An exhaust gas analyser or exhaust carbon monoxide (CO) analyser is an instrument for the measurement of

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...

among other gases in the exhaust, caused by an incorrect combustion, the Lambda coefficient measurement is the most common. The principles used for CO sensors (and other types of gas) are infrared gas sensors and chemical gas sensors. ''Carbon monoxide sensors are'' used to assess the CO amount during an Ministry of Transport test. In order to be used for such test it must be approved as suitable for use in the scheme. In the UK, a list of acceptable exhaust gas analysers for use within the MOT test is available via the

Driver and Vehicle Standards Agency The Driver and Vehicle Standards Agency (DVSA) is an executive agency of the UK Department for Transport (DfT). It carries out driving tests, approves people to be driving instructors and MOT testers, carries out tests to make sure lorries an ...

website.

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 gasoline involved in 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 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 coefficient If Lambda > 1 = lean mixture, excess of air. If Lambda < 1 = rich mixture, excess of gasoline. *A lean mixture contains an excess of oxygen. The surplus oxygen will react with nitrogen to (

oxides of nitrogen Nitrogen oxide may refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds: Charge-neutral *Nitric oxide (NO), nitrogen(II) oxide, or nitrogen monoxide *Nitrogen dioxide (), nitrogen(IV) oxide * Nitrogen trioxide (), or n ...

), if the temperature is high enough (around 1600 °C) for enough time to permit so. *A rich mixture contains a deficit of oxygen. This makes it impossible for all fuel to combust completely to carbon dioxide and water vapour. Hence, some fuel will remain as a hydrocarbon, or it will react only to carbon monoxide (CO). The carbon monoxide concentration in exhaust gases is closely related, and almost proportional to the air fuel ratio in the rich regions. It is, therefore, of great value when tuning an engine. *Carbon dioxide emitted is theoretically directly proportional to the fuel consumed at a given and constant air fuel ratio. Less carbon dioxide will be emitted per litre of fuel if λ < 1, since some fuel won't be able to combust completely.

Types of sensors

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 nondispersive infrared sensor measurement principle.Reliable CO Sensors Based with Silicon-based Polymers on Quartz Microbalance Transducers, R. Zhou, S. Vaihinger, K.E. Geckeler and W. Göpel, Conf.Proc.Eurosensors VII, Budapest (H) (1993); Sensors and Actuators B, 18–19, 1994, 415–420. *Another method (

Henry's Law In physical chemistry, Henry's law is a gas law that states that the amount of dissolved gas in a liquid is directly proportional to its partial pressure above the liquid. The proportionality factor is called Henry's law constant. It was formulate ...

) also can be used to measure the amount of dissolved CO in a liquid, if the amount of foreign gases is insignificant.

Nondispersive infrared CO sensors

Nondispersive infrared sensors are

spectroscopic Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...

sensors to detect CO in a gaseous environment by its characteristic absorption. The key components are an

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

source, a

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

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 In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...

of light. Sensors are most often used for measuring carbon monoxide.Carbonate Based CO Sensors with High Performance, Th. Lang, H.-D. Wiemhöfer and W. Göpel, Conf.Proc.Eurosensors IX, Stockholm (S) (1995); Sensors and Actuators B, 34, 1996, 383–387. 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 sensors cost in the (US) $100 to $1000 range.

Cambridge indicator

Used by older aircraft, the Cambridge Mixture Indicator displayed air-fuel ratio by measuring the

thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

of exhaust gas. It was manufactured by the

Cambridge Instrument Company Cambridge Scientific Instrument Company was a company founded in the late 1870s by Robert Fulcher. The original use of the company was to service instruments for the Cambridge physiology department. In the beginning, the company was financially dri ...

. This device was installed on airplanes in the 1930s, including the

Lockheed Model 10 Electra The Lockheed Model 10 Electra is an American twin-engined, all-metal monoplane airliner developed by the Lockheed Aircraft Corporation in the 1930s to compete with the Boeing 247 and Douglas DC-2. The type gained considerable fame as one was f ...

flown by Amelia Earhart on her last flight.

References