Exhaust System: Description and Operation
Exhaust SystemThe vehicle exhaust system consists of the following components:
^ muffler
^ tailpipe
^ muffler inlet pipe
^ muffler outlet pipe and resonator
^ catalytic converter
^ heat shields
^ hangers
^ hanger insulators
^ flexible pipe
The exhaust system provides an exit for exhaust gases and reduces engine noise by passing exhaust gases through the Three-Way Catalytic Converter (TWC), muffler inlet pipe and resonator, and muffler. Rubber exhaust hanger insulators attach the exhaust system to mounting hangers.
Three Way Catalytic Converter (TWC)
The catalytic converter and exhaust system work together to control the release of harmful engine exhaust emissions into the atmosphere. The engine exhaust gas consists mainly of nitrogen, carbon dioxide and water vapor. However, it also contains carbon monoxide, oxides of nitrogen, hydrogen, and various unburned hydrocarbons. Carbon monoxide, oxides of nitrogen, and hydrocarbons are major air pollutants, and emission into the atmosphere must be controlled.
The exhaust system consists of an exhaust manifold combined with a pre-catalytic converter, Upstream Heated Oxygen Sensor (HO2S 11), (located just in front of the pre-catalytic converter), Three Way Catalytic Converter (TWC), downstream heated oxygen sensor (HO2S 12) (located in front of the TWC), a muffler and an exhaust tail pipe.
Three Way Catalytic Converter (TWC):
The engine exhaust consists mainly of Nitrogen (N2); however, it also contains Carbon Monoxide (CO), Carbon Dioxide (CO2), Water Vapor (H2O), Oxygen (O2), Nitrogen Oxides (NOx), and Hydrogen (H2) as well as various Unburned Hydrocarbons (HC). Three of these exhaust components - CO, NOx, and HC - are major air pollutants, so their emission to the atmosphere must be controlled.
The Three Way Catalytic Converter (TWC) (Figure 2), mounted in the engine exhaust system, works as a gas reactor to convert and reduce the pollutant levels to within legally prescribed limits.
The TWC reduces the air pollutants HC, CO, and NOx. The TWC removes these pollutants from the exhaust gases by means of a chemical reaction. The remaining gases are transferred to the muffler.
The catalyst metals are thinly coated onto and supported by a honeycomb shaped high temperature ceramic, mounted inside the converter shell. The result is a highly effective converter design having minimum restriction to exhaust gas flow and good durability.
The Three-Way Catalytic Converter (TWC) plays a major role in the emission control system. The TWC operates as a gas reactor. Its catalytic function is to speed the heat-producing chemical reaction of components in the exhaust gases in order to reduce air pollutants.
The catalyst material inside the TWC consists of a ceramic substrate. The substrate is coated with a high-surface area alumina and contains chemically active palladium. The front catalytic converter is integrated into the left side exhaust manifold.
Pre-Catalytic Converter:
Pre-Catalytic Converter
The pre-catalytic converter (Figure 3) is designed to reduce HC, CO, and NOx.
The pre-catalytic converter is mounted ahead of the TWC, causing it to warm up faster upon vehicle start up. This results in less air pollutants escaping during warm up.
Typical Heated Oxygen Sensor(HO2S):
Heated Oxygen Sensors
The Villager is equipped with both upstream and downstream Heated Oxygen Sensors (HO2S) (Figure 1). The upstream heated oxygen sensor (HO2S 11) is used to control engine fuel flow. The sensor detects the concentration of oxygen in the exhaust gas and sends a signal to the Powertrain Control Module (PCM). In a rich condition, the HO2S will output a voltage of 0.6 volts or greater, and in a lean condition a voltage of 0.4 volts or less will be output.
The downstream heated oxygen sensor (HO2S 12) is used primarily for monitoring the efficiency of the catalytic converter system. The HO2S 12 works in a similar way to the HO2S 11 but only indicates to the PCM that the catalyst is functioning properly. In the event that the HO2S 11 fails, fuel control will be switched over to the HO2S 12 and the Malfunction Indicator Lamp (MIL) will be illuminated. This allows the vehicle to operate with fewer emissions, even with a sensor failure.
Precautions
WARNING: The normal operating temperature of the exhaust system is very high. Never work around or attempt to repair any part of the exhaust system until it has cooled. Use special care when working around the three-way catalytic converter. The three-way catalytic converter heats to a high temperature after only a short period of engine operation.
WARNING: Exhaust gases contain carbon monoxide which can be harmful to health and is potentially lethal. Exhaust system leaks should be repaired immediately. Never operate the engine in enclosed areas.
In a vehicle that is continually misfueled, lead in the fuel will be deposited in the TWC and completely blanket the catalyst. Lead reacts with platinum to poison the catalyst. Continuous use of leaded fuel can destroy the catalyst and render the TWC useless. The addition of lead to the TWC can also solidify the catalyst, causing excessive back pressure in the exhaust system and possibly causing engine damage.
Extremely high temperatures of 1,100°C (2,012°F) or above due to misfiring or an over-rich fuel/air mixture will cause the ceramic base to sinter or burn, destroying the TWC.
The TWC is designed to provide a long service life. No maintenance is necessary for the TWC. Avoid the following conditions:
^ driving with a misfiring engine
^ continuously running the engine with a misfiring spark plug
^ parking or stopping the vehicle over combustible material such as dry grass (The material can be ignited by the high temperatures of the TWC and muffler.)
Sound Insulators and Shields
Sound insulators and shields protect the vehicle from exhaust system heat and should be inspected at regular intervals to make sure they are not dented or out of position. If a sound insulator and shield is damaged or shows evidence of deterioration, a new one should be installed. The sound insulators and shields for the muffler, muffler pipe, resonator, and TWC pipe are installed separately.
Sound insulators and shields that are attached to the TWC are not repaired other than to install new attaching parts such as clamps or screws, and to remove foreign material that may collect in the shield area.