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Johansson2004-HCCI.pdf
Homogeneous Charge Compression Ignition – the future of IC
engines?
Prof. Bengt Johansson
Lund Institute of Technology at Lund University
ABSTRACT
The Homogeneous Charge Compression Ignition Engine, HCCI, has the potential to combine the best of the Spark
Ignition and Compression Ignition Engines. With high octane number fuel the engine operates with high compression
ratio and lean mixtures giving CI engine equivalent fuel consumption or better. Due to premixed charge without rich or
stoichiometric zones, the production of soot and NOx can be avoided. This paper presents some results from advanced
laser diagnostics showing the fundamental behaviour of the process from a close to homogeneous combustion onset
towards a very stratified process at around 20-50% heat released. The need for active combustion control is shown and
possible means of control are discussed. Results with multi-cylinder engines using negative valve overlap, variable
compression ratio, fast inlet temperature control as well as dual fuel are given.
INTRODUCTION
The internal combustion engine is the key to the
modern society. Without the transportation performed
by the millions of vehicles on road and at sea we would
not have reached the living standard of today. We have
two types of internal combustion engines, the spark
ignition, SI, and the compression ignition, CI. Both
have their merits. The SI engine is a rather simple
product and hence has a lower first cost. This engine
type can also be made very clean as the three-way
catalyst, TWC, is effective for exhaust aftertreatment.
The problem with the SI engine is the poor part load
efficiency due to large losses during gas exchange and
low combustion and thermodynamical efficiency. The
CI engine is much more fuel efficient and hence the
natural choice in applications where fuel cost is more
important than first cost. The problem with the CI
engine is the emissions of nitrogen oxides, NOx, and
particulates, PM. Aftertreatment to reduce NOx and
particulates is expensive and still not generally
available on the market. The obvious ideal combination
would be to find an engine type with the high efficiency
of the CI engine and the very low emissions of the SI
engine with TWC. One such candidate is named
Homogeneous Charge Compression Ignition, HCCI.
The fuel efficiency of HCCI has been compared to
that of normal SI operation by Stockinger et al. [1].
Figure 1 shows that they noted an improvement of fuel
efficiency from 15% to 30% at 1.5 bar BMEP. This is
an improvement of 100% equivalent to a reduction of
fuel consumption with 50%. More recently Yang et al.
presented a comparison between HCCI, denoted OKP,
and normal SI and direct injected SI concepts, DISI. He
found a much higher fuel consumption benefit for
HCCI than for DISI concepts.
The major benefit of HCCI compared to CI is the
low emissions of NOx and PM. The CI engine normally
has a trade-off between particulates and NOx. If the
engine operates at conditions with higher in-cylinder
peak temperature, the oxidization of soot will be good
but the thermal production of NO will increase. If on
the other hand the engine is operated with lower
temperature NO can be suppressed but PM will be high
due to bad oxidation. Figure 3 shows this trade-off and
also the allowed emissions in EU and US today and in
the near future. Clearly the CI engine must use exhaust
aftertreatment of NOx and/or PM.
In the CI engine, NO is formed in the very hot
zones with close to stoichiometric conditions and the
soot is formed in the fuel rich spray core. The incylinder average air/fuel ratio is always lean but the
combustion process is not. This means that we have a
large potential to reduce emissions of NOx and PM by
simply mixing fuel and air before combustion. In
Figure 3 the normal emission level from an HCCI
engine is also displayed. The NOx is normally less than
1/500 of the CI level and no PM is generated by
combustion.
HCCI FUNDAMENTALS
THE HCCI PRINCIPLE – HCCI means that the
fuel and air should be mixed before combustion starts
and that the mixture is autoignited due to the increase
in temperature from the compression stroke. Thus
HCCI is similar to SI in the sense that both engines use
a premixed charge and HCCI is similar to CI as both
rely on autoignition for combustion initiation.
However, the combustion process is totally different for
the three types.
Figure 4 shows the difference between (a) SI
combustion and (b) HCCI. In the SI engine we have
three zones, a burnt zone, an unburned zone and
between them a thin reaction zone where the chemistry
takes place. This reaction zone propagates through the
combustion chamber and thus we have a flame
propagation. Even though the reactions are fast in the
reaction zone, the combustion process will take some
time as the zone must propagate from spark plug (zero
mass) to the far liner wall (mass wi ). With the HCCI
process the entire mass in the cylinder will react at
once. The right part of Figure 4 shows HCCI, or as
Onishi called it Active Thermo-Atmosphere
Combustion, ATAC. We see that the entire mass is
active but the reaction rate is low both locally and
globally. This means that the combustion process will
take some time even if all the charge is active. The total
amount of heat released, Q, will be the same for both
processes. It could be noted that the combustion
process can have the same duration even though HCCI
normally has a faster burn rate. Initial tests in Lund on a
two stroke engine revealed the fundamental difference
between these two types of engines. Figure 5 shows
normal flame propagation from two spark plugs at the
rated speed of 9000 rpm. We see two well defined
flames and a sharp border between burned and
unburned zones. Figure 6 shows the same engine when
HCCI combustion was triggered by using regular
gasoline (RON 95) instead of iso-octane. The engine
speed was increased up to 17000 rpm and a more
distributed chemiluminescense image resulted.
REQUIREMENTS FOR HCCI – The HCCI
combustion process puts two major requirements on the
conditions in the cylinder:
(a) The temperature after compression stroke
should equal the autoignition temperature of
the fuel/air mixture.
(b) The mixture should be diluted enough to give
reasonable burn rate.
Figure 7 shows the autoignition temperature for a
few fuel as a function of λ. The autoignition
temperature has some correlation with the fuels’
resistance of knock in SI engines and thus the octane
number. For iso-octane, the autoignition temperature is
roughly 1000K. This means that the temperature in the
cylinder should be 1000 K at the end of the
compression stroke where the reactions should start.
This temperature can be reached in two ways, either the
temperature in the cylinder at the start of compression
is controlled or the increase in temperature due to
compression i.e. compression ratio is controlled. It
could be interesting to note that the autoignition
temperature is a very weak function of air/fuel ratio.
The change in autoignition temperature for iso-octane
is only 50K with a factor 2 change in λ. Figure 7 also
shows the normal rich and lean limits found with HCCI.
With a too rich mixture the reactivity of the charge is
too high. This means that the burn rate becomes
extremely high with richer mixtures. If an HCCI engine
is run too rich the entire charge can be consumed within
a fraction of a crank angle. This gives rise to extreme
pressure rise rates and hence mechanical stress and
noise. With a high autoignition temperature like that of
natural gas, it is also possible that formation of NOx
can be the load limiting factor. Figure 8 shows the NO
formation as a function of maximum temperature. Very
low emission levels are measured with ethanol. If the
combustion starts at a higher temperature like with
natural gas, the temperature after combustion will also
be higher for a given amount of heat released.
On the lean side, the temperature increase from the
combustion is too low to have complete combustion.
Partial oxidation of fuel to CO can occur at extremely
lean mixtures; λ above 14 has been tested. However,
the oxidation of CO to CO2 requires a temperature of
1400-1500 K. As a summary, HCCI is governed by
three temperatures. We need to reach the autoignition
temperature to get things started; the combustion
should then increase the temperature to at least 1400 K
to have good combustion efficiency but it should not be
increased to more that 1800 K to prevent NO
formation.
HCCI COMBUSTION PROCESS IN DETAIL
The above description of HCCI gives just a rough
idea about the requirements and conditions of the
combustion process. It is also of greatest interest to
acquire detailed knowledge of the process. In order to
get such information, laser based diagnostics is of
crucial importance. Some of the activities in this field
from Lund University will thus be presented.
INHOMOGENEOUS COMBUSTION – The first
experiments with laser based diagnostics were
performed to analyze the difference in combustion
between a perfectly homogeneous fuel/air mixture and
one with small gradients. Laser induced fluorescence of
fuel tracer or OH was used to mark the combustion
process. Figure 9 shows the system setup with a laser
generating a vertical laser sheet. Figure 10 shows the
fuel distribution for the two cases with an
inhomogeneity of approximately 5% in the case of port
fuel injection and homogeneity within the detection
limit for the case with a mixing tank and fuel injection
far upstream. Figure 11 and Figure 12 shows the fuel
concentration with half the heat released. We can from
these images conclude that the combustion is far from
homogeneous. There are islands with much fuel
remaining and close to them regions with very little fuel
left. Figure 13 shows the same behavior for the
concentration of OH. Zones with much OH are close to
zones with no OH and the gradients are steep. Each
individual cycle was also found to be unique. The four
cycles displayed are randomly picked samples. No
preferred type of structure could be detected.
SINGLE CYCLE INFORMATION – A major
limitation with the information from Figure 11 to
Figure 13 is that only one image can be captured from
each cycle. Due to the very large cycle to cycle
variation in the process, it is impossible to extract
information on possible expansion of zones with
intense reactions i.e. flame propagation. To overcome
this problem a unique laser system was used. Four
individual lasers which can generate eight laser pulses
were combined with a framing camera using eight
individual CCD chips. This system was used in an
optical Scania engine with transparent liner and a
window in the extended piston. The setup can be seen
in Figure 14. The measured area was 95x 55 mm thus
enabling distinction between local and global effects.
Figure 15 shows a sequence of fuel LIF images
captured with 0.5 CAD time separations at 1200 rpm.
The images are from 20% to 50% heat release. From
these and numerous similar mini-movies it was possible
to conclude that the combustion changed behavior
during the process. In the initial phase a slow but stable
decrease in the fuel LIF signal was detected. This was
interpreted as a slow and rather homogeneous start of
the process. At around 20-30% heat released the fuel
LIF image changed. Then even the smallest structures
found before were amplified to give an image with
more intense gradients. The gradients were found to be
amplified even more as the process evolved and at
approx. 50% heat release the structures found earlier
during the single shot experiments were clear. From
50% heat release and onwards the structures were
stable and the fuel signal disappeared not long after
that. This single cycle observation of the process leads
to a phenomenological description of the HCCI
combustion process.
THE PHENOMENOLOGICAL MODEL OF HCCI
COMBUSTION – The HCCI combustion process is
assumed to start with a gradual decomposition of the
fuel with well distributed reactions. The reactions will
become significantly exothermic when a critical
temperature is approached. At this critical condition the
reaction rate will be very sensitive to the temperature of
the charge. Even the smallest variations in temperature
will thus influence the reaction rates. As we will have
random variation in temperature in the cylinder, some
locations will have more favorable conditions. In those
locations, sometimes denoted “hot spots” the reactions
thus will start a bit earlier. As the exothermic reactions
start the temperature is increased and thus reactions
become even faster. We thus have a local positive
feedback in temperature. Figure 16 shows an attempt
to illustrate this. As the local positive feedback is fast,
there will not be sufficient time to distribute all the heat
to the surrounding cold bulk. Thus we have a gradual
amplification of small inhomogeneities generating the
very large structures seen in the experiments. The size
of the hot spots was found to be of the same order as
the integral length scale of turbulence in the cylinder. In
the Scania engine, this was 4-6 mm.
Flame propagation? - It could be argued that the
“hot-spots” grow as a function of time and this growth
rate could be translated to a reaction zone propagation
or in other words flame front. However, after studying
numerous individual cycles it was concluded that the
concept of flame propagation in HCCI could not be
supported. There will be a time lag between
combustion starting point at different zones but new
“hot-spots” show up randomly and the structures seen
in the images are rather fixed i.e. do not move from
image to image. If we would use the term flame speed
for a case where two hot spots show up at exactly the
same time we would also have a problem as the flame
speed then would be infinity.
THE NEED FOR CONTROL
For better understanding of the combustion process,
laser diagnostics is needed and this knowledge can be
used to optimize the system. However, the HCCI
process is very sensitive to disturbances. It can be
sufficient to change the inlet temperature 2˚C to move
from a very good operating point to a total misfire. This
sensitivity makes the HCCI engine require closed loop
combustion control, CLCC. Closed loop control
requires as always a sensor, control algorithm and
control means.
The main parameter to control for HCCI is the
combustion timing i.e. when in the cycle combustion
takes place. Figure 17 shows the rate of heat release
for a range of timings. With early phasing the rate of
heat release is higher and as it is phased later the burn
rate goes down. With combustion before top dead
center, TDC, the temperature will be increased both by
the chemical reactions and the compression due to
piston motion. Thus for a given autoignition
temperature, combustion onset before TDC will result
in faster reactions. With the conditions changed to give
combustion onset close to TDC, the temperature will
not be increased by piston motion, the only temperature
driver would be the chemical reactions. This gives a
more sensitive system and the later the combustion
phasing the more sensitive the system is. This is the
underlying problem with HCCI combustion control.
We want a late combustion phasing to reduce burn rate
and hence pressure rise rate and peak pressure but on
the other hand we can not accept too much variations in
the combustion process. How late we can go depends
on the quality of the control system. With a fast and
accurate control system we can go later and hence
reduce the noise and mechanical loads of the engine.
COMBUSTION SENSOR – The most accurate and
reliable signal for combustion is the in-cylinder
pressure. With the standard heat release equation it is
very easy to extract the combustion onset etc. The most
usable parameter for combustion phasing is the crank
angle of 50% of the heat released. Figure 18 shows the
procedure to extract this 50% heat released point
denoted CA50.
The cylinder pressure is a very stable and robust
signal but the cost of such sensors is still too high for
production engines. One alternative could be an ion
current measurement system. The ion current can be
measured by applying a voltage on the electrodes of a
normal spark plug. The technique has been used by
SAAB Automobile in production since 1993 for the
detection of knock and misfire in SI engines, but the
application on HCCI is not straight forward. The signal
intensity is very sensitive to the temperature in the
cylinder and thus lean burn HCCI give low signal.
Figure 19 shows a typical ion current measurement
system and Figure 20 shows the typical signal obtained
in HCCI mode. The best representation of combustion
phasing was found by extracting the crank angle at
which 50% of the maximum amplitude was detected.
This gave good correlation to the crank angle of 50%
heat released, CA50 as shown in Figure 21. Two
individual operating points are shown, one with a
relatively early timing and hence less cycle to cycle
variations and one with late timing. For both cases, a
small phase difference was detected between the crank
angle at 50% of maximum ion signal and CA50 but this
can easily be compensated by the controller.
CONTROL MEANS - The HCCI combustion
control can be considered as a balance in temperature.
With low temperature at TDC the combustion will be
late and with high temperature at TDC the combustion
will start early. To control temperature, three major
parameters can be used. Inlet temperature and
compression ratio will directly change the TDC
temperature. The third parameter is the amount of
residual gas retained in the cylinder from the previous
cycle. A fourth possible way of controlling the process
is to change the required autoignition temperature by
adjusting the fuel quality. Figure 22 shows possible
combinations of inlet temperature, compression ratio
and fuel octane number for combustion onset at TDC
for a 1.6 liter single cylinder Volvo Truck engine. The
figure shows that a higher octane fuel needs higher inlet
temperature or higher compression ratio to reach
autoignition at TDC. Figure 23 shows similar
combinations but here the two fuels are regular gasoline
and diesel oil instead of the primary reference fuels nheptane and iso-octane.
A very popular concept for achieving HCCI in SI
engines at part load is the use of negative valve
overlap. With this concept the exhaust valves close
early and thus hot burnt gas is trapped in the cylinder.
After a short compression and expansion the inlet valve
is opened late. This type of process often denoted
Controlled Autoignition, CAI, gives good performance
but in a limited operating range. Figure 24 shows the
operating range of a 6-cylinder 3 litre Volvo Cars
engine. A better way of controlling the process is by
applying variable compression ratio or fast inlet air
temperature control. With this concept it is possible to
run at idle at all engine speeds between 600 and 5000
rpm. Maximum load is the same as for CAI but it can
be maintained also for higher engine speeds. Figure 26
shows the operating range for a SAAB 1.6 liter 5cylinder variable compression engine using fast thermal
management as shown in Figure 25. It should be noted
that the BMEP is presented in contrast to the IMEP for
CAI in Figure 24.
A possible way of HCCI combustion control can
also be the use of dual fuels. Using two fuel tanks could
cause some problems with costumer acceptance but it is
possible to generate two fuels from one using a
reformer. Experiments with dual fuel in Lund have
shown that it is a very powerful control means. Figure
27 shows the operating range possible with a Scania
12-liter 6-cylinder truck engine running on a mixture of
ethanol and n-heptane.
CONTROLLER - In order to achieve the high loads
reported for the SAAB and Scania multi cylinder
engines, it is absolutely necessary to use closed loop
control with a well tuned controller. To make the
controller usable over the entire speed and load range,
the gain of the controller must be changed in
accordance with the change of gain of the process.
Figure 28 shows the combustion phasing, CA50, as a
function of octane number for the Scania dual fuel
engine at different operating conditions. With early
combustion timing and conditions requiring low octane
number, the slope of the curves are low. This means
that a large change of octane number is needed to
change the combustion timing one crank angle. Thus
we should have a large gain of the controller in these
operating conditions. If we then look at conditions with
high octane number and late combustion phasing, the
required change in octane number to change phasing a
crank angle is much less. With this higher gain of the
process we must reduce the gain of the controller;
otherwise the system will become unstable. Tuning the
gain of the controller to compensate for changes in the
process can be done by using gain scheduling. With
this it is possible achieve close to optimal performance
for all operating conditions. In fact it is even possible to
operate an HCCI engine at unstable operating points
with the closed loop combustion control active. Figure
29 shows one such case.
TABLES AND FIGURES
Figure 1: The fuel efficiency of HCCI and SI engine
configurations. Open diamond =SI at λ=1, Rc=18.7,
Open triangle = HCCI lean burn, Filled diamond=
HCCI with EGR and Filled circle= SI at λ=1 and
Rc=9.5:1 [1]
Figure 2: Net indicated specific fuel consumption of
four different combustion types [2]
Figure 5: SI flame propagation in 2-Stroke
engine at 9000 rpm [4].
USA 2007
0,01
P
M
*
0,00
0
0,05
NOx
0,5
Figure 3: The NOx-PM trade-off for a standard
diesel engine, the future emission regulations
and the emissions of HCCI (Green)
Figure 6: HCCI combustion in 2-Stroke engine
at 17000 rpm.
1200
Natural gas
Iso−octane
Ethanol
Methanol
Ignition Temperature [K]
1150
1100
1050
1000
950
900
Figure 4: The difference between SI and HCCI
combustion process. Q= total amount of heat,
q=heat per mass unit, w=mass [3]
850
2
2.5
3
3.5
λ
4
4.5
5
Figure 7: Ignition temperature for a few fuels
as a function of dilution (λ
λ).
Figure 8: NOx as a function maximum
temperature evaluated from the pressure-trace
[5].
Figure 11: Fuel distribution at approx. 50%
heat released with port fuel injection [6].
Figure 9: First laser system [6].
Figure 12: Fuel distribution at approx. 50%
heat released with mixing tank [6].
Figure 10: Fuel distribution with port fuel
injection (left) and far upstream (right) just
before combustion starts. Four individual
cycles are shown. [6]
Figure 13: OH signal at approximately 50%
heat released. To the left with port fuel
injection and to the right with mixing tank [6].
Figure 17: Rate of heat release with a change
in inlet temperature and thus combustion
phasing [9].
Figure 14: Optical system for high speed fuel
LIF [7].
Visualized area
Figure 15: Fuel concentration from 2 CAD
ATDC with 0.5 CAD step [7].
Reactions at T(t=1),
releasing significant
heat
Ignition at T (t=1)
Critical ignition
temperature
Temp
T (x,t=0)
Wall
Ignition at T (t=0)
T (x,t=1)
Figure 18: Cylinder pressure trace and
corresponding heat release [10].
Arbitrary distance x
Figure 16: Temperature at three instants of
time [8].
Figure 19: Ion current measurement system
[11].
Ion current [µA]
Crank Angle [CAD]
Figure 20: Ion current signal with a change in
combustion timing. The average of 300 cycles
is shown [11].
Figure 23: Combinations of percentage
gasoline, compression ratio and inlet
temperature to give combustion onset at TDC
[12].
CA50
Figure 21: Crank angle for 50% of maximum
ion signal vs. crank angle at 50% heat released
for two individual operating points [11].
Figure 24: Operating range of Controlled
Autoignition type of HCCI in a 6-cylinder Volvo
Car engine [13].
Air in
Exhaust
heat
Catalyst
Exhaust out
Figure 22: Combinations of fuel octane
number, compression ratio and inlet
temperature to give combustion onset at TDC
[12]
Figure 25: Fast Thermal Management, FTM
[14].
Figure 26: Operating range with compression
ratio and inlet temperature control. Minimum
load is 0 bar (idle) at all engine speeds [15].
16
CONCLUSION
14
12
BMEP (bar)
10
8
6
4
2
0
1000
Figure 29: Operation at stable and unstable
conditions after closed loop combustion
control is switched off [10].
1200
1400
1600
Engine Speed (rpm)
1800
2000
Figure 27: Operating range with dual fuel
control [16].
The Homogeneous Charge Compression Ignition,
HCCI, combustion process is an interesting alternative
to the conventional Spark Ignition and Compression
Ignition processes. The potential benefit of HCCI is
high with simultaneous ultra low emissions of NOx and
PM and low fuel consumption. Thus it can combine the
best features of the SI (with TWC) and CI engines. To
better understand the process, laser based techniques
must be used. Such measurements in Lund have
revealed that the combustion process is rather
homogeneous in the initial stage but it gradually
transfers into a highly inhomogeneous process with
steep gradients between reacting and non-reacting
zones.
The HCCI engine requires active control of the
combustion process. Such closed loop combustion
control has been demonstrated in a number of multicylinder HCCI engines in Lund. Use of negative
overlap is possible but often generates a limited
operating range. The use of variable compression ratio
is a very powerful control means but can have some
problems to reach production for cost reasons. Fast
Thermal Management can perhaps be the key
technology to be used for HCCI combustion control.
Figure 28: Combustion phasing vs. octane
number for a range of operating conditions
[16].
The maximum engine speed for HCCI in Lund is
17000 rpm and the maximum load is 20.4 bar IMEP/
16 bar BMEP. This indicates that most interesting
speeds and loads can be reached with HCCI.
ACKNOWLEDGMENTS
The results presented in this paper are a summary of
results of the HCCI activities in Lund. I thank all fellow
researchers, Ph.D. students and technicians for
generating the results. I would also like to thank our
sponsors: The Swedish Energy Administration, The
Swedish Gas Centre, Volvo Cars, Volvo Trucks, Volvo
Penta, Scania CV, Saab Automobile, Fiat-GM
Powertrain, Caterpillar, Cummins, Toyota, Nissan and
Hino.
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