Instantaneous IMEP Method

The Instantaneous IMEP method employs a force balance between the measured cylinder pressure force, the compressive force in the connecting rod, the inertia force and the friction force. Considering the components of these forces acting along the cylinder axis, the sum of the other three equals the friction force of the piston, rings and piston pin assembly. Figure 1 shows a single cylinder version of the Cummins 903 V-8 engine modified for the Instantaneous IMEP method. The large pipe emanating from the cylinder head region is a vent for the crankcase. This provides atmospheric pressure on the backside of the piston. A crankangle encoder is mounted on the front of the engine. Simultaneous measurements of cylinder pressure and rod compressive force are made at each crank angle over the 720° cycle.

In applying this method, inertia forces are

Figure 1. Single cylinder Cummins 903 engine equipped for Instantaneous IMEP Method.

commonly calculated from the theoretical equation, assuming a constant crank speed. As discussed later in the paper, for this one cylinder engine, and where inertia forces are quite large, crank speed variation must be taken into account in the calculation of the inertia forces. In the determination of the inertia forces it is necessary to consider the reciprocating mass of the piston, rings, the pin and that portion of the connecting rod which reciprocates with the piston. As reported in reference (4) a distributed model of the connecting rod inertia is necessary since it was found that the error associated with the use of a lumped model was excessive.

Figure 2 shows a strain gauged connecting rod used with the Instantaneous IMEP method. Pairs of 90° rosettes are installed on either side of a machined section located at approximately one-third of the connecting rod web length near the small end. Wires from the strain gauges,

Figure 2. Strain -gauged connecting rod.
Figure 3. Grasshopper linkage.

power supply, and thermocouple to measure the bridge temperature are attached to the connecting rod. From the connecting rod the wires are routed exterior to the engine by the use of a grasshopper linkage shown in Figure 3.

For purposes of illustration Figure 4 shows signals for Instantaneous IMEP method measurements made on a two liter gasoline engine at 800 rpm motoring, part-throttle. The top trace is the cylinder pressure, the center trace is the compressive force in the connecting rod and the lower trace is the inertia force. Note that the strain gauge signal includes abrupt jumps at the dead centers, indicated by the circles in the figure. These jumps arise from the change in static friction as the piston reverses direction.

Differencing these forces to obtain piston ring assembly friction results in the top curve of Figure 5. That curve shows piston ring assembly friction force as a function of crank angle. The static friction reversals at the dead centers are very apparent.

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