An ML-Enhanced Laser-Based Methane Slip Sensor Using Wavelength Modulation Spectroscopy

Natural gas (NG) is a promising alternative to diesel for sustainable transport, potentially reducing GHG and air quality emissions significantly. However, the GHG benefits hinge on managing methane slip, the unburned methane in the exhaust of NG engines, which carries a significant global warming potential. The CH₄ slip from NG engines is highly dependent on engine type and operation, and effective greenhouse gas emission mitigation requires that the actual operation of real-world engines is monitored. This requires suitable instrumentation for online robust CH₄ measurement in engine exhaust. Traditional methane slip measurement methods need frequent calibration, may not be suited to dynamic operational conditions, carry significant costs, or require expert users. Furthermore, the significant computational demands associated with calibration-free spectroscopic methods and the prevalent noise uncertainty underscore the urgent requirement for innovative sensors. These sensors must not only respond rapidly but also have low uncertainty in their readings. This paper presents a machine learning (ML)-enhanced, laser-based methane slip sensor using wavelength modulation spectroscopy (WMS) for rapid, accurate, and calibration-free CH₄ measurements for application in the exhaust of NG engines. The sensor utilizes a distributed feedback (DFB) laser diode emitting around 1.65 μm propagated through a multipass optical cell. An ML-based approach is used to invert the recorded WMS signal, which reduces computational cost and uncertainty due to noise vulnerabilities inherent in traditional measurement inversion approaches. A Gaussian process regression (GPR) model, trained on measured and simulated WMS signals, was selected for its high predictive accuracy, where it achieved a mean absolute percent error (MAPE) of 0.24%. For exhaust measurement on an in-use natural gas marine vessel, a mean absolute difference of 3.95% was observed, relative to simultaneous reference Fourier transform infrared spectroscopy measurements. The ML-based WMS inversion system marks a significant advancement in methane slip measurement, offering real-time monitoring capabilities with reduced computational demands. Its development supports the realization of NG environmental benefits for transport by providing accurate CH₄ slip data, which are essential for engine performance optimization, regulatory adherence, and sustainable policy decisions.