When you arrive in Seoul and see the cloudy sky, you realize the severity of air pollution, which is caused by the large population and the exhaust emitted by vehicles. Diesel engines in particular produce high levels of pollutants, and while various aftertreatment devices have been developed to reduce them, there is still much work to be done. The world is demanding more efficient emission reduction technologies for cleaner air.
Seoul is always exciting. I’m always surprised when I get off the KTX train at Seoul Station after a 2.5-hour ride from Busan. In particular, the first thing I notice when I get off at Seoul Station is the overcast sky. The sky that was crystal clear just three hours ago in Busan is often ashen in Seoul. This air pollution is a result of Seoul’s large population and the fact that the number of vehicles is much higher than in Busan. In fact, Seoul residents have become somewhat accustomed to seeing ashen skies on a daily basis, but it’s not just a problem in Seoul. Poor air quality and smog have long since become a serious environmental issue facing large cities around the world.
In fact, air pollution from automobile exhaust has been a topic of public debate since the early 20th century. For example, an automotive museum in Alaska displays the protective gear worn by ladies in the early 1900s to prevent exhaust from contaminating their clothes or skin, indicating that air pollution was recognized as a problem from the very beginning of the automobile. The problem of air pollution from automobiles has only gotten worse over time, and in the modern era, the United States, Europe, and other countries have tightened regulations on automobile emissions.
In particular, the growing demand for diesel engines, which emit more pollutants than gasoline, has increased air pollution concerns. Diesel engines boast high fuel efficiency, but at the cost of emitting more pollutants. As a result, various aftertreatment devices and technologies are being developed to reduce pollutants from diesel engines.
The difference between diesel and gasoline engines starts with the physical properties of the fuel. The relatively light component of petroleum is classified as gasoline and the heavier component as diesel, and the difference in weight between the two fuels leads to a difference in the boiling point at which they gasify, which makes a difference in how the fuel is injected into the engine. Gasoline is pre-mixed with air before it enters the engine, and combustion occurs in a homogeneous mixture. This makes gasoline relatively close to complete combustion. On the other hand, diesel, which has a higher boiling point, enters the engine separated from the air, where the fuel is injected under high pressure and burns after the air is injected. Because of this difference, diesel engines emit fuel particles that have not fully reacted with oxygen, and these particles are harmful to human health, making them particulate matter (PM), one of the main air pollutants from diesel vehicles.
Another pollutant produced by diesel engines is nitrogen oxides (NOx). Because diesel engines operate at high temperatures and pressures, nitrogen oxides are formed when stable nitrogen molecules in the atmosphere combine with oxygen. Nitrogen oxides cause chemical reactions in the atmosphere, leading to the formation of ozone and fine particulate matter, which can cause respiratory and cardiovascular diseases in the long term. Meanwhile, substances such as nitrogen oxides in automobile emissions are easily overlooked because they are generally not visible, but when pollution accumulates, it can cause smog in the atmosphere and lead to long-term environmental problems.
Various aftertreatment devices have been developed to mitigate air pollution problems from diesel vehicles. One such device is the diesel particulate filter (DPF), which is a device that traps particulate matter in the exhaust through a filter, reducing the million particulate matter emitted by the engine to ten thousand. DPFs work on the principle that as exhaust enters the filter, particulate matter builds up inside the filter, which is plugged at the end, allowing only clean exhaust to escape. To keep the filter functioning, it needs to be periodically “regenerated” by raising the temperature inside to burn off the particulate matter that builds up on the filter. While these devices can effectively reduce particulate matter, they need to be well maintained, as neglecting to do so can lead to clogging of the filter, which can result in more pollutants being emitted.
Another aftertreatment device is the exhaust gas recirculation system (EGR). EGR reintroduces some of the exhaust gas back into the engine’s combustion chamber to lower the combustion temperature, which in turn reduces the production of nitrogen oxides. While EGR is effective in reducing nitrogen oxide emissions, it has drawbacks that can affect engine efficiency. In addition, selective catalytic reduction (SCR) has been developed to chemically neutralize nitrogen oxides. SCRs work by injecting a substance containing a reducing agent to break down nitrogen oxides in the exhaust gas into water and nitrogen. This technology is particularly popular among European automakers such as Mercedes-Benz and BMW.
However, the presence of these aftertreatment devices does not solve all problems. They only work well above a certain temperature, and if you drive for short periods of time or repeatedly drive at low speeds in the city, they may not function properly. For example, most pollutants are concentrated in the first 10 minutes or so after a vehicle is started, which is when aftertreatment devices are less likely to work effectively. In addition, certain aftertreatment devices have issues such as increasing fuel consumption or causing other pollutants to be generated, so there are still many challenges to overcome.
Growing concerns about air pollution have led to tighter regulations on vehicle emissions around the world, and technology developments to reduce pollutants are being actively pursued. Improving the technology to ensure that diesel engine aftertreatment units remain efficient and reliable in a variety of driving conditions will continue to be a major challenge. In order to leave a better environment for the next generation, the automotive industry needs to develop more advanced emission reduction devices and innovative technologies, which will gradually contribute to cleaner air in our daily lives.
