EXHAUST GAS ANALYSIS

THEORY

The gases that come out of the exhaust provide us with very important information. The main gases that are used in exhaust gas analysis are:-

·       CO₂

·       O₂

Other exhaust emissions used are H₂O, SO₂, HO and C.

CARBON DIOXIDE  (CO₂)

Carbon dioxide is a desirable by-product that is produced when the carbon from the fuel is fully oxidised during the combustion process. As a general rule, the higher the carbon dioxide reading, more the efficient the engine is operating. Therefore, air/fuel imbalances, misfires or engine mechanical problems will cause CO₂ to decrease. Remember, “Ideal” combustion produces large amounts of CO₂ and H₂O (water vapour).

OXYGEN (O₂)

Oxygen readings provide a good indication of a lean running engine, since O₂ increases with leaner air/fuel mixtures. Generally speaking, O₂ is opposite of CO, i.e., O₂ indicates leaner mixtures while CO indicates richer air/fuel mixtures. Lean air/fuel mixtures and misfires typically cause high O₂ output from the engine.

OTHER EXHAUST EMISSIONS

There are a few other exhaust components which impact driveability and/or emissions diagnosis, that are not measured by shop analyzers. They are:

·       Water vapour (H₂O)

·       Sulphur dioxide (SO₂)

·       Hydrogen (H₂)

·       Particulate carbon soot (C)

Sulphur dioxide (SO₂) is sometimes created during the combustion process from the small amount if sulphur present in gasoline. During certain conditions the catalyst oxidizes sulphur dioxide to make SO₃, which then reacts with water to make H₂SO₄ or sulphuric acid. Finally, when sulphur and hydrogen react, it forms hydrogen sulphide gas. This process creates the rotten egg odour you sometimes smell when following vehicles on the highway. Particulate carbon soot is visible black “smoke you see the tailpipe of a vehicle that’s running very rich.”

PROCESS

First all I took an Exhaust Analyser. It should be warmed up and recently calibrated. When it’s time to start taking tailpipe readings, put the analyser probe into the tailpipe and install exhaust tube over probe to capture probe to capture exhaust fumes. Make sure there is adequate ventilation.

·       First of all I noted the analyser readings when it was sensing normal air:-

                    I.        CO: - 0.001

                  II.        HC: - 12

                III.        CO₂: - 0.001

                IV.        O₂: - 20.40

·       Then I started the engine idling cold and then took the reading

I.      CO: - 2.5

II.     HC: - 788

III.    CO₂: - 12.6

                  V.        O₂: - 5.15

·       Then when the engine was warmed up then I took  the recording again

I.      CO: - 0.001

II.     HC: - 78

III.    CO₂: - 13.01

IV.    O₂: - 0.54

·       Then I took the readings with the engine warmed up and revving at 2500 rpm

I.      CO: - 0.234

II.     HC: - 121

III.    CO₂: - 13.10

IV.    O₂: - 20.32

·       Then I took the readings engine running at idle with running the mixture rich with extra propane, LPG, or carburettor cleaner,

I.      CO: - 0.961

II.     HC: - 224

III.    CO₂: - 14.32

IV.    O₂: - 0.18

·       Then with the engine running at idle, with a lean condition created by an air leak or vacuum leak, I took the readings

I.      CO: - 0.029

II.     HC: - 470

III.    CO₂: - 7.79

IV.    O₂: - 5.93

·       Then i accelerated the engine, by blipping the throttle a few times, and watched the gas readings change

I.      CO: - 7.5

II.     HC: - 434

III.    CO₂: - 10

IV.    O₂: - 1.3

·       Then I disconnected one spark plug wire, and grounder it with a jumper wire and recorded the readings

I.      CO: - 0.01

II.     HC: - 10.33

III.    CO₂: - 13.6

IV.    O₂: - 4.53

·       Then I disconnected the injector harness connector from one injector on an engine that has one injector for every cylinder. Then I recorded the reading.

I.      CO: - 0.008

II.     HC: - 57

III.    CO₂: - 11.61

IV.    O₂: - 5.45

·       Then I switched on the air/con and then I recorded the readings.

I.      CO: - 0.009

II.     HC: - 18

III.    CO₂: - 15.44

IV.    O₂: - 0.04

REFLECTIONS

CAUSES OF EXCESSIVE EXHAUST EMISSIONS

As a general rule, excessive HC, CO, and NOx levels are most often caused by the following conditions:-

·       Excessive HC results from the ignition misfire or misfire due to excessively lean or rich air/fuel mixtures

·       Excessive CO results from rich air/fuel mixtures

·       Excessive NOx results from excessive combustion temperatures.

Causes of excessive hydrocarbons

·       Faulty ignition secondary component

·       Leaky intake manifold gasket

·       Worn throttle shaft

·       EGR valve stuck open or excessive EGR flow rate

·       In correct spark timing

·       Exhaust leakage past exhaust valve

Causes of Excessive Carbon Monoxide

·       Excessive fuel pressure at the injector

·       Leaking fuel injector

·       Ruptured fuel pressure regulator diaphragm

·       Loaded or malfunctioning EVAP system.

·       Crankcase fuel contamination

·       Plugged PCV valve or hose

·       Closed loop control system incorrectly shifted rich

Causes of Excessive Oxides of Nitrogen

·       Insufficient radiator airflow

·       Low coolant  level

·       Slow rich to lean switch mode

·       Incorrect base timing

·       False input signal to ECM

·       Poor cooling fan operation