![]() ![]() Your engine uses intake valves to feed clean air into the cylinders and exhaust valves to kick spent combustion gases out. ![]() Say your engine is ticking like a time bomb, especially in the morning when it’s cold. Though it saves your engine from destroying itself, performance and fuel economy can suffer. The resulting shock wave rattles the piston inside the cylinder, creating the knock, ping or can-of-marbles sound you hear.ĭetonation has the same effect, except it occurs after the plug fires.Ĭomputers in modern vehicles can detect engine knock and compensate by adjusting engine timing. In certain conditions, they can clash with the upward-moving piston. Then, when the plug does fire a fraction of a second later, the two flame fronts collide. Or, chunks of carbon can heat up and create a hot spot that effectively ignites the fuel/air before the plug fires. ![]() Sometimes low-octane fuel is to blame sometimes it’s deposits on the piston crown.įuel with too low an octane rating for your engine can sporadically ignite prior to the piston reaching TDC. Under certain conditions, the fuel/air can spontaneously ignite too early in the combustion cycle. Pre-ignition (and its cousin, low-speed pre-ignition ) are abnormal combustion events that throw off this precise balance. This careful timing ensures the downward force of the exploding fuel/air mixture works in tandem with downward piston momentum, resulting in optimum efficiency and power. In a properly running engine, spark-triggered ignition typically occurs a few degrees before the piston reaches top dead center (TDC). This is likely due to either pre-ignition or detonation. They’re effectively the same phenomenon, but they occur at different times. When you mash the accelerator out of the parking lot, that’s when you hear an engine knocking sound. Say the clock has struck 5:00 and you make a bee-line to your truck and take off for home. Engine knock typically occurs during low-speed, high-torque conditions, like when you’re accelerating. ![]()
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