202 Auto Engine Performance and Driveability Copyright by Goodheart-Willcox Co., Inc. corrosion. Exhaust systems with a catalytic converter tend to last longer, since the extra heat produced by the converter prevents condensation and the contaminants exit the sys- tem. Dual exhaust systems run cooler than single exhaust systems and are more prone to rust. Non-Stock Exhaust System Alterations Many people modify the exhaust system to gain extra horsepower. Vehicles with V-type engines and single exhaust systems can sometimes be modified with a dual exhaust system. Another common modification is to replace the entire exhaust system with a system that has larger-diameter pipe. Perhaps the most common modification is to replace the stock muffler with a performance muffler. Another common modification to the exhaust system is to replace the stock exhaust manifolds with headers. Headers are designed to reduce exhaust backpressure and increase engine power. The best headers use the scavenging principle in which exhaust pulses from each cylinder are routed to produce a suction effect that further reduces back- pressure. While headers are usually aftermarket products, many vehicle manufacturers have designed cast iron exhaust manifolds to obtain some of the flow characteristics of a header. A few manufacturers equip their high- performance vehicles with cast iron headers as original equipment. Note: Many states and municipalities restrict certain non-stock exhaust parts to off-road use only due to emissions and noise. Modifying the exhaust system or installing aftermarket parts that are not approved by the manufacturer are considered a form of emission control system tampering. Also, the use of non-stock exhaust system parts or modifications could void any warranty that may apply to the vehicle. Be sure to check all state and local regulations and manufacturer stipulations before replacing any exhaust system part. Emissions Inspections In certain areas, inspection and maintenance (I/M) programs require all cars and light trucks to be inspected and tested for exhaust emissions. This is designed to ensure exhaust emissions are kept low in these regions. The test procedure and interval varies from state to state. Many states simply check for the presence of the catalytic converter and fuel inlet restrictor and perform an exhaust gas analysis. In other states or regions where high airborne-pollution rates are a problem, a more stringent program called IM 240 is used. More information on emissions testing programs is given in Chapter 18. Summary The three pollutants the emission control system must reduce are carbon monoxide (CO), unburned hydrocarbons (HC), and oxides of nitrogen (NO X ). CO is caused by a rich air-fuel mixture. HC is caused by unburned fuel and any lubricating oil that escapes from the crankcase as a vapor. NO X is caused by high combustion-chamber temperatures that force oxygen to combine with nitrogen. Internal components of the engine have been rede- signed over the years to balance the needs of performance, emissions, and driveability. Some of these modifications were first developed many years ago, but are still used today. Many parts of the emission control system are external to the engine. The PCV system reduces air pollution and keeps the internal parts of the engine clean. Heated oxy- gen sensors allow the sensor to begin functioning earlier. Thermostatic air cleaners heat the incoming air when the engine is cold, using exhaust manifold heat. The throttle body has coolant passages to warm the incoming air. The evaporative emission control system prevents gasoline vapors from escaping into the atmosphere from the fuel system. Air-injection systems cause unburned fuel in the exhaust manifold to combust. Using an EGR valve directly reduces combustion temperatures and NO X formation by recycling exhaust gases into the intake manifold. The exhaust system includes the exhaust manifold, exhaust pipe, catalytic converter, muffler, and tailpipe. The exhaust system removes exhaust gases and heat from the engine. It also muffles the noise of the escaping exhaust gases without causing excessive flow restrictions. The catalytic converter in the exhaust system is also considered part of the emission control system. It helps to clean up the exhaust gases by reducing HC, CO, and NO X . Most cur- rent vehicles use a three-way catalytic converter. On diesel engines, the converter has a particulate (soot) trap and cata- lysts to reduce NO X . Review Questions—Chapter 10 Answer the following questions using the information provided in this chapter. 1. Name the three pollutants that the emission control system reduces. 2. What air-fuel ratio does the ECM try to maintain under all engine conditions? (A) 8:1. (B) 12:1. (C) 14.7:1. (D) 16.5:1.