Cf are obtained from the data of the beginning of expansion stroke (θc=8°ATDC) and the midstroke of expansion stroke (θc=82°ATDC). Four kinds of lubricants mentioned previously are employed for the correlation. With the increase in μU/W', the Cf value at the beginning of stroke decreases and that at the midstroke increases. This indicates the boundary and hydrodynamic friction characteristics respectively.
4. Experiment
A floating liner method17) 19) 20) was adopted as a means of measuring the friction force of the piston assembly and the experimental results were compared to the calculations. A schematic of the test apparatus is illustrated in Fig.14. The specifications of the engine are the same as those used in the calculations. The cylinder liner ? was isolated from the cylinder block and the cylinder head to detect the instantaneous friction force exerted on the cylinder liner by the piston assembly. Eight hydrostatic bearings ? were incorporated in the apparatus so that the floating liner was firmly supported in thc radial direction while the liner was allowed to move freely in the axial direction. The axial force of piston assembly exerted on the cylinder liner was measured by a piezo-electronic sensor ? at the base of the liner. The technique of a force-balancing gas seal is employed to establish an effective gas seal without preventing the axial movement of the liner. A typical friction diagram for piston assembly under the firing condition is depicted in Fig.15. The calculated friction diagram of piston ring pack based on the present mixed lubrication model is also superimposed as the dotted line in the figure. As to the feature of measured friction diagram, the occurrence of friction spike is observed at the beginning of each stroke due to the mixed or boundary friction, particularly at the beginning of the expansion stroke, the sharp friction spike occurs. After the friction spike, the friction force decreases and then varies corresponding to the variation of sliding speed. The calculation result can explain the qualitative tendency of the measured friction diagram except that the friction spike at the end of the compression stroke. However, the calculated value of Pf is smaller than the experimental value and is around 70% of experimental one. This seems reasonable value considering the experimental value of Pf contains extra friction forces besides the one between rings and a liner. Fig.16 shows the effects of engine load. Experimental and calculated values of Fmax increase with the increase in the engine load.
Fig. 15 Typical Friction Diagram
