In the United States at least there are 3 grades of unleaded gasoline at most pumps. Does it really matter what grade of gas you use? Can I use the lowest grade one week and the next week get premium if I can afford it? Does it help with milage or does it keep your engine clean? What is the difference?
In: Engineering
Higher octane gas can withstand more compression in the cylinder than lower octane gas. The higher the compression ratio, the higher octane you need to avoid premature combustion of the fuel/air mixture, also known as spark knocking.
A compression ratio is the measure of how much air is squeezed to the top of your cylinder during the Squeeze stroke of a 4 stroke engine: Suck, Squeeze, Bang, Blow. The ratio is measured by comparing the amount of air in an unpressurized cylinder, 1, to the amount of air in a pressurized cylinder, usually 8-12. Engines designed during the days of leaded gas require the highest octane at the pumps nowadays because the tetraethyl lead in the gas greatly improved the gasoline’s stability, allowing for engines to run compression as high 16:1. As an aside, adding a turbo or supercharger to your engine, or building something like a [Mini-me](http://www.crx.honda-perf.org/articles/moreminime/moreminime.html) increases compression and can sometimes require premium gas, racing fuel with an octane rating of 100+, or an octane booster additive to avoid spark knocking.
Aside from being required in higher compression engines, premium fuel usual contains lots of additives like detergents to clean carbon build up in your engine, oxygenates to allow cleaner burning, antioxidants to add stability to the fuel, and anti-knock additives, previously including lead compounds. Racing fuel companies tend to treat their blend recipe like Coke and their secret formula, and for good reason. Different blends offer different performance, and racing blends run above $40 a gallon. It even smells different than normal gas.
To expand upon octane ratings, a common misconception is that higher octane gas, the premium stuff, will increase performance. To an extent this is true, due to additives in the gas, but not due to octane. Octane is a measurement of how much compression the fuel can take before it ignites, and compression is generally a determining factor on how much power an engine makes. Higher octane gas does not affect compression, and therefore does not affect power output. Putting jet fuel in a Miata doesn’t change the fact that it’s still a 1.8L Miata.
On the coal rolling side of things, diesel has a cetane rating that tells you how fast the fuel combusts, rather than how much compression it can take. Compression in a diesel engine can average from 14:1 to 23:1; the heat from the extremely compressed air ignites the diesel and produces the Bang, rather than a spark from a sparkplug. Higher cetane ratings mean the diesel combusts faster; a higher cetane is better for faster diesel engines, like those in road cars.
Huge shoutout to [Donut Media](https://youtube.com/c/DonutMediaTV), they make learning about cars and the ins and outs of engine much more entertaining than your dad yelling at you about your flashlight position. Would highly recommend to anyone that wants to learn something new or just want something entertaining to watch.
Latest Answers