What Causes Knocking Sounds When Accelerating the Engine?
2 Answers
Knocking sounds when accelerating the engine may be caused by carbon buildup in the starter motor's internal carbon brushes. Cleaning the carbon deposits should resolve the issue. Introduction to Carbon Brushes: Carbon brushes, also known as electric brushes, are sliding contact devices widely used in many electrical equipment. The materials used in carbon brushes mainly include graphite, impregnated graphite, and metal (containing copper or silver) graphite. How Carbon Brushes Work: Carbon brushes are components that transfer energy or signals between the stationary and rotating parts of motors, generators, or other rotating machinery. They are typically made of pure carbon with a binder, shaped into blocks that fit into metal holders. A spring inside presses the brush against the rotating shaft. When the motor operates, the brush delivers electrical energy to the coils via the commutator. Since the primary component is carbon, it is called a carbon brush. These brushes are prone to wear and require regular maintenance, replacement, and carbon deposit cleaning.
With over a decade of car repair experience, I often encounter this issue of engine knocking when accelerating. This is typically detonation, where the gasoline in the cylinder ignites spontaneously before the spark plug fires. Excessive carbon buildup increasing the compression ratio is the most common cause, especially in older cars frequently driven at low speeds. Using low-octane fuel or having ignition timing too advanced can also lead to this—last time, a turbocharged car running on 92-octane gas even cracked a piston. A faulty knock sensor is even more troublesome, as it's supposed to automatically adjust ignition timing. This problem really shouldn't be ignored: at best, it can melt spark plugs; at worst, it can punch holes in pistons. The most reliable approach is to read the trouble codes immediately—don't just let the repair shop talk you into carbon cleaning.
