MBT timing for an internal combustion engine is also called minimum spark timing for best torque or the spark timing for maximum brake torque. Unless engine spark timing is limited by engine knock or emission requirements at a certain operational condition, there exists an MBT timing that yields the maximum work for a given air-to-fuel mixture. Traditionally, MBT timing for a particular engine is determined by conducting a spark sweep process that requires a substantial amount of time to obtain an MBT calibration. Recently, on-line MBT timing detection schemes have been proposed based upon cylinder pressure or ionization signals using peak cylinder pressure location, 50 percent fuel mass fraction burn location, pressure ratio, and so on. Because these criteria are solely based upon data correlation and observation, both of them may change at different engine operational conditions. Therefore, calibration is still required for each MBT detection scheme. This paper shows that MBT timing can be achieved by locating the maximum net pressure acceleration point at top dead center. This result is developed based upon the physical aspects of the combustion process, and therefore, it should be independent of engine operational conditions and valid for all spark-ignited engines that have one peak heat release rate during the combustion process. Experimental validation of this result over certain engine operational conditions is completed, and validation of this result over whole engine operational map is the subject of future work.
The second part of this paper develops an MBT timing closed-loop control using the detected MBT timing criteria as a feedback signal. The benefit of closed loop control of MBT timing is improved robustness over open-loop MBT timing calibration with respect to engine-to-engine variations, engine aging, engine operational conditions, etc. A two-way filtering algorithm, combined with the derivative calculation, is developed to improve the robustness of MBT timing detection scheme without the penalty of filter phase delay. A PI (proportional and integral) controller is used to illustrate closed-loop control of MBT timing, where the reference signal is used to control the engine ignition timing at its set point. The closed-loop control system is implemented in dSpace and prototyped on a two liter four cylinder engine. The test results show that the closed-loop MBT timing controller based upon the maximum net pressure acceleration point not only maintains the engine average ignition timing at its MBT timing but also reduces the cycle-to-cycle variations. For comparison purpose, three MBT timing feedback signals are used in the study: peak cylinder pressure location, 50 percent burn location, and maximum net pressure acceleration location.