Why are unburnt hydrocarbons released from engines?

Question

thattts it

Answer 1

Because internal combustion engines are not efficient!

Answer 2

Several reasons that may be present on varying degrees:

1- complete combustion requires a thorough mixing of air and fuel; such mixing is not that easy to achieve when a piston takes a load, compresses it and exhausts the spent gasses 40 times a second, especially when a car has to adapt to very different regime, from idle to wide open throttle

2- cold engines (at startup) have area where fuel pools instead of mixing, and which gets dumped.  Catalytic converters are supposed to handle unburned hydrocarbons, but cannot work effectively when cold

3- a car may have dirty spark plugs, be off in their timing, have inaccurate oxygen sensors, or in need of service; this also means unburned hydrocarbon rejected

4- driving habits: jack rabbit acceleration increase the fuel load to a point where the mix may be less than stoichiometric, leading to incomplete combustion

5- it is also a question of balance.  Increase the air proportion (lean engine) and one produces more nitrous oxyde.  Make the engine run rich, and you have unburned hydrocarbons.

Answer 3

It is CO and it is deadly.