In this blog post, we will look at the pros and cons of diesel engines and the development of emission reduction technologies (DPF, EGR, SCR) to examine the viability of diesel.
Pros and cons of diesel
Compared to gasoline engines, diesel engines generally emit less carbon dioxide, have higher thermal efficiency, and are more durable. However, diesel engines generate a lot of particulate matter, known as fine dust, and nitrogen oxides such as nitrogen monoxide and nitrogen dioxide. These substances are a major cause of various respiratory diseases such as bronchitis and pneumonia, as well as photochemical smog and acid rain. Accordingly, technologies to reduce the pollutants emitted by diesel engines are constantly being developed.
Characteristics of diesel engines and emission reduction technologies
A representative technology for treating particulate matter is the diesel particulate filter (DPF) system. This system collects particulate matter generated from exhaust gas with a filter and restores the filter’s function by burning the accumulated matter at a certain point in time. In order to burn the collected particulate matter, fuel must be supplied to the engine cylinder so that it can flow to the collection filter.
When fuel is supplied, it mixes with the exhaust gas and is combusted in the filter. The DPF system is easy to manufacture because it does not require any special improvements to the engine, only the supply of additional fuel, but it has poor fuel efficiency. In addition, separate technology is required because it is difficult to reduce nitrogen oxides. One technology for reducing nitrogen oxides is the exhaust gas recirculation (EGR) system.
This system recirculates exhaust gas back into the engine and then combusts it together with fuel to lower the combustion temperature. The reason for recirculating exhaust gas back into the engine to lower the combustion temperature is that nitrogen oxide emissions are reduced when fuel is combusted at lower temperatures. However, lowering the combustion temperature results in high emissions of particulate matter, so the EGR system is used in conjunction with the DPF system. The EGR system has the disadvantage of causing impurities to accumulate in the engine, which can reduce output.
Recently, the SCR (selective catalytic reduction) system, which is more efficient at reducing nitrogen oxides than the EGR system, has been developed and is replacing the EGR system. The SCR system does not recirculate exhaust gas, so the combustion temperature in the engine is higher than in the EGR system. This results in less particulate matter but more nitrogen oxides. The SCR system uses ammonia to reduce nitrogen oxides. However, ammonia is explosive, can corrode metals, and has a distinctive pungent odor at room temperature, which can cause discomfort. Therefore, its use is restricted and caution is required when handling it.
How the SCR system works and its advantages
To solve these problems, the SCR system uses a separate urea tank that supplies urea dissolved in water and a compressed air injector that supplies air to induce the following chemical reaction in the SCR device. The urea is decomposed into ammonia and isocyanic acid through thermal decomposition, and the isocyanic acid is hydrolyzed to produce carbon dioxide and ammonia.
The nitrogen monoxide reacts with oxygen in the air and is converted into nitrogen and water. The nitrogen dioxide reacts with ammonia and is also converted into nitrogen and water. The interior of the SCR device, where the chemical reaction takes place, is made of a metal catalyst such as platinum or vanadium, which adsorbs the reactants.
In the SCR method, the reduction efficiency can be improved by allowing the exhaust gas to remain on the surface of these catalysts for a long time. In other words, the space velocity must be slowed down to allow sufficient time for the chemical reaction to take place. Here, the space velocity is the amount of exhaust gas supplied per unit time divided by the volume of the catalyst in the SCR device.
Challenges and future prospects of the SCR method
The SCR system is gradually expanding in use due to its high reduction efficiency, but there are still issues to be resolved. Ammonia slip, in which ammonia is emitted along with exhaust gas, can occur, and if urea decomposes at low temperatures, solid amine or melamine can be generated and adhere to the inside of pipes and device surfaces. In addition, the accumulation of these solid substances can reduce system efficiency and increase maintenance costs. To prevent this, it is necessary to maintain the system temperature at an appropriate level and perform regular inspections and cleaning.
Diesel engine emission reduction technologies continue to evolve in line with stricter environmental regulations. Each of the DPF, EGR, and SCR technologies has its own advantages and disadvantages, so it is important to find the optimal combination to minimize emissions. Although the role of diesel engines is likely to decline due to the increase in electric and hybrid vehicles, they still play an important role in large vehicles and commercial vehicles. Efforts will continue to improve the efficiency of diesel engines and reduce environmental pollution in harmony with environmentally friendly technologies.
