On-Board Diagnostics (OBD) regulations impose missing combustions detection within a wide portion of the engine operating range. Missing combustions can be caused either by ignition (misfire) or injection (misfuel) system failures. Missing combustions can damage the catalyst and cause abrupt pollutants increases (especially HC), but misfuels are not as detrimental as misfires, both from the emissions and the after treatment system life point of view. It would be important for the Electronic Control Unit (ECU) to be informed not only about the fault event, but also about its type, for the purpose of setting the right recovery strategy. The aim of this paper is to analyze missing combustion phenomena, in order to find out if a fault recognition strategy able to distinguish between misfire and misfuel can be setup. Different approaches can be found in the literature to diagnose missing combustions: many of them are based on the speed signal analysis, both in time and frequency domains, others use the knock accelerometer signal, or the exhaust manifold pressure information. A Universal Exhaust Gas Oxygen (UEGO) sensor can also be used. Usually diagnosis methodologies consist in observing signals perturbations subsequent to the malfunction event. Observable consequences of missing combustions are, for example, a sudden lack of indicated torque, causing vibrations and speed fluctuations, an increasing in exhaust gases Oxygen content, anomalous exhaust pressure ripples, etc. Many phenomena interact influencing in different ways the engine behavior, during and after the fault event: their effect can depend on the fault cause, thus helping the recognition. The first combustion taking place in the faulty cylinder after a misfire (post-misfiring cycle) usually leads to higher indicated pressure and torque levels if compared to standard values for the same operating conditions, while the same cannot be said for the post-misfueling combustion. On the other side, Air-Fuel Ratio (AFR) assumes different trends during the misfiring and post-misfiring cycles, with respect to misfueling and post-misfueling cycles. A 4 cylinders 1.2 liters spark ignition port injected engine, equipped with a programmable Electronic Control Unit (ECU) has been tested on the test bench, inducing both misfires and misfuels, over a wide engine operating range, while monitoring the engine faulty behavior. Misfire and misfuel-related phenomena have been analyzed showing their “signature” on indicated pressure and torque, engine speed and Air-Fuel Ratio measured signals, in order to define the most reliable recognition strategy.

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