Chao F. Daniels

MBT Timing Detection and its Closed-Loop Control Using In-Cylinder Pressure Signal

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 openloop MBT timing calibration with respect to engine-toengine 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.

Inaudible Knock and Partial-Burn Detection Using In-Cylinder Ionization Signal

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. Detection of inaudible knock is important since it will improve the existing knock control capability to allow the engine run at its inaudible knock limit. Partial-burn detection using ionization is also shown in this paper. A comparison of both in-cylinder pressure and ionization sensor signals are used in this analysis. The test results show that some light partial-burn cases can only be detected by ionization signals. The partialburn information appears to be difficult to observe under some conditions using the pressure trace directly.

MBT Timing Detection and its Closed-Loop Control Using In-Cylinder Ionization Signal

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. The test results show that the control, using the ionization current based feedback signal, not only maintains the engine average ignition timing at its MBT timing but also reduces the cycle-to-cycle variations.

On Combustion Invariants for MBT Timing Estimation and Control

An MBT timing control criterion has been studied through burn rate analysis using pressure measurements. The optimal spark timing which maximizes the power output mainly depends on engine speed, load and mixture composition among others. A combustion invariant at MBT timing is sought to characterize the MBT combustion. Specifically, it has been shown that the maximum mass fraction acceleration point occurs around TDC at MBT timing by analytically processing the wide range spark sweep data.Copyright

Closed Loop Maximum Dilution Limit Control using In-Cylinder Ionization Signal

This paper presents a combustion stability index derived from an in-cylinder ionization signal to control the engine maximum EGR limit. Different from the existing approaches that use the ionization signal values to gauge how much EGR was added during the combustion, the proposed method concentrates on using the ionization signal duration and its stochastic properties to evaluate the end result of EGR on combustion stability. When the duration index or indexes are higher than pre-determined values, the EGR limit is set. The dynamometer engine test results have shown promise for closed loop EGR control of spark ignition engines. INTRODUCTION Exhaust Gas Recirculation (EGR) is a well-known practice to improve engine fuel economy and reduce NOx emissions in certain operating regimes. For EGR, a portion of the exhaust gas is either recirculated back to intake manifold through a link between the intake and exhaust manifolds (external EGR) or trapped inside the cylinder through valve timings (internal EGR) for engines having a variable valve timing mechanism in order to mix with the fresh air for the next combustion event. Dilution of the fresh air-charge mixture with the inert exhaust gas lowers the combustion temperature and therefore suppresses the NOx formation. Currently, the EGR setting for a given engine operating condition is generally pre-determined by extensive engine calibrations and implemented in real time utilizing the stored maps in an open loop control setting. The amount of EGR for each operating condition needs to be determined based on the emission and combustion stability considerations. The addition of EGR not only reduces NOx emissions, but also allows better fuel economy until excessive dilution starts to deteriorate the combustion quality. Generally speaking, as long as the combustion stability is within the desired operating range, the higher the EGR content, the better the fuel economy and the lower the NOx emission results for a steady state condition. Combustion stability is often measured by the COV of IMEP (COVariance of Indicated Mean Effective Pressure). The lower the COV value, the better the combustion stability. However, there is no direct method to obtain the COV of IMEP for an operating condition without an in-cylinder pressure sensor. Therefore, the closed loop control of EGR, lean limit, idle spark timing, or any other stability related engine control, is challenging in the absence of an in-cylinder pressure measurement. Without having an online measurement, all the settings are pre-calibrated and generally applied as they are for all the engines of the same type during the engine’s lifetime. For an SI (Spark Ignition) engine, when the combustion stability is beyond the desired stability limit, the crank angles for the combustion to reach a certain fraction of fuel burned or for the combustion to complete usually become greater compared to a normal combustion event. The standard deviations for the corresponding angles also get larger [1]. Furthermore, both the initial flame development (05% burned) and main combustion durations (10-90% burned) increase when EGR increases (see [2], among others). Therefore, the COV of IMEP is not the only indicator of combustion stability for an SI engine, how long the combustion process takes in crank angle degrees could also be an alternative indicator as well. However, the burn duration calculation still relies on an in-cylinder pressure measurement. On the other hand, in-cylinder ionization signals have recently gained a lot of attention for combustion/engine control purposes. An example of a 300-cycle average ionization signal is shown in Figure 1. It usually consists of two peaks following the ignition pulse. The first peak represents the flame kernel growth and development around spark plug (chemical ionization), and the second peak is the re-ionization (thermal ionization) due to the in-cylinder temperature increase as a result of both pressure increase and flame development in the cylinder. The thermal ionization may disappear at operating conditions with light loads or high EGR rates. Nevertheless, an ionization signal provides a detailed fingerprint about the combustion process. It shows when a flame kernel is formed and propagates away from the spark plug gap, when the combustion is 2005-01-3751 Closed Loop Maximum Dilution Limit Control using In-Cylinder Ionization Signal Ibrahim Haskara, Guoming Zhu, Chao Daniels and Jim Winkelman Visteon Corporation Downloaded from SAE International by Brought To You Michigan State Univ, Saturday, April 04, 2015