Internal combustion engine heat transfer-transient thermal analysis

Internal combustion engine heat transfer-transient thermal analysis

Heat transfer to the cylinder walls of internal combustion engines is recognized as one of the most important factors that in influences both engine design and operation.
Research eff�orts concerning heat transfer in internal combustion engines often target the investigation of thermal loading at critical combustion chamber
components. Simulation of internal combustion engine heat transfer using low dimensional thermodynamic modelling often relies on quasi-steady heat transfer correlations. However unsteady thermal boundary layer modelling could make a useful contribution because of the inherent unsteadiness of the internal combustion engine environment.

In this study, a computational and experimental study is presented. The experiments are performed on a spark-ignition, single-cylinder engine under motored
and �red conditions. In the present study, decoupled simulations are performed, in which quasi-steady heat transfer models are used to obtain the gas properties
in the core region. A scaled Eichelberg's model is used in the simulation of the motored test under wide open and fully closed throttle settings. In the �red case
the scaled Woschni's model was used. The scaling factor is used to achieve a good agreement between measured and simulated pressure histories.

An unsteady heat transfer model based on the unsteady thermal boundary layer is presented in this study. Turbulent kinetic energy in the core of the cylinder is
modelled by considering the balance between production and dissipation terms as suggested by previous authors. An effective variable thermal conductivity is applied to the unsteady model with diff�erent turbulent Prandtl number models and turbulent viscosity models, and a �xed value is assumed for the thermal boundary layer thickness. The unsteady model is run using the gas properties identif�ed from the quasi-steady simulation.

The results from the quasi-steady modelling showed that no agreement was achieved between the measured and the simulated heat flux using the scaled Eichelberg's model for the motored case and the scaled Woschni's model for the
�red case. A signifi�cant improvement in the simulation of the heat flux measurements was achieved when the unsteady energy equation modelling of the thermal boundary layer was applied. The simulation results have only a small sensitivity to the boundary layer thickness. The simulated heat flux using the unsteady model with one particular turbulent Prandtl number model, agreed with measured
heat flux in the wide open throttle and fully closed throttle cases, with an error in peak values of about 6 % and 35 % for those cases respectively. In the
�red case, a good agreement was also observed from the unsteady model and the error in the peaks between the measured and the simulated heat flux was found to be about 9 %.

The turbulent Prandtl number and turbulent viscosity models are derived from quasi-steady flow experiments and hence their general applicability to the unsteady internal combustion engine environment remains uncertain. The thermal
boundary layer thicknesses are signifi�cant relative to the internal combustion engine clearance height and therefore, the assumption of an adiabatic core is questionable.
Investigation of a variable thermal boundary layer thickness and more closely coupled simulation to account for heat loss from the entire volume of the gas should be targeted in the future.