Gasoline Versus Diesel Engines: A Fuel Efficiency Comparison
Gasoline vs diesel engines — with respect to fuel efficiency — is not even a debate. Unlike in Europe and the vast majority of the rest of the developed world, it is not uncommon for people in the United States to be unaware that diesel is a better fuel than gasoline with respect to fuel economy. Though not common knowledge among automobile consumers in the U.S., it is true. Diesel engines are better with respect to fuel consumption than gasoline engines.
And not by just a little, diesel engines are far more fuel efficient than gasoline-powered engines of comparable size.
Diesel engines get much better “gas” mileage than gasoline engines.
A comparison between diesel and gasoline fuel efficiency is not even close. Diesel engines are typically anywhere from 25% to 35% more fuel efficient than gasoline engines. Diesel engines get between a quarter and a third better “gas” mileage than gas engines.
If a gas-powered vehicle gets 30 miles to the gallon, a comparable diesel-powered vehicle will get anywhere from 37.5 to 40.5 miles per gallon. At worse, a diesel engine travels 40 miles for every 30 miles a gasoline travels on the same volume of fuel. At best, for every 65 miles a gasoline engine travels, a diesel engine will travel 100 miles on the same volume of fuel.
As such, a comparison of the sum of emissions a gallon of diesel produces per gallon vs the number of emissions from a gallon of gas is irrelevant.
The reason being, even though diesel engines produces 13% more carbon dioxide per gallon than comparable gasoline engines, that fact has very little to do with the reality of how much either engine generates during practical use. While — according to the European Automobile Manufacturers Association — “1kg of diesel burnt under ideal conditions will produce 2.65kg of CO2. 1kg of petrol burnt under ideal conditions will produce 2.3kg of CO2,” a volume scale comparison offers very little usable information.
Per mile, therefore, a gasoline engine produces between 12% and 22% more carbon dioxide than a comparable diesel engine. In other words, the fuel efficiency of a diesel engine determines realities of diesel vs gasoline emissions to a far greater degree than a per-volume comparison.
Why Diesel Engines get Better “Gas” Mileage than Gasoline-Powered Engines
Understanding why diesel engines are so much more efficient than gasoline engines does not require test driving two comparably sized engines — one with a diesel-powered engine and the other with a gasoline-powered engine — and comparing the mileage per gallon. Instead, understanding why diesel engines are so much more fuel efficient than gasoline engines is a matter of understanding three things about gasoline and diesel and gasoline engines and diesel engines: energy density, thermal efficiency, and compression ratio.
Energy Density, Thermal Efficiency, and Compression Ratio of Diesel vs Gas
The first major difference between diesel and gasoline is energy density. There is more energy in a gallon of diesel than there is in a gallon of gasoline. The energy density of diesel is at a minimum 13% greater than that of gasoline. Often the energy density of diesel is more than 13% greater than that of gasoline.
Coincidentally, there is a direct correlation between energy density and pollution. The more energy dense a fuel, the more pollution it generates on a volume scale. Diesel produces 13% or more energy and 13% more emissions. But again, comparing emissions generated per measure of volume does not provide useful information. If a trip from one place to another is 50 miles and a gallon of gasoline is necessary to cover that distance in a gasoline vehicle, only 6.5 gallons are necessarily using a diesel engine.
The second factor that separates diesel vs gasoline powered engines — a.k.a. “heat engines” — is thermal efficiency. The thermal efficiency of a diesel engine can be twice that of a gasoline-powered engine. Thermal efficiency increases fuel efficiency.
The third factor that influences the fuel efficiency and emissions of combustion engines is compression ratio. The compression ratio of an engine is determined by the compression resistance of a fuel. Compression resistance is how much compression pressure a fuel can sustain without combusting. The higher the compression ratio of an engine, the better. The higher the compression ratio, the greater the combustion efficiency, how much of the fuel burns up during combustion.
In combination, the fuel density of diesel, the thermal efficiency of diesel engines, and the compression ratios of diesel engines make diesel-powered vehicles and machinery considerably more fuel efficient than gasoline engines. And, these three factors are also the reason diesel engines pollute less than gasoline-powered engines.
Energy Density of Gasoline vs Diesel
Diesel has an energy density of between 15% and 25% greater than that of gasoline. Energy density is potency of a fuel, how much energy per unit of measure — gallon, liter, cubic foot or meter, etc. As Isaac Ramos of Stanford University explains in a paper titled, “A Comparison of Diesel and Gasoline in Consumer Automobiles,”
“Relating to energy, another important metric to keep in mind is the energy density of diesel and gasoline. Diesel fuel is heavier and oilier than gasoline and its takes less refining to create, its chemical compound is C14H30. Gasoline, on the other hand, is C9H20.  When burned, these chemical compounds correspond to energy densities of approximately 155 million Joules per gallon for diesel and 132 million Joules per gallon for gasoline. Thus, in terms of energy density, diesel is clearly chemically ahead.”
The reason diesel has a higher energy density than gasoline is that of the structures of the hydrocarbons within it. Hydrocarbons are the valuable component in fossil fuels. Hydrocarbons are the molecules in fossil fuels that ignite, burn, combust, and explode — oxidize. It is the oxidation of hydrocarbons that make the modern world go around.
Not all hydrocarbons are equal. The higher the hydrogen-to-carbon ratio in hydrocarbons, the lighter the fossil fuel. Also, the higher the ratio of hydrogen-atoms-to-carbon-atoms in fossil fuel’s hydrocarbon molecules, the less energy the fossil fuel per measure of volume, per gallon for example. It is because of methane — a.k.a., “natural gas” — has very small hydrocarbon molecules with very high hydrogen-to-carbon ratios that it is a gas-state fossil fuel.
High carbon-atom-to-hydrogen-atom ratios, on the other hand, produce heavy, high-energy-density molecules. Diesel hydrocarbons have a high carbon-to-hydrogen ratio. Gasoline, on the other hand, has a medium carbon-to-hydrogen ratio in relation to other fossil fuels. So, gasoline hydrocarbons have a very low carbon-to-hydrogen ratio in relation to diesel hydrocarbons. Because of diesel’s high carbon-to-hydrogen ratio, it is more energy-dense than gasoline.
Because diesel produces more energy per gallon, liter, or cubic feet or a meter, that means gasoline engines require more gallons of gas to travel the same distance as a diesel engine can on less fuel.
But, fuel density is not the only reason diesel-powered cars are more fuel efficient than their gasoline counterparts. The combustion efficiency of diesel engines also makes them more fuel efficient.
Combustion efficiency is determined by two factors: oxygen-to-fuel ratio and compression ratio.
Combustion Efficiency of Diesel vs Gasoline Engines
Combustion efficiency — the rate of hydrocarbon oxidation — is in part a product of the ratio between hydrocarbons and oxygen. The greater the amount of oxygen added to a fuel/oxygen mixture, the higher percentage of fuel that burns. Without oxygen, hydrocarbons will not burn irrespective of how much heat or flame — spark — exposure.
That is not to say, however, exposure to heat and flame will not change un-oxygenated hydrocarbons. Hydrocarbons that are not mixed with oxygen but that are exposed to heat and/or flame will undergo a chemical reaction. But, they will not oxidize — ignite, combust, burn, explode. The chemical reaction un-oxygenated hydrocarbons undergo when exposed to heat or flame is typically one of a chemical bond.
A completely burned fossil fuel will only produce two emissions: water and carbon dioxide.
In an engine, un-oxygenated hydrocarbons exposed to heat and/or flame bind together to produce emissions, a wide variety of different emissions. The products of bonded, unburned hydrocarbons from a combustion engine include carbon monoxide, sodium oxides, nitrogen oxides, ozone, acetaldehyde, acetone, benzene, methylbenzene, ethylbenzene, and xylenes.
The obvious question, then, is why not simply engineer all vehicles to operate on an extremely lean fuel mix, one that is highly oxygenated? It is possible to run diesel engines on an extremely lean fuel mix.
But, it is not possible gasoline-powered engines to run lean fuel-to-air mixtures.
Why Gasoline Engines Can Not Burn Hyper-Oxygenated Fuel
Gasoline engines cannot run on a lean fuel-to-air mixture. Gasoline engines must always run rich. The perfect blend of air to fuel is called the stoichiometric ratio. The stoichiometric ratio is the point at which there is exactly the correct amount of oxygen to burn a sum of fuel. If there is less oxygen than the amount required to reach the stoichiometric ratio, a vehicle is running rich. If there is excess oxygen — more oxygen than is required to reach the stoichiometric ratio — the engine is running lean.
Gasoline engines can neither run at the stoichiometric ratio nor lean. The reason is, when a car runs at the stoichiometric ratio, the engine cannot deal with the heat generated.
“The stress on most internal combustion engines is maximum when they are running at the stoichiometric ratio. The flame front propagates rapidly, maximizing peak pressure and temperature and generates the maximum power for a given airflow. [Gasoline] engines, when generating high power, are designed to run significantly rich of the peak temperatures and internal pressures. [They are designed to run] at about a 12:1 ratio, [rather than] at stoichiometric which is 14.7:1.”
Running a gasoline engine at stoichiometric will cause engine overheating, can destroy pistons, and melt gaskets. Running lean — an excess of air that pushes the mixture beyond the stoichiometric ratio — causes problems as well including backfiring, erratic idle, hard cold starts, etc.
While running a gasoline engine with an air-to-fuel mix that is equal stoichiometric ratio means combustion efficiency is at its highest and emissions are at their lowest, it is not possible to do so without destroying the engine.
Diesel, on the other hand, can run extremely lean.
Why it is Possible to Run Diesel Engines on Extremely Lean Fuel Mixture
Diesel, on the other hand, is so energy dense that much greater amounts of air can be added to the mixture. The stoichiometric ratio of diesel is much higher than that of gasoline because, again, diesel is considerably more energy dense. “Typical operating ranges of diesel engines spread between an air/fuel ratio of 18 and up to 70, depending on the operation point.”
Diesel start their combustion in a locally rich environment (right near the injector), but the fuel is within an overall quite lean mixture, so once the flame starts in the rich area, it burns in a lean environment with a slow and sustained flame front, but the high flame temperatures associated NOx production from a lean peak combustion process. It is the high pressure and sudden energy release of running at the peak that causes the overheating of engines, not actually the flame temperature, hence why diesel don’t overheat.
Knock, a.k.a., pre-combust. The reason being, lean-fuel mixtures will pre-combust in a gasoline engine is because of weak compression resistance of gasoline. Because gasoline is a light, low-energy, highly-volatile fuel, gasoline will combust under less pressure than a heavy, stable fuel like diesel. That means gasoline engines have a much lower compression ratio than diesel engines.
The greater the pressure hydrocarbons are under prior to combustion, the more efficient the combustion. The combustion efficiency, therefore the fuel economy, of gasoline engines is poor because the compression resistance of gasoline is low when gasoline is highly oxygenated.
High-compression, lean-burn engines generate the highest fuel efficiency. But, in gasoline engines, those two variables are mutually exclusive. So, in order to increase combustion efficiency by increasing the compression ratio of a gasoline engine, gasoline engines must run on rich fuel mixtures, a mixture that prevents pre-combustion.
While a lab study followed by a field study — two studies that compare the fuel efficiency of diesel vs gasoline engines — provides the best evidence that diesel engines are superior with respect to “gas” mileage, energy density, thermal efficiency, and compression ratio explain why. Diesel engines are more fuel efficient in both practice and principal. And it is the theory behind these comparisons that explain why diesel engines will probably always be superior to gasoline engines with respect to fuel efficiency.