In order to reduce fuel cost and CO2 emissions, modern spark ignition (SI) engines need to lower as much as possible fuel consumption. A crucial factor for efficiency improvement is represented by the combustion phase, which in an SI engine is controlled acting on the spark advance. This fundamental engine parameter is currently controlled in an open-loop by means of maps stored in the electronic control unit (ECU) memory: such kind of control, however, does not allow running the engine always at its best performance, since optimal combustion phase depends on many variables, like ambient conditions, fuel quality, engine aging, and wear, etc. A better choice would be represented by a closed-loop spark timing control, which may be pursued by means of combustion phase indicators, i.e., parameters mostly derived from in-cylinder pressure analysis that assume fixed reference values when the combustion phase is optimal. As documented in literature (Pestana, 1989, “Engine Control Methods Using Combustion Pressure Feedback,” SAE Paper No. 890758; BERU Pressure Sensor Glow Plug (PSG) for Diesel Engines, http://beru.federalmogul.com; Sensata CPOS SERIES—Cylinder Pressure Only Sensors, http://www.sensata.com/download/cpos.pdf; Malaczynski et al., 2013, “Ion-Sense-Based Real-Time Combustion Sensing for Closed-Loop Engine Control,” SAE Int. J. Eng., 6(1), pp. 267–277; Yoshihisa et al., 1988, “MBT Control Through Individual Cylinder Pressure Detection,” SAE Paper 881779; Powell, 1993, “Engine Control Using Cylinder Pressure: Past, Present, and Future,” J. Dyn. Syst., Meas. Control, 115, pp. 343–350; Muller et al., 2000, “Combustion Pressure Based Engine Management System,” SAE Paper 2000-01-0928; Yoon et al., 2000, “Closed-Loop Control of Spark Advance and Air-Fuel Ratio in SI Engines Using Cylinder Pressure,” SAE Paper 2000-01-0933; Eriksson, 1999, “Spark Advance Modeling and Control,” Dissertation N° 580, Linkoping Studies in Science and Technology, Linköping, Sweden; Samir et al., 2011, “Neural Networks and Fuzzy Logic-Based Spark Advance Control of SI Engines,” Expert Syst. Appl., 38, pp. 6916–6925; Cook et al., 1947, “Spark-Timing Control Based on Correlation of Maximum-Economy Spark Timing, Flame-Front Travel, and Cylinder Pressure Rise,” NACA Technical Note 1217; Bargende, 1995, “Most Optimal Location of 50% Mass Fraction Burned and Automatic Knock Detection,” MTZ, 10(56), pp. 632–638.), the use of combustion phase indicators allows the determination of the best spark advance, apart from any variable or boundary condition. The implementation of a feedback spark timing control, based on the use of these combustion phase indicators, would ensure the minimum fuel consumption in every possible condition. Despite the presence of many literature references on the use combustion phase indicators, there is no evidence of any experimental comparison on the performance obtainable, in terms of both control accuracy and transient response, by the use of such indicators in a spark timing feedback control. The author, hence, carried out a proper experimental campaign comparing the performances of a proportional-integral spark timing control based on the use of five different in-cylinder pressure derived indicators. The experiments were carried out on a bench test, equipped with a series production four cylinder spark ignition engine and an eddy current dynamometer, using two data acquisition (DAQ) systems for data acquisition and spark timing control. Pressure sampling was performed by means of a flush mounted piezoelectric pressure transducer with the resolution of one crank angle degree. The feedback control was compared to the conventional map based control in terms of response time, control stability, and control accuracy in three different kinds of tests: steady-state, step response, and transient operation. All the combustion phase indicators proved to be suitable for proportional-integral feedback spark advance control, allowing fast and reliable control even in transient operations.
