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Real-Time Impedance Measurement and Frequency Control
in an Automotive Plasma Ignition System
Roger Williams∗ and Dr. Yuji Ikeda†
∗ NXP
Semiconductors, Smithfield, RI, USA. Email: roger.williams@nxp.com
† Imagineering, Inc., Kobe, Japan. Email: yuji@imagineering.jp
Abstract—We describe a method for optimizing plasma matching in a pulsed 2.45 GHz automotive plasma ignition system.
The large-signal impedance into the spark plug feed network is
continuously monitored, and the frequency for optimal match is
estimated and changed in less than 100 µs to accomodate changes
in gas pressure. This method is also used to “learn” the frequency
response of the system to compensate for initial manufacturing
variations, ageing, and temperature-related changes in the feed
network and spark plug. We describe the performance of a system
using a modified coaxial transmission line resonator (CTLR)
spark plug operating at pressures from 0.1 to 1 MPa, and discuss
a low-cost implementation.
Index Terms—Automotive applications, plasma applications,
impedance measurement, frequency control, ignition.
I. I NTRODUCTION
Over the past decade, many efforts have been undertaken to
improve the efficiency and reduce the emission of greenhouse
gases in automotive internal combustion engines. Microwave
plasma ignition has been shown to have great potential in
expanding the limits of lean-burn and exhaust gas recirculation
(EGR) technologies necessary for improving fuel economy,
particularly in its ability to stabilize combustion and initial
flame development [1] [2] [3].
Laser-generated plasmas have similar benefits. The drawbacks of laser ignition systems are cost, efficiency, and reliability issues related to the optical windows required in
the engine cylinders [3]. Moreover, the ability to deliver
microwave energy into a much larger volume of plasma than
either spark or laser, and the ease of controlling microwave
power level and timing give microwave ignition an advantage
in generating non-thermal plasma over long bursts [4].
For practical reasons, the most complete development on
plasma ignition has involved plasma-assisted combustion in
gasoline engines, using microwaves to pump plasma seeded
by a standard spark discharge from an ignition coil. In this
technology, special spark plugs are used which include both
a standard spark gap and a (generally coaxial) microwave
radiator [5]. Microwave energy emitted into the engine cylinder is absorbed by the free electrons in the spark discharge,
generating non-thermal plasma. Microwaves are applied as a
burst of narrow pulses to keep gas temperature low to reduce
undesirable NOX generation [6]. This microwave-generated
plasma stabilizes the combustion of leaner, higher-EGR gas
mixtures than can be supported by spark discharge alone.
As the technology has matured, attention has shifted to
engineering challenges related to microwave transistor and amplifier development, packaging, spark plug design, and plasma
control techniques. In this paper we discuss an automotive
plasma ignition system which uses a closed-loop mechanism
for stabilizing plasma generation in real time, thus improving
combustion stability.
II. T HE AUTOMOTIVE P LASMA I GNITION S YSTEM
In this plasma ignition system, the microwave generator,
power amplifier, and control circuitry for each cylinder is in a
self-contained module (Figure 2), permanently attached to the
engine-head-mounted feed network into which the spark plug
and high-voltage ignition coil are plugged.
A. ECU and Power Supply
The engine control unit (ECU) which manages ignition timing and fuel mixture communicates with each plasma ignition
module over a standard vehicle controller area network (CAN
bus), sending data about the current engine operating point to
the module and monitoring its status. Because the microwave
power transistors in the modules require a supply voltage
higher than the nominal automotive 12 V, a common power
supply generates 28 to 50 VDC which is distributed to all
plasma ignition modules.
B. Control
A microcontroller (MCU) handles communication, housekeeping, and the plasma control algorithms and maps. It contains hardware to generate all module timing signals needed to
implement the ignition timing shown in Figure 1 and support
fast data acquisition. It monitors various module conditions,
including temperature, supply voltage and drain currents, and
includes simple bias voltage sources for the microwave power
transistors. The MCU contains nonvolatile memory used to
store both initial control parameters and updates “learned”
during operation to compensate for feed network aging and
normal wear in the spark plug.
Crankshaft
Top Dead Center
θ ig (ignition timing)
Ignition
Trigger
Spark ignition occurs
at falling edge
MW delay = -200 ~ 200 us
Microwave
Burst of MW pulses,
each 0.1 ~ 10 us width
Burst width = 0.2 ~ 2 ms
Fig. 1. Ignition timing (not to scale).
978-1-4799-8275-2/15/$31.00 ©2015 IEEE
+28~50VDC
+12V
HV
COIL/
IGNITER
RF ASIC
0~20dB
2.4~2.5GHz
MCU
TRIGGER
LDMOS
CAN BUS
Feed
Network
TIMER
LDMOS
Spark
Plug
Reference
A
CAN
Re
ECU
4x 10−bit A/D
Im
B
To Other
Cylinders
SERIAL
Re
Im
TIMER
Vector Receivers
Plasma Ignition Module
Fig. 2. Simplified block diagram of automotive plasma ignition system (one cylinder shown).
C. Microwave Generation
The microwave signal is generated by a fast phase-locked
loop (PLL) synthesizer locked to a crystal oscillator, then gated
by a high-speed switch to create the burst signal, and amplified
to roughly 1 W by a medium-power variable-gain amplifier.
Both frequency and power level can be modified by the MCU
burst-by-burst to provide optimum plasma conditions for the
changing plasma conditions in the engine cylinder. These
functions are contained in a low-cost CMOS applicationspecified integrated circuit (ASIC).
The signal is further amplified by two stages of laterally
diffused MOS (LDMOS) microwave transistors to reach the
peak power (250 to 750 W) required to generate the microwave
plasma. Matching networks are hybrid microstrip/lumped element (capacitors and inductors) structures to realize the low
loss and small size needed for the engine-mounted module.
D. Impedance Measurement
A key element in the module’s ability to monitor and
optimize the plasma is its ability to measure large-signal
impedance (not just forward and reflected power) in real time.
The incident and reflected voltages at the module output
are sampled by a dual directional coupler incorporated into
the structure of the multilayer circuit board. Each of these
incident and reflected voltage samples is resolved by an I/Q
demodulator into a pair of DC components representing the
real and imaginary voltage. The two mixers in each demodulator are driven in quadrature by 0° and 90° reference signals
generated by the synthesizer.
These four DC samples are digitized by a four-channel A/D
converter. This has a 30 ns aperture time and a 1 Ms/s sampling
rate, which allows it to measure the complex incident and
reflected voltages in every microwave pulse in the burst. The
demodulator and A/D gains are very stable over temperature,
so these measurements are also used to stabilize power level.
For the purposes of plasma monitoring, it is adequate to
know the measured reflection coefficient, which can be easily
computed from incident and reflected voltages A and B as
B
(1)
Γ= .
A
Obviously, physical calibration of this embedded impedance
measurement system is difficult compared to (say) a benchtop
network analyzer. Fortunately, absolute accuracy is not critical
in this application, and a worst-case analysis of coupler,
demodulator, and ADC gain variations shows that it is possible
to readily achieve uncalibrated |Γ| errors less than 5%.
The only calibration needed is to periodically reestablish
a phase reference for Γ, as the electrical delay from the
coupler to the end of the spark plug will change over time
and temperature. This is easily accomplished by making
impedance measurements over the frequency range with no
plasma present (e.g. following the exhaust stroke), and using
this Γopen data to establish an arbitrary zero-phase reference.
Then the corrected reflection coefficient can be calculated as
B
ΓA =
cos(arg(Γopen )) − i sin(arg(Γopen )) . (2)
A
E. Feed Network and Spark Plug
The feed network and spark plug together comprise a
modified coaxial quarter-wave transmission line resonator [7],
schematically illustrated in Figure 3. It can be shown [8] that
the efficiency of the total matching structure is given by
π
η=
Pplasma
(1 − |Γ|2 )e− 2Q
≈
,
π
Pin
1 − |Γ|2 e− Q
(3)
where Q is the quality factor of the resonator. (3) indicates
that both the Q and the match are important for efficient
transfer of the input power into the plasma. Since resonant
frequency varies with plasma impedance, an iterative process
of modelling, prototype construction, and experimental testing
HV IN
M
RF IN
50 Ω
Zr
Cc
≈ λ /4
Copen
Zplasma
Fig. 3. Simplified schematic of the feed network and spark plug.
978-1-4799-8275-2/15/$31.00 ©2015 IEEE
+j1.0
0
+j0.5
+j2.0
No Plasma
−5
+j0.2
+j5.0
5.0
2.0
0.5
0.2
1.0
2.5 GHz
2.5 GHz
0.0
1 MPa
2.5 GHz
∞
2.4 GHz
Return Loss (dB)
2.4 GHz
1 MPA
100 kPA
−10
100 kPa
−j0.2
−j5.0
2.4 GHz
−j0.5
No Plasma
−15
−j2.0
−20
2.4
−j1.0
(a) Γ for various pressures.
2.41
2.42
2.43
2.44
2.45
2.46
Frequency (GHz)
2.47
2.48
2.49
2.5
(b) Return loss for various pressures.
Fig. 4. Γ measured at the input of the experimental feed network/spark plug during operation into plasma at various gas pressures.
was performed to find the location of the 50 Ω feed point
and the characteristic impedance and length of the resonator
structure that will yield in-band resonant matches with Q > 10
and |Γ| < 0.25 for gas pressures from 0.1 to 1 MPa.
III. P LASMA S TABILIZATION
According to (3), a spark plug and feed network designed
for most efficient microwave power transfer to the plasma will
exhibit both a high Q and low |Γ| when driving plasma. However, such a network will also have a resonant frequency that
varies with plasma impedance (e.g. Figure 4b). Since the gas
pressure, and thus the plasma impedance, varies widely (from
100 kPa to as much as 3 MPa) during an engine cycle [9], good
power transfer with a high-Q network cannot be achieved for
the entire combustion stroke with a fixed frequency.
A. Principle
The frequency response of Γ for any specific feed network/spark plug construction is well defined over a range
of plasma impedances. For example, the impedance of the
network and plug used in the experimental setup is shown in
Figure 4a. This known frequency response makes it possible,
given a measured plasma impedance, to estimate a new operating frequency which will result in an improved match for the
current plasma. Figure 4 also illustrates why a simple scalar
measurement of reflected power is insufficient to indicate the
direction in which frequency needs to be changed.
If the response of the feedback loop is fast compared to
the combustion time, this operating frequency can be updated
many times during combustion, optimizing the plasma match
as the gas pressure and plasma density change.
The 1 kW pulsed power amplifier (PA) module is mechanically integrated with a dual directional coupler and impedance
measurement module which contains the vector receivers and
A/D converters contained in the ASIC in Figure 2. The PA
is driven by a controller module which includes an MCU
that communicates with the impedance measurement module
and four channels of the synthesizer, switch, and variablegain microwave amplifier contained in the ASIC. The PA and
controller are powered by an ordinary 32 VDC, 200 W bench
power supply (not shown) in parallel with a 13 mF electrolytic
capacitor to supply sufficient current to the PA during the
microwave burst. Because the average duty cycle of the burst is
less than 1% in these experiments, conduction cooling through
the test stand provides adequate PA cooling.
For safety and ease of measurement, the spark plug is
mounted in a constant-volume chamber filled with a (noncombustible) atmospheric gas mixture.
Microwave bursts with a nominal pulse power level of
800 W are generated with a burst repetition rate of 10 Hz.
Each burst (see Figure 1) contains 100 pulses of 1 µs duration
and 2 µs period (i.e. 200 µs total burst length). The plasma is
seeded by a spark discharge generated by a standard ignition
coil. The microwave burst is started 50 µs after the spark.
B. Experimental Setup
Figure 5 shows the electronics, feed network, and spark
plug used in the experimental setup. This uses development
hardware designed and fabricated as the predecessor of the
self-contained module used in the vehicle.
Fig. 5. Experimental setup (DC power supply and constant-volume chamber
not shown).
978-1-4799-8275-2/15/$31.00 ©2015 IEEE
D. Experiment
The high-voltage spark ignition and microwave bursts were
turned on at an initial gas pressure of 100 kPa, and the starting
frequency was adjusted for the best match at this pressure
(2.46 GHz, for this setup). Then the gas valve was opened
enough to allow the pressure to rise to 1 MPa in 25 s. At the
same time, logging of all calculated Γ data was started on the
MCU. When the pressure reached 1 MPa, data logging was
paused, and the gas feed was turned off. The bleed valve was
then opened to allow gas pressure to decrease to atmospheric
pressure over the course of the next 25 s, while data logging
ran again. In total, this yielded approximately 50,000 data
points while gas pressure was changing.
In the first part of the experiment, the operating frequency
was fixed at the starting frequency throughout changes in gas
pressure. In the second part, the plasma stabilization script on
the MCU was allowed to run while the gas pressure changed.
The experiment was repeated ten times to ensure repeatability.
E. Results
Figure 6 presents histograms of |Γ| collected during one run
of the experiment, where Figure 6a shows the distribution of
|Γ| with a fixed frequency, and Figure 6b shows |Γ| achieved
with the plasma stabilization technique, clearly demonstrating
that the technique results in an improved match over changes
in gas pressure.
(In both cases, the second |Γ| distribution near 0.85 corresponds to microwave pulses generated with no plasma present.
These are mostly clustered near the beginning of the bursts,
i.e. immediately after the plasma was seeded by the spark.)
6000
6000
5000
5000
4000
4000
Count
Incident and reflected voltages are measured in every pulse
in every burst, and Γ for each pulse is computed by the
MCU after the burst using (2). To find the best frequency,
samples with no plasma are discarded, and the remaining
samples are averaged and used to estimate a new frequency if
|Γ| > 0.25. To do this, the MCU uses a table of Γ curves (i.e.
Γ vs. frequency) for a range of values of plasma impedance
corresponding to different gas pressures, calculated from the
model in Figure 3. The MCU finds the curve which is closest
to the measured Γ for the current operating frequency, and
then selects the new frequency as the point for which Γ on
that curve is lowest. For instance, referring to Figure 4a, if
measured Γ ≈ 0.46 −135° at 2.45 GHz, the MCU will select
a curve corresponding to gas pressure near 100 kPa, and will
select a new operating frequency closer to 2.47 GHz. The
next burst, with the new frequency, will then be delivered into
a better match.
The MCU takes less than 100 µs to execute this algorithm
and set the new frequency, making it suitable for burst rates far
higher than the highest engine speed. If it were implemented in
an ASIC or field-programmable gate array (FPGA) combined
with a faster frequency synthesizer, it could adjust for changing
gas pressure during individual combustion cycles.
Count
C. Algorithm
3000
3000
2000
2000
1000
1000
0
0
0.2
0.4
0.6
0.8
1
0
0
0.2
|Γ|
0.4
0.6
0.8
1
|Γ|
(a) |Γ|, frequency = 2.46 GHz.
(b) |Γ| with frequency control.
Fig. 6. Experimental results for plasma stabilization using frequency control.
6a and 6b show the distribution of |Γ| during operation with gas pressure
varying from 100 kPa to 1 MPa: in 6a the microwave frequency is fixed at
2.46 GHz, in 6b the frequency control technique previously described is used.
IV. C ONCLUSION
We have designed, fabricated, and characterized prototype
automotive microwave plasma ignition systems, including
CTLR-based microwave spark plugs. We have incorporated a
low-cost high-speed impedance measurement system that we
can use to monitor plasma behavior in real time. We have
designed and tested a frequency control technique for optimizing the plasma match during operation, which was shown
experimentally to double the amount of power transferred to
the plasma over a simulated automobile drive cycle.
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978-1-4799-8275-2/15/$31.00 ©2015 IEEE