Chapter 4 Principles of Engine Operation 63 Copyright Goodheart-Willcox Co., Inc. major subassemblies of a typical diesel engine. The largest engine subassembly is the cylinder block and its related parts, such as the cylinder liners (if used), pistons, con- necting rods, crankshaft, vibration damper, flywheel, cam- shaft, bearings, and camshaft followers. In order to work on the internal components of the cylinder block, the top and bottom of the engine must be removed. All of the parts above the cylinder block are nor- mally referred to as the head group. This group includes the cylinder head, rocker arm assemblies, rocker arm covers, engine brake, air intake manifold, and exhaust manifold. When the crankshaft must be removed from the cyl- inder block, the gear cover, oil pan, and flywheel housing must also be removed. The cylinder head, oil pan, gear cover, and flywheel housing are all bolted to the cylinder block. Most joining surfaces must be sealed with gaskets that are designed to withstand the temperatures and pres- sures generated inside the engine. Accessory items mounted to the outside of the engine include the water pump, fuel pump, oil cooler, turbocharger, supercharger, charge air cooler, and fan hub. A number of other systems are needed for engine operation. These include the cooling, exhaust, fuel metering, fuel injection, and governing systems. All of these systems will be covered in greater detail in the chap- ters related to their service and repair. The parts of a diesel engine are so closely related that it is almost impossible for any single part to wear without affecting other components. A skilled diesel technician must be able to visualize all of the engine parts in relation to each other. Cylinder Block and Cylinder Liners The cylinder block is the foundation of the diesel engine, Figure 4-2. It supports all of the other engine com- ponents. The block contains openings for each cylinder, internal passages for coolant and lubricating oil, bores for the crankshaft and camshaft, and openings for the push tubes or push rods and cam followers. Cylinder openings are cast in the block. In most diesel engines, cylinder liners (sleeves) are placed in these open- ings to form the walls of the combustion chamber. The top of each liner is sealed by the cylinder head, while the bottom is sealed by the piston. As the engine operates, the moving pistons contact the liners, not the cylinder block itself. When the liner becomes worn, it can be removed from the block and replaced. This is one of the keys to long diesel engine life. Dry and Wet Liners Cylinder liners are classified as either dry or wet liners. Dry liners fit into the cylinder counterbore in the block and do not make direct contact with the circulating coolant in the water jackets. Water jackets are passages in the cylinder block that allow coolant to move around the outside of the cylinder and carry off the excess heat gener- ated by combustion, Figure 4-3. In an engine with dry liners, the heat of combustion must pass through the liner and the cylinder block wall before it reaches the coolant in the water jacket, Figure 4-4A. In contrast, wet liners are inserted into the cylinder block and make direct contact with the coolant. This allows for the quickest possible dissipation of excess cyl- inder heat. The top of the liner is designed to form a tight seal with both the cylinder block and the cylinder head. The bottom of the liner is sealed with an O-ring that pre- vents the coolant in the water jacket from leaking into the oil pan, Figure 4-4B. Pistons The pistons move up and down in the cylinder liners. When the fuel ignites in the combustion chamber, the hot expanding gases force the piston down. The downward motion of the piston is transferred through a connecting rod to the engine’s crankshaft. See Figure 4-5. Figure 4-2. Typical inline six-cylinder cylinder block. Note the various openings and passages. (Cummins Engine Co., Inc.) Figure 4-3. The arrows indicate coolant circulation around the outside of the cylinder bores. Coolant also travels through the cylinder head to keep the valves cool. (Cummins Engine Co., Inc.)