Document baRZaQVZo52mvMNgY04dwb23k

39 CHAPTER 5/THE EMISSIONS SYSTEM i A pelletized type converter is a shell that con preventing the exhaust gases from contacting it. tains a bed of small catalyst-coated ceramic The result is a less active chemical reaction or, beads. The exhaust gases are forced to flow in the event of severe lead contamination, no around these beads in their path to the outlet of chemical reaction at all. the exhaust system. Sometimes, returning to unleaded fuel will There are four basic types of converter allow the converter to regain some of its func functions--oxidation, reduction, dual bed three- tional efficiency. The efficiency regained way and single-bed threeway. Each one of these depends on how much and how long leaded converter functions (or a combination of two or fuel has been used. more) is designed to meet the requirements of It is a violation of federal law to sell leaded the specific emission control system. fuel for use in vehicles equipped with con Oxidation--The carbon monoxide (CO) and verters. To help regulate this, converter- hydrocarbons (HC) that have not been equipped cars and trucks have a restrictor in the totally consumed by combustion are chan fuel tank filler that will only allow the use of an neled into an oxidation converter and then unleaded gasoline pump nozzle. Unleaded gaso changed into harmless water (H20) and car line pump nozzle spouts are smaller in diameter bon dioxide (COz). than are spouts of leaded gasoline nozzles. Reduction--The converter reduces the nitrous oxides (NOx) produced during combustion into nitrogen and carbon diox ide (COz). Reduction and Oxidation (Dual Bed)--A reduction catalyst requiring an oxygen defi cient environment is followed by an oxida tion catalyst requiring an oxygen rich environment. Threeway--Reduction and oxidation func tions take place simultaneously. Used only on cars with electronic fuel injection, the mixture cycles from rich to lean and the converter operates alternately in a reduc tion mode when the mixture is rich and in an oxidation mode when the mixture is lean. Heat and Engine Conditions Heat is a normal part of converter operation. The optimum temperature for high converter efficiency is 1500 degrees F. If the temperature in the catalyst area rises, however, the substrate of the converter may break up or melt. Poor engine conditions usually cause this problem. Engines in need of a tune-up will deliver a too-rich exhaust gas to the converter, having either not enough air or too much fuel. When this rich fuel mixture reaches the catalyst, the converter becomes a furnace. The catalyst converts the pollutants, heat is released in the process, and combustion takes place while raw fuel is present. The additional heat Converter Problems A catalytic converter can be harmed or des troyed in many ways. Under normal conditions, a catalytic converter will last at least 50,000 miles, or even longer. But with the wrong fuel, excessive combustion heat, or an incorrect airfuel mixture, a catalytic converter can be seri ously damaged or destroyed. ruins the converter substrate. A similar problem can be caused by spark plugs that misfire. When more than two spark plugs misfire simultaneously, raw fuel is pumped into the converter. If this misfiring con dition occurs over an extended period, the raw fuel causes the internal temperature of the con verter to rise quickly. This can lead to a melt down, as we've already described. Leaded Fuel Even an engine in good operating condition can have converter problems. These are nor Catalytic converters must be used with mally caused by a severe service condition, such unleaded fuel only. Lead and phosphorus will as a long idling period. More heat builds up everely reduce the catalyst effect and perfor when idling; so if more than ten minutes of mance. When leaded fuel is used, lead coats the idling is required it's usually better to turn the catalyst in the converter and forms a barrier, engine off and restart it later.