Mercedes Mullins
Diesel fuel is a type of fuel with a high energy content that is used in diesel engines. The heat value of diesel fuel is measured using a calorimeter to determine how much energy the fuel provides when burned. The heat value of diesel fuel is important because it indicates how much power an engine can generate, and therefore how much fuel is needed to do a certain amount of work. The heat content of diesel fuel is influenced by factors such as its cetane rating, which measures the ease of ignition and the manner in which it burns, as well as its viscosity and sulfur content.
| Characteristics | Values |
|---|---|
| Heat Value | Number 1 diesel fuel: 125,500 BTUs per gallon. Number 2 diesel fuel: 139,500 BTUs per gallon. |
| Heat Value compared to gasoline | Diesel fuel produces 11% more heat energy than gasoline on a volume basis. |
| Carbon Dioxide Production compared to gasoline | Diesel fuel produces 12% more carbon dioxide than gasoline. |
| Cetane Rating | Cetane rating is a measure of the ignition quality of a fuel. The recommended minimum cetane rating for modern diesel engines is 40. |
| Viscosity | Viscosity is a measure of resistance to flow. Low viscosity fuel produces a fine, atomized mist of fuel which improves its mix with incoming air to encourage complete combustion. |
| Sulphur Content | The sulphur content of diesel fuel has been known to cause increased wear on certain engine components. |
| Water and Sediment Content | Water and sediment content in diesel fuel can cause rusting and damage to fuel system components. |
| Flash Point | The flash point of diesel fuel is the minimum temperature to which the fuel must be heated before flammable vapour is produced that could ignite. |
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What You'll Learn
Diesel vs. gasoline heat content
The heat value of a fuel is the amount of heat released during its combustion. It is also referred to as the energy or calorific value of the fuel. The heat value of diesel fuel is measured using a calorimeter to determine how much energy the fuel provides when burned.
Diesel and gasoline have very similar heating values (energy content) on a mass basis. This is because the carbon content of diesel and gasoline are very similar. However, diesel fuel produces about 11% more heat energy than gasoline on a volume basis. This is because the straight-chain higher C-number liquid alkanes in diesel pack more densely than the branched-chain lower C-number alkanes in gasoline. The difference in liquid densities is about 11%.
The heat value of Number 2 diesel fuel is 139,500 BTUs per gallon. The heat value of Number 1 diesel fuel is approximately 10% lower at 125,500 BTUs per gallon. In comparison, the heat value of gasoline is 124,500 BTUs per gallon. The higher the heat value of the fuel, the more power the engine can generate, and the less fuel it consumes to do the same amount of work.
Cetane rating is a measure of the ignition quality of a fuel. The ease with which diesel fuel ignites and the manner in which it burns influence engine starting and combustion roughness. Pure cetane is a colourless liquid hydrocarbon with excellent ignition qualities and is rated at 100. The higher the cetane rating, the shorter the lag time between the time the fuel enters the combustion chamber and the time it begins to burn. A good quality diesel fuel with a high cetane rating has a lag time of approximately 0.001 seconds. The recommended minimum cetane rating for modern diesel engines is 40.
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Measuring diesel heat content
The heat content of diesel fuel is important to measure as it indicates how much energy the fuel will provide when burned. The higher the heat content, the more power an engine can generate, and the less fuel it will consume to do the same amount of work.
The heat value of diesel fuel is measured using a calorimeter, which calculates the amount of heat produced when a specific amount of fuel is burned. The unit of measurement used for this is British Thermal Units (BTUs). One BTU is the quantity of heat required to raise the temperature of one pound of liquid water by 1° Fahrenheit (F) at the temperature that water has its greatest density (approximately 39°F). This is roughly equivalent to the energy released by burning a match.
There are two types of heat of combustion: gross heat of combustion (GHC) and net heat of combustion (NHC). GHC is the quantity of energy released when a unit mass of fuel is burned in a constant volume enclosure, with the products being gaseous, other than water that is condensed to a liquid state. NHC is the quantity of energy released when a unit mass of fuel is burned in a constant pressure enclosure, with all the products, including water, being gaseous. NHC is the appropriate value to use when comparing fuels as engines exhaust water as a gas. GHC and NHC are calculated using the fuel density, sulphur, water, and ash content.
The cetane rating of diesel fuel is also an important factor in determining its quality. This measures the ignition quality of the fuel, or how easily it ignites and burns, which influences engine starting and combustion roughness. The higher the cetane rating, the shorter the lag time between the fuel entering the combustion chamber and it beginning to burn.
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Diesel fuel properties
Diesel fuel, also known as diesel oil, heavy oil, or simply diesel, is a liquid fuel designed for use in a diesel engine. Diesel engines are internal combustion engines that use fuel ignition without a spark, instead relying on the compression of inlet air and the injection of fuel. The most common type of diesel fuel is a fractional distillate of petroleum fuel oil, but alternative types that are not derived from petroleum, such as biodiesel, biomass-to-liquid (BTL), or gas-to-liquid (GTL) diesel, are becoming more common.
Diesel fuel has several important properties that contribute to its effectiveness in modern diesel engines. One key property is its high energy content, which provides more power to the engine, resulting in increased efficiency. The heat value of diesel fuel is typically measured using a calorimeter, and it is expressed in British Thermal Units (BTUs) per gallon. Number 2 diesel fuel, for example, has a heat value of 139,500 BTUs per gallon, while Number 1 diesel fuel has a slightly lower heat value of 125,500 BTUs per gallon.
Another important characteristic of diesel fuel is its cetane rating, which indicates the combustion speed and ignition quality of the fuel. Cetane rating is the inverse of the octane rating used for gasoline. A higher cetane rating results in a shorter lag time between the fuel entering the combustion chamber and the ignition of the fuel. The recommended minimum cetane rating for modern diesel engines is 40, and increasing it beyond 48 does not improve performance and may even cause fuel knocking.
Diesel fuel also has specific viscosity and density properties. Viscosity refers to the resistance to flow, and it decreases as temperature increases. Low viscosity fuel produces a fine mist that improves combustion and reduces emissions. Density, on the other hand, is measured in g/mL, with iso-octane having a density of 0.692 g/mL and cetane having a density of 0.770 g/mL.
Other properties of diesel fuel that are important to consider include its low-temperature operability, sulfur content, fuel volatility, lubricity, and cold-flow properties. Additionally, the water and sediment content in diesel fuel can impact engine performance and cause damage to fuel system components if not properly managed.
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Carbon dioxide production
Diesel fuel is a type of fuel with a high energy content that is used in modern diesel engines. The heat value of diesel fuel is measured using a calorimeter, which determines the amount of energy produced when the fuel is burned. This is measured in British Thermal Units (BTUs). The heat value of Number 2 diesel fuel, for example, is 139,500 BTUs per gallon, while Number 1 diesel fuel has a lower heat value of 125,500 BTUs per gallon.
The combustion of diesel fuel produces carbon dioxide (CO2) as a byproduct. The amount of CO2 produced is directly related to the carbon content of the fuel. Diesel fuel has a higher carbon content compared to gasoline, with carbon percentages of 85.0% for diesel compared to 84.2% for gasoline. This results in a higher production of CO2 from diesel fuel on a per-litre basis.
According to calculations, the ratio of carbon dioxide production for one litre of diesel fuel to one litre of gasoline is approximately 1.12, or 12% greater for diesel. This variation in carbon dioxide production contributes to the carbon pricing of diesel fuel. The US Energy Information Administration estimates that the combustion of a gallon of gasoline that does not contain ethanol produces about 19.64 pounds of carbon dioxide.
The carbon dioxide emissions from diesel fuel combustion have significant implications for the environment. In 2023, diesel fuel consumption in the US transportation sector contributed to about 9% of total US energy-related carbon dioxide emissions. The black, sooty material that remains after diesel fuel is burned is referred to as carbon residue, and its presence can vary depending on engine manufacturers and running conditions.
To summarize, diesel fuel combustion results in the production of carbon dioxide, with diesel fuel producing more carbon dioxide per litre compared to gasoline. This contributes to its carbon pricing and environmental impact, as reflected in the carbon dioxide emissions estimates in the transportation sector.
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Cetane rating
The cetane rating of a specific diesel mixture is based on how much cetane—a clear, colourless hydrocarbon that ignites under high pressure—is in the fuel. Pure cetane represents the highest purity of diesel fuel possible and has a cetane rating of 100. The higher the cetane rating, the shorter the lag time between the time the fuel enters the combustion chamber and the time it begins to burn. A good-quality diesel fuel with a high cetane rating has a lag time of approximately 0.001 seconds.
The cetane number is defined by finding a blend of cetane and isocetane with the same ignition delay. Cetane has a cetane number of 100, while isocetane's measured cetane number is 15. Once the blend is known, the cetane number is calculated as a volume-weighted average, rounded to the nearest whole number, of cetane's 100 and isocetane's 15. The cetane number measures the delay in the ignition time of the fuel. In other words, it is how minimised the delay is between when the fuel is injected into the chamber and when the combustion begins.
The recommended minimum cetane rating for modern diesel engines is 40. Increasing the cetane rating above 48 is not recommended as it will not increase engine performance and may cause fuel knocking. Generally, diesel engines operate well with a CN from 48 to 50. Modern highway diesel engines tend to require fuel with a cetane number ranging from 45 to 55. Regular no. 2 diesel has a cetane number of 40 to 42, while Cenex Premium Diesel has a cetane number of 47 to 52. Premium diesel fuel can have a cetane number as high as 60.
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Frequently asked questions
The heat content of diesel fuel, also known as the heat value, is measured using a calorimeter to determine how much energy the fuel provides when burned. The heat value of Number 1 diesel fuel is 125,500 BTUs per gallon, while the heat value of Number 2 diesel fuel is 139,500 BTUs per gallon.
Diesel and gasoline have similar carbon contents and are very close in heating value (energy content) on a mass basis. However, diesel produces more heat energy and carbon dioxide than gasoline on a volume basis.
The heat content of diesel fuel is influenced by various properties such as energy content, density, lubricity, cold-flow properties, and sulfur content. Maintaining these properties is crucial for the effective operation of modern diesel engines.
The higher the heat value of the fuel, the more power the engine can generate, resulting in reduced fuel consumption for the same amount of work. A higher cetane rating, which indicates the ignition quality of diesel fuel, also contributes to improved engine performance by reducing the lag time between fuel ignition and combustion.
The heat content of diesel fuel is measured using a calorimeter. A specific amount of fuel is burned, and the amount of heat released is carefully measured in British Thermal Units (BTUs). This process helps determine the energy density or calorific value of the fuel, expressed in energy (joules) per specified amount (e.g., kilograms).
