Document ZBDVjj7m1Qy7oX5jpYYNzGKgO

In order to maintain equilibrium in our free body, intuitively one can see that either L, & Lz must increase and /or P,r Pz, P3 & P4 must decrease in some proportion to the magnitude of Fv. Pi P2 Fv P3 P. FIG. 2 Considering the elasticity of the bolts, Lx = Lz = k8 A^. = Lb, where kB is the spring constant of the bolts and A/ is the incremental cha nge in bolt length. Further, the gasket being elastic (ideally) has a similar characteristic: Z Pn = kn /\ G = 5IPqi n= 1 where kG is the spring rate of the gasket and Ag is the incremental change in gasket thickness. From the foregoing, we know that Fv + Z Pg -- Z LB, or, Fv = Z Lb - ZPg Thus, when Fv = 0, or approximately so, Z Lb = Z PG ; As mentioned earlier, the cylinder head gasket must fulfill the requirements pre sented under the basic theory of sealing and maintain the seal during combustion within the chamber. Recognizing that the sealing stress (on the gasket) is at a minimum dur ing peak firing pressure, one might intuitive ly feel that the gasket should sea l as long as any amount of sealing stress remains. Un fortunately, such is not indicated in labora tory work conducted at Victor. Rather, data observed agree very closely with the theory of static friction: f --J*- N* That is, leakage past the gasket occurs when the sealed fluid pressure produces a force, FH, against the edge of the gasket equal to or greater than the frictional forces, f, on the gasket oppos ing it. (FIG. 3). One would theorize that micromotion occurs between the gasket and mating surfaces at such a time which dis turbs the plastically formed gasket surface, thereby allowing leakage to progress along FIG. 3 various leakage paths. One can validly'con clude, therefore, that a seal can be main tained provided the sealing toad is always sufficient to allow a frictional component at least equal to the fluid force exerted on the edge of the gasket. but, when Fv is at its maximum, ZLB will be at its maximum and Z PG will be at its minimum. The precise magnitudes of these forces, of course, depend entirely on the contributing engine parameters and the particular gasket design used. Their magni tudes do, as mentioned previously, influence the system deflections in the assembly as well as the sealing stresses on the gasket. If the forces were known and an ideally rigid assembly (except for the bolts and gasket) were considered, the resulting deflection common to the bolts and gasket could be calculated as well as the change in the seal ing stresses. The real case, however, usually presents the most challenge, so further dis cussion will be given that. The apparent simplicity of the above is not intended to be misleading. As stated earlier, the sealing stress on the gasket is at a minimum during peak firing pressure. Therefore, the effective sealing load at this time must be sufficient to allow a frictional component adequate to resist the fluid force on the gasket edge. I n order to achieve this, the rigidity of the cylinder head and block gasket surfaces must be considered as well as the initial load on the gasket, its thickness and its spring rate, the spring rate of the bolts, and the combustion force acting on the cylinder head. Precisely determining the effective values for each of these factors would be an almost insurmountable task, especially in view of the fact that each will assume a number of values depending on In the notation of this discussion, f would read f =v ZPg No. 10106 B 2 1-70