What is "knock"?
Options:
- When the fuel in a combustion chamber ignites due to pressure before the flame can reach it
- When a car lurches forward when the clutch is released too quickly
- The term that auto technicians use for "backfire"
- A unit of measurement used to describe wheel alignment
Explanation: Knock – also sometimes called "pinging" or "detonation" – occurs when fuel in the combustion chamber of a gasoline-powered engine ignites prematurely due to pressure. This can cause anything from a harmless, barely audible noise to complete engine destruction. High octane gas can reduce the probability of "knocking". Knocking is never a problem in diesel engines because diesel fuel is designed to ignite without a spark, due solely to pressure, in the first place.