An Overview of the Emission Terms


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PCV System:

When the engine is running, intake manifold vacuum is supplied to the PCV valve. This vacuum moves air through the clean air hose into the rocker arm cover. From this location, air flows through cylinder head openings into the crankcase where it mixes with blow-by gases that escape from the combustion chamber past the piston rings. The mixture of blow-by gases and air flows up through cylinder head openings to the rocker arm cover and PCV valve.

Intake manifold vacuum moves the blow-by gas mixture through the PCV valve into the intake manifold. The blow-by gases are then moved through the intake valves into the combustion chambers where they are burned. Since blow-by gases contain hydrocarbons and other pollutants, these gases must not be allowed to escape to the atmosphere.

The PCV system prevents the escape of blow-by gases to the atmosphere. On many engines, the PCV system delivers blow-by gases to one location in the intake manifold. This type of system may not deliver these gases equally to all the cylinders.

This action may result in an air-fuel ratio variation between the cylinders, which results in rougher idle operation. Some engines, such as the Ford 4.6 L V8, have passages from the PCV valve system through the intake manifold and gaskets that supply blow-by gases equally to each cylinder, resulting in smoother idle operation. PCV Valve Position with the Engine Not Running The PCV valve contains a tapered valve. When the engine is not running, a spring keeps the tapered valve seated against the valve housing.

PCV Valve Position During Idle or Deceleration

During idle or deceleration, the high intake manifold vacuum moves the tapered valve upward against the spring tension. Under this condition, there is a small opening between the tapered valve and the PCV valve housing.  Since the engine is not under heavy load during idle or deceleration operation, blow-by gases are minimal and the small PCV valve opening is adequate to move the blow-by gases out of the crankcase.

PCV Valve Position at Part Throttle

The intake manifold vacuum is lower during part-throttle operation than during idle operation. Under this condition, the spring moves the tapered valve downward to increase the opening between this valve and the PCV valve housing. Since engine load is higher at part-throttle operation than at idle operation, blow-by gases are increased. The larger opening between the tapered valve and the PCV valve housing allows all the blow-by gases to be drawn into the intake manifold.

PCV Valve Position During High Engine Load and Engine Backfire

When the engine is operating under heavy load conditions with a wide throttle opening, the decrease in intake manifold vacuum allows the spring to move the tapered valve further downward in the PCV valve. 

This action provides a larger opening between the tapered valve and the PCV valve housing. Since higher engine load results in more blow-by gases, the larger PCV valve opening is necessary to allow these gases to flow through the valve into the intake manifold. When worn rings or scored cylinders allow excessive blow-by gases into the crankcase, the PCV valve opening may not be large enough to allow these gases to flow into the intake manifold. Under this condition, the blow-by gases create a pressure in the crankcase, and some of these gases are forced through the clean air hose and filter into the air cleaner. When this action occurs, there is oil in the PCV filter and air cleaner. This same action occurs if the PCV valve is restricted or plugged.

If the PCV valve sticks in the wide-open position, excessive air flow through the valve causes rough idle operation. If a backfire occurs in the intake manifold, the tapered valve is seated in the PCV valve as it is when the engine is not running. This action prevents the backfire from entering the engine where it could cause an explosion.






Evaporative (EVAP) Emission Control Systems

Since gasoline vapors contain HC, these vapors must not be allowed to escape from the fuel tank to the atmosphere. In the EVAP system, a hose is connected from the top of the fuel tank through the tank pressure control valve (TPCV) to a charcoal canister. Gasoline vapors from the fuel tank are stored in the charcoal and then purged into the intake manifold when the engine is running under certain conditions.

A fresh air line is connected to the charcoal canister. Some EVAP systems have a fresh air intake directly on the bottom of the canister with a filter in the air intake. EVAP systems vary depending on the vehicle make and model year. Always obtain the exact system details from the vehicle manufacturer's service manual. A canister purge solenoid is connected in the purge hose from the canister to the intake port near the edge of the throttle.   The PCM provides a ground for the canister purge solenoid winding to operate the solenoid.

How the system works

The PCM energizes the canister purge solenoid and allows vacuum to purge vapors from the canister under these conditions:

  1. 150 seconds have elapsed since the PCM entered closed loop.
  2. Coolant temperature is above 176 degrees F. (80 degrees C.).
  3. When the PCM is not enabling injector shut-off, such as on vehicles with traction control while one drive wheel is spinning.
  4. The idle contact switch in the IAC motor is open.
  5. Vehicle speed is above 20 mph.
  6. Engine speed is above 1,100 rpm.
  7. The engine metal temperature sensor is not indicating excessive temperature.
  8. Low coolant level is not indicated.

If any of these conditions are not present, the PCM does not energize the canister purge solenoid, and the gasoline vapors from the fuel tank are stored in the canister.

When the engine is running, intake manifold vacuum is supplied to the TPCV valve. This vacuum opens the valve and allows vapors to flow through the valve into the canister. When the engine is not running, the TPCV valve closes, and fuel vapors are contained in the fuel tank. If the tank pressure exceeds 15 inches of water with the engine not running, this pressure forces the TPCV valve open and allows vapor flow to the canister.

The canister contains a liquid fuel trap that collects any liquid fuel entering the canister.  Condensed fuel vapor forms liquid fuel. This liquid is returned from the canister to the tank when a vacuum is present in the tank. This liquid fuel trap prevents liquid fuel from contaminating the charcoal in the canister. The EVAP system reduces the escape of HC evaporative emissions from the gasoline tank to the atmosphere.

Mechanically Operated EVAP System

In some EVAP systems, the purge hose between the charcoal canister and the intake manifold is opened and closed by a thermal vacuum valve (TVV) that is mounted in the cooling system.   The TVV contains a thermowax element and a plunger. When the thermowax is heated, it expands and moves the plunger.

If the engine coolant temperature is below 95 degrees F. (35 degrees C.), the plunger in the TVV closes the purge hose between the intake manifold and the canister. Above this temperature, the TVV plunger opens the purge hose.

Three check valves are located in the top of the charcoal canister. When the throttle is open enough so the edge of the throttle uncovers the purge port, and the TVV is open, check valve #1 in the canister is opened by vacuum. Under this condition, fuel vapors are purged from the canister through the TVV into the intake manifold.

Check valve #2 in the canister is open with pressure in the fuel tank and closed with vacuum in the tank. Check valve #3 operates in the opposite way to check valve #2. A vacuum valve is located in the fuel tank filler cap. This valve opens and allows air into the tank if a specific amount of vacuum develops in the tank.















Catalytic Converters

Catalytic converters may be pellet-type or monolithic-type. A pellet-type converter contains a bed made from hundreds of small beads, and the exhaust gas passes over this bed.    In a monolithic-type converter, the exhaust gas passes through a honeycomb ceramic block. The converter beads, or ceramic block are coated with a thin coating of platinum, palladium, or rhodium, and mounted in a stainless steel container.

An oxidation catalyst changes HC and CO to CO2 and water vapor (H2O). The oxidation catalyst may be referred to as a two-way catalytic converter.

In a three-way catalytic converter, the three-way converter is positioned in front of the oxidation catalyst. A three-way catalytic converter reduces NOx emissions as well as CO and HC. The three-way catalyst reduces NOx into nitrogen and oxygen.   Some catalytic converters contain a thermo-sensor that illuminates a light on the instrument panel if the converter begins to overheat. Unleaded gasoline must be used in engines with catalytic converters. If leaded gasoline is used, the lead in the gasoline coats the catalyst and makes it ineffective. Under this condition, tail pipe emissions become very high. An engine that is improperly tuned causes severe overheating of the catalytic converter. Examples of improper tuning would be a rich air-fuel mixture or cylinder misfiring.

Many catalytic converters have an air hose connected from the belt-driven air pump to the oxidation catalyst. This converter must have a supply of oxygen to operate efficiently. On some engines, a mini-catalytic converter is built into the exhaust manifold or bolted to the manifold flange.

During oxidation catalyst operation, small amounts of sulfur in the gasoline combine with oxygen in the air to form oxides of sulfur (SOx). Sulfur dioxide (SO2) gas is also formed during oxidation converter operation. This SO2 gas is the same gas produced by rotting eggs. Although this gas has an unpleasant smell, it is not considered a major pollutant at present.

The SOx combines with the water vapor in the converter to form small amounts of sulfuric acid (H2SO4). There is some concern among environmental agencies about these small amounts of H2SO4 from many vehicles contributing to acid rain.
















 Exhaust Gas Recirculation Valves

Port Exhaust Gas Recirculation Valve

The port exhaust gas recirculation (EGR) valve contains a diaphragm with a sealed vacuum chamber above the diaphragm. A vacuum outlet from this chamber is connected to control vacuum. A stem extends from the diaphragm to a tapered valve in the lower part of the EGR valve. A spring above the diaphragm forces the diaphragm downward and seats the tapered valve on a matching seat in the lower valve body. A passage is connected from the exhaust manifold to the tapered valve and seat.

A passage is connected from the top of the tapered valve to the intake manifold. Vacuum is usually supplied to the EGR valve diaphragm chamber through a solenoid controlled by the PCM. When vacuum is supplied to the diaphragm chamber, the diaphragm, stem, and valve are lifted upward, which allows some exhaust gas to recirculate from the exhaust manifold into the intake manifold.  Since this exhaust gas contains very little oxygen, it does not burn in the combustion chambers, and combustion temperature is reduced. This action decreases oxides of nitrogen (NOx) emissions.

Positive Backpressure EGR Valve

The positive backpressure EGR valve has a bleed port and valve positioned in the center of the diaphragm. A light spring holds this bleed valve normally open (NO), and an exhaust passage is connected from the lower end of the tapered valve through the stem to the bleed valve. The area under the diaphragm is vented to the atmosphere. When the engine is running, exhaust pressure is applied to the bleed valve. At low engine speeds, exhaust pressure is not high enough to close the bleed valve. If control vacuum is supplied to the diaphragm chamber, the vacuum is bled off through the bleed port, and the valve remains closed. 

As engine and vehicle speed increase, the exhaust pressure also increases. At a preset throttle opening, the exhaust pressure closes the EGR valve bleed port. When control vacuum is supplied to the diaphragm, the diaphragm and valve are lifted upward, and the valve is open. If vacuum from an external source is supplied to a positive backpressure EGR valve with the engine not running, the valve will not open, because the vacuum is bled off through the bleed port.

Negative Backpressure EGR Valve

In a negative backpressure EGR valve, a normally closed (NC) bleed port is positioned in the center of the diaphragm. An exhaust passage is connected from the lower end of the tapered valve through the stem to the bleed valve. 

When the engine is running at lower speeds, each time a cylinder fires and an exhaust valve opens, there is a high-pressure pulse in the exhaust system. However, between these high-pressure pulses, there are low-pressure pulses. As the engine speed increases, more cylinder firings occur in a given time, and the high pressure pulses become closer together in the exhaust system. At low speed with fewer cylinder firings in a given amount of time, the negative exhaust pulses are more predominant compared to higher engine speeds.

At lower engine and vehicle speeds, the negative pulses in the exhaust system hold the bleed valve open. When the engine and vehicle speed increase to a preset value, the negative exhaust pressure pulses decrease, and the bleed valve closes. Under this condition, if control vacuum is supplied to the diaphragm chamber, the EGR valve is opened. When vacuum from an external source is supplied to a negative backpressure EGR valve with the engine not running, the bleed port is closed, and the vacuum should open the valve.

Negative or positive backpressure EGR valves may be identified by an N or a P stamped on top of the valve with the part number and plant code identification.

Linear EGR Valve

The linear EGR valve contains a single electric solenoid that is operated by the PCM. A tapered pintle is positioned on the end of the solenoid plunger. When the solenoid is energized, the plunger and tapered valve are lifted, and exhaust gas is allowed to recirculate into the intake manifold 

Five terminals on the linear EGR valve are connected to the PCM. The EGR solenoid winding is connected to terminals A and E.  The EGR valve contains an EGR valve position (EVP) sensor that has a ground terminal (B), a signal terminal (C), and a 5-V input terminal (D).  The EVP sensor contains a linear potentiometer. The signal from this sensor varies from approximately 1 V with the EGR valve closed to 4.5 V with the valve wide open.

The PCM pulses the EGR solenoid winding on and off with a pulse width modulation (PWM) principle to provide accurate control of the plunger and EGR flow. The EVP sensor acts as a feedback signal to the PCM to inform the PCM if the commanded valve position was achieved.

EGR Valve with Exhaust Gas Temperature Sensor

Some EGR valves, particularly on vehicles sold in California, contain an exhaust gas temperature sensor. This sensor contains a thermistor that changes resistance in relation to temperature. An increase in exhaust temperature decreases the sensor resistance. Two wires are connected from the exhaust gas temperature sensor to the PCM.  The PCM senses the voltage drop across this sensor. Cool exhaust temperature and higher sensor resistance cause a high-voltage signal to the PCM, whereas hot exhaust temperature and low sensor resistance result in a low-voltage signal to the PCM.

This page last updated May 30, 2001.  Send us your feedback, and come join the Imperial Mailing List - Online Car Club