Some Useful Additions to Calculate the Wall Heat Losses in Real Cycle Simulations 2012-01-0673
More than 20 years after the first presentation of the heat transfer equation according to Bargende [1,2], it is time to introduce some useful additions and enhancements, with respect to new and advanced combustion principles like diesel- and gasoline- homogeneous charge compression ignition (HCCI).
In the existing heat transfer equation according to Bargende the calculation of the actual combustion chamber surface area is formulated in accordance with the work of Hohenberg. Hohenberg found experimentally that in the piston top land only about 20-30% of the wall heat flux values from the combustion chamber are transferred to the liner and piston wall. Hohenberg explained this phenomenon that is caused by lower gas temperature and convection level in charge within the piston top land volume. The formulation just adds the existing piston top land surface area multiplied by a specified factor to the surface of the combustion chamber.
In this work, it is shown that an analytical calculation of this heat transfer problem is possible. Furthermore, it is also possible to analytically describe the influence of a possible leakage on this wall heat loss component, which results in a significant better description of the wall heat losses in the piston top land area with better results when simulating the wall temperature field of this area.
The combustion-generated convection, which increases the heat transfer during combustion, was formulated empirically by squaring the heat-transfer-increasing effect, taking into account the different driving temperature gradients from the unburned and burned zones to the combustion chamber wall. This - in principle - analytical formulation has only proven to be sufficient for heat-releasing processes close to TDC. When the heat release is moved towards the expansion stroke, with the consequence of a reduced combustion related pressure increase, an underestimation of the combustion-generated convection can be observed. This underestimation occurs by principle in all heat transfer equations, in which the combustion-induced pressure change is used as a measure of the combustion-generated convection (e.g. Woschni and Hohenberg).
This paper presents a new formulation for the combustion-generated convection that avoids this disadvantage and seems to be valid for all common combustion principles, including low temperature combustion without the existence of turbulent flame propagation.
With these adaptations, the modified wall heat transfer equation correctly reflects the transient, spatially averaged wall heat losses for conventional, homogeneous operated SI-engines, stratified combustion in SI-engines, heterogeneous DI-diesel combustion, as well as diesel- and gasoline-HCCI combustion processes without any specific adjustments of empirical calibration factors. The accuracy of this enhanced heat transfer equation was carefully verified by evaluation of intensive in-cylinder pressure and fast-response surface temperature measurements.