Optically-accessible engines are a key tool for the study of sprays, mixing, and ignition and combustion phenomena in internal combustion (IC) engines. Due to their construction, they are typically operated for limited durations, resulting in significant thermal transients in the in-cylinder surface temperatures and cycle-to-cycle in-cylinder gas temperature. This makes collection of highly repeatable data difficult and can introduce considerable uncertainty in the in-cylinder thermal conditions. In this paper, rigorous analyses of transient in-cylinder boundary conditions and in-cylinder gas temperature were performed in an optically-accessible compression-ignition engine. Piston surface thermometry, in-cylinder pressure measurements, and in-cylinder gas thermometry were employed to determine the engine warmup time required to reach a quasi-steady thermal state for motored operation over a range of intake air temperatures and pressures from 300-420 K and 100-300 kPa, respectively. The effect of fueling on thermal stability was investigated for single-injection mixing-controlled compression-ignition combustion utilizing a range of skip-fire ratios from 5:1 to 50:1 and loads from 0.8 to 10.1 bar gross indicated mean effective pressure. Both optical and metal pistons were studied. A motored warmup time of 300 s was found to be sufficient for the engine to reach a thermally stable condition regardless of piston material or intake condition. Piston temperature was found to increase with load, but the increase was modest for even the highest load studied (~4 K) for 10 injection sets typical of late direct-injection optical engine studies.
