I saw this post on fuel octane on LifeHacker. I work at Ford Motor Company as an Advanced Engine Development Engineer and I’d like to clear some things up, since there is some inaccurate advice on that post,

My views/comments are my own and do not necessarily represent those of my employer (We have these monthly online training sessions, and one of them told me to put this if I identify who I am, so hopefully I don’t get in trouble...)

First Some Background on how Octane is determined:

Octane on the fuel pump is known as Anti Knock Index or AKI, and is the average of a fuels Research Octane Number and Motor Octane Number [(R+M)/2]. The MON and RON are determined by using a special engine with variable compression ratio, and comparing the knocking qualities of a given fuel to that of reference fuels, isooctane and n-heptane. MON and RON have different engine operating conditions, like engine speed, intake temperature, etc. The slope created by these two numbers are important to us engineers in understanding a fuel’s knocking quality in various engine conditions (like boosted, vs non boosted). To the average Joe, the average of these two numbers (what you see on the pump) is what matters.

Next, some background on what knock actually is, and what Octane does to prevent it:

In a spark ignited engine, combustion does not occur instantly at all points in the combustion chamber. Combustion starts at the spark plug and propagates outward roughly spherically. Flame speed is determined by many factors, such as tumble, swirl, piston design, etc. The biggest impact on flame speed is engine speed, so as the engine spins faster, combustion occurs faster. (This is very convenient in terms of designing ignition advance and valve timing)

So, what is knock? Well, as the flame front is propagating across the combustion chamber, the unburnt portion, or end gas, experiences and increase in pressure and temperature. If the increase is high enough, the end gas will spontaneously combust. This spontaneous combustion is very fast and rather violent, so it causes the engine structure to resonate. The resonance is what we hear as knock.

What does Octane do? An increase in octane rating of a fuel represents an increase in the fuel’s auto ignition temperature and pressure. It does not change combustion speed or anything like that. So, if the fuel has a higher auto-ignition pressure and temperature, it takes higher pressure and temperature for knock to occur. The actual mechanics of how this works is complicated chemistry, and this post is long enough already...

Now some background on knock sensors.

A knock sensor is a piezoelectric device. When the piezo crystal inside experiences a force, it generates a voltage. That voltage is then measured by the PCM. The important part of this signal is the frequency of the signal. All engine components create vibration, so the PCM must analyze the signal, looking for specific frequencies associated with engine knock. The PCM performs an FFT (Fast Fourier Transform), or some other proprietary algorithm, to translate the signal from time domain to frequency domain. The results of this FFT are then analyzed to determine if the engine is knocking. If it is determined the engine is knocking, timing is reduced to prevent knocking. This is computationally intensive, so it is difficult to do for transients, like tip-ins, tip-outs, engine shifts, etc. Also, if an engine component creates frequencies near the knocking frequency, it can be difficult to distinguish. So, knock detection is mathematically difficult and computationally intensive, so it can be difficult to do in all circumstances.

So, what happens if you put a higher octane than your car requires?

In short, nothing. Absolutely nothing (except waste money). If the engine calls for 87 octane, then the timing is set for 87 octane. If you put in higher octane, say 93, there is no cause for knock concerns, since it is designed for 87 octane.