Luke
05-25-2004, 09:23 AM
Emissions Systems
When positive crankcase ventilation (PCV) became standard in 1968, the recycling of crankcase vapors eliminated blowby emissions as a major source of automotive pollution. When evaporative emission controls were added in 1971, charcoal canisters and sealed fuel systems eliminated fuel vapors as another factor that contributed to air pollution. Exhaust gas recirculation (EGR) was added in 1973, which lowered harmful oxides of nitrogen (NOX) emissions. But the most significant add-on came in 1975 when the auto makers were required to install catalytic converters
on all new cars.
The catalytic converter proved to be a real breakthrough in controlling emissions because it reduced both unburned hydrocarbons (HC), and carbon monoxide (CO). The converter slashed the levels of these two pollutants nearly 90%!
In 1981, three-way" converters were introduced. Three-way converters also reduced NOX concentrations in the exhaust, but required the addition of a computerized feedback fuel control system to do so. The catalyst inside a three-way converter that reduces NOX, requires a rich fuel mixture. But a rich fuel mixture increases CO levels in the exhaust. So to reduce all three pollutants (HC, CO and NOX), a three-way converter requires a fuel mixture that constantly changes or flip flops back and forth from rich to lean. This, in turn, requires feedback carburetion or electronic fuel injection, plus an oxygen sensor in the exhaust to keep tabs on what’s happening with the fuel mixture.
Note:
Phosphorus, which is found in motor oil, can foul the converter, as the engine is burning oil, because of worn valve guides or rings. Converters may also fail if they get too hot. This can be caused by unburned fuel (HC) in the exhaust.
A "sluggish" oxygen sensor, for example, may not allow the fuel mixture to change back and forth quickly enough, to keep the converter working at peak efficiency. Though this might not lead to a meltdown, it could cause enough of an increase in pollution to make the vehicle fail and emissions test. If the oxygen sensor has died altogether, the fuel mixture will remain fixed, and the engine will probably run too rich, causing an increase in fuel consumption, as well as emissions. If the oxygen sensor has died altogether, the fuel mixture will remain fixed, and the engine will probably run too rich, causing an increase in fuel consumption, as well as emissions.
Unheated 1 or 2 wire wire O2 sensors on 1976 through early 1990s applications should be replaced every 30,000 to 50,000 miles.
Heated 3 and 4-wire O2 sensors on mid-1980s through mid-1990s applications should be changed every 60,000 miles. And on 1996 and newer OBD II equipped vehicles, the recommended replacement interval is 100,000 miles.
EGR
lowers combustion temperatures, to keep it under 2500 degree F, so little NOX is formed (( the higher the flame temperature, the higher the rate at which oxygen and nitrogen react to form NOX )). As an added benefit, EGR also helps prevent detonation.
An EGR valve that has failed, refuses to open (or the EGR passageway in the manifold is clogged) will allow elevated NOX emissions, and may even also cause a detonation (spark knock) problem.
OBD II
is designed to detect emission problems. When a problem is detected, the Check Engine light comes on, and a diagnostic trouble code is stored in the vehicle’s powertrain computer. With OBD II, the Check Engine light will come on anytime emissions exceed federal limits by 50% on two consecutive trips, or there’s a failure of a major emissions control system. Unlike an emissions test, which may only be given once very year or two, OBD II is monitoring emissions performance every time the vehicle is driven. And now that OBD-II regulations are here (1996 and newer vehicles), many vehicles are now equipped with multiple O2 sensors, some as many as four!
O2 Sensor
This sensor is the master switch in the fuel control feedback loop. The O2 sensor monitors the amount of unburned oxygen ( O2 ) in the exhaust, and produces a voltage signal that varies from about 0.1 volts (lean) to 0.9 volts (rich).
The O2 sensor is mounted in the exhaust manifold to monitor how much unburned oxygen is in the exhaust, as the exhaust exits the engine. Monitoring oxygen levels in the exhaust is a way of gauging the fuel mixture. It tells the computer if the fuel mixture is burning rich (less oxygen) or lean (more oxygen).
The O2 sensor is the master monitor for what’s happening with the fuel mixture. Consequently, any problems with the O2 sensor can throw the whole system out of whack.
The result is a constant flip-flop back and forth from rich to lean, which allows the catalytic converter to operate at peak efficiency, while keeping the average overall fuel mixture in proper balance, to minimize emissions.
When no signal is received from the O2 sensor, as is the case when a cold engine is first started (or the 02 sensor fails), the computer orders a fixed (unchanging) rich fuel mixture. If the engine fails to go into closed loop, when the O2 sensor reaches operating temperature, or drops out of closed loop because the O2 sensor’s signal is lost, the engine will run too rich causing an increase in fuel consumption and emissions.
A bad coolant sensor can also prevent the system from going into closed loop,
because the computer also considers engine coolant temperature when deciding whether or not to go into closed loop.
An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen in the exhaust. When the mixture is lean, the sensor’s output voltage will drop down to about 0.1 volts. When the air/fuel mixture is balanced, or at the equilibrium point of about 14.7 to 1, the sensor will read around 0.45 volts.
The transition rate is slowest on engines with feedback carburetors, typically once per second at 2500 rpm. Engines with throttle body injection are somewhat faster (2 to 3 times per second at 2500 rpm), while engines with multiport injection are the fastest (5 to 7 times per second at 2500 rpm).
It should be noted, on my 95SL2 Saturn, the PCM operates in two different Fuel Control Modes: Open Loop and closed Loop. Whenever the vehicle is first started, the PCM
operates in Open Loop fuel control. When the PCM determines taht the engine coolant temperature (ECT sensor has reached at least 68F, and the oxygen sensor has reached 600F, it will go into Closed Loop fuel control operation.
In Open Loop, the MIL lamp will flash two and one-half times per second, while in Closed Loop, the MIL lamp should flach once per second. Also, in Closed Loop the lamp will stay off most of the time, if the system is running lean. It will stay on most of the time if the system is running rich.
Unheated 1 or 2 wire wire O2 sensors, on 1976 through early 1990s vehicles, can be replaced every 30,000 to 50,000 miles. Heated 3 and 4-wire O2 sensors, on mid-1980s through mid-1990s applications, can be changed every 60,000 miles.
On OBD II equipped vehicles (1996 & up), a replacement interval of 100,000 miles is recommended.
So what exactly does the O2 sensor do? It monitors the fuel mixture so the engine computer can adjust the air/fuel ratio to maintain the lowest possible emissions and best fuel economy. The O2 sensor does this by reacting to unburned oxygen in the exhaust. The sensor generates a small voltage signal (usually less than 1 volt) that increases when the air/fuel mixture goes rich, and drops when the air/fuel mixture goes lean. It's acts like a rich/lean switch, that signals the computer every time the fuel mixture changes, which is constantly. This back-and-forth balancing act creates an average mixture that is pretty close to ideal. This is the "fuel feedback control loop"
that allows today's vehicles to maintain extremely low emission levels, and the O2 sensor
is the key sensor in this loop. Typically, a bad O2 sensor will read low (lean),
which causes the engine to run too rich, pollute too much and use too much gas.
The increased lag time makes the sensor sluggish and prevents the engine from keeping the air/fuel mixture in close balance.
Source:
http://www.autotap.com
...
When positive crankcase ventilation (PCV) became standard in 1968, the recycling of crankcase vapors eliminated blowby emissions as a major source of automotive pollution. When evaporative emission controls were added in 1971, charcoal canisters and sealed fuel systems eliminated fuel vapors as another factor that contributed to air pollution. Exhaust gas recirculation (EGR) was added in 1973, which lowered harmful oxides of nitrogen (NOX) emissions. But the most significant add-on came in 1975 when the auto makers were required to install catalytic converters
on all new cars.
The catalytic converter proved to be a real breakthrough in controlling emissions because it reduced both unburned hydrocarbons (HC), and carbon monoxide (CO). The converter slashed the levels of these two pollutants nearly 90%!
In 1981, three-way" converters were introduced. Three-way converters also reduced NOX concentrations in the exhaust, but required the addition of a computerized feedback fuel control system to do so. The catalyst inside a three-way converter that reduces NOX, requires a rich fuel mixture. But a rich fuel mixture increases CO levels in the exhaust. So to reduce all three pollutants (HC, CO and NOX), a three-way converter requires a fuel mixture that constantly changes or flip flops back and forth from rich to lean. This, in turn, requires feedback carburetion or electronic fuel injection, plus an oxygen sensor in the exhaust to keep tabs on what’s happening with the fuel mixture.
Note:
Phosphorus, which is found in motor oil, can foul the converter, as the engine is burning oil, because of worn valve guides or rings. Converters may also fail if they get too hot. This can be caused by unburned fuel (HC) in the exhaust.
A "sluggish" oxygen sensor, for example, may not allow the fuel mixture to change back and forth quickly enough, to keep the converter working at peak efficiency. Though this might not lead to a meltdown, it could cause enough of an increase in pollution to make the vehicle fail and emissions test. If the oxygen sensor has died altogether, the fuel mixture will remain fixed, and the engine will probably run too rich, causing an increase in fuel consumption, as well as emissions. If the oxygen sensor has died altogether, the fuel mixture will remain fixed, and the engine will probably run too rich, causing an increase in fuel consumption, as well as emissions.
Unheated 1 or 2 wire wire O2 sensors on 1976 through early 1990s applications should be replaced every 30,000 to 50,000 miles.
Heated 3 and 4-wire O2 sensors on mid-1980s through mid-1990s applications should be changed every 60,000 miles. And on 1996 and newer OBD II equipped vehicles, the recommended replacement interval is 100,000 miles.
EGR
lowers combustion temperatures, to keep it under 2500 degree F, so little NOX is formed (( the higher the flame temperature, the higher the rate at which oxygen and nitrogen react to form NOX )). As an added benefit, EGR also helps prevent detonation.
An EGR valve that has failed, refuses to open (or the EGR passageway in the manifold is clogged) will allow elevated NOX emissions, and may even also cause a detonation (spark knock) problem.
OBD II
is designed to detect emission problems. When a problem is detected, the Check Engine light comes on, and a diagnostic trouble code is stored in the vehicle’s powertrain computer. With OBD II, the Check Engine light will come on anytime emissions exceed federal limits by 50% on two consecutive trips, or there’s a failure of a major emissions control system. Unlike an emissions test, which may only be given once very year or two, OBD II is monitoring emissions performance every time the vehicle is driven. And now that OBD-II regulations are here (1996 and newer vehicles), many vehicles are now equipped with multiple O2 sensors, some as many as four!
O2 Sensor
This sensor is the master switch in the fuel control feedback loop. The O2 sensor monitors the amount of unburned oxygen ( O2 ) in the exhaust, and produces a voltage signal that varies from about 0.1 volts (lean) to 0.9 volts (rich).
The O2 sensor is mounted in the exhaust manifold to monitor how much unburned oxygen is in the exhaust, as the exhaust exits the engine. Monitoring oxygen levels in the exhaust is a way of gauging the fuel mixture. It tells the computer if the fuel mixture is burning rich (less oxygen) or lean (more oxygen).
The O2 sensor is the master monitor for what’s happening with the fuel mixture. Consequently, any problems with the O2 sensor can throw the whole system out of whack.
The result is a constant flip-flop back and forth from rich to lean, which allows the catalytic converter to operate at peak efficiency, while keeping the average overall fuel mixture in proper balance, to minimize emissions.
When no signal is received from the O2 sensor, as is the case when a cold engine is first started (or the 02 sensor fails), the computer orders a fixed (unchanging) rich fuel mixture. If the engine fails to go into closed loop, when the O2 sensor reaches operating temperature, or drops out of closed loop because the O2 sensor’s signal is lost, the engine will run too rich causing an increase in fuel consumption and emissions.
A bad coolant sensor can also prevent the system from going into closed loop,
because the computer also considers engine coolant temperature when deciding whether or not to go into closed loop.
An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen in the exhaust. When the mixture is lean, the sensor’s output voltage will drop down to about 0.1 volts. When the air/fuel mixture is balanced, or at the equilibrium point of about 14.7 to 1, the sensor will read around 0.45 volts.
The transition rate is slowest on engines with feedback carburetors, typically once per second at 2500 rpm. Engines with throttle body injection are somewhat faster (2 to 3 times per second at 2500 rpm), while engines with multiport injection are the fastest (5 to 7 times per second at 2500 rpm).
It should be noted, on my 95SL2 Saturn, the PCM operates in two different Fuel Control Modes: Open Loop and closed Loop. Whenever the vehicle is first started, the PCM
operates in Open Loop fuel control. When the PCM determines taht the engine coolant temperature (ECT sensor has reached at least 68F, and the oxygen sensor has reached 600F, it will go into Closed Loop fuel control operation.
In Open Loop, the MIL lamp will flash two and one-half times per second, while in Closed Loop, the MIL lamp should flach once per second. Also, in Closed Loop the lamp will stay off most of the time, if the system is running lean. It will stay on most of the time if the system is running rich.
Unheated 1 or 2 wire wire O2 sensors, on 1976 through early 1990s vehicles, can be replaced every 30,000 to 50,000 miles. Heated 3 and 4-wire O2 sensors, on mid-1980s through mid-1990s applications, can be changed every 60,000 miles.
On OBD II equipped vehicles (1996 & up), a replacement interval of 100,000 miles is recommended.
So what exactly does the O2 sensor do? It monitors the fuel mixture so the engine computer can adjust the air/fuel ratio to maintain the lowest possible emissions and best fuel economy. The O2 sensor does this by reacting to unburned oxygen in the exhaust. The sensor generates a small voltage signal (usually less than 1 volt) that increases when the air/fuel mixture goes rich, and drops when the air/fuel mixture goes lean. It's acts like a rich/lean switch, that signals the computer every time the fuel mixture changes, which is constantly. This back-and-forth balancing act creates an average mixture that is pretty close to ideal. This is the "fuel feedback control loop"
that allows today's vehicles to maintain extremely low emission levels, and the O2 sensor
is the key sensor in this loop. Typically, a bad O2 sensor will read low (lean),
which causes the engine to run too rich, pollute too much and use too much gas.
The increased lag time makes the sensor sluggish and prevents the engine from keeping the air/fuel mixture in close balance.
Source:
http://www.autotap.com
...