Thermal efficiency, compression ratio, and fuel density are the primary factors that determine the fuel efficiency — a.k.a. fuel economy — of an engine. If mounted in a car, pick up, truck, boat, ship, a piece of heavy equipment, etc., even more, variables come into play with respect to the fuel efficiency of an engine. Concerning the fuel efficiency of an engine used for locomotion, transport, and mobility, factors like the weight of a vehicle, terrain, and air-flow dynamics play a role. But, while those variables play a role in determining fuel efficiency, they are by no means the most influential factors.

The three variables that influence the fuel efficiency of an engine to the greatest degree are fuel density, combustion efficiency, and thermal efficiency. Of the three most relevant variables which determine fuel economy, thermal efficiency is the most influential.

No other variable plays a bigger role in determining fuel economy than thermal efficiency. The reason being, thermal efficiency is a byproduct of all the other combustion-specific variables including fuel density, the energy density of a fuel, the compression ratio of an engine, and the air-to-fuel mix ratio fed into an engine.

Thermal efficiency, for all practical purposes, is “gas” mileage.

What Thermal Efficiency Is

Both the layman’s definition and the strict definition of thermal efficiency are two of the simplest explanations in physics to understand. Thermal efficiency is the percentage of energy — fuel — that generates work. dictionary.com explains, “Thermal efficiency definition, the ratio of the work output of a heat engine to the heat input expressed in the same units of energy.” Thermal efficiency is the portion of energy an engine produces during combustion that pushes a car down the road, spins the propeller on a boat, lifts the boom and bucket of a backhoe, etc.

With respect to combustion engines, thermal efficiency is a measure of what percentage of the heat — heat being synonymous with energy/fuel — put into an engine that same engine can convert into work. Thermal energy is a measure of the percent of the heat in a gallon of fuel that an engine can use to push a vehicle down the road or perform some other mechanical task like lift a bucket or boom, the percentage of energy in fuel that an engine does not waste.

Another Way of Thinking of Thermal Energy

Thermal energy can also be thought of as the amount of energy an engine uses versus the amount of energy it wastes, how much of the energy in a gallon of gas goes toward locomotion and how much of the heat is blown out the exhaust or lost to the environment surrounding an engine.

To understand the basics of engine-related thermal efficiency, it is necessary to understand the fundamentals of combustion engines.

Thermal Efficiency of Diesel vs Gasoline Engines

Combustion engines are also called “heat engines.” Combustion engines convert energy —  the energy from fuel — into heat and the heat generates work. But, it is a small portion of the heat/energy/fuel that becomes work, far less than half.

There are two types of combustion engines used in vehicles and machinery: spark-ignition engines and compression-ignition engines. Diesel and biodiesel engines are compression engines and gasoline, ethanol, and propane-powered engines are spark-fired engines.

Mechanics of a Spark-Ignition Engine

Spark-fired engines ignite the air-fuel mixture with a small electric charge. As the piston begins to fall following the exhaust stroke — the stroke in which a piston pushes the exhaust from the previous exhaust cycle out of the cylinder, — injectors fill the cylinder with an air-fuel mixture. From the bottom of its stroke, the piston begins to rise, compressing the air-fuel mixture. At the top of the piston cycle, the spark fires and ignites the mixture.

Mechanics of a Compression Engine

Unlike spark-fired engines that add an air-fuel mixture at the bottom of the piston cycle, only air is in the cylinder at the bottom of a piston cycle in a compression engine. The piston rises and compresses the air — increasing the temperature inside the cylinder — and at the top of the piston stroke, injectors inject diesel into the hot compressed air. The temperature of the air is so hot, that it causes the diesel to ignite.

While both compression engines and spark-fired engines are surprisingly inefficient, diesel engines are considerably more efficient than gasoline engines.

Heat engines — especially gasoline, ethanol, and gas-state-fuel engines — are extraordinarily inefficient. Even the most thermally efficient gasoline engines lose about 70 percent of the energy they produce. Though slightly better, even the most thermally efficient diesel engines still waste between 50 and 60 percent according to GreenCarReports.com. “The efficiency by which they do so is measured in terms of “thermal efficiency”, and most gasoline combustion engines average around 20 percent thermal efficiency. Diesel is typically higher–approaching 40 percent in some cases.”

Why Heat Engines are Inefficient

There are different types of heat/combustion engines — diesel, gasoline, ethanol, natural gas, propane, biodiesel, etc. But, to different degrees, all combustion engines are inefficient. And, the reason combustion engines are inefficient is universal. Simply, the engine technologies required to convert 100 percent of the heat an engine produces during combustion does not exist.

A very large portion of the heat generated during combustion blows out the exhaust pipe. Convection and conduction are responsible for the remainder of the heat lost; the heat engines produce that does not become mechanical energy. The engine block soaks up the heat because the coolant in a radiator keeps an engine cool so it will not overheat and seize. The air outside the engine also soaks up the heat because it too saps heat from the engine block.

To be fair though, there isn’t an energy conversion system that is 100 percent efficient. Wood burning stoves and electric power plants, for example, both waste tremendous amounts of energy. The majority of the energy simply rises out of the chimney or smokestack.

Heat engines, however, are particularly inefficient.

But, there are means of improving the thermal efficiency of combustion engines. Increasing the compression ratio of a combustion engine is the first means.

What Compression Ratio Is

It is the compression ratio, more than any other engineering feature of an engine, that determines thermal efficiency — or more accurately, thermal inefficiency. Compression ratio is the difference in the volume of a cylinder between the time a piston is at the bottom of its cycle and the time that the piston is at the top of its cycle.

Again, when the piston is at the bottom of the cycle, the cylinder is full of air in the case of a compression engine and full of an air-fuel mixture in the case of a spark-fired engine and as the piston moves up, the air or air-fuel mixture begins to compress and the more the air or air-fuel mixture compresses, the more the temperature inside the cylinder increases and once the piston reaches the top of its cycle, the air-fuel mixture combusts.

The more the air or air-fuel mixture heats as the result of being compressed prior to combustion, the greater the thermal efficiency.

How Compression Ratio Affects Thermal Efficiency

The greater the compression ratio, to a certain point, the greater the thermal efficiency of an engine. Thermal efficiency, defined, is the amount of heat or heat potential — i.e. fuel — that an engine converts into mechanical energy, work. Thermal efficiency, in layman’s’ terms, is the percent of fuel an engine uses to push a vehicle down the road.

The formula for thermal efficiency is simple. The formula for thermal efficiency is the amount of heat an engine puts out divided by the amount of heat — again, in the form of fuel — put into the engine. The closer the two temperatures, the greater the thermal efficiency of an engine. If the temperature of compressed air or air-fuel mixture in a cylinder is the same as the air-fuel combustion temperature, the thermal efficiency is 100 percent.

Theoretically, compressing the air or air-fuel mixture until the heat generated is equal to the combustion temperature of the air-fuel mixture would be ideal. However, that is not possible.

Limits of Compression Ratio

Increasing the compression ratio of an engine design is not possible beyond a certain degree. Engineers can make the compression ratio of a diesel engine much higher than that of a gasoline engine. The reason being, a diesel engine’s cylinder only has air in it as the piston rises. Diesel is injected into the cylinder once the piston reaches the top of its stroke. Once injected, the diesel auto-ignites and the pressure generated from the diesel combusting pushes the piston back down, an action that turns the crankshaft.

The cylinders of spark-ignition gasoline engines, on the other hand, fill with an air-gasoline mix at the bottom of the piston cycle. So, when the piston begins to rise, the heat generated by compressing air — at a certain point — will cause the gasoline in the air-fuel mixture to auto-ignite.

Auto-ignition in a gasoline engine is a catastrophic event. Auto-ignition, a.k.a. pre-ignition is not to be confused with detonation. Detonation is when pockets of the air-fuel mixture in a cylinder ignite at different times. Detonation causes a pinging sound, so detonation is often referred to as “knocking.” Auto-ignition is completely different than detonation. Detonation occurs on the down stroke of a piston cycle. Auto-ignition occurs on the upstroke. There is no sound associated with auto-ignition. The engine simple explodes. Auto-ignition destroys piston heads and rods, destroys rings and seals, and can even blow the spark plugs out the side of an engine.

To prevent auto-ignition in a spark-fired engine — to prevent the gasoline in the air-fuel mixture from igniting as a result of the heat generated as a piston compresses the mixture inside the cylinder, — engineers must keep the compression ratio between 8:1 and 12:1.

But, since diesel is fed into the cylinder of a compression engine at the end of the piston cycle — top dead center — as opposed to the beginning of the piston cycle as the fuel is in a spark-fired engine, the compression ratio of diesel engines can be much higher: between 14:1 and 25:1. That means the temperature inside a diesel engine gets much hotter than that of a gasoline engine which means the input temperature and the output temperature are closer. Therefore, diesel engines are much more thermally efficient than a gasoline engine.   

Thermal efficiency, along with fuel density, determines the fuel efficiency of an engine. Diesel engines are more fuel efficient than gasoline engines because they are more thermally efficient and because diesel is a denser fuel. Diesel engines have greater thermal efficiency than gasoline engines because diesel engines have higher compression ratios. Diesel engines can have higher compression ratios because compression engines inject fuel into the engine cylinder at the end of the piston cycle.

Fuel Density and Fuel Efficiency

Even without greater compression ratio leading to higher thermal efficiency, diesel engines would still be considerably more fuel efficient. Diesel engines are naturally more fuel efficient because diesel is a higher density fuel than gasoline. While diesel and gasoline have the same energy density — an equal sum of energy when measured by weight, — diesel has more energy when measured by volume. And, liquid fossil fuels are sold in volume units of measure, gallons or liters.

“The calorific value of diesel fuel is roughly 45.5 MJ/kg (megajoules per kilogram), slightly lower than petrol which is 45.8 MJ/kg. However, 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 of fuel density alone, a diesel engine will travel five (5) miles for every four (4) miles a comparably sized gasoline engine will travel.

“Gas” mileage — and the reason diesel engines are more fuel efficient than gasoline engines — is a product of thermal efficiency and thermal efficiency is a product of compression ratio. Thermal efficiency and compression ratio combined with fuel density is the reason that a diesel engine gets between 25 and 35 percent better fuel economy than a gasoline engine.