Chapter 15 Fuel and Emission Control Systems 317 Copyright by Goodheart-Willcox Co., Inc. Any factor that reduces the amount of oxygen present during combustion increases carbon monoxide emissions. For example, a rich air-fuel mixture increases CO levels. As the mix- ture is leaned, CO emissions are reduced, Figure 15-17. Oxides of Nitrogen (NOx) Oxides of nitrogen (NO X ) are emissions produced by high temperatures during combustion. Air consists of about 80% nitrogen and 20% oxygen. With enough heat, above approximately 2500°F (1370°C), nitrogen and oxygen in the air-fuel mixture combine to form NO X emissions. Particulates Particulates are solid particles of carbon soot and fuel additives that blow out of a vehicle’s tailpipe. Carbon particles make up the largest percentage of these emissions. About 30% of all particulate emissions are heavy enough to settle out of the air in a relatively short period of time. The other 70%, however, can float in the air for an extended period of time. Particulate emissions are not usually a prob- lem with gasoline engines. Engine Modifications for Emission Control The best way to reduce exhaust emissions is to burn all of the fuel inside the engine. For this reason, several engine modifications have been introduced to improve efficiency. Today’s engines can have the following modifications to lower emissions: • Lower compression ratios. Lower compression stroke pressure reduces combustion temperatures and NO X emissions. • Leaner air-fuel mixtures. In a lean mixture, more air is present to help all of the fuel burn. This lowers HC and CO emissions. • Heated air intake systems. These systems speed engine warmup and permit the use of leaner mixtures during initial startup. • Smaller combustion chamber surface volumes. A smaller chamber increases combustion efficiency by lowering the amount of heat dissipated out of the fuel mixture. Less combustion heat enters the cylinder head and more heat is left to burn the fuel. This results in reduced HC emissions. • Increased valve overlap. A camshaft with more overlap dilutes the incoming air-fuel mixture with inert exhaust gases. This reduces peak combustion temperatures and, thus, NO X emissions. • Hardened valves and seats. Hardened valves and seats are needed to prevent excessive wear when using unleaded fuels. • Wider spark plug gaps. Wider gaps produce hot- ter sparks that can ignite hard-to-burn, lean air-fuel mixtures. • Reduced quench areas. When the piston is too close to the cylinder head, there is a tendency to quench (put out) combustion, which results in increase emissions due to unburned fuel. Modern engines have cylinder heads and pistons designed to prevent high quench areas. • Higher operating temperatures. If the metal parts in an engine are hotter, less combustion heat will dissipate out of the burning fuel and into the parts of the engine. This improves combustion and reduces HC and CO emissions. Higher-temperature thermostats are used to achieve higher operating temperatures. Vehicle Emission Control Systems Several systems are used to reduce the pollution pro- duced by the engine and its fuel system. These include: • PCV system. • Heated air inlet system. • Evaporation emissions control system. • EGR system. • Air injection system. • Catalytic converter. Variations of these systems and computer control are all used to make the modern engine very efficient. Positive Crankcase Ventilation (PCV) The positive crankcase ventilation (PCV) system keeps engine crankcase fumes out of the atmosphere. It uses engine vacuum to draw toxic blowby gases into the intake manifold for reburning in the combustion chambers, Figure 15-18. Blowby gases can cause: • Air pollution. • Corrosion of engine parts. • Dilution of engine oil. • Formation of sludge. The PCV system is designed to prevent these problems. It helps keep the inside of the engine clean and also reduces air pollution. A PCV valve is commonly used to control the flow of air through the crankcase ventilation system. It may be located in a rubber grommet in the valve cover or in a breather opening in the intake manifold. The PCV valve changes airflow for idle, cruise, accel- eration, wide open throttle, and engine-off conditions. This is shown in Figure 15-19. In case of an engine backfire, the PCV valve plunger is seated against the body of the valve. This keeps the backfire from entering the crankcase and igniting the fumes within. Heated Air Inlet System The heated air inlet system, also called a thermo- static air cleaner system, speeds engine warmup and keeps the temperature of the air entering the engine constant. A thermostatically controlled air cleaner maintains a constant temperature of air entering the engine, which improves combustion. An air temperature control valve is used to open and close the inlet to the air cleaner, Figure 15-20. By maintaining a more constant inlet air temperature, the