Pressure and the price of gas


in Geek stuff, Science, The environment

The tendency of gasoline to increase in price during the summer is well known. Partly, this reflects increased demand (which leads to an increased quantity sold at an increased price, given a particular supply curve). Partly, this is the consequence of how summer gasoline is a different blend of hydrocarbons. The reason for this is the need to prevent too much pressure from building up inside gas tanks as more of the liquid turns to vapour in the summer heat. This is standardized in terms of Reid vapour pressure (RVP): the pressure of any particular gasoline blend at 100°F (37.8°C) expressed in kilopascals, calibrated to a standard atmospheric pressure of 101.3 kPa.

RVP is used to specify which blends of gasoline are acceptable for sale at different ambiant temperatures. Gasoline with an RVP of over 14.7 will fairly easily pressurize gas tanks and gas cans in summer heat. It will also boil if left in open containers. As such, regulations require summer gasoline to contain less butane than the winter sort. This is on account of how butane is relatively inexpensive (making companies want to include more of it), but is also the most active contributor to vapour pressure. As such, the butane content of summer gasoline must be very low – one factor behind the higher price.

I learned all this from R-Squared, an energy blog that seems to be commonly cited. The blog makes one other important point: anyone considering storing cheap winter gasoline for use in the summer should consider the dangers of having the butane therein turn to vapour and start pressurizing the container in which it has been stored.

{ 2 comments… read them below or add one }

Anon September 21, 2007 at 9:41 am

In terms of the supply and demand argument, isn’t it relevant that this happens every year? Since it is predictable, it seems inevitable that self-interested firms and individuals will try to manipulate it to their advantage, if possible. Firms seem a lot more likely to actually be able to do so.

. April 16, 2008 at 12:32 pm

It’s Time for Summer Gasoline

By Robert Rapier on summer gasoline

Another misconception that some have is that they can save money by buying cheap gas in the winter and storing it for the summer. Remember that winter gasoline will pressure up as the weather heats up, and the contained butane will start to vaporize out of the mix. You will end up with less gasoline than you paid for, and you will be contributing to the air pollution problem that summer gasoline was designed to avoid. If, on the other hand, you were to buy summer gasoline and try to store it until winter, you might find yourself having problems getting the fuel to ignite, due to the lower vapor pressure. This would be like putting a little bit of diesel in your gasoline – not very good for your car. So buy and use gasoline in the correct season.

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