Diesel emissions compared to gasoline, biodiesel, ethanol, propane, and natural gas is debated with regard to least/most and safest/worse. With respect to fuel efficiency, there is no debate. Diesel engines are far more fuel efficient than gasoline engines of comparable size. In fact, diesel engines are anywhere from 25 to 35 percent more fuel efficient than gasoline engines. Additionally, gasoline engines produce around 40 percent more carbon dioxide (CO2) than diesel engines. 

However, diesel engines are not better than gasoline engines in every respect. Diesel engines produce more nitrogen oxides, more sulfur oxides, and more particulate matter than gasoline engines typically do. In all there are six emissions from diesel engines that concern scientists with respect to diesel-engine emissions:

1. 1. Carbon Monoxide
   2. Nitrogen Oxides
   3. Sulfur Oxides
   4. Lead
   5. Hydrocarbons
   6. And Particulate Matter

The amount of lead emissions produced by an engine is a product of the amount of lead in the diesel fuel fed into an engine. With respect to sulfur oxides, the amount found in diesel emissions is also a product of the amount of sulfur in the fuel fed into an engine. As such, to reduce the amount of lead and sulfur oxides found in diesel emissions, the two contaminants are typically removed during the refining process of diesel. 

However, carbon monoxide, nitrogen oxides, hydrocarbon emissions, and particulate matter are the consequence of diesel engines and diesel engine technologies. The reduction of these four emission contaminants occurs as diesel engine and diesel emissions technologies improve. 

Carbon Monoxide and Its Effects

Carbon monoxide is a toxic gas that negatively affects almost all the living species in the biosphere. Carbon monoxide can affect everything from the nervous system of people to heart problems. “Breathing CO can cause a headache, dizziness, vomiting, and nausea. … Exposure to moderate and high levels of CO over long periods of time has also been linked with increased risk of heart disease. People who survive severe CO poisoning may suffer long-term health problems”

While not a greenhouse gas itself, carbon monoxide contributes greatly to the generation of greenhouse gases and the destruction of atmospheric elements that reduce greenhouse gases. 

“Carbon monoxide readily reacts with the hydroxyl radical (OH) forming a much stronger, greenhouse gas–carbon dioxide. This, in turn, increases concentrations of methane, another strong greenhouse gas, because the most common way methane is removed from the atmosphere is when it reacts with OH. So, the formation of carbon dioxide leaves fewer OH for methane to react with, thus increasing methane’s concentration. 

A NASA report indicates that carbon monoxide is responsible for a 13% reduction in hydroxyl concentrations and through other reactions, a 9% drop in sulfate concentrations. Sulfates are credited for offsetting some of the global warming due to greenhouse gases by reflecting incident solar radiation back to space.”

Gasoline-powered engines produce considerably more carbon monoxide than diesel-powered engines, but the amount of CO generated by diesel combustion is still a concern. With respect to how much of each emission both types of engines — gasoline engines vs diesel, — diesel engines produce far more nitrogen oxides.

Effects of Nitrogen Oxides

There are three types of nitrogen oxides commonly found in diesel emissions: nitric oxide (NO) and nitrogen dioxide (NO2), and nitrous oxide (N2O). Nitric oxide and nitrous oxide are innocuous to humans, but can still have devastating effects on the environment. 

Nitrogen oxides contain nitrogen atoms and oxygen atoms. While neither nitrogen nor oxygen is found in abundance in fossil fuels, engines produce nitrogen oxides during the combustion process by creating a chemical bond between the two elements that constitute a large portion of the air used to oxidize the hydrocarbons in fossil fuels. Compression-ignition diesel engines produce considerably more nitrogen oxides than spark-ignition gasoline engines produce during combustion.

The reason diesel engines produce more nitrogen oxides is that gasoline engines and diesel engines combust fuel is very different ways. Because diesel engines combust fuel at a much higher temperature than gasoline engines, diesel engines produce more nitrogen oxides. High engine temperatures are necessary for the development of nitrogen-oxygen bonds. 

Today’s diesel engines are more efficient than ever. That means they burn diesel more efficiently. The more efficiently a fossil fuel burns, the hotter the combustion temperatures. As a result of increased combustion efficiency, today’s diesel engines generate larger sums of nitrogen oxides than in the past. “Modifications in engine operating conditions which increase the peak cycle temperature, or the oxygen concentration in the burnt gases increase the nitric oxide concentration in the exhaust. The measured nitric oxide levels are found to be nearer the equilibrium concentration corresponding to the peak cycle temperature and pressure, then to the equilibrium concentration at exhaust conditions.”

Of the nitrogen oxides diesel engines produce, nitric oxide is the most common.

Nitric oxide and Its Effects

Nitric oxide (nitrogen monoxide or NO) is a completely innocuous gas that is used as a dietary supplement for people and animals. However, NO is also a precursor for other nitrogen oxides that are not innocuous, nitrous oxides that are toxic and other nitrous oxides that are greenhouse gases. “Nitrogen gas and oxygen gas are present in our atmosphere, and nitrogen monoxide is created naturally when lightning strikes. The super-heated air breaks apart nitrogen molecules (N2) and oxygen molecules (O2), which causes nitrogen and oxygen atoms to bond. After nitrogen monoxide gas is generated, it immediately reacts with another oxygen molecule to form nitrogen dioxide (NO2).”

While nitric oxide is not a problem with respect to health or the environment, the fact it is highly reactive with oxygen and the two almost invariably produce a chemical bond — a bond that creates nitrogen dioxide, — nitrogen monoxide is an issue.

Nitrogen Dioxide and Its Effects

Unlike nitrogen monoxide, nitrogen dioxide is a dangerous diesel engine emission. “Nitrogen dioxide is toxic to various animals as well as to humans. Its toxicity relates to its ability to form nitric acid with water in the eye, lung, mucous membrane and skin. In animals, long-term exposure to nitrogen oxides increases susceptibility to respiratory infections lowering their resistance to such diseases as pneumonia and influenza. Epidemiological studies [of people] have also shown associations between NO2 concentrations and daily mortality from respiratory and cardiovascular causes and with hospital admissions for respiratory conditions.”

Nitrogen dioxide is also one of the two primary contributors to acid rain. “Acid rain results when sulfur dioxide (SO2) and nitrogen oxides (NOX) are emitted into the atmosphere and transported by wind and air currents. The SO2 and NOX react with water, oxygen and other chemicals to form sulfuric and nitric acids. These then mix with water and other materials before falling to the ground.”

The third nitrogen oxide of great concern to environmental scientist is nitrous oxide.

Nitrous Oxide and Its Effects

Nitrous oxide (N20) is non-toxic to humans but extremely detrimental to the atmosphere. Used as an anesthetic — laughing gas, — nitrous oxide has a global warming potential of roughly 300 times that of carbon dioxide. According to the United Nations, nitrous oxide has a global warming potential 280 times greater than CO2 over a 20 year period. Over a 100-year period, the global warming potential of N20 is 310 times that of carbon dioxide. After 500 years, the global warming potential of nitrous oxide is still 170 times that of carbon dioxide. 

In comparison, methane — which is considered potentially the most dangerous of all greenhouse gases because of its potency; the sheer amounts released into the atmosphere annually, and because its stability — only has a global warming potential of 56 times that of carbon dioxide over a 20-year period; 21 times the GWP of CO2 over a 100-year period; and 6.5 times over a 500 year period.  

In addition to nitrogen oxides, diesel engines also produce greater sums of sulfur oxides. 

Effects of Sulfur Oxides

Diesel engines also produce more sulfur oxides than gasoline engines. That is because diesel — the fuel itself — contains more sulfur than gasoline generally does. It is because of the chemical composition of diesel that diesel engines produce greater sums of sulfur oxide emissions. The most concerning sulfur oxide is sulfur dioxide.

Sulfur Dioxide and Its Effects

Sulfur dioxide is a gas made of molecules containing one sulfur atom and two oxygen atoms. While sulfur dioxide is generated by the earth, it is also a byproduct of fossil fuel emissions and can cause health problems for people. 

“Sulfur dioxide irritates the skin and mucous membranes of the eyes, nose, throat, and lungs. High concentrations of SO2 can cause inflammation and irritation of the respiratory system, especially during heavy physical activity. The resulting symptoms can include pain when taking a deep breath, coughing, throat irritation, and breathing difficulties. High concentrations of SO2 can affect lung function, worsening asthma attacks, and worsen existing heart disease in sensitive groups. This gas can also react with other chemicals in the air and change to a small particle that can get into the lungs and cause similar health effects.” according to the U.S. National Park Service.

Hydrocarbon emissions are another fossil fuel combustion pollutant that affects people. While gasoline vehicles produce far more hydrocarbon emissions than diesel engines, diesel engines still produce them.

Hydrocarbon Emissions and Their Effects

Hydrocarbon emissions occur when an engine fails to burn a fuel completely. The result is that the hydrocarbons in a fuel — hydrocarbons being the valuable part of fossil fuels that ignite/combust/burn — exit the exhaust partially burned or unburned and enter the atmosphere. There are four components of hydrocarbon emissions that concern scientists most: aldehydes, alkyl nitrates, benzene, and benzene compounds.

“Aldehydes are toxic chemicals that result from the combustion of hydrocarbons, such as burning car fuel and plywood. They’ve been shown to inhibit photosynthesis in plants, cause eye, and lung irritations, and even possibly cause cancer. Alkyl nitrates are products of hydrocarbons that chemically react with molecules in the atmosphere. They can chemically react again to produce nitrous oxide, which can affect blood vessels, the liver, the kidneys and the nervous system. Benzene [has] been found to deplete red blood cells, cause cancer in mammals and damage bone marrow. Polynuclear aromatic compounds [have] been shown to cause cancer also.”

Particulate matter — the soot that causes black smoke — is possibly the most concerning of all diesel emissions. While it does very little with respect to global warming, like sulfur oxides and nitrogen oxides, particulate matter can affect the health of people, fauna, and plant life.

Effects of Particulate Matter

Particulate matter from a diesel engine is akin to the smoke that comes out of a fireplace chimney. Particulate matter is a combination of sulfur oxides and nitrogen oxides and is visible to the naked eye. Measured in micrometers, the particulate matter from a diesel engine can be anywhere from 2.5 – 10 micrometers. By comparison, the diameter of a human hair is typically between 50 and 70 micrometers.

According to the EPA, the smaller the particulate matter, the greater the potential of it to cause health problems in people and animals because they are capable of entering the lungs and getting into the bloodstream. The size of particles is directly linked to their potential for causing health problems. “Numerous scientific studies have linked particle pollution exposure to a variety of problems, including: 

• premature death in people with heart or lung disease
• nonfatal heart attacks 
• irregular heartbeat 
• aggravated asthma 
• decreased lung function 
• increased respiratory symptoms, such as irritation of the airways, coughing or difficulty breathing. 

While diesel engines are more fuel efficient and produce far less carbon dioxide and hydrocarbon emissions than their gasoline counterparts, the emissions from diesel engines — particularly nitrogen oxides and sulfur oxides — are of concern. However, there are measures that can be taken to reduce diesel emissions. Diesel fuel catalysts, catalytic converters, and particle filters can reduce emissions greatly. 

The Rentar Fuel Catalyst can reduce particulate matter particulate matter by up to 19 percent. The Rentar can also reduce nitrogen oxides by up to 19 percent, hydrocarbon emissions by up to 35 percent and black smoke by up to 44 percent. A particulate matter filter can reduce particulate matter by up to 40 percent. Catalytic converters and selective catalytic reduction can reduce emissions to an even greater extent.