ANALYSIS OF TEMPERATURE SWING THERMAL INSULATION ON THE PERFORMANCE OF DIESEL ENGINES
In a reciprocating internal combustion engine, the heat transfer from combustion gases to the surfaces of combustion chamber walls, namely, cylinder head, piston and liner, depends on the instantaneous temperature difference between the gases and the surface walls. Thus, for the combustion chamber walls insulated with low conductivity material such as ceramics, the profile of surface temperature across 4-stroke of an engine cycle are very critical. 'Temperature Swing' insulation refers to the insulation material applied on the surfaces of combustion chamber walls, that enables selective manipulation of its surface temperature profile over the 4 strokes of an engine cycle consisting of 720 crank angle degrees. , An ideal 'Temperature Swing' insulation closely follows the temperature of the gases in the combustion chamber, thereby minimizing the instantaneous temperature difference between the gas and the wall over each engine cycle. The 'Temperature Swing' insulations/coatings are characterized by low thermal conductivity and low specific heat materials, however the time varying thermal properties such as thermal diffusivity and thermal effusivity also have significant influence on 'Temperature Swing' characteristics which have not been discussed much in details earlier.
The objective of this work is to develop a numerical simulation methodology and thereby investigate the effects of thermal diffusivity and effusivity in combination with insulation thickness on the 'Temperature Swing' characteristics. The approach is extended to assess the impact of predicted temperature profiles on the key performance parameters of a diesel engine. A one-dimensional transient heat conduction analysis and engine cycle simulations are performed using scaled down thermal properties of Yttria Stabilized Zirconia.
An ideal thermal insulation material should have lower effusivity at high temperatures, i.e. during power stroke, and higher diffusivity at intake stroke to reduce preheating of incoming fresh air. Further, the impact of heat insulation on thermal efficiency is quantified and it is found that when insulation rate of 48% is achieved by applying coatings only on the piston top surface, engine thermal efficiency can be improved by up to 1.5%. Additionally, sensitivity of 'the coating properties on engine thermal efficiency and volumetric efficiency at different engine speeds (frequency) and loads is discussed.