How Efficient are Engines: Thermodynamics and Combustion Efficiency (2022)

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 X-Engineer.org. 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 LearnThermo.com, “Thermal efficiency is a measure of the performance of a power cycle or heat engine.”

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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

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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.

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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.

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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.

FAQs

How efficient are combustion engines? ›

Modern gasoline engines have a maximum thermal efficiency of more than 50%, but road legal cars are only about 20% to 35% when used to power a car.

What is the most efficient engine in thermodynamics? ›

The electric engine is most efficient. Electric motors are very efficient at converting electricity into work. Physicists throw around abstract numbers like 90%, whereas when discussing combustion engines, they use numbers less than 40%.

Are combustion engines getting more efficient? ›

Now, engines have gotten significantly more efficient over the past few decades, including in the thermal department. Most modern gasoline engines have a thermal efficiency of around 40%. That means that 40% of the energy created when its fuel is burned gets turned into motion.

How efficient is the most efficient internal combustion engine? ›

To date, the most thermally efficient automotive internal combustion engine belongs to Mercedes-AMG's Formula 1 team, with an efficiency of 50 percent; AMG hopes the F1-derived engine in the Project One street-legal supercar will achieve 41-percent thermal efficiency, which would make it the most thermally efficient ...

Why is the efficiency of combustion engines so low? ›

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.

Which engine is more efficient? ›

Petrol engine has higher efficiency as compared to diesel engine. Q. Why CNG is more preferred than Petrol and Diesel ? Give any 2 reason.

Why is 100 efficiency impossible? ›

It is impossible for heat engines to achieve 100% thermal efficiency () according to the Second law of thermodynamics. This is impossible because some waste heat is always produced produced in a heat engine, shown in Figure 1 by the term.

Why is it not possible to 100% efficiency in heat engines? ›

A heat engine is considered to be 100% efficient if only all the heat is converted into useful work or mechanical energy. Since heat engines cannot convert all the heat energy into mechanical energy, their efficiency can never be 100%.

What is efficiency in thermodynamics? ›

In thermodynamics, efficiency is one of the most frequently used terms to indicate how well energy is converted into useful work. Generally it is defined as the ratio of desired output to required input.

How engines have become more efficient? ›

Engines Modern vehicles approach the goal of increased efficiency in many ways, including increasing the compression ratio (how much the air–gas mixture is compressed in the cylinder before ignition); replacing carburetors with fuel injectors that pump atomized fuel into the cylinders; and doubling (generally from two ...

Why combustion engines are better than electric? ›

When comparing the electric motor and the internal combustion engine, the internal combustion engine has a higher energy density, which means it produces a higher energy output per density of fuel. The combustion engine also takes less time to refuel than the electric motor.

Why are ICE cars so inefficient? ›

ICE vehicles are inefficient in how they consume fuel. Only 16% – 25% of the energy from the fuel is actually converted into the energy to move the vehicle. The rest of the energy is either lost to exhaust heat, friction and combustion or used to power accessories like headlights.

What is the most efficient car engine in the world? ›

Toyota is believed to offer the most thermally efficient internal-combustion automotive engine in production today, a 2.0-liter naturally aspirated four-cylinder that achieves 41 percent thermal efficiency.

Why are F1 engines so efficient? ›

F1 engines are some of the most fuel-efficient petrol engines on the planet in large part thanks to the turbo hybrid nature of the power units. The ERS provides an electrical boost of around 160 horsepower for 33 seconds every lap, so the engine uses less fuel while producing more power.

Which car has the greatest efficiency? ›

Most Fuel-Efficient Upscale/Luxury Cars
RankMake & ModelOverall mpg
1Tesla Model 3 Long Range1301
2Tesla Model S 100D1021
3Polestar 2921
4Porsche Taycan 4S691
10 more rows
5 Aug 2022

How can combustion efficiency be increased? ›

An often stated rule of thumb is that boiler efficiency can be increased by 1% for each 15% reduction in excess air or 40°F reduction in stack gas temperature. Operating your boiler with an optimum amount of excess air will minimize heat loss up the stack and improve combustion efficiency.

Which lists the engines from most efficient to least efficient? ›

  • Answer: (III). The engines from most efficient to least efficient is Z,X,W,Y.
  • Explanation:
  • Given that,
  • Using formula of efficiency.
  • Hence, The engines from most efficient to least efficient is Z,X,W,Y.
15 Oct 2018

What is the average efficiency of internal combustion? ›

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.

Which type of engine is fuel-efficient? ›

Diesel engines generally achieve greater fuel efficiency than petrol (gasoline) engines. Passenger car diesel engines have energy efficiency of up to 41% but more typically 30%, and petrol engines of up to 37.3%, but more typically 20%.

Why are Carnot engines not 100% efficient? ›

The efficiency is 100% only when T2 is zero or T1 is infinite. This is not possible since some amount of heat gets rejected to the sink.

Is there a machine that is 100% efficient? ›

No,Efficiency of a machine is always less than 100% as output energy is always less than the input energy, because some energy is lost to overcome friction. Was this answer helpful?

Does a heat engine with 100% efficiency violate any of the laws of thermodynamics? ›

In order to operate, a heat engine must reject some of the heat it receives from the high-temperature source to a low-temperature sink. A heat engine that violates the second law converts 100 percent of this heat to work. This is physically impossible.

Which thermodynamic cycle has maximum efficiency? ›

Explanation: Carnot engine: A theoretical thermodynamic cycle proposed by Leonard Carnot. This engine gives the maximum possible efficiency that a heat engine can have during the conversion process of heat into work or vice-versa, working between two reservoirs.

Which system has lowest efficiency? ›

Class A amplifier has lowest efficiency and the efficiency of different amplifiers are given below.

What is meant by efficiency of an engine? ›

The efficiency of a heat engine is defined as the ratio of work done by the heat engine to heat absorbed per cycle. If a heat engine absorbs Q1 amount of energy from the source and dissipates Q2 amount of energy to sink, the efficiency η, is given by, η=QQ−Q=1−QQ.

What is a good thermal efficiency? ›

When expressed as a percentage, the thermal efficiency must be between 0% and 100%. Efficiency must be less than 100% because there are inefficiencies such as friction and heat loss that convert the energy into alternative forms.

Can a heat engine be 100 efficient? ›

It is impossible for heat engines to achieve 100% thermal efficiency () according to the Second law of thermodynamics. This is impossible because some waste heat is always produced produced in a heat engine, shown in Figure 1 by the term.

Why is it not possible to 100% efficiency in heat engines? ›

A heat engine is considered to be 100% efficient if only all the heat is converted into useful work or mechanical energy. Since heat engines cannot convert all the heat energy into mechanical energy, their efficiency can never be 100%.

How efficient is an F1 engine? ›

F1 regulations have had a massive push towards fuel efficiency in recent years. The 2020 Mercedes engine is now over 50% thermal efficient, meaning that over half of the energy in the fuel is used to propel the car, which is an increase from around 44% in 2014 when these engines were introduced.

How efficient is a turbine engine? ›

Modern units can have compressor efficiencies of 86–88 percent and turbine efficiencies of 88–90 percent at design conditions.

Does a heat engine with 100% efficiency violate any of the laws of thermodynamics? ›

In order to operate, a heat engine must reject some of the heat it receives from the high-temperature source to a low-temperature sink. A heat engine that violates the second law converts 100 percent of this heat to work. This is physically impossible.

Why are Carnot engines not 100% efficient? ›

The efficiency is 100% only when T2 is zero or T1 is infinite. This is not possible since some amount of heat gets rejected to the sink.

Why is efficiency always less than 100%? ›

Efficiency is the percent of work put into a machine by the user (input work) that becomes work done by the machine (output work). The output work is always less than the input work because some of the input work is used to overcome friction. Therefore, efficiency is always less than 100 percent.

Can you design an engine of 100% efficiency justify your answer? ›

As absolute zero and infinite temperature cannot be achieved in practice, the efficiency of a heat engine cannot be 100%.

Which thermodynamic cycle has maximum efficiency? ›

Explanation: Carnot engine: A theoretical thermodynamic cycle proposed by Leonard Carnot. This engine gives the maximum possible efficiency that a heat engine can have during the conversion process of heat into work or vice-versa, working between two reservoirs.

How can you increase the efficiency of a heat engine? ›

Therefore, the efficiency of the heat engine can be increased by decreasing the temperature of the cold sink or increasing the temperature of the hot source.

Why are F1 engine so efficient? ›

F1 engines are so efficient due to the research and development that has been put into them over the years. A complex hybrid system makes them so efficient, as it recovers energy while using less fuel, helping produce 1000+ HP. F1 car engines are some of the most efficient engines on the planet.

What octane is F1 fuel? ›

Formula One fuel would fall under high octane premium road fuel with octane thresholds of 95 to 102. Since the 1992 season onwards all Formula One cars must mandatorily utilize unleaded racing gasoline fuel. F1 Blends are tuned for maximum performance in given weather conditions or different circuits.

Why do F1 cars need 1 liter of fuel? ›

The rule is in place to ensure teams are running the fuel that they have registered with the FIA, and one liter is required in order to carry out the relevant checks. In effect, it is one of many mechanisms Formula 1's governing body, the FIA, has in place to make sure no one is tampering with fuel.

Are turbines more efficient than internal combustion engines? ›

The gas turbine is most efficient at the maximum power output of any practical internal combustion engine. Diesel engines use much higher compression ratios than gas turbines and that higher ratio compensates for air pumping losses within the engine.

How efficient is an aircraft engine? ›

In the U.S., domestic flights average 0.54 miles per gallon of jet fuel, an improvement of more than 40% since 2000.

What is the most efficient aircraft engine? ›

The GE9X engine is the most fuel-efficient commercial engine ever built and is a key contributor to what makes the 777X the largest and most efficient twin-engine jet in the world according to Boeing.

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