270 360 450 540 630 720

Figure 10. Firing piston and ring friction. 1200 rpm, 29.17 psia manifold pressure, 200° F water and oil. Friction mep= 2.25 psi.

The more than 50 rpm variation in speed over the cycle of this one cylinder engine affects the inertia force significantly. Thus consideration of the effect of instantaneous crank speed is necessary in this case. The friction trace is distorted as shown in Figure 12 if this is not done.

Our conclusion is that accounting for the crankshaft magnetism and taking into account the instantaneous crank speed variation of the engine improves the Instantaneous IMEP results significantly. At the least results are qualitatively meaningful and can show unusual engine behavior. If the friction force at 360° crankangle were to suddenly become large due to problems of scuffing or breaking of piston rings, that change is expected to be very evident. General Comments. The Instantaneous IMEP method, when properly implemented, has the potential of determining piston and ring assembly friction with good accuracy over a large portion of the 720° cycle. It is estimated that about 90% of the cycle is reported with good accuracy. At the very highest pressures and combustion temperatures the uncertainties become greater. Thus about 10% of the crank angles of the cycle, around top dead center combustion, have relatively high uncertainty with respect to friction forces. The method is projected to be able to assess changes in friction coming about from engine or lubrication failures. A major plus for the Instantaneous IMEP method is that essentially no engine structural modifications are required. Our overall experience with this method in both gasoline and diesel engines shows that it has very good repeatability.

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