Exhaust gas analyzer

Lambda coefficient measurement

The presence of oxygen in the exhaust gases indicates that the combustion of the mixture was not perfect resulting in contaminant gases. Thus measuring the proportion of oxygen in the exhaust gases of these engines can monitor and measure these emissions. This measurement is performed in the MOT test through Lambda coefficient measurement.

The Lambda coefficient (λ) is obtained from the relationship between air and involved in gasoline combustion of the mixture. It is a measure of the efficiency of the gasoline engine by measuring the percentage of oxygen in the exhaust.

When gasoline engines operate with a stoichiometric mixture of 14.7: 1 the value of LAMBDA (λ) is "1".

Mixing ratio = weight of the fuel / weight of air

- Expressed as mass ratio: 14.7 kg of air per 1 kg. of fuel. - Expressed as volume ratio: 10,000 liters of air per 1 liter of fuel.

With this relationship theoretically a complete combustion of gasoline is achieved and greenhouse gas emissions would be minimal. The coefficient is defined as Lambda

If Lambda > 1 = lean mixture, excess of air. If Lambda < 1 = rich mixture, excess of gasoline.

A poor mixture will generate a high content of oxygen in the exhaust and therefore a high content of Nitrogen Oxide.

A rich mixture will generate a small content of oxygen in the exhaust and increased emissions of carbon monoxide and hydrocarbons.

Carbon dioxide emitted is directly proportional to the fuel consumed.

Chemical CO sensors

Chemical CO gas sensors with sensitive layers based on polymer- or heteropolysiloxane have the principal advantage of a very low energy consumption, and can be reduced in size to fit into microelectronic-based systems. On the downside, short- and long term drift effects as well as a rather low overall lifetime are major obstacles when compared with the NDIR measurement principle.

Another method (Henry's Law) can be also be used to measure the amount of dissolved CO in a liquid, if the amount of foreign gases is insignificant.

Nondispersive Infrared (NDIR) CO Sensors

NDIR sensors are spectroscopic sensors to detect CO in a gaseous environment by its characteristic absorption. The key components are an infrared source, a light tube, an interference (wavelength) filter, and an infrared detector. The gas is pumped or diffuses into the light tube, and the electronics measures the absorption of the characteristic wavelength of light. NDIR sensors are most often used for measuring carbon monoxide. The best of these have sensitivities of 20–50 PPM.

Most CO sensors are fully calibrated prior to shipping from the factory. Over time, the zero point of the sensor needs to be calibrated to maintain the long term stability of the sensor. New developments include using microelectromechanical systems to bring down the costs of this sensor and to create smaller devices.Typical NDIR sensors cost in the (US) $100 to $1000 range.