Of Gas Stations and Octane Numbers

There are so many things in the world that we just pass over without giving it a second thought. A small walk in the woods could introduce you to new smells you’ve never smelled before; a boat-ride could reveal completely new animals. Even a flight through the clouds could introduce you to the myriad of cotton balls in the sky. All that you need is a little bit of curiosity; a little bit of time set aside everyday to research on the topic, and a little bit of ingenuity to apply it to you everyday life. Something similar happened to me one day, when I took a drive to the gas station and came across three numbers. They kind of looked like this:

Three numbers on the gas station pump

Hmm. I wondered what those numbers mean. I am sure it had something to do with the grade of gasoline I usually put into my car. The bigger number probably makes it more expensive. But this lead to bigger questions: What do they mean? How do they apply to my car? Is bigger the better? Curious, I went online as always. These numbers are the octane ratings as is described in the bigger picture itself. An octane number is a measure of the compressibility of the gasoline fluid, specifically how much the fluid can be compressed until it detonates. Generally speaking, the bigger number implies a higher compressibility and thus makes the energy conversion more efficient; but it doesn’t necessarily lead to better car performance, better energy savings, better fuel economy, or make your car go faster. I cover some of this topics in more detail in this blogpost. The starting point for this discussion would be the way an engine works in a car. It usually consists of a piston, a chamber through which the piston moves back and forth, a sparkplug for providing the initial spark, entry points for fuel and air, and an exit for the combustion exhaust. It takes four-strokes of the crankshaft to clear one cycle of the combustion, as demonstrated in the .

How a 4 stroke engine works. (Click picture to open video on new tab)

The piston pulls in air and fuel when it is lowered and compresses it on its way up. The compression act brings the mixture to its ignition temperature, and a small spark through the spark plug ignites the mixture and pushes the piston back down. On its way back up, the piston pushes the exhaust out while clearing out space for the next air-fuel set. The piston then repeats the cycle again and again. A high compressible fluid (e.g. with an octane rating of 93) does two things: it allows a smaller volume of fluid to reach the ignition temperature of the mixture and allows more space for the piston to slide, thus resulting in higher mechanical energy conversion. A fluid of lower compressibility might not get completely ignited at the same compression. This would result in a buildup of fluid in the chamber over time which might ignite at some other point of time in the four stroke cycle, leading to the phenomenon called knocking, which could potentially damage the engine. It seems apparent that the extent of compression of the fuel plays an important part in the entire process.

On the left, a normal combustion process ignites at the spark plug, and that to when the piston is closed on the fuel-air mixture. On the right, knocking leads to the fluid ignite at several spots when the piston is not fully closed. A good fuel choice can lead to more efficient combustion. Most cars today come with a lot of automation which can control spark plug timing to minimize the knocking effect with just some loss in fuel economy and vehicle performance. So is it worth paying extra to recover the extra performance? Probably not, several websites say. FuelEconomy.gov, for example, states that for vehicles which carry heavy loads (a tow truck, or boat for instance) could benefit slightly in terms of fuel economy, combustion efficiency and performance. But for most cars… it really doesn't seem to be worth it. Unless the manufacturers recommend otherwise, normal grade gasoline (e.g. 87) is probably okay. This has generally been what auto-experts recommended for a while now. In conclusion, being a bit more curious and inquisitive can definitely be more helpful, but having better octane gas probably isn't necessary. Unless you drive either of these cars: