We have adapted a standard Fourier-transform infrared (FTIR) spectrometer (Thermo-Scientific Nexus 670) for in-cylinder measurements of gas spectra. During engine operation, the engine shaft encoder signal is logged continuously along with the FTIR's He–Ne laser signal and infrared interferogram signal. The engine piston and FTIR mirror move in an uncorrelated fashion, so that after many minutes of engine operation, a complete interferogram is populated at each piston position. Afterward, the data are compiled into a series of spectra versus crank-angle degree. Here, we present a near-infrared H2O thermal emission spectrum measured through a fiber-optic spark plug connected to an engine (Briggs 128603-OHV).