Sealing Prediction and Improvement at Cylinder Head Block Interface under Thermo-Mechanical Loading involving Multi- Layer Steel Gasket

摘要

An inadequate sealing of the combustion chamber gasket interface may have severe consequences on both the performance & emission of an engine. In this investigation, both the distribution of the contact pressure on the gasket and the stresses of the cylinder head at different loading conditions are explored and improved by modifying the design. A single cylinder gasoline engine cylinder head assembly has been analyzed by means of an uncoupled FEM simulation to find the sealing pressure of the multi-layer steel (MLS) gasket, strength & deformation of the components involved. The thermal loads are computed separately from CFD simulations of cylinder head assembly. The cylinder head assembly consisting of head, blocks, liner, cam shaft holder, bolts, gaskets, valve guides & valve seats, is one of the most complicated sub-assembly of an IC engine. It is also most difficult to analyze being subjected to a variety of loading conditions like valve guides & seats press fitting, bolt tightening combustion pressure, valve train & piston loads and thermal loads. Non-linear material properties along with geometric non-linearity are considered. The analysis is done in two stages. First stage simulates the assembly including pressing & bolt tightening in order to have their effect on the subsequent stage of analysis. In second stage, temperatures obtained from steady-state thermal analysis are mapped on the assembly using predefined field. The mechanical loads arising from valve train & crank train are applied. This simulates engine hot running condition. This study reveals that under the operating conditions with gas pressure acting opposite to the pretension applied to the bolts, possibility of gas escaping increases. Also contact pressure contours on the gasket are greatly transformed when the thermal and mechanical loading are taken into account. Sealing deficient regions were identified and corrective measures were taken to improve them at very beginning of the design stage. Proper pre-stressing force of the bolts and the gasket bead pattern are critical factors in enhancing the efficiency of the sealing of the gasket. The capacity of gasket sealing mainly depends upon the pretension of the bolts, but there is a limit to which it could be increased. Therefore, an effective method was proposed to enhance the sealing capacity of the gasket by changing the design of the beads in the MLS ensuring enough pre-compression is present even in the hot running condition. In addition, bolt placement configuration was changed to square pattern from an irregular pattern. The bolts were brought closer to the combustion chamber as compared to the base design. Sealing pressure results were correlated with the Fuji Film Test, a good correlation is achieved between test and simulation. This methodology can be advantageously used to predict the sealing during hot running condition which is the not possible with Fuji Film test.