## 1 Introduction
### Methods for Measuring Instantaneous Piston Assembly Friction  
Part of the challenge of evaluating the contribution of the piston ring and skirt boundary friction is the fact that it is almost impossible to measure in a practical firing engine. The floating liner method is one of the methods that has shown promise in isolating this term, but unfortunately, the increasing pressures also affect the sealing mechanism of the floating liner while piston side loads also interfere with the axial force balance via deflections and friction. As a result, the measured piston assembly liner force does not necessarily represent the actual friction. Another factor that limits the accuracy of floating liner testing is the fact that the state of wear of the cylinder liners and piston rings is generally in the close to new condition with the honing marks still fresh, and the piston ring profile is still mostly intact as it was optimized by its designers. However, a real heavy-duty engine as it operates in the field will reach a state relatively early in its useful life where the honing marks have disappeared from much of the top piston ring stroke, and the piston rings have worn out and reached their steady-state profile. It would be impractical to run the floating liner for sufficient hours (possibly over 1000) in order to reach this state. Therefore, this method has value in isolating changes in the frictional characteristics with different operating or design parameters. However, these results may not be sufficiently accurate to draw conclusions on the absolute energy contribution of different friction terms. Another method that has been proposed in the past to isolate the individual components in a firing engine, the P-w method, was demonstrated by [PERSON] et al. in 1991. The primary value of this approach is that it gives the overall instantaneous friction torque of a single cylinder engine without any modifications to the engine itself. One of the downsides of this method is that it only worked for low load cases and for single cylinder engines with a low moment of inertia. While the overall instantaneous torque cannot by itself identify and separate individual components, since we know, when in the cycle, high instantaneous friction torque takes place, we can deduce the likely source. Figure 1 shows a sample of the instantaneous friction torque. The conclusion from this work is that the friction torque is dominated by the high-pressure part of the cycle, where the piston assembly boundary terms are active (note, according to [PERSON] et al. [1], the frictional losses of the crank bearings are almost independent of instantaneous bearing load, and are more or less constant throughout the cycle). This trend remains even when the engine load is zero, but not as pronounced as when the engine produces torque. This was the case, despite the fact that the oil sump temperatures for those tests were fairly low, amplifying the hydrodynamic terms. At increasing loads, the dominating spike increases, increasing the overall FMEP by 20% (despite the rather low brake mean effective pressure (BMEP) that this air-cooled, naturally aspirated Diesel can generate). It can be safely deduced that the piston assembly boundary friction (plus the injector pump, but in these type of engines, the injection pressure was fairly low and, therefore, this contribution was small) is the primary contributor for this 20% increase in total FMEP. It can also be deduced that while this term is important even at idle, at the much higher BMEP that modern heavy-duty Diesel engines operate, this term has a considerable contribution and possibly even dominates the mechanical losses. An additional method of measuring friction losses in firing Diesels is the measurement of IMEP (indicated mean effective pressure) and BMEP. This is also not an easy  
Figure 1: Instantaneous frictional torque of a single cylinder Diesel [2]. This particular case examined the effect of lubricant viscosity in instantaneous friction.