What is diesel?
Diesel is a widely used fuel, but its combustion emits carcinogenic compounds that are harmful to the environment.
Image: Itaro
what is diesel
Diesel is a fuel used in road and sea transport of passengers and cargo. In Brazil, the use of diesel engines in light vehicles has been prohibited by law since 1976, and is currently used in the country only in trucks, buses and 4×4 traction vehicles (which include medium pickup trucks, SUVs and crossovers).
Diesel is an oil derived from petroleum. In its composition there are atoms of carbon, hydrogen and, in lower concentrations, sulfur, nitrogen and oxygen. Diesel is denser (has a longer hydrocarbon chain) and less volatile than other petroleum components, such as gasoline, which facilitates its separation by distillation.
Understand how diesel is separated from oil in this short video.
In the combustion process, diesel engines emit gases and particulate matter that reduce air quality. These emissions were classified as carcinogenic to humans by the International Agency for Research on Cancer (IARC), linked to the UN. The agency concluded that high exposure to diesel emissions causes lung cancer.
Because it has a larger hydrocarbon chain, diesel has greater heating power (generates more heat when burned). This makes vehicles that use the fuel more economical, that is, they consume less fuel per kilometer driven. However, this does not guarantee that it pollutes the air less.
In diesel engines, the mixtures of air and fuel are less homogeneous than in gasoline. Diesel is a less volatile fuel and its engine has the characteristic of operating by spontaneous ignition - both characteristics make mixing difficult. This means that, in diesel engines, to ensure complete combustion, there must be excess air in the combustion chamber. In the absence of this excess, there is emission of soot, carbon monoxide (CO), and hydrocarbons (HC), due to incomplete combustion, and this engine pollutes the environment seven times more than gasoline.
The gases generated
Emissions from diesel engines are composed of gases, vapors and particulate matter. The constituent gases and vapors include carbon dioxide, carbon monoxide, nitric oxides, nitrogen dioxide, sulfur oxides, and various hydrocarbons - some of which are volatile organic compounds. These air pollutants can also interact with each other or undergo photolysis, forming the so-called secondary pollutants, such as ozone, peroxyacetyl nitrates, among others.
According to a study, more than 95% of solid particulates derived from diesel engine exhaust are smaller than 1 cubic micrometer (μm³ - the millionth part of a cubic meter) which facilitates their inhalation and penetration into the lungs. Elemental carbon (a particulate material) generates the black soot in the photo below.
Inhalable particulate matter and ozone are hazardous agents that, in the world's large cities, derive 40% and 80% respectively of diesel burning by the vehicle fleet.
NOx is one of the compounds emitted in higher concentrations by diesel engines. Tunnel studies show that these engines produce five times more NOx than gasoline vehicles, and that trucks are responsible for most of the particulate matter emission.
The sulfur concentration in diesel is also a concern. According to a study, if the concentration of sulfur in the fuel is high, the emissions of polluting gases will also be high, especially those of sulfur dioxide (SO2) and sulfuric oxide (SO3), which cause damage to human health. In contact with atmospheric moisture, SO2 generates sulfuric acid (H2SO4), which contributes considerably to the formation of acid rain, it can acidify the soil and water, harming the development of small algae and insects.
Health risks
SOx and NOx affect the respiratory system causing short-term asthma attacks and airway irritation, and long-term chronic cardiovascular and respiratory diseases. CO reduces the blood's oxygen-carrying capacity and particulate matter causes respiratory allergies, as well as transporting other pollutants such as heavy metals and carcinogenic organic compounds.
In 2002, the United States Environmental Protection Agency (EPA) released a report that warned of the risks of prolonged exposure to diesel oil vapors. According to the report, long-term inhalation of these particulate materials, as well as sulfur and nitrogen oxides, can cause cancer in humans. In 2013, Iarc concluded that diesel engine emissions do indeed cause lung cancer, and probably bladder cancer as well.
According to Paulo Saldiva, a researcher at the Laboratory of Atmospheric Pollution at the Faculty of Medicine of the University of São Paulo (FM-USP), among the pollutants, the most harmful are particulate materials. According to the researcher, these particles accumulate in the lung alveoli, aggravating respiratory diseases, and enter the bloodstream, potentially affecting other organs. In the city of São Paulo, for example, it is estimated that for each increase of 10 micrograms per cubic meter (µg/m³) in the concentration of inhalable particulate matter in the air (smoke, soot, etc.) there is an increase of 1.5 % in hospitalizations for ischemic heart disease in the elderly and more than 4% for lung diseases in children and the elderly.
Emission control
There are some measures taken by the public sector that aim to reduce the emission of these pollutants into the atmosphere. Among them, we can mention the Environmental Vehicle Inspection and the Air Pollution Control Program.
Environmental Vehicle Inspection
The Environmental Vehicle Inspection was created with the objective of inspecting the emission of pollutants from automobiles. During the inspection, tests are carried out on the exhaust system to check the levels of gases, pollutants and noise. It is up to the states and municipalities to carry out the inspection.
Air Pollution Control Program (Proconve)
In 1986, the National Environmental Council (Conama) created the Air Pollution Control Program by Motor Vehicles (Proconve) with the objective of reducing and controlling atmospheric contamination from mobile sources (motor vehicles). Deadlines, maximum emission limits and technological requirements for national and imported motor vehicles were then set.
Technological advancements
Several technologies have been created to minimize the generation of pollutant gases by cars. They help make the fuel cleaner and create low-emission engines. Among the existing ones, some deserve to be highlighted:
Catalysts and particulate material filters
These technologies emerged to treat and/or retain exhaust gases. The catalyst is composed of two chemical substances (palladium and molybdenum), which react with the gases, converting them into water vapor, carbon dioxide and nitrogen (non-toxic gases). There are different types of catalysts. The particulate material filter has the function of filtering some of the gases generated during combustion in the engine. By law, since 1983, all cars are required to have a catalytic converter. However, there are still diesel-powered vehicles (such as buses and trucks) circulating without good catalysts, due to the advanced age of the fleet.
direct injection
This technology allows fuel to be injected directly into the combustion chamber. Thus, the mixture between air and fuel is smaller and the waiting period in the intake manifold is skipped. In this type of engine, the fuel is injected into the hottest part of the combustion chamber with the least amount of air. The way the fuel disperses inside the chamber allows for more regular and complete burning.
This option of direct injection, for diesel engines, has been around since the 1950s. Before, there was only indirect injection, in which there is a combustion pre-chamber. This pre-chamber is designed to ensure that the mixture between fuel and compressed air takes place correctly.
Direct injection increases engine efficiency and reduces fuel consumption. However, in the diesel engine it can produce more NOx as a reaction by-product. Some automakers have been trying to solve this problem through measures such as the production of specific catalysts, recirculation of exhaust gases, among other measures, which increase the price of the engine's production.
In the case of Volkswagen fraud - confirmation that the company was tampering with the emission of polluting gases from diesel engines became public -, nitrogen oxide emissions were between ten and 40 times higher than the limit established by the Agency for the Protection of the Environment (EPA), and the first 11 million cars confirmed to have the fraudulent software ran on a 2.0 direct injection turbodiesel engine.
Low sulfur diesel
In 2012, environmental legislation and Proconve 07 forced the beginning of the process of creation and use of diesel with low sulfur content in its composition, diesel S10 and S50 - with 10 parts per million (ppm) and 50 ppm of sulfur respectively - in the country. In 20 years, diesel in Brazil went from a composition of 13 thousand ppm to the current 10 ppm. This, together with engine technologies, make the emission levels similar to those in Europe.
A lower concentration of sulfur in the fuel reduces the emission of sulfur oxides and also helps to reduce the emission of other pollutants, such as NOx and particulate materials. This is because the sulfur trioxide generated in the combustion process can form sulfuric acid when it joins with water. This acid corrodes metal parts of the engine. In other words, sulfur attacks engine components, such as the catalytic converter, and as a consequence there is a loss of efficiency in this equipment.
The initiative to reduce the sulfur content is very good, but the fleet must be renewed (in old engines the expected effect does not occur), and, as mentioned above, there must be inspection. Find out more details about the S10 and S50 diesel in Brazil.
