17. Camshafts and the Valve Train

The poppet valve, as previously detailed in Fig. 9.5, is now used universally in four-stroke vehicular engines—both to draw fresh charge into the cylinder, and to exhaust the spent products. The valves face an especially harsh environment. Because they are exposed directly to the combustion chamber, and provide very restrictive heat transfer paths they operate at especially high temperatures. The demand for rapid opening and closing results in high impact loads, and a requirement for high hardness valves and seats. The combination of high hardness and high temperature requirements drives the selection of special steel alloys, typically with high nickel content for both the valve head and the valve seat. Most automobile valves are made as a single piece, while the valves in heavy-duty engines generally have the nickel alloy head inertia welded to a mild steel stem. Hollow stem two-piece valves are generating interest in automobile applications, for savings of both weight and cost. A valve spring and retainer assembly as shown in Fig. 9.5 completes the installation. The retainer is typically stamped from mild steel, and holds the spring in a partially compressed position with two hardened steel keepers fitted near the top of the valve stem.