Combustion Chamber Deposit Interference Effects in Late Model Vehicles

Certain late model vehicles have experienced an audible clattering during cold engine-start that is caused by the piston top striking the cylinder head after combustion chamber deposits build up to a level that exceeds the available clearance between the piston and the head. This clattering is referred to variously as “carbon knock,” “carbon rap,” “deposit induced noise,” and “combustion chamber deposit interference (CCDI).” The audible clattering intensity increases with mileage as deposits continue to accumulate.

An extensive vehicle fleet test program used vibration intensity to quantify the individual impact events that cause the clattering noise. This quantification was used to examine the effects of base fuel, various deposit control additive packages, engine operating conditions, and engine mechanical clearances. The research effort described here involved the testing of 20 vehicles of 5 different makes and accumulated over 900,000 road test kilometers.