Installation Play and Running Play. Piston Masses. Operating Temperatures

One attempts to keep installation play at the piston skirt as small as possible so that uniformly smooth running is achieved in all operating situations. When working with light-alloy pistons, this objective can be achieved only with special engineering efforts. This is because of the high coefficient of thermal expansion for lightweight alloys. In the past steel strips were often cast in place to influence expansion in response to heat ("regulating piston").

Figure 7-4 provides an overview of the amount of play found at the skirt and fire land for various piston designs.

Fig. 7-4. Normal installation play dimensions for light-alloy pistons in vehicular engines (as ‰ of nominal diameter; installation in gray cast engine block)

The amount of play at the wristpin, inside the wristpin boss, is important for smooth piston running and low wear at these bearing points. When determining the minimum play (Fig. 7-5), it is necessary, in the case of gasoline engines, to determine whether a floating wristpin is used or whether it is fixed in the small-end eye by shrink fit.

The floating wristpin is the standard design and the version that can handle the highest loads in the piston bosses. The "shrink-fit" conrod, which according to statements by some engine builders is more economical, is used only in gasoline engines. The shrink-fit conrod design is not suitable for modern diesel engines and for turbocharged gasoline engines.

Fig. 7-5. Minimum wristpin play in gasoline engines, in mm (not for racing engines)

Piston Masses. The piston and its accessories (rings, wristpin, circlips) form, together with the reciprocating share of the conrod, the reciprocating masses. Depending on the engine design, free mass inertias and/or free moments occur; in some cases these can no longer be compensated for or may be compensated for only with considerable effort. It is because of this phenomenon that, above all in the case of high-speed engines, the need to achieve the lowest possible reciprocating masses arises. The piston and the wristpin account for the largest share of the reciprocating masses. Consequently, weight optimization has to start here.

About 80% of the piston weight is located between the center of the wristpin and the upper surface of the head. The remaining 20% is located between the center of the wristpin and the end of the skirt. Of the major dimensions previously discussed, the determination of the compression height obtains decisive significance; with the determination of the compression height, about 80% of the piston weight is predetermined.

When dealing with direct-injection gasoline engines the piston head is used to deflect the stream and is shaped accordingly; see Fig. 7-6. The pistons are both taller and heavier. The center of gravity shifts upwards.

Fig. 7-6. Piston for a gasoline engine with direct injection

The piston's masses G_N can best be compared when one references them to the comparison volume V ~ D³ (without piston rings and the wristpin). It should be noted here, however, that the length of the compression height is always to be included in any analyses of the engine.

The mass indices G_N/D³ (without rings and the wristpin) for proven piston designs are shown in Fig. 7-7.

Fig. 7-7. Mass indices for passenger car pistons <100 mm diameter

Operating Temperatures. An important factor regarding operational reliability and safety and service life is the component temperature for both the pistons and the cylinders. The piston head, exposed to the hot combustion gases, absorbs varying amounts of heat, depending on the operating situation (engine speed, torque). These volumes of heat, where the pistons are not oil cooled, are given off to the cylinder wall primarily through the first piston ring and, to a far lesser degree, through the piston skirt. When piston cooling is affected, by contrast, a major part of the heat volume is transferred to the motor oil.

Because of the material cross sections determined by the engineering, there appear heat flows that result in characteristic temperature fields. Figures 7-8 and 7-9 show typical temperature distributions at pistons for gasoline and diesel engines.

Fig. 7-8. Temperature distribution at a piston for a gasoline engine. (See color section.)

Fig. 7-9. Temperature distribution at a piston with cooling channel for a diesel engine. (See color section.)

Severe thermal loading, on the one hand, reduces the durability of the material from which the piston is made. The critical points in this regard are the zenith of the boss and the edge of the recess in direct-injection diesel engines, and the transitional area between the hub connection point and the piston head in gasoline engines.

On the other hand, the temperatures in the first piston ring groove are significant in regard to oil carbonization. Whenever certain limit values are exceeded, the piston rings tend to stick and as a result are limited in their functioning. In addition to the maximum temperatures, the dependency of piston temperatures on engine operating conditions (such as engine speed, mean pressure, ignition angle, and volume injected) is of significance. Figure 7-10 shows typical values for gasoline and diesel engines used in passenger cars, in the area around the first piston ring groove, depending on the operation conditions.

Fig. 7-10. Influence of engine operating conditions on the piston groove temperatures