What does it mean?
For the engine to generate power most efficiently, depending on engine speed and load and driver demand, it relies on the precisely-timed delivery of fuel and spark for a given amount of air. The engine control module (ECM) constantly monitors a number of sensors, using this data to modify, amongst other things, injector pulse timing and width and ignition spark timing.
Generally speaking, the best time to ignite the air-fuel mixture is so that it generates the most pressure just as the crankshaft reaches a few degrees after top dead center (ATDC) for that cylinder, 14° ATDC for most engines.
Because it actually takes a few hundred milliseconds for the entire air-fuel charge to ignite, the ignition spark needs to take place at some point before the cylinder reaches top dead center (BTDC), up to 30° BTDC for an engine at idle, and up to 45° BTDC for an engine while cruising.
Ignition spark timing advance, however, is limited by a combustion phenomenon called “knocking,” also referred to as “pinging” or “spark knock.” Knock is caused by pre-ignition, in which the fuel is ignited by a hot-spot in the cylinder instead of by the spark plug. The result is two flames, one started earlier by the hot-spot, the other started later by the spark plug.
When the two flame fronts collide, they cause a sudden spike in cylinder pressure, resulting in a sound which some describe as a bouncing marble or ball bearing. The effects of knocking range from an inconsequential pinging sound to total destruction of head gaskets, pistons, rings, connecting rods, bearings, or valves. To be clear, for the purposes of this discussion, this article refers to knocking due to abnormal combustion, specifically pre-ignition.
This article is not referring to engine knocking due to mechanical problems, such as worn crankshaft connecting rod bearings or excessive valve lash, for example. The knock control system, an ECM sub-routine, uses “knock sensors” to detect the specific frequency generated by engine knock. Usually, knock sensors are attached to a solid part of the engine block, but some may be connected to the cylinder head, where they can sense vibrations in the engine.
A straight engine, i4 or i6, will have one sensor, while “V” engines, V6 or V8, will have one sensor on each bank. Generally, if the ECM detects that the engine is knocking, it will “retard” ignition spark timing, that is, fire the spark plugs later – 25° instead of 28° BTDC, for example. For a given engine speed and load, the ECM will adjust spark timing to keep it just outside of the knock threshold, or the range in which the engine knocks.