Why diesel engines get better gas mileage and burn cleaner than petrol engines is because diesel is a cleaner more energy dense fuel. A diesel engine is between 25 and 35 percent more fuel efficient than a gasoline vehicle of comparable size. With a diesel engine, you go to the pump much less often than you do with a gasoline engine. Explains BusinessInsider.com, “Diesel may cost more than regular gasoline (about $0.24 more per gallon, per AAA’s current figures) but it’s much richer in energy — between 25% and 30% richer. [For example], the new BMW 328d (d for diesel) gets better mileage than the gas-powered Smart Fortwo Coupe. And it produces 180 horsepower, compared to the Smart’s measly 70.”

Even greater is the difference between the fuel efficiency of a diesel engine and a “clean” or “alternative” fuel engine. The difference in fuel economy between diesel and alternative fuel engines is so big that alternative fuels are untenable without government subsidies. 

The reasons diesel engines get better gas mileage than gasoline engines are the same reasons diesel engines get better gas mileage than alternative fuel engines. 

Energy Density of Diesel

There are two reasons diesel engines get better “gas” mileage than all other combustion engines. First, diesel has a higher energy density than gasoline and alternative fuels. Diesel is as — or more — energy dense than almost all other fossil fuels used in combustion engines. Only marine fuel oil has a higher energy density. 

In fact, of all fossil fuels, only marine-use fuel oil and residual fuel oil — bunker fuel — have higher fuel densities than diesel. Diesel has a higher energy density than all alternative combustion engine fuels. It has a higher energy density than gasoline and kerosene and jet fuel. Diesel has a higher energy density than coal. And the energy density of diesel is exponentially greater than gas-state fossil fuels like natural gas (methane) and propane. 

Fuel density is not the only reason a diesel engine is more fuel efficient than a gasoline engine. 

Thermal Efficiency of Diesel Engines

Diesel engines are better engines than gasoline engines with respect to combustion efficiency. The reason diesel engines have a higher combustion efficiency than gasoline engines is that diesel engines have higher thermal efficiency. And, the reason diesel engines have higher thermal efficiency than other combustion engines is that diesel has a higher compression resistance than other fossil fuels. 

Thermal Efficiency of Diesel Engines Because of Compression Resistance of Diesel

The higher the compression resistance of fuel, the greater the compression ratio a fuel can withstand. For example, gasoline can resist a compression ratio of between 8:1 and 10:1 depending on the gasoline’s octane rating. Diesel, on the other hand, can withstand extreme compression. Diesel engine’s function with a compression ratio of 18:1 or higher. And, the higher the compression ratio of an engine, the greater that engine’s thermal efficiency. 

Thermal Efficiency and Fuel Economy

Thermal efficiency, technically, works input versus work output. It is the difference between how much energy — fuel — goes into an engine versus how much of that energy becomes mechanical work. Measuring thermal efficiency is as simple as measuring the difference between the temperature of a cylinder before it is injected with fuel and the combustion temperature. 

Again, the greater the thermal efficiency, the greater the fuel economy. And the greater the compression ratio of an engine, the greater the thermal efficiency. That is because higher compression ratios generate more heat. The reason being, when gases compress, they generate heat. The greater the compression of a gas, the more heat generated. 

Because diesel engines have higher compression ratios, they generate more heat. Because diesel engines generate more heat than other combustion engines, they have higher thermal efficiencies. And, because diesel engines have higher thermal efficiencies, they put more of the energy put into them to work. Again, the reason diesel engines can operate at higher compression ratios than other combustion engines is that diesel has greater compression resistance than other combustion engine fuels.  

Diesel engines are better than other combustion engines because diesel is a better fuel than other fuels combustion engine fuels. 

Diesel and Diesel Engines Also Pollute Less

Diesel engines are cleaner than gasoline engines and alternative fuel engines as well. Diesel engines do not produce as much carbon dioxide as gasoline. They may produce more carbon dioxide than alternative fuels. However, throughout the entire lifecycle of fossil and alternative fuels, diesel produces less carbon dioxide. And, both alternative fuels and gasoline produce more toxic emissions than diesel, carbon monoxide, and nitrous oxides, for example. 

The National Academy of Sciences, Engineering, and Medicine explains, 

“On a volume basis, diesel has a higher energy content, called heat of combustion, and higher carbon content than gasoline; thus, on a per gallon basis diesel produces almost 15 percent more [energy]. However, on a weight basis the heat of combustion of diesel and gasoline is about the same, and so is the carbon content. One needs to keep in mind that this difference in energy content is one of the reasons why CI engines have lower fuel consumption when measured in terms of gallons rather than in terms of weight.”

And it isn’t only carbon dioxide that diesel and diesel engines produce less of. Gasoline and alternative fuels produce many emissions that diesel do not. For example, the combustion of gasoline produces large quantities of carbon monoxide. Diesel produces almost none. Alternative fuels generate large amounts of nitrous oxides, far more than diesel. And, diesel does not require them.  

The general worry about diesel is the sum of sulfur oxides it produces when combusted. However, low-sulfur fuel reduces sulfur emissions by more than 97 percent. Additionally, 

Why Diesel and Diesel Engines Produce Less Pollution than Other Combustion Engine Fuels

In addition to being more fuel efficient, diesel engines pollute less than gasoline and alternative fuel engines. For the same reasons diesel engines are more fuel efficient than gasoline engines, diesel engines pollute less. The reason diesel engines pollute less is two-fold. For one, diesel engines pollute less because of diesel itself. And two, diesel engines pollute less because diesel engines burn diesel fuel more efficiently than gasoline and alternative fuel engines burn fuel. Those are two of the reasons why diesel engines get better gas mileage and burn cleaner than petrol engines are because diesel is a cleaner more energy dense fuel.

Differences in Density: Specific Energy Vs Fuel Density

Diesel is considerably more dense than gasoline and far more energy dense than alternative fuels. There are two measures of density that pertain to the amount of energy in fossil fuels. The first is specific energy. Specific energy is the amount of energy in a fossil fuel on a weight scale. The second measure of density that pertains to a diesel vs gasoline comparison is energy density. Fuel density is the amount of energy in a fossil fuel measured on a volume scale. 

Neutrium.net explains why the difference between specific energy and fuel density is important when determining fuel efficiency, “The specific energy and energy density of fuel provide practical measures of the energy content of fuel in units more commonly used in the storage and handling of these substances (energy per weight and volume).”

The distinction between energy density and specific energy is significant. There are relevant differences between the amount of energy in a fossil fuel when measured by weight versus the amount of energy when measured on the volume scale.

Energy Densities of Gasoline and Diesel Vs Specific Energies

The specific energies of gasoline and diesel are roughly the same. The specific energy of gasoline is 47.3 megajoules per kilogram. The specific energy of diesel is 44.8 mg/kg. However, the specific energy of gasoline vs diesel is not relevant to fuel economy. Nor is the specific energy of a fossil fuel relevant to personal or business accounting. 

Gasoline and diesel sold in units of volume: gallons, liters, cubic meters, cubic feet. Gasoline and diesel are not priced per pound or kilogram. As a result, a comparison of the specific energy of gasoline vs diesel irrelevant when determining fuel economy and fuel costs. When calculating fuel efficiency and fuel costs, what is of significance is the energy a fuel contains per liter or gallon. 

The Role of Fuel Density in the Calculation of Energy Density

Fuel density is the weight of fuel on a volume scale, how much a gallon or liter of fuel weighs.

Diesel is a heavier fuel than gasoline in terms of weight per unit of volume. A gallon of diesel weighs around seven (7) pounds per gallon. Gasoline weighs about six (6) pounds per gallon. The exact weights of both vary because there are different types of both gasoline and diesel. 

Roughly, diesel is between 11 and 14 percent denser than gasoline. 

As a result of the difference in fuel density, the energy density of diesel and gasoline are different. The energy density of conventional gasoline is roughly 124,340 Btu per gallon. The Btu is high octane gasoline is lower than that of conventional diesel. That is to say, the higher the octane rating of gasoline, the less energy it contains per unit of measure. 

The Btu per gallon of diesel is around 137,380. And, low sulfur diesel has an energy density of 138,490. So, the energy density of diesel is between 9.5 and 15 percent greater than gasoline.

However, fuel density on accounts for about half of the difference in fuel economy that exists between gasoline engines and diesel engines. The remaining difference between the 25 to 35 percent superiority of diesel engines over gasoline engines with respect to “gas” mileage is why diesel engines get better gas mileage and burn cleaner than petrol engines is because diesel is a cleaner more energy dense fuel.

Spark-Fired Versus Compression-Fired Engines

Gasoline engines and diesel engines are, fundamentally, very different. Gasoline engines are spark-fired engines. The fuel inside the engine cylinder that ignites and pushes the piston down as it burns ignites as the result of exposure to a flame, a spark. 

Diesel engines are compression-fired. The diesel inside the engine cylinder ignites as the result of exposure to extremely high temperatures. The temperatures are a byproduct of the compression of air in the piston cylinder. All gases, when compressed, generate heat. The heat generated in compression engine ignites the fuel in the cylinders. Diesel engines are compression engines. 

Explains Transport Topics News, “The most significant difference [between spark-fired and compression engines] is in the way the fuel is ignited in the combustion chamber. Either the engine is built to run solely on natural gas, requiring it — unlike a diesel — to use spark plugs for ignition, or it is dual-fuel, combining the natural gas with a small amount of diesel fuel, which is compressed in the combustion chamber until it ignites, a process known as high-pressure direct injection, or HPDI.”

Diesel engines are more fuel efficient than gasoline for several reasons. However, all the reasons come down to the qualities of diesel fuel. Because diesel fuel has high energy density and because diesel fuel has high compression resistance, diesel engines are more fuel efficient than other engine types. And, because diesel fuel burns more efficiently — more completely — diesel engines pollute less.