Blending (vapor pressure)
To better understand the need for gasoline blending, it helps to understand the basic workings of your car's engine:
- Gasoline vapor mixes with air in the cylinder chamber.
- The cylinder compresses the air/gasoline mixture.
- The spark plug fires when the cylinder achieves maximum compression, which ignites air/gasoline mixture.
- The force of the combustion pushes the cylinder down, which turns the drive shaft, generating useable power.
- The byproducts of combustion are let out of the chamber when the cylinder is in its down cycle.
- Fresh gasoline vapor enters the chamber on the up cycle and the process repeats.
Vapor pressure - If you leave a dish of water out at room temperature, it will eventually evaporate - turn from liquid to vapor - even though the water never approaches its boiling point. Just how quickly the water evaporates depends on the air pressure in the room. At high altitudes, where the air is thinner, the air pressure is lower, so the water will evaporate faster than it would at the same temperature in a room at sea level.
Gasoline also evaporates, and your car’s engine relies on a controlled amount of vapor flowing into the combustion chamber in order to start and run efficiently. Vapor pressure - measured in terms of RVP - determines the amount of surface pressure required to keep gasoline from changing from liquid to gas. If the RVP is too low, there won't be enough hydrocarbon vapor available to start a cold engine. If the RVP is too high, too much of the gasoline will evaporate before reaching the engine, causing vapor lock.
Because the gasoline recovered from the distilling, hydrocracking, catalytic cracking and reforming processes have distinct RVPs, the RVP of the combined gasoline will depend on the make-up of the mixture.
During the winter, even the highest-RVP gasoline won't meet the higher-RVP demands of your car’s engine. That's why refineries add butane, which has a very high RVP, to raise the RVP of the blended gasoline to the desired level.