SAE Technical Paper 2009-01-1984 A new simple friction model for S. I. engine Emiliano Pipitone Department of Mechanics – University of Palermo Copyright © 2009 SAE International DOI: 10.4271/2009-01-1984 ABSTRACT Internal combustion engine modeling is nowadays a widely employed tool for modern engine development. Zero and mono dimensional models of the intake and exhaust systems, combined with multi-zone combustion models, proved to be reliable enough for the accurate evaluation of in-cylinder pressure, which in turn allow the estimation of the engine performance in terms of indicated mean effective pressure (IMEP). In order to evaluate the net engine output, both the torque dissipation due to friction and the energy drawn by accessories must be taken into consideration, hence a model for the friction mean effective pressure (FMEP) evaluation is needed. One of the most used models accounts for engine speed dependent friction by means of a quadratic law, while the effect of engine load (i.e. the thrust that the gas exercises on the piston surface) is considered by means of a linear dependence from the maximum in-cylinder pressure: hence the model requires the calibration of four constants by means of experimental data. The author, on the basis of data acquired during an extensive experimental campaign carried out on the engine test bed, found this model to give an unsatisfying prediction, above all for retarded pressure cycles (i.e. with peak pressure positions higher than 20 crank angle degrees after top dead centre): hence, by means of analysis performed using these experimental data, the author arrived at a new formulation of the friction model, which substantially take into account the effect of engine load by means of the Location of Pressure Peak (LPP). The new model, once calibrated, proved to be effectively more accurate in the prediction of the FMEP than the Chen-Flynn model. INTRODUCTION Engine modeling is nowadays one of the most employed tools for internal combustion engines development. One of the most common output of an internal combustion engine model is the in-cylinder pressure, which allow the evaluation of the indicated mean effective pressure (IMEP). In order to obtain the brake mean effective pressure (BMEP), which is the real engine output, a sub- model for the friction mean effective pressure (FMEP) evaluation must be employed. Different approach to this problem can be followed, as proposed in literature: complex models, such as those from [1, 2, 3], estimate the instantaneous torque losses taking into account the different contribution to the total energy dissipated by friction or drawn from accessories (valve train, pumps, etc..); in these kinds of models, the friction losses at the piston-wall interface are evaluated separately from the losses at the bearings, and even the different kind of lubrication that may occur are considered (boundary, mixed or hydrodynamic lubrication). When calibrated, these models succeed in giving a precise prediction of the FMEP (obtained integrating the total torque losses), but this normally requires the estimation of several constants, possible only by means of accurate and detailed experimental data on instantaneous in-cylinder pressure and crankshaft speed. Simpler models, instead, aim to estimate the overall FMEP, making use of few global variable, typically one related to the engine load and the other related to the engine speed, in order to separately account both the energy dissipated by friction due to gas thrust and the energy losses influenced by the speed (e.g. those related to inertia forces). In this second category, one of the most encountered in literature and employed in commercial software is known as the Chen & Flynn model [4], according to which the FMEP depends on in- cylinder maximum pressure and engine speed by means of the following law: 2 max n D n C P B A FMEP        (1) As shown, this model accounts for the engine speed effect by means of a quadratic law (through the constants C and D), while the load effect is represented by the maximum in-cylinder pressure through the constant B; the constant A instead accounts for the energy drawn by accessories and all the other invariable factors. MAIN SECTION During the set-up phase of an engine model for the prediction of engine performances, the author experienced the necessity to use an FMEP model. Lacking of detailed data on instantaneous speed and acceleration, the author decided to follow the common approach of the Chen-Flynn model, whose four