Table of contents: Exhaust system ↓ Emission control systems ↓
Exhaust system
The exhaust gas (EG) exhaust system consists of exhaust manifolds with catalytic converters and a particulate filter (DPF, only on some diesel models), mufflers and exhaust pipe. In addition to rigid fastening in certain places, the exhaust system has rubber mounting rings-hangers. They provide anti-vibration and tension-free fastening of the system in a horizontal position, as well as the necessary distance to the bottom of the car.
Emission control systems
The principle of operation of the engine management system is designed to obtain maximum output from the engine with minimum fuel consumption and content of toxic components in the EG. The following exhaust gas toxicity reduction systems are installed on the vehicles in question: crankcase ventilation system (PCV), fuel vapor recovery system (EVAP, gasoline models only), exhaust gas circulation system (EGR, diesel models only), particulate filter (only some diesel models), as well as lambda probes and a catalytic converter.
To warn of malfunctions of the emission control systems, a warning lamp "MIL" is provided in the instrument cluster (see chapter "Controls and operating techniques"). This K/L lights up if one or more of the following emission limits are exceeded:
Controlled Crankcase Ventilation (PCV) System
To eliminate leaks of unburned hydrocarbons into the atmosphere, the engine is completely sealed. Gases and oil vapors formed in the crankcase are fed into the intake manifold and burn in the cylinders together with the fuel (except for oil vapors retained in the oil separator).
Gases are removed from the crankcase due to the difference in pressure in the crankcase and the intake manifold (crankcase pressure is higher) through the PCV valve, which is opened by vacuum from the intake manifold. The amount of crankcase gases depends on the crankshaft speed.
Evaporative Emissions (EVAP) System
The EVAP system is installed only on gasoline models and is designed to reduce the emission of unburned hydrocarbons into the atmosphere. The main element of the EVAP system is an adsorber with activated carbon granules that adsorb fuel vapors formed in the tank when the car is parked. The filler neck of the fuel tank is hermetically sealed with a cap. Fuel vapors are retained in the carbon adsorber until the ECM signals the purge of the adsorber. During the purge, fuel vapors are fed through the purge valve into the intake manifold, where they mix with the working mixture and then burn in the usual way in the combustion chambers (see Part A).
To ensure normal engine operation at idle and during warm-up, the engine control unit keeps the valve closed. This prevents unburned fuel from entering the catalytic converter (at high idle speed the mixture is too rich). After the engine warms up, the valve begins to open and close, regulating the supply of fuel vapors into the intake tract.
Since diesel fuel is difficult to volatilize, diesel models do not use an activated carbon filter. Air is removed from the fuel tank directly into the atmosphere.
Exhaust Gas Recirculation (EGR) System
This system reduces the amount of nitrogen oxides (NO) in the exhaust gases. To do this, a small portion of the exhaust gases is fed back into the intake manifold through a special EGR valve.
The exhaust gases are introduced into the charge of air entering the intake manifold, where they mix with the air-fuel mixture and reduce the maximum temperature of the gases, reducing the content of nitrogen oxides (NOx) in the exhaust gases.
The EGR valve is a stepper motor with a position sensor, completely controlled by the duty cycle signal from the ECM. The ECM uses the mass of air sucked into the engine as an input signal.
To reduce the temperature of the returning exhaust gases on diesel models, an EGR cooler is used. Reducing the EGR temperature does not have a negative effect on the intensity of air injection at the intake, therefore, optimal fuel characteristics are maintained.
Diesel particulate filter (diesel models)
Note: The use of chemical fuel additives is not permitted (for example, valve cleaner or cold start accelerator). They contain metal compounds that, when burned, produce ash, which leads to contamination of the particulate filter.
The diesel particulate filter absorbs soot particles contained in the exhaust gases of diesel engines. At exhaust gas temperatures below 300°C, soot particles accumulate in the diesel particulate filter. Regeneration of the diesel particulate filter is required to remove them (burn them).
Continuous regeneration occurs in operating ranges in which the exhaust gas temperature exceeds the ignition temperature of soot (more than 350°C). Oxidation of soot begins under the influence of nitrogen dioxide (NO₂) contained in the exhaust gas, without the release of heat. Nitrogen dioxide is formed in the oxidation catalyst (see subsection below) from nitrogen oxide (NO). During combustion, the soot particles are slowly oxidized into carbon monoxide (CO) and carbon dioxide (CO₂). The effect of continuous regeneration is enhanced by the location near the engine.
During cyclic regeneration, the exhaust gases are heated to approximately 600°C to burn off the filtered soot particles. No special additives are required. This process is regulated using signals from the exhaust backpressure sensor and the exhaust temperature sensors. Like continuous regeneration, cyclic regeneration occurs without any noticeable effect on the vehicle's driving characteristics. Cyclic regeneration occurs depending on the engine load. In this case, the intake air flow is specifically limited by the throttle valve in combination with one or two additional injections. As a result, the exhaust gas temperature rises to approximately 600°C. The residual oxygen (O₂) contained in the exhaust gases promotes the combustion of the soot. The regeneration period of the particulate filter can be approximately 20 minutes. Cyclic regeneration of the particulate filter occurs at least every 1,000 km. If the vehicle is driven over short distances, regeneration begins earlier. The ECM calculates the regeneration time based on the distance travelled after each engine start, as well as the average driving speed.
Catalytic converter and lambda probes
To reduce the amount of harmful emissions into the atmosphere, a catalytic converter is built into the exhaust system: on gasoline models - a three-functional converter, and on diesel models - an oxidizing converter. Three-functional catalytic converters are used to carry out the following chemical reactions: 2CO + O₂ → 2CO₂; 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O; 2NO + 2CO → N₂ + 2CO₂, and oxidation catalytic converters convert only CO and C₂ to H ₆.
In petrol models, the fuel injection control system has a feedback loop, which includes lambda probes that constantly inform the control unit about the composition of the exhaust gas. Depending on the data received, the control unit adjusts the quality of the mixture supplied to the combustion chambers and thus optimizes the fuel combustion conditions.
The working surface of the lambda probes is sensitive to changes in the oxygen content in the exhaust gas. Depending on its concentration, the output voltage of the sensor changes. If the mixture is over-enriched (the oxygen content in the exhaust gas is very low), the lambda probe sends signals with low voltage. The voltage increases as the mixture becomes leaner and the oxygen content in the gases increases. The converter works most effectively with an optimal composition of the combustible mixture (14.7 parts air to 1 part gasoline).
On diesel models, a wideband lambda probe can be used to correct the amount of fuel injected and optimize the operation of the EGR system.
(Content source: the specified website: «AudiManual»)