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

Abstract

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