Transient plasma ignition for clean, fuel-
efficient, transportation vehicle engines
Final Report
METRANS project 07-20
Reporting Period: September 2006 – August 2008
Prof. Paul D. Ronney, Principal investigator
Prof. Martin Gundersen, co-investigator
University of Southern California
Department of Aerospace and Mechanical Engineering
Olin Hall of Engineering 430
Los Angeles, CA 90089
Disclaimer
The contents of this report reflect the views of the authors, who are responsible for the facts and
the accuracy of the information presented herein. This document is disseminated under the
sponsorship of the Department of Transportation, University Transportation Centers Program, and
California Department of Transportation in the interest of information exchange. The U.S.
Government and California Department of Transportation assume no liability for the contents or
use thereof. The contents do not necessarily reflect the official views or policies of the State of
California or the Department of Transportation. This report does not constitute a standard,
specification, or regulation.
Abstract
The use of non-thermal transient plasma ignition (TPI) as an alternative to spark ignition (SI) for
internal combustion engines was investigated. A 2.5 liter 4-cylinder test engine was used that has
two spark plug ports per cylinder to enable direct, instantaneous and valid comparisons of spark and
corona ignition at the same time under identical engine operating conditions. A ceramic corona
electrode was designed that fits in the new test engine and is capable of withstanding the pressures
and temperatures encountered inside the combustion chamber. The corona ignition system was
tested on the engine and an increase (compared to spark ignition) in both peak pressures and
indicated mean effective pressure (IMEP) were observed. Testing showed typically 15 - 20%
increases in IMEP and much shorter burn durations at identical operating conditions. Moreover,
the tradeoff between thermal efficiency and brake specific NOx emissions was found to improve
with TPI. It is proposed that these advantages of corona ignition may be exploited by either (1) the
use of leaner fuel-air ratios or (2) by designing engines with lower turbulence levels, thereby reducing
heat loss to cylinder walls and increasing thermal efficiency, and employing corona ignition to obtain
sufficiently rapid burning.
List of figures and tables
Figure 1. Experiment setup for investigation of flame ignition by pulsed corona discharge
Figure 2. Corona generator connected to one cylinder on test engine
Figure 3. Top: Cutaway diagram of turbulent test chamber; bottom: photograph of chamber
Figure 4. Experimental setup for engine cylinder test chamber
Figure 5. Dimensioned drawing of Engine Cylinder Test Chamber
Figure 6. Exploded view of Engine Cylinder Test chamber assembly, shown with circular ring
electrode
Figure 7. Engine cylinder test chamber in open configuration for visualizing corona discharges
Figure 8. Test chamber using piston and cylinder head from engine (assembled).
Figure 9. Test chamber using piston and cylinder head from engine (disassembled).
Figure 10. Test engine mounted to dynamometer.
Figure 11. Main panel for data acquisition and control equipment. Top left AMUX-64T, top right
SCB-68, bottom left MID-7604, bottom right CB68-LPR
Figure 12. Screen capture of PC engine Control Panel
Figure 13. Control cabinet for cylinder pressure monitoring system
Figure 14. Modified spark plug with pressure transducer installed
Figure 15. Ceramic corona electrode for running in engine show in exploded view (left), assembled
(center) and shielded (right).
Figure 16. Picture of pulsed corona discharge (end view). Diameter of central electrode: 6.35mm,
energy: 251 mJ.
Figure 17. Picture of pulsed corona discharge (side view). Diameter of central electrode: 6.35mm ,
energy: 356 mJ.
Figure 18. Photograph of corona discharge in engine cylinder test chamber with open top plate
installed. .5mm diameter electrode 50mm circular ring
Figure 19. Plot of voltage, energy and current delivered to the gas for corona discharge plus arc.
Test performed in engine cylinder test chamber.
Figure 20. Plot of Voltage, Energy and Current delivered to the gas for corona discharge only. Test
performed in engine cylinder test chamber.
Figure 21. Typical pressure waveform with definitions of ignition delay time, pressure rise time and
peak pressure.
Figure 22. Delay time versus equivalence ratios for corona ignition with various electrode
geometries, and comparison with spark ignition.
Figure 23. Rise time versus equivalence ratios for corona ignition with various electrode geometries,
and comparison with spark ignition.
Figure 24. Rise time versus equivalence ratios for corona ignition with various electrode geometries,
and comparison with spark ignition.
Figure 25. Plot of Pressure vs. Time for Corona, Corona + arc and conventional spark plug with
associated electrical energy. Tests performed in engine cylinder test chamber.
Figure 26. Pressure-time histories of spark-ignited and plasma-ignited quiescent and turbulent
flames. Top: stoichiometric mixture; bottom: lean mixture.
Figure 27. Engine test results (natural gas fuel): typical (not “best case” comparison of spark-
ignited and plasma-ignited P-V diagrams.
Figure 28. Engine test results (natural gas fuel): gross heat release rate profile (no heat loss model
has been incorporated, thus values indicate (heat generation by combustion) – (heat
loss/gain to cylinder walls)).
Figure 29. Engine test results (natural gas fuel): integrated heat release rates.
Figure 30. Engine test results (natural gas fuel): comparison of “envelope” of brake specific NOx
emissions vs. indicated efficiency for spark-ignited and transient plasma (“corona”) ignited
engines.
Figure 31. Engine test results (natural gas fuel): comparison of “envelope” of brake specific
hydrocarbon emissions vs. indicated efficiency for spark-ignited and transient plasma
(“corona”) ignited engines.
Figure 32. Engine test results (natural gas fuel): comparison of “envelope” of brake specific CO
emissions vs. indicated efficiency for spark-ignited and transient plasma (“corona”) ignited
engines.
Table of contents
1! INTRODUCTION ........................................................................................................ 1!
2! EXPERIMENTAL APPARATUS ................................................................................. 2!
2.1! Corona Generator .................................................................................................................................................2!
2.2! Test Chambers .....................................................................................................................................................3!
2.3! Dual Plug Test Engine and Instrumentation ....................................................................................................7!
2.4! Cylinder Pressure Monitoring System ................................................................................................................9!
2.5! Ceramic Corona Electrode ..................................................................................................................................9!
3! RESULTS .................................................................................................................. 11!
3.1! Electrical Characteristics ...................................................................................................................................11!
3.2! Combustion Characteristics ..............................................................................................................................13!
3.3! Tests in turbulent, constant-volume chambers ...............................................................................................16!
3.4! Engine tests ........................................................................................................................................................17!
4! CONCLUSIONS AND RECOMMEDATIONS ......................................................... 21!
5! IMPLEMENTATION ............................................................................................... 21!
6! PUBLICATIONS ....................................................................................................... 22
1 Introduction
The electric arc has been the ignition source of choice for automotive combustion engines for
over 100 years. It has many advantages including simplicity, low cost, size and weight of the
electronics, and it produces sufficiently high temperatures to dissociate and partially ionize most fuel
and oxidant molecules. Nevertheless, there are also numerous disadvantages of arc discharges,
including the limited size of the discharge, the low energy of the electrons that are created and the
low "wall-plug" efficiency (i.e., ratio of energy deposited in the gas to the electrical energy consumed
in producing the discharge.) For these reasons, many investigations of ignition by laser sources have
been conducted in recent years. Still, laser ignition sources present many practical difficulties,
especially the need for reliable, low-attenuation optical access, extremely low wall-plug efficiency,
and extremely high optical intensities needed to induced breakdown in the gas which in turn makes
it difficult to control the location and intensity of the discharge.
In the proposed investigation transient plasma discharges (sometimes called pulsed corona discharges)
will be used for the initiation of combustion in low-turbulence premixed-charge internal combustion
engines to “restore” burning rate lost by reduction of turbulence levels. The transient plasma
discharge is defined as the portion of an electric discharge before the onset of the low-voltage, high
current arc discharge. Only the earliest, most efficient transient plasma discharge stage, is used for
ignition. The discharge is stopped before the later, less efficient (and geometrically less desirable)
stage begins. The electrical energy that would have been used in these later inefficient stages is
saved for the next combustion cycle.
With recent advances in pulsed power electronics developed at USC, for the first time transient
plasma discharges can be produced with very high wall-plug energy efficiencies in a system of
reasonable cost, size and weight. Effectively these electronics produces only the transient plasma
phase of the discharge; the later arc discharge is a less efficient source for transferring energy to
ionization, electronic excitation, and dissociation, does not occur, and thus less energy is required for
a given ignition event. Consequently, in addition to the inherent increase in burn rate possible with
transient plasma ignition, the transient plasma generators proposed here are much more energy-
efficient than any advanced ignition system producing arc-type discharges, and thus introduce less
parasitic losses in terms of the shaft power needed to drive the electrical generator supplying the
ignition system. Furthermore, we note that there is much less heating and erosion of the electrodes
in the transient plasma phase than in the arc phase and thus transient plasma ignition systems are
likely to be more reliable than conventional ignition systems.
It should be emphasized that the transient plasma discharges discussed here are entirely different from that
used in any prior IC engine studies outside of our group and cannot be produced by conventional IC engine ignition
systems. Unlike so-called “plasma igniter” spark plugs that have been around for many years, which
simply produce and expel a minute amount of low energy electrons a short distance, the transient
plasma generator produces streamers of high energy electrons that span the entire combustion
chamber volume and are generated with high wall-plug efficiencies.
Consequently, the objective of the current work was to assess the feasibility of employing
pulsed corona discharges in internal combustion engines. Initially, burn-rate measurements were
made in constant-volume chambers under quiescent and turbulent condition, comparing spark-
ignited and corona-ignited flames. Based on these encouraging results, performance in a laboratory
engine was measured, again comparing conventional ignition to corona discharge ignition.
2
2 Experimental Apparatus
2.1 Corona Generator
The experimental apparatus for the constant-volume tests consists of an electronic system to
generate pulsed corona discharges and a combustion cylinder with measurement and gas handling
system (Fig. 1). The pulsed power includes a high voltage pulse generator utilizing a thyratron
(Triton F-211) and a Blumlein transmission line to create a high voltage pulse (typically 30 - 60 kV)
with narrow pulse width (typically 100 nsec). The maximum pulse energy employed was 1.8 J. For
engine tests a pseudospark-switched circuit having higher repetition rate capability was used instead
of the thyratron circuit. All signals were displayed and recorded by a digital oscilloscope (Tektronix
TDS 420A).
Oscilloscope
Trigger
Pulse
Generator
HV DC
Power
Supply
HVH
V
Transformer
HV Probe
V Signal
P Signal
I Signal
Pressure Transducer
Air
Fuel
Vacuum
Central Electrode
Cylindrical Combustion Chamber
Figure 1. Experiment setup for investigation of flame ignition by pulsed corona discharge
The same equipment is used in static combustion chambers and in the engine experiments. The
firing is triggered at the desired timing from an optical encoder on the engine.
3
Figure 2. Corona generator connected to one cylinder on test engine
2.2 Test Chambers
Three different test chambers have been developed for testing of the corona discharge
ignition. The first test cylinder is constructed from a 2.5” ID stainless steel tube that acts as the
ground electrode. This chamber has an interchangeable metal rod placed at its central axis acting as
the (central) electrode. The outer electrode is always grounded and the central electrode is connected
to high voltage. In this report all reported experiments were done with positive corona (central
electrode is anode) unless otherwise noted. There are gas inlets, outlet and vacuum pump inlet in
one end plate and pressure gauge (Omega, DPG1000B), pressure transducer (Omega PX4201) and
conventional car sparkplug (Bosch, platinum) on the other end plate. The sparkplug was switched by
a standard automotive ignition circuit. A transparent plastic end plate was used for end view photos.
This chamber also has a configuration that allows for testing under turbulent conditions (See Fig. 3).
The second test chamber used for the corona ignition experiments was modeled after the
dimensions of the combustion chamber in the test engine. This chamber was constructed from
6061 T6 aluminum and consists of a bottom half that is machined into the shape of the piston dome
of the test engine, and a top half that is machined to closely match the volume of the cylinder head.
The two pieces are bolted together with six 5/16” bolts and are sealed with an O-ring (seen in Figs.
6 and 7). With this test chamber various electrode shapes and configurations could be tested much
more quickly than in the engine. For tests performed at 1 atmosphere, the corona generator and the
measurement equipment are identical to those used with the cylindrical shaped chamber. For tests
run at elevated pressures a Kistler model 603B1 spark plug-mounted pressure transducer and model
5004 dual mode amplifier were used to measure the pressure. When performing ignition tests in this
chamber the gas and air mixture was premixed in a larger chamber (Fig. 4). Combustion tests were
performed at initial pressures up to 9.5 atmospheres. An alternate chamber top plate was fabricated
from a ¼” plate of aluminum that leaves the sides open for photographing the test chamber (Fig. 7).
4
Fan
HV Anode
Grounded Cathode
Figure 3. Top: Cutaway diagram of turbulent test chamber; bottom: photograph of chamber
Figure 4. Experimental setup for engine cylinder test chamber
5
Figure 5. Dimensioned drawing of Engine Cylinder Test Chamber
Figure 6. Exploded view of Engine Cylinder Test chamber assembly, shown with circular ring electrode
6
Figure 7. Engine cylinder test chamber in open configuration for visualizing corona discharges.
The third test chamber used in testing is constructed using a cylinder head and piston from
the test engine described in below in section C. An aluminum block was machined to accept the
piston from the test engine and holds the piston in a TDC position (see Fig. 9). The block has four
holes that are tapped to accept the head bolts that secure the piston/block assembly to the head.
The seal between the head and the block is held by using a head gasket from the engine. Gas
handling is done through fittings installed in block-off plates that are installed over the intake and
exhaust ports. Measurements are done with the same equipment used on the other test chambers.
This chamber allows electrodes to be tested in a chamber with the exact same dimensions of the
engine.
Figure 8. Test chamber using piston and cylinder head from engine (assembled).
7
Figure 9. Test chamber using piston and cylinder head from engine (disassembled).
2.3 Dual Plug Test Engine and Instrumentation
The test engine as seen in Figure 10 was required to simplify the testing of the corona
discharge ignition system in the engine. The engine purchased was a 2.5 liter 4 cylinder engine from
a 2000 Ford Ranger Pickup. This engine was chosen because it has two spark plug ports per
cylinder. This allows the corona electrode to be inserted into one of the ports and a conventional
spark plug with cylinder pressure transducer to be inserted in the other. The engine was adapted to
run on natural gas using a control valve and natural gas flow meter.
Figure 10. Test engine mounted to dynamometer.
8
Figure 11. Main panel for data acquisition and control equipment. Top left AMUX-64T,
top right SCB-68, bottom left MID-7604, bottom right CB68-LPR
Figure 12. Screen capture of PC engine Control Panel
Data acquisition and control hardware was purchased from National Instruments for the
engine lab. This equipment consists of a PCI-6031E DAQ card, a PCI-7344 motion control card,
an AMUX-64T terminal block for thermocouple inputs, a SCB-68 terminal block for analog inputs,
a MID-7604 stepper motor driver and a CB68-LPR terminal block for digital IO (Fig. 11). The
engine is fully instrumented and all critical pressures, temperatures and other parameters are
monitored.
Software was developed using LabView to interface with the new hardware. The software
displays all of the pertinent engine parameters and incorporates PID control for the air/fuel ratio
9
and dyno load control. The user interface was developed to be easy to read and user friendly (Fig.
12). Warning alarms and safety shutdowns were incorporated to protect the engine when
parameters (pressures, temperatures or speed) get out of a safe range.
2.4 Cylinder Pressure Monitoring System
In order to quantify the performance characteristics of the corona discharge ignition system it
was necessary to monitor the pressure of the cylinder that the corona electrode is installed in. A
cylinder pressure monitoring system was designed to accomplish this task. The system consists of a
Kistler pressure transducer with amplifier, an optical encoder coupled to the crankshaft with 2048
pulses per revolution (providing 0.18 degree resolution), and a high speed data acquisition card from
National Instruments (NI 6070E). The amplifier was located in a shielded enclosure as was the
terminal block for the wiring connections (Fig. 13).
Figure 13. Control cabinet for cylinder pressure monitoring system
The pressure transducer reads the cylinder pressure through a spark plug that was modified by
Kistler with an added port for the transducer as seen in Fig. 14.
A program was created in LabView that collects cylinder pressure traces based on the trigger
inputs received from the encoder that is mounted on the crankshaft. Cylinder pressure data is
logged vs crank angle. This data is then imported into a spreadsheet where IMEP is calculated and
PV diagrams are created.
2.5 Ceramic Corona Electrode
The electrodes that were used for testing the corona in the test chambers were constructed of
a plastic that could withstand the pressures of single shot combustion tests but not the temperatures
encountered in a running engine. It was necessary to design an electrode that could withstand the
temperatures and pressures of a running engine. A conventional spark plug could not be used as the
conductor for the corona electrode as the electrical resistance is much too high for this application.
The resistance should be as low as possible for the corona electrode to perform properly.
10
Figure 14. Modified spark plug with pressure transducer installed.
Figure 15. Ceramic corona electrode for running in engine show in exploded view (left), assembled (center) and
shielded (right).
A machinable ceramic (Macor™) was chosen that has both high temperature resistance,
good electrical insulating properties and is easily machinable. The steel body of the corona electrode
uses a spark plug body with the original ceramic portion removed. The swaged lip was machined off
the body that holds the ceramic in place. The Macor is purchased in ½” diameter rods and
11
machined down to fit the steel spark plug casing. A shoulder is machined into the top of the
ceramic rod which is captured by a steel ring installed from the top which is then welded to the
spark plug body. A 1/8” brass rod is drilled from both ends to accept the electrode tip and the
conductor. The brass rod is installed from the bottom and is held in place with a high temperature
epoxy. This design allows different electrode tips to be used with one electrode body. The
electrodes were hydro-tested at 1000 psi before put service in the engine.
3 Results
3.1 Electrical Characteristics
Photographs of various pulsed corona discharge are shown in Figs. 17 and 18. They were taken in a
completely dark environment. Fig. 17 is a pulsed corona discharge with a plain electrode viewing
from the end of cylindrical chamber. This shows many streamers distributed along the length of
central electrode and superimposed with each other. If viewed from side many discrete streamers
evenly distributed on the surface of the central electrode are evident. The average separation
between streamers is 5mm and average diameter is 0.7mm. An estimated number of streamers on
the central electrode is 600. When pulse energy was reduced from 356mJ/pulse to 44mJ/pulse the
total number of streamers did not change noticeably, but the brightness of streamers decreased.
Figure 18 shows the corona discharge from the side view in the open top chamber as seen in Fig. 7.
Figure 16. Picture of pulsed corona discharge (end view). Diameter of central electrode: 6.35mm, energy: 251 mJ.
Figure 17. Picture of pulsed corona discharge (side view). Diameter of central electrode: 6.35mm , energy: 356 mJ.
12
Figure 18. Photograph of corona discharge in engine cylinder test chamber with open top plate installed. .5mm
diameter electrode 50mm circular ring
Figures 19 and 20 shows typical voltage, current and energy waveforms of pulsed corona
discharges with and without arcing. Current waveforms clearly show the difference between corona
discharges with arcing and without arcing. Both dashed and solid curves have a first peak that
corresponds to the corona discharge. Only the dash curve has the second current peak which
correspondences to arc discharge. One can see that after arcing (at 150 ns after trig where current
started to rise rapidly) voltage drops rapidly. Consequently, no rapid energy increase was observed
on energy waveform implying that arcing does not contribute significantly to energy input to plasma
at the voltages at which arc is going to start, during times on the order of 100s of nanoseconds. In
these experiments the experimental conditions (mainly the applied voltage and electrode structure)
were so controlled that no arcs were observed to ensure that only the transient phase occurred
before the formation of arcing. Typical voltage pulse width (FWHM) is 140 ns, and current pulse
width 80 ns.
Figure 19. Plot of voltage, energy and current delivered to the gas for corona discharge plus arc. Test performed in
engine cylinder test chamber.
13
Figure 20. Plot of Voltage, Energy and Current delivered to the gas for corona discharge only. Test performed in
engine cylinder test chamber.
Pulse energy as a function of peak voltage was measured for various electrodes. The pulse
energy increases non-linearly and is faster at higher peak voltages. There is an intercept peak
voltage, below which no energy output to plasma, i.e. pulse corona discharge starts at a certain value
of peak voltage. The intercept peak voltage decreases as decreasing of diameter of central electrode.
At the same peak voltage, a thinner central electrode provides higher pulse energy than a thick
central electrode does. A threaded electrode produces higher pulse energy than smooth electrode
(plain electrode) with the same diameter and peak voltage. In our particular conditions, brush-like
electrodes provide best electrical performance. Positive corona discharge (central electrode is
anode) is much better than negative corona discharge (central electrode is cathode). In the
experiments reported below, only positive pulsed corona discharge is used.
3.2 Combustion Characteristics
The experimental procedure for bench testing the corona discharge in the test chambers were
as follows. The test cylinder was evacuated with a vacuum pump until the pressure was less than
0.01 psi. Then fuel (CP grade) and air (dry cylinder air) were filled into the cylinder using the partial
pressure method. Composition was controlled with a digital pressure gauge with precision of 0.01
psi. After igniting with either pulsed corona or spark discharge, the pressure was measured by the
pressure transducer. Pressure signals were read and recorded by the digital oscilloscope.
Figure 21 shows a typical pressure waveform. The pressure rose slowly after trigger for a
period of time, then suddenly increased rapidly at a certain point, reached its maximum and finally
dropped gradually. We define ignition delay time as the interval between trigger and the moment at
which the pressure rises to 10% of its maximum value. Pressure rise time is defined as the interval
between moments at which pressure rises to 10% and 90% of its maximum value.
14
2
4
6
8
10
12
14
16
-0.02 0 0.02 0.04 0.06 0.08 0.1
Pressure (atm., abs)
Time (s)
Delay
Time
Rise Time
Discharge trigger
10% of total pressure rise
90% of total
pressure rise
Figure 21. Typical pressure waveform with definitions of ignition delay time, pressure rise time and peak pressure.
10
100
0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05
corona, 1 pin, 75 mJ
spark, 75 mJ
corona, 3.9 mm dia rod, 710 mJ
corona, 2 ring x 2 pin, 170 mJ
corona, 4 ring x 2 pin 170 mJ
Delay Time (ms)
Equivalence ratio
CH
4
/Air
P = 1 atm
Figure 22. Delay time versus equivalence ratios for corona ignition with various electrode geometries, and comparison
with spark ignition.
15
10
100
0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05
corona, 1 pin, 75 mJ
spark, 75 mJ
corona, 3.9 mm dia. rod, 710 mJ
corona, 2 ring x 2 pin, 170 mJ
corona, 4 ring x 2 pin, 170 mJ
Rise Time (ms)
Equivalence ratio
CH
4
/Air
P = 1 atm
Figure 23. Rise time versus equivalence ratios for corona ignition with various electrode geometries, and comparison
with spark ignition.
3
3.5
4
4.5
5
5.5
6
0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05
corona, 1 pin, 75 mJ
spark at center, 75 mJ
corona rod, 710 mJ
corona, 2 ring x 2 pin, 170 mJ
corona, 4 ring x 2 pin, 170 mJ
Peak P/P
o
Equivalence ratio
CH
4
/Air
1 atm
Figure 24. Rise time versus equivalence ratios for corona ignition with various electrode geometries, and comparison
with spark ignition.
Figure 22 shows the delay time as a function of equivalence ratio, electrode geometry and
discharge type (spark or corona) in the coaxial cylinder. The superiority of the corona ignition or
spark ignition can be seen, and the maximum improvement occurs for the rod electrode as
compared to the various pin-type electrode. Similar tendencies are found in curves of rise time (Fig.
23) and peak pressure (Fig. 24).
16
In the engine-like test cylinder, several different electrode geometries were tested in the
engine cylinder test chamber, a ring shaped electrode, a straight electrode and a single pin point
electrode. The ring electrode allowed the most electrical energy to be deposited to the gas before
arcing so it was used for the corona combustion tests. Due to the smaller chamber size of the
engine cylinder test chamber the center electrode is much closer to the chamber walls than in the
larger cylindrical test chamber and is more likely to arc. Arcing usually occurs at the tip of the
electrode or at a sharp bend. In order to prevent the electrode from arcing the end of the electrode
was insulated. Without this insulation the energy of the corona was too low to ignite the air/fuel
mixture in the cylinder.
Figure 25 shows the pressure rise time in the engine cylinder test chamber for corona only
ignition, corona plus arc and for a conventional spark plug. It can be seen that the corona ignition
delivers a steeper pressure rise than the conventional spark plug and a higher peak pressure. When
the power was increased to allow the corona to arc the peak pressure was higher than the corona
alone but it comes at the expense of additional electrical energy needed to produce the arc.
Figure 25. Plot of Pressure vs. Time for Corona, Corona + arc and conventional spark plug with associated electrical
energy. Tests performed in engine cylinder test chamber.
3.3 Tests in turbulent, constant-volume chambers
While the mechanisms determined by us to be responsible for the improvements of transient
plasma ignition in quiescent mixtures should carry over to turbulent conditions, we felt it was
necessary to prove this point to lay any doubt to rest. Previously, it could have been argued that
with turbulence, all flames will burn quickly and that the benefit of transient plasma ignition would
be lost, but the results below discredit this superfluous argument. To test the effect of turbulence
on the benefit of transient plasma ignition, one of our existing cylindrical transient plasma ignition
systems (Fig. 3) was used. Exactly the same apparatus was used for all tests (plasma and spark,
quiescent and turbulent). The turbulence intensity (u’) was about 1 m/s, about 2.5 times the laminar
burning velocity (S
L
) of stoichiometric hydrocarbon-air flames. Thus, the effect of turbulence on
burning velocity should be substantial. Figure 26 shows that (as expected) the turbulent cases have
faster rise times, higher peak pressures and faster decay (due to heat losses) in the burned gas. The
17
results are reasonably repeatable, even though the mixtures are turbulent. The delay times are about
the same with or without turbulence for both transient plasma and spark (transient plasma vs. spark
affects delay time, but turbulent vs. quiescent does not), thus transport does not play a significant
role on delay time. The rise times are faster with transient plasma than with spark, for both
turbulent and quiescent cases. Most importantly, the "improvement factor" (ratio of transient plasma rise
time to spark rise time) is nearly the same with or without turbulence. So the advantages of transient plasma
ignition still occur in turbulent flames. The peak pressures are significant higher with transient
plasma than with spark, for both turbulent and quiescent cases. In a 'real' combustor, burning faster
using transient plasma ignition in the presence of strong turbulence and thus strong heat losses
could yield significantly higher thermal efficiency due to the higher peak pressures. Figure 28 (right)
shows that similar but even more pronounced trends are exhibited for lean mixtures because of their
lower S
L
.
1
1.5
2
2.5
3
3.5
4
-0.02 0 0.02 0.04 0.06 0.08 0.1
Pressure (atm)
Time (s)
CH
4
/Air
! = 1.0
1 atm
Quiescent, spark
Turbulent, spark
Turbulent, corona
Quiescent, corona
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
-0.05 0 0.05 0.1 0.15 0.2 0.25 0.3
Pressure (V)
Time (s)
CH
4
/Air
! = 0.7
1 atm
Quiescent, spark
Turbulent, spark
Turbulent, corona
Quiescent, corona
Figure 26. Pressure-time histories of spark-ignited and plasma-ignited quiescent and turbulent flames. Top:
stoichiometric mixture; bottom: lean mixture.
3.4 Engine tests
18
With these encouraging results, tests were conducted using a 2000 Ford Ranger 2.5 liter
dual-plug engine running on natural gas. As discussed in the Experimental Apparatus section , one
cylinder was fitted with the TPI electrode in one plug port. The same discharge generator used for
bench testing was also used for on-engine testing. The other spark plug port was fitted with a
modified spark plug containing a Kistler pressure transducer. This cylinder’s exhaust gas was
collected and analyzed using a Horiba emissions bench. An optical encoder was used to measure
crank angle information and in concert with the cylinder pressure data to create P-V diagrams. The
plasma electrode was a simple “spike” design, home-built from a modified commercial spark plug;
we expect greater improvements may result with more sophisticated electrode designs.
Figures 27 - 29 show, respectively, the P-V diagram, heat release rate and total integrated
heat release (not including any heat loss model, which accounts for the drop during expansion) for a
typical (not “best case”) spark-ignited and plasma-ignited engine cycle. Note that the peak pressure,
peak burn rate, total burn duration and peak gas thermal energy (i.e. peak on the heat release curve)
are substantially higher for the plasma-ignited than the spark-ignited cycle. The indicated thermal
efficiencies are 33.6% and 29.3%, indicating a 33.6/29.3 – 1 = 15% improvement. The
improvement is brake efficiency is even larger, about 21%, since a constant friction loss must be
subtracted from the indicated work to obtain the brake work. Note also that at the end of the
expansion stroke, the “total heat release” has dropped about 40%, meaning that about 40% of the
thermal energy generated by combustion was lost to the cylinder walls. Figure 30 is a comparison of
“envelope” of brake specific NO
x
emissions vs. indicated efficiency for spark-ignited and transient
plasma ignited engines. Clearly the tradeoff is more favorable for plasma ignition in that there are
points to the “southeast” of any points on this plot for spark ignition. Figures 31 and 32 show that
CO and hydrocarbon emissions are similar for the two modes of ignition.
Figure 27. Engine test results (natural gas fuel): typical (not “best case” comparison of spark-ignited and plasma-
ignited P-V diagrams.
19
Figure 28. Engine test results (natural gas fuel): gross heat release rate profile (no heat loss model has been
incorporated, thus values indicate (heat generation by combustion) – (heat loss/gain to cylinder walls)).
Figure 29. Engine test results (natural gas fuel): integrated heat release rates.
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Figure 30. Engine test results (natural gas fuel): comparison of “envelope” of brake specific NO
x
emissions vs.
indicated efficiency for spark-ignited and transient plasma (“corona”) ignited engines.
1
10
100
0 0.1 0.2 0.3 0.4
Indicated Efficiency
BSHC (g/hp-hr)
spark
corona
Figure 31. Engine test results (natural gas fuel): comparison of “envelope” of brake specific hydrocarbon emissions vs.
indicated efficiency for spark-ignited and transient plasma (“corona”) ignited engines.
21
1
10
100
1000
0 0.1 0.2 0.3 0.4
Indicated Efficiency
BSCO (g/hp-hr)
spark
corona
Figure 32. Engine test results (natural gas fuel): comparison of “envelope” of brake specific CO emissions vs.
indicated efficiency for spark-ignited and transient plasma (“corona”) ignited engines.
4 Conclusions and recommedations
Flame ignition by transient plasma (“pulsed corona”) discharges is a promising technology
for ignition of natural-gas fueled internal combustion engines. Constant-volume chamber laboratory
experiments show that such discharges yield shorter ignition delay and rise times (thus shorter burn
times) than flames ignited by conventional sparks under otherwise identical conditions. The rise
time is a more significant issue because unlike delay time, rise time can’t be compensated by “spark
advance” in IC engines. The benefits of corona discharges were also found to apply to turbulent
flames in constant-volume chambers. Based on this groundwork, a corona ignition system was
tested in an IC engine. For identical conditions, corona ignition was found to yield significantly
higher Indicated Mean Effective Pressure (IMEP) for same conditions with same or better Brake
Specific NO
x
(BSNO
x
) emissions than spark-ignition, with comparable CO and hydrocarbon
emissions. Cylinder pressure analysis showed that corona ignition yielded shorter burn times and
faster heat release rates than spark ignition at identical engine operating conditions.
5 Implementation
Transient plasma ignition is an attractive technology for the ignition of small internal
combustion engines because of engine data showing significant reductions in ignition delay, ignition
of leaner fuel-air mixtures, lower specific fuel consumption, and faster burn rates. The inherently
faster burn rates produced by Transient plasma ignition may be used in conjunction with intake port
and combustion chambers intentionally shaped to provide reduced turbulence levels and thus reduce
thermal losses while still obtaining rapid burn rates. Moreover, the reduced cooling system load
means that smaller cooling systems can be used, thus reducing aerodynamic drag. This double
benefit (increased efficiency and reduced drag) is potentially enabling technology for long-distance
small engines having large combustion chamber surface area to volume ratios.
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6 Publications
Singleton DR, Sinibaldi JO, Brophy CM, Kuthi A, Gundersen MA, “Compact Pulsed-Power System
for Transient Plasma Ignition,” IEEE TRANSACTIONS ON PLASMA SCIENCE, Volume 37,
Pages 2275-2279 (2009)
Shiraishi, T; Urushihara, T; Gundersen, M, “A trial of ignition innovation of gasoline engine by
nanosecond pulsed low temperature plasma ignition,” JOURNAL OF PHYSICS D-APPLIED
PHYSICS, Volume 42, Article Number 135208 (2009)
Singleton, D; Cathey, C; Kuthi, A, Gundersen, M., “Applications of Power Modulator Technology
to Ignition and Combustion,” PROCEEDINGS OF THE 2008 IEEE INTERNATIONAL
POWER MODULATORS AND HIGH VOLTAGE CONFERENCE, Pages 174-177 (2008).
Cathey, C; Cain, J; Wang, H, Gundersen, M., “OH production by transient plasma and mechanism
of flame ignition and propagation in quiescent methane-air mixtures,” COMBUSTION AND
FLAME Volume 154 Pages 715-727 (2008).
Tang, T; Singleton, DR; Cathey, CD, Gundersen, M. A., “Solid state pulse adding system for
transient plasma ignition,” 2007 IEEE PULSED POWER CONFERENCE, Pages 995-999 (2007).
Cathey, CD; Tang, T; Shiraishi, T, Gundersen, M., “Nanosecond plasma ignition for improved
performance of an internal combustion engine,” 3rd International Workshop and Exhibition on
Plasma-Assisted Combustion, IEEE TRANSACTIONS ON PLASMA SCIENCE Volume 35
Pages 1664-1668 (2007).
Memarzadeh, S., Rossi, J. Neiman, R, Ronney, P. D., Gundersen, M. A., “Transient Plasma Ignition
for Internal Combustion Engines,” 2007 U. S. National Meeting, Combustion Institute, La Jolla,
CA, March 26 – 28, 2007.
