Unburned hydrocarbon

For example, in piston engines, some of the fuel-air mixture "hides" from the flame in the crevices provided by the piston ring grooves.

These regions will cause intermediate species such as formaldehyde and alkenes to be emitted.

Sometimes the term "products of incomplete combustion," or PICs, is used to describe such species.

The hydrocarbon is an auspicious way to reach low NOx (nitrogen oxide) emissions in diesel engines but one of its disadvantages is drastic increasing amount of unburned hydrocarbons.

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