They are pretty cool.  This post was driven (pun intended) by the need for determining whether a 2003 BMW 325i can be safely driven with regular gas rather than higher octane gas.

The compression ratio of the bmw is 9:1.  This ratio tells us the volume of the combustion chamber at its largest capacity to its smallest capacity.  Essentially, the larger the ratio the more heat we use to actually do work the better.  However, this requires higher octane gas in order for knocking (the gas detonating) not to occur.  Basically higher octane gas has a higher activation level (meaning it can stand a much higher temp/pressure before it combusts).  So, when a lower octane gas is put into a higher compression ratio engine the fuel will self detonate (knock) before the piston compresses the whole way possibly causing damage to the piston.  When a knock sensor detects knocking it adjusts when it ignites the fuel so that knocking won’t occur.  However, fuel will be wasted.

Knock sensors are required on vehicles after 1996.  So, it seems like it is ok to put lower octane gas into the bmw.  Maybe keep an eye on the mpg and compare it to what the manual says it should be getting.

 

My mom taught all this to me when I was homeschooled!  Wish I had a better memory.

http://upload.wikimedia.org/wikipedia/commons/d/dc/4StrokeEngine_Ortho_3D_Small.gif