- If you go to the gas station and you see regular gas and premium gas, can you tell me the difference between the types of gas?

- Uh.

- I feel like I am being told it's slightly better for my car.

- It's the octane.

- Yes.

- But I don't know what that means.

(laughing) The chemical makeup of the gasoline is slightly different.

- I always assumed it was, premium gas was more pure.

Like it didn't have, it had less junk in it or something.

- Interesting, 'cause it actually has more junk in it.

- What?

(dynamic music) - Good?

Now I know which of these I'm supposed to use.

It is 87.

And I know that because I have read my car's user manual.

Yeah.

87.

But to figure out the difference between regular and premium gas, we actually have to get a little closer to the gas pump.

A little closer.

All right, so see here on the buttons there's something that says minimum octane rating and then an equation.

The difference between regular and premium gas is something called the octane rating.

It's usually somewhere between 87 and 94 on a typical gas pump.

So what is it?

A fuel's octane number is actually a measure of how much compression the fuel can take when mixed with air inside an engine before it self ignites and rapidly releases its energy.

To explain, let's look at a cylinder in a typical gasoline engine.

A mixture of fuel and air enters each cylinder.

A piston then compresses that mixture.

And then, at just the right moment, a spark plug ignites the compressed gases.

The force of the resulting explosion moves the piston down, which turns a crank shaft and moves your car.

The spent gases exhaust out and then the cycle starts again.

This is the normal gasoline engine combustion that we want.

But sometimes, after the spark plug starts a normal combustion process, the rest of the fuel ignites prematurely, consuming the yet to burn mixture ahead of the propagating flame.

As the mixture burns, it expands, adding even more pressure to the compression from the piston.

This fire syringe illustrates how compression can lead to self ignition.

I hate the fire syringe.

(horn blowing) All I have here is a chamber, a plunger, and a tiny piece of cotton inside.

There's no spark plug or wires.

But if I push this plunger down really quickly I can dramatically increase the pressure in the chamber.

This increases the temperature of the air and causes the cotton to ignite.

This is how diesel engine combustion starts.

The piston compression alone causes the fuel to catch on fire.

And without that spark plug or a timing belt there are fewer parts than in a gasoline engine, contributing to a diesel engine's often greater longevity.

And they're usually also more efficient than gasoline engines too.

But the gasoline engines that are more typical in consumer cars here in the United States can provide faster acceleration and higher speeds than diesel engines can.

In these engines, auto-ignition from compression before combustion has completed is bad.

It can cause strong sound waves within the cylinder creating an annoying noise we call knock, as well as big spikes in pressure that can damage your engine if they persist.

So how can you prevent that?

I'll give you a hint.

This is a video about octane.

So, it's probably that.

The gasoline your car engine is burning is a complex mixture of chemicals made when petroleum is distilled into various types of fuel, think gasoline, kerosene, diesel, et cetera.

Basically, you heat petroleum up and then feed the vapor into a big tall container with hotter temperatures at the bottom and cooler temperatures at the top.

You can separate out the different components based on the temperature at which they condense.

The resulting gasoline is composed of about 150 different hydrocarbons, including things like butane, pentane, and isopentane.

Typically, this gasoline is sub-octane gasoline, meaning it's below the octane required to work in your engine.

So manufacturers need a way to measure how much compression that gasoline can take when it reaches your engine cylinders.

And the scale they've come up with is called the octane rating.

That's the number on the button that you push to fill up your car.

It's measured on a scale of zero to 100.

As I mentioned, gasoline is composed of a lot of different molecules, but to define this scale scientists use only two.

Heptane and isooctane.

Heptane ignites at lower pressures and temperatures, and octane at higher pressures and temperatures.

So a fuel air mixture that was 100% octane would be able to handle a lot of compression, and a mixture of 0% octane and 100% heptane would ignite with relatively little compression.

We use the octane number as a handy equivalent to know whether or not the gas you're putting into your car is compatible with your engine.

So when you hit the 92 octane button at the pump that rating means that the gas that comes out can handle a similar amount of compression as a mixture that's 92% octane and 8% heptane.

And remember that little equation on the gas pump button, R + M /2?

Long story short, it just means that the octane number is an a average of how the gasoline will perform under idling low temperature conditions, and how it will perform under high speed high temperature conditions.

And back to some more stuff you might've ignored at the gas pump.

Why do some pumps advertise unleaded gasoline?

Lead doesn't naturally show up in the oil refining process that makes gasoline.

But remember, that when oil refineries distill gasoline what comes out is typically sub-octane gas, which has a lower octane than is needed in your car.

So to increase the octane rating and make it usable they have to add something to it before they could sell it to you.

And in ye old days of poor consumer regulations, manufacturers would add tetraethyl lead to gasoline to increase its octane.

And when the gasoline burned the tetraethyl lead would form lead and lead oxide particles and vapor, which slowed the gasoline auto ignition chemistry down.

So basically, it slowed down how quickly it would self ignite.

The problem, of course, is that lead is a poison.

But it was also cheap, so into the gas it went.

The EPA began phasing out lead in gas in the US in the 1970s in favor of other safer additives like ethanol.

And it was completely phased out worldwide in 2021.

Nowadays, manufacturers can also use aromatic hydrocarbons like toluene to increase the octane rating because they can take a lot of compression before they ignite.

They also have a bit of a sweet smell, which is why a lot of people like the smell of gasoline.

But please don't just go around sniffing gasoline, that's bad.

And if you've ever seen octane boosters sold at a gas station, they work by adding a bunch of inhibitors to your fuel tank that are harder to ignite, artificially raising your octane rating.

Some cars with really high compression engines do require high octane gas.

Think your fancy sports cars and some turbocharged engines.

Typically these higher compression engines have a higher power output.

But if you're just driving a regular old car and your manual tells you that you should just use regular gas with the lowest octane level, believe it.

Higher octane gas contains a little bit more energy with its higher aromatic content, but it's only a percent or so, and totally not worth the extra cost.

And I feel like we've also left out the final remaining elephant at the gas pump.

What the heck is mid-grade gas?

Some cars take regular 87 octane gas and some take premium 91 octane gas.

And yeah, some cars and motorcycles do take mid-grade 89 octane gas.

But only 10 car models sold in 2018 required mid-grade.

It's pretty rare.

And it's usually just a blend of the regular and premium gas.

It's like a soft serve machine.

There's a tank under the gas station for regular and a tank under the gas station for premium.

And if you hit that middle button you're just getting a blend of the two.

Like a chocolate vanilla swirl cone.

Of course the gas coming outta the gas pump would be much better blended, but a little less tasty.

So the best thing that you can do for your car is just read the manual.

Put in regular if it asks for regular, and premium if it asks for premium.

And don't waste your money buying the good stuff if your car isn't built to handle it.

I promise, you're not helping.

You're just literally lighting money on fire.

Just like pouring gasoline on money and lighting it on fire.