[L1] 1 Overview [L2] 1) Definition of Knocking [L4] - A phenomenon accompanied by noise like a knocking sound occurring in an abnormal state or specific condition of the engine. [L4] - Knocking causes damage to the engine, creating temporary power loss or permanent degradation of normal engine performance. [L2] 2) Occurrence of Knocking [L4] - Knocking occurs in both gasoline and diesel engines, but the causes and effects are different. [L5] * Gasoline: Occurs when ignition timing is too early. [L5] * Diesel: Occurs when ignition timing is too late (ignition delay). [L4] - The frequency of knocking has decreased due to the development of engine technology, but it occurs during improper engine maintenance or abnormal situations. [L1] 2 Knocking in Gasoline Engines [L2] 1) Phenomenon [L4] - A phenomenon where abnormal loud noise and vibration occur when the compression ratio is high or the engine is overheated. [L4] - Black smoke and sparks are discharged through the exhaust pipe, and output decreases. [L2] 2) Effect [L4] - When knocking occurs, the temperature in the cylinder area rises abnormally, causing damage or deformation to the piston head and exhaust valve due to overheating. [L4] - Reduction in mean effective pressure, loss of effective work (pre-ignition occurs). [L4] - Rise in combustion chamber temperature and drop in exhaust temperature. [L2] 3) Cause [L4] - When intake air temperature and pressure are high. [L4] - When the cylinder is overheated. [L4] - When ignition timing is too early. [L2] 4) Prevention Measures [L4] - Use fuel with high ignition delay (High Octane Number). [L4] - Improve flame propagation speed. [L4] - Make the fuel mixture lean, around a ratio of 12.5. [L1] 3 Knocking in Diesel Engines [L2] 1) Phenomenon [L4] - In the case of diesel, combustion should occur in multiple stages. However, if it burns all at once in the latter half after injection, pressure and temperature rise rapidly due to massive heat generation, causing a loud noise due to rapid shock waves inside the cylinder. [L2] 2) Effect [L4] - Impossible to form effective pressure due to rapid combustion in the latter half. [L4] - Deformation and damage of parts occur due to rapid temperature rise inside the cylinder. [L2] 3) Cause [L4] - Problems with fuel ignitability. [L4] - Drop in pressure inside the cylinder. [L4] - Low air temperature. [L4] - Poor spray condition. [L4] - Poor atomization of fuel. [L2] 4) Prevention Measures [L4] - Increase the compression ratio. [L4] - Promote ignition (Raise Cetane Number). [L4] - Retard injection timing. [L4] - Use a supercharger. [L4] - Increase intake, coolant, and combustion chamber temperatures. [L4] - Increasing RPM brings it closer to adiabatic conditions, increasing air flow and reducing delay time. [L1] 4 Difference Between Diesel and Gasoline Knocking [L2] 1) Difference in Causes Based on Ignition Method [L4] - Gasoline knocking is caused by the formation of abnormal pressure due to ignition occurring before Top Dead Center (TDC). Diesel knocking occurs because ignition does not happen until the situation reaches from TDC to Bottom Dead Center (BDC). In other words, the conditions for knocking occurrence in gasoline and diesel are opposite. [L4] - Gasoline knocks if the fuel catches fire too easily; Diesel knocks if the fuel does not catch fire easily. [L4] - Gasoline knocks if the temperature and pressure of the mixture are high; Diesel knocks if the temperature and pressure of the compressed air are low. [L2] 2) Difference in Countermeasures [L4] - Gasoline uses fuel with low flammability properties (High Octane). [L4] - Diesel uses fuel with high ignitability properties (High Cetane). [L5] * Diesel can prevent knocking by increasing the pressure of compressed air to promote self-ignition. [c]