This blog post examines the air pollution problem of diesel vehicles and the limitations and possibilities of post-treatment technology to solve it.
Seoul is always an exciting place. I am always surprised when I get off the KTX train at Seoul Station after a two-and-a-half-hour ride from Busan Station. The first thing that catches my eye when I get off at Seoul Station is the hazy sky. The clear, transparent sky in Busan just three hours ago has turned gray in Seoul. This air pollution is caused by the fact that the population of Seoul and the number of vehicles are much higher than those of Busan. In fact, Seoul citizens, who are exposed to the gray sky every day, are somewhat accustomed to this situation, but this is not just a problem in Seoul. Air quality deterioration and smog have long been serious environmental problems facing large cities around the world.
In fact, the problem of air pollution caused by automobile exhaust has been the subject of social discussion since the beginning of the 20th century. For example, a car museum in Alaska displays protective equipment that ladies wore in the early 1900s to prevent their clothes and skin from being contaminated by exhaust fumes, which shows that air pollution was recognized as a problem from the very beginning of the advent of the automobile. The problem of air pollution caused by automobiles is becoming more serious as time goes by, and in the present day, many countries, including the United States and Europe, have tightened regulations on automobile emissions.
In particular, concerns about air pollution are growing due to the increasing demand for diesel engines, which emit more pollutants than gasoline. Diesel engines boast high fuel efficiency, but in return, they emit more pollutants. Accordingly, various aftertreatment devices and technologies have been developed to reduce pollutants emitted by diesel engines, and their importance is becoming increasingly apparent.
The difference between diesel and gasoline engines starts with the physical properties of the fuel. The lighter components of petroleum are classified as gasoline, while the heavier components are classified as diesel. The difference in weight between the two fuels leads to a difference in the boiling point (the point at which they vaporize), which in turn leads to a difference in the way the fuel is injected into the engine. Gasoline is mixed with air beforehand and enters the engine, and combustion occurs in a uniformly mixed state. This makes gasoline relatively close to complete combustion. On the other hand, diesel with a high boiling point enters the engine in a separate state from air, and the fuel is injected and burned after being injected under high pressure. This difference causes diesel engines to emit fuel particles that do not fully react with oxygen, and these particles are particulate matter (PM) that are harmful to the human body and are one of the main air pollutants emitted by diesel vehicles.
Another pollutant emitted by diesel engines is nitrogen oxide (NOx). Since diesel engines operate at high temperatures and high pressures, stable nitrogen molecules in the atmosphere combine with oxygen to form nitrogen oxide. Nitrogen oxide causes chemical reactions in the atmosphere, leading to the generation of ozone and fine dust, which can cause respiratory and cardiovascular diseases in the long term. On the other hand, substances such as nitrogen oxides contained in automobile exhaust are generally invisible, so they are easily overlooked, but if pollution accumulates, it can cause smog in the atmosphere and cause long-term environmental problems.
Various aftertreatment devices have been developed to alleviate the air pollution problem of diesel vehicles. A typical device is the diesel particulate filter (DPF), which filters particulate matter from exhaust gas through a filter, reducing the number of particulate matter emitted from the engine to 10,000. The DPF works by using the principle that exhaust gas enters the filter, where it accumulates particulate matter inside the filter with a closed end, and only emits clean exhaust gas. To maintain the function of this filter, it is necessary to periodically perform a “regeneration” process that increases the internal temperature to burn off particulate matter that has accumulated on the filter. Such a device can effectively reduce particulate matter, but if filter maintenance is neglected, the filter can become clogged and release more pollutants, so it is necessary to maintain it thoroughly.
Another post-treatment device is the exhaust gas recirculation (EGR) system. EGR is a device that reduces the combustion temperature by re-injecting a portion of the exhaust gas back into the engine combustion chamber, thereby suppressing the generation of nitrogen oxides. EGR has the effect of reducing nitrogen oxide emissions, but it has the disadvantage of affecting engine efficiency. In addition, a selective catalytic reduction (SCR) system that chemically neutralizes nitrogen oxides has also been developed. SCR increases the purification rate of exhaust gas by injecting a substance containing a reducing agent to decompose nitrogen oxides in exhaust gas into water and nitrogen. This technology is especially actively used by European car manufacturers such as Mercedes-Benz and BMW.
However, the presence of these aftertreatment devices does not solve all problems. Automotive aftertreatment devices only function properly above a certain temperature, and may not function properly if the vehicle is driven for a short period of time or if it repeatedly drives at low speeds in urban areas. For example, most pollutants are emitted in a concentrated manner within about 10 minutes after the vehicle is started, which is when the aftertreatment device is unlikely to function effectively. In addition, there are still many issues to be addressed, as certain post-treatment devices can increase fuel consumption or cause the generation of other pollutants.
As concerns about air pollution grow, regulations on vehicle emissions are being tightened around the world, and technology development to reduce pollutants is being actively pursued. Improving technology to ensure that the aftertreatment system of diesel engines maintains a constant efficiency and stability under various driving conditions will remain an important task in the future. The automotive industry requires the development of more advanced emission reduction devices and innovative technologies to pass on a better environment to the next generation, which will gradually contribute to providing cleaner air in our daily lives.
