Katerina Swanson
Diesel fuel is denser than regular gasoline. This is due to the large, long-chain hydrocarbons that make up diesel, which give it a higher energy density. Diesel fuel has about 13% higher energy density per volume than gasoline, resulting in more energy per gallon. This makes diesel-powered vehicles more fuel-efficient, as they get more miles per gallon. However, diesel engines are more expensive to build and have trouble starting in cold temperatures due to their reliance on temperature for detonation.
| Characteristics | Values |
|---|---|
| Energy Density | Diesel has a higher energy density than gasoline |
| Energy Density (figures) | Diesel: 45.6 MJ/kg, Gasoline: 46.4 MJ/kg |
| Energy Density per Volume | Diesel has 13% higher energy density per volume |
| Compression Ratio | Diesel engines have a higher compression ratio than gasoline engines |
| Spark Plug | Gasoline engines require spark plugs, diesel engines do not |
| Fuel Viscosity | Gasoline is lighter and thinner, diesel is heavier and more oily |
| Flammability | Gasoline is more flammable |
| Volatility | Diesel is less volatile |
| Fuel Economy | Diesel provides a better fuel economy |
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What You'll Learn
Diesel fuel's higher energy density
Diesel fuel has a higher energy density than regular fuel. This is due to the large, long-chain hydrocarbons that make up diesel, which give it a high energy density because of their size and length. The density of diesel and its heating values provide an ideal balance for fuel economy and tractive effort. This is why diesel is used in large vehicles like trucks, as the diesel engines produce more torque, which is necessary when hauling or towing heavy cargo.
Diesel engines also have a higher compression ratio compared to gasoline engines, which contributes to their higher efficiency in converting chemical energy into mechanical energy. This higher compression ratio also means that diesel engines do not require spark plugs to ignite the fuel, unlike gasoline engines. The absence of spark plugs further contributes to the efficiency of diesel engines.
While the energy density of diesel is only marginally higher than gasoline (45.6 vs 46.4 MJ/kg), the difference in efficiency between the two types of engines becomes more pronounced when used at larger scales. At smaller scales, gasoline engines have a better power-to-weight ratio. However, as the scale increases, diesel engines become more efficient in terms of the power obtained for the weight and volume of the engine.
The higher energy density of diesel fuel translates to more energy per gallon, which is why diesel-powered vehicles achieve more miles per gallon. This higher energy density also contributes to better fuel economy, making diesel a preferred choice for certain applications despite the higher cost of building diesel engines.
In summary, the higher energy density of diesel fuel, combined with the efficiency of diesel engines, makes diesel a popular choice for large vehicles and applications requiring high torque and fuel efficiency.
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Gasoline's lower energy density
Diesel fuel has a higher energy density than gasoline. One source states that diesel has about 13% higher energy density per volume. Diesel engines are more efficient in terms of the power you get for the weight and volume of the engine. This is due to their higher compression ratio, which results in a higher efficiency Diesel cycle compared to the Otto cycle of a petrol engine. Diesel engines do not require spark plugs as the compression ratio is high enough to raise the temperature of the fuel vapour to the ignition point.
Gasoline, on the other hand, is a lighter petroleum distillate. It is a smaller, lighter hydrocarbon molecule and is thinner and more flammable than diesel. Gasoline engines require spark plugs to ignite the gasoline vapour inside the engine. While gasoline has a lower energy density than diesel, it has a better power-to-weight ratio at smaller scales.
The energy density of a fuel relates to the amount of stored energy per volume of the storage equipment. Fuels with high energy density can store more energy in a given volume. This is advantageous for vehicles as it provides a longer range. For example, lithium-ion storage has a much lower range than gasoline.
While gasoline has a lower energy density than diesel, it has a higher energy density than other fuels such as coal, wood, and ammonia. Methane and hydrogen have higher energy densities than gasoline, but their gaseous form creates storage difficulties.
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Diesel engines' higher efficiency
Diesel engines are widely recognised for their efficiency and reliability under heavy loads, making them the preferred choice for industries that rely on long-haul drives and high-torque applications. They are more fuel-efficient than gasoline engines, with an average fuel efficiency of 20–35% more than older engines.
The higher efficiency of diesel engines is mainly due to their higher compression ratio. Diesel engines have a compression/expansion ratio between 14:1 and 25:1. This higher compression ratio results in a higher temperature that ignites the fuel vapour, eliminating the need for spark plugs. In comparison, gasoline engines require spark plugs to ignite the gasoline vapour inside the engine.
The higher compression ratio in diesel engines also contributes to their higher energy density. The energy density of diesel fuel is slightly lower than gasoline (45.6 vs 46.4 MJ/kg). However, the higher compression ratio in diesel engines compensates for this marginal difference, resulting in higher overall energy density.
Additionally, diesel engines do not suffer from efficiency losses at small throttle openings like gasoline engines. Gasoline engines experience high turbulence and frictional loss when the incoming air must pass through a nearly closed throttle. In contrast, diesel engines do not restrict the incoming air, avoiding this type of efficiency loss.
Modern turbo-diesel engines further enhance efficiency through electronically controlled common-rail fuel injection systems. These advancements in engine design and technology have improved fuel efficiency ratings and reduced CO2 emissions.
While diesel engines offer higher efficiency, they also come with a higher price tag due to higher fuel costs, purchase prices, and inefficiency with light loads.
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Gasoline engines' better power-to-weight ratio
The power-to-weight ratio is a simple metric that gives a general idea of how well an engine is paired with its body. It is calculated by taking the power figures of the car and dividing them by their curb weight. In the Philippines, for example, the power-to-weight ratio is measured in horsepower per kilogram (hp/kg).
At smaller scales, gasoline engines have a better power-to-weight ratio than diesel engines. This is because gasoline is a lighter petroleum distillate than diesel. Gasoline is a smaller, lighter hydrocarbon molecule, and it is thinner and more flammable. Diesel, on the other hand, is more oily and energy-dense but harder to ignite.
The difference in flammability means that gasoline engines require a spark plug to ignite, whereas diesel engines use high compression to raise the temperature of the fuel vapour to the ignition point. This compression ratio means that diesel engines need more material to handle the pressure, so the engine is heavier.
While power-to-weight ratios give an indication of how well an engine is matched to its chassis, they are not the only metric to determine how fast a car is. A diesel-powered SUV, for example, can outpace a gasoline-powered hatchback with a lower power-to-weight ratio because diesel engines produce more torque. Torque is important for hauling or towing something heavy, which is why very large vehicles typically use diesel engines.
Other factors that can affect power-to-weight ratios include the choice of power transmission system, such as variable-frequency drive versus direct-current drive, which can support a higher power-to-weight ratio by better managing propulsion power.
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Diesel's higher compression ratio
Diesel engines have a higher compression ratio than petrol engines. This is because diesel fuel is harder to ignite than petrol, so it requires a higher pressure to self-ignite. This is achieved through adiabatic heating, where the air is compressed until the kinetic energy of the molecules turns into heat.
Compression ratios are typically between 14:1 and 23:1 for direct injection diesel engines, and between 18:1 and 23:1 for indirect injection diesel engines. A higher compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture, resulting in higher thermal efficiency. This means that the same combustion temperature can be reached with less fuel, while also creating a longer expansion cycle, which generates more mechanical power output.
The higher compression ratio in diesel engines is achieved through a longer stroke and a smaller combustion chamber. This results in heavier and sturdier components, including the piston, its rod, crankshaft, and shaft bearings. The most common method of increasing the compression ratio is to "skim the head", or lower the ceiling of the combustion chamber. However, this can lead to issues with other components, such as the spark plug, coming into contact with the piston.
While diesel engines traditionally had higher compression ratios, modern diesel engines tend to have lower compression ratios due to improvements in fuel injection and the use of turbocharging. Lower compression ratios can make engines run smoother and reduce friction, but they may also require a heavier flywheel. Variable compression ratio technology can also be used to adjust the compression ratio while the engine is in operation, improving fuel efficiency under varying loads.
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Frequently asked questions
Yes, diesel fuel has a higher density than gasoline. Diesel fuel has a density of 0.820 to 0.845 kg/L (6.84 to 7.05 lb/US gal) at 15 °C (59 °F), while gasoline has a density of 0.720-0.775 kg/L (6.01-6.47 lb/US gal) at the same temperature.
Diesel fuel is made up of large, long-chain hydrocarbons, which contribute to its high energy density due to their size and length. The structure of these hydrocarbons also influences their heating values, with longer hydrocarbons having higher heating values.
Yes, the higher density of diesel fuel contributes to its higher energy density compared to gasoline. Diesel engines use a high compression ratio to raise the temperature of the fuel vapor to the ignition point, and the higher energy density of diesel fuel results in greater efficiency in terms of the power output relative to the engine's weight and volume. Therefore, diesel fuel is commonly used in larger vehicles such as trucks, where high torque and towing capacity are required.
