What Does an Octane Rating Indicate?

gas pumps


The octane rating varies between gasoline grades.

An octane rating is something you notice at the gas pump. But what does it mean?

Simply put, the octane rating shows how difficult it is for said gas to ignite from temperature and pressure. The higher the octane rating, the more challenging it is for it to ignite. Indeed, you may realize that an engine can become quite hot, hotter than what your temperature dial indicates. Furthermore, it can also reach very high pressures. For these reasons, certain cars require a specific octane rating.

Octane Rating

Now, what are these octane ratings, why do they matter, and what do they prevent precisely?

Octane ratings are measures of fuel stability. Specifically, these ratings are based on the pressure at which fuel will self-ignite, such as under low volume, high pressure, and high temperature. The higher the rating, the more resistant to self-ignition it is.

An engine works by moving a piston up and down, following certain strokes: intake, compression, ignition/expansion, and exhaust. The ignition is variable, based on RPM and engine load, but it should be in the piston’s top dead center.

This is highly variable, allowing the electronic control unit (ECU) to ignite said fuel before the top dead center, or even after the top dead center. Thermodynamics and physics say that when volume decreases, pressure increases, and when pressure increases, temperatures increase.

Engine Knock

A piston’s top dead center is the time where the piston has the smallest amount of volume. As such, it also boasts its largest temperature and pressure. What happens when both are large enough that the fuel ignites before the spark is fired? Well, that’s what people know as engine knock.

For example, let’s assume that the crankshaft is spinning clockwise and that the top dead center of the piston is a mark that indicates 0-degrees of crankshaft rotation. If the spark happens before that top dead center, they are considered positive degrees, such as plus 10 degrees. Everything else after those are considered negative degrees, like -10 degrees. We want the spark to fire at that exact 0-degrees of rotation.

But what if the piston is hot enough that it ignites the fuel at plus-40 degrees of crankshaft rotation? Well, the explosion pushes the piston down, but the piston wants to ove upwards due to inertia and work. Moreover, it wants to compress the air properly in its compression stroke.

In this case, when the piston still moves upwards, the explosion happens, which pushes the piston down. Crankshaft degrees of rotation is negative. As such it will want to spin the engine oppositely. The engines spin in one way, clockwise in this case. Oppositely spinning an engine with an explosion will result in a large shock which can and will damage the engine, along with a loss of power.

ECU Tuning

To prevent this, car manufacturers tune the ECU and create injection and ignition maps. These maps tell the engine when to fire the spark, so knock doesn’t happen. They choose this timing based on the octane rating.

Notably, a higher octane allows engineers to pick more strict degrees of crankshaft rotation, like plus-2 degrees at high RPMs. A lower octane is followed by a looser timing of crankshaft rotation, such as plus-15 degrees. Therefore, “engine timing” is a concern, and what “retarded timing” and “advanced timing” mean.

Advanced timing indicates that the spark fires before its top dead center, while retarded timing indicates that the spark fires after its top dead center.

If engineers pick higher octane ratings, then they can use the engine’s mechanical work better. For example, an engine produces work for just 180-degrees of crankshaft rotation. Remember that a 4-stroke engine needs 720-degrees of crankshaft rotation for it to finish its stroke order. Thus, if the ignition happens after those 180-degrees of crankshaft rotation is done, that retarded timing uses up some of the engine’s torque as it has lost some of its stroke lengths.

Knock Sensor

Some engines can alter their ignition timing, while most others cannot. For example, if the engine has a knock sensor, it can tell when the engine is starting to knock. Consequently, the ECU has all the information needed to know what gasoline was used. If it starts to knock later, then better gasoline is used, changing the ignition timing and producing more power in the process.

If you have no such features or sensors, then the ECU can’t know what gas you’re using. Knowing this, the engineers will pick a single octane rating and create an ignition timing map based on that octane alone.

Choose the Correct Octane

Importantly, if you fuel the car with a higher octane than the car was designed, you are wasting money. For this reason, the regular octane rating in the US is 87. Most engineers tune cars to run on 87 octane. Again, using gasoline with a higher octane rating doesn’t do anything.

Some cars use an 89 octane rating, mostly sports cars and vehicles with a little bit of power, or manufacturers who simply prefer to use that for the most part such as Ford, or the Dodge Charger/Challenger series.

As for premium gasoline, it has a 91/92/93 octane rating, which is used for sporty cars, like supercars. Racing fuels used in Formula 1 and Indycar have over 100 octane ratings, with fuel ratings of 112 octane or higher. Also, the Dodge Demon can utilize a 100 octane rating gasoline to achieve maximum performance. In this situation, the higher octane is preferred to extract all the necessary power, while the ignition timing has to be at the top dead center exclusively.

Lastly, ongoing engine knock can be catastrophic, damaging pistons and the cylinder wall.


References

  1. U.S. Energy Information Administration. Octane in Depth. Gasoline Explained. [Online] https://www.eia.gov/energyexplained/gasoline/octane-in-depth.php.
  2. ChemistryGod. Graphs of Gay-Lussac’s Law. chemistrygod.com. [Online] https://chemistrygod.com/gay-lussac-law-graph.
  3. ScienceDirect. Ignition Timing. [Online] https://www.sciencedirect.com/topics/engineering/ignition-timing.
  4. Motorsport Electronics. Tuning of Ignition Timing. [Online] https://motorsport-electronics.co.uk/onlinehelp/html/4100.html .

Image Attribution

Inset photo by OpenClipart-Vectors from Pixabay

Main photo by Gustavo Fring from Pexels

See AlsoFuel Types and Your Car

Author: Cristian Puscasu
Cristian lives in Romania and is studying to become an automotive engineer. Currently in his third year, he enjoys writing about cars as well as tinkering with them. We are pleased to welcome him as a contributing writer, knowing that he will pursue far greater things in the years to come.

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