In an optical accessible 4-stroke engine laser-induced fluorescence (LIF) imaging measurements of fuel tracer (3-pentanone) and formaldehyde were performed during the compression stroke and combustion. Formaldehyde (HCHO) is intermediately present at high concentrations within the cool flame and is burned later on when the "hot" combustion proceeds. It can be used as an internally generated tracer to observe the boundaries of the hot combustion zones. Despite the fact that a frequency-tripled Nd:YAG laser excites only weak transitions in the HCHO molecule, the high concentration (several thousands ppm) provide for sufficient signal intensity when detecting fluorescence above 395 nm. Using formaldehyde LIF, auto-ignition (occurring close to 356°ca) and the further development of combustion were observed. Combustion was completed within the field of view by 360°ca in most observed engine cycles. 3-pentanone which has been used frequently for fuel-concentration imaging in spark ignited engines was of limited use in the CAI engine due to laser and signal attenuation prior to top dead center as a result of tracer destruction during and UV absorption by cool-flame intermediates.