Blog: More on octane and knock

By Susanne Retka Schill | September 19, 2016

My blog last week was an attempt to give a simplified, plain English description of ethanol’s Btu rating, and its compensating qualities that make up for a lot of that lower energy content. I got a complimentary email from Steve VanderGriend that means a lot, since as technical manager fuel and engine technology at ICM, he’s put a lot of effort into understanding ethanol’s fuel characteristics. He expanded on the octane and knock discussion.

“For the most part, Dan is correct about pushing octane,” Steve wrote,  “though I will say, higher horsepower and or torque is much different than talking higher mileage benefits.  I also find the most don’t understand that, yes, ethanol burns cooler in your car or in these race engines, but this is due to adjusting the fuel flow.  Ethanol blends like E15 and higher will burn hotter in a lawn mower.” 

Knock, he adds, isn’t an explosion. From a technical view, he says, “knock is actually a sonic shock wave when two or more flame fronts collide with each other.  This occurs above the speed of sound and causes the damage. Increased combustion temperature, which occurs mostly by increased cylinder pressure, creates the opportunity for knock if the octane of the fuel isn’t high enough.  Lower octane fuels, though, don’t create increased temperature.” 

It’s interesting to see how many ways knock gets explained, he says. What we can hear is generally hard knock. Sensors, data loggers and computers help with detecting light knock.

There also was an interesting comment made last week about EPA’s lackluster support for ethanol, which would logically be the first-order solution to reducing emissions and carbon footprint – the infrastructure, heritage vehicles and cost to car owners would make for an effortless transition.