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PhD Student Lars
Eriksson.
This project is performed in collaboration with Mecel AB and
SAAB Automobile AB, and participates in the ISIS competence center with the
project Signal Interpretation and
Control in Combustion Engines. A shorter
description of the project and the results are also available.
November 25, 1998
Environmental issues and lower fuel consumption require improved combustion engines. Several trends desire use of feed back control directly from the combustion instead of using indirect measurements as is mostly done today. The availability of computing power has revolutionized the possibilities of sensor interpretation and combination. The development is based on new sensors or improved interpretation of available sensor signals. One example is ionization current sensing which is obtained by applying a sense voltage on the spark plug when it is not used for firing. The sensed current depends on the ions created, on their relative concentration and recombination, on pressure, and on temperature to mention some of the more important factors. The signal is very rich in information but also complex to analyze.
The main results in this project are
Spark advance control is treated in Section 2, especially principles relating pressure information to efficiency. Section 3 deals with the basics of ionization currents. Section 4 presents the structure of the ion-sense spark advance controller. Experimental demonstrations are found in Section 5, and the results are summarized in Section 6.
Engines are difficult and complex, but before ruling out interpretation of complex signals one could consider the progress in human medicine. A medical doctor can draw conclusions from measurements like EEG or EKG, that are indirect crude clues to what is going on inside the body. Engine measurements, like e.g. ionization currents being in-cylinder engine measurements, are signals that are more directly coupled to the physics and chemistry of the process of interest i.e. the combustion (see Figure 1).
Figure 1: A medical doctor can from
measurements like EEG or EKG, that are crude compared to human complexity, draw
many conclusions. Ionization currents, like the one in the figure, are
in-cylinder engine measurements that are directly coupled to the combustion.
Virtual engine-doctors and virtual engine-fine-tuners are now being
developed.
Virtual engine-doctors that detect and diagnose serious malfunctions like knock that will destroy the engine and misfire that will destroy the catalyst, are not a farfetched idea in that perspective. They also already exist. Ionization current interpretation can be used for both purposes. Knock is a pressure oscillation in the cylinder with a frequency determined by the geometry of the combustion chamber. The oscillation is present in the current measurement and can be extracted mainly by using a band pass filter in a well chosen time window of the current signal. When there is a misfire, then there are no resulting ions and hence no current which is easily detected. These systems are already used in production cars [1, 2]. Therefore, the basic hardware is already available and to develop a virtual engine-doctor for combustion requires only additional signal interpretation in the electronic engine control unit (ECU), Figure 2.
Figure 2: The introduction of
computerized engine controllers (here above the engine) has revolutionized the
engine control era. Already today they represent an impressive computing power
and the development continues.
The term virtual engine-fine-tuner is more inspired by a skilled auto mechanic than a medical doctor. A human performing the task of tuning an engine, e.g. for best performance, would use several clues like test measurements and the sound of the engine, but also experience, e.g. about the actual weather situation. The result can typically be an increase of several percent in engine efficiency. One way to achieve engine tuning that has been shown previously is to use feedback schemes that use a pressure sensor [3, 4, 5], but these systems have not yet been proven cost effective due to expensive pressure sensors.
With the increasing computational power it is now becoming possible to do engine tuning by feed back control from more advanced interpretation of signals to take care of circumstances previously not possible to easily measure. A multi-sensor idea is developed where a basic signal, like engine speed or ionization current, is measured and several other sensor signals can be deduced from it (Figure 3).
Figure 3: The spark plug can, using
signal interpretation, function as sensor for several parameters. Knock
intensity and misfire are already implemented in production cars as a basis for
virtual engine-doctors. Lambda sensing and peak pressure position estimation can
be used in virtual engine fine tuners. The peak pressure position (and a quality
measure of it) is the information concentrated on in this project.
Variations in engine speed together with crank shaft models can be used to conclude misfire by for example lacking torque pulse or to estimate cylinder pressure from derived torque fluctuations [6, 7]. Usage of the spark plug as an integrated actuator and sensor leading to ionization current interpretation is the path taken here.
Spark-advance control deals with determination of the engine position where the spark plug shall ignite the air-fuel mixture and start the combustion. It is thus used to position the combustion and pressure trace relative to the crank shaft motion. Engine efficiency and emissions are directly affected by the spark advance, due to its influence on the in-cylinder pressure. Work is lost to heat transfer and to the compression if it is placed too early, and expansion work is lost if it is placed too late. The optimal spark advance setting depends on several parameters, e.g. engine speed, engine load, air/fuel ratio, fuel characteristics, air humidity, EGR, air temperature, and coolant temperature. Emission regulations and engine knock also affect the best spark advance setting, but this is not a topic here.
Today, most spark-advance controllers are open-loop systems, which measure a number of parameters that affect the spark advance and compensate for their effects. Extensive testing and calibration, during the design phase of the engine, results in a nominal spark-advance schedule. Such a calibrated schedule is conservative since it has to guarantee good performance over the entire range of the non-measured parameters, and also be robust to aging. If all parameters that affect the spark advance were measured, and their effects and interactions were properly accounted for, it would be possible to determine the best spark advance. However, such a system would be too expensive due to the measurements and testing required to incorporate it in a production car.
A different approach is to use closed-loop spark-advance control. Such a system measures the result of the spark setting rather than measuring all the parameters known to affect the spark advance. This requires measurement of parameters directly resulting from the actual combustion, such as the in-cylinder pressure or the ionization current. It is an accepted fact that the position for the pressure peak is nearly constant with the optimal spark advance, regardless of operating condition [3]. A spark-advance control algorithm that maintains a constant peak pressure position (PPP) is therefore close to optimum. Even for large changes in parameters that affect the flame speed, such a feedback scheme still maintains the optimal spark advance. This has been shown previously by using feedback schemes that utilize a pressure sensor [3, 4, 5], but these systems have not yet been proven cost effective due to expensive pressure sensors.
The spark advance is used to position pressure development in the cylinder
such that the combustion produces maximum work. Under normal driving conditions
the mixture is ignited around 
in crank angle before the piston has reached top dead
center (TDC), and the pressure peak comes around 20 degrees after TDC. In
Figure 4
three different pressure traces, resulting from three different spark timings,
are shown. Earlier spark advance normally gives higher maximum pressures and
maximum temperatures that appear at earlier crank angles.
Figure 4: Three different pressure
traces resulting from three different spark advances. The different spark
advances are; SA1: spark advance 
before top dead center (TDC), SA2: 
before TDC, SA3: 
before TDC. The optimal spark advance is close to
SA2.
The optimal spark advance for maximum output torque is close to SA2 for the
operating point in the figure, and the resulting peak pressure position lies
around 
after TDC. With too early ignition timing the pressure
rise starts too early and counteracts the piston movement. This can be seen for
the pressure trace with spark advance SA1 where the pressure rise starts already
at 
due to the combustion. There are also losses due to heat
and crevice flow from the gas to the combustion chamber walls, and with an
earlier spark advance the loss mechanisms start earlier reducing the work
produced by the gas. Higher pressures give higher temperatures which also
decrease the difference in internal energy between the reactants and products in
the combustion, thus resulting in lower energy-conversion ratios. The heat loss
mechanisms and the lower conversion ratio can be seen in Figure 4,
at crank angles over 
, where the pressure trace from the SA1 spark advance is
lower than the others.
Too late ignition gives a pressure increase that comes too late so that work
is lost during the expansion phase. In Figure 4,
the pressure increase for spark advance SA3 starts as late as at TDC. But work
is also gained due to the later start of the effects mentioned above, which also
can be seen in the figure. The pressure trace from the spark advance, SA3, is
higher than the others at crank angles over . However, this gain in produced work can not compensate
for the losses early in the expansion phase.
Thus, optimal spark advance positions the pressure trace in a way that compromise between the effects mentioned above. To define the position of the in-cylinder pressure relative to TDC, the peak pressure position (PPP) is used, Figure 5. The PPP is the position in crank angle where the in-cylinder pressure takes its maximal value. There also exist other ways of describing the positioning of the combustion relative to crank angle, e.g. based on the mass fraction burned curve.
Figure 5: The PPP (Peak Pressure
Position) is the position in crank angles for the pressure peak. It is one way
of describing the position of the pressure trace relative to crank angle.
Development of an engine-fine-tuner for efficiency requires experiments to
describe optimal engine output. Such a description is the basis for determining
the set-point values to be used in the feed back scheme. In Figure 6, mean
values, over 200 cycles, of the PPP are plotted together with the mean value of
the produced torque at four different operating points covering a large part of
the road load operating range for the engine. Two of the operating points have
an engine speed of 1500 rpm with different throttle angles, and for the two
other operating points the engine speed is doubled to 3000 rpm. The PPP for
maximum output torque in the figure lies around 
ATDC (after TDC) for all these operating points.
Figure 6: Mean PPP (Peak Pressure
Position) and output torque for 1500 rpm and 3000 rpm and two different engine
load conditions. Each circle is a mean value from 200 consecutive cycles with
the same ignition timing. The optimal mean PPP is close to for all loads, even though the spark advance differs a
lot.
Note that the load and speed are changed over large intervals, and that the
PPP for maximum output torque at the different operating points does not differ
much. The PPP versus torque curve is flat around the position for the maximum.
Therfore a spark schedule that maintains a constant PPP at is close to optimum. Considering only the work produced,
this motivates that an optimal spark schedule maintains almost the same position
for the peak pressure [3].
However, the optimal PPP changes slightly with the operating points. The
efficiency can thus be improved a little bit further by mapping the optimal PPP
for each operating point, and provide these values as reference signal to the
spark timing controller. The peak pressure positioning principle can also be
used for meeting emission standards. In [4] this
question is addressed by rephrasing the emission regulations on the spark
advance to desired peak pressure positions.
The experiments in Figure 6 are interesting not only for determining the optimal point. They can also be used to illustrate the effect of cycle-by-cycle variations, which limits the performance of SI Engines [8, 9]. Recall that these variations are significant as previously illustrated in Figure 8.
The following principle study illustrates that variations in the output
torque are smaller when the mean PPP is held at its optimum. In Figure 7, a
quadratic polynomial is plotted, which is the same as those in Figure 6. The
polynomial represents an idealized relation between the PPP, 
, and the output torque, 
. The polynomial can be parameterized as
Figure 7: The figure illustrates
that when the mean PPP (peak pressure position) is at optimum the variations in
the output torque are minimal. At a) the mean peak pressure position lies at
optimum which give small variations in output torque at a1). At b) the mean peak
pressure position lies some degrees off from optimum and the resulting
variations are larger at b1).
Using this equation the standard deviation of the variations in the output
torque, 
, can be derived as a function of the standard deviation
of PPP, 
, and the deviation of PPP from the optimal, d,
[10]
Equation 1 gives a useful rule of thumb, and another useful quantification of the value of spark advance feedback control. The interpretation is that the influence of cycle-to-cycle variations in PPP on the output torque is minimal if the mean peak pressure position is controlled to its optimal value d=0.
The conclusion is that if an engine is not kept at its optimum point then not only is efficiency lost. It also increases variability that leads to harsher operation, which of course is not desired for driveability reasons.
In an ideal combustion reaction, hydrocarbon molecules react with oxygen and generate only carbon dioxide and water, e.g. isooctane gives
In the combustion there are also other reactions present, that include ions, which go through several steps before they are completed; some examples are [11]
These ions, and several others, are generated by the chemical reactions in the flame front. Additional ions are created when the temperature increases as the pressure rises.
The processes creating the ionization current are complex and are also varying from engine cycle to engine cycle. Figure 8 shows ten consecutive cycles of the cylinder pressure and the ionization current operating at constant speed and load.
Figure 8: Cycle to cycle variations
are always present in the combustion. The plots show ten consecutive cycles at
stationary engine operation that clearly exhibit the cyclic variations.
As can be seen, the cycle-by-cycle variations are significant. An important part of this paper is to derive pressure characteristics from ionization current.
To detect the ions, a DC bias is applied to the spark plug, generating an electrical field. The electrical field makes the ions move and generates an ion current. A schematic illustration is shown in Figure 9 (a). The current is measured at the low-voltage side of the ignition coil, and does not require protection from the high-voltage pulses in the ignition, Figure 9 (b). Ionization current measurement systems are already in use in production engines for: individual cylinder knock control, cam phase sensing, pre-ignition detection, and misfire/combustion quality/lean limit [1]. Also, work on detection of spark plug fouling by using the ionization current has been reported [12].
Figure 9: Measurement of the
ionization current. (a) The spark plug-gap is used as a probe. (b) Measurement
on the low voltage side of the ignition coil.
The ionization current is an interesting engine parameter to study. It is a direct measure of the combustion result that contains a lot of information about the combustion, and several challenges remain in the interpretation of it. Some of the parameters that affect the ionization current are: temperature, air/fuel ratio, time since combustion, exhaust gas recycling (EGR), fuel composition, engine load, and several others.
The ionization current typically has three phases: a phase related to ignition, a phase related to ions from the flame development and propagation, and a phase related to pressure and temperature development. In Figure 10, the three phases of the ionization current are displayed. Each of these phases has varying characteristics and they also mix together in complicated ways. In the ignition phase, the ionization current is large, with reversed polarity. Due to the high current in the ignition the measured signal shown in the figure is limited. What can be seen in Figure 10 is the ringing phenomenon in the coil after the ignition.
Figure 10: Ionization current
showing three clear phases, ignition, flame front, and post flame.
In the flame-front phase, the high level of ions associated with the chemical reactions in the flame produces one or more characteristic peaks. The ions generated by the flame have different recombination rates. Some ions recombine very quickly to more-stable molecules, while others have longer residual times. The result is a high peak which after some time decays as the ions recombine.
In the post-flame phase the most stable ions remain, generating a signal that follows the cylinder pressure due to its effect on the temperature and molecule concentration. Ions are created by the combination of the measurement voltage and the high temperature of the burned gases, since the temperature follows the pressure during the compression and expansion of the burned gases, when the flame propagates outwards and the combustion completes. The ionization current thus depends on the pressure.
The ionization current can be studied by thermodynamical and chemical kinetic modeling [13, 14, 15]. Concentrating on the pressure-related post-flame phase, an analytical expression for the ionization current has been presented. Some of the fundamental assumptions in the model are that the gas in the spark plug is: fully combusted, in thermodynamic equilibrium, undergoes adiabatic expansion, and that the current is carried in a cylinder extending from the central electrode of the spark plug [13]. Given the cylinder pressure, the analytical expression for the ionization current is
Where:
A key step in our method for deducing information is to use parameterized functions based on a phenomenological description of the ionization current, i.e. the signal consists of two combustion related phases. These functions must be rich enough to capture the different variations, but they must also be such that the relevant information can be extracted. The parameterized functions are used to separate out the different phases of the ionization current, and to get an estimate of the pressure. As a model, with 6 parameters, a sum of two Gaussian functions is used
Note that this model is not based on combustion physics with respect to the flame-front phase. Even though this may seem ad hoc, the model is physically motivated in [16] with regard to pressure information. Measured pressure traces are recalculated to ionization currents using Equation 2, and the result is shown to be close to a Gaussian function.
For ionization current interpretation, the model, Equation 3, is fitted to the measured ionization current. Figure 11 shows two ionization currents together with the Gaussian components of the model. The first component corresponds to the flame-front phase and the second to the post-flame phase.
Figure: Components of the model
(Equation 3)
that captures the appearance and the phases of the ionization current.
This second part, corresponding to the post-flame phase, is the experimentally and physically motivated basis for obtaining pressure information.
The developed engine-fine-tuner relies on ionization current interpretation to obtain an estimate of the peak pressure position (PPP), and it relies on the analysis in mainly Section 2.3 to obtain set-points and feed forward values.
The ionization current interpretation method is presented in somewhat more
detail in [16].
The phenomenological model in Equation 3 is
fitted to the measured ionization current, and the model parameters 
corresponding to the flame front and post-flame phases
are extracted. The second phase, the post-flame phase, is used as the estimate
of the in-cylinder pressure development.
In Figure 12, the peak pressure position (PPP) estimate from the ionization interpretation algorithm is compared to the measured PPP.
Figure 12: The peak pressure
position estimated from the ionization current compared to the measured. Each
point corresponds to the estimated and true PPP for one cycle. Close to 500
cycles are displayed in the plot. One to one correspondence is indicated by the
solid line.
For the experiments shown in the figure the engine speed and the throttle
angle are held constant, and the ignition timing is positioned at six different
spark timings from 
BTDC (before TDC) to 
BTDC. The resulting PPPs range from 
ATDC (after TDC) to 
ATDC as can be seen in the figure. The estimate
correlates quite well with the measured peak pressure position. The correlation
is best around the point of optimal efficiency at 15 degrees after TDC, which is
yet another way of pointing out the increase in engine variations when moving
away from optimal position. The correlation is improved by further filtering
which is discussed in Section 4.3.
The implementation to obtain the model parameters, , can be done in different ways, but there is a
real-time requirement since it is pattern recognition in a fast inner loop. The
algorithm used in the real-time implementation [17]
estimates bimodal functions based on the Kullback directed divergence.
The controller structure for the spark timing is shown in Figure 13. The
spark plug that is used is a conventional spark plug. The ionization current is
produced by the integrated ignition and measurement system, described
in [1], and
the interpretation algorithm gives an estimate of the PPP. The reference value
for the PPP gives a possibility to have different spark schedules for different
operating points, i.e. meeting other goals than to maximize the work. For
example in mid-load mid-speed ranges it is desirable to have a spark advance
close to MBT, with PPP around , and in high load ranges a more conservative schedule,
with late PPP, for reducing engine noise and 
emissions. The feed forward structure shown in
Figure 13
incorporates information about how changes in reference value and engine
transients affect the spark advance. This structure is similar to the ones used
in conventional lambda controllers.
Figure 13: The structure of the
spark advance control structure, where the spark plug operates as an integrated
actuator and sensor. Information is extracted from the raw ionization current,
and the estimate of the PPP is the input to the spark timing controller.
Reference values and feed forward signals are obtained using other sensors, e.g.
engine speed and load.
The spark advance controller measures the on-going combustion and updates the spark timing to the next combustion. Without the feed forward the spark timing update is done through the following, PI like, control law
where ST is the spark timing, 
the desired peak pressure position, 
the PPP estimation from the ionization current, and
C a gain that has to be tuned.
The gain C in Equation 4 is
selected as a balance between attenuation of cycle-to-cycle variations and
response speed. The filtering comes at the price of slowing down the feedback
loop, but this can be compensated by using feed forward schemes, shown in
Figure 13,
based on a nominal spark advance table. Since environmental parameters like
humidity do not change rapidly, very quick responses is not an issue. One
criterion is that the spark timing shall not move more than 
due to the cyclic variations [18].
For this engine the cycle to cycle variations for the estimate of the PPP is
around 
.
Another consideration to take into account is how well the PPP estimate
correlates with the actual PPP. Moving averages of different lengths have been
computed for the measured and the estimated peak pressure positions [10].
This indicates that a good choice for the gain in the feedback control law is
, which is the gain used in the on-line tests.
Experiments with the engine-fine-tuner will be presented. Responses to set-point changes are presented together with measurements from an extra pressure sensor to prove that the pressure trace is correctly positioned. The high light of the experiments is the demonstration in Section 5.4 where the engine is being exposed to increased humidity. There is an increase in power and efficiency when the engine-fine-tuner is turned on.
The engine used for measurement and validation is a spark-ignited, SAAB 2.3 l, 16 valve, four-stroke, four-cylinder, fuel-injected, normally aspirated, production engine equipped with the Trionic engine control system. The ionization current measurement system is the production system developed by Mecel AB [1], which is used in the SAAB engine. A pressure transducer and amplifier from AVL, for in-cylinder pressure measurement, is used for algorithm validation.
The ionization current interpretation scheme is implemented in a PC that is connected to the ECU by a CAN bus. Ionization current and pressure data are sampled into the PC synchronously with the crank shaft rotation, and a new updated spark advance is calculated and sent to the ECU using the CAN bus.
In Figure 14, it is shown that the ionization current based controller achieves the goal of controlling the peak pressure position to the desired values.
Figure 14: Closed loop control of
spark advance with changing reference value, showing that the PPP can be
controlled to the desired positions. Dash dotted - reference signal, solid - PPP
measured by an extra pressure sensor, dashed - PPP estimated from ionization
current
The reference value (dash dotted) shifts every 250'th engine cycle, from the
initial value of 
to 
to to 
to 
and back to . The mean values for the PPP estimate from the
ionization current (dashed) and the PPP (solid) are computed using a first order
LP filter with unity static gain, 
.
The results are very good, taking into account that the cycle-to-cycle
variations of the PPP and its estimate are of the order , and the actual mean PPP is controlled to within 
of the desired position, as can be seen in Figure 14. It
is thus demonstrated that the peak pressure position can be controlled to
desired positions using only information from the ionization current signal.
The response time for the controller has been evaluated using a reference square wave with a fast duty cycle, showing that the step response time is approximately 30 cycles without feed forward compensation [10]. Since no feed forward compensation is used this step response time for the reference signal will be the same as for environmental disturbances. With a feed forward loop the step response can be made faster to fit the needs during engine transients e.g. quick changes in the manifold pressure.
To create a change in air humidity in the laboratory a water sprayer is used. The sprayer is originally a color sprayer that has a valve which delivers a liquid spray. This liquid spray is further atomized by two opposing holes that blows pressurized air on the spray. In Figure 15 a photo of the sprayer with the water spray is displayed. The figure also shows a schematic figure that displays an enlargement of the nozzle with the liquid spray and the pressurized air. The liquid is not fully atomized by the pressurized air but the droplets are made significantly smaller. By directing the water spray towards the throttle plate the water is drawn into the induction system by the lower pressure in the intake manifold.
Figure: Left: A picture of the
sprayer spraying water. Right: A schematic figure of the sprayer nozzle
with the liquid spray, pressurized air, and the atomized liquid drops.
The amount of water sprayed into the engine was not measured during the tests but it had no audible effect on the engine during the tests. Nevertheless, there was enough water present to change the in-cylinder pressure trace so that the mean peak pressure position moved to a position four to five degrees later than optimal.
Humidity slows down combustion speed, leading to delayed pressure development and thus decreased power and efficiency. This is normally not possible to compensate for, and the ultimate test of the engine-fine-tuner is of course if it really has an effect on the overall engine output in terms of power and efficiency when subjected to an air humidity change.
During the water injection tests, the throttle angle, fuel injection time,
and engine speed are held constant. The engine is running at steady state and
the A/F ratio is tuned to 
before the test cycle starts. Then the injection time is
frozen and held constant during the test cycle. A controller structure that
includes a feed-forward coupling, Figure 13,
using a conventional look-up table with engine speed and manifold pressure as
inputs was used during the tests.
Figure 16 shows
a part of a test cycle where water is sprayed into the engine air intake, and
the closed loop spark advance controller is switched on and off. The speed and
load condition is 1500 rpm and 55 Nm. Initially in the test cycle, the
closed-loop spark-advance controller is running and it changes the spark advance
controlling the peak pressure position to a position close to MBT, i.e. after TDC. The ionization current is used as input to the
controller, and the in-cylinder pressure is only used for validation.
Figure 16: The interesting part of
the test cycle. The spark advance controller is switched off at cycle 50 and the
water injection starts at cycle 250, which leads to increased PPP. The
controller is switched on again around cycle 500, controlling PPP to MBT which
increases the output torque.
The signals PPP and output torque have been filtered off-line with the filtering procedure with zero phase shift, which is included in the signal processing toolbox in Matlab. The filter that is used is a Butterworth filter with order 3, and normalized cut-off frequency at 0.3.
At cycle 50 the closed-loop controller is turned off and the spark advance is
held constant, changing only slightly due to the measurement noise in the
manifold pressure signal used for feed forward. At cycle 250 the water spraying
is started, and two things can be noted at this point: Firstly, the most
important point is that the PPP moves 4 degrees. Secondly, the actual spark
advance changes slightly, 
, in the wrong direction due to a change in intake
manifold pressure. When the controller is turned off, the spark advance can be
viewed as a conventional pre-calibrated schedule with a spark advance close to
MBT. The parameters that affect the spark advance is then the engine speed and
the manifold pressure. Note that a conventional scheme changes the spark advance
in the wrong direction, since increased manifold pressure indicates higher load
and therefore would requires a smaller spark advance.
The spark advance controller is switched on again at cycle 500. The PPP is
controlled to ATDC by using information from the ionization
current. Note that the output torque increases by 
when the controller is switched on. It is thus shown that
the engine-fine-tuner can handle external disturbances such as air humidity, and
control the engine to an optimal operating condition.
Developments of virtual engine-doctors and virtual engine-fine-tuners are trends that add to the challenges and joys of modern research in engine control. Here an ion-sense engine-fine-tuner has been presented. It is a feed back scheme, not a calibration scheme, based on ionization current interpretation. The method is very cost effective since it uses exactly the same hardware and instrumentation (already used in production cars) that is used to utilize the spark plug as sensor, to detect misfire and for knock control. The only addition for ignition control is further signal interpretation in the electronic engine control unit.
Humidity significantly changes the burn-rate in the combustion, and thus the peak pressure position which in turn affects power and efficiency. Humidity is not easily measured, and is therefore usually not compensated for. Both experimental and theoretical studies (Figures 6 and 16, and Equation 1) clearly demonstrate the value of spark advance control regarding power and efficiency. The ion-sense engine-fine-tuner has a response time more than sufficient to follow environmental changes. And it was shown, as a main result, that it can control the engine back to its optimal operation when subjected to humidity in the intake air.
Lars Eriksson
