On Optical Semi-Quantitative Spectral Study of Low-Speed Pre-Ignition Sources in Spark Ignition Engines

Low-Speed pre-ignition (LSPI) in modern-day, heavily downsized, boosted, and direct-injection spark ignition (SI) engines is a well-known problem. Several mechanisms contribute towards stochastic pre-ignition (SPI), the most prominent being crevice material droplet induced and deposit induced pre-ignition mechanisms. The droplet mechanism is typically dominated by the detergent additives present in the lubricant formulation; more specifically calcium and sodium-based detergent additives correlate strongly with the increased LSPI rates. Deposits flaking off the combustion chamber surfaces can also induce LSPI under certain conditions.

This study aimed to develop an optical method designed to investigate the nature of pre-ignition precursors. Southwest Research Institute (SwRI) utilized an optically accessible GM 2.0 L LHU engine to study the pre-ignition phenomenon and studied the nature of pre-ignition precursors using spectral information from one of the cylinders in this engine. A custom, simplified borescope was employed to gather visible light from the combustion chamber to detect the glowing pre-ignition precursors. Further, this light signal was sent to an array of four photomultiplier tube (PMT) detectors which were tuned to visible light emission wavelength bands of interest. Three of the four wavelengths (555, 623, 645 nm) were selected to be representative of calcium and CaOH while the fourth wavelength of 715 nm (red emission) was selected to be representative of glowing soot-like aggregates. A high calcium first-generation Dexos™ 1 compliant oil with moderate LSPI activity was used in this study. The analysis revealed that the precursors contained both the signals for calcium as well as the deposit wavelengths in different proportions. More specifically, it was observed that in a series of LSPI events, the first event was usually more biased towards calcium wavelengths suggesting a pre-ignition initiated by the droplet ejecting from the crevice. A relative semi-quantitative metric was developed to compare the light signals for different frequencies to further delve into the pre-ignition mechanism. Two additional fuels were tested with different particulate matter index (PMI) values to assess the impact of PMI on pre-ignition precursors. A bulk comparison of around 30,000 combustion cycles of the two additional test fuels revealed that the high PMI fuel showed more bias towards a deposit induced mechanism while the lower PMI fuel exhibited signs of both droplet and deposit induced pre-ignition precursors.