Emissions and engine control systems
1. To prevent pollution of the atmosphere from incompletely burned and evaporating gases, and to maintain good driveability and fuel economy, a number of emission control systems are incorporated. They include the:
Catalytic converter
2. A catalytic converter is an emission control device in the exhaust system that reduces certain pollutants in the exhaust gas stream.
There are two types of converters: oxidation converters and reduction converters.
3. Oxidation converters contain a monolithic substrate (a ceramic honeycomb) coated with the semi-precious metals platinum and palladium. An oxidation catalyst reduces unburned hydrocarbons (HC) and carbon monoxide (CO) by adding oxygen to the exhaust stream as it passes through the substrate, which, in the presence of high temperature and the catalyst materials, converts the HC and CO to water vapor (H2O) and carbon dioxide (CO2).
4. Reduction converters contain a monolithic substrate coated with platinum and rhodium. A reduction catalyst reduces oxides of nitrogen (NOx) by removing oxygen, which in the presence of high temperature and the catalyst material produces nitrogen (N) and carbon dioxide (CO 2).
5. Catalytic converters that combine both types of catalysts in one assembly are known as «three-way catalysts» or TWCs. A TWC can reduce all three pollutants.
Evaporative Emissions Control (EVAP) system
6. The Evaporative Emissions Control (EVAP) system prevents fuel system vapors (which contain unburned hydrocarbons) from escaping into the atmosphere. On warm days, vapors trapped inside the fuel tank expand until the pressure reaches a certain threshold. Then the fuel vapors are routed from the fuel tank through the fuel vapor vent valve and the fuel vapor control valve to the EVAP canister, where they’re stored temporarily until the next time the vehicle is operated. When the conditions are right (engine warmed up, vehicle up to speed, moderate or heavy load on the engine, etc.) the PCM opens the canister purge valve, which allows fuel vapors to be drawn from the canister into the intake manifold. Once in the intake manifold, the fuel vapors mix with incoming air before being drawn through the intake ports into the combustion chambers where they’re burned up with the rest of the air/ fuel mixture. The EVAP system is complex and virtually impossible to troubleshoot without the right tools and training.
Powertrain Control Module (PCM)
7 The Powertrain Control Module (PCM) is the brain of the engine management system. It also controls a wide variety of other vehicle systems. In order to program the new PCM, the dealer needs the vehicle as well as the new PCM. If you’re planning to replace the PCM with a new one, there is no point in trying to do so at home because you won’t be able to program it yourself.
Positive Crankcase Ventilation (PCV) system
8. The Positive Crankcase Ventilation (PCV) system reduces hydrocarbon emissions by scavenging crankcase vapors, which are rich in unburned hydrocarbons. A PCV valve or orifice regulates the flow of gases into the intake manifold in proportion to the amount of intake vacuum available.
9. The PCV system generally consists of the fresh air inlet hose, the PCV valve or orifice and the crankcase ventilation hose (or PCV hose). The fresh air inlet hose connects the air intake duct to a pipe on the valve cover.
The crankcase ventilation hose (or PCV hose) connects the PCV valve or orifice in the valve cover to the intake manifold.
Accelerator Pedal Position (APP) sensor — as you press the accelerator pedal, the APP sensor alters its voltage signal to the PCM in proportion to the angle of the pedal, and the PCM commands a motor inside the throttle body to open or close the throttle plate accordingly
Camshaft Position (CMP) sensor — produces a signal that the PCM uses to identify the number 1 cylinder and to time the firing sequence of the fuel injectors
Crankshaft Position (CKP) sensor — produces a signal that the PCM uses to calculate engine speed and crankshaft position, which enables it to synchronize ignition timing with fuel injector timing, and to detect misfires
Engine Coolant Temperature (ECT) sensor — a thermistor (temperature-sensitive variable resistor) that sends a voltage signal to the PCM, which uses this data to determine the temperature of the engine coolant
Fuel tank pressure sensor -measures the fuel tank pressure and controls fuel tank pressure by signaling the EVAP system to purge the fuel tank vapors when the pressure becomes excessive
Intake Air Temperature (IAT) sensor — monitors the temperature of the air entering the engine and sends a signal to the PCM to determine injector pulse-width (the duration of each injector’s on-time) and to adjust spark timing (to prevent spark knock)
Knock sensor — a piezoelectric crystal that oscillates in proportion to engine vibration which produces a voltage output that is monitored by the PCM. This retards the ignition timing when the oscillation exceeds a certain threshold
Manifold Absolute Pressure (MAP) sensor — monitors the pressure or vacuum inside the intake manifold. The PCM uses this data to determine engine load so that it can alter the ignition advance and fuel enrichment
Mass Air Flow (MAF) sensor -measures the amount of intake air drawn into the engine. It uses a hot-wire sensing element to measure the amount of air entering the engine
Oxygen sensors — generates a small variable voltage signal in proportion to the difference between the oxygen content in the exhaust stream and the oxygen content in the ambient air. The PCM uses this information to maintain the proper air/fuel ratio. A second oxygen sensor monitors the efficiency of the catalytic converter
Throttle Position (TP) sensor -a potentiometer that generates a voltage signal that varies in relation to the opening angle of the throttle plate inside the throttle body. Works with the PCM and other sensors to calculate injector pulse width (the duration of each injector’s on-time)
