In this blog post, we’ll explain the role and key factors of the catalytic converter, which effectively purifies exhaust gases in gasoline-powered vehicles.
The cold is receding, and the spirit of spring is gradually growing stronger. While spring is generally associated with positive imagery as a time of new beginnings, South Korea faces an unwelcome visitor: yellow dust. This uninvited guest, which disrupts the vibrant energy of nature and unsettles our spirits, has far-reaching effects, including an increase in yellowish-brown atmospheric dust, the onset of respiratory and eye diseases, and damage to industry and agriculture. It’s only natural that people pay closer attention and take an interest in air quality during the yellow dust season in April.
Although not yellow dust, there is a case that raised awareness of environmental issues in a similar context. In the 1940s in the United States, smog—resembling fog—began to appear on clear days, causing serious harm such as eye irritation and unpleasant odors. Investigations revealed that the cause was nitrogen oxides and hydrocarbons emitted by automobiles, which subsequently led to the world’s first implementation of automobile exhaust regulations.
Automotive exhaust is produced when fuel is burned in the engine. To meet emissions standards, either the amount of exhaust generated must be minimized from the outset, or the exhaust must be purified; this requires a variety of technologies. Harmful substances detected in exhaust include hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), and particulate matter (PM). Since the causes of each pollutant and the methods for purifying them vary, vehicles are equipped with a variety of purification devices tailored to their specific purposes.
In this article, we will examine the catalytic converter—a key component in gasoline-powered vehicles—among the various technologies used to purify automotive exhaust gases.
A catalytic converter is a device that purifies harmful components from exhaust gases emitted by the engine before they pass through the exhaust pipe and are released into the atmosphere. It uses a catalyst to convert harmful substances in the exhaust into harmless ones; here, the catalyst acts like a stepping stone that helps a person clear a hurdle, creating an environment conducive to chemical reactions.
Most gasoline engines are equipped with a three-way catalytic converter. As the name suggests, a three-way catalytic converter purifies three harmful substances: carbon monoxide, hydrocarbons, and nitrogen oxides. These substances are converted into harmless substances through oxidation or reduction reactions on the catalyst’s surface; oxidation refers to a reaction in which a substance combines with oxygen, while reduction refers to a reaction in which a substance is separated from oxygen. Therefore, oxidation reactions are more likely to occur when there is plenty of oxygen, and reduction reactions are more likely to occur when there is little oxygen.
The problem is that both oxidation and reduction reactions are necessary to purify all three of these harmful substances. So, what is the appropriate level of oxygen in exhaust gas?
The oxygen content in exhaust gas is closely related to the air conditions during combustion inside the engine. When oxygen in the air combines with fuel to burn, if there is too much oxygen relative to the fuel, the uncombined oxygen is simply emitted as exhaust gas. This can be explained by the mass ratio of air to fuel—that is, the engine’s air-fuel ratio. To control the air-fuel ratio, the amount of fuel supplied to the engine must be adjusted, and in a gasoline engine, the fuel supply is proportional to the amount of torque required by the vehicle.
For reference, a vehicle’s torque can be thought of as the force that drives the wheels. No matter how fast the wheels spin, if there isn’t enough torque, the vehicle will struggle to climb an incline. The driver shifts gears depending on driving conditions, and torque changes accordingly. When a specific amount of fuel corresponding to a certain torque is supplied, the amount of air required to completely combust that fuel is also determined; this is called the stoichiometric air-fuel ratio. The stoichiometric air-fuel ratio refers to the optimal amount of air—neither too much nor too little relative to the fuel. To ensure both oxidation and reduction reactions occur, the air supply is typically aimed at a value close to the stoichiometric air-fuel ratio.
Another important variable affecting purification performance is the temperature of the catalytic converter.
The performance of the catalytic converter is highly sensitive to temperature; as the temperature rises, the conversion efficiency increases sharply beyond a certain point. Generally, it must reach a certain temperature (commonly referred to as the “light-off” temperature) to function properly; if the temperature is lower than that, efficiency drops, resulting in high emissions of harmful substances. This is why starting the engine immediately before it has warmed up leads to higher emissions of harmful substances in the exhaust. Furthermore, even if the temperature is high and efficiency is good, the effect is limited if a significant amount of pollution has already been emitted at the time of startup. Consequently, various technologies are being applied to quickly raise the catalyst to the desired temperature. Once the temperature conditions are met, the catalytic converter can achieve a purification efficiency of over 95%, which is quite remarkable.
So far, we have examined the performance of catalytic converters used in gasoline-powered vehicles in terms of oxygen content and temperature. While most car buyers do not pay much attention to exhaust emission technology, a great deal of engineering knowledge and technology is applied to purify these emissions. Furthermore, even when considering just the catalytic converter alone, there are many technical challenges to overcome, and as environmental regulations continue to tighten, more advanced technologies will likely be adopted in the future. Perhaps in the future, we can even look forward to devices that go beyond the current 95% purification efficiency to purify the air itself.
