Measurements and Correlations of Local Cylinder-Wall Heat-Flux Relative to Near-Wall Chemiluminescence across Multiple Combustion Modes 2020-01-0802

Minimizing heat-transfer (HT) losses is important for both improving engine efficiency and increasing exhaust energy for turbocharging and exhaust aftertreatment management, but engine combustion system design to minimize these losses is hindered by significant uncertainties in prediction. Empirical HT correlations such as the popular Woschni model have been developed and various attempts at improving predictions have been proposed since the 1960s, but due to variations in facilities and techniques among various studies, comparison and assessment of modelling approaches among multiple combustion modes is not straightforward. In this work, simultaneous cylinder-wall temperature and OH* chemiluminescence high-speed video are all recorded in a single heavy-duty optical engine operated under multiple combustion modes. OH* chemiluminescence images provide additional insights for identifying the causes of near-wall heat flux changes. To measure spray, flow, and combustion effects on heat transfer using a cylinder-wall probe, a portion of the piston bowl is cut out to expose the cylinder wall to sprays, flows, and combustion that are normally confined to the bowl. The cylinder-wall heat flux is derived from the measured transient temperature and compared with Woschni HT correlation predictions using estimated local gas-temperatures. The local Woschni correlation predictions of heat flux and the HT coefficient for spark-ignition direct-injection (SIDI) and homogeneous charge compression-ignition (HCCI) match relatively well with measurements. For impinging jets of conventional diesel combustion (CDC), the Woschni HT correlation vastly under-predicts both heat flux and the HF coefficient. For CDC, the Woschni correlation underpredictions are likely due to its basis on internal-pipe flow, for which the flow is generally parallel rather than perpendicular to the wall as for impinging jets. Hence, for CDC, the predictions of local heat flux by empirical correlations like Woschni can be improved by using an impinging-jet basis.