Abstract
Research and/or Engineering Questions/Objective
The maximum attainable load of Homogeneous Charge Compression Ignition (HCCI) engines is limited by ringing combustion. This occurs when the combustion rate is too fast, leading to pressure oscillations in the combustion chamber accompanied by a knocking sound. Some researchers have hypothesized that these pressure oscillations increase the heat transfer from the combustion gases to the cylinder wall, due to the breakup of the thermal boundary layer. The goal of the current work is to investigate how the occurrence of a ringing combustion alters the heat transfer and which physical phenomena are responsible for it, in order to propose the best modeling approach.
Methodology
The heat flux is measured with a Thin Film Gauge sensor in the cylinder head of a CFR-engine converted to HCCI operation. HCCI operation is obtained by using a low octane fuel (n-heptane) and by heating the inlet air. Variations in the inlet air temperature and mass fuel rate are performed so the heat flux is measured in three different cases: no, light and severe ringing. The occurrence and the intensity of the ringing are determined with the amplitude of the cylinder pressure oscillations and by calculating the ringing intensity. Furthermore, the heat flux is measured at the cylinder head and at the cylinder wall to investigate the spatial variation of the heat transfer.
Results
In this work the measured heat flux traces for the different operating points are presented and discussed. A comparison is made between different pathways to obtain ringing and between different measurement locations. The effect of ringing on the maximum value of the heat flux is analysed. The heat flux is split up in the convection coefficient and the temperature difference between the gas and the wall. This decomposition makes it possible to determine which phenomena affect the heat flux.
Limitations of this study
The main limitation of this study is the test engine. The CFR engine is a standardized research engine, and is not a good representation of a modern internal combustion engine. However, it is a commonly used research engine that can be found in engine test facilities around the world.
What does the paper offer that is new in the field including in comparison to other work by the authors?
Few studies exist in which the heat transfer has been measured experimentally during HCCI operation. Even fewer studies have discussed the effect of ringing on the heat transfer. Contrary to other work of the authors, this work focusses on ringing instead of normal HCCI combustion.
Conclusions
This works shows the effect of ringing on the instantaneous heat flux. It also demonstrates that the decomposition of the heat flux in the convection coefficient and the temperature difference between the gas and the wall makes it possible to determine which physical phenomena cause the change in heat flux.
KEYWORDS : HCCI, heat transfer, experimental, ringing, knock