Convert pound/horsepower-hour [lb/(hp·h)] to gram/kilowatt-hour [g/(kW·h)] • Specific Energy, Heat of Combustion (per Mass) Converter • Thermodynamics — Heat • Compact Calculator • Online Unit Converters (2024)

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Overview

The energy that is measured for a given unit of mass of fuel is called specific energy. This article discusses the energy that is generated by combustion and metabolism. For example, combustion (burning) of a given mass of hydrocarbon, for example, propane, will generate a particular amount of specific energy or heat. It is measured in joules per kilogram (J/kg) in the SI system. Specific energy is most commonly calculated for heat generated through the combustion of hydrocarbons, although many other fuels can be combusted. Methane and butane are some examples of hydrocarbons.

Oxygen has to be present for the combustion to happen — in most cases oxygen from the air is used. When energy is generated by the combustion of hydrocarbons, the byproducts are water and carbon dioxide. The latter has a negative effect on our environment, which is why the alternative energy industry that generates energy without this byproduct is developing rapidly. While carbon dioxide is harmful, water produced during combustion, on the other hand, is useful — some animals employ it as an internal source of water — for example, camels, as described below.

Measuring Specific Energy

Specific energy can be measured by calorimeters — devices that measure heat. Bomb calorimeters are most commonly used with energy generated through combustion. A bomb calorimeter consists of an insulated inner chamber, also known as a “bomb” where the oxygen is supplied and fuel combusts; a device for igniting the fuel, which usually consists of electric wires; and an insulated outer chamber with a water container around the inner chamber that heats up as the fuel is combusted. The water temperature in this outer chamber is measured.

Applications: Fuel

People depend on fuels in everyday life. They are used for cooking, heating, powering machinery and vehicles, illumination, and other purposes. At the moment most fuels are hydrocarbon-based, and calculating their heat of combustion as specific energy per mass is useful for comparison of various fuels and their efficiency. The more energy can be produced with a given mass of fuel — the more efficient this fuel is.

Vehicles that are powered by fuel need to carry it on board, and there are often limitations on the amount of additional mass in the fuel that these vehicles can carry. Therefore, as a result of these limitations, fuel’s specific energy per mass determines how far they can travel. For such vehicles, it is important to have fuel with as high specific energy per a given unit of mass as possible. This is especially true for aircraft and hydrofoils.

Aircraft Weight Restrictions

Fuel in aircraft is stored in the wings, and if more fuel is needed, then it is stored in additional tanks in the fuselage. Weight restrictions on an aircraft often result in the need to bring only as much fuel as necessary for a given distance, to be able to use the rest of the allowed weight to carry passengers and cargo. Aircraft routes, especially those of commercial airlines are often calculated in such a way that the airplanes can carry enough fuel without having to make a stop-over for refueling. The distances are thus selected to be as long as the fuel supply onboard allows. The weight restriction is also why passengers are allowed a limited amount of luggage, and why they are charged high fees for additional or overweight luggage. In some cases, due to the price of fuel at the destination airport, the aircraft may be fueled for the return journey as well — in this case, the weight restrictions may be especially tight.

Cargo Carriers

Weight considerations are especially important for space shuttle carrier aircraft and other similar models, which are designed to carry another flying vehicle, such as a spacecraft. A spacecraft is very heavy, compared to the weight of conventional cargo and passengers, and spacecraft carriers need to be able to accommodate this extra weight and have enough fuel to cover the necessary distance.

The largest spacecraft and overweight cargo aircraft, Antonov An-225 Mriya, currently operated by a Ukrainian carrier Antonov Airlines is an example of such a vehicle. It was designed to transport Buran, which was a Soviet orbital vehicle, similar to NASA’s Space Shuttle. An-225 weighs 250 tons when empty, and can carry up to 300 tons of fuel. The total maximum weight it can lift is 640 tons, including its own weight. Thus, if it carries full tanks of fuel, it can only carry additional cargo weighing 640 – 250 – 300 = 90 tons. If it were carrying passengers, 50 tons of that weight would have been used on 500 passengers with luggage, if we estimate an average passenger with the luggage to weigh 100 kilograms. On the other hand, with minimum fuel used to travel a short distance, An-225 can carry up to 250 tons of cargo.

The heaviest recorded load that An-225 has ever carried included 4 combat tanks weighing a total of 254 tons. With the amount of fuel of 640 – 254 – 300 = 86 tons it flew a distance of 1,000 km. Only one An-225 aircraft is built at the moment, the second one being incomplete. This aircraft has transported disaster relief supplies, military supplies and meals, locomotives, generators, wind turbines, and other heavy or large items.

Passenger Aircraft

An example of a calculation for a passenger aircraft is as follows. For the Boeing 777-236/ER pictured, the total weight of an empty aircraft is 138 tons. The maximum weight that it can carry on takeoff is 298 tons. It can seat up to 440 passengers, rendering the maximum passenger + cargo weight to 400 × 100 kg = 40,000 kg or 40 tons. This leaves 298 – 40 – 138 = 120 tons for additional cargo and fuel.

Fuel consumption varies throughout the flight and for different flights, depending on the total weight carried, the type of flight, and many other variables. By a very rough estimate, Boeing 777-236/ER uses about 8,000 kg or 8 tons of fuel per hour. This means that this aircraft can fly for up to 15 hours if it uses all of the 120 tons for fuel and has all 440 passengers with luggage on board. Let us check the accuracy of our calculations. Specs on the Boeing website indicate that at maximum load 777-236/ER can fly a distance of up to 14,310 km. This is about 8892 miles. Its cruising speed is 905 km/h (562 mph), which means that it can fly for about 14,310 / 905 = 15.8 hours. This is close to the estimate above.

For comparison, an intercontinental flight between London and New York is about 7 hours. Currently one of the longest non-stop flights is between Singapore and Newark (18 hours and 50 minutes).

Another example of a passenger carrier is Airbus A310. Pictured is its cabin during the flight between Montreal, Canada, and Paris, France. It is smaller than Boeing 777-236/ER: 46.66 meters or 153 feet 1 inch in length (compared to 63.7 meters or 209 feet 1 inch) and 15.80 meters or 51 foot 10 inches in height (compared to 18.5 meters or 60 feet 9 inches). It can carry up to 150 tons during take-off, and its weight without the fuel (zero fuel weight) is 113 tons. This means that the additional weight that it can carry is 150 – 113 = 37 tons. It can have up to 220 passengers (220 × 100 kg = 22,000 kg or 22 tons) on board, so with a full load, it can have 37 – 22 = 15 tons of fuel. The Airbus specifies that the maximum payload (cargo + passengers) that it can carry is 21.6 tons, and our estimate for the weight of passengers and luggage gives us 22 tons. This means that the crew on the Airbus has to make sure that the weight restrictions are followed by the passengers.

The maximum weight allowed for the operation of the aircraft is specified in the operation manual. Aircraft are not allowed to operate with weights that exceed these maximum allowances, because it will jeopardize their safety. These weight limits might be further reduced by the airlines because of the costs associated with airport usage for heavier aircraft.

Hydrofoils

Hydrofoil vehicles are just as weight-sensitive as aircraft. They generally need to have the properties of a boat to remain afloat, but also some aerodynamic properties of an aircraft, to “fly” above the surface of the water. The foils stay submerged in the water and generate lift, raising the hull out of the water. Air drag is much smaller than the drag of water, and this makes these vehicles move faster than conventional boats because the drag of water on the hull is minimized.

Engineers constantly work on improving their structure, to make it lighter, while still remaining structurally strong. This helps increase the amount of fuel and cargo (or the number of passengers) that a hydrofoil can take. Aluminum alloys are often used for the body of hydrofoils because they are lightweight.

Pictured is a hydrofoil of type Voskhod, built in Feodosiya, Russia, at the shipbuilding plant Morye (from Russian “Sea”). The hydrofoil pictured is now in operation in Canada. It is designed for use on rivers, lakes, and in coastal areas, and can achieve speeds of up to 65 km/hour. Voskhod is one of the popular hydrofoil models, and besides Russia and Ukraine, it is operated in a range of European countries, in China, Vietnam, and Thailand. Locally built models based on the Voskhod design also exist, for example in Cambodia.

Some of the most fuel-efficient hydrofoils are human-powered ones because the passenger becomes the fuel source, thus, no fuel mass is added. Most of them require skill and practice to navigate and can move up to 30 km/h. These are popular vehicles to create, because of the relative simplicity of their design, for the speed that they can provide. There are several different designs and they can be powered by pedaling or by bouncing on it. Many websites provide videos and step-by-step instructions with photos and diagrams on how to make different types of human-powered hydrofoils.

Applications: Energy for Metabolism

Food — Energy for the Animal Body

Energy is vital for all living beings. It is generated through metabolism — a process similar in some ways to conventional combustion. While there is no “real” fire burning inside the organism, as with combustion, oxygen is still required for metabolism, and the energy is generated with byproducts of carbon dioxide and water. This is why oxygen is so vital to all living beings.

Energy sources in food include carbohydrates and proteins both at 17 kJ/g, fats at 38 kJ/g, and alcohols at 30 kJ/g. Metabolism breaks down nutrients in foods into components such as glucose, amino acids, and fatty acids, and turns them into chemical energy in the form of the synthesized enzyme adenosine triphosphate (ATP). ATP transports its energy to the cells that need to use it and is absorbed by these cells.

The specific energy of food can be measured. In some cases, it is calculated as joules per kilogram, but more often calories per gram are used. Usually, the measurements of specific energy of food are done by combustion in a bomb calorimeter, just like with other types of fuel. If food is burned in a calorimeter, its energy is released with the carbon dioxide and water as byproducts — the same as during metabolism.

Foods that produce a high amount of energy per a given unit of mass are said to have a high energy density. As the amounts of water and low-energy nutrients such as fiber increase, the energy density of foods decreases. Fat has a high specific energy value, therefore foods high in fat have a high energy density, meaning that they have a high specific energy per mass.

Energy Intake in Extreme Conditions

It is useful to know the specific energy of foods when making a meal plan for hikes and other trips where food has to be carried by people or animals, especially for extended periods. Food’s nutrition content is extremely important as well, but if water can be found along the path that people take, then dehydrated foods are preferred since they have a higher specific energy.

Explorers in the Arctic and the Antarctic often use dogs to carry their equipment and supplies, and minimizing weight is very important for them. Often they also do not have good conditions for cooking the food. The explorers need at least three times more energy for their daily activities than people under normal conditions. This is because they generally have a high rate of physical activity, and additional energy is needed by the body to keep constant temperature. Due to these limitations and needs, they eat a lot of food high in specific energy content, such as chocolate (high in carbohydrates and fats), butter, and nuts, as well as dehydrated meats.

It is believed that one of the reasons why the five explorers of the 1912 Terra Nova Expedition to the South Pole, headed by Robert Falcon Scott, did not survive the return journey was the inaccurate estimate of the total daily energy needs per person. They also did not use enough foods high in specific energy. In particular, some researchers believe that the men only brought enough food to afford 4,500 calories a day, while the current estimates call for 6,000 calories or more. They could not carry more food with them because they chose the lower energy density foods like proteins. While they did eat butter, researchers now believe that they needed to eat more fats and carbohydrates, but did not need as much protein as they actually ate.

Storing Fat as Energy Supplies

Fat is what some animals use to survive food and water shortages. Water is a byproduct of metabolism, this is why storing fat also gives animals access to extra water. Because fat renders more energy per gram than do proteins and carbohydrates, it is a preferred fuel to store internally. Camels, for example, store fat in their hump so that they have access to energy and water during long trips in the desert. They have about 15–20 kilograms of fat. Whales, seals, polar bears, and many other creatures also store excess food as fat.

Some scientists hypothesize that people’s tendency to store fat originated from the same evolutionary need to sustain themselves in times of food shortage. Some also believe that women have a higher percentage of body fat because they were evolutionally restricted in hunting and gathering when caring for the children, so the additional fat sustained them at those times. If the men that cared for them and the children did not catch enough food, it is possible that they ate this food themselves, and the women did not get any. Hence the need for extra fat reserves. Now most people do not have this need, but the body still accumulates fat when people eat more than they need — this is one of the possible causes for the obesity epidemic in most of the developed world where food is abundant, cheap, and easily accessible.

The Energy in Other Organisms and Plants

Most animals derive energy from organic components, namely proteins, carbohydrates, and fats. Microbes, on the other hand, extract energy from inorganic components such as ammonia, hydrogen, sulfides, and iron monoxide. Plants derive energy from solar radiation and convert it during the process of photosynthesis to chemical energy that their cells can use. ATP is used during photosynthesis in plants and microbial metabolism in a similar way as it is used by animals.

Specific Energy of Commonly Used Fuels and Foods

References

This article was written by Kateryna Yuri

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Compact Calculator Full Calculator Unit definitions

Online Unit Converters Thermodynamics — Heat

Thermodynamics — Heat

Thermodynamics is the branch of physics concerned with heat and its relation to other forms of energy and work. It defines thermodynamic variables (such as temperature, entropy, and pressure; they are also referred to as macroscopic variables) that describe average properties of material bodies and radiation, and explains how they are related and by what laws they change with time.

Specific Energy, Heat of Combustion (per Mass) Converter

Specific energy (per mass) is defined as the energy per unit mass. Common units are J/kg or cal/kg. The concept of specific energy applies to a particular (e.g. transportation) or theoretical way of extracting useful energy from the fuel. The specific energy of the fuel is also called the energy content of the fuel.

Power-Specific Fuel Consumption (PSFC) is a fuel efficiency measure of an engine defined as the rate of fuel consumption divided by the power produced. It allows comparing the fuel efficiency of different engines. In SI units, the power-specific fuel consumption is expressed in kilograms per kilowatt-hour.

Using the Specific Energy, Heat of Combustion (per Mass) Converter Converter

This online unit converter allows quick and accurate conversion between many units of measure, from one system to another. The Unit Conversion page provides a solution for engineers, translators, and for anyone whose activities require working with quantities measured in different units.

You can use this online converter to convert between several hundred units (including metric, British and American) in 76 categories, or several thousand pairs including acceleration, area, electrical, energy, force, length, light, mass, mass flow, density, specific volume, power, pressure, stress, temperature, time, torque, velocity, viscosity, volume and capacity, volume flow, and more.
Note: Integers (numbers without a decimal period or exponent notation) are considered accurate up to 15 digits and the maximum number of digits after the decimal point is 10.

In this calculator, E notation is used to represent numbers that are too small or too large. E notation is an alternative format of the scientific notation a · 10^x. For example: 1,103,000 = 1.103 · 10⁶ = 1.103E+6. Here E (from exponent) represents “· 10^”, that is “times ten raised to the power of”. E-notation is commonly used in calculators and by scientists, mathematicians and engineers.

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