You’ll undoubtedly grumble as, in the coming months, gasoline prices creep upward again, forcing you to shell out more cash. This is a likely scenario, since refineries switch to producing “cleaner” but more expensive summer blends. Here’s a short overview that will provide you with a better understanding of the cause of your thinner wallet:

In an internal combustion engine, the piston squeezes a mixture of air and gasoline, which the spark plug then ignites. For a vehicle to run properly, its gasoline must be of a suitable grade so that the heat of compression doesn’t ignite it before the spark plug does; if it does, this causes “engine knock.”

Octane prevents engine knocking. Gasoline comes in octane ratings of 87, 88, 89 and 93 (premium). The lower the octane number, the easier it is for the fuel to burn. We advise that you use the lowest octane grade that your car can run on without it causing engine knocking, unless your vehicle’s owner’s manual specifically tells you to use premium gasoline.

To produce the octane ratings that most cars require, refineries mix various additives with gasoline to increase its octane level and make it cleaner-burning. During the winter, refineries add ethanol to reduce carbon monoxide, a particular problem in the cold. As summer approaches, refineries remove heavy metals and add detergents to create a summer mixture known as RFG (reformulated gasoline), which emits less pollution. There are more than a dozen blends of RFG used around the nation, based on the varying weather conditions and local pollution restrictions of states.

As refineries produce these summer fuels, they leave less capacity to make gas and cause temporary shortfalls and price spikes. Meanwhile, demand for gasoline rises in the summer when more people travel, further spiking the price of gasoline.