Machine Design

Why is the cylinder head warping? FEA has evidence

The U.S. military has so many diesel-powered vehicles, it's natural for them to want to improve their efficiency.

The solid model of the cylinder shows the detail necessary for the thermal and stress simulations.

Steady-state heat-transfer analysis, conducted with Algor FEA software, revealed high temperatures between the two exhaust ports. This simulation was the first part of a multiphysics analysis.

The linear static-stress analysis predicted thermal stresses well beyond the material's yield point between the two exhaust ports, especially on the inner face in the inset.

One research program with Adiabatics Inc., Columbus, Ind., used multiphysics FEA software from Algor Inc., Pittsburgh, to study the thermal and structural behavior of a prototype diesel-engine cylinder head designed for lower heat rejection and increased power density.

In the prototype, temperatures on a stainlesssteel cylinder head were measured at 1,600°F, almost twice the normal operating temperature of 800 to 900°F. Distortion of the cylinder head at such high temperatures was causing leaks in valve inserts and the head gasket. Finite-element analysis was needed to evaluate the head's design and materials, and verify that high temperatures were, in fact, causing the leaks.

Vishwas Bantanahal, an engineer at Adiabatics, began with a Pro/E model of the cylinder head and six washers under the bolt heads. He used the FEA developer's InCAD to capture the geometry and create a 3D finite-element mesh. “This direct CAD to CAE data exchange is easier than other methods, such as an intermediate file format,” he says.

Small holes and cuts were suppressed to prevent stress concentrations and for a better understanding of the overall stress distribution. Bantanahal also refined the mesh around the holes and fillets.

The model was set up for a steady-state heattransfer analysis. AISI 410, a stainless steel, was specified for the cylinder head and carbon steel for the washers. Bantanahal modeled the washers as well for circular areas on which to apply bolt loads.

Temperatures on different surfaces of the head (600°F) were determined by previous tests. Bantanahal used related convection coefficients as input for the heat-transfer analysis. It revealed high temperatures on exhaust port walls ranging from 750 to almost 1,500°F. The region between the two exhaust valves reached the highest temperatures.

A subsequent linear-static-stress analysis considered only the temperature distribution as loading. “Preliminary analyses showed that other effects such as the mechanical loads from bolt tightening on the cylinder head contributed little towards the stresses when compared to the high temperatures,” says Bantanahal.

He found the highest stresses between the two exhaust ports where temperatures were also highest. “Stresses across other regions of the cylinder head fell well below the yield point of the material,” says Bantanahal. “But near the exhaust port, stresses ranged from 85 to over 200 ksi, well above the yield point of the material at the predicted temperatures. So, some local yielding could be expected.”

Stress analysis confirmed behavior seen in prototype tests. “Highest thermal stresses coincided with the part of the cylinder head that leaked in the preliminary prototypes,” says Bantanahal. “It's clear from these analyses that either the cylinder head or the operating parameters have to change to ensure the final design performs adequately.” Although stress analysis predicted failure, Bantanahal chose not to do a nonlinear analysis. “Ideally, the part should stay well within the linear range,” he says.

Strength for most metals drops considerably at temperatures over 1,400°F. One way to increase the cylinder head's durability is to use a different material, one with appreciable strength (100 to 150 ksi) at 1,600 to 1,800°F. Another option uses a thermal-barrier coating, an Adiabatic specialty, to protect-the head from wear due to the thermal stresses. The team continues trying to make the cylinder head more durable while maintaining operating parameters that minimize heat rejection and increase the engine's power density.





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