Answer:
  Dr. Mitsuharu Koguma in Clean Power System Research Group, who is actually studying the car engine, kindly answered this question.
 
 
The mechanism of a gasoline engine
  The following shows how gasoline works in a car:
  

• Gasoline is mixed with air by the optimum-mixing ratio (the condition where there is a minimum amount of oxygen, which is necessary for burning 1 g of gasoline completely). The mixed gas is burned in the engine later.
    

• The mixed gas is expelled into the engine. The gas is compressed by the piston raised with inertial force, which elevates the temperature and pressure of the expelled gas.
    

• The mixed gas is ignited by the spark plug at the most suitable timing (just before the temperature and pressure of the gas reaches the highest point).
    

• The mixed gas begins to burn near the spark plug.
    

• The flame is propagated gradually in the mixed gas, then spread throughout the interior of the engine (inner combustion chamber).
    

• The pressure inside the combustion chamber is increased by the combustion of the gas, which pushes down on the piston.
    

• This work is passed to the outside, which moves the wheels.
    

  These are the mechanisms of a gasoline engine. The mechanism is also the reason why the gasoline engine is called “spark ignition engine”. 
  
  
About the components in auto emissions
  Regarding auto emissions, No. 1 to No. 3 following, are harmful components discharged from gasoline engines:
 

1. HC (unburned hydrogen carbon)
    

2. CO (carbon monoxide)
    

3. NOx (nitrogen oxide)
    

4. SOx (sulfur oxide) would be generated when sulfur (S) in fuel is bound with oxygen (O) in air by burning, if the fuel contains sulfur. However, since current gasoline is desulfurized almost completely, SOx is not measured as the component of the auto emissions now. 
    

5. As for soot, not only soot but also micro particles (PM), which are even smaller than soot, are regulated for diesel engines; however, they are not regulated for gasoline engines. Although I think that the soot might be discharged from gasoline engines as well, it is one of the components which are unregulated.
     
   In addition,
     

6. CO₂ (carbon dioxide)
     

7. H₂O
     
   are also components discharged from gasoline engines.

  
The reason why components in auto emissions are generated
  Because the reaction paths of these components are so complicated, we are unable to explain them in this article. Instead, we will summarize briefly how the components in auto emissions are generated and discharged:
  

1. HC (unburned hydrogen carbon)
     This (HC) is the part of gasoline which is discharged either in unburned form or in incompletely broken down form. Some factors cause HC; for example, imperfect combustion by insufficient oxygen, suppressed flame near the interior engine wall, the drop in temperature caused by low concentration of gasoline, etc. In other words, we can say that HC is a gasoline component that remained unburned or changed its form without being burned completely.
     

2. CO (carbon monoxide)
     “Burning something” is an oxidative reaction. When there is a lack of O₂ (oxidizer), burned compounds are not oxidized completely, that is, they do not become CO₂, but only become CO.
     

3. NOx (Nitrogen oxide)
     The two above (HC and CO) are products that are generated because they are not burned completely, so that they do not become CO₂ during the process of burning gasoline (oxidative reaction). On the other hand, the mechanism of generation of NOx is completely different from these two. N and O in NOx come from air. N₂ and O₂ in air are inert in themselves, however, they react with one another and produce NOx at the high-temperature conditions produced by burning gasoline. Therefore, the higher the temperature, the more NOx is generated.
     

6. CO₂ (carbon dioxide)
     CO₂ is a final product in the oxidation process of gasoline. This is generated from combining C in gasoline with O₂ in air. CO₂ itself is not a harmful component. However, if the concentration of CO₂ is high on earth, it prevents heat on the surface from escaping into outer space, which increases the temperature on earth. Gases, such as CO₂, which have an effect of increasing temperature on earth, are called “green house gases”.
    

7. H₂O (water)
     Water, as well as CO₂, is a final product in the oxidation process of gasoline. You may have seen that water dripping from the muffler when a car in front of your car starts moving at an intersection. The water is generated when vapor in auto emissions is fully cooled down at the muffler.
    

Recent trend (This comment was added in August 2003)
In addition to the regulated components in auto emissions such as HC, CO, NOx and PM (for diesel engines), minor components, which are not regulated but might be harmful for the human body, are more recently also under examination. For example, toluene, benzene, formaldehyde, acetaldehyde, 1,3-butadiene, SO₂, formic acid, N₂O, and so on. This would be a trend corresponding to PRTR (in Japan), which may regulate minor components in auto emissions in the future.
 
  
Acknowledgement
  We would like to thank Dr. Mitsuharu Koguma in Clean Power System Research Group for his kind answer. We also would like to thank Dr. S.G in the same group for introducing us to Dr. Mitsuharu Koguma.
 
 
This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp.  Please let us know if you find any errors.