Smoke and Control of Smoke

Formation of smoke is basically a process of conversion of molecules of hydrocarbon fuels into particles of soot. It should be noted that soot is not carbon but simply an agglomeration of very large polybenzenoid free radicals. It is also observed that soot formation during the early part of the actual combustion process is common to all diesel engines but is consumed during the latter part of combustion.

Pyrolysis of fuel molecules is thought to be responsible for soot formation. Fuel heated with insufficient O₂ will give carbonaceous deposits. It is believed that the ‘heavy ends’ of diesel fuel may pyrolyse to yield the type of smoke that is observed from a diesel engine. This is believed to be the path of formation of polycyclic aromatic hydrocarbons (benzo-pyrene) found in soot.

Many theories have been put forward for the formation of smoke but the basic reactions leading to the formation of smoke are not fully known.

There is hardly any successful method to control the formation of soot except the engine has to run at lower load and maintain the engine at best possible condition.

Some methods suggested for the control of smoke are as follows.

1. Smoke-suppressing additives: It has been found that some barium compounds added in fuel reduce the temperature of combustion and avoid soot formation. It is further observed that if the soot is found, the barium compounds break them in very fine particles and reduce the smoke. However, berium salts added in fuel form the deposit on engine parts and reduce the filtering capacity of the filter.
2. Fumigation: Fumigation is a method of injecting a small amount of fuel in the intake manifold. This helps pre-combustion reactions during compression stroke and reduces the chemical delay because the intermediate products such as peroxides and aldehydes react more rapidly with O₂ than hydrocarbons. Reducing the chemical delay curbs thermal cracking which is responsible for soot formation.The cracking may not even happen which is mainly responsible for soot formation when fumigation is used because it requires about 350 kJ/mole to break C-C bond and 425 kJ/mole to break C—H bond. The energy required may not be available due to easy oxidation during pre-combustion reaction.
3. Catalytic convertors: Catalytic convertors are not effective like in a petrol engine because of large soot formation which interferes in the oxidation of HC, CO, and NO_x. Therefore, these catalysts have a very small effect on engine smoke. Extensive research is being carried on to use catalysts for effective removal of soot as well as for removal of other emissions.