Auto Service World
Feature   April 1, 2004   by Auto Service World

Knowledge Building: Emissions Systems Components

Modern emissions systems are truly wonders of automotive technology. Not only do modern vehicles produce a fraction of the exhaust emissions of their predecessors, they do so while producing more horsepower per litre and better driveability.

A large part of the push behind the increasing complexity of emissions and fuel systems is the increasing demands to reduce emissions and improve fuel mileage. This is to the point that the fuel system as a separate entity is no longer, as it is so closely tied in to ignition and emissions systems and powertrain controls. Without tightly controlling spark advance, combustion temperature and fuel delivery, meeting government-mandated emissions limits would be impossible.

In general, all fuel systems seek to maintain a fuel-air ratio of 14.7:1. This is referred to as stoichiometric. While some engine systems sold in other markets around the world can burn ratios as lean as 30:1 and up by employing gasoline direct injection, these will not be the norm for some time.

From a controls standpoint, however, modern systems can mask combustion and fuel system problems by maintaining driveability in the face of increasing problems. Until a “Check Engine” light comes on, the driver may be completely unaware of a problem, and there is no guarantee that this will occur.

For the more conventional systems, however, there is a fairly standard set of components. It should be noted that not all systems have all components and that newer, OBD II equipped vehicles may have more than one, particularly oxygen sensors.

The Electronic Control Module’s (ECM) job includes the fuel system controls, but also stretches to ignition and other systems, depending on the vehicle. It receives signals from the sensors and issues commands. It is also too often misdiagnosed as the culprit when driveability and starting problems arise.

The Air Charge Temperature Sensor converts air temperature to a voltage signal and operates similar to the engine coolant sensor. It helps the ECM properly meter the air-fuel ratio.

The Engine Coolant Temperature Sensor (ECT) also converts temperature into a voltage signal. The signal is processed by the ECM to control fuel mixture, spark advance, and cold start idle, as well as other parameters.

The Cold Start Valve provides an engine with additional fuel for better cold starting. Its operation is controlled by the Thermal (or Thermo) Time Switch. Mounted on the block, it reads the coolant temperature and closes the Cold Start Valve when the temperature is above a predetermined point.

The Crankshaft Position Sensor/Camshaft Position Sensor reads the position of the crankshaft or camshaft using a magnetic field and sends a signal to the computer. There are three types: magnetic, Hall Effect and photo-optical. The principles of the first two are based on fluctuating magnetic fields; the third relies on a light source (an LED). The computer uses the resulting signal to time ignition spark, injectors, and other components.

The Exhaust Gas Recirculation Valve and the EGR Valve Position Sensor work together to control NOx (nitrous oxide) emissions. Feeding exhaust gases back through the combustion chamber has the effect of cooling combustion, curbing the production of the NOx by-products. The EGR Temperature Sensor provides control signals and the EGR Valve Position Sensor reads how open or closed the EGR valve is and sends a signal to the computer which, depending on the signals from a number of sensors, sends a signal to a solenoid to either open or close the EGR’s vacuum control or directly open or close the EGR valve for those systems so equipped. The EGR Valve Position Sensor may also be referred to as an EGR Pintle Position Sensor.

A Manifold Absolute Pressure (MAP) Sensor uses a pressure-sensitive disc to convert manifold air pressure to a voltage or frequency signal for the ECM. Its function is to allow the ECM to monitor engine load, to accurately control ignition timing and the air-fuel ratio.

A Mass Air Flow (MAF) Sensor or Meter performs essentially the same function as a MAP sensor, but uses a vane which is forced open by engine vacuum/air flow rather than reading pressure.

The Oxygen (O2) Sensor is, as its name would imply, a device for measuring the oxygen content in the exhaust manifold or exhaust pipe. It supplies a varying voltage signal to the ECM to control the air-fuel ratio. There are a variety of types, with OBD II sensors being both more sophisticated and more expensive than their predecessors. From a parts replacement standpoint it has emerged as a key part of the modern “tune-up,” and is considered by many in the aftermarket to be a market unto its own.

A Throttle Position Sensor can be found on either fuel injected or some of the later carburetor-equipped vehicles. This sensor sends a variable signal depending on throttle position. The computer uses this signal to set air-fuel mixture, spark timing, torque converter lockup, air conditioning operation, EGR flow rate, and idle.

The Fuel Injection System delivers fuel to the engine. Fuel injection systems can vary greatly but are generally divided into Throttle Body Injection (TBI), and Multi-port. TBI systems locate the injectors in essentially the same place as a carburetor, with the air-fuel mixture following a similar route to the cylinders, through an intake manifold or plenum. Multi-port systems put the injector as near to the cylinder as possible, leading to better balanced and timed fuel delivery. On these systems, the intake manifold carries only air for most of its length, with fuel being added only at the lower intake manifold. Direct injection systems, not currently offered in North America, place the injector directly in the cylinder, as in diesel systems.

The Fuel Pump is an obviously necessary part of a fuel system. Old-style fuel pumps for carbureted vehicles were most often mechanical and operated at about 5 to 15 pounds per square inch (psi) pressure. Mechanical pumps were driven by the camshaft and mounted on the engine. Low pressure electric fuel pumps can also be found on carbureted vehicles. Fuel pumps for fuel-injected vehicles operate at much higher pressures, usually 30 to 50 psi, but some pumps deliver more than 100 psi. High pressure electric pumps are very often located in the gas tank, although many systems use a low pressure pump in the tank and a high pressure in-line pump attached to the vehicle frame.

The Fuel Pressure Regulator’s job is to maintain the correct pressure to the fuel injectors. While the pump’s outlet pressure may vary somewhat in normal operation, keeping the pressure supplied to the fuel injectors within tight limits is necessary for accurate air-fuel mixture. In a Throttle Body Injection system, it is located at the throttle body housing. In a multi-port injection system, it is commonly located at the outlet end of the fuel rail, allowing excess fuel to return to the tank. There are some systems which incorporate it into the fuel pump assembly inside the fuel tank and do away with the fuel return line.

Fuel Filters are an important part of the fuel system. There are types that fit into the fuel line, usually with a paper element, as well as filters that are fitted to the bottom of in-tank fuel pumps, often referred to as strainers. Clogged or dirty fuel filters or strainers are a common cause of poor fuel pressure and volume.

The Evaporative Emissions Control System is a method of recapturing fuel vapour that would otherwise end up in the atmosphere. Generally this is in the form of a canister with a charcoal filter. Vapours collect there, where they condense. The canister is purged during normal engine operation and the fuel routed to the fuel system.

The Idle Air Control Valve , also known as an Air Bypass Valve, is a motor/solenoid which varies the amount of air passing around the throttle plates on injected vehicles. It is controlled by the ECM, which uses this valve to co
ntrol engine idle speed.

The Idle Speed Control (ISC) controls the idle speed during periods of closed throttle. It is an electric motor-operated plunger located adjacent to the throttle body. A motor in the ISC extends and retracts a plunger which limits the closing position of the throttle lever.

The Catalytic Converter uses precious metals in a honeycomb-type structure or as beads to convert exhaust gases to less harmful gases and water. There are two types: single bed and dual-bed, also referred to as three-way. In both, a chemical reaction caused by these precious metals causes oxygen (O2 ) to be combined with unburned hydrocarbons (HC, essentially unburned fuel) and carbon monoxide (CO) to produce water (H2O) and carbon dioxide (CO2) as well as Nitrogen (N2). The dual-bed type adds a stage ahead of this to convert nitrogen oxides (NOx ) and oxygen. These gases then travel along with the HC and CO to the oxidation catalyst, where the reaction mentioned first takes place. The net effect is to minimize, and preferably eliminate, harmful emissions from entering the atmosphere.

The Air Pump is designed to feed air into the exhaust gases to cause unburned fuel to burn. Some systems inject air into the exhaust stream at the manifold, others inject air at the catalytic converter to aid in the conversion of exhaust gases. Air pumps are protected from hot exhaust gases by an Air Pump Check Valve, which only allows air to flow from the pump.

Air Diverter Valves (or Air Management Valves) re-route the compressed air from an Air Pump under certain conditions. This air may be vented to the outside or, on some vehicles, it may direct air upstream of the O2 sensor on cold starts, to clean up HC and help heat the O2 sensor.

Pulse Air Injection Valves perform the same function as air pumps but use the natural pressure variations in the exhaust stream to draw in fresh air.

The PCV Valve is the oldest emissions control item. It replaced the old dump tubes that vented crankcase vapours to the atmosphere. The PCV Valve is a one-way, check valve that vents these vapours (mostly HC from unburned fuel) back through the induction system to the combustion chamber for burning.

Images and graphics courtesy NGK Spark Plugs of Canada, Robert Bosch and Delphi Product & Service Solutions.

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