How Efficient are Engines: Thermodynamics and Combustion Efficiency

How efficient are engines? Combustion engines are stupefyingly inefficient. Most diesel engines do not even have a thermal efficiency of 50%. Of every gallon of diesel burned by a combustion engine, less than half of the energy generated becomes mechanical energy. That is to say, of the energy produced by the diesel engine in a pickup truck, for example, less than half of the energy produced actually pushes the pickup down the road. 

And, gasoline-powered vehicles are even more inefficient, considerably more inefficient. 

While it may sound like a vehicle that only converts 50% of the thermal energy it produces during combustion into mechanical energy is extraordinarily inefficient, many vehicles on the road actually waste close to 80% of the energy produced during the combustion of fuel. Gasoline engines often blow more than 80% of the energy produced out the tailpipe or lose that energy to the environment around the engine. 

The reasons combustion engines are so inefficient are consequences of the laws of thermodynamics. Thermodynamics determine the thermal efficiency — or inefficiency — of a combustion engine. 

“Internal combustion engines produce mechanical work (power) by burning fuel. During the combustion process the fuel is oxidized (burned). This thermodynamic process releases heat which is transformed partly in mechanical energy,” according to But, a great deal of the energy produced is lost. A great deal of the energy produced by a combustion engine is wasted. 

While even a short explanation of why combustion engines necessarily require a somewhat lengthy explanation of thermodynamics, a Twitter feed length explanation is easy to understand: the difference in temperature between fuel combustion, the engine, and the air outside the engine determines thermal efficiency — i.e. combustion engine inefficiency. 

What is Thermal Efficiency and what are the Laws of Thermodynamics

The efficiency of a combustion engine is measured as the sum of thermal efficiency. Thermal efficiency is a consequence of thermodynamics. There is both a definition and a formula for thermal efficiency. According to, “Thermal efficiency is a measure of the performance of a power cycle or heat engine.” 

The strict definition of thermal efficiency, according to the Merriam-Webster Dictionary, is, “the ratio of the heat utilized by a heat engine to the total heat units in the fuel consumed.” A more practical layman’s definition of thermal efficiency is to the effect of the amount of energy produced when a combustion engine burns fuel in relation to the amount of that energy that becomes mechanical energy. 

The formula for thermal efficiency, however, may provide the simplest explanation. Thermal energy is the amount of heat lost divided by the amount of heat put into a system, heat being synonymous with energy. The result of dividing loss by input is the thermal efficiency rate of that system. The thermal efficiency rate is the amount of energy that goes into powering the crankshaft of a combustion engine — at least those with pistons.

There are two laws of thermodynamics that determine the thermal efficiency of a combustion engine.

First Law of Thermodynamics

Thermal efficiency — hence, the efficiency of a combustion engine — is determined by the laws of thermodynamics. According to the first law of thermodynamics, the energy output cannot exceed the energy input. In other words, the energy an engine produces — whether it is energy lost or energy used for locomotion — will never be greater than the energy potential of the fuel fed into the combustion chamber. 

The first law of thermodynamics is intuitive to understand. The first law of thermodynamics is part and parcel to the law of conservation of energy. Energy cannot be created nor can it be destroyed. The first law of thermodynamics is simply another formula proving that energy cannot be created. Using money as a metaphor for the first law of thermodynamics, you can’t get more than four quarters out of a dollar. 

While the first law is relevant to combustion engine efficiency, it is the second law of thermodynamics that explains why combustion engines are so inefficient. 

Second Law of Thermodynamics

According to the second law of thermodynamics, 100% thermal efficiency is impossible to achieve. 

There is a limit to the potential efficiency of a combustion engine. The second law of thermodynamics, called the Carnot’s Theorem states, “Even an ideal, frictionless engine can’t convert anywhere near 100% of its input heat into work. The limiting factors are the temperature at which the heat enters the engine, and the temperature of the environment into which the engine exhausts its waste heat.” 

An extremely large percentage of the energy produced during fuel combustion is lost. Lost energy is the reason an engine heats up. The engine heating is a result of conductive heat transfer. Lost energy in the form of heat is the reason the air around an engine heats, through convective heat transfer. Rather than producing mechanical energy, the heater heats the engine and the atmosphere around the engine. As a result of heat convection and conduction, energy is lost to the air around the engine and to the engine, because both the engine and the air around the engine have a lower temperature than the temperature of fuel combustion. 

Additionally, a huge portion of the energy produced by a combustion engine simply blows out the exhaust, again, never becoming mechanical energy. 

Heat — Energy — Loss and Carnot’s Theorem

The greater the difference in temperature between a fuel’s combustion temperature and that of its surroundings, the lower the thermal efficiency of an engine. In other words, the greater the difference between the temperature of burning fuel and the metal and air around it, the greater the energy loss. The greater the difference in temperature, the greater the inefficiency of an engine is a fact proven by Carnot’s Theorem. 

The Carnot Limit is the amount of energy produced during combustion that becomes mechanical energy. That limit is determined by the difference in the heat of combustion and the temperature of the elements and atmosphere around the combustion process. The greater the difference between the temperature of burning fuel and the ambient temperature of the environment around the combustion process, the lower the Carnot Limit. 

What is the Thermal Efficiency of a Gasoline Engine Versus a Diesel Engine?

The thermal efficiency of a gasoline engine is extremely low. While there are companies making strides to improve the thermal efficiency of gasoline engines, to even match the combustion efficiency of older diesel engines is extremely difficult. According to Toyota, a company attempting to increase the thermal efficiency of its vehicles, “Most internal combustion engines are incredibly inefficient at turning fuel burned into usable energy. 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 typically has a higher thermal efficiency, a thermal efficiency approaching 40 percent in some cases. Toyota is in the process of developing a new gasoline engine which the company claims have a maximum thermal efficiency of 38 percent, a thermal efficiency that is “greater than any other mass-produced combustion engine.”

Another way of thinking about thermal efficiency is in respect to fuel costs. For every dollar of gasoline a person purchases, almost 80 cents is lost as waste. Only 20 cents out of every dollar actually moves a gasoline engine down the road. While still shockingly low, even normal diesel engines put at least 40 cents per dollar to mechanical use. 

While 60 cents out of every dollar of diesel is lost to thermal inefficiency, that is still twice as good as the average gasoline engine. 

Why the Thermal Efficiency of a Diesel Engine is Greater than that of a Gasoline Engine

While Toyota claims the thermal efficiency of gasoline engines is 20% and that of diesel engines is 40%, MDPI of Basel, Switzerland believes those numbers are actually higher. According to MDPI, gasoline engines have a thermal efficiency of between 30% and 36% while diesel engines can reach a thermal efficiency of almost 50%. “Current production spark-ignition engines are working with brake thermal efficiency (BTE) about 30–36% [12], compression-ignition engines have long been recognized as one of the most efficient power units, the current BTE of diesel engines can achieve to 40–47%.

Still, that means the thermal efficiency of a diesel engine is about 25% greater than that of a gasoline engine. According to Popular Mechanics, the reason diesel engines have a higher thermal efficiency than gasoline engines is that of two factors: compression ratios and lean-burn combustion. “When it comes to traversing great distances at highway speeds, the diesel engines higher compression ratios and lean-burn combustion provide an efficiency that no gas engine can currently match—at least not without a major assist from an expensive hybrid system.”

Thermal Efficiency and Combustion Ratios

In a combustion engine, thermal efficiency is determined in part by the compression ratio. Compression ratio is the difference between the greatest amounts of volume in a combustion chamber — when the piston is down — and the volume in the combustion chamber as it comes up to the point where the fuel injected into the chamber explodes. The compression ratio of a gasoline engine is much lower than that of a diesel engine. 

The combustion ratio of a typical gasoline engine is between 8:1 and 12:1. “If the gasoline engine compression is above about 10.5, unless the octane number of the fuel is high, knocking combustion occurs.” Knocking is the result of pre-combustion when gasoline ignites because of compression pressure as opposed to compressing as the result of exposure to a spark. 

Diesel engines have a much higher compression ratio. There are two reasons why. First, diesel engines are compression engines. Compression is what makes the diesel in a combustion chamber explode. There is no spark that ignites the diesel in a compression engine. Additionally, diesel engines have a higher compression ratio because diesel is a more stable fuel. More pressure — a higher compression ratio — is necessary to ignite diesel. The compression ratio of most diesel is between 14:1 and 25:1. 

Solutions for Improving Engine Efficiency

There is very little the owner of a vehicle can do to increase the thermal efficiency of an engine. Design limitations and the limitations of technology prevent owners from making major improvements to a vehicle with respect to thermal efficiency. However, it is possible to make improvements with regard to combustion efficiency. 

Combustion efficiency is the rate at which an engine converts fuel into energy. Particularly with heavy fuels with high-energy densities — diesel, fuel oil, bunker fuel, etc. — there are technologies available that make it possible to improve combustion efficiency dramatically. Because of the nature of high-energy density fuels, namely that high-energy density fuels are composed of big and long hydrocarbon molecules, heavy fuels can have poor combustion efficiency rates. 

Low-energy-density fuels like gasoline and natural gas typically have consistent combustion rates relative to heavier fuels because they are composed of smaller, short-chain hydrocarbon molecules. But, the bigger, longer hydrocarbon molecules and molecule chains in heavy fuels have a tendency to bundle together in clusters which means the molecules on the inside of a cluster are not exposed to air. Without air, hydrocarbons will not ignite.

Fuel catalysts are one of the simplest means of improving the combustion efficiency of heavy fuels. The noble metals — a.k.a., catalysts — in noble metals breakup fuel clusters by depolarizing the inherent charges that cause hydrocarbons to cluster together. 

The Rentar Fuel Catalyst, for example, can increase combustion efficiency — and therefore, fuel efficiency — by between 3% and 8% in over-the-road vehicles. On heavy machinery, the fuel efficiency increase is even more dramatic. By adding a Rentar Fuel Catalyst to a furnace or boiler that burns heavy fuels, the increase can be 30% or more.

While it is difficult to prevent the waste of energy that is innate to all combustion engines, it is still possible to increase fuel efficiency. Until we can produce engines with higher thermal efficiencies, the best we can do is improve combustion efficiency.


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