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Five Gas Diagnostics

By: Steve Zack

The four pollutants emitted by internal combustion engines are Oxides of Nitrogen (NOx), Carbon Monoxide (CO), Hydrocarbons (HC), and Sulfur Dioxides (SO2). NOx and CO are the byproducts of engines operating under less than ideal conditions. HC is the unused portions of the fuel and SO2 is a byproduct of combustion. The catalytic converter also produces SO2. In theory, a perfect running engine would create only nontoxic byproducts of Carbon Dioxide (CO2), and Water Vapor (H2O). However, too much CO2 is not good for the atmosphere, as this has helped to create global warming. Even under the best of circumstances, engines do not operate under ideal conditions, and it only gets worse as the engine wears.

The production of the five gases.

The PCM controls fuel delivery and ignition timing based on air demand, which in turn will cause an exchange of elements creating energy. As the intake valve opens, atmospheric pressure of 14.7 Psi over takes the low-pressure area formed in the cylinder. This air combines with Hydrocarbons (HC), gasoline to form the basis for the combustible. Gasoline is made up of Benzene (C6H6), Olefins (CnH2n), Sulfur (S), Methyl Tertiarybutyl Ether (MTBE) and Oxygen (O2). The air that the engine breathes is made up of 21% Oxygen, and 77 to 78% Nitrogen. Oxygen consists of two elements of O, (O2), and the engine uses it to create energy through an exchange of elements, combining Oxygen with either Carbon or Hydrogen, known as oxidation. Nitrogen consists of two elements of N, (N2), and it is a stable gas used in the engine to absorb heat in the combustion chamber. There are also Helium (He), Argon (Ar), Neon (Ne), Hydrogen (H2), Water (H2O), Carbon Dioxide (CO2) and Hydrocarbons (HC) in the air.

As air and fuel enter the combustion chamber during the intake stroke, the valves will close as the piston rises during compression, sealing the chamber. As the piston rises, the pressure will increase causing the elements of air and fuel to impact. The impact is a result of trying to push the elements together, bouncing off one another, and bouncing off the combustion chamber walls. The elements begin to absorb the heat created by the friction from the impact, and they begin to expand and start to separate, becoming a vapor. The O2 (air) separate into individual Oxygen elements, and the HC (fuel) will separate into individual Hydrogen (H) and Carbon (C) elements. Because of the increase in pressure during compression and the impact of the elements, the speed of the element increases. Remember that the separated elements are now lighter and unstable, bouncing all over looking for a place to attach.

As the spark plug ignites, electrons cross the two electrodes of the spark plug under pressure. This extreme electron pressure created by the spark plug in the combustion chamber forces the elements of O and O and H and C towards a compatible bond. With spark ignition, one Oxygen element will oxidize (combine) with two Hydrogen elements to create Water, and two Oxygen elements will combine with one Carbon element to create Carbon Dioxide. As oxidation increases, the newly created Water and Carbon Dioxide create additional pressure, which the engine converts into usable power. As the elements attach to their pair, another collision has occurred and the speed of the elements increases. This oxidation or attachment of Oxygen elements to its mate, and their resulting collision appears as a spark, and that spark multiplied by the many collisions appears as fire. Energy occurred as a result of the collisions, and CO2, and H2O have formed.

As long as the combustion chamber temperature remains below 2300 degrees Fahrenheit, the Nitrogen elements will not separate. However, as pressure increases closer to top dead center (TDC) the temperature rises above 2300 degrees and Nitrogen will separate into individual Nitrogen elements. One individual Nitrogen element will oxidize with one or two Oxygen elements forming a bond known as NOx.

Obviously the solution would be to keep cylinder temperatures below 2300 degrees Fahrenheit, and certainly considerable progress has been made in that regard. However, the combustion area in the cylinder contains many "pockets" where temperatures will be significantly higher than in the rest of the cylinder. Any area not directly surrounded by a cooling jacket is an obvious candidate. To help prevent temperatures that create NOx, engine manufacturers have installed an EGR (Exhaust Gas Recirculation) valve and passage to recycle exhaust gas to cool the combustion chamber down. Over the years, they have also lowered compression ratios to reduce the pressure, thereby reducing the friction of the gases, thus lowering combustion temperature. A richer fuel mixture also creates additional hydrocarbons to absorb the heat, and retarded timing advance is another part of the strategy to reduce combustion chamber pressures. The catalytic converter will convert NOx to N2 and CO2, and oxidize HC and CO to H2O and CO2.

All these collision under pressure are movement or vibration that occurs beyond piston speed. The cylinder wall cooling passages will reduce some of that speed by absorbing the temperature from the elements. The piston crown will absorb the impact of the elements, while the oxidizing gases expand under pressure pushing against the piston. As the piston absorbs the impact of the elements, it will reduce some of the vibration or speed of the elements. As the elements exit through the exhaust valve the speed of the elements or vibration is still high. The O2 sensor in the exhaust stream will see the vibrating exhaust gas; this appears on the oscilloscope as an oscillation.

Some people are thinking that if this were how it works the engine would have a massive vibration. The impact occurs against multiple planes that absorb or dampen some of the vibration through heat reduction. This does not, nor is it powerful enough to cause engine vibration. Engine horizontal vibration occurs from momentum created by the rotation of the crankshaft and connecting rods. The crankshaft webs cancel the vibration. The piston reciprocating weight causes engine vertical vibration. The crankshaft webs also cancel this.

What are the three pollutants?

By understanding how the gases are formed, you will be able to analyze what the 5 gas analyzer is reading to determine the cause of the excessive NOx, CO, and HC and it's mechanical or electrical failure.

What is NOx?

NOx is an odorless colorless gas that is a byproduct of combustion and it is a toxicant. Approximately 77 to 78 percent of the air used by the engine contain two elements of Nitrogen. This gas absorbs heat in the combustion chamber. When temperatures approach 2300 degrees Fahrenheit, because of high pressure, the Nitrogen separates to N and N, and it will combine with Oxygen elements.

NOx is one element of Nitrogen and unknown (or "x") quantities of Oxygen elements. NOx actually contains NO and NO2. NO is one element of Nitrogen and one element of Oxygen and it is colorless, it makes up the bulk of the NOx. NO2 is one element of Nitrogen and two elements of Oxygen and it is brown in color. Your meters read NOx due to the high cost of a meter to read NO and NO2.

What is CO?

Carbon Monoxide is an odorless colorless gas, it is a byproduct of combustion, and it is a toxicant. Carbon Monoxide forms because of insufficient oxygen. With a fuel, enriched condition there is not enough Oxygen to create CO2. One Oxygen element is missing, therefore creating CO.

CO is one element of Carbon and one element of Oxygen.


What is HC?

HC is the result of unused fuel and it is a toxicant. A rich fuel mixture will form HC because of limited Oxygen reducing oxidation of the H and C. A lean condition forces the HC back together because of the high pressure from too much air. Poor compression pressure can cause high HC, because of low pressure resulting in poor separation of the HC. Ignition system problems can cause high HC, because of not creating the additional pressure to cause the H and C to combine with O. Under pressure, the vaporized H and C move towards the cylinder walls, therefore the cooling system absorbs the gases high temperature causing the gases to condense back to HC.

Your 5-gas analyzer measures the HC or Hexane value of the gasoline due to the high cost of a gas chromatograph.

HC is one element of Hydrogen and one element of Carbon.


How to diagnose the failures of the five gases with a five gas analyzer.

Many component failures and system malfunctions can cause the five gases. For example, if the cooling system is utilizing pure water, it will absorb combustion heat and obtain its boiling point rapidly, leaving an air pocket around the combustion chamber. The air pocket will become a hot spot allowing the combustion chamber temperature to rise. Equally bad is 100% antifreeze, which forms a blanket around the combustion chamber, keeping the heat in and allowing the combustion chamber temperature to rise. Rust surrounding the cooling jacket surface will create the same blanket around the combustion chamber. Poor flow through the radiator, because of a blockage or poor circulation, or a partially closed thermostat or limited flow from the water pump, will prevent the high temperatures from escaping through the cooling system. The temperature increase will cause preignition, allowing the Oxygen to prematurely oxidize the Hydrogen, reducing the available air. A loose fan belt, worn clutch fan or an electrical fan working incorrectly can cause the heat in the radiator not to cool because of insufficient air passing by the radiator. Therefore, Oxygen will not properly complete the oxidation of the Hydrogen and Carbon into Carbon Dioxide and water. In this case, the gas meter will read high HC, high O2, low CO, low CO2, and high NOx.

As the piston rises during the compression stroke, the force of compression in the upward direction and the downward force created because of preignition will cause the piston to rock against the cylinder wall, causing a knock. Because of an engine knock, we should see the scan tool knock reading indicate yes, with a command to retard ignition timing. The firing line voltage will exceed 12 kV. With excessive preignition, the early flame front is exposed to the spark ignition flame front causing the two flame fronts to increase pressure. This will result in high HC, high O2, low CO, low CO2, and high NOx. The horsepower of the engine will suffer as well.

Carbon build-up on top of the piston or on the cylinder walls may also cause preignition. This would give similar results to the cooling system issue. The carbon buildup could be because of running extremely rich for any length of time. Therefore, when repairing high CO emissions failures, always assume that carbon has formed. Oil consumption will also cause this type of carbon, which will also cool the combustion chamber temperature. Carbon on the throat of the valve will absorb fuel, causing a lean condition and giving a similar result as to the previous lean condition. This type of diagnosis may require a borascope to visually inspect pistons, cylinder bores or valves. To repair this, a chemical top end cleaner may help. If the vehicle is not running rich, the gas analyzer will read high HC, high O2, low CO, low CO2, and high NOx. The reason for the high O2 is that the carbon will assimilate a lean ignition misfire. The scan tool would read low oxygen sensor voltage. The secondary oscilloscope would read high firing voltage, exceeding 12 kV, and a possible appearance of second firing line in the spark line.

Mechanical engine problems such as poor valve seating, or worn piston rings cause compression pressure to be low. This results in the O2 and HC not effectively separating, limiting oxidation. Engine vacuum may be low causing MAP sensor voltage to be higher than normal allowing the PCM to increase injector pulse width, resulting in a rich condition. The gas analyzer will read high HC, low O2, high CO, low CO2, and low NOx.

The exhaust valve-seating surface is not just to seal the cylinder airtight, but to provide a means of removing heat from the valve and disbursing it to the cooling system. An incorrectly seated exhaust valve will not transfer heat. Consequently, the valve and its seat will rise in temperature, causing preignition. This will result in high HC, high O2, low CO, low CO2, and high NOx.

A worn or slipped timing belt can increase internal temperatures. If the timing belt or timing chain has excessive slack, the cams timing retards. The camshaft will be behind the crankshaft resulting in the camshaft lobes not opening the valves in the proper relationship to the piston. The intake valves during the intake stroke will open late, causing the air to continue entering the cylinder later than required. Therefore, the compression pressure will increase at the top of the compression stroke and temperature will reach its maximum later into the stroke. This is because of later oxidation that results in extreme combustion temperature. Because of late intake valve opening, the vacuum will be low, causing the MAP sensor voltage to be high adding more fuel, causing high CO. The gas analyzer will read high HC, low O2, high CO, low CO2, and high NOx. NOx may be low because of the high CO.

If the EGR system were inactive, because of a plugged passage, inoperative valve, inoperative vacuum control system, or an electronic malfunction, the controlled exhaust flow would not occur. This would allow the combustion temperature to rise creating NOx. As the EGR opens, exhaust flows to the combustion chamber, acting as a combustion coolant. The gas analyzer would see high HC, low O2, low CO, low CO2, and high NOx. Many PCM strategies reduce injector pulse width when the EGR command to open occurs because the exhaust gas contains unused HC, an enriched condition. If the EGR passage did not open, the PCM reduced injector pulse width creating a lean condition that aids in creating NOx. The gas analyzer would see high HC, high O2, low CO, low CO2, and high NOx.

Rich conditions caused by MAP sensor out of calibration, TPS out of adjustment, IAC closed, injectors leaking, low vacuum or a dirty air cleaner will cause high HC and result in high CO. An evaporative purge system passing fuel at the wrong time, caused by vacuum control, electrical control or evaporative canister filled with gas can also result in a rich mixture. The gas analyzer will read high HC, low O2, high CO, low CO2, and low NOx. An engine crankcase filled with gasoline as it comes to operating temperature will turn the gasoline to a vapor. The HC vapor pulls through the PCV system by engine vacuum causing high HC resulting in high CO. Whenever the gas analyzer reads high CO, remove the PCV valve or block the passage, if CO drops 1 to 1.5 percent change the engine oil. The gas analyzer will read high HC, low O2, high CO, low CO2, and low NOx.

A lean condition because of a false signal from the oxygen sensor, an out-of-calibration MAP sensor, plugged injector, low fuel pressure and/or low pump volume, open IAC, or a vacuum leak will increase HC, O2 and NOx. In a lean condition, excess Oxygen will increase pressure, causing high temperatures. As the temperature climbs the Nitrogen elements will separate and form with the Oxygen elements to create NOx. As the Oxygen oxidizes to produce NOx, the available Oxygen reduces, causing many Hydrocarbons to be unused. The gas analyzer will read high HC, high O2, low CO, low CO2, and high NOx. The secondary ignition oscilloscope should have a higher than normal firing line, exceeding 12 kV, and shorter than normal spark duration. The scan tool would read zero to low oxygen sensor voltage and a lean condition. The CO element needed in the catalytic converter to cause the catalyst to reduce NOx to N2 and CO2, and oxidize HC to CO2 and H2O is not available. Consequently, the catalytic converter ceases to function.

Secondary ignition voltage that is low because of spark plugs, spark plug wire, ignition coil or the distributor cap and rotor will reduce the second wave of pressure causing poor oxidation. This results in high HC, high O2, low CO, low CO2, and low NOx. High secondary voltage because of spark plugs, spark plug wires, and distributor cap and rotor will cause high HC, high O2, low CO, low CO2 and high NOx. NOx maybe be high because of the pressure increase created by the high secondary voltage.

The primary ignition system supplies the secondary ignition system with current. If the primary ignition circuits are in trouble because of low current due to high resistance or problems with the battery and charging system, secondary voltage will suffer. This can also include problems with the coil, ignition module and crank sensor. As result of poor primary condition causing low secondary voltage, oxidation suffers because the second wave of pressure is limited. This results with gas readings of high HC, high O2, low CO, low CO2, and low NOx.

If the base timing is advanced too far, the spark plug will ignite the air fuel mixture early, causing the combustion pressure to be higher as the compression stroke continues, causing NOx to form. With a very early flame front in the compression stroke, as a result of advanced ignition timing, the pressure created as a result of early oxidation will cause the temperature created during compression to rise extensively. The gas analyzer will read high HC, high O2, low CO, low CO2, and high NOx. Retarded ignition timing will cause engine vacuum to be low resulting in high MAP sensor voltage. Therefore, injector pulse width will be higher causing a richer mixture resulting in high HC, low O2, high CO, low CO2, and low NOx.

The last and final area of concern, and just as major as the rest is the charging system. The charging system replenishes the battery after starting and then through the battery it feeds the primary ignition circuit, fuel pump, injectors and the PCM. Low charging voltage to the injectors or the fuel pump will result in a lean condition. The gas readings would be high HC, high O2, low CO, low CO2, and high NOx. If the charging system voltage to the ignition coil is low, it will cause low secondary voltage. The gas analyzer readings would be high HC, high O2, low CO, low CO2, and low NOx. Incorrect charging voltage, such as a diode spike, will alter PCM reference voltage, which will alter sensor values affecting injector pulse width and ignition timing resulting in a rich condition.

An exhaust system that is leaking will read high O2 and possible low CO2 on the gas analyzer. Repair any exhaust leaks because this false condition will have you chasing inaccurate O2 readings.

To verify the catalytic converter functions increase engine speed to 2000 rpm to allow the catalytic to reach operating temperature. At a steady 2000 rpm with the catalytic converter at operating temperature, the Oxygen reading should be less than .5 percent, and ideally .2 percent.

As you can see, the causes of the five gases are by mechanical failures, on board computer electrical errors or ignition system problems. With a 5-gas analyzer you can isolate the problem, and replace the faulty components or make adjustments as required. The most important thing to remember is that the PCM adjusts fuel and ignition timing based on air volume. Therefore, the conditions reported in the diagnosis section affect driveability and fuel mileage as well as emissions. The 5-gas analyzer is your best attack on driveability complaints, as it will lead you to the area at fault. Use it first to diagnose the problem and then follow with either the scan tool, oscilloscope or compression gauge or borascope to determine the cause. If you follow this approach, your flat rate time improves and you will be more profitable.

DRIVEABILITY SPECIFICATIONS

TEST AT 1000 RPM, ENGINE AT OPERATING TEMPERATURE

HC - 100 PPM MAXIMUM
O2 - .5% MAXIMUM
CO - .5% MAXIMUM
CO2 - 12% TO 15% MAXIMUM
NOx - 150 PPM TO 300 PPM MAXIMUM

DIAGNOSTIC CHART

HC

O2

CO

CO2

NOx

COOLING

HIGH

HIGH

LOW

LOW

HIGH

PREIGNITION

HIGH

HIGH

LOW

LOW

HIGH

CARBON

HIGH

HIGH

LOW

LOW

HIGH

CARBON INTAKE VALVE

HIGH

HIGH

HIGH
OR
LOW

LOW

HIGH
OR
LOW

MECHANICAL

HIGH

LOW

HIGH

LOW

LOW

EXHAUST VALVE

HIGH

HIGH

LOW

LOW

HIGH

CAM TIMING

HIGH

LOW

HIGH

LOW

HIGH

CAM TIMING EXCESSIVE

HIGH

LOW

HIGH

LOW

LOW

EGR

HIGH

LOW

LOW

LOW

HIGH

RICH

HIGH

LOW

HIGH

LOW

LOW

CONTAMINATED OIL

HIGH

LOW

HIGH

LOW

LOW

LEAN

HIGH

HIGH

LOW

LOW

HIGH

EXTREME LEAN

HIGH

HIGH

LOW

LOW

LOW

SECONDARY LOW VOLTS

HIGH

HIGH

LOW

LOW

LOW

SECONDARY HIGH VOLTS

HIGH

HIGH

LOW

LOW

HIGH

PRIMARY

HIGH

HIGH

LOW

LOW

LOW

BASE TIMING ADVANCED

HIGH

HIGH

LOW

LOW

HIGH

BASE TIMING RETARDED

HIGH

LOW

HIGH

LOW

LOW

CHARGING SYSTEM LOW VOLTS FUEL

HIGH

HIGH

LOW

LOW

HIGH

CHARGING SYSTEM LOW VOLTS IGNITION

HIGH

HIGH

LOW

LOW

LOW

EXHAUST LEAK

LOW

HIGH

LOW

LOW

LOW

 

 
 
 
 
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