Diesel engines are more fuel efficient than gasoline engines. That is to say, diesel engines get better “gas” mileage than comparably sized gasoline powered engines. There are two reasons why. First, diesel is a better fossil fuel than gasoline with respect to energy density. And second, diesel engines are better engines with regard to thermal efficiency. 

And while gasoline and diesel — as fuels — are not getting better, at least not with respect to energy density, both gasoline engines and diesel engines are becoming more thermally efficient. But, diesel engines are becoming considerably more thermal efficient while gasoline engines are only making moderate strides. 

Considering the strides gasoline engineers would have to make with respect to thermal efficiency in order to make gasoline engines comparable to diesel engines — even if diesel engine engineers were to make zero progress, — it is almost a certainty that gasoline engines will never be as fuel efficient as diesel engines. 

Why Diesel is a Better Fuel than Gasoline

With respect to the amount of energy produced per unit of volume, diesel fuel is a better fuel than gasoline. Per gallon or liter of fuel, there is more energy in diesel than there is in gasoline. The amount of energy in a gallon of fossil fuel varies. Just like there is a difference between gasoline and diesel, there are differences between the diesel and gasoline’s extracted from different parts of the world. Some diesels have more energy per gallon than other diesel and the same is true for different types of gasoline. 

But, generally, it is agreed that there is roughly 15% more energy in a gallon of diesel than there is in a gallon of gasoline. The measure of energy in a volume of fuel is called energy density. Energy density is directly related to the number of hydrocarbon molecules in a fuel. 

Energy Density

Gasoline has an energy density of 33,867 megajoules per meter cubed. Diesel has an energy density of 37,184 mj/m3. Again, those are rough estimates. Fuels with higher sulfur counts have lower energy densities. High octane gasoline and high cetane diesel also have lower energy densities as well. 

Nevertheless, it is fair to say that diesel fuel is anywhere from between 10% and 20% more energy dense than gasoline. 

Why Diesel is More Energy Dense than Gasoline

The energy density of fossil fuels is a product of hydrocarbons. The greater the number of large molecular hydrocarbons in a fossil fuel, the heavier the fuel and the more energy rich. That is why, for example, methane — a.k.a., natural gas — has such an extremely low energy density. Natural gas is comprised primarily of very small energy molecules. So small are the molecules and molecule chains in methane, in fact, that methane is a gas-state fossil fuel at room temperature. 

Gasoline, too, is also made up of smaller fuel molecules. While a liquid-state fuel, gasoline is relatively low in energy density, particularly when compared to diesel. Diesel, on the other hand, is extremely high in energy in relation to most other fossil fuels. Diesel is made of large, long-chain hydrocarbons. Because of the size and length of the hydrocarbons in diesel, diesel has a high energy density. 

But, the energy density of diesel is not the only reason diesel engines are more fuel efficient than gasoline-powered engines. Not only is diesel a better fuel, diesel engines are better engines with respect to thermal efficiency.

Why Diesel Engines are better than Gasoline Powered Engines

Diesel engines are more fuel efficient than gasoline engines because diesel engines have a higher thermal efficiency than gasoline engines. Thermal efficiency is, in layman’s terms, how much of the energy in a fuel becomes mechanical energy. Thermal efficiency is a measure of how much of the total energy potential of a fuel an engine can convert into energy that moves a vehicle down the road or that a piece of machinery converts into work. 

All heat engines, be they steam, gasoline, diesel, natural gas, etc. are surprisingly inefficient. 

The Thermal Efficiency of Diesel vs Gasoline Powered Engines

Thermal efficiency is technically defined as the combination of two laws of physics: the first and second law of thermodynamics. The first law of thermodynamics states that the energy output of a system cannot exceed the energy put into a system. The second law of thermodynamics states that no system can be 100% efficient because heat will always be lost. 

So, thermal efficiency can be defined with a formula. Thermal efficiency is the amount of useful energy put into a system divided by the total amount of energy put into a system. The word “useful” is important because the laws of thermodynamics state that there is always a waste. 

Of all the energy put into a combustion engine, only a portion is converted into useful energy. The rest blows out the tailpipe or is lost as heat into the environment around the engine. 

Diesel combustion engines have a far higher thermal efficiency than gasoline combustion engines. 

Why Gasoline Powered Engines Will Never Get Better Mileage than Diesel Powered Engines

Currently, gasoline engines have an efficiency of between 20% and 35%. According to Toyota, a company that is attempting to produce a high thermal efficiency gasoline engine, the thermal efficiency of, “most gasoline combustion engines average around 20 percent thermal efficiency.”  According to the same, “Diesels are typically higher–approaching 40 percent in some cases.”

However, the Swedish vehicle engineering company MDPI, the thermal efficiency of both gasoline and diesel engines is considerably higher, though hardly as high as a layperson might assume, “Current production spark-ignition engines are working with brake thermal efficiency (BTE) about 30–36%, compression-ignition engines have long been recognized as one of the most efficient power unit, the current BTE of diesel engines can achieve to 40–47%.”

Toyota claims that it is developing a gasoline engine with a considerably higher thermal efficiency than typical gasoline engines, one with a thermal efficiency of 38%. If the MDPI is correct, the thermal efficiency of Toyota’s new engine is still more than 20% less efficient than a typical diesel engine. 

Why Diesel Engines Have a Higher Thermal Efficiency than Spark-Fired Gasoline Engines

There are two reasons that diesel engines have a higher thermal efficiency than gasoline engines: compression ratio and air-to-fuel mixture. Compression ratio is a measure of the difference between the total volumes of a combustion chamber when the piston is at the bottom of its stroke to the volume of the combustion chamber at the moment the fuel combusts. 

Compression Ratio and Energy Output

An increase in compression ratio increases fuel efficiency because of the higher the compression ratio, the greater the amount of energy released from a given volume of fuel. “Theoretically, increasing the compression ratio of an engine can improve the thermal efficiency of the engine by producing more power output.” Diesel engines have a compression ratio of between 14:1 and 25:1. A gasoline engine has a compression ratio of between 8:1 and 12:1.

And the problem is, the compression ratio is not something gasoline engine engineers can increase, at least not without decreasing fuel efficiency. It is possible to make gasoline engines with higher compression engines. However, as gasoline is highly volatile and unstable in relation to diesel engines, higher compression ratios mean that the gasoline will combust prior to the spark igniting it in the fuel chamber. The pre-combustion of gasoline is commonly called “knocking.” 

While it is possible to prevent pre-combustion ignition by putting additives in gasoline in order to increase the octane of the fuel, high octane fuel is less energy dense than low octane fuel. The reason being, the additives that prevent pre-combustion do so because they have a lower energy density and, therefore, do not ignite as easily as gasoline.

In other words, increasing the compression ratio of gasoline engines is a catch 22. The purpose of increasing compression ratio is to increase the energy output per unit of volume of fuel. But in order to increase the compression ratio of gasoline, it is necessary to reduce the amount of energy in it. So, increasing the octane of gasoline in order to increase the compression it can withstand without combusting — something is done in order to increase the total energy output — actually reduces the energy output. 

High Oxygen to Fuel Ratio

The second reason that diesel engines have a higher thermal efficiency than gasoline engines is that of the oxygen to fuel ratio. The greater the amount of air in an air-to-fuel ratio, the cleaner the fuel burns and the more energy it produces. Diesel engines can run on an extremely lean mixture. However, in a gasoline engine, a lean mixture causes problems. 

The ideal air-to-fuel mixture of combustion engines is called the stoichiometric air-to-fuel ratio. The stoichiometric air-to-fuel ratio of gasoline engines is typical, “14.7:1, which means 14.7 parts of air to one part of the fuel,” according to Honeywell Garret. The stoichiometric air-to-fuel ratio of diesel engines can vary, widely. While the stoichiometric air-to-fuel ratio is 14:6, “diesel engines are not running under stoichiometric conditions normally. Typical operating ranges of diesel engines spread between an air/fuel ratio of 18 and up to 70, depending on the operation point.”

While gasoline engines almost always run as close to the stoichiometric air-to-fuel ratio as possible, Dieselnet.com’s Hannu Jääskeläinen and Magdi K. Khair explain in their article, Combustion in Diesel Engines, “Diesel combustion is characterized by lean overall A/F ratio. The lowest average A/F ratio is often found at peak torque conditions. To avoid excessive smoke formation, A/F ratio at peak torque is usually maintained above 25:1, well above the stoichiometric (chemically correct) equivalence ratio of about 14.4:1. In turbocharged diesel engines, the A/F ratio at idle may exceed 160:1. 

Therefore, excess air present in the cylinder after the fuel has combusted continues to mix with burning and already burned gases throughout the combustion and expansion processes.” 

 Because gasoline engines always run close to the stoichiometric air-to-fuel ratio, just slightly richer, and diesel engines can run as lean as 160 parts air to one part fuel, the efficiency at which diesel burns is considerably greater than the efficiency at which gasoline burns. The cleaner the burn, the more energy produced per gallon. So, not only does diesel have more energy per unit of measure, a diesel engine uses a greater percentage of that energy to produce mechanical power.