Abstract
Keywords Spark advance, feedback control, proportional integral control, cylinder pressure, spark ignition engine
Abstract In order to reduce the impact of both increasing fuel cost and CO2 emission, modern spark ignition engines need to lower as much as possible fuel consumption; as is known, a fundamental parameter for efficiency improvement is spark advance. The control of this parameter is actually realized in open loop by means of maps stored in the ECU, but since optimal spark timing depends on many variables, like ambient conditions (temperature, pressure and humidity), fuel quality, engine ageing and wear, this kind of control does not allow the engine to run always at its best performance. The optimal choice would be an adaptive spark advance control system, which would ensure the minimum fuel consumption in every condition. In the last twenty years several researchers have studied techniques for the closed-loop control of spark advance, mainly by means of cylinder pressure analysis or engine speed fluctuation, and various methods have been proposed. As far as the pressure analysis is concerned, some researchers focused on the pressure peak phase during the expansion stroke, others on the combustion pressure, which yields the mass fraction burned and allows optimal combustion timing. In the present work the authors tested the performances of a proportional-integral spark timing feedback 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 four-cylinder series production engine and an eddy current dynamometer, using two data acquisition boards. The 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 quickness in reaching the optimal spark advance, 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 PI feedback spark advance control, allowing fast and reliable control even in transient operations.