Cycle-Resolved Detection of Combustion Start in SI Engines by Means of Different In-Cylinder Pressure Data Reduction Techniques

An original technique for the detection of combustion start in SI engines on a cycle-by-cycle basis was proposed and applied to the analysis of pressure time-histories taken on a bi-fuel engine fueled by either gasoline or CNG. Such a technique locates the onset of combustion on the basis of the earliest release of chemical energy. It stems from the fact that, during the compression stroke, changes in the charge sensible energy, and thus in the cylinder pressure, are ruled only by work and heat exchanges with the combustion chamber walls. Hence, an imbalance in these three energies indicates the correspondent release of chemical energy, identifying combustion onset. The results of this technique were compared to those obtained through a direct analysis of in-cylinder pressure time-histories on logarithmic-coordinates p-V diagrams. More specifically, compression stroke appears like a segment on such diagrams and thus combustion onset can be defined by the detachment of in-cylinder pressure curve from linearity. The results of the different approaches for combustion start detection were compared on a wide range of working conditions of the bi-fuel SI engine under both gasoline and CNG fueling. The experimental matrix covered different engine speeds (N = 2000–4600 rpm), loads (bmep = 200–790 kPa), relative air-fuel ratios (RAFR = 0.80–1.60) and spark advances (SA ranging from 8 deg retard to 2 deg advance from MBT timing). 100 consecutive in-cylinder pressure traces were analyzed for each point in the test matrix. Particular attention was also given to the techniques applied for in-cylinder pressure filtering, which proved to be fundamental for accurate cycle-by-cycle investigation. Finally, on the basis of the experimental results obtained through the chemical-energy approach, two correlations for flame-development angle prediction are proposed, one for gasoline and the other for CNG. These correlations are based on cylinder-average thermodynamic properties at SA and can be usefully applied for triggering the flame propagation routines in indicated-cycle simulation codes.