There are pros and cons of diesel engines. With respect to the pros of diesel engines, diesel engines pollute less than gasoline and biofuel engines. And, diesel engines are more fuel efficient than gasoline and biofuel engines. That is to say, diesel engines get better “gas” mileage than other engines. And, diesel engines last longer than gasoline and biofuel engines, at least twice as long. 

However, diesel engines have issues that gasoline engines do not. For example, cold weather starts are considerably more difficult for diesel engines. And, traditionally, diesel engines produced more particulate matter than gasoline engines. Also, in the past, diesel engines produced greater numbers of sulfur and nitrogen oxides.

However, that was more than 30 years ago. It is easy to argue that today’s diesel engines are the cleanest running, most fuel-efficient engines on the road. More important than asking what the pros and cons of diesel engines are is asking what the pros and cons of today’s diesel engines are. 

But, to understand the advantages of today’s diesel engines, it is necessary to understand the disadvantages of yesterdays. 

Cons of Traditional Diesel Engines

Up until the 1990s, diesel engines had a number of issues that made them unattractive as passenger vehicles. Traditional diesel engines were loud, smokey, and slow. And because traditional diesel engines produce black smoke, they were the scourge of environmentalists — or so they thought. 

Not only was black smoke highly visible, but it also smelled bad. Black smoke, scientists began to realize, also contributed heavily to health issues, respiratory tract irritation particularly. But it turns out, traditional diesel engines were not nearly as polluting as the gasoline engines of the same era. Nevertheless, it is difficult for anyone to ignore the pollution they can see and smell.

What Cause(d) Black Smoke

Most irritating about black smoke is the fact that it is a product of engine inefficiency. The black smoke produced by traditional diesel engines consisted almost entirely of unburned and partially burned hydrocarbons. Black smoke is vaporized diesel that goes from the fuel tank, through the engine, and out the exhaust without combusting. 

The reason traditional diesel engines burned diesel so inefficiently is because of poor diesel oxygenation. Diesel —  and all fossil fuels for that matter — requires oxygen to burn. More precisely, each individual hydrocarbon molecule that composes diesel requires oxygen to burn. Until the last 30 years, diesel engine injectors were inefficient at oxygenizing diesel. 

The finer the diesel aerosol injectors produce, the greater the oxygenation potential of the diesel. The greater the oxygenation, the greater the combustion efficiency. The greater the combustion efficiency — the percentage of the fuel that burns as opposed to simply blowing out the tailpipe — the less black smoke that is produced. 

Because traditional fuel injectors produced large-droplet diesel aerosol, an unacceptable percentage of diesel hydrocarbons went unoxygenated. Simply, when the injectors sprayed the diesel into the combustion chamber of the engine, the engines could not combust the diesel completely.

Another issue that was common in traditional diesel engines was the production of two highly toxic emissions: sulfur oxides and nitrogen oxides.

Sulfur Oxide Emissions from Traditional Diesel Engines

Of combustion engine emissions, sulfur oxides are some of the most toxic and harmful. Sulfur dioxide is especially dangerous.  It is a highly toxic, colorless, non-flammable gas. While other combustion engine fuel types produce sulfur dioxide as well, it is the most dangerous of those produced by diesel engines.

Dangers of Sulfur Dioxide

Sulfur dioxide is the cause of a variety of environmental ills. The reason being, when sulfur dioxide bonds with water, the effect is sulfuric acid. It is because of the interaction of sulfur dioxide with water that acid rain exists. Other environmental effects of sulfur dioxide include:

• • Deforestation. 
  • Acidify waterways to the detriment of aquatic life. 
  • Corrode building materials and paints.

Causes of Sulfur Oxide Emissions

Unlike hydrocarbon emissions, it is not a technological shortcoming that produces sulfur oxides. Traditional diesel engines produced sulfur oxide emissions because sulfur is inherent contamination in much of the oil produced around the world. Eliminating sulfur oxide emissions requires the elimination of sulfur from fossil fuels. 

Sulfur Oxide Emissions No Longer a Major Diesel Engine Emissions Concern

Today, the sulfur content of diesel is extremely low. According to the EPA, “Before EPA began regulating sulfur in diesel, diesel fuel contained as much as 5,000 parts per million (ppm) of sulfur. EPA began regulating diesel fuel sulfur levels in 1993. Beginning in 2006, EPA began to phase-in more stringent regulations to lower the amount of sulfur in diesel fuel to 15 ppm. This fuel is known as ultra-low sulfur diesel (ULSD).”

Since producing ultra-low-sulfur diesel became a government mandate, the amount of sulfur dioxide produced by diesel engines in the U.S. has fallen significantly. Unfortunately, not all countries in the world operate under the same standards for sulfur in fossil fuels as the U.S., Canada, and Western Europe. India, for example, is not nearly as strict. “India currently has two fuel quality standards: one that applies to places that meet BS-IV and other for the once that follows BS III. Nearly half of the country now requires 50 ppm sulfur and gasoline. The rest of the country allows up to 150 ppm sulfur gasoline and 350 ppm sulfur diesel.” 

Overall, however, the levels of sulfur oxides emitted from fossil fuel engines are far lower than they were in the past. As a result, the most dangerous emissions produced by the diesel engines of today are nitrogen oxides. 

Biggest Negative of Today’s Diesel Engines: Nitrogen Oxides

The most toxic of all diesel engine emissions, nitrogen oxides are extremely difficult to remove from diesel engine emissions. For one, 78 percent of the atmosphere is made of nitrogen. That means 78 percent of the air that feeds diesel engines is made of nitrogen. So, eliminating nitrogen from the fuel mixture that feeds a combustion engine is virtually impossible. 

As a result of the nitrogen content of air, 67 percent of the emissions produced by a diesel engine is nitrogen. That is not an issue. However, when nitrogen atoms bond with oxygen atoms to produce nitrogen oxides, there is an issue. “Nitrogen oxides are compounds of nitrogen N2 and oxygen O2 (z. B. NO, NO2, N2O, etc.). Nitrogen oxides are produced by high pressure, high temperature and a surplus of oxygen in the engine during the combustion cycle. Several oxides of nitrogen are harmful to health.”

Harmful consequences of nitrogen oxides do not include health problems. However, “NOx gases react to form smog and acid rain as well as being central to the formation of fine particles (PM) and ground-level ozone, both of which are associated with adverse health effects.”

Causes of Nitrogen Oxide Emissions

Again, nitrogen oxides are a consequence of the nitrogen in the air. When the nitrogen in the air and the oxygen in air bond, the consequences are nitrogen oxides. But the reason nitrogen oxide is more prevalent in diesel engines than in gasoline engines is temperature. Diesel burns at extremely high temperatures. The ideal environment for nitrogen and oxygen to bond is one with extremely high temperatures. 

The prevalence of nitrogen oxide production in a high-temperature environment is the reason nitrogen oxides are more common in the emissions of today’s gasoline-powered engines than those of the past. One of the major factors in fuel economy is thermal efficiency, the amount of energy that goes into an engine in relation to the amount of energy that becomes work. 

Thermal Efficiency and Nitrogen Oxide Production

Thermal efficiency is a measure of the difference between the energy that goes into an engine in relation to the amount of energy that an engine produces to propel a vehicle down the road. The greater the temperature difference between the intake stroke temperature of an engine and the combustion stroke, the greater the thermal efficiency. In other words, mechanical engineers want the combustion stroke of an engine to be as hot as possible. The problem is, the hotter the combustion stroke, the more nitrogen oxides produced. 

“Action taken to reduce fuel consumption has, unfortunately, often led to a rise in nitrogen oxide concentrations in exhaust emissions because a more efficient combustion process generates higher temperatures. These high temperatures, in turn, mean higher nitrogen oxide emission.” 

But nitrogen oxides are not the only negative component of today’s diesel engine emissions. Of course, diesel engine emissions will produce water and carbon dioxide. That is a given. But, the production of particulate matter is not a consequence of diesel combustion. Particulate matter is a consequence of diesel not combusting.

Other Biggest Negative of Diesel Engine Combustion: Particulate Matter

Particulate matter is one of the most preventable emissions diesel engines produce. However, though today’s engines produce far less particulate matter than those of the past, it is still an issue. Particulate matter is nothing more than partially burned or unburned hydrocarbons. When produced by gasoline engines, particulate matter is called “hydrocarbon emissions.”

Contrary to what one might think, no combustion engine burns all the fuel fed into it. Ever. There is always a portion of the fuel that goes from the fuel tank, through the engine, and out the exhaust without combusting. The black smoke once produced by diesel engines was little more than vaporized diesel. Soot is nothing more than partially burned, vaporized hydrocarbons. 

Again, today’s diesel engines produce far less particulate matter than the diesel engines of the past. In fact, there is now evidence suggesting that gasoline engines actually produce up to 100 times more particulate matter than diesel engines produce. 

“A study by researchers at the Materials, Science, and Technology Laboratory in Switzerland claims that particulate emissions from gasoline engines can be far greater than those from diesel engines. The laboratory studied the emissions of 7 gas engine vehicles equipped with direct-fuel-injection systems. The research found that they emit from 10 to 100 times more particulates than modern diesel engines. In fact, they have higher particulate emissions than older diesel without particulate filters.”

So, while particulate matter is major concern scientists and environmentalists have for diesel engines, gasoline engines are becoming far more concerning.

Pros of Diesel Engines

There are a large number of advantages to diesel engines. However, the two biggest advantages of diesel emissions are emissions and fuel economy. Diesel engines produce less carbon dioxide than gasoline and biofuel engines. And, diesel engines produce a smaller number of different types of emissions. 

Emissions from Diesel Engine in Relation to Gasoline Engines

Diesel engines produce almost no carbon monoxide. Carbon monoxide is one of the most dangerous emissions to human health associated with fossil fuel. Additionally, diesel has a very high compression resistance. Gasoline, on the other hand, does not. Gasoline will auto-ignite under relatively low pressure. As a result, gasoline manufacturers increase the octane rating of gasoline. 

In order to increase the octane rating of gasoline, manufacturers add additives to gasoline. In the past, lead was the additive. But, in 1996, because of the serious health and environmental issues vaporized lead produced, Congress banned the use of tetraethyllead as an octane booster. Today, benzene, toluene, and xylene — aromatic hydrocarbons — are the additives used to boost octane. However, major concerns are arising regarding aromatic hydrocarbon additives.

“Members of the scientific community, are concerned that aromatics exist in the environment at unsafe levels. The chemicals get released into the air as nano-sized particles – ultrafine particulate matter, or UFPs – that can be absorbed through the lungs or skin. Studies in peer-reviewed journals like the Journal of Environmental Science and Health, Environmental Health Perspectives and Particle and Environmental Toxicology, have linked these particles from aromatics to diseases ranging from ADHD to asthma.”

Because diesel has a very high compressive resistance to autoignition, there is no need to add additives. So, diesel combustion produces almost no carbon monoxide and no ultrafine particulate matter. And, diesel engines get far better fuel efficiency than their gasoline counterparts. 

Additionally, even though diesel produces 12.5 percent more carbon dioxide per gallon than gasoline, diesel engines produce between 10 and 20 percent less carbon dioxide per mile than gasoline engines. 

Fuel Efficiency of Diesel Engines Biggest Pro

The fuel efficiency of diesel engines, compared to their comparably-sized gasoline counterparts, is considerably better. Considerably may be an understatement. A diesel engine will get between 25 and 35 percent better fuel efficiency than its gasoline counterpart. There are two reasons why. First, diesel has a fuel density of 12.5 percent greater than gasoline. In addition to diesel having a higher density than gasoline, diesel engines are more efficient. 

The thermal efficiency of diesel engines is anywhere from 10 to 20 percent greater than that of gasoline engines. Thermal efficiency is the amount of energy fed into an engine that becomes work. It is the amount of fuel an engine converts into the mechanical energy that pushes a vehicle down the road. 

The only way to increase the thermal efficiency of an engine is to increase its compression ratio. And the only way to produce gasoline that can survive a higher compression ratio without auto igniting is to increase the octane rating. But, increasing the octane rating of fuel decreases its fuel density. So, high octane fuels mean greater thermal efficiency, but less energy. 

As a result of the limitations of gasoline with respect to fuel density and compression resistance, gasoline will likely always be inferior to diesel.