LO 9-5 Identify and describe the purpose, construction, and operation of components used in the valve train
1. Valve Train Components
1.1. Poppet Valves
LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train. |
|
Poppet Valves Poppet valves are the most common type of valve used in diesel engines. A large head and a long stem characterize poppet valves. The head of the intake valves is usually larger than exhaust valves because pressure forcing the air charge into the cylinder is much lower than forces pushing exhaust gases out Figure 25. On diesel engines, the valves are often designed in two pieces that are spun-welded together. The weld, though durable under most operating conditions, can break when valves become overheated. The valve head will separate from the stem and the head will embed itself in a piston if a weld failure occurs Figure 26. The valve stem is typically chrome-plated steel to reduce its requirement for lubrication and to minimize wear and friction. The head is made of specialized alloys to withstand high temperatures and mechanical forces that slam the head into the valve seat. These materials improve the valve’s wear resistance, the ability to withstand high temperature, and the absence of lubricant when using dry fuels, such as natural gas. Stellite ® , Crutonite ® , or Inconel ® are alloys commonly used to make the valve head. These materials are made from a combination of chromium, nickel, tungsten, and cobalt to give the metal great strength as well as corrosion and wear resistance at high temperatures. Crutonite ® is the latest valve material developed with less than half the amount of costly nickel as traditional high-temperature alloys. |
FIGURE 09-25: Nomenclature associated with poppet valves. Note that valve seats are typically cut with three angles to improve gas flow across the seat. |
FIGURE 09-26: A broken weld of a two-piece valve is evidenced by a valve head embedded in the top of a piston. Overheating a valve can cause the weld to break. |
FIGURE 09-27: Two-valve engines use a larger intake valve and a smaller exhaust valve. Fuel distribution is not symmetrical and creates more noxious emissions compared with a four-valve engine. |
Two and four valves per cylinder are common, but a number of engines may use three and five valves. An extra intake valve, usually the third valve (such as used in the Caterpillar 3126B or C9), is used to improve intake airflow for better combustion efficiency, power density, and lower emissions. Four smaller intake and exhaust valves rather than two valves—a single large intake and a single exhaust valve—are used in central direct injection combustion chambers to accommodate a centralized location for the injector nozzle. A centrally located injector provides better uniform fuel distribution in the combustion chamber for lower emissions Figure 27. This emissions reduction feature of four-valve cylinder heads explains why most diesel engines now use four valves rather than two. The downside of four-valve technology is that it adds more complexity to the valve train, and smaller valve heads with less mass cannot withstand intermittent peak exhaust temperatures as well as a larger valve head used in two-valve engines. Note that in two-valve engines, the injector is offset to one side of the combustion chamber and leaned over to make room for the larger single exhaust and intake valves.
©2026 - CDX |