Irrelevant factoids cloud the question, “do diesel engines pollute more than petrol engines?”  Gasoline engine manufacturers and oil refining companies would like to have you believe so, but it does not. Simply, the answer is, no, diesel engines do not pollute more than petrol engines. In fact, they don’t even pollute as much as petrol engines. 

Gasoline engines pollute more than diesel engines. 

Confusing the issue is the fact that the words “pollute” and “pollution” are vague and ambiguous terms. Even strict dictionary definitions do very little to provide clear parameters relating to gasoline vs. diesel pollution. Further confusing the issue is the fact that groups and pundits posing as environmental crusaders, typically, do little to focus the blurry lines surrounding emissions pollution either. In fact, some people and corporations claiming to have the best interests of the environment — and by extension, us — at heart are doing the most damage by perpetuating misinformation. 

The Realities of Diesel Engine vs Petrol Engine Emissions

It is hardly even a debate within the scientific community anymore. Gasoline-powered engines produce more toxic emissions than comparably-sized diesel engines. Gasoline engines produce greater sums of high global warming potential emissions. And, gasoline engines produce greater quantities of emissions than comparably-sized diesel engines. With respect to only one emission type — nitrogen oxides — do diesel engines produce more emissions than gasoline engines. 

Understanding why petrol engines pollute more than diesel engines is a matter of understanding the difference between diesel and gasoline and understanding the mechanical differences between diesel and gasoline engines. Though hydrocarbons makeup both diesel and gasoline, diesel and petrol are two very different fuels. And, even though gasoline engines and diesel engines are both combustion engines, they burn the fuel completely differently. 

It is because of both the differences in fuel types and engine types that petrol engines pollute more than diesel engines. 

Differences Between Petrol and Diesel

Petrol — gasoline — is light liquid fuel relative to diesel. In addition to weight, there are significant differences in the chemistry of the two different fuels. Principally, gasoline is made of short-chain hydrocarbons. The average hydrocarbon molecule in gasoline has between 5 to 12 carbon atoms. The average hydrocarbon molecule in diesel has between 8 to 21 carbon atoms. 

The differences in hydrocarbon sizes between gasoline and diesel are the biggest factor in the pollution rates of the two fuels. There are two reasons why. For one, diesel fuel is between 10 and 15 percent more energy dense than gasoline per gallon or liter. Additionally, the longer and larger the hydrocarbons in a fuel, the more stable that fuel. 

The stability of a fossil fuel — in particular, its compression resistance — determines what type of engine the fuel can power. 

Spark-Fired vs Compression Engines

There are two types of compression engines: spark-fired and compression-fired engines. Compression-fired engines are more efficient, less polluting, and generate more low-end torque thank spark-fired engines. But, only stable fuels with high compression resistance are capable of powering compression engines. Volatile fuels with low compression resistance are not capable of powering a compression engine. That means gasoline, ethanol, propane and natural gas only work in spark-fired engines. 

A spark-fired engine ignites the fuel in an engine’s cylinder using the spark from a spark plug. A compression engine ignites the fuel in an engine’s cylinder by compressing the fuel until it auto-ignites. The autoignition of fuel is far results in both less pollution and greater energy efficiency. That is to say, compression engines are better engines with respect to energy efficiency and pollution. 

The reason autoignition via the compression of fuel is an advantage is because autoignition generates greater thermal efficiency. Thermal efficiency is the measure of how much energy put into an engine becomes work. In reality, neither gasoline engines nor diesel engines are particularly thermally efficient. The thermal efficiency of a gasoline engine is only about 20 to 25 percent. The rest of the energy is lost to the atmosphere surrounding the engine or blown out the exhaust. 

While not efficient either, a diesel engine — a compression-fired engine — is almost twice as thermally efficient as a gasoline spark-fired engine. A diesel engine has a thermal efficiency of between 30 and 45 percent. That means a diesel engine puts between 10 and 25 percent more of the energy it produces to work.

Two Factors of Compression Engine Pollution

Those are the two factors— energy density and compression resistance — that determine pollution in combustion engines. At least, that is to say, with respect to amounts of pollution. The energy density of diesel is between 10 and 15 percent greater than that of gasoline. And, diesel engines have a thermal efficiency of between 10 and 25 percent greater than gasoline engines.

As a result, diesel engines are roughly 30 percent more fuel efficient than gasoline engines per mile. The problem is — the confusion with respect to the amounts of diesel and gasoline generated pollution — is how pollution is measured. Proponents of gasoline want to compare pollution generated per gallon. 

The amount of pollution an engine generates per gallon fails to account for energy efficiency. 

Pollution per Mile vs Per Gallon

The vogue measure of pollution is carbon footprint, the number of carbon-based molecules found in emissions. Of particular interest to pollution, pundits are C02. Carbon dioxide is the standard against which scientists measure other pollutants. That is especially the case for greenhouse gases. The global warming potential of an emission gas is measured against C02. Therefore, proponents of gasoline engines like to point out that gasoline produces less carbon dioxide per gallon than diesel. 

The EPA explains, “About 19.64 pounds of carbon dioxide (CO2) are produced from burning a gallon of gasoline that does not contain ethanol. About 22.38 pounds of CO2 is produced by burning a gallon of diesel fuel.” That is a difference of a little over 13 percent. 

Diesel engines get between 25 and 35 percent better fuel efficiency. That means that per mile diesel produces between 12 and 23 percent less carbon dioxide. 

In addition to producing more C02 per mile, between 9 and 12 percent of gasoline emissions are carbon monoxide. Diesel produces virtually zero, only trace amounts of CO. In order to increase the compression resistance of gasoline, refineries add amines to gasoline. But, just like their predecessors — lead additives — amines produce extremely toxic emissions. As diesel inherently has high compressive resistance, there is no need to add amines. 

So, no, diesel engines do not pollute more than petrol engines.