【正文】 directly. For this method extensive engine modification is required. Forbes and Taylor made the first attempt to measure the pistonassembly friction in a firing engine. The experiment was carried out using an elastically mounted cylinder liner. A very small amount of liner movement was allowed in the axial direction, and the axial movement of the liner was recorded photographically. Leary and Jovellanos and Livengood and Wallour later continued this work. Furuhama and Takiguchi measured pistonassembly friction measurement using the floating liner method on a modified singlecylinder diesel engine. Richez et al. , Parker et al. , Sherrington and Smith, Cerrato et al. , Wakuri et al. , Kikuchi et al. And Rogowski in their individual work, used the floating liner method for pistonassembly friction measurements. They all faced similar problems in their experiments, such as gas sealing at the liner top end, the effect of piston side force on the force sensor, and vibration. Cho et al. used the floating liner method with a straingagetype load cell instead of the more normal piezoelectric type to determine absolute friction force under motored conditions. The floating liner method requires extensive modification of the engine, isolating the cylinder liner from the engine block. This is carried out by having a special gassealing arrangement at the cylinder headliner interface and, as a result, may not give the true picture of an actual firing engine. In the IMEP method the pistonassembly friction force is determined indirectly by measuring the forces acting on the piston assembly and the connecting rod. That is, measuring the connecting rod strain to determine the small end axial reaction force and subtracting this from the axial forces due to cylinder pressure and piston inertia, thus, subtracting large numbers to determine the friction force, which is normally less than % of the maximum pressure force. As this method is liable to considerable potential inaccuracies, a very accurate measurement system and sensors are required. However, the advantage of the IMEP method is that it requires almost no modification of the engine and therefore measures the true pistonassembly friction in a firing engine. Uras and Patterson used the IMEP method for the first time to measure pistonassembly friction force. A special device called a grasshopper linkage was used to lead the wires from the strain gages on the connecting rod to the side of the crankcase. Although this method was successful in achieving results, there was a limitation on the engine operating speed and load because at high speeds the wires broke. Also the errors accumulated from various input data measurements and processing resulted in some inaccuracies. As the IMEP method allows true measurement of pistonassembly friction without any major engine modification, this method was adopted to study pistonassembly friction under different engine operating conditions.2 PistonAssembly Friction ForceThe forces acting on the piston assembly in a real firing engine in the direction along the central axis of the cylinder liner are the gas force acting on the piston crown, pistonassembly inertial force, connecting rod force, weight of the piston assembly, and the pistonassembly friction force. Using the information from a 720 pulse per revolution encoder connected to the front end of the crankshaft and simple geometry, the direction of all the above forces can be determined. Thus if any of the three forces mentioned above are known, the fourth can be calculated. This is the basic principle of the IMEP method used for measuring pistonassembly friction loss. To measure pistonassembly friction, very accurate measurement of gas force and connecting rod force is required. Normally the gas and connecting rod force are on the order of 10,000–30,000 N, whereas the friction force is around 50–300 N. Thus any small error in measuring gas or connecting rod force will result in high inaccuracy, for example, an error of % in measuring gas or connecting rod force can result in a nearly 15% error in instantaneous pistonassembly friction force. To measure instantaneous pistonassembly friction in a real firing engine, experiments were carried out on a singlecylinder Ricardo Hydra gasoline engine, the design of this being based on a real engine, a GM liter Cavalier engine. The engine specification is given in the Appendix . A detailed description of all the forces acting on the piston assembly and connecting rod is shown in Fig. 1. Forces Acting on the Piston Assembly. According to Fig. 1, the sum of all the forces acting on the piston assembly in the axial direction is given by …… (1)P1?P2 is the net gas pressure acting on the piston assembly, A is the calculated average cylinder bore area (bore = mm). ao is the piston axial acceleration calculated every deg crank angle. mpg, is the pistonassembly weight (mp= g). CY is the force acting at the center of the small end bearing. mpao is the pistonassembly inertial force and is calculated as Piston accelerationmass of [piston + piston rings + piston pin + connecting rod section A, (Fig. 2)].For measuring pistonassembly inertial force, the mass of connecting rod section A (87 g) is added to pistonassembly mass as it is always in translational motion. Forces Acting on the Connecting Rod. According to Fig. 1 and the full bridge straingage circuit fitted at a distance of mm from the bigend bearing center (Fig. 2), the ponent of forces in the axial direction at the point where strain gages are fitted is given by …… (2)FSTG is the force acting along the connecting rod principal axis measured by the strain gauge circuit. FC is the inertial force of connecting rod section B _Fig. 2_ and can be calculated in two different ways. One option is to assume that the mass of the connecting rod is concentrated at the ends, whereas the alternative is to treat the connecting rod as a tr