Recently, several theories have been offered as possible explanations for claimed increases in diesel engine heat transfer when combustion chamber surface temperatures are raised through insulation. A multi-dimensional computational fluid dynamics (CFD) analysis, using a recently developed near wall turbulent heat transfer model, has been employed to investigate the validity of two of these theories. The proposed mechanisms for increased heat transfer in the presence of high wall temperatures are:
- 1
piston-induced compression heating of the near wall gas which increases the near wall temperature gradient when wall temperatures are high;
- 2
increased penetration of hot, burned gases into the near wall flow during combustion through reduction of the flame quench distance.
Calculations of heat transfer in a motoring engine demonstrated that the piston-induced pressure work energy source did have a noticeable effect on the shape of the temperature profiles in the thermal boundary layers on all combustion chamber surfaces. However, it was not enough to cause heat transfer to increase with wall temperature. Calculations of heat transfer during combustion in a constant volume chamber revealed that, in spite of changes in quench distance, total heat transfer during the combustion period decreased with increasing wall temperature for quench distances in the range expected to occur in diesel engines. The study shows that neither of the above mechanisms can cause heat transfer to increase with wall temperature, which is an important result for the ongoing development of insulated diesel engines.
IMPROVEMENT OF DIESEL ENGINE efficiency through the insulation of the combustion chamber using ceramic materials has been the subject of several significant research efforts in recent years. Engine measurements have been made which indicate that significant reductions in heat transfer can be achieved which should result, in a corresponding improvement in engine efficiency (Morel et al 1988, Jackson et al 1990). However, other experimental studies claim that heat transfer may actually increase when the combustion chamber is insulated and surface temperatures rise to high levels (Furuhama and Enomoto 1987, Woschni et al 1987).
The following theories have been formulated by others to explain the claimed increase in heat transfer with higher wall temperatures:
increased temperature gradient in the near wall gas caused by piston-induced compression heating of the in-cylinder gas;
increased penetration of high temperature burned gases into the near wall flow due to a reduction in flame quench distance (the distance from the wall at which the flame is extinguished at the end of combustion);
evaporation of impinged liquid fuel off the hot combustion chamber surface and subsequent slow burning near the surface;
absorption of impinged liquid fuel into porous ceramic insulation, evaporation of the fuel into the near wall gas and slow burning of the evaporated fuel very near the combustion chamber surface.
It would be extremely difficult (if not impossible) to determine the validity of these theories through experimental investigation. This is because of the very detailed near wall flow information required to construct an accurate picture of the governing heat transfer mechanisms. Multi-dimensional, computational fluid dynamic (CFD) codes for simulating in-cylinder engine flows provide a powerful analytical tool for obtaining such detailed near wall flow information. By performing highly resolved CFD calculations in simplified engine-like geometries and carefully interpreting the results in light of known shortcomings in turbulence and combustion models, an understanding of heat transfer phenomena relevant to insulated diesel engines can be obtained and an assessment of high wall temperature heat transfer theories can be made. This paper presents the results from a study of insulated diesel engine heat transfer using a CFD based analysis in the manner described. The focus of the study was on the first two heat transfer mechanisms listed above.
It is important to note that the resolution of the conflicting claims regarding the effect of insulation on heat transfer is critical to the viability of the insulated diesel engine concept. In order to settle this issue it is necessary to develop an understanding of the fundamental engine combustion and heat transfer processes that are sensitive to increases in combustion chamber surface temperatures. The present work is directed at contributing to this understanding.
Discussion of High Wall Temperature Heat Transfer Theories. Figure 1 presents a schematic illustrating the effect of compression heating on the near wall thermal boundary layer in the combustion chamber of an engine. At the beginning of the compression stroke, the combustion chamber walls are at a higher temperature than the gas temperature in the core region of the cylinder. A thermal boundary layer characterized by smoothly varying, monotonically increasing gas temperatures separates the cool bulk gas from the surrounding hot walls. As the compression stroke proceeds, the in-cylinder gas is heated as a result of the pressure work being performed on it by the piston. This energy source is evenly distributed throughout the combustion chamber so that a nearly equal net energy gain is felt by the core and near wall gases. Since the near wall gases began the compression stroke at a higher temperature than the gases in the core region, the distorted boundary layer temperature profile shown in the figure can develop by the end of the compression stroke. At this point in the cycle, the boundary layer is characterized by gas temperatures which exceed the temperature of the gas in the core region. The result is an enhancement of wall heat transfer by the action of compression heating of the near wall gases. If this behaviour is extrapolated to the case of an insulated diesel engine in which the wall temperatures at the start of the compression stroke are much higher than those in a conventional engine, it has been hypothesized that the near wall temperature overshoot will be so great that the resulting heat transfer will be higher than the same engine with lower wall temperatures (without insulation).
Figure 2 presents a schematic illustrating the effect of increased wall temperature on near wall flame propagation during combustion. The period shown corresponds to the end of combustion in an engine when the flame approaches the wall. In a conventional engine (denoted by the cold wall case in the figure), the wall temperature is substantially lower than the burned gas temperature and a thermal boundary layer containing relatively low temperature gas exists near the wall. During flame propagation through the wall layer (before the flame arrives at the wall), the combustion rate is attenuated and eventually stopped as the flame is quenched primarily as a result of heat loss from the flame front to the low temperature near wall gas. The region adjacent to the wall where no combustion takes place is referred to as the quench region as indicated in the figure. When the combustion chamber is insulated (denoted by the hot wall case in Figure 2), the wall temperature is high (relative to the uninsulated case) and so is the temperature of the wall layer gas. As a result, the flame will propagate even closer to the wall and the high temperature burned gases will penetrate further into the wall layer, which will have an impact on wall heat transfer. The primary issue to be resolved is whether, for conditions which exist in insulated diesel engines, the reduction in the size of the quench region will be so large that the net heat transfer will be higher than in the uninsulated case, where the wall temperatures are lower and a thicker boundary layer exists between the burned gases and the wall.