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Hi Guys,

There has been a lotof talk around emissions on here lately, I though I'd post up a bunch of info I've obtained from numerous sources to give anyone interested an overview of what it all means and why it's so important.

EMISSIONS.....basic information.


OZONE..what is it?


This is a form of molecular oxygen that consists of three oxygen atoms linked together. The \"ozone layer\" or upper atmosphere occurs naturally and filters out ultraviolet radiation from the sun. Ozone at ground level is a noxious pollutant and is the major component of smog. Ozone is an irritant. It causes respiratory problems. Aggravation of respiratory disease and respiratory infections to children and susceptible adults is a major effect of ozone at ground level. At higher elevations ozone can inhibit plant growth and cause crop and forest damage. Ozone is formed in the atmosphere through chemical reactions involving hydrocarbons,oxides of nitrogen and sunlight.


CARBON MONOXIDE..what is it?


Carbon Monoxide consists of a Carbon atom and an Oxygen atom linked together. It is a colourless,odourless and poisonous gas. The production of Carbon Monoxide is via the vehicle tailpipe and is due to the incomplete burning of hydrocarbon base fuels. Carbon Monoxide carried by the blood stream to the lungs creates a compound called Carboxhemoglobin. This compound influences the bloods capacity to carry oxygen to organs and tissue. Carbon Monoxide poisoning affects infants,the elderly and persons with heart ailments. Even healthy people are susceptible to Carbon Monoxide. Visual perception,manual dexterity,the ability to perform tasks and learning abilites are just some of the effects of Carbon Monoxide poisoning. Incomplete combustion of fuel results in Carbon Monoxide being emitted from the vehicle tailpipe. Typically a low air to fuel ratio such as during cold starts or a badly tuned vehicle will produce Carbon Monoxide. Vehicles dríven at higher altitudes where effectively the \"thinner\" air reduces the amount of oxygen available for combustion also causes higher Carbon Monoxide tailpipe output.


NOx..what is it?


This is a collective term which refers to the combustion by products Nitric Oxide (NO) and Nitrogen Dioxide (NO2). So termed Nitrogen Oxides (NOx). NOx affects the respiratory system, in particular, children and asthmatic sufferers. Nitrogen Oxides are precursors to the formation of ozone and contribute to the formation of acid rain. Due to the high pressures and temperatures generated in a vehicles combustion chamber,nitrogen and oxygen atoms form to produce the various Nitrogen Oxides.


Hydrocarbons..what is it?


Hydrocarbons is a term that refers to a large group of chemical compounds made up of Hydrogen and Carbon. Gasoline contains several compound groups,which when combined with oxygen will burn when ignited. Unburned or partially burned fuel when emitted from a vehicle tailpipe is an \"Exhaust Hydrocarbons Emissions\". Another source of Hydrocarbons emissions are fuel vapours escaping to the atmosphere from a vehicles fuel system. Hydrocarbons emissions when reacting with elevated temperatures,sunlight and oxides of nitrogen form ground level ozone. This hazardous form of air pollution contributes to eye,throat and lung irritation and possibly cancer.


Particulates..what is it?

Particulates are the solid particles of carbon soot and fuel additives that are emitted from a vehicles tailpipe. Carbon particles make up a large percentage of these emissions. A vehicle emitting black smoke or a diesel blowing black smoke emitts such particulates into the atmosphere. A large proportion of these particulate emissions can float in the air for extended periods,causing possible health hazards.

EMISSIONS...basic information


GAS ANALYSER..... 4 gas exhaust emissions theory


HYDROCARBONS (HC) are unburned or partly burned fuel. High levels of hydrocarbons, measured as parts per million ( ppm ) in the exhaust gas, are often related to problems in the ignition system. Fouled plugs or defective ignition wires, improper dwell or timing, vacuum leaks or incorrect air/fuel ratio all can cause improper combustion and high HC readings.


CARBON MONOXIDE (CO) is formed when there is not enough oxygen present during combustion. High levels of carbon monoxide ( measured as a percentage of the exhaust gas ) can be caused by a too rich fuel mixture, incorrect idle speed, faulty air cleaner, PCV valve, incorrect fuel pressure or faulty carburetor/injection system.


OXYGEN (O²) indicates the accuracy of the carburetor/injection system settings. If oxygen content is measured as the air fuel ratio is adjusted from rich to lean (or vice versa), a sharp change in the reading of at least 0.5% shows the crossover point from rich to lean. O² measurement is also useful in detecting vacuum leaks and ignition problems causing misfire. Oxygen in the 1.5% is considered ideal, with HC and CO at or near zero and CO² in the 13-15% range.


CARBON DIOXIDE (CO²) is an important diagnostic clue to the efficiency of the combustion process. Carbon monoxide ( one part carbon to only one part oxygen ) is the result of too little oxygen in the combustion process. Carbon Dioxide ( one part carbon to two parts oxygen ) is the result of more perfect combustion. The greater percentage of CO² in the exhaust gas,the more efficiently the engine is running. A range of 13-15% is considered ideal, with HC and CO at or near zero and oxygen in the 1.5% range.


USING THE GAS ANALYSER


Why test HC? As the air/fuel mixture ignites inside the combustion chamber not all of the fuel will be burned, since the internal combustion engine is not 100% efficient. The fuel that remains unburnt will be expelled through the exhaust as HYDROCARBONS. If any condition occurs which allows incomplete burning of the air/fuel mixture, excessive HYDROCARBONS will be emitted.


Why test CO? When the air/fuel mixture is ignited by a spark plug, the resulting explosion produces some carbon monoxide as a normal condition.


Why test CO² and O²? Carbon Dioxide and Oxygen, are not considered gross pollutants of the atmosphere. Instead these gases are used to verify correct engine operation. When catalytic convertors became standard fitment to the vehicle exhaust system HC and CO could no longer be used to accurately diagnose engine malfunctions. The catalytic convertor \"reduces\" the HC and CO concentrations, resulting in lower-than-actual levels at the tailpipe. Since the tailpipe HC and CO levels do not represent their true concentrations, they offer minimal diagnostic information about engine operation. Therefore since HC and CO cannot be accurately measured at the tailpipe, CO² and O² are used in diagnostics because the convertor does not drastically affect their readings at the tailpipe. The tailpipe CO² and O² levels will be nearly identical to the engine CO² and O², although the convertor does produce CO².





Choosing the most favourable setting for a particular engine is a compromise as shown in the tablebelow. When one pollutant is low the others are high.
Obviously a balance has to be achieved and engine manufacturers go to considerable lengths to calibrate carburettors and fuel injection systems to give the best compromise.

Using the GAS ANALYSER.


How do we use use the exhaust gas analyser?

TESTING PROCEDURES.


1..Make sure the analyser is warmed up and properly calibrated before start of testing.


2..Insert exhaust pick-up probe as far as possible into end of tailpipe.


On dual-exhaust cars equipped with manifold heat valve, insert pick-up probe in the tailpipe without heat valve. Be sure the heat valve is free to operate.


3..Some manufacturer's have a provided exhaust gas analysis test openings between the engine and catalytic convertor. Remove the plug and insert the analyser exhaust gas pick-up as far as possible.
Be sure to check the manufacturer's repair manual for the exact locations of the test openings. On vehicles fitted with exhaust downstream air devices i.e. pulse air/air pump, be sure to disconnect and plug the air entry.


4..With the analyser connected, start engine and warm up to normal operating temperature. Accelerate slightly to assure release of any fast-idle cam linkage and ensure any auxillary air devices have closed. Allow engine speed to settle and note emissions reading. Now run the engine at 2500 rpm. Hold the rpm steady and note emissions readings. Release the throttle and allow the engine idle to stabilize. Reconnect any air diverting devices and re run the idle and 2500 rpm tests. Note the emissions readings. When comparing these readings to the previous, there should be a difference in the HC, CO and 0² figures. HC should be low, CO should be low and 0² should be high. CO² levels will diminish slightly.
NOTE...Some manufacturer's recommend that the engine be warmed up under load ( i.e. not at idle ) either by driving the vehicle or running it on a dynamometer. Always follow the manufacturer's recommended procedure.


5..If there is a specification label under the hood of the vehicle, adjust the engine emission levels to those standards.
If there is no emissions specification label, follow published specifications for your state.
If there are no published state specifications and no specification label under the hood follow the vehicle manufacturer's specifications for emission control adjustment procedures regarding how the adjustments are to be made.

Using a dynamometer for a \"loaded\" 2500 rpm test will yield a more accurate emissions reading. In conjunction with the analyser and an ignition scope the use of a dynamometer provides a 'realtime drive' and a fuller 'picture' is obtained during the 2500 rpm test.





Typical pre-cat emissions reading at 800 rpm. (air pump/pulse air disconnected)



Typical pre-cat emissions reading at 2500 rpm. (air pump/pulse air disconnected)




Typical cat. emissions reading at 900 rpm. (pulse air/air pump connected)




Typical cat. emissions reading at 2500 rpm. (pulse air/air pump connected)

Incorrect Emissions Reading?


The exhaust analyser is an excellent tool to aid in the diagnosing of :
1..Engine mechanical problems.
2..Carburetor or fuel injection faults.
3..Ignition system problems.
4..Vacuum leaks.
5..Faulty air injection systems.
6..Computer control system failures.
7..Catalytic convertor condition.
8..Evaporative control system problems.
9..Faulty emission control components.


High Hydrocarbons.
Higher than normal HC readings can be caused by :
Rich or lean air-fuel mixture....carburetor or injection system problems.
Incorrect ignition timing..........distributor,computer or adjustment problem.
Ignition system faults.............spark plug fouling,high tension lead failure,distributor cap fault.
Engine problems....................worn rings,burned valve,head gasket failure.
Emission components............PCV faulty, catalytic convertor faulty, evaporative control system problem.
note : Because the cylinder head and combustion chamber walls are cool (in comparison to the burning fuel ) there is often a thin layer of fuel that doesn't burn. This causes a small HC reading which cannot be eliminated by adjusting the AIR/FUEL RATIO.


High O²
High O² readings may be caused by :
1..Air injection system not disabled and supplying air downstream to exhaust.
2..Exhaust system leakage.
3..Engine misfires in one or more cylinders.
4..An intake leak into the manifold causing a lean condition.
5..Analyser probe not inserted far enough into the tailpipe.
6..A leak in the analyser hosing or probe.
7..A leak in the analyser sample and filtering system.
note :If a leak occurs in the exhaust system or analyser sampling system, high O² readings will be indicated and repairs should be effected before continuing with testing. A diluted gas sample could cause incorrect HC, CO and CO² readings.
If all cylinders are firing correctly most of the oxygen will be consumed by combustion and the O² reading will be close to zero.


High CO
High CO readings can be caused by :
1..Faulty fuel system components.... injector fault, fuel pressure incorrect, restricted air cleaner element, choke system adjustment incorrect, high carburetor float level, computer malfunction, faulty engine sensor, incorrect carburetor jetting.
2..Emission control components.... almost any emission control system can upset CO.
3..Incorrect ignition timing..... ignition timing too far advanced, improper vacuum going to vacuum advance unit.
4..Low engine idle speed..... carburetor or injection system setting incorrect.
note : The analyser's CO reading is related to the air-fuel ratio. A high CO reading would indicate an over-rich mixture ( too much fuel compared to air ). A low CO reading would indicate a lean air-fuel mixture ( not enough fuel compared to air ).


High NOx
1..High compression pressures....cylinder head faults, modified camshafts, ignition timing incorrect.
2..High combustion temperatures....overheating, ignition timing fault, modified camshafts.
3..Lean mixtures....carburetor or fuel injection system problem, vacuum leaks.
4..Exhaust gas re circulation....incorrect operation of the EGR valve and any of the associated components including vacuum valves, hosing, exhaust port restriction and electrical control faults.
note : NOx emissions are produced when combustion temperature exceeds 1370 degrees C (2500 degrees F).

High CO, HC and NOx readings obtained at the tail pipe are an indication of a failure in at least one part of the system, but a reading that appears \" normal \" or slightly elevated is not necessarily a reliable indicator that all is well with a vehicle that is equipped with a catalytic convertor. If the convertor is functioning well, the emissions may be being \" masked \" and the potential for a systems problem may be missed. Further evaluation should be effected and testing of the emissions upstream from the convertor is suggested.


PROBLEM
Rich Mixture

CO High
HC Low
CO² Low
O² Low

POSSIBLE CAUSES
1......Contaminated crankcase
2......Dirty air cleaner element
3......Bowl vent/canister purge
4......Incorrect carburetor adjustment
5......Malfunction of on-board computer system
6......Leaking injectors

PROBLEM
Lean Mixture

CO Low
HC High
CO² Low
O² High


POSSIBLE CAUSES

1......Vacuum leaks
2......Leaking EGR or misrouted vacuum hoses
3......Accelerator pump not working
4......Power valve not working
6......Carburetor adjusted incorrectly
7......Fuel injector fouled/poor spray pattern
8......Malfunction of on-board computer system

PROBLEM
Exhaust/Sample Leak

CO Normal
HC Normal
CO² Low
O² High


POSSIBLE CAUSE
1......Damaged exhaust system
2......Leak at analyser sample hose or probe
3......Analyser filter bowl leak

What are some of the emission control systems currently being used?

POSITIVE CRANKCASE VENTILATION ( PCV )

To recycle blow-by gases from worn rings in an engine a PCV system is used to pass the gases into the intake manifold. The gases which contain HC and CO are routed by the PCV system from the crankcase back into the combustion process. Inoperative or restricted PCV systems can increase engine wear, engine sludging and increase exhaust emissions. A leaking PCV system can cause lean mixtures at idle.

THERMOSTATIC AIR CLEANERS ( TAC )

To reduce tailpipe emissions during engine warm up periods the TAC provides the engine with warm air. The warm air aids in better fuel atomisation, reduces choke operation time and thereby lower fuel consumption and tailpipe emissions. If the airflap in the TAC snorkel stays closed after warm up it can cause over lean mixtures/ detonation and overheating. If the flap stays open during the warm up period the engine could miss, stumble or stall.

IDLE STOP SOLENOIDS

Some vehicles require a higher idle speed to reduce HC emissions. An electric solenoid is used to raise the idle speed. To reduce the possibility of engine run-on an electric solenoid is used to either close the throttle plate, shut off the fuel supply or open an air passage to lean the mixture when the ignition is switched off.

VACUUM DELAY VALVE ( VDV ) / VACUUM SUSTAIN VALVE ( VSV )

A Vacuum Delay Valve delays the operation of a device such as an Exhaust Gas Recirculation ( EGR ) valve by restricting the vacuum supply. The time period of the delay can be altered by differing the size of the internal restricted orifice within the valve.
Vacuum Sustain Valves maintain vacuum to a device such as an ignition distributor vacuum advance unit by restricting the depletion of vacuum. The sustain time is altered by varying the internal restricted orifice of the valve.

DECELERATION DEVICE

Deceleration has a major impact on emissions and fuel consumption. When the throttle plate is closed abruptly the high vacuum developed by the engine can cause a rich mixture in the intake manifold. High HC and CO emissions are produced as a result. Unburned fuel ( HC ) passing to the hot exhaust will also cause exhaust back fire and/or catalytic convertor damage.
Decelerating systems have two basic categories; throttle plate position control systems and lean off of mixture on deceleration systems. These devices can be electronically, mechanically operated or vacuum controlled.
Some types include:

Leaning the Mixture


Coaster Leaner system
Boost Controlled Deceleration Device
Coaster Richer System
Anti-Afterburn Valve Control
Throttle Controllers


Throttle Opener Control System
Deceleration Control System
Throttle Nudger Control
Dash pot Control
Throttle Positioner System
EVAPORATIVE EMISSION CONTROL SYSTEMS

The Evaporative Emission Control system is used to contain all fuel vapour emitted by the vehicle. Sealing the entire fuel system, collection and routing the stored vapours to be re-combusted prevents HC emissions. Manufacturer's use different operating procedures and components within the evaporative control system but basic operation is the same. Systems will store vapour in two possible ways: charcoal canister or crankcase storage. Typical of the components within the evaporative control system include: fuel cap with pressure and vacuum relief valves, liquid/vapour separators, overfill limiting valves, roll over valves, electronically or vacuum controlled purge valves, charcoal canister, and bi-metallic switching valves.

EXHAUST GAS RECIRCULTION (EGR)

When combustion temperatures rise NOx emissions also rise. By using recycled exhaust gas in the combustion cycle these temperatures can be lowered. The EGR valve either vacuum operated or electronically controlled is used to achieve this. When the valve is open the recirculated exhaust gas is released into the intake manifold to be drawn into the combustion chamber. The EGR system when functioning correctly should: prevent EGR operation until the engine is at operating temperature, allow EGR operation only at part throttle and vary the EGR operation according to engine load.

CATALYTIC CONVERTORS

Control of the HC, CO, and NOx tailpipe emissions can be controlled by three types of convertors: Oxidation, Dual- Bed and 3-way.
Oxidation type convertors uses excess O² to oxidise the CO and HC remaining from the combustion process. NOx is not controlled. Downstream air devices or leaner mixtures are used with this system.
Dual-bed type convertors utilise two catalysts in series. The first in line is a reduction catalyst to control NOx. The second catalyst is an oxidation catalyst. By injecting air in front of this catalyst control of HC and CO emissions is effected.
3-way convertors are used on an engine that produces a near perfect air-fuel ratio. The use of an oxygen sensor with this system aids in the control HC, NOx and CO emissions.

My recommendation.. Forget the NOx, get the NO2 and outrun them all...

Love your detailed posts, they should all be in the Tech archive as well, saves searching through hundreds of posts

Largely from other sources, so I can't take the credit, just thought a few here might find it interesting.