Use of a four or five Gas Exhaust Analyser can be helpful in troubleshooting both emissions and driveability concerns. Presently, shop grade analysers are capable of measuring from as few as two exhaust gasses, HC and CO, to as many as five. The five gasses measured (for petrol emissions) by the latest technology exhaust analysers are: HC, CO, CO2, O2 and NOx. All five of these gasses, especially O2 and CO2, are excellent troubleshooting tools. Use of an exhaust gas analyser will allow you to narrow down the potential cause of driveability and emissions concerns, focus your troubleshooting tests in the area(s) most likely to be causing the concern, and save diagnostic time. In addition to helping you focus your troubleshooting, an exhaust gas analyzer also gives you the ability to measure the effectiveness of repairs by comparing before and after exhaust readings. In troubleshooting, always remember the combustion chemistry equation: Fuel (hydrogen, carbon, sulphur) + Air (nitrogen, oxygen) = Carbon dioxide + water vapour + oxygen + carbon monoxide + hydrocarbon + oxides of nitrogen + sulphur oxides. When we do exhaust analysis, we are being a detectives. We look at what came out of the exhaust and figure out what could have happened before to create those emissions. What happened in the combustion chamber, or before the combustion chamber, to create these results?

We can use clues and patterns of exhaust readings to figure out if we have a problem in one of the following areas:

• Air/Fuel Ratio

• Combustion

• Ignition

• Emission Control Devices

Interpretation can be drawn from following gases:

HC = Hydrocarbons, concentration of the exhaust in parts per million (ppm). = Unburned Petrol, represents the amount of unburned fuel due to incomplete combustion exiting through the exhaust. This is a necessary evil. We don't want it so try to keep it as low as possible. An approximate relationship between the percentage of wasted fuel through incomplete combustion and the ppm of HC is about 1/200 (1.0% partially burned fuel produces 200 ppm HC, 10%=2000 ppm HC, 0.1%=20 ppm HC)

CO = Carbon Oxide, concentration of the exhaust in percent of the total sample. = Partially Burned Petrol, This is the petrol that has combusted, but not completely. This gas is formed in the cylinders when there is incomplete combustion and an excess of fuel. Therefore excessive CO contents are always a sign of an overly rich mixture preparation. (The CO should have become CO2 but did not have the time or enough O2 to become real CO2 so it is exhausted as CO instead.) CO is HIGHLY POISONOUS ODORLESS GAS! Always work in well ventilated areas!

CO2 = Carbon Dioxide, concentration of the exhaust in percent of the total sample. = Completely Burned Petrol, represents how well the air/fuel mixture is burned in the engine (efficiency). This gas gives a direct indication of combustion efficiency. It is generally 1-2% higher at 2500 RPM than at idle. This is due to improved gas flow resulting in better combustion efficiency. Maximum is around 16%. At night the trees convert CO2 in to Oxygen. Preserve them!

O2 = Oxygen, concentration of the exhaust in percent of the total sample. Free O2 occurs in the exhaust when there is an excess of air in the mixture. The O2 content increases sharply as soon as Lambda rises above 1. Taken with the CO2 maximum, the oxygen content is a clear indicator of the transition from rich to lean mixture range, or leaks in the manifold or exhaust systems or combustion failures. With rich mixture most of the oxygen is burned during combustion. Whit very lean mixture more O2 escapes "un-combusted" so the level rises.

NOx = Oxides of Nitrogen (This is only seen by a 5-gas analyser) only seen with dynamometer or engine under load. NOx emissions rise and fall in a reverse pattern to HC emissions. As the mixture becomes leaner more of the HC's are burnt, but at high temperatures and pressures (under load) in the combustion chamber there will be excess O2 molecules which combine with the nitrogen to create NOx. NOx increases in proportion to the ignition timing advance, irrespective of variations in A/F ratio. This gas is related to the exhaust gas detoxification systems (in conjunction with Co and HC) , exhaust gas recirculation systems. Those systems bring some of the inert (processed) exhaust gas back in to the engine to be burned again. This time around this gas has no O2 extra molecules and prevents high combustion temperatures and further increase in NOx formation. NOx is Very Dangerous Lethal Gas and air pollutant!

A/F ratio or Lambda = Calculated Air/Fuel Ratio or Lambda value based on the HC, CO, CO2 and O2 concentrations. Remember the ideal (Stoichiometric) A/F is 14.7 liters air to 1 liter fuel or 14.7/1. The ideal Lambda value is 1(one) below that the A/F mixture is rich and above - lean. For example, lambda=0.8 corresponds to an air/fuel ratio of (0.8x14.7):1=11.76:1 (e.g. lambda 0.8 = A/F ratio of 11.76/1 or very rich air fuel mixture). General rules from emission analysis:

• If CO goes up, O2 goes down, and conversely if O2 goes up, CO goes down. Remember, CO readings are an indicator of a rich running engine and O2 readings are an indicator of a lean running engine.

• If HC increases as a result of a lean misfire, O2 will also increase

• CO2 will decrease in any of the above cases because of an air/fuel imbalance or misfire

• An increase in CO does not necessarily mean there will be an increase in HC. Additional HC will only be created at the point where rich misfire begins (3% to 4% CO)

• High HC, low CO, and high O2 at same time indicates a misfire due to lean or EGR diluted mixture.

• High HC, high CO, and high O2 at same time indicates a misfire due to excessively rich mixture.

• High HC, Normal to marginally low CO, high O2, indicates a misfire due to a mechanical engine problem or ignition misfire.

• Normal to marginally high HC, Normal to marginally low CO, and high O2 indicates a misfire due to false air or marginally lean mixture.