1998-02-23

Modeling the Structural Thermal Response of an Air-Cooled Diesel Engine under Transient Operation Including a Detailed Thermodynamic Description of Boundary Conditions 981024

A comprehensive structural analysis simulation model is used for describing the thermal condition of a four-stroke, air-cooled, DI diesel engine under steady and transient operation. Two- and three- dimensional finite element analyses are implemented for the representation of the complex geometry metal components (piston, liner, cylinder head), in a way that the temperature and heat flux variations are calculated during any transient event. A detailed thermodynamic simulation model of engine operation is utilized for the determination of boundary conditions on the combustion chamber sides of each component.
During an engine transient, processing of experimental cylinder pressure diagrams on a cycle to cycle basis resulted in the estimation of heat resease rate and boundary conditions (gas temperature, heat transfer coefficient) variation from the initial to the final engine thermodynamic state. Consequently, the power and specific fuel consumption curves can be accurately determined. In this way any individual characteristic of engine performance (resulting for example from fuel pump injection operation) can be analysed and its effect on structural transients revealed (thermal shock calculations).
An extensive set of experimental measurements was conducted on a Lister LV1, direct injection, single cylinder, air-cooled, diesel engine in order to test the model's validity. Cylinder pressure and structure temperatures were continuously recorded, among other variables, during any transient operation with the aid of two on-line computers through analog-to-digital converters and sub-multiplexer cards. A satisfactory degree of agreement is found between theoretical predictions and experimental measurements under all conditions tested. Presentation of results provides insight into the mechanism of heat propagation inside the combustion chamber components and reveals the connection of various thermodynamic parameters with the thermal response of engine structure. The method gives better results, under all kinds of engine operating conditions (increment or reduction of load and/or speed), than a fully theoretical description of transient operation used in the past by the present research group.

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