Figure 4-7. The crankshaft protrudes through the cylinder block at the front and rear of the engine. Oil seals are used at these Cylinder block Cylinder block A B Liner is only the wall of the cylinder Liner is the cylinder Piston Piston O-rings Coolant passages Figure 4-4. The two types of liners used in diesel engines. A—Dry liners are thin and do not make contact with the coolant. B—Wet liners are much thicker since they do not have the walls of the block for support. They come into direct contact with the engine coolant. Piston ring Piston ring Connecting rod Figure 4-5. The piston, connecting rod, and crankshaft work together to change the energy of combustion into usable motion. (Cummins Engine Co., Inc.) Figure 4-6. The diesel engine uses three types of piston rings. The top ring is the compression ring and is designed to contain combustion chamber pressures. The middle compression/ scraper ring also holds in compression and helps to scrape excess oil from the cylinder walls. The lower ring is called the oil ring or oil control ring. Its job is to keep excess oil from reaching the combustion chamber. Compression ring Oil control ring Intermediate ring points to prevent the oil inside the block from escaping. The crankshaft has a number of key parts and features, including journals, oil galleries, and timing gears. Figure 4-7. Typical diesel engine crankshaft. Note the names of the various crankshaft parts. (Cummins Engine Co., Inc.) Connecting rod journals Crankshaft throws Main bearing journals