68 Diesel Technology Copyright Goodheart-Willcox Co., Inc. into a reciprocating motion. This motion is then transmitted through the push tubes (or push rods) and rocker arms to the valves, and in some engines, the fuel injectors. There is one cam follower for each lobe on the camshaft. The push tubes or push rods connect the cam fol- lowers to the rocker arms (levers). While some engines use solid push rods, many diesels use hollow push tubes made of high-strength alloys. The rocker arms are levers that transfer motion from the push tubes to the valve stems. Cylinder Head The cylinder head, Figure 4-16, is a large casting that bolts to the cylinder block and forms the top of the com- bustion chamber. The cylinder head contains an elaborate system of passageways that permit coolant to circulate, intake ports (four-cycle engines) that allow air to enter the cylinder, and exhaust ports which channel the burned combustion gases from the cylinder. The cylinder head also has openings for each cylinder’s fuel injector. See Figure 4-17. The cylinder head supports the valve seats, valve guides, valve springs, fuel injectors, push rods, and rocker arm assemblies. Since coolant enters the engine from the bottom and travels toward the top, the cylinder head is the last major engine component to receive coolant. By the time the coolant reaches the head, it has very little cooling capacity left. Nevertheless, a constant, high volume of coolant flowing through the cylinder head cools it enough to keep it from warping or distorting. This is why the cyl- inder head, head gasket, valves, and related parts are often the first components damaged when an engine overheats. When the cylinder head is bolted to the cylinder block, it must form an airtight and watertight seal. A head gasket is installed between the head and block to maintain this seal. Valves Valves are used to seal the air passages in the cylinder head. In a four-cycle engine, the intake valves open to admit fresh air into the cylinders and then close to hold the air in during the compression, power, and exhaust strokes. The exhaust valves are closed during the intake, compres- sion, and power strokes, but are opened during the exhaust stroke to allow the exhaust gases to exit the cylinder, Figure 4-18. Two-, Three-, and Four-Valve Heads When an engine is equipped with only two valves for each cylinder, the cylinder head is referred to as a two- valve head. In order to provide greater airflow through the cylinder, many engines are designed with three or four valves per cylinder. The cylinder heads for these engines are commonly called three- and four-valve heads. Valves operate at very high speeds, opening and closing many times per second. They are also subjected to operating temperatures that often exceed 1400°F (760°C). Valves must be made of extremely high-grade metals in order to withstand these high temperatures. In most cases, Figure 4-16. Typical cylinder head. Note the numerous passages cast into the head. (Cummins Engine Co., Inc.) Air Fuel Coolant Exhaust Figure 4-17. The cylinder head passages are used for routing combustion air and coolant. The head also has mounting openings for the fuel injectors. (Cummins Engine Co., Inc.) Air Exhaust Combustion chamber Figure 4-18. Action of intake and exhaust valves in a four-stroke engine.