Engine exhaust smoke is the result of incomplete combustion. Smoke from exhaust is a visible indicator of the combustion process within the engine. It is generated at any volume in the engine where the mixture is rich. The fuel-air ratio greater than 1.5 and at pressures developed in diesel engine produces soot. Once soot is formed, it can burn if it finds sufficient O2; otherwise it comes out with exhaust. It becomes visible if it is dense. The size of the soot particles affects the appearance of smoke. The soot particles agglomerate into bigger particles which have an objectionable darkening effect on diesel exhaust.
The turbo-charged engine emits less smoke with increasing fuel air ratio as the mixing is much better in this engine as well as sufficient O2 is also available in the engine cylinder at all times.
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