How to Measure the Density of Gasoline

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Measuring the density of gasoline can give you a better understanding of the uses of gasoline for various purposes in different types of engines.

Density of Gasoline

The density of a liquid is the ratio of its mass to volume. Divide the the mass by its volume to calculate it. For example, if you had 1 gram of gasoline that measures 1.33 cm3 in volume, the density would be 1/1.33 or about .75 g/cm3.

The density of diesel fuel in the United States depends on its class 1D, 2D or 4D. 1D fuel is better for cold weather because it has a lower resistance to flow. 2D fuels are better for warmer outside temperatures. 4D is better for low-speed engines. Their densities, respectively, are 875 kg/m3, 849 kg/m3 and 959 kg/m3. The European density of diesel in kg/m3 .ranges from 820 to 845.

Specific Gravity of Gasoline

Density of gasoline can also be defined using the specific gravity of gasoline. Specific gravity is an object's density compared to the maximum density of water. The maximum density of of water is 1 g/ml at around 4°C. This means, if you know the density in g/ml, that value should be the specific gravity of gasoline.

A third way of calculating density of a gas uses the ideal gas law: PV = nRT, in which P is pressure, V is volume, n is the number of moles, R is the ideal gas constant and T is temperature of the gas. Rearranging this equation gives you nV = P/RT, in which the left-hand side is a ratio between n and V.

Using this equation, you can calculate ratio between the number of moles of gas that are available in a quantity of gas and the volume. The number of moles can then be converted to mass using the atomic or molecular weight of the gas particles. Because this method is meant for gases, gasoline in liquid form will deviate much from the results of this equation.

Experimental Density of Gasoline

Weigh a graduated cylinder using a metric scale. Record this amount in grams. Fill the cylinder with 100 ml of gasoline and weigh it in grams with the scale. Subtract the mass of the cylinder from the mass of the cylinder when it contains gasoline. This is the mass of the gasoline. Divide this figure by the volume, 100 ml, to get the density.

Knowing equations for density, specific gravity and the ideal gas law, you can determine how density varies as function of other variables such as temperature, pressure and volume. Making a series of measurements of these quantities lets you find the way density varies as a result of them or how density varies as result of one or two of these three quantities while the other quantity or quantities are held constant. This is often handy for practical applications in which you don't know all the information about every single gas quantity.

Gases in Practice

Keep in mind that equations such as the ideal gas law may work in theory, but, in practice, they don't account for the proper of gases in practice. The ideal gas law doesn't take into account the molecular size and intermolecular attractions of the gas particles.

Because the ideal gas law doesn't account for the sizes of the gas particles, it is less accurate at lower densities of gas. At lower densities, there is greater volume and pressure such that the distances between gas particles becomes much larger than particle size. This makes the particle size less of a deviation from the theoretical calculations.

Intermolecular forces between the gas particles describe the forces caused by differences in charge and structure between the forces. These forces include dispersion forces, forces between the dipoles, or charges, of atoms among the gas particles. These are caused by the electron charges of the atoms depending on how the particles interact with their environment among non-charged particles such as noble gases.

Dipole-dipole forces, on the other hand, are the permanent charges on the atoms and molecules that are used among polar molecules such as formaldehyde. Finally, hydrogen bonds describe a very specific case of dipole-dipole forces in which molecules have hydrogen bonded to oxygen, nitrogen, or fluorine that, due to the difference in polarity between the atoms, are the strongest of these forces and give rise to qualities of water.

Density of Gasoline by Hydrometer

Use a hydrometer as a method of experimentally measuring density. A hydrometer is a device that uses the principle of Archimedes to measure specific gravity. This principle holds that an object floating in a liquid will displace a quantity of water that's equal to the weight of the object. A measured scale on the side of the hydrometer will provide the specific gravity of the liquid.

Fill a clear container with gasoline and carefully place the hydrometer on the surface of the gasoline. Spin the hydrometer to dislodge all of the air bubbles and allow the hydrometer's position on the surface of the gasoline to stabilize. It's essential that the air bubbles be removed because they will increase the buoyancy of the hydrometer.

View the hydrometer so that the surface of the gasoline is at eye level. Record the value associated with the marking at the surface level of the gasoline. You'll need to record the temperature of the gasoline since the specific gravity of a liquid varies with the temperature. Analyze the specific gravity reading.

Gasoline has a specific gravity between 0.71 and 0.77, depending on its precise composition. Aromatic compounds are less dense than aliphatic compounds, so the specific gravity of gasoline can indicate the relative proportion of these compounds in the gasoline.

Gasoline Chemical Properties

What's the difference between diesel and gasoline? Gasolines are generally made of hydrocarbons, which are strings of carbons chained together with hydrogen ions, that range in length from four to 12 carbon atoms per molecule.

The fuel used in gasoline engines also contains amounts of alkanes (saturated hydrocarbons, meaning they have the maximum amount of hydrogen atoms), cycloalkanes (hydrocarbon molecules arranged in circular ring-like formations) and alkenes (unsaturated hydrocarbons that have double bonds).

Diesel fuel uses hydrocarbon chains that have greater numbers of carbon atoms, with the average being 12 carbon atoms per molecule. These larger molecules increase its evaporation temperature and how it requires more energy from compression before igniting.

Diesel made from petroleum also has cycloalkanes as well as variations of benzene rings that have alkyl groups. Benzene rings are hexagon-like structures of six carbon atoms each, and alkyl groups are extended carbon-hydrogen chains that branch off of molecules such as benzene rings.

Four-Stroke Engine Physics

Diesel fuel uses an ignition of the fuel to move a cylindrical-shaped chamber that performs the compression that generates energy in automobiles. The cylinder compresses and expands through the steps of the four-stroke engine process. Diesel and gasoline engines both function using a four-stroke engine process that involves intake, compression, combustion and exhaust.

  1. During the intake step, the piston moves from the top of the compression chamber to the bottom such that it pulls a mixture of air and fuel into the cylinder using the pressure difference generated through this process. The valve remain open during this step such that the mixture flows freely through.
  2. Next, during the compression step, the piston presses the mixture in itself, increasing the pressure and generating potential energy. Valves are closed such that the mixture remains inside the chamber. This causes the cylinder contents to heat. Diesel engines use more compression of the cylinder contents than gasoline engines do.
  3. The combustion step, involves rotating the crankshaft through the mechanical energy from the engine. With such a high temperature, this chemical reaction is spontaneous and doesn't require external energy. A spark plug or the compression step's heat either ignite the mixture. 
  4. Finally, the exhaust step involves the piston moving back to the top with the exhaust valve open such that the process may repeat. The exhaust valve lets the engine remove the ignited fuel that it has used. 

Diesel and Gasoline engines

Gasoline and diesel engines use internal combustion to generate chemical energy that's converted to mechanical energy. The chemical energy of combustion for gasoline engines or air compression in diesel engines is converted to mechanical energy that moves the piston of the engine. This movement of the piston through different strokes creates forces that power the engine itself.

Gasoline engines or petrol engines use a spark-ignition process to ignite a mixture of air and fuel and create chemical potential energy that is converted to mechanical energy during the steps of the engine's process.

Engineers and researchers look for fuel-efficient methods of performing these steps and reactions to conserve as much as energy as possible while remaining effective for the purposes of gasoline engines. Diesel engines or compression-ignition ("CI engines"), by contrast, use an internal combustion in which the combustion chamber houses the fuel ignition caused by high temperatures when the fuel is compressed.

These increases in temperature are accompanied by decreased volume and increased pressure in accordance with laws that demonstrate how gas quantities change such as the ideal gas law: PV = nRT. For this law, P is pressure, V is volume, n is the number of moles of the gas, R is the ideal gas law constant and T is temperature.

Though these equations may be true in theory, in practice engineers have to take into account real-world constraints such as the material used to build the combustion engine and how the fuel is much more liquid than a pure gas would be.

These calculations should account for how, in gasoline engines, the engine compresses the fuel-air mixture using pistons and the spark plugs ignite the mixture. Diesel engines, in contrast, compress the air first before injecting and igniting the fuel.

Gasoline and Diesel fuels

Gasoline cars are more popular in the United States while diesel cars make up almost half of all car sales in European countries. The differences between them show how the chemical properties of gasoline give it the qualities necessary for vehicle and engineering purposes.

Diesel cars are more efficient with mileage on the highway because diesel fuel has more energy than gasoline fuel. Automobile engines on diesel fuels also have more torque, or rotational force, in their engines which means that these engines can accelerate more efficiently. When driving through other areas such as cities, the diesel advantage is less significant.

Diesel fuel is also typically more difficult to ignite because of its lower volatility, the ability of a substance to evaporate. When it is evaporated, however, it's easier to ignite because it has lower autoignition temperature. Gasoline, on the other hand, requires a spark plug to ignite.

There is hardly any cost difference between gasoline and diesel fuels in the United States. Because diesel fuels have better mileage, their cost with respect to miles driven is better. Engineers also measure the power output of automobile engines using horsepower, a measure of power. While diesel engines may accelerate and rotate more easily than gasoline ones do, they have a lower horsepower output.

Diesel Advantages

Along with high fuel efficiency, diesel engines typically have lower fuel costs, better lubrication properties, greater density of energy during the four-stroke engine process, less flammability and the ability to use biodiesel non-petroleum fuel that's more environmentally friendly.

References

About the Author

S. Hussain Ather is a Master's student in Science Communications the University of California, Santa Cruz. After studying physics and philosophy as an undergraduate at Indiana University-Bloomington, he worked as a scientist at the National Institutes of Health for two years. He primarily performs research in and write about neuroscience and philosophy, however, his interests span ethics, policy, and other areas relevant to science.