Combustion knock happens when pockets of unburned gas ignite spontaneously before the spark plug’s flame front reaches them, creating destructive pressure spikes.

• Low Methane Number (MN). The Methane Number measures a gaseous fuel’s resistance to knock, similar to octane in gasoline. Heavy hydrocarbons (ethane, propane, butane) or hydrogen lower the MN and cause rapid detonation, while fluctuating biogas compositions can drop the MN below the engine’s safe threshold (typically 75–85 MN).
• High Intake Manifold Temperature. Elevated charge air temperatures—often caused by fouled intercoolers, low coolant levels, or high ambient temperatures—push the air-fuel mixture closer to its auto-ignition point.
• Incorrect Air-Fuel Ratio (AFR). In lean-burn engines, a mixture that is too rich burns faster and hotter, increasing knock risk. Conversely, in stoichiometric (λ = 1) engines, operating near full load leaves very little margin before detonation occurs.
• Over-Advanced Ignition Timing. Firing the spark plug too early raises peak cylinder pressure and temperature, triggering end-gas detonation.
• In-Cylinder Hot Spots. Ash deposits from lube oil, glowing carbon buildup, or incorrect spark plug heat ranges can ignite the gas mixture prematurely before the spark fires.
• Cylinder Imbalance. If one cylinder misfires or loses power due to a bad spark plug or fuel valve, the engine controller increases manifold boost pressure to maintain the target load, overloading the remaining cylinders into knock.

Modern stationary gas engines rely on knock sensors and engine control units (ECUs) to detect vibration frequencies and protect the engine automatically.

• Reduce Engine Load. Immediately derate the engine power output to lower cylinder pressures and temperatures until the root cause is found.
• Check Control System Logs. Review ECU fault codes, individual cylinder knock sensor readings, and exhaust gas temperatures (EGTs) to isolate whether the knock is localized to one cylinder or affecting the whole bank.
• Retard Ignition Timing. If the controller has not already done so, retarding the ignition timing by a few degrees reduces peak combustion pressure and stops detonation.
• Analyze Fuel Gas Composition. Use the BlueEye™ Ex-D or BlueEye™ Mobile to verify that the fuel’s MN and Wobbe Index match the manufacturer’s specifications.
• Inspect Cooling and Ignition Hardware. Verify charge-air cooler (intercooler) efficiency, check oil pressure and filters for metal shavings, and inspect spark plugs for ash fouling or eroded electrodes.