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Maximum Brake Torque (MBT) timing for an internal combustion engine is the minimum advance of spark timing for best torque. Traditionally, MBT timing is an open loop feedforward control whose values are experimentally determined by conducting spark sweeps at different speed, load points and at different environmental operating conditions. Almost every calibration point needs a spark sweep to see if the engine can be operated at the MBT timing condition. If not, a certain degree of safety margin is needed to avoid pre-ignition or knock during engine operation. Open-loop spark mapping usually requires a tremendous amount of effort and time to achieve a satisfactory calibration. This paper shows that MBT timing can be achieved by regulating a composite feedback measure derived from the in-cylinder ionization signal referenced to a top dead center crank angle position. A PI (proportional and integral) controller is used to illustrate closed-loop control of MBT timing.

Internal combustion engines are designed to maximize power subject to meeting exhaust emission requirements and minimizing fuel consumption. Maximizing engine power and fuel economy is limited by engine knock for a given air-to-fuel charge. Therefore, the ability to detect engine knock and run the engine at its knock limit is a key for the best power and fuel economy. This paper shows inaudible knock detection ability using in-cylinder ionization signals over the entire engine speed and load map. This is especially important at high engine speed and high EGR rates. The knock detection ability is compared between three sensors: production knock (accelerometer) sensor, in-cylinder pressure and ionization sensors. The test data shows that the ionization signals can be used to detect inaudible engine knock while the conventional knock sensor cannot under some engine operational conditions.

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 focuses on comparing the mass fraction burned obtained by pressure sensing and ion sensing through a conventional spark plug. The mass fraction burned curves obtained from both methods at different engine operating conditions are compared. In addition, the 0-10%; 50%, and 90% mass fraction burn durations are also compared. Traditionally, the mass fraction burned for a spark ignition engine is obtained through the measurement of pressure signal and the well known Rassweiler-Withrow method, which requires an expensive pressure sensor and some rather complex instrumentation. Ion sensing through a spark plug is very cost effective and reveals much information about the combustion process in the cylinder. Instead of making assumptions to attain the mass fraction burned, ion sensing by a spark plug allows us to obtain the mass fraction burned information through the Wiebe function constructed by several critical events shown in the ion signal during the combustion.

The ignitability of an air/fuel mixture by a spark plug is crucial for spark ignition engine performance, especially at idle and lean fuel conditions. It has been demonstrated that electrode geometry plays an important role in mixture ignitability. Typical industry practice for comparison of a mixture's ignitability by a spark plug is measured by the coefficient of variance (COV) of the indicated mean effective pressure (IMEP). However, the COV of IMEP is influenced by many other factors. Often the COV of IMEP fails to identify the influence of different electrode geometries on mixture ignitability, which hinders the determination of an optimal electrode design. This paper makes use of the 2% mass burn duration and location (when the spark timing is known) to gage mixture ignitability by a spark plug. Using this parameter, results obtained are consistent with the COV of IMEP.