Gasoline and diesel are derived from crude oil and are known as petroleum, but their molecular structures and properties are different. Gasoline is lightweight and extremely volatile. Diesel fuel is heavier than gasoline and has the appearance of oil. Diesel fuel takes less refining than gasoline and is therefore often cheaper. Because it is less expensive and provides better mileage than gasoline, diesel fuel is a popular fuel choice around the world.
Combustion Ratio
The main difference between gasoline and diesel fuel is their combustion ratios. Gasoline has a low compression ratio and therefore does not self-ignite when compressed because it is not able to create enough heat. Diesel fuel has a higher compression ratio and is able to create enough heat when compressed to self-ignite. The higher the compression ratio, the more power is generated. Gasoline engines have compression ratios of about 10:1 to 11:1, while the compression ratios in diesels can be as high as 25:1.
Chemical Structure
Crude oil is a mixture of hydrocarbon chains (molecules that only contain hydrogen and carbon) in varying lengths. Each hydrocarbon chain of a specific length has different chemical properties that define its use in engines. The greater the number of carbons in the molecule, the higher the boiling point. Because the boiling points of each of these molecules is different, they are able to be separated by distillation in an oil refinery. Hydrocarbon molecules that contain between 7 and 11 carbons in their chain are pooled together and used for gasoline. Hydrocarbon molecules that contain between 10 and 15 carbons in their chain are pooled together and used for diesel.
Fuel Efficiency
Diesel tends to get better mileage than gasoline because its high density leads to more energy per ignition. According to Charlotte Country Day School, 1 gallon of diesel fuel contains approximately 155x106 joules of energy, while 1 gallon of gasoline contains 132x106 joules of energy. The diesel cycle is more efficient because the combustion ratio and the combustion temperature are higher than that of gasoline. Also, because gasoline and air are mixed before they reach the cylinder and ignite before the piston gets to the optimal position, the gasoline cycle does not exploit the best use of the energy produced and is less efficient than the diesel cycle.
Gasoline Engines
Gasoline engines use spark plugs to ignite the fuel because the heat created from the compressed gasoline and air is not enough to ignite the mixture. The process of creating energy for a gasoline-powered automobile starts with gasoline combining with air, the mixture is forced into the cylinder and compressed, the spark plug ignites and explodes, which moves the piston up and moves the car. The resulting burned mixture is then forced out as exhaust.
Diesel Engines
Diesel engines have a high compression ratio, therefore they do not require spark plugs to ignite the fuel because the heat created from the compression is enough to ignite the diesel/air mixture. The process of creating energy for a diesel powered car starts with air being compressed in the cylinder, diesel fuel is added to the compressed air, the compression then causes the fuel to ignite and push the piston up and move the car.
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