Diesel or diesel fuel (pronounced /ˈdiːzəl/) in general is any fuel used in diesel engines. The most common is a specific fractional distillate of petroleum fuel oil, but alternatives that are not derived from petroleum, such as biodiesel, biomass to liquid (BTL) or gas to liquid (GTL) diesel, are increasingly being developed and adopted. To distinguish these types, petroleum-derived diesel is increasingly called petrodiesel. Ultra-low sulfur diesel (ULSD) is a standard for defining diesel fuel with substantially lowered sulfur contents. As of 2007, almost every diesel fuel available in America and Europe is the ULSD type.
The word “diesel” is derived from the German inventor Rudolf Christian Karl Diesel who in 1892 invented the diesel engine.
Diesel fuel is produced from petroleum and from various other sources. The resulting products are interchangeable in most applications.
Petroleum diesel, also called petrodiesel, or fossil diesel is produced from the fractional distillation of crude oil between 200 °C (392 °F) and 350 °C (662 °F) at atmospheric pressure, resulting in a mixture of carbon chains that typically contain between 8 and 21 carbon atoms per molecule.
Fuel value and price
The density of petroleum diesel is about 0.85 kg/l (7.09 lbs/gallon(us)), about 18% more than petrol (gasoline), which has a density of about 0.72 kg/l (6.01 lbs/gallon(us)). When burnt, diesel typically releases about 38.6 MJ/l (138,700 BTU per US gallon), whereas gasoline releases 34.9 MJ/l (125,000 BTU per US gallon), 10% less by energy density, but 45.41 MJ/kg and 48.47 MJ/kg, 6.7% more by specific energy. Diesel is generally simpler to refine from petroleum than gasoline. The price of diesel traditionally rises during colder months as demand for heating oil rises, which is refined in much the same way. Due to recent changes in fuel quality regulations, additional refining is required to remove sulfur which contributes to a sometimes higher cost. In many parts of the United States and throughout the UK and Australia diesel may be higher priced than petrol. Reasons for higher priced diesel include the shutdown of some refineries in the Gulf of Mexico, diversion of mass refining capacity to gasoline production, and a recent transfer to ultra-low sulfur diesel (ULSD), which causes infrastructural complications.
Use as vehicle fuel
Unlike petroleum ether and liquefied petroleum gas engines, diesel engines do not use high voltage spark ignition (spark plugs). An engine running on diesel compresses the air inside the cylinder to high pressures and temperatures (compression ratios from 15:1 to 21:1 are common); the diesel is generally injected directly into the cylinder near the end of the compression stroke. The high temperatures inside the cylinder cause the diesel fuel to react with the oxygen in the mix (burn or oxidize), heating and expanding the burning mixture in order to convert the thermal/pressure difference into mechanical work; i.e., to move the piston. (Glow plugs are used to assist starting the engine to preheat cylinders to reach a minimum operating temperature.) High compression ratios and throttleless operation generally result in diesel engines being more efficient than many spark-ignited engines.
This efficiency and its lower flammability and explosivity than gasoline are the main reasons for military use of diesel in armoured fighting vehicles like tanks and trucks. Engines running on diesel also provide more torque and are less likely to stall as they are controlled by a mechanical or electronic governor.
A disadvantage of diesel as a vehicle fuel in some climates, compared to gasoline or other petroleum derived fuels, is that its viscosity increases quickly as the fuel’s temperature decreases, turning into a non-flowing gel at temperatures as high as -19 °C (-2.2 °F) or -15 °C (+5 °F), which can’t be pumped by regular fuel pumps. Special low temperature diesel contains additives that keep it in a more liquid state at lower temperatures, yet starting a diesel engine in very cold weather still poses considerable difficulties.
Another rare disadvantage of diesel engines compared to petrol/gasoline engines is the possibility of runaway failure. Since diesel engines do not require spark ignition, they can sustain operation as long as diesel fuel is supplied. Fuel is typically supplied via a fuel pump. If the pump breaks down in an “open” position, the supply of fuel will be unrestricted and the engine will runaway and risk terminal failure.