Characterizing Features. Radial Pressure Characteristics
Tangential force. The tangential force Ft is that force which must be present at the ends of the ring, at the outside diameter, in order to compress the piston ring to the specified gap at the joint (Fig. 7-46).
Fig. 7-46. Tangential force at a piston ring
This is the determinant magnitude for the contact pressure. The contact pressure influences the sealing function and is the force with which the piston ring presses against the cylinder wall. It is calculated as shown below, where p = contact pressure, d = nominal diameter, h =ring height:
Radial pressure characteristics. Contact pressure can be set up to be constant around the circumference of the ring or to correspond to specified graduations in radial pressure. Figure 7-47 shows three typical forms for radial pressure characteristics.
Fig. 7-47. Radial pressure characteristics
The four-cycle characteristic (positive oval) (Fig. 7-47a) with increased radial pressure at the ends of the rings helps to "damp" piston ring flatter, which, in general, starts at the ends of the rings.
Rings with this characteristic show greater wear at the gap than those with uniform distribution of pressure (circular characteristic) (Fig. 7-47b).
Diesel engines, which do not run at such high speeds but which develop greater pressures, are thus equipped with rings that exhibit uniform radial pressure characteristics.
Where wear near the gap is to be reduced even further, rings with two-cycle characteristics (negative oval) (Fig. 7-47c) may also be used. Here the radial pressure at the ends of the rings is greatly reduced.
Installed flexure tension. This is the flexure load to which the piston ring is subjected when installed in the cylinder. Maximum tension is found at the back of the ring and is calculated as follows:
Rectangular ring:
The piston ring parameter k characterizes the elastic property of the piston ring. For rings with a rectangular cross section, for instance, it is defined as
where tangential force Ft is used, or
when using gap width m (Fig. 7-40).
The capacity to fill the space is the capability of the piston ring to adapt even to cylinders that are out of round. Good shape-filling capability ensures correct sealing against the gas and the lubricating oil and thus good performance and low oil consumption.
Taking ui as the radial, harmonic deformation of the cylinder of the ith order (Fourier analysis), the shapefilling capacity at which the ring is just in contact with the cylinder wall, exerting radial pressure of p = 0, is calculated as follows:
Ring gap. The ring gap is the space left between the ends of the ring after installation; this space is necessary to allow for thermal expansion in the piston ring. It is to be laid out for temperature differentials of at least 100°C between the compression rings and the cylinder and for 80° at oil control rings. If the ring gap is too large, then gas loss (blow-by) will result; if it is too narrow, then ring expansion can exert pressure on the ends of the rings and cause ring failure.
Butt joints with straight end surfaces are normally used. Bevel joints and lap joints are not used for passenger car engines and do not offer any advantages in regard to the tightness of the seal. Ring gaps with increased sealing quality (roll-shaped or beveled) improve the sealing quality of the rings in comparison with the butt joint.
These joint designs are recommended for use in two-ring piston concepts and have been employed with differing degrees of success.
Date added: 2022-12-29; views: 701;