190 Auto Engine Performance and Driveability Copyright by Goodheart-Willcox Co., Inc. External Systems and Components Many external emission control systems are installed on engines. Some of these systems have been in use for many years. These systems are discussed in the following sections. However, all emission controls are part of a single, integrated emission control system. Positive Crankcase Ventilation (PCV) The PCV system is one of the earliest emission-control devices installed on engines. The positive crankcase ventilation (PCV) system reduces HC emissions and keeps the internal engine components clean. Positive crankcase ventilation is a controlled vacuum leak from the engine crankcase into the intake manifold. This leak draws gases from the crankcase into the intake manifold and then into the engine where they are burned. PCV systems are used on both gasoline and diesel engines. The PCV system is necessary because piston rings, although providing a tight fit to the cylinder wall, do not perfectly seal. Pressure in the combustion chamber forces some blowby gases into the crankcase. These blowby gases are composed of unburned fuel (HC), exhaust gases, and water vapor. If not removed, blowby gases will create condi- tions that result in engine damage. Blowby gases are also explosive. A typical PCV system is shown in Figure 10-10. In a typical system, a large vacuum hose connects the crank- case to the intake manifold. A flow-control valve called a PCV valve is installed in the hose. Spring pressure tries to open the PCV valve, while manifold vacuum tries to keep it closed. The PCV valve is held closed when manifold vacuum is high at idle and during light loads. During these condi- tions, piston speed is low and the amount of gases blowing by the rings is low. The small amount of blowby gases flows through a small hole in the valve. As engine load increases, piston speed increases. As a result, more gases blow by the rings. At the same time, manifold vacuum begins to decrease. As the vacuum drops, spring pressure centers the shuttle in the valve. This allows more blowby gases to flow into the intake manifold. If the engine backfires, a pressure surge occurs in the intake manifold. This pressure pushes the shuttle onto a Figure 10-9. A—Low engine temperatures allow unburned fuel to collect on the combustion chamber walls. Some of this unburned fuel escapes into the exhaust as hydrocarbon emis- sion. B—Using a high-temperature thermostat raises the engine temperature, which prevents the fuel from condensing. Unburned fuel (HC) collects on cooler cylinder head, walls, and piston heads Fuel completely burned A B Figure 10-10. The PCV system draws explosive gases out of the crankcase, but prevents them from entering the atmosphere. Combined blowby gases and air Air cleaner housing Sealed oil filler Fresh air intake Valve cover Intake manifold PCV valve