Evaluation of operating parameters and fuel composition on knock in large bore two-stroke pipeline engines

Legacy integral compressor engines are crucial to the operation of the North American natural gas pipeline network. Many of these engines are uncontrolled utilizing mechanical scavenging or light turbocharging. The engines typically operate between ∼10 and 20 g/bhp-hr of NOx with a minimal knock margin. While suitable for typical pipeline gases, high ethane shale gas can erode the knock margin with the added risk of performance degradation and engine damage. Phase one of the project investigated the impact of variable fuel quality and determined which engine parameters (ignition timing, air manifold temperature, and trapped equivalence ratio) were most effective in preventing and controlling knock. Phase two of the project evaluated the ability of an accelerometer and a NOx emissions sensor to detect engine knock as well as the development of knock prediction models.Testing confirmed elevated fuel ethane could induce continuous engine knock but ignition timing was the most effective method to mitigate engine knock, followed by trapped equivalence ratio. Air manifold temperature did not significantly affect knock onset. When properly tuned, a typical off the shelf accelerometer knock detection system could detect and protect against the onset of knock. Persistent knock lead to a steady increase in NOx production, but NOx values alone were inadequate to detect and protect against knock. The knock prediction models accurately predicted the onset of knock based on ignition timing, trapped equivalence ratio, and fuel composition, suggesting a fundamental relationship between operating parameters and engine knock.