184 Auto Engine Performance and Driveability Copyright by Goodheart-Willcox Co., Inc. This chapter concentrates on the emission control and exhaust systems. Emission control systems are often inspected as part of state emissions inspections. Other inspections are performed in some regions as part of feder- ally mandated inspection and maintenance programs. In addition, almost all emission control systems can fail in a way that affects vehicle driveability. The driveability techni- cian must know how these systems operate. Note: Carbon dioxide (CO 2 ) is a naturally occurring gas considered harmless in normal concentrations. Increasing amounts of in CO 2 in the atmosphere, however, may be contributing to global warming. Although CO 2 emission controls may be developed in the future, the only current method of reducing tailpipe CO 2 emissions is to reduce fuel consumption. Common Pollutants The three pollutants that the emission control system reduces are carbon monoxide, hydrocarbons, and oxides of nitrogen. Carbon monoxide (CO) is an odorless, colorless, poisonous gas. It is a product of incomplete combustion. When a fuel is burned in the atmosphere, each carbon atom combines with two oxygen atoms to form carbon dioxide (CO 2 ). CO 2 is a relatively safe compound. In the combustion chamber, however, oxygen is scarce. This causes each car- bon atom to combine with only one oxygen atom during the burning process, forming carbon monoxide. A rich air-fuel mixture leads to CO emissions. Most hydrocarbons (HC) emissions are the result of unburned fuel that passes through the engine and enters the atmosphere. However, any lubricating oil that escapes from the crankcase as a vapor also results in HC emissions. Oxides of nitrogen (NO X ) emission is caused by exces- sively high combustion chamber temperatures. This forces oxygen to combine with nitrogen, forming NO X . In sunlight, oxides of nitrogen combine with unburned hydrocarbons to form the air pollution called smog. Emission Control Systems The first emission control devices were grafted onto existing engines. In many cases, there was not a good under- standing of how these controls would affect other engine systems. Early emission control devices often lowered one type of emission, while increasing another. The air-fuel ratio was sometimes adjusted lean to eliminate CO, but the lean mixture caused the engine to misfire, increasing HC levels. Increasing the temperature in the combustion chambers reduced HC, but created more NO X and caused detonation. One emission control device often created a need for another device to overcome new driveability or emission problems. Over the years, manufacturers have developed a com- prehensive understanding of emissions and designed a fully integrated emission control system. Now, emission control devices work together with each other and with the basic engine systems. Much of this has been accomplished by the use of engine-control computers. The ECM manages the emission control system and fuel system to maintain a stoichiometric air-fuel ratio, which is 14.7:1, while allowing good starting, idling, accelerating, decelerating, and cruis- ing in all weather conditions, Figure 10-1. The ECM keeps the ignition timing advanced as far as possible for good fuel economy and power, but not so far as to cause excessive emissions or spark knock. The action of the ECM, fuel system, cooling system, and emission control system keeps the combustion chamber hot enough to lower HC levels, but not hot enough to cause excessive NO X or pinging. Some manufacturers provide lambda specifications. Lambda is simply another way to express the air-fuel ratio. A lambda of 1 is equivalent to a 14.7:1 air-fuel ratio. Leaner air-fuel ratios have a lambda that is greater than 1, while richer ratios have a lambda that is less than 1. The ECM precisely controls ignition timing for maxi- mum efficiency. ECM control of automatic transmission/ transaxle shifting and the air conditioner compressor clutch Figure 10-1. A stoichiometric air-fuel ratio must be maintained for optimum fuel economy and low exhaust emissions. A nar- row window around 14.7:1 provides the best performance. Note how emissions increase as the air-fuel mixture goes lean or rich. Oxygen (O 2 ) and carbon dioxide (CO 2 ) are not considered pol- lutants, but they can be measured as an aid to diagnosis. Rich CO2 NOX O2 CO HC Lean 14.7:1 20% 0 PPM Stoichiometric air-fuel ratio 1000 PPM 2000 PPM 3000 PPM 4000 PPM 15% 25% 10% 5% 0%