Double spark ignition system: spectroscopic diagnostic of the plasma
B. Hnatiuc *, D. Astanei *, S. Pellerin**, N. Cerqueira**, M. Hnatiuc***,
* - Technical University « Gh. Asachi » Iasi, Romania
** - GREMI Laboratory - Site of Bourges, Orleans University-CNRS, France
*** - Constanta Maritime University, Romania
Abstract: The ignition sparks provided by the classical spark plug do not always assure a fast and complete
combustion of the mixture hydrocarbon-air. For this reason we propose a new type of plug with a double spark
using two simultaneous discharges generated by a pulsed high voltage power supply. This work presents the
spectroscopic analysis of the plasma generated by the classic plug and the proposed plug in different positions in
the space between the electrodes. These are supplied with waveforms containing one pulse by period. The
waveform has adjustable pulse duration. The experiments have been done in air at atmospheric pressure.
In each case it was pursued to identify the species (molecules and atoms) of spectrum in order to study the
influence they may have on the development of the combustion. Because the spectra obtained are clear, stable and
repeatable, it was possible to determine the temperatures in the plasma column by using two different
spectroscopic diagnostic methods based on the molecular band spectra.
Keywords:: ignition spark, spectroscopic diagnostic
1. Introduction
The general objectives of this study are referring to
the testing, validation, implementation and
spectroscopic investigation of a plasma ignition
device for an internal combustion motor, in order to
increase the rapidity and the quality of the fuel
combustion, therefore the reducing of pollutant
emissions. The development of an ignition system
for internal combustion motors requires a fast and
complete burn of the poor hydrocarbon-air mixtures
at high pressure.
The aim of the proposed system consists in the
generation of an electrical discharge between the
spark plug electrodes able to assure a larger and
more homogenous volume of the plasma in the
engine cylinder. Nakamura, [1], has proposed to use
more ignition points for a cylinder equipped with
several classical spark plugs and he noticed that the
combustion cycle evolves faster, the compression
ratio of the motor increases and the poor mixture of
hydrocarbons reduces the concentration of some
pollutants.
Taking into account the conclusions mentioned
above, a new ignition system has been proposed and
tested in a high pressure air reactor (values up to 10
bars pressure), on an engine testing stand (EX1000
type), [2]. The system consists in a double spark
system, with three electrodes, that used two quasi-
simultaneous sparks generated by a pulsed high
voltage power supply, Figure 1. The first electrode,
1, is connected at the high voltage, the second, 3, at
the ground and the third, 2, located between the two
others, free of potential. The electrode free of
potential was made from a washer cut and then
shrinks on its place between other two electrodes. In
the end the washer was welded in place, Figure 1.
Figure 1. Photo of the double spark plug with the electrodes and the
analysis regions.
The power supply used for the experiments permits
the shift control of the pulses and therefore the
possibility to control the ignition timing of the
combustion process.
The main aim of the present research was focused on
comparative spectroscopic analyze of the plasma
produced by two types of spark plugs for different
durations of the pulses and in different regions of the
produced plasma.
2. Experimental set-up
The testing set up of two-spark plug required a
pulsed power supply that uses an ignition coil driven
by a microsystem with AT89S52, which permits the
shift control of the pulses and therefore the
possibility to control the ignition timing of the
combustion process. In this study we present the
results for the three elected pulse widths of ∆t2 = 1.5,
3.0 and 3.4 msec, see Figure 2. The command source
provides also a trigger signal for the spectroscopic
acquisitions system.
processed, which allow spatial and temporal study of
the plasma, with a more precise diagnosis,
associated with different operating modes. In order
to determine the rotational temperature of the plasma
a diagnostic method [3], based on the comparison
between experimental and theoretical rotational
structure of the molecular emission spectra of the
OH band at 306.357 nm, by identification of the
optical apparatus function has been used.
The double spark plug has been divided in 5 regions
of study as shown above in Figure 1: I – inferior part
of the high voltage electrode, II – the superior part of
the potential free electrode, III – inferior part of the
potential free electrode, IV – the middle distance
between the potential free electrode and the ground
electrode, V – superior part of the ground electrode.
The classic spark plug has been divided in 2 regions
of study, see Figure 4: I – superior part of the high
voltage electrode, II – inferior part of the ground
electrode and the space between electrodes.
Figure 2. Pulses wave forms with variable time duration, one pulse per
period (∆t1 - the delay, ∆t2 = 1.5, 3.0 and 3.4 msec, T = 10 msec).
The spectroscopic analysis of the spark electrical
discharges was made in air using the experimental
set-up shown in Figure 3.
Figure 4. The analysis regions of the classic spark plug.
On can observe in Figure 5, that is a picture recorded
on the spectrometer used in the zero order, by
accumulation of N gates, that the sparks of the
double spark system ignite mostly in the same place
and therefore on can consider that for a complete
recording a similar plasma zone have been studied.
Figure 3. Experimental set-up.
It has been used a spectrometer ACTON 750i
(ROPERS Scientifics) (750 focal length, resolution
lower than 0.10nm) equipped with intensified CCD
camera. The plasma image was then focused directly
on the entrance slit of the spectrometer using a
quartz lens (focal length f = 15 cm). The spark plug
was positioned on the step-by-step moving device.
The spectra obtained provide data easier to be
500000
450000
400000
Intensity (au)
350000
300000
250000
200000
150000
100000
50000
0
305
310
315
320
325
Wavelength (nm)
Figure 7. OH system (∆ν = 0) for two different pulse durations.
Figure 5. Image of the plasma of the double spark applied to the
spectrometer.(texp=10 msec; accumulations in N=100 gates)
3. Results
The first portion of the OH molecular spectrum (see
Figure 7) has been taken into account for the
spectroscopic analysis. The first step was the
extraction of the offset, considered linear, from the
experimental spectra using a quite isolated line,
Figure 8.
The data analysis is referring to the identification of
the species formed in plasma (molecular spectra,
atomic spectra), in order to compare them as a
function of the region of the signal acquired and the
duration of the pulses. In addition the diagnostic of
the plasma concerning temperature starting from the
OH molecular band from 306.357 nm has been
considered.
Thus were identified molecular spectral band
corresponding to N2 and OH, see Figure 6 and
Figure 7, and atomic spectral lines for Fe I, W I and
O I.
Figure 8. Typical recorded spectrum with the offset extraction and
peaks considered for calculation of rotational temperature.
Figure 6. The identification of the N2 molecular species.
In Figure 6 the band heads of the second positive
system N2 are corresponding to: P1 - ∆ν = 0 at 337.1
nm, P2 - ∆ν = -1 at 357.5 nm, P3 - ∆ν = -2 at 380.4
nm, P4 - ∆ν = -3 at 405.94 nm and P5 - ∆ν = -4 at
434.36 nm.
The apparatus function was also determined by
using some calibration lamps and compared with the
values obtained by analyzing the peaks that can be
considered like relatively well-isolated. Each of the
peaks was examined by matching with a Gauss
profile, obtaining an optical apparatus function of
∆λapp = 0.06 nm (Full Width at Half-Maximum).
Rotational temperatures obtained in the five different
areas of the double spark plug, and in both areas of
the classic spark, based on reports between
amplitudes of peaks I01, I02, I22 and I24 are presented
in Table 1. The identification of temperature was
made by interpolation method using different
characteristics corresponding to several apparatus
function, as shown in reference [3].
Table 1: Measured rotational temperatures in the different analysis
regions of the spark plugs
Temperature [K]
Double spark
Classic spark
Region Region Region Region Region Region Region
I
II
III
IV
V
I
II
∆λ [nm]
0.055
0.06
0.06
0.06
0.06
0.06
0.06
∆t1=1.5ms
3130
3830
2390
2630
2640
3700
3630
∆t1=3.0ms
2860
3550
2710
3080
2190
3360
3380
∆t1=3.4ms
2710
3480
2850
2280
2800
3590
3460
4. Discussion
On the regions of the washer (II, III and IV),
considering the double spark, have been identified
much more atomic lines (W I, Fe I, O I) than in the
others regions. In fact the lines appear for all the
durations of the pulses but it seems there are more
intensive for a shorter duration of the applied pulses.
On region I there are very few atomic lines and only
for the pulse duration of 1.5 msec. On region V there
are few Fe I atomic lines between 500 and 850 nm.
In the case of the classic spark, for both analyzed
regions, the lines are very few lines. In fact in this
case only the O I was identified around 777.4 nm
and 844.6 nm both of them corresponding to the
region 2 and for the shortest durations of the pulses.
Regarding the comparison of rotational temperature
values obtained for the two types of plugs it can be
seen that they are higher in case of the classic type.
In this case there is no difference between the
analyzed regions. In the case of the double spark the
region II seems to be the more solicited from the
heat point of view. For the regions I and II the
rotational temperature increase with the decreasing
of the pulse duration.
5. Conclusion
The experiments performed on the engine stand and
also in the reactor with air at high pressure have
demonstrated the applicability in real condition of
the proposed double spark plug. This new type of
spark plug provides a longer and a bigger volume of
the plasma that should assure a better combustion
process.
The spectroscopic analyze emphasized differences in
the composition of the species formed in plasma
(identification on the spectral lines) but also in their
energy.
We can note that the values of the rotational
temperature for the double spark plug is decreasing
from the first zone to the fifth one because the
distance between the electrodes is larger and occurs
a higher thermal exchange between the plasma of the
electrical discharge (the spark) and the environment.
The composition of the spectra and the higher
temperature found in the zone II of the double spark
system impose the chose the composition of the
material for the third electrode free of potential and a
special fixture.
The rotational temperatures obtained for the classical
spark plug are higher than in the case of the double
spark plug because in first case the discharge is
produced into a smaller volume of gas and the
distance between the electrodes is smaller. That
means that the electric current is lower in the case of
the double spark system.
The next step of this work will be the determination
of the vibrational and rotational temperatures based
on the N2 molecular spectra and comparison with the
rotational temperatures values obtained from the OH
molecular spectra.
References
[1]. N. Nakamura, T. Baika, and Y. Shibata: Multipoint spark
ignition for lean combustion, SAE paper 852/092.
[2]. B. Hnatiuc, S. Pellerin, E. Hnatiuc, R. Burlica, The study of
an electric spark for igniting a fuel mixture, 12th International
Conference on Optimization of Electrical and Electronic
Equipment, OPTIM 2010, Brasov, May 20 – 22, ISBN 978-973131-080-0, CPDA 9.1.09.
[3]. S. Pelllerin, J.–M. Cormier, F. Richard, K. Musiol, J.
Chappelle, A spectroscopic diagnostic method using UV OH
band spectrum, J. Phys. D.: Appl. Phys. 29, p. 726 – 739 (1996).