Gasoline is more volatile than diesel oil, Jet-A or kerosene, not only because of the base constituents, but because of the additives that are put into it. The final control of volatilityis often achieved by blending with butane. The Reid Vapor Pressure (RVP) test is used to measure the volatility of gasoline. The desired volatility depends on the ambient temperature. In hot weather, gasoline components of higher molecular weight and thus lower volatility are used. In cold weather, too little volatility results in cars failing to start.
In hot weather, excessive volatility results in what is known as "vapor lock", where combustion fails to occur, because the liquid fuel has changed to a gaseous fuel in the fuel lines, rendering the fuel pump ineffective and starving the engine of fuel. This effect mainly applies to camshaft-driven (engine mounted) fuel pumps which lack a fuel return line. Vehicles with fuel injection require the fuel to be pressurized, to within a set range. Because camshaft speed is nearly zero before the engine is started, an electric pump is used. It is located in the fuel tank so that the fuel may also cool the high-pressure pump. Pressure regulation is achieved by returning unused fuel to the tank. Therefore, vapor lock is almost never a problem in a vehicle with fuel injection.
In the United States, volatility is regulated in large cities to reduce the emission of unburned hydrocarbons. In large cities, so-called reformulated gasoline that is less prone to evaporation, among other properties, is required. In Australia, summer petrol volatility limits are set by state governments and vary among states. Most countries simply have a summer, winter, and perhaps intermediate limit.
Volatility standards may be relaxed (allowing more gasoline components into the atmosphere) during gasoline shortages. For example, on 31 August 2005, in response to Hurricane Katrina, the United States permitted the sale of non-reformulated gasoline in some urban areas, effectively permitting an early switch from summer to winter-grade gasoline. As mandated by EPA administrator Stephen L. Johnson, this "fuel waiver" was made effective until 15 September 2005.
Modern automobiles are also equipped with an evaporative emissions control system (called an EVAP system in automotive jargon), which collects evaporated fuel from the fuel tank in a charcoal-filled canister while the engine is stopped and then releases the collected vapors into the engine intake for burning when the engine is running (usually only after it has reached normal operating temperature.) The evaporative emissions control system also includes a sealed gas cap to prevent vapors from escaping via the fuel filler tube. Modern vehicles with OBD-II emissions control systems will illuminate the malfunction indicator light, (MIL) "check engine" or “Service Engine Soon” light if the leak detection pump (LDP) detects a leak in the EVAP system. If the Electronic Control Unit (ECU) or Powertrain Control Module (PCM) detects a leak, it will store an OBD-II code representing either a small or large leak, thus illuminating the MIL to indicate a failure. Some vehicles can detect whether the gas cap is incorrectly fitted, and will indicate this by illuminating a gas cap symbol on the dash.
Octane rating
Internal combustion engines are designed to burn gasoline in a controlled process called deflagration. But in some cases, gasoline can also combust abnormally by detonation, which wastes energy and can damage the engine. One way to reduce detonation is to increase the gasoline's resistance to autoignition, which is expressed by its octane rating.
Octane rating is measured relative to a mixture of 2,2,4-trimethylpentane (an isomer of octane) and n-heptane. There are different conventions for expressing octane ratings so a fuel may have several different octane ratings based on the measure used.
The octane rating became important as the military sought higher output for aircraft engines in the late 1930s and the 1940s. A higher octane rating allows a higher compression ratio, and thus higher temperatures and pressures, which translate to higher power output.
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