As part of a Small Business Innovation Research (SBIR) grant from TACOM (Tank-Automotive & Armaments Command) - a division of the U.S. Army -Adiabatics used multiphysics finite element analysis (FEA) software from ALGOR to study the thermal and structural behavior of a cylinder head for a diesel engine that would provide reduced heat rejection and increased power density, while maintaining superior fuel economy and lower operating costs.
Diesel engines have better fuel economy and lower operating costs than the turbine engines typically used in military tanks, which have a higher power density and higher fuel consumption. The project involved a design modification of an existing diesel engine.
In the prototype of the altered engine, temperatures at certain portions of the stainless steel cylinder head were measured at 1600°F - almost twice the normal operating temperature of 800 to 900°F. At these high temperatures, the cylinder head within the diesel engine was developing valve-insert and head-gasket combustion leaks, all due to local head distortion. Finite element analysis was necessary to evaluate the design and material used for the cylinder head, and hence verify that the high temperatures were causing the leaks.
Adiabatics engineers began with a Pro/ENGINEER solid model consisting of the cylinder head and six washers, and used ALGOR's InCAD technology to capture the geometry and create a 3D finite element mesh. Small holes and cuts were suppressed to prevent stress concentrations and to obtain a better understanding of the overall stress distribution in the model.
The model was set up for a steady-state heat transfer analysis. Stainless steel AISI 410 was the material used for the prototype cylinder head and steel was used for the washers. The washers were modeled to obtain a uniform, circular, cross-sectional area for the application of the loads caused due to the tightening of the bolts.
The temperatures on the different surfaces of the cylinder head were determined using data obtained through previous cycle simulation tests for an intake air temperature of 600°F. These temperatures, along with the related convection coefficients, were applied as input for the heat transfer analysis.
As expected, the heat transfer analysis revealed high temperatures on the walls of the exhaust port, ranging from 750°F to almost 1500°F, with the interface (bridge-like) region between the two exhaust valves reaching the highest temperatures. The temperatures obtained from the steady-state heat transfer analysis were then applied to a linear static stress analysis where only thermal stresses were considered. Through preliminary analyses, engineers found that other effects such as the mechanical loads caused by the tightening of the bolts on the cylinder head did not contribute much towards the stresses when compared to the high temperatures.
The highest stresses were found at the interface between the two exhaust ports, where temperatures also were the highest. The stresses across other regions of the cylinder head fell well below the yield point of the material. Near the exhaust port region, stresses ranged from 85 KSI to more than 200 KSI - well above the yield point of the material at the temperatures predicted by the heat transfer analysis. Some local yielding could be expected.
The linear static analysis confirmed the behavior that engineers saw in the initial prototype tests. The highest thermal stresses coincided with the part of the cylinder head that had been leaking in the preliminary prototypes. The analyses showed that either the cylinder head or the operating parameters would have to be changed to ensure that the final design would perform adequately.
Although the linear static stress analysis predicted failure, engineers did not choose to do a nonlinear analysis, believing that the part should stay well within the linear range. They also felt that once the part yielded, one would not want it to still be operating inside the engine.
The strength for most metals such as stainless steel drops considerably at temperatures greater than 1400°F. The most obvious way to increase the durability of the cylinder head is to choose a different material that has appreciable strength at very high temperatures (around 100 to 150 KSI at 1600°F to 1800°F). Another option would be to use a specialized thermal barrier coating to protect the head from wear and tear due to the thermal stresses. Research is currently in progress to make the cylinder head more durable while maintaining the operating parameters that reduce heat rejection and increase the diesel engine's power density.
This work was performed by Adiabatics, Inc. of Columbus, IN, in collaboration with ALGOR, Inc. For more information, contact Julie Halapchuk of ALGOR at 412-967-2700, ext. 3029; e-mail:
Vishwas Bantanahal of Adiabatics, Inc. in Columbus, Indiana performed a multiphysics analysis of a cylinder head designed under an SBIR program contracted from TACOM.
