Gasoline is more volatile than diesel oil, Jet-A, or kerosene, not only because of the base constituents, but also because of additives. Volatility is often controlled by blending with butane, which boils at −0.5 °C. The volatility of petrol is determined by the Reid vapor pressure (RVP) test. The desired volatility depends on the ambient temperature. In hot weather, petrol 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 state 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 within a set range. Because the camshaft speed is nearly zero before the engine is started, an electric pump is used. It is located in the fuel tank so 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.
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