There are two answers to the question how to manufacture air pollution control car. First answer is to make ‘complete engine’ not to discharge harmful gas, and the second answer is to ‘convert into harmless gas’ before harmful gas generated from the engine is discharged into the air. Of course, now many engineers have made every effort to the first answer(electric car ,hydrogen-fuel automobile etc.) and a solution to the second answer is automobile catalytic system being currently used. Automobile catalyst is divided into three way catalyst(TWC) for gasoline fueled cars and oxidation catalyst for diesel cars. TWC is named from the meaning of ‘simultaneous reaction with 3 harmful elements’, and the three harmful substances are CO, HC and NOx. These three substances have their unique harmful characteristics; CO phagocytizes hemoglobin carrying oxygen in blood, HC causes hallucination similar to bond, and NOx causes dyspnoea by stimulating the mucous membrane of the respiratory system. Therefore TWC is necessary for automobiles not to discharge harmful exhaust.

The TWC system is installed between the exhaust manifold(collecting exhaust from the combustion chamber) of the engine and the noise suppresser in a long hexagonal shape. As if 'catalyst' does not participate in a reaction but just promote it, ‘catalyst’ in the TWC system helps harmful gas to be readily converted into harmless gas. As catalysts, several additives are added to metal catalysts of 'Platinum'(Pt), 'Palladium'(Pd) and 'Rhodium'(Rh), and harmful exhaust is purified as it passes through the honeycomb catalyst system. CO is combined with oxygen into CO2, CH into CO2 and water, and NOx into nitrogen and oxygen, then consequently nearly harmless gases(95% at least of harmful gas is purified) are exhausted into the air. For reference, water falls from the exhaust pipe of a car, which implies that fuel is not leaked but exhaust is purified properly.

Purification of the TWC system may differ according to the ‘concentration of oxygen’ in exhaust. Because the concentration of oxygen may vary by 'air-fuel ratio', it shall be precisely controlled to make the TWC system properly function. However, existing carburetor-type engine(mechanically mixing air and fuel) has limitation in controlling air-fuel ratio, thus current electronically controlled engine has appeared. In conclusion, electronically controlled engine has shown up to reduce harmful exhaust.