A mixture of petrol vapour and air is ignited in a cylinder in a car engine. The vapour-air mixture is made in the carburettor, but when it's cold, petrol is difficult to vapourise, so it's difficult to start the car.
Petrol companies make different blends for different times of the year. In winter, they put more volatile components so it vaporises more readily. This means putting in more hydrocarbons with small molecules - eg butane and pentane.
In hot weather, you don't want too many volatile components, because you'd lose petrol from the tank by evaporation - which is costly and polluting. The fuel pump then delivers a mixture of liquid and vapour to the carburettor, and this means that not enough fuel gets through to keep the engine running - vapour lock.
Another important property that blenders must take into accound is the octane number of the petrol. This is the tendency of a petrol-based fuel to auto-ignite. The fuel-air mixture in the engine has to ignite at the right time. Many hydrocarbons auto-ignite under high compression. The mixture catches fire as it's compressed, and two explosions occur - one due to the compression, and another when the spark occurs (when engine is turned on). This produces a problem - 'knocking' sound - in the engine.
Comments
No comments have yet been made