Compression ratio plays as large a role in fuel efficiency as any other engine combustion factor. Diesel engines are between 25 percent and 35 percent more fuel efficient than gasoline engines of comparable size. If two vehicles have engines that are equal in size, but one has a diesel engine and the other has a gasoline engine, the vehicle with the diesel engine will travel four (4) miles on the same amount of fuel the gasoline engine will travel three (3). And those numbers are modest.  

It is not uncommon for smaller diesel engines to travel 50 miles on a gallon of gas. It is equally common for a comparably-sized gasoline engine to only travel 32.5 miles per gallon. That is to say, it is not uncommon for a gasoline engine to have two-thirds (⅔) the fuel efficiency of a diesel engine. 

“Diesel engines are more fuel-efficient and have more low-end torque than similar-sized gasoline engines, and diesel fuel contains roughly 10% to 15% more energy than gasoline. So, diesel vehicles can often go about 20% to 35% farther on a gallon of fuel than their gasoline counterparts. Plus, today’s diesel vehicles are much improved over diesel of the past,” explains the U.S. Department of Energy.

Suffice it to say, statistics showing that diesel engines get considerably better “gas” mileage than gas-powered engines of comparable size are not controversial. Diesel engines are more fuel efficient than gasoline engines. The reason? Diesel itself and diesel engines, the compression ratio of diesel engines specifically.   

Fuel Density and Fuel Efficiency

As to why diesel engines are more efficient than gasoline engines, the complete answer includes physics and engineering concepts like fuel ignition type, flame diffusion, compression ratio, and thermal efficiency. But simply, there are two factors that determine fuel efficiency. The first is fuel density, the amount of energy in a gallon or liter of fuel. The energy density of a fuel is extremely difficult to change on a mass scale. A fuel — diesel, gasoline, ethanol, biodiesel, natural gas, propane, etc. — either has a high density, or it does not. 

It is difficult to dramatically change the density of a fuel. A fuel is either light or heavy. 

Not only is fuel density important because it is the amount of energy in a gallon or liter of fuel, fuel density is important because it determines the design and engineering of combustion engines. The density of fuel allows for engine engineering designs that increase fuel efficiency and reduce emissions. But, it is also fuel density that limits engine design possibilities with respect to fuel efficiency and emissions reduction.   

Fuel Density Defined

Also known as API gravity, fuel density is the amount of mass in a fuel on a volume scale. The reason fuel density is important with respect to the chemical makeup of a fuel is that fuel density is an indication of the contents of a fossil fuel, the types and sizes of hydrocarbons as well as a fuel’s contaminants. Heavy fuels like fuel oil and bunker fuel are extremely dense, but a considerable portion of the density comes from contaminants. Light fuels like gas-state fuels and gasoline have low fuel density but are also low in contaminants. In relation to other fossil fuels, diesel has medium weight. Typically, the only contaminant in diesel of significance is sulfur. 

Diesel is a compilation of large hydrocarbon molecules and molecular chains. Larger and longer hydrocarbons are the reason diesel is less volatile than gasoline, propane, natural, gas, and ethanol, a.k.a. light distillate fuels. Light distillate fuels have small molecules and short molecule chains, molecules, and chains that are highly volatile. 

“Gasoline-like fuels are typically composed of relatively small (i.e. carbon number in the range of 5–10) branched or cyclic hydrocarbons. These molecular structures have a high bond strength and thus low chemical reactivity (e.g. high octane number). On the contrary, diesel fuels are highly reactive due to the long, saturated molecular structures (i.e. carbon number in the range of 10–20) and thus auto ignite readily, making it difficult to achieve LTC.”

While it is easy to assume the more combustible a fuel is, the more energy it produces, that is not the case. 

While gasoline is more volatile and ignites/combusts/burns easier than diesel, diesel has more energy. On a weight scale, diesel and gasoline have roughly the same energy density. But, diesel is denser than gasoline which means on a volume scale — gallon or liters — diesel contains more energy. The fuel density of diesel is between 13 and 18 percent greater than gasoline. 

According to the European Automobile Manufacturers Union, “diesel fuel is denser than petrol and contains about 15% more energy by volume (roughly 36.9 MJ/liter compared to 33.7 MJ/liter). Accounting for the difference in energy density, the overall efficiency of the diesel engine is still some 20% greater than the petrol engine, despite the diesel engine also being heavier.”

Because diesel is more energy-rich than gasoline, each gallon of diesel can generate more power, more work. Because diesel fuel contains more energy per gallon, diesel engines travel farther per gallon. That is to say, because diesel contains more energy than gasoline, diesel engines are more fuel efficient than gasoline engines. 

Fuel Density, Compression Ratio, Thermal Efficiency, Fuel Efficiency

In addition to the fact that diesel has more energy on a volume scale, because diesel is a more dense fuel than gasoline, has greater compression resistance. In other words, diesel is a more stable fuel than gasoline. Because diesel has a higher compression resistance — because it is denser than light distillate fuels — engineers can design and develop diesel engines with higher compression ratios than gasoline engines. The higher the compression ratio of an engine, the greater the thermal efficiency. The greater the thermal efficiency of an engine, typically, the more energy efficient the engine. 

Diesel engines get better gas mileage than gasoline engines because of the density of diesel fuel and because of the compression ratios of diesel engines, compression ratio being a catalyst of thermal efficiency and thermal efficiency playing a major role in fuel economy.

Simply, because diesel is denser than gasoline and most other fossil fuels, diesel engines get better “gas” mileage.

Compression Ratio Defined

The dictionary definition of compression ratio is, “the ratio of the maximum to minimum volume in the cylinder of an internal combustion engine,” according to Google. While true, the definition does not explain the value of compression ratio, why compression ratio has such an impact on fuel efficiency. 

Layman’s Definition of Compression Ratio

In more tangible terms, the compression ratio is how much the pistons in an engine compress a fuel — diesel, gasoline, biofuel, etc. — inside a cylinder before the fuel combusts. Simply, the compression ratio is the volume difference in a cylinder between the moment the piston is at the bottom of a cycle — when the piston is at the bottom of a cycle, the cylinder has the most volume — and the moment the fuel combusts as the piston is moving up.

In an interview with Engine Builder Magazine, Diamond Piston’s Ron Beaubien explains, “An engine’s compression ratio is calculated by taking the total swept volume (with the piston at bottom dead center) and dividing it by the total compressed volume (with the piston at top dead center). For example, if the total swept volume of a 632 cid big-block Chevrolet is 1380.34cc (cubic centimeters) and the total compressed volume is 86.69cc the compression ratio would be stated as 15.92:1.”

The higher the compression ratio of an engine, the more efficiently the engine burns fuel. 

Effect of Compression Ratio on Thermal Efficiency

To say, the higher the compression ratio of an engine of an engine, the more complete an engine burns fuel is synonymous with saying the higher the compression ratio, the greater the combustion efficiency. But, combustion efficiency is not the only advantage of a high compression ratio. A high combustion ratio also means a higher thermal efficiency. 

Thermal efficiency, in a nutshell, is the percentage of the energy that goes into an engine that the engine converts into mechanical energy. Mechanical energy is the energy that generates torque, the energy that pushes a vehicle down the road. The greater the compression ratio of an engine, the greater the thermal energy meaning the greater the compression ratio of an energy, the higher percentage of the total energy fed into an engine which becomes power as opposed to waste. 

Why Compression Ratio Increases Thermal Efficiency

Compression ratio increases thermal efficiency because the greater the compression ratio, the more work each unit of energy does. The greater the compression ratio of an engine, the less energy an engine loses to convection and conduction and the more that goes into putting pressure on a crankshaft. The more pressure on the crankshaft, the more torque on the drive shaft. The more torque on the drive shaft, the more power to the wheels. 

Another way of thinking about thermal efficiency is that the greater the thermal efficiency of an engine, the less energy an engine wastes. As explained in Nuclear-Power.net, 

“It is desirable to achieve a high compression ratio to extract more mechanical energy from a given mass of the air-fuel mixture. A higher compression ratio permits the same combustion temperature to be reached with less fuel while giving a longer expansion cycle. This creates more mechanical power output and lowers the exhaust temperature. Lowering the exhaust temperature causes the lowering of the energy rejected to the atmosphere.”

The formula for thermal efficiency explains why compression ratio increases thermal efficiency. Thermal efficiency is a measure of energy in the form of heat. The higher the temperature of the energy going into an engine vs the temperature of the energy going out, the greater the energy efficiency. 

When an engine’s piston compresses the air inside the cylinder of an engine, the air heats. There is so much pressure generated by the upstroke of an engine’s piston that the heat generated is sufficient to cause fossil fuels to auto combust, to ignite without exposure to a flame. 

In a compression engine — a diesel or biodiesel engine, — auto ignition is an intended consequence of compression ratio. 

In a spark-fired engine, autoignition will destroy an engine. When autoignition occurs in a spark-ignition engine — a gasoline or propane-powered engine, — it is called pre-ignition. Pre-ignition is a different engine malfunction than detonation. Detonation the consequence of pockets of air-fuel mixtures combusting at different times. Detonation is also known as “knocking” and is not uncommon nor is it always a major problem. 

Pre-ignition in a spark-ignition engine, on the other hand, destroys piston heads, O-rings, and will blow spark plugs out the side of an engine. For this reason, gasoline engines must always have a considerably lower compression ratio than diesel engines. The air-fuel mixture in a gasoline must always be low enough that the pressure generated during the upstroke of a piston will not create pre-ignition. 

Because gasoline engines are not capable of withstanding pre-ignition that means the compression ratio of spark-fired engines is necessarily lower than the compression ratio of compression-ignition diesel engines. Because the compression ratio of gasoline engines must be lower in gasoline engines, that means that difference between the heat of the energy going into the engine vs the heat of the energy going coming out of the engine is less than the temperature difference between the heat input and heat output of a diesel engine. Because the difference in input vs output heat of a gasoline engine is less than that of a diesel engine, gasoline engines are less thermally efficient. Gasoline engines are necessarily less thermally efficient than diesel engines because of the engineering of engines, something that is determined by fuel density.

Because thermal efficiency is one of the two variables that determine fuel economy, gasoline engines are necessarily less fuel efficient than diesel engines. At its core, fuel efficiency relates to thermal efficiency because the compression ratio determines thermal efficiency and fuel density determines the compression ratio.