Gasoline vs diesel emissions is a comparison measured by an accounting for fuel economy, fuel composition, and engine-type combustion characteristics. It is not unreasonable to assume that gasoline and diesel engines produce the same emissions. After all, both gasoline and diesel are hydrocarbon based fossil fuels. However, while gasoline and diesel are made of the same component parts, how gasoline and diesel engines combust fossil fuels are completely different. The result, gasoline and diesel engines produce extremely different types of emissions. 

For example, a gasoline engine produces 40 percent or more carbon dioxide (CO2) than diesel engines of comparable size. Gasoline engines also produce considerably more hydrocarbon emissions — unburned and partially burned hydrocarbons. Diesel engines, while cleaner with respect to most emissions and considerably more fuel efficient, produce more nitrous and sulfur oxides than gasoline engines as well as more particulate matter.

While there is a variety of mechanical reasons diesel and gasoline engines are different, very different in fact, there are really only two reasons that the two engine types produce different quantities of different types of emissions. The first reason gasoline and diesel engines generate different quantities of emissions is combustion efficiency. By design, gasoline engines have extremely low combustion efficiencies. That means gasoline engines produce large quantities of hydrocarbon emissions. Low combustion efficiency — which plays a large role in fuel efficiency — also means higher carbon dioxide emissions on a per-mile scale. 

The second reason diesel and gasoline engines produce different sums of emissions, and different types are because of temperature. Diesel engines combust diesel at much higher temperatures than gasoline engines burn gasoline. That means diesel engines produce higher quantities of nitrogen oxides. 

The third reason diesel and gasoline engines produce different quantities of different types of emissions has nothing to do with how one engine type combusts fuel in relation to the other. Diesel engines typically produce more sulfur dioxides than gasoline engines because diesel — the fuel itself — contains more sulfur than gasoline normally. 

Most Impactful Gasoline-Engine Emissions

In reality, there are hundreds of emissions associated with spark-ignition gasoline engines. In terms of sheer amount, there is no other emission from gasoline engines that account for a greater percentage of combustion emissions than nitrogen (N2). Nitrogen makes up 71 percent of the composition of exhaust emissions of petrol engines. The reason being, nitrogen constitutes 78 percent of the air in our atmosphere.

However, the number of emissions that have a major impact on the environment and/or people, fauna, and flora is limited to less than 20. Less than 5 petrol exhaust emissions generate the vast majority of concern for scientist with respect to gasoline-powered engines. 

The most concerning petrol exhaust emission is carbon dioxide. 

Impact of Carbon Dioxide

Carbon dioxide constitutes 14 percent of exhaust emissions of petrol engines. Carbon dioxide is a non-toxic emission that does not harm people and is extremely valuable for plants during the process of photosynthesis. However, carbon dioxide has a greater impact on global warming than any other vehicle emission because petrol engines produce more CO2 than any emission aside from nitrogen. 

According to the Environmental Protection Agency (EPA), “A typical passenger vehicle emits about 4.6 metric tons of carbon dioxide per year. This assumes the average gasoline vehicle on the road today has a fuel economy of about 22.0 miles per gallon and drives around 11,500 miles per year. Every gallon of gasoline burned creates about 8,887 grams of CO2. The average passenger vehicle emits about 404 grams of CO2 per mile.”

That is to say, the typical passenger car emits 10,141 pounds of CO2 per year burning 3,136 pounds of gasoline or 523 gallons.

It may seem counterintuitive that a car creates more emissions in pounds than it burns in pounds of gasoline. But, part of the hydrocarbon combustion process is gaining weight. The energy produced during hydrocarbon combustion occurs when the carbon-hydrogen molecules split. But, once split, both the hydrogen and carbon atoms create chemical bonds with other atoms. Hydrogen atoms bond with oxygen atoms to make water and carbon atoms bond with oxygen to make carbon dioxide — under ideal settings. 

In practice, the conditions under which hydrocarbon combustion occur are never ideal, So, one to two percent of the emissions from a vehicle are hydrocarbons, nitrous oxides, and carbon.

Impact of Hydrocarbons

Between one and two percent of gasoline engines are not nitrogen, carbon dioxide, or water. One of the emissions produced by petrol engines is hydrocarbons. Petrol engines, by design, generate hydrocarbon emissions. 

Hydrocarbons are the valuable component in fossil fuels. Hydrocarbons are the part of gasoline, diesel, ethanol, natural gas, biodiesel, propane, etc. that ignite/combust/burn. However, a gasoline engine cannot burn all the hydrocarbons that feed into an engine. If an engine burns all the hydrocarbons completely, the engine will overheat and the consequences are catastrophic for the engine. 

As such, a gasoline engine — all gasoline engines — run on a rich fuel-to-air mixture. A rich fuel mixture is one in which there is not enough oxygen to oxygenate all the hydrocarbons in the mix. As a result of a lack of oxygenation, a percentage of the hydrocarbons blow out the exhaust partially burned or unburned.   

According to the Illinois EPA, “Hydrocarbon emissions result when fuel molecules in the engine do not burn or burn only partially. Hydrocarbons react in the presence of nitrogen oxides and sunlight to form ground-level ozone, a major component of smog. Ozone irritates the eyes, damages the lungs, and aggravates respiratory problems. It is our most widespread and intractable urban air pollution problem. A number of exhaust hydrocarbons are also toxic, with the potential to cause cancer.”

But it is carbon monoxide that is the most abundant of gasoline-engine emissions. 

Effects of Carbon Monoxide

Of the gasoline engines emissions that would not occur under ideal combustion conditions — water, CO2, and nitrogen — carbon monoxide is the most abundant. And, carbon monoxide is one of the most dangerous emissions that exist. “There is no question that carbon monoxide is a pollutant with the potential to harm all living things,” explains the Earth System Science Education Alliance. 

It is a greenhouse gas, though it does not have a direct impact on global warming, “Although carbon monoxide is only a weak greenhouse gas, its influence on climate goes beyond its own direct effects. Its presence affects concentrations of other greenhouse gases including methane, tropospheric ozone, and carbon dioxide.”

The last of the major non-ideal-combustion emissions produced by gasoline engines are nitrogen and sulfur oxides. However, nitrogen and sulfur oxides are more prevalent in diesel engine emissions than gasoline. 

Most Impactful Diesel-Engine Emissions

Unlike gasoline engines which would only produce three types of emissions under ideal conditions, diesel engines would produce four. In addition to nitrogen, water, and carbon dioxide, diesel engines also produce oxygen as an emission. Oxygen accounts for 10 percent of diesel emissions; water constitutes another 11 percent; 12 percent of diesel engine emissions are carbon dioxide, and nitrogen makes up the remaining 67 percent. 

Unlike gasoline emissions of which one to two percent is made of carbon monoxide, partially burned and/or unburned hydrocarbons, and nitrous and sulfur oxides, only .3% of the emissions generated by a diesel engine are made of pollutants besides oxygen, water, carbon dioxide, and nitrogen. However, that .3% is made of five pollutants rather than three. The most concerning of the five non-ideal-combustion emissions that diesel engines produce are nitrogen oxides and sulfur oxides. 

Effects of Nitrogen Oxides

While nitrogen is an innocuous gas that constitutes 78 percent of the atmosphere, nitrogen compounds — particularly nitrogen/oxygen compounds — are greenhouses gases, toxic, or both. There are three types of nitrogen oxides present in diesel engine emissions that are concerning: Nitric Oxide, Nitrogen Dioxide, and Nitrous Oxide.

Effects of Nitric Oxide

NO — nitric oxide — is an innocuous emission by itself, but once it enters the atmosphere, NO easily pairs with oxygen to create nitrogen dioxide. By itself, NO is used as both a food supplement and as fertilizer. However, nitric oxide is extremely reactive, “Nitric oxide (NO) is a reactive gas that plays an important role in atmospheric chemistry by influencing the production and destruction of ozone and thereby the oxidizing capacity of the atmosphere. NO also contributes by its oxidation products to the formation of acid rain.” 

As a result of the fact that nitric oxide almost always pairs with oxygen to create nitrogen dioxide, NO and NO2 are often explained in conjunction with one another. “When nitrogen is released during fuel combustion it combines with oxygen atoms to create nitric oxide (NO). This further combines with oxygen to create nitrogen dioxide (NO2). Nitric oxide is not considered to be hazardous to health at typical ambient concentrations, but nitrogen dioxide can be. Nitrogen dioxide and nitric oxide are referred to together as oxides of nitrogen (NOx).”

Effects of Nitrogen Dioxide

Unlike nitric oxide, nitrogen dioxide is not innocuous. With respect to ecosystems, “High levels of NOx can have a negative effect on vegetation, including leaf damage and reduced growth. It can make vegetation more susceptible to disease and frost damage.” And nitrogen dioxide is dangerous for the health of people as well. 

NOx produces respiratory conditions in people and animals. At high enough levels, nitrogen dioxide causes inflammation of the airways and lungs. And, “long-term exposure can decrease lung function, increase the risk of respiratory conditions and increases the response to allergens.”

Effects of Nitrous Oxide

Laughing gas is the layman’s term for nitrous oxide. Commonly used as an anesthetic and innocuous, nitrous oxide is possibly the most dangerous of all nitrogen oxides with respect to the atmosphere and greenhouse gases. Nitrous oxide has a global warming potential that is exponentially more dangerous to the atmosphere than carbon dioxide.

The global warming potential of nitrous oxide makes it one of the most dangerous greenhouse gases. 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. 

Of equal concern to scientist are sulfur oxides.

Effect of Sulfur Oxides

Molecules made of sulfur atoms in combination with oxygen atoms, sulfur oxides also contribute to acid rain. In addition to their acidic pH balance, sulfur oxides are also a major component in the development of particulate matter. Of all the sulfur oxides, sulfur dioxide is the biggest culprit with respect to acid rain and particulate matter generation.

Sulfur Dioxide Effects

Each molecule having one sulfur atom and two oxygen atoms, sulfur dioxide occurs as an emission when a fossil fuel has high sulfur content. As such, great measures have been taken to reduce the sulfur content found in fossil fuels. As of 1993, the maximum allowable amount of sulfur in ultra-low sulfur diesel fuel is 15 parts per million. In gasoline, the maximum allowable amount is 10 parts per million. 

Sulfur dioxide is not a greenhouse gas. In fact, as opposed to trapping thermal heat in the atmosphere, sulfur dioxide actually blocks sunlight before it enters the atmosphere. However, sulfur dioxide is extremely hazardous to the health of people. The Canadian Centre for Occupational Health and Safety describes sulfur dioxide as,” colorless gas, suffocating odor and VERY TOXIC. It is fatal if inhaled and corrosive to the respiratory tract. A severe, short-term exposure may cause long-term respiratory effects (e.g., Reactive Airways Dysfunction Syndrome (RADS)). CORROSIVE, it can cause severe skin burns and eye damage an may cause frostbite. SUSPECT MUTAGEN Suspected of causing genetic defects.”

In addition to sulfur oxides and nitrogen oxides, diesel emissions generally have a higher particulate matter count than gasoline.

Effects of Particulate Matter

Particulate matter (PM) can be anything from dust or pollen to smog. Particulate matter from diesel engines has the same chemical composition as the smoke that comes off a campfire. Engine emissions particulate matter is dangerous because it causes respiratory and blood issues. Particulate matter is visible because it is large enough to create gas clouds — smog — as well as black smoke. 

“PM pollution can cause lung irritation which leads to increased permeability in lung tissue; [it] aggravates the severity of chronic lung diseases, causing rapid loss of airway function; causes inflammation of lung tissue, resulting in the release of chemicals that can impact heart function and PM causes changes in blood chemistry that can result in clots that may lead to heart attacks. PM can [also] increase susceptibility to viral and bacterial pathogens leading to pneumonia in vulnerable persons who are unable to clear these infections.”

While the emissions from diesel engines are of a greater number, the advantage diesel refineries, diesel engine manufacturers, and the owners of diesel-powered vehicles and machines have over those associated with gasoline-powered engines is the capacity to reduce the most dangerous of the emissions dramatically. 

Diesel refineries can reduce the sulfur content of fuel to extremely low levels. Fuel catalysts can reduce nitrogen oxide emissions, black smoke, and particulate matter. Particulate matter filters can reduce PM by up to 40 percent as well. Catalytic converters drop levels of nitrogen oxides, particulate matter, and hydrocarbon emissions significantly. Unfortunately, very little can be done about neither the amount of carbon monoxide gasoline engines produce nor the number of hydrocarbon emissions and carbon dioxide.