BACK to TUNING

Exhaust Gas Analysers

Petrol is a Hydrocarbon (HC) made up of Hydrogen and Carbon. These two elements mix readily with Oxygen. When Hydrogen unites with Oxygen, water (H₂O) is formed. When carbon unites with Oxygen, Carbon Monoxide (CO) and Carbon Dioxide (CO₂) are formed. If all the fuel burns completely in an engine, the only component to come out the tailpipe would H₂O and CO₂. However perfect combustion cannot be achieved so some CO and HC are present in the exhaust as well as a third compound, Nitrogen Oxide (NO_x). These are the three pollutants that we are trying to eliminate with various pollution control devices.

By measuring and analysing the exhaust gas of an internal combustion engine using an exhaust gas analyser, the degree of combustion and efficiency can be determined and is a must have piece of equipment for any Tuning specialist. In the old days somewhere around about 1974, my boss, Brian was using a Vane analog air fuel ratio (AFR) meter that measured A/F ratio obviously and displayed it on an analog gauge. The background of the gauge, behind the needle was colour coded for easy identification of lean, rich and optimum. No other information on exhaust content was available. Within a few years Brian purchased a new oscilloscope which incorporated a new measurement system to cater for tighter pollution laws that have been around for a few years. These new machines had a two way exhaust analyser which now measured Carbon Monoxide and Hydrocarbons. After a little learning adjustment from using AFR as a reference, we soon became accustomed to now using Carbon Monoxide as a reference for fuel mixture strengths. In 1979 I started my own business and the first thing I bought was a new Allen oscilloscope with an analog 2 way infrared exhaust gas analyser. With the introduction of unleaded fuel and the catalytic converter more complex and sophisticated analysers were necessary for diagnosis and tuning. The new breed of analysers were now digitally processed and started with the three way gas analyser and then the four gas. I purchased a 4 gas analyser in about 1988 when I found it necessary to keep up with technology. There are actually five gases that need to be sampled to be able to get the full picture of how well the combustion process has taken place but until only recently the cost of a five gas analyser had been out of reach for many workshops because of the cost. The five common gases produced by the internal combustion engine are listed below:

The basic exhaust gas analyser at a minimum must be 2 gas, CO and HC. This analyser is all that is required for diagnosis and tuning of older vehicles, pre catalytic converter. To successfully diagnose, tune and adjust modern engines however, a 4 or 5 way analyser is required to achieve optimum results. To be honest, I have to say that I never really got much experience in using the 4 way analyser as I should have. On most post 1986 dyno work, I tapped into the exhaust in front of the catalytic converter and used only CO & HC to diagnose and tune these vehicles. After the tune-up, I would then place the sniffer into the tailpipe and check how well the catalytic converter was doing its job in reducing pollutants. The efficiency of the catalytic converter cannot be checked without the use of a 4 or 5 way gas analyser. It was only in the last 6 months of business that I was regularly measuring the exhaust content at the tailpipe as I became more familiar with understanding CO₂ and O₂

Since I don't have a vast experience level of all the gases produced, only the theory of it, I will only discuss the two commonly used in a 2 gas analyser, CO & HC and are explained in a little more detail below.

Carbon Monoxide (CO)

CO is a colorless, odorless gas that is formed when carbon in fuel is not burned completely due to lack of oxygen. CO reduces the ability of the blood to absorb oxygen, this fact, together with it also being colourless, odourless, and tasteless, makes CO extremely dangerous. A proportion as low as 0.3% by volume can prove fatal within 30 minutes and hence the reason why strict regulations were introduced and getting tighter each year to control the emission level of this gas.

High concentrations of CO cause oil dilution, resulting in engine damage and can also form carbon deposits in the combustion chamber and on spark plugs causing pre-mature failure. The carbon deposits can also cause pre-ignition when they glow red hot and ignite the mixture before the spark plug does.

Some causes of excessive CO are:

On the other end of the scale is of course a too low a reading. Older cars were not designed to run lean mixtures and the most common consequences of running lean was valve damage, spark plug failure, engine over-heating and the vehicles would surge while driving under light load. In saying this I had discovered that even the later model vehicles benefitted from having the idle and cruise mixtures set just slightly higher than recommended settings. This improved idle quality and gains in fuel economy, but this is another topic for discussion. CO in essence is really how proportionally correct the fuel and air are mixed and delivered to the combustion chambers.

HydroCarbon (HC)

As stated earlier, petrol is almost pure hydrocarbon, made up of Hydrogen and Carbon. On modern vehicles, measurements stated by the manufacturers are generally given in grams per kilometre. However in a workshop, HC is measured in parts per million (PPM) when using an infra-red emission analyser. Because of combustion chamber designs and variations in air fuel ratios, a certain amount of HC will always be exhausted as unburnt fuel. A reading of 50 to 300 PPM in an emission controlled vehicle of the 1972 era or earlier was about the norm. Pre-emission vehicles it was not uncommon to have a HC reading of up to 800PPM of HC. However the idea was to have the HC as low as possible as this maximises fuel conversion to power and also improved economy. Raw fuel also emits HC fumes and will send the readout on the analyser to overload because as mentioned before, petrol is almost a pure hydrocarbon.

Some causes of a high HC readings, There are many other less likely causes that are not in the list, I have only listed the most common causes:

Whatever the cause may be, when the HC is high, loss of performance and fuel economy can be expected. HC in essence is really how well the engine burns the fuel.

The infra-red analyser can be used for other very useful diagnostics. The most common is of coarse, a fuel leak. When the probe of the analyser comes anywhere near raw fuel vapour, it goes off its tree. This makes it useful to detect the tiniest of leaks. An example would be a faint smell of fuel inside the car. The probe will pick up fuel vapour long before the human nose will smell it. Placing the probe in obvious places will narrow down where the leak is originating from as the readings would get higher closer to the source.

Another use is checking for blown head gaskets between the water jackets and the cylinders. The radiator cap is removed and the probe is placed very close to the header tank opening without submerging it in the water. The engine is revved hard a few times and if any HC is present in the water it will travel up the filler neck of the radiator and will be measured by the analyser. The instrument will measure as low as 10PPM, now that is very sensitive. Any HC reading registered, indicates that exhaust gas is entering into the coolant system via the combustion chamber either by a blown head gasket (most common) or possibly a crack in the head.