Diesel engines, can we clean up our act? (Technologies and challenges to address air pollution)

Air pollution from diesel engine exhaust is still a significant problem, but various aftertreatment devices are being developed to reduce it. However, many technical challenges remain to be solved in order for these devices to maintain optimal efficiency.

 

Seoul is always exciting. After a two-and-a-half-hour KTX ride from Busan Station, I’m always surprised when I get off at Seoul Station. It’s so cloudy. The sky that was so clear just three hours ago in Busan is ashen-colored. Even Busan, the second capital of South Korea, can’t compare to the number of people and cars in Seoul. Ashen skies have long been a problem in big cities around the world. In an automobile museum in Alaska, you can see what ladies used to wear in the 1900s to avoid getting dirty from exhaust fumes, which shows that people were aware of the problem from the time they first saw cars on the road. Air pollution from cars is getting worse and worse over time, which has led to tighter regulations on car emissions around the world, including the United States and Europe. However, the growing demand for diesel engines, which emit more pollutants than gasoline engines, has raised concerns about air pollution and the need for proper emission treatment of these substances.
The difference between diesel and gasoline engines starts with the fuel. Petroleum, the fuel used in automotive engines, is categorized by weight per unit volume. The lighter form of petroleum is called gasoline, while the heavier form is called diesel. The difference in weight between the two fuels leads to a difference in the boiling point at which the liquid fuel becomes a gas, which makes a difference in how the fuel is injected into the engine. Gasoline enters the engine mixed with air, while diesel, which has a relatively higher boiling point, is injected and mixed with air as it enters the engine. Also, when the same weight of both fuels is burned, a higher percentage of air enters the engine in a diesel engine. These differences result in more pollutants in a diesel engine.
Inside the engine, combustion occurs when fuel reacts with oxygen at high temperatures to produce energy. Because diesel is mixed with air after it enters the engine, it encounters less oxygen than gasoline, which is mixed first. Fuel particles that do not react with oxygen are released, and these particles are called particulate matter (PM). In addition, nitrogen, which makes up 78% of the air, is a very stable substance and is normally very unreactive, but inside the engine under high temperature and pressure, it cannot react with the fuel and reacts with the remaining oxygen to form nitrogen oxides (NOx). When comparing one cycle to another, diesel engines produce more nitrogen oxides than gasoline engines because they have a higher ratio of air to fuel.
Our hair is 100 μm thick, which is very thin. However, particulate matter has a size of 10 μm or less. So, diesel vehicles are equipped with an aftertreatment device consisting of several layers of plates that block particles larger than 10 μm from passing through, reducing the million particulate matter emitted by the engine to 10,000 particles, called a Diesel Particulate Filter (DPF). The principle is that when exhaust from the engine enters the DPF, particulate matter accumulates at the end of the plates and only exhaust escapes. Once a certain amount of particulate matter builds up on the walls of the DPF, it stops working properly, but by heating it up to a higher temperature, the particulate matter is burned off and blown into the gas, allowing the DPF to be reused. Other ways to reuse the DPF include the installation of a thermal converter, which is a high-temperature combustion chamber, or EGR, which feeds the exhaust back into the engine. To treat NOx, various aftertreatment devices are used. Currently, the most commercialized is the Lean NOx Trap (LNT), which traps NOx for a certain period of time and reduces it by instantaneously increasing the amount of fuel when a certain amount is collected. German companies such as Mercedes-Benz and BMW are choosing Selective Catalytic Reduction (SCR), which uses highly reducible ammonia to reduce nitrogen oxides, as an aftertreatment device. There are also ternary catalytic converters that use a catalyst to oxidize hydrocarbons (HC) and carbon monoxide (CO) that remain from incomplete combustion while simultaneously reducing NOx.
Despite the use of various aftertreatment devices to reduce pollutant emissions from diesel engines, there are still many issues that need to be addressed. The water purifiers we use to drink our water ensure that we can always have clean water as long as we change the filter. However, the aftertreatment devices in your car need to be above a certain temperature in order to function properly and efficiently. This is why most pollutants are generated within the first 10 minutes of starting the car. There are also still issues to be resolved with LNT, such as the richness of the fuel at the time of reduction, which can reduce NOx but results in unburned fuel being emitted, SCR is not reliable, and EGR changes engine efficiency.
With growing concerns about air pollution, regulations on automobile emissions are tightening around the world, and various methods are being researched to reduce the emissions of pollutants from diesel engines, which are in high demand despite the large number of pollutants they produce. However, challenges remain, such as the inability of diesel engine aftertreatment devices to function consistently depending on driving conditions, the increase in pollutants that can be generated during the aftertreatment process, and ensuring reliability. Technologies to effectively reduce air pollutants will continue to be required and developed in order to leave a better environment for the next generation.