Emissions and Ignition systems are an important part to have knowledge of when explaining repairs to your customers. Most customers don’t understand these systems so it’s critical to have a good interpretation of these systems(from the technician, to you, then to the customer). Vehicle emission systems are designed to minimize three primary pollutants; HC (Hydrocarbons), CO (Carbon Monoxide) and NOx (Oxides of Nitrogen). Automobile HC emissions can come from the exhaust, the crankcase ventilation system and from fuel system leaks. Excessive CO emissions are caused by a lack of air or too much fuel. NOx is caused when combustion temperatures climb too high under loads or when the engine us running too lean.
HC & CO Emissions Reduction
HC and CO emissions from the tailpipe are reduced in a number of ways. Computer control of fuel injection and ignition timing helps to reduce emissions by optimizing combustion. Air Injection Reaction (AIR) systems help to further reduce HC and CO.
The AIR system works by forcing fresh air into the exhaust system. The oxygen in the fresh air combines with the post-combustion HC and CO to provide secondary oxidation, converting the residual HC and CO to water vapor and C02. The fresh air is fed into the exhaust stream at the cylinder head exhaust ports or in the exhaust manifold just past the exhaust ports, and on some vehicles, the catalytic converter.
Catalyst System (catalytic converter)
The catalytic converter is located in the exhaust system. The outside of a catalytic converter resembles a muffler, but inside it contains special metal alloys and precious metals such as platinum, palladium and rhodium, which function as catalysts. Catalysts are agents of chemical change which themselves do not change. The converter transforms smog-creating exhaust emissions into relatively harmless gases. Catalytic converters are expensive and can be ruined by engine misfires, overly rich air/fuel mixtures or by engine oil and antifreeze leaking into the combustion chamber.
NOx Emissions Reduction – EGR (exhaust gas recirculation)
NOx emissions are the result of excessively high combustion temperatures, which cause the nitrogen in the air to combine with oxygen. An EGR system is used to reduce NOx by recirculating some of the exhaust gas into the intake manifold. Because exhaust gases are basically inert and won’t support combustion, blending a slight amount of exhaust into the intake stream will reduce combustion temperature.
Note: On late model vehicles they use variable valve timing control to reduce NOx emissions and have eliminated the EGR valve.
HC (hydrocarbon) Emissions Reduction – EVAP (evaporative emissions control)
The Evaporative Emissions Control System (EVAP) helps prevent HC emissions. The fuel system is built to keep all fuel vapor sealed in the tank. An evaporative system with pressure sensors and other controls is utilized by the PCM/ECM to control system vapor pressure and introduce vapors back into the engine to be burned. Some vehicles even use vacuum stored in the brake booster to draw fuel vapor into the evaporative system during refilling with the engine off.
PCV (Positive Crankcase Ventilation) systems prevent emissions from the engine crankcase. The engine crankcase must be ventilated to allow the removal of fumes from blowby (unburned or partially burned air/fuel that leaks past the piston rings) and oil evaporation. The PCV system ducts these fumes into the intake manifold where they are drawn into the cylinder and burned. The PCV valve meters the flow so that it does not upset the air/fuel mixture and protects the crankcase from any backfire into the intake manifold. The PCV valve should be checked periodically as indicated in the maintenance schedules.
Emissions Failure Detection
All vehicles manufactured today have computerized systems that manage the ignition, fuel injection, emission control and other aspects of power delivery such as transmission gear selection. These systems have a PCM/ECM that processes input from various sensors and compares it to parameters pre-programmed at the factory. The on-board computer is capable of monitoring all of the sensors and actuators to determine whether they are working as intended. It can detect a malfunction or deterioration of the various sensors and actuators, usually well before the driver becomes aware of the problem through a loss in vehicle performance or driveability. The OBD II system is designed to identify problems with the emissions control system before the vehicle becomes an excessive polluter, allowing time to repair the vehicle before emissions become a problem. When the OBD II system determines that a problem exists, a DTC (Diagnostic Trouble Code) is stored in the computer’s memory and a light on the dashboard called an MIL (Malfunction Indicator Light) is illuminated. This light is for emissions problems only. It will have the words Service Engine Soon or Check Engine or a symbol of an engine. An illuminated MIL is intended to inform the driver of the need for service, not of the need to stop the vehicle. Service should be sought as soon as possible. If the MIL is flashing or blinking, it is indicating that there has been a fault that could cause serious damage to the catalytic converter. The driver should reduce speed and seek service as soon as possible.
Ignition System Basics
Once the air/fuel mixture is in the cylinder, it must be ignited to be of any use. A diesel engine will ignite the fuel from the heat of compression alone, except for cold-start conditions, which may require an electrically heated glow plug or inlet air heater to achieve ignition . However, a gasoline engine needs a spark for ignition. The ignition system components can vary greatly depending on the vehicle and manufacturer, but their basic function is to deliver a spark to the correct cylinder at the correct time and ignite the air/fuel charge in the cylinder. Most systems in use today are distributorless, meaning that instead of a distributor with wires running to each cylinder, they have ignition coils that provides power directly to a few cylinders or have one coil fo r each cylinder called a COP (Coil On Plug) system. The PCM/ECM uses crank and cam signals from the engine to synchronize the fuel and ignition and controls timing of spark from its software calibration. Older vehicles usually have a distributor ignition system, which has a distributor, a distributor cap and rotor as well as a triggering mechanism, such as a pole piece and pickup coil. The high voltage current coming from the coil is directed to the proper spark plug at the proper time by the distributor and the rotor, the triggering mechanism and the ignition control module. The spark plugs thread into the cylinder head(s) and extend into the combustion chamber. They provide the spark that ignites the air/fuel mixture. The coil or coils provide high voltage electricity to the spark plugs via the cables. All spark plugs are not the same. They differ in physical size and heat range and each engine requires spark plugs that are specific to the application.