The blend changes daily, depending on refinery conditions, so that the gasoline has a constant octane number. Manufacturers also blend gasolines to make engines run better at various altitudes, in different climates, and during different seasons. For example, in summer or in hot climates, gasoline refiners prepare blends containing few hydrocarbons that boil at low temperatures. The heat turns such hydrocarbons into a vapor. If the gasoline contained too many of these hydrocarbons, the resulting vapor could cause vapor lock, an interruption in the flow of gasoline to the engine.
Gasolines contain a number of additives.
One important group of additives are special oxygen-containing chemicals called oxygenates. These chemicals have high octane numbers and may make up as much as 10 percent of a gasoline. Gasolines also contain smaller amounts of a number of other additives. Antioxidants keep the gasoline from becoming gummy. Anti-icers prevent ice from clogging gas lines in winter.