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On the Rate of Heat Release for High-Boost, Low-Temperature Combustion Schemes: Accounting for Compressibility Effects

A methodology has been developed to account for compressibility effects (i.e., non-ideal-gas behavior) in the analysis of heat release rates. These effects may be important for advanced, high-boost, low-temperature combustion schemes currently under investigation, as the high pressure and overall lower temperatures can lead to compressibility factors (Z = Pυ/RT) that are significantly greater than 1.0. The rate of heat release (ROHR) formulation developed here uses a generalized energy conservation equation where the cylinder is segregated into three distinct regions (i.e., fresh, burned, and mixed); an arbitrary equation of state can be applied to these zones. Crevice/ringpack flows, which can be substantial under high-boost/high-compression ratio (CR) operation, are taken into account using a source/sink term. The new methodology is demonstrated using pressure-volume traces for six different engine cycles; these traces have been generated using a real gas engine simulator where the heat release is prescribed by a Wiebe-type function. Results are presented using the Redlich-Kwong-Soave equation of state, with comparisons made between the new formulation and conventional ideal gas ROHR results and computed mean gas temperatures. Noticeable differences in ROHR values are seen for cases where Z becomes greater than 1.05; however, computationally significant differences appear important only for very high or extreme CR operation. A real gas equation of state should be used to estimate the mean charge temperature for all boosted LTC operation as the ideal gas temperature can be off by ∼ 70–300 K under the cases investigated here.