115 octane?

[Maelfyn]

I have read that 87 octane gas is 87% octane and 13% heptane. So what the hell does 115 octane (for jets) mean? 115% makes no sense = l

[vvtlikick]

Octane number only tells you the knock restistance of the fuel. Not the composition.

By definition, pure n-heptane(C_{<small>7</small>}H_{<small>16</small>}) is 0 Octane. By definition, pure Octane (C_{<small>8</small>}H_{<small>18</small>}) is 100 Octane.

However, gasoline that's 100 octane isn't pure Octane, it just happens to have the same knock resistance. 115 octane would have a higher knock resistance than pure octane (I'm not sure if the scale is linear, so it might not be 15% higher)

[Slant]

What do you mean by knock resistance? I never understood when people say "the engine was knocking..."

[vvtlikick]

Originally posted by Slant
What do you mean by knock resistance? I never understood when people say "the engine was knocking..." The way I describe the difference between knock/pinging/detonation and normal combustion is "knock is when the air/fuel mixture explodes, while in normal combustion the mixture merely burns". This is probably too much of a oversimplification, but it works for me :D.

The Bosch Automotive Handbook 4th Ed. states:Although higher compression ratios provide increased part-load efficiency, they also increase the chance of knock (preignition) under full load. Combustion knock occurs when flame propagation rates approach the speed of sound. It occurs primarily toward the end of the combustion process when the end gas is already highly compressed and at high temperature. Preignition is characterized by extreme pressure peaks not present in normal combustion...

[VZV21]

Joe, do a search on this site. There was a while back, and interesting thread about octane numbers, which involved a British chemist and someone else (they hit below the belt many times though)..