Q I understand that carbon monoxide
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in the exhaust gas can be eliminated by various means before it reaches the outlet. What are the measures that can be applied and can the same means be used to remove diesel smoke from the exhaust fumes?
AA catalyst can be used to facilitate
conversion of carbon monoxide to carbon dioxide and this normally comprises a cartridge of finely granulated metals located in the silencer. The carbon monoxide is absorbed by the catalyst while the cartridge is fed with air from the atmosphere by using a venturi in the system. Time is required for the air to act on the carbon monoxide when the gas has cooled down and the catalyst "holds" the gas until it is converted to carbon dioxide when it is then discharged through the outlet.
The catalyst cartridge does not, of course, undergo a change of state, i.e. it does not react with the gas. But in practice it has to be renewed at intervals because of contamination by oil and so on.
Carbon monoxide may also be converted into carbon dioxide by pumping air into the exhaust tracts close to the valves and in this system a catalyst is unnecessary because of the heat of the gas. Alternatively, an after-burner can be employed which is fed with extra air and incorporates a sparking plug. This can also be used to remove diesel smoke by "burning" the finely-divided carbon; no other method appears to be practical.
Petrol engines are the chief offenders in the production of carbon monoxide, while the type of combustion in a diesel engine is such that
incomplete combustion tends to create free carbon rather than carbon monoxide. Whereas the fuel charge of a petrol engine comprises pre-mixed fuel and air, the fuel injected into a diesel engine combustion chamber burns as each surface area unites with the surrounding air. The use of petrol injection can almost eliminate carbon monoxide by virtue of the efficient fuel-metering control it provides, notably at tick-over speed.