Tag Archives: naca

Fuel octane myths

Higher octane fuel will give me more power!

This is the big one.  Unfortunately, higher octane fuels do not generally give you more power.  If only it were so easy.  A higher octane fuel has almost exactly the same energy as a lower octane fuel, and if all goes well, it burns identically.

There are exceptions to this in the automotive world — quite a few modern cars run knock sensors, and with a lower octane fuel, the computer will be forced to retard timing to prevent knock, which will typically cause power to fall off.  HOWEVER, this does not mean that a higher octane fuel will bring more power, only that running the grade of fuel the engine was designed for will provide optimum power on a car equipped with a knock sensor.  Once you reach a point where octane is sufficient such that the computer doesn’t have to pull timing, there are no more gains to be had.

Higher octane fuels burn hotter.

One common myth concerns the ignition/burn temperature of higher octane fuels.  Many people, including quite a few who should know better by now, continue to perpetuate the myth that higher octane fuels require more energy to ignite, or burn at a different temperature or rate (usually “more slowly”).  This is simply not true.  Though burn rate can vary between fuels with all other things being equal, this is not linked to the fuel’s octane rating, and other variables like mixture quality and distribution have a MUCH greater effect on burn rate.
Octane is also not directly a measure of the amount of compression it takes to initiate burn (preignition or detonation).  Although they are often connected, knock is not necessarily directly linked to preignition and detonation.  Knock, specifically, is a violent resonance of the gases in the combustion chamber, causing severe spikes in pressure and temperature, and usually audible from outside the engine.

It is possible for detonation (spontaneous ignition and simulaneous burn of the entire air-fuel mix ahead of the flame front created by the spark plug) or preignition (spontaneous ignition of pockets of flame ahead of the main flame front, caused by heat and pressure, which then burns at a normal rate) to occur without knock, and it is also possible for knock to occur without preignition or detonation.

This phenomenon was studied extensively in the 40’s via high speed camera by NACA (predecessor to NASA) when they were developing many of the more sophisticated late WWII era piston aircraft engines.  However, with the massive shift in research from piston to turbine engines at the end of the war, a lot of the NACA research was filed away and forgotten.  Their information, which is now freely available from NASA over the web, is a treasure trove of information on detonation, supercharging (including turbocharging, which is simply a form of supercharging), fuels, water injection, and even things as odd as using nozzles on jetted exhaust pipes to gain thrust (don’t get too excited, it doesn’t benefit much below about 250mph, and is useful mostly because propellers start to lose efficiency at higher speeds).

For more information, the NACA article covering knock and detonation can be found on the NASA Technical Reports server.  Search for NACA-TR-912 and enjoy!