HCCI operation of an optically accessible diesel engine fuelled with RME fuel
Ezio Mancaruso, Bianca M. Vaglieco
Istituto Motori
CNR – Napoli (Italy)
Introduction
In order to overcome the pollutant emission li-
mitations the use of homogeneous charge com-
ICCD
pression ignition (HCCI) mode must be considered
in direct injection diesel engine using biodiesel
fuel. HCCI mode reduces PM and NOx emissions
without penalize the performances [1]. This occurs
because the combustion develops with low tem- Spectrograph
perature and burns a premixed air/fuel mixture. Filter
Biodiesel is a renewable fuel that can be produced ICCD
from a variety of vegetable oils including rapeseed
oil, soybean oil, sunflower oil and palm oil [2]. The
goal of this paper is the evaluation of the HCCI CCD
combustion in an optically accessible diesel engine
realized with four small early injections using RME Fig. 1: Experimental setup.
fuel.
In particular, the ICCD camera was used and
Experimental apparatus coupled with narrow filters corresponding to λ=310
The optically accessible engine used during ex- nm for OH, λ=330 nm for HCO* and λ=431nm for
periments was a single cylinder, direct injection, CH, respectively. The second CCD was sensible in
four-stroke, diesel engine, with EURO IV multi the visible. Moreover, two colour pyrometry method
valves production head. The engine used a classic was applied by using an “ad hoc” filter in order to
extended piston with piston crown window (46 mm evaluate soot temperature and concentration [4].
diameter) and a 45° UV-visible mirror located in- Synchronization of engine with ICCD and CCD
side the elongated piston. The engine was camera was obtained by the unit delay connected
equipped with Common Rail injection system and a to the engine shaft encoder. The effect of RME fuel
fully flexible Electronic Control Unit (ECU) that with respect to reference fuel (REF) at two engine
controlled the number of injections up to 5 per speeds, 1500 and 2000 rpm, respectively, and
cycle. To analyse the injection signals, a Hall-effect varying the injection pressure was analysed.
sensor was applied to the line of the solenoid cur-
rent and a piezoelectric pressure transducer was Results and discussions
located in the injection line between the rail and In figure 2 typical histories of cylinder pressure,
the injector. To acquire the cylinder pressure in rate of heat release and drive current of HCCI at
motored and fired condition, a piezoelectric pres- 2000 rpm for REF and RME fuels, respectively, are
sure transducer was set in the glow plug seat of reported. In particular, in order to realize a well
the engine head. Imaging measurements from mixed air/fuel charge in the cylinder four early in-
ultraviolet (UV) to visible were performed by means jections were performed during the compression
of the optical set up shown in figure 1. stroke. The same fuel amount was injected varying
Two different CCD cameras, with a different the injection pressure, even if higher quantities for
capability, were used. The first (ICCD) had high the engine operation with RME fuel because of its
sensibility both in the UV and visible range. The lower energy content. It can be noted that the rates
intensifier-gate duration of 41 µs was used in order of heat release have typical shape of HCCI com-
to have a good accuracy in the timing of the onset bustion with two well resolvable peaks not corre-
of the combustion process. Previous investigations lated with the injection strategy investigated. The
showed the presence of characteristics radicals first is characteristic of low temperature reactions
during the low temperature and premixed combus- that occur in the chamber at autoignition; and the
tion development [3]. second is due to the development of high tempera-
ture reactions [3].
60
REF
Combustion
UV images were recorded by means a filter
400
500
pressure
(bandwidth 10nm) at the characteristic emission
50
600 wavelength of OH (310 nm). In figure 3, the OH
Rate Of Heat Release [kJ/kg °]
700
40 images and the relative maximum intensity at the
lower left corner are reported.
Cylinder pressure [bar]
150
30
100
Drive injector current [A]
20
50
10
ROHR 30
0 0
20
10
Current
0
-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40
Crank angle [°]
60
RME
Combustion
400 pressure
50 500
600
Rate Of Heat Release [kJ/kg °]
700
40
Cylinder pressure [bar]
150
30
100
Drive injector current [A]
20
50
10
ROHR 30
0 0
20
10
Current
0
-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40
Crank angle [°]
Fig. 2: Combustion pressure, rate of heat release
and drive current at 2000 rpm for REF (up) and RME
(down) fuels at several injection pressures.
To examine the temporal and spatial evolution
of HCCI combustion digital imaging measurements
from UV to visible were performed. In figure 3 the
images of visible combustion and the soot KL fac-
tor are reported. They are related to RME fuel, the
engine speed of 2000 rpm and an injection pres-
sure of 500 bar. In figure 3, the first column shows
the flame evolution, it can be noted the presence
of luminous spots randomly distributed in the bowl.
Increasing the crank angle, the luminous spots
move in the cylinder, due to the air swirl motion,
and increase their density in the whole chamber.
These bright spots are distributed not only in the
bowl but also in the space between the engine
head and the top of the piston [3]. By means of the
two colour pyrometry method, the soot KL factor
proportional to the soot mass concentration were
calculated and reported in figure 3. The soot mass
concentration in the cylinder is very low during the
whole combustion process. In particular, the max- Fig. 3: HCCI combustion images, KL factor propor-
imum intensity is detected at 6° before top dead tional to soot mass concentration, and OH intensity for
center (BTDC). RME fuel at 2000 rpm.
Previous analysis of extinction and chemilumi-
nescence measurements showed that the pre- They show very low emission because only
mixed combustion process is widely dominated by 15% of incident light passed through the filter. The
the presence of OH radical [3]. For this reason, the OH radical is detected close to the bowl wall at the
start of combustion, then it fills the whole chamber, This is due to the high unburned hydrocarbon con-
and it shows maximum intensity at 4° BTDC, three centrations of the homogeneous combustion.
crank angle degrees later than the heat release In conclusion, the use of oxygenated fuel helps
peak. to keep low the production and emission of HC
with consequently positive effect on the emission
4.E+07
soot REF
of particulate matter.
1.E+08
4.E+07 soot RME
Integral soot mass concentration [a.u.]
OH REF 1.E+08
3.E+07
OH integral emission [a.u.]
OH RME
1.E+08
3.E+07
2.E+07
8.E+07 Acknowledgments
6.E+07
The authors wish to thank Mr. Carlo Rossi and
2.E+07
Bruno Sgammato for maintaining the experimental
1.E+07 4.E+07
apparatus and for their precious help.
5.E+06 2.E+07
0.E+00 0.E+00
400 500 600 700
Fig. 4: Integral OH radical and soot mass concentra- References
tion measured in the combustion chamber at 2000 rpm [1] Assanis, D. N., Najt, P. M., Dec J. E., Eng J. A.,
for REF and RME fuels at different injection pressure. Asmus T. N., Zhao F., “Homogeneous Charge Com-
pression Ignition (HCCI) Engines”. Key research and
In figure 4 the integral OH measured in the Development Issues. SAE International 2003
[2] Sustainable biofuels: prospects and challenges-The
combustion chamber at 2000 rpm and for several Royal Society, 2008.
injection pressures are reported. The integral con- [3] Merola, S.S., Vaglieco, B. M., Mancaruso, E., “Ex-
centrations were computed for both REF and RME tinction and Chemiluminescence Measurements in
fuels. It can be noted that increasing the injection CR DI Diesel Engine Operating in HCCI mode”, SAE
pressure the OH intensity decreases for both fuels Paper n° 2007-01-0192, 2007.
and the lowest intensity is detected for RME at 700 [4] Mancaruso, E., Merola, S.S., Vaglieco, B.M. “Study
bar. Previous paper showed that OH radical was of the Multi-Injection Combustion Process in a
responsible of the soot oxidation in the chamber Transparent DI Common Rail Diesel Engine by
[5]. For this reason the integral soot concentrations means of Optical Techniques”. Int. Journal of Engine
Research, vol.9, n.6, pp. 483-498, 2008.
are reported in same figure. The higher injection [5] Heywood, J. B., “Internal Combustion Engine Fun-
pressure produces better atomization of the fuel, damentals” - Mc Graw-Hill, NewYork, 1988.
thus lower soot production. Moreover, the oxygen
content of the RME fuel contributes strongly to the
reduction of soot. Finally, the lowest OH concen-
tration is detected at the lowest soot concentration,
due to the higher in-cylinder soot reduction.
8 7.E+07
REF
Integral soot mass concentratiuon [a.u.]
RME
7 6.E+07
REF cc
RME cc
6
5.E+07
5
PM [mg/m3]
4.E+07
4
3.E+07
3
2.E+07
2
1 1.E+07
0 0.E+00
400 500 600 700
Fig. 5: Comparison of in-cylinder soot mass concen-
tration and PM exhaust emission at 2000 rpm for REF
and RME fuels at different injection pressure.
Figure 5 compares the integral value of soot
concentration curves with the PM measured at the
exhaust pipe of the optical engine by means of an
opacimeter. The data in Figure 5 indicate similar
trends in soot emission even if the difference be-
tween the two fuels is less evident at the exhaust.