1：Experimental Design and Method Selection:

A rapid compression machine (RCM) was used to perform time-resolved quantitative measurements of HCHO in the low-temperature oxidation of iso-octane. A two-color detection scheme was applied to HCHO detection due to the weak interference of the broadband absorption of iso-octane.

2：Sample Selection and Data Sources:

Homogeneous gas mixtures of fuel/oxidizer were prepared using ultra-high-purity O2 and N2, and high-purity iso-octane.

3：List of Experimental Equipment and Materials:

A 3.561-μm DFB-ICL (Nanoplus, Gerbrunn, Germany) was used as a light source. The optical setup included CaF2 windows, an AR-coated CaF2 lens, and an InSb photodiode detector (HAMAMATSU P4631-03, Hamamatsu, Japan).

4：561-μm DFB-ICL (Nanoplus, Gerbrunn, Germany) was used as a light source. The optical setup included CaF2 windows, an AR-coated CaF2 lens, and an InSb photodiode detector (HAMAMATSU P4631-03, Hamamatsu, Japan). Experimental Procedures and Operational Workflow:

4. Experimental Procedures and Operational Workflow: The beam was multi-passed seven times in the combustion chamber using two cylindrical mirrors with a focal length of 180 mm. The effective optical path length was 581 mm.

5：Data Analysis Methods:

The cross-sections of HCHO and iso-octane at selected absorption lines were measured, and the time profiles of the HCHO and iso-octane concentration were obtained using the measured cross-sections, absorption length, and measured absorbance.