An Investigation of the Relationship Between Measured Intake Temperature, BDC Temperature, and Combustion Phasing for Premixed and DI HCCI Engines

Combustion phasing is one important issue that must be addressed for HCCI operation. The intake temperature can be adjusted to achieve ignition at the desired crank angle. However, heat-transfer during induction will make the effective intake temperature different from the temperature measured in the runner. Also, depending on the engine speed and port configuration, dynamic flow effects cause various degrees of charge heating. Additionally, residuals from the previous cycle can have significant influence on the charge temperature at the beginning of the compression stroke. Finally, direct injection of fuel will influence the charge temperature since heat is needed for vaporization.

This study investigates these effects in a systematic manner with a combination of experiment and cycle simulation using WAVE from Ricardo. The results show that the amount of charge heating/cooling that occurs due to heat-transfer during the induction period can be computed from changes in the volumetric efficiency. The amount of charge cooling associated with vaporization of directly injected fuel changes with injection timing and can be related to the volumetric efficiency. A simple procedure for estimating the charge mixture temperature at the end of the intake stroke is presented. Several examples are given where this procedure can be used beneficially to explain experimental observations for fired HCCI operation where the charge temperature at the end of the intake stroke was related to the combustion phasing. Iso-octane was used as a gasoline surrogate since it facilitates comparison with chemical-kinetics models, in this case the detailed iso-octane mechanism from LLNL.

Although this procedure is developed for HCCI operation, it can be applied to other types of engines as well. For example, the changes to the charge temperature that occur during the induction process are important for both the occurrence of knock in SI engines and NO_x formation in Diesel engines.