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SAE TECHNICAL
PAPER SERIES
2004-01-2977
IC Engine Retard Ignition Timing Limit Detection
and Control using In-Cylinder Ionization Signal
Ibrahim Haskara, Guoming G. Zhu and Jim Winkelman
Visteon Corporation
Reprinted From: SI Engine Experiment and Modeling
(SP-1901)
Powertrain & Fluid Systems
Conference and Exhibition
Tampa, Florida USA
October 25-28, 2004
400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760 Web: www.sae.org
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2004-01-2977
IC Engine Retard Ignition Timing Limit Detection
and Control using In-Cylinder Ionization Signal
Ibrahim Haskara, Guoming G. Zhu and Jim Winkelman
Visteon Corporation
Copyright © 2004 SAE International
ABSTRACT
Internal combustion engines are designed to maximize
power
subject
to meeting exhaust emission
requirements and minimizing fuel consumption.
However, the usable range of ignition timing is often
limited by knock in the advance direction and by
combustion instability (partial burn and misfire) in the
retard direction. This paper details a retard limit
management system utilizing ionization signals in order
to maintain the desired combustion quality and prevent
the occurrence of misfire without using fixed limits. Incylinder ionization signals are processed to derive a
metric for combustion quality and closeness of
combustion to partial burn/misfire limit, which is used to
provide a limiting value for the baseline ignition timing in
the retard direction. For normal operations, this assures
that the combustion variability is kept within an
acceptable range. During start-up operations, the retard
limit management can be used as a rapid catalyst lightoff strategy by maximally delaying the combustion as
long as misfire and partial-burn are avoided. This
improved start-up strategy reduces cold-start HC
emissions by reducing the time required to increase the
catalysts temperature to its light-off level. The closed
loop nature of the system provides maximum usage of
the possible ignition timing range in the retard direction
at any given operating condition.
INTRODUCTION
In a conventional spark-ignition (SI) engine, combustion
is initiated at the spark plug by an electrical discharge.
Recent advances in the powertrain electronic controls
make it possible to employ online spark adjustment to
optimize the engine operation in terms of power, fuel
economy and emissions. However, the range of ignition
timing one would like to use is often limited by knock in
the advance direction and by combustion instability
(partial burn and misfire) in the retard direction. Since
the feasible ignition timing range depends on the engine
operating conditions, it is beneficial to extract this online
through in-cylinder combustion information in order to
optimally adjust the operational boundaries.
The use of in-cylinder sensors for combustion control is
a promising way of improving the fuel economy, power
output and emissions of production vehicles (see
4,5,6,7,8 and the references cited therein among
others). In this paper, in-cylinder ionization signals are
studied to determine a combustion stability metric for
ignition retard limit. Spark sweeps are conducted at
different operating points. Stochastic properties (mean,
variance and probability distribution functions) of peak
ionization location and the ionization energy content
distribution are correlated to the combustion quality
(COVariance of Indicated Mean Effective Pressure
IMEP) and combustion type (normal, late but complete,
incomplete and misfire). A feedback parameter is
derived from the ionization signal as an indicator of the
closeness to the retard limit. Pressure measurements
are used to confirm information for these correlations.
The data is then used to generate stochastic relations
from ignition timing to the processed ionization
parameter at different operating conditions.
A stochastic ignition retard limit control utilizing the
derived retard limit ionization parameter is also proposed
in the paper. Experimental data from dynamometer tests
is included to demonstrate that the controller can limit
and correct the ignition timing to keep the combustion
quality/stability from exceeding a user-specified level. It
is further shown that the control system is able to
operate the engine at its retard limit despite the cycle-tocycle combustion variability and inherent ionization
signal variations owing to that stochastic nature.
RETARD LIMIT MANAGEMENT
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During engine warm-up, the retard limit management
can seek the maximum retard possible while assuring
that misfire is avoided with the objective of increasing
the catalyst temperature rapidly. Delaying the
combustion through high values of ignition retard can
shorten the time that it takes the catalyst to reach its
light-off temperature. Therefore, the conventional threeway catalyst becomes effective much sooner in reducing
tail-pipe emissions (1, 2, 3). However, if the ignition
retard is too much, engine-out HC emissions become
excessive due to incomplete combustion as well as
misfire. An open loop retard calibration needs to provide
enough margins to avoid misfire at all conditions and
with all types of fuels. It therefore is inherently
conservative. On the other hand, a real time retard limit
indicator as part of a closed loop strategy alleviates this
conservatism by further being able to push the timing in
the retard direction if things are favorable. That way, the
catalyst light-off time is minimized and the tail-pipe
emissions can be reduced.
ignition timing is too early (advanced), useful combustion
work is wasted during the compression stroke. On the
other hand, if the combustion process starts too late
(retard), the peak pressure occurs later in the expansion
stroke and its resulting torque diminishes. The optimum
spark, for which the maximum brake torque is obtained
in the presence of these opposing trends, is called MBT
timing. Maximum brake power and minimum brake
specific fuel consumption are also achieved with MBT
timing. A typical cylinder pressure behavior during a
spark sweep is shown in Figure 2. This data was
gathered from a 3L, 6-cylinder engine operated in an
o
engine dyno. Spark timing was swept from 37 BTDC to
o
25 ATDC at 1500 RPM and 2.5 bar BMEP for MBT
spark. For each spark, the plotted pressure signal is the
average of 300 consecutive cycles.
The architecture of a stochastic retard limit management
system is shown in Figure 1. ,RQL]DWLRQGHWHFWLRQ ,RQ
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The ignition coil in this system is both a sensor and an
actuator. As a sensor, it has one ionization output signal.
As an actuator it has one dwell control input signal for
ignition. The ionization feedback signals of all cylinders
are fed into the signal conditioning circuit, and signals
are merged. The conditioning signal is then sampled and
processed to determine a retard limit indicator, as it will
be outlined in the following sections. The further details
of the set-up can also be found in 5,6 and 7.
COMBUSTION BEHAVIOR WITH IGNITION
RETARD
For a spark-ignition (SI) engine, as ignition timing is
varied relative to TDC (top dead center), the cylinder
pressure, and in turn, combustion torque varies. If the
As shown in Figure 3, the combustion torque peaks at a
o
spark around 29 BTDC (indicating the MBT timing for
this operating point) and starts falling when the spark is
further retarded.
The covariance of IMEP is also included in Figure 4 as a
measure of combustion quality. As a practical
convention, a covariance value around 3% is regarded
as an indication of good quality combustion. For this
operating point, ignition timing should be kept before 17
o
BTDC in order to maintain the covariance below 3%.
The covariance goes beyond 5% with an ignition timing
o
of 11 BTDC. As the ignition is retarded beyond that
value, the combustion variations and roughness further
increase. Before that, the combustion can be
categorized as normal. When the ignition timing is
delayed, combustion starts to take place later in the
expansion stroke but initially can still be completed
before the exhaust valves are opened (late but
complete). With further retard, the allotted duration
becomes too small for the combustion to be completed
and the combustion further extends to the exhaust
stroke (incomplete). Eventually, for some cycles,
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combustion does not grow at all and the cylinder
contents go to the exhaust manifold directly (misfire).
other hand, for the cycle of Figure 6, the combustion
duration is longer compared to that of Figure 5 based on
its burn profile. However, the burning still ends much
before the exhaust valves are opened. Therefore, the
combustion is late but still complete. The ionization
signal is a bit noisier than that of the Figure 5, but that
itself could not be a robust indicator of the lateness of
the combustion. It is also important to note that there is
no distinctive thermal and chemical ionization peaks this
time. Instead, the thermal peak is folded into the
chemical ionization region. However, the strength of
ionization signal (peak magnitude and area) is even
more for the late combustion case. Therefore, there is
not a clear correlation between the strength of ionization
and its torque output either. Figure 7 with a spark of 5
o
ATDC depicts a cycle with incomplete combustion.
Ionization signal extends well beyond the exhaust
stroke. Similarly, burn profile extends towards the
exhaust stroke as well. However, the ionization signal is
very wide and tails down over a long crank interval.
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Figure 5-Figure 7 show a typical ionization current and
the corresponding burn profiles from a sample
o
combustion cycle at three spark timings: 21 BTDC, 11
o
o
BTDC and 5 ATDC at the same speed and throttle
position. For burn profile, the pressure signal is used to
compute the mass fraction burned from a heat release
model. Note that, for the cycle of Figure 5, the
combustion is normal and the ionization signal has
distinct chemical and thermal ionization regions. The
thermal ionization is less noticeable as it is usually the
case with light load conditions. Note that, there is no
o
significant ionization signal beyond roughly 50 ATDC.
This coincides with the burn profile, from which no
further burning is noticeable beyond that location. On the
In summary, the following observations can be made as
the behavior of ionization signal with varied ignition
timing:
x
For normal combustions, the ionization signal
has a typical shape with two distinctive peaks
after the spark charging duration. While the
ignition is being retarded, these two peaks
merge and the signal shows a single peak
shape with a tapering falling edge.
x
The ionization signal becomes wider as the
spark is retarded. However, relative values of
the signal or its total area/energy (square area)
do not have correlations with the amount of
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ignition retard. On the other hand, a distribution
measure such as the location at which the first
90% of the total area under ionization signal has
been achieved is a good indicator. This is
because the range that the ionization signal
extends is affected by the ignition timing.
For normal combustions, the peak location does
not go much beyond TDC. Furthermore, the first
peak location in crank angle coordinates
referenced to the end of spark event increases
rapidly when the combustion starts leaving the
normal range.
RETARD LIMIT INDICATORS FROM IONIZATION
SIGNAL
Based on the previous observations, two retard limit
indicators (peak location and distribution location) from
ionization signal are proposed and studied in this section
A typical ionization signal is shown in Figure 8. The peak
location is defined as the crank angle at which the
chemical-ionization part of the ion signal takes its
maximum value. In other words, it is when the ionization
signal peaks out first after the spark charging window.
The ionization signal is also integrated over a userspecified window and the crank angle at which the ion
integral reaches a calibratable percentage of the total
area is determined. This parameter is defined as the
distribution location. The normalized ion integral is also
shown in Figure 8. It is further possible to use the
integral of the square (energy) in this computation
instead. The resulting area/energy signal is normalized
relative to the total area/energy. Note that, 100%
distribution location is ideally reached when the
ionization signal completely dies-out. Similarly, X%
distribution location is the crank degree for which the
normalized ion integral is X/100. A percentage close to
100 is used to approximately locate the crank location
after which the ionization signal strength (combustion
activity) is minimal.
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x
x
At the ultimate case, the location at which the
ionization signal dies out also indicates how
much the combustion is really retarded. For
example, there is still significant ionization signal
in Figure 7 even after the exhaust valves are
o
opened (124 ATDC for this particular engine
and valve timing), which points out some
combustion activity going on at the exhaust
stroke.
The first peak location of ionization signal can
also be correlated to the amount of spark retard.
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Figure 9-Figure 11 show the stochastic properties of the
o
o
ion peak location at spark timings 21 BTDC, 11 BTDC
o
and 5 ATDC, respectively. At each firing event, the
ionization signal is processed to obtain the peak location
number for that particular combustion. For each case,
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300 cycles (number of consecutive firing events at the
same conditions) of data are used to create the PDF
(probability density function) or histogram of the peak ion
signal location. The PDFs of distribution location
parameter are also quite similar to those of peak location
and therefore not included here for brevity.
a number of cycles increase rapidly with the retard.
Thus regulating the statistics of the ionization signal, not
each individual cycle, will also control the combustion
quality. Another observation is that for a normal
combustion (Figure 9), the peak location displays a
Gaussian (normal) distribution. When the ignition is
retarded, the PDF of peak location starts skewing
towards the retard direction (Figure 10). Note that the
misfire/partial burn cycles are the primary interest and
they are the ones extending in the retard direction.
Therefore, a derived parameter from PDF, such as the
worst-case data (highest peak location) in the buffer or
among a percentage of it when the data are sorted from
most advanced (lowest peak location) to most retard
(highest peak location) is superior to using variance
information itself in capturing these limiting cycles. This
point is later going to be utilized in the control strategy
development.
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Based on the PDFs, the cycle-to-cycle variability of
combustion and the statistics of the ionization signals
seem to be coupled. However, the relation is in terms of
their stochastic properties rather than being a cycle-tocycle correspondence. For example, a peak location
o
value of 10 ATDC can be seen in both Figure 9 and
Figure 10, but it’s most probable to have this value with
o
a spark timing of 11 BTDC among the three plots
included. Similarly, a peak location value close to 60
o
ATDC has been observed only in one cycle in Figure 10
while that number was almost the mean value of all data
in Figure 11. In other words, peak location from a single
cycle may not be that informative. However, both the
mean and the standard deviation of peak location along
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Figure 12-Figure 15 show the stochastic properties of
the introduced ionization retard limit parameters during a
spark sweep at 1500 RPM and 2.5 bar BMEP at MBT
spark. For distribution location, the location where 95%
of ionization signal area occurs was used. Both the
worst-case ion peak location and distribution location are
calculated with respect two references: one with respect
to the TDC location and the other with respect to the
spark timing, as shown in Figure 12 and Figure 14. The
mean value of each parameter increases as the timing is
moved in the retard direction, demonstrating consistent
gradient (or slope).
Note that, the worst-case peak location (the maximum
peak location among 300 cycles) and mean value with
respect to ignition timing, the duration from ignition
timing crank location to the peak location, was almost
flat for the first 4-5 ignition timings (Figure 12 and Figure
13). After that, there is a sharp increase in the
parameters with further ignition retard. This observation
is in accordance with Figure 4, for which the combustion
o
quality is affected beyond a spark timing of 11 BTDC.
The standard deviation of peak location in Figure 13 also
shows an abrupt increase after that timing. Based on
o
Figure 14, ignition timing of 5 BTDC, the worst-case
ionization signal distribution location (largest number
o
among 300 cycles) was before roughly 100 ATDC,
which basically indicates that the combustion is still
complete at that ignition timing although it’s late.
The desired values for these parameters need to be
calibrated to the engine operating points. A mean value
o
of 5 ATDC for peak location appeared to be sufficient
for good combustion quality for a variety of operating
points. The PDF of a signal such as the worst-case peak
location can further be used to adapt this target online
based on the achieved performance (second order
statistics). This simplifies the calibration task. On the
other hand, for cold-start the worst-case distribution
location should be properly placed before the exhaust
valve opening angle.
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Next a retard limit management control system is
discussed. This system computes the introduced
parameters from the measured ionization signal at each
cycle and derives the stochastic properties of the data
online from a stored buffer. Based on these online
feedback signals, the control provides a closed loop limit
value for the ignition timing in the retard direction.
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This analysis has been performed at different speed
/load conditions and the general trend hold at all
conditions. Basically, the mean value of peak/distribution
location can be used as the feedback signal for
regulation (first order statistics). It has been observed
that the peak location shows a better correlation when
the ignition retard is not too extensive (which is the case
when around MBT) whereas the distribution location
shows a better correlation when the retard level is
significant (this is the case for example at cold start).
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provide the desired settling time and steady-state
accuracy for the response.
RETARD LIMIT CONTROL
Figure 16 shows the stochastic closed-loop retard limit
controller. It's a part of an overall spark controller, which
manages the spark timing for best fuel economy, power
and emissions by employing a closed loop MBT timing
strategy (7). The ionization signal from each cylinder is
sampled and saved in a buffer at each combustion
event. The overall spark control is triggered at every
firing event and retard limit control processes the
ionization signal from the most recent cycle to compute
the retard limit feedback parameters. The objective of
retard limit controller is to provide a retard limit for the
overall spark controller to assure combustion stability.
The key part of the retard controller is the stochastic
analyzer block. The derived ionization parameters from
each firing cycle are gathered in a buffer of a userselected size for stochastic analysis of the data.
Basically, the mean, standard deviation and PDF of data
are constantly updated at the end of each firing event.
Using the PDF, an achieved user-specified percentage
confidence level number is also computed. This number
(crank location) is defined as follows: saying 90%
confidence number for peak location is 20 degrees
means that for the 90% of the combustion events in the
buffer, the measured peak locations are more advanced
o
than that particular location, 20 ATDC.
Three main feedback actions are proposed in the
stochastic retard limit controller. Their functionalities are
listed below:
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Regulation controller: The regulation loop is used to
regulate the mean value of the retard limit feedback
(peak location or distribution location) to a mean target
value. The regulation controller is structured as a PI
controller with an additional sliding mode control term
and a feed-forward term based on engine operating
conditions. Despite the variability of the retard limit
feedback, its mean value is a well-behaved signal for
regulation purposes. The regulation controller is tuned to
Adaptive seeking feedback: The purpose of this loop is
two-fold: reducing the calibration conservativeness of
providing the engine with its "TRUE" retard limit target
and improving robustness of retard limit controller when
the engine operates under different conditions. This is
accomplished by using an error signal between the
desired confidence level target and the achieved. Note
that the confidence level is a second-order property of
PDF like variance. The adaptive seeking algorithm
reduces the mean target for the regulation controller if
the confidence number is greater than the specified;
otherwise, increases the mean target value.
Instant correction feedback: This block calculates an
instant correction signal to be fed into the integration
portion of PI controller. When the error between
confidence level target and retard limit feedback is
greater than zero, the output is zero. That is, no
correction is required, and when the error is less than
zero, the error is fed into a one dimensional lookup table
that outputs an instant correction in the advanced
direction for the integration portion of the PI controller.
With these three loops: the ideal action of the retard limit
controller can be explained as follows: Suppose that we
want to make sure that the ionization peak location will
o
not go beyond 20 ATDC. This location is then the
desired confidence level target. Using the standard
deviation of the measured data, one can back-calculate
a nominal target for the regulation controller by
subtracting a certain multiple of the standard deviation of
the measured data in the buffer. That initial mean target
is then increased by the adaptive seeking loop slowly if
the resulting, say 90% confidence level number
computed from the measured data is less than desired
o
confidence level target of 20 ATDC. That way, if the
initial mean target was too conservative; i.e., the worsto
case peak location is well below 20 ATDC, then the
mean target will be increased. On the other hand, the
instant correction feedback acts as a safety since
o
whenever the feedback goes beyond 20 ATDC it will
instantaneously advance the retard limit. Then the
seeking will start again to push the retard limit as long as
the things are favorable. Since the mean and stochastic
properties are used as feedback signals, the controller
will not react aggressively to each combustion variation,
which would be the case if the feedback signals from
each cycle were used directly.
The interaction of the retard limit controller with the
overall spark controller is as follows: If the baseline
spark is more advanced than the current retard limit,
then the baseline spark is used as it is. In that case, the
retard limit controller pushes the limit in the maximum
retard direction by itself. This is because the integration
will keep integrating till the maximum retard allowed is
reached (an anti-windup scheme is used) as it was
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designed. If the baseline spark controller pushes the
ignition timing to a level at which the feedback signals
generate corrections, then the retard limit moves from its
maximum limit to a new level as a variable saturation
limit on the baseline spark. On the other hand, if the
baseline controller still tends to push the spark in the
retard direction, the seeking and instant correction
actions of the retard controller will adjust the retard limit
online.
To illustrate the performance of the retard limit
controller, some preliminary results from dyno runs have
been included in Figure 17-Figure 20. The stochastic
analyzer block and the regulation/adaptive seeking loops
were tested at several fixed operating points. The main
emphasis so far was given to the evaluation of the
control features rather than demonstrating its projected
benefits. A 3L, 6-cylinder engine equipped with
ionization feedback coils was used for these
experiments. Due to our current dyno set-up, the
experiments were run at a constant MAP (manifold
absolute pressure) with fixed throttle openings, rather
than fixing load. Therefore, a fixed engine load cannot
be maintained during a spark retard. Although this may
not be the case for real operations, it is considered to be
sufficient for control algorithm evaluations.
Figure 17 and Figure 18 demonstrate the controllability
of the combustion through the proposed controller. In
Figure 17, first the mean ion peak location was regulated
o
to 10 ATDC to show the regulation performance of the
retard limit controller. Around 50 seconds, the adaptive
seeking is enabled with the goal of bringing the
o
confidence level to 30 ATDC. Note that the mean target
is adapted to drive ignition to a new level at which the
o
confidence level is in the vicinity of 30 ATDC as desired.
In Figure 18, the system is started at a retarded ignition
timing, which results in high combustion variability. From
correlations, it has been observed that the ion peak
o
location should be located before roughly 5-10 ATDC to
assure a good combustion quality. To this end, the
retard limit controller was activated to regulate the mean
o
value of ion peak location to 10 ATDC as an initial mean
target. At the resulting ignition timing, the confidence
o
level was about 17 ATDC. Around 60 seconds, the
adaptive seeking loop was enabled to bring the
o
confidence level to 10 ATDC this time. At the new spark
level, the covariance of IMEP reduces to below 3% at
the steady state.
Figure 19-Figure 20 show responses from a cold-start
run. For that, the ionization distribution location
parameter was used as the feedback signal and the
control was activated with all features to drive the
o
confidence level to 110 ATDC. The confidence level
was also included in Figure 19 as a performance
o
measure. Note that it was kept around 110 ATDC at the
o
steady state and did not exceed 124 ATDC, which was
the exhaust valve opening timing for the particular
engine. Therefore, the combustion was over before the
exhaust valves were opened and the controlled high
retard was safe in terms of engine-out HC emissions.
Figure 20 demonstrates the corresponding fast exhaust
temperature rise-up during the run. An open loop
temperature profile was also included in Figure 20 to
show the improved temperature rise-time with the
control. For the open loop case, the ignition timing was
held at TDC, which was the initial ignition timing for the
closed loop controller. Based on Figure 20, the time it
takes the exhaust temperature to reach 500 C was
reduced to 12 seconds from 17 seconds with the closed
loop controller.
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Downloaded from SAE International by Brought To You Michigan State Univ, Thursday, April 02, 2015
REFERENCES
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1. N. A. Henein and M. K. Tagomari, Cold-start
Hydrocarbon Emissions in Port-Injected Gasoline
Engines, Progress in Energy and Combustion
Science, 25, pp. 563-593, 1999.
2. J. Zhu and S. C. Chan, An approach for rapid
automotive catalyst light off by high values of ignition
retard, Journal of the Institute of Energy, pp. 167173, 1996.
3. P. Tunestal, M. Wilcutts, A. T. Lee and J. K.
Hedrick, In–Cylinder Measurement for Engine ColdStart Control, Proceedings of the 1999 IEEE
International Conference on Control Applications,
pp. 460-464, 1999.
4. R. Isermann and N. Muller, Design of Computer
Controlled Combustion Engines, Mechatronics, pp.
1067-1089, 2003.
5. Guoming G. Zhu, Chao F. Daniels and Jim
Winkelman, MBT Timing Detection and its Closedloop Control Using In-Cylinder Pressure Signal, SAE
2003-01-3266, 2003.
6. Chao F. Daniels, Guoming G. Zhu and Jim
Winkelman, Inaudible Knock and Partial Burn
Detection Using In-Cylinder Ionization Signal, SAE
2003-01-3149, 2003.
7. Guoming G. Zhu, Chao F. Daniels and Jim
Winkelman, MBT Timing Detection and its Closedloop Control Using In-Cylinder Ionization Signal,
Submitted to SAE Powertrain and Fluid Systems
Conference, Tampa, FL, 2004.
8. Lars Eriksson, Spark advance modeling and control,
Ph.D. Dissertation, Linkoping University, 1999.
9. J. B. Heywood, Internal Combustion Engine
Fundamentals, McGraw-Hill, 1988.
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CONTACT
CONCLUSION
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Ibrahim Haskara, Visteon Corporation, 17000 Rotunda
Drive B-360-172, Dearborn, MI 48185. E-mail:
ihaskara@visteon.com.
DEFINITIONS, ACRONYMS, ABBREVIATIONS
MBT:
TDC:
o
ATDC:
o
BTDC:
BMEP:
IMEP:
EGR:
SI:
PDF:
HC:
PI:
MAP:
Minimum spark advance for Best Torque
Top Dead Center
Degrees After Top Dead Center
Degrees Before Top Dead Center
Brake Mean Effective Pressure
Indicated Mean Effective Pressure
Exhaust Gas Recirculation
Spark Ignition
Probability Distribution Function
Hydrocarbons
Proportional and Integral
Manifold Absolute Pressure