Investigation of Correlations Between COV of Ion Integral and COV of IMEP in a Port-Injected Natural-Gas Engine
Correlations between the coefficient of variations (COV) of ion current integrals and COV
of indicated mean effective pressure (IMEP) are investigated. The investigations are made
to see if there is a possibility to use ion sense technique for closed loop combustion control
to maximize Exhaust Gas Recirculation (EGR) level with remaining combustion stability.
The engine studied in this thesis is a stoichiometric natural gas engine with a three way
catalyst working with different EGR levels. One way to increase the efficiency in a stoichiometric
natural gas engine is to maximize the EGR level [3]. With a higher EGR level
it is possible to open the throttle even further with a result of lower pumping losses. Also
temperature losses to chamber walls will be smaller with high EGR levels. A closed loop
system controlling the EGR valve against COV of IMEP around 5 % is used for this engine.
For COV of flame peak integral in range from 0 to 15-20% of COV of IMEP a linear
correlation was found for lambda less than 1.3. For COV of two peaks integral a linear
correlation was found for the entire measured range. This correlation may not be used for
a robust EGR control system due to a small increase of COV of ion integral and due to a
high variance in COV of ion integral. Correlations are discussed for different ion integrals
with different air/fuels ratios and EGR levels. A control strategy is also presented.
Johan Molin
2008
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