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Chapter 1.
INTRODUCTION
In the quest for ever improving fuel efficiency and emissions reduction, an oldand very promising idea has found new life. HCCI (Homogeneous Charge Compression
Ignition) technology has been around for a long time, but has recently received renewed
attention and enthusiasm. While the early years saw many insurmountable (at the time)
obstacles whose answers would only come as sophisticated computer controlled
electronics were developed and matured into reliable technologies, progress stalled. Time
has, as it always does, worked its magic and nearly every problem has been solved. HCCI
is an idea whose time has come with nearly all of the parts and pieces of technology and
know-how in place to make a real go of it. [2]
1.1 HOMOGENEOUS CHARGE
Definition: Homogeneous charge, as it relates to internal combustion engines, is a
thoroughly and completely mixed (so that every molecule is evenly distributed) charge of
air and fuel across the combustion chamber. This absolute mixing occurs well before the
start of ignition. The idea behind homogeneous charge is to create an easily ignitable fuel
mixture that is easy to manage and burns smoothly and evenly across the entire
combustion chamber. It does this well, but at the expense of excessive NOx build-up that
must then be captured and processed by the vehicle's catalytic converter. [3]
1.2 WHAT IS HCCIENGINE?
An HCCI engine is a mix of both
conventional spark-ignition and diesel
compression ignition technology. The
blending of these two designs offers highefficiency like diesel engine and very low
NOx and particulate matter emissions as that
of spark ignition engine. In its most basic
form, it simply means that fuel (gasoline orFig. 1.1 SI,CI and HCCI Engine
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E85) is homogeneously (thoroughly and completely) mixed with air in the combustion
chamber (very similar to a regular spark ignited gasoline engine), but with a very high
proportion of air to fuel (lean mixture). As the engine's piston reaches its highest point
(top dead center) on the compression stroke, the air/fuel mixture auto-ignites
(spontaneously and completely combusts with no spark plug assist) from compression
heat, much like a diesel engine. The result is the best of both worlds: low fuel usage and
low emissions. [2]
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Chapter 2.
HISTORY AND LITERATURE SURVEY
HCCI engines have a long history, even though HCCI has not been as widely
implemented as spark ignition or diesel injection. It is essentially an Otto combustion
cycle. In fact, HCCI was popular before electronic spark ignition was used. One example
is the hot-bulb engine, which used a hot vaporization chamber to help mix fuel with air.
The extra heat combined with compression induced the conditions for combustion to
occur.[1]
Fig. 2.1Some early results gave piston damage
Onishi et al initially investigated the concept of HCCI for gasoline applications, in
order to increase combustion stability of two-stroke engines. They found that significant
reductions in emissions and an improvement in fuel economy could be obtained by
creating conditions that led to spontaneous ignition ofthe in-cylinder charge. Stable HCCI
combustion could be achieved between low and high load limits with gasoline at a
compression ratio of 7.5:1 over the engine speed range from 1000 to 4000 rpm. Noguchiet al. performed a spectroscopic analysis on HCCI combustion by experimental work on
an opposed piston two- stroke engine. Building on previous work on two-stroke engines,
Najt and Foster extended the work to four-stroke engines and attempted to gain additional
under- standing of the underlying physics of HCCI combustion. They concluded that
HCCI auto-ignition is controlled by low temperature (below 1000 K) chemistry and the
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bulk energy release is controlled by the high temperature (above 1000 K) chemistry
dominated by CO oxidation.[8]
As discussed above, initial efforts with HCCI involved gasoline- fuelled engines,
and this technology continues to be strongly pursued today.2.1 FOLLOWING ARE SOME SUMMERY POINTS COLLECTED ON HCCI
FROM DIFFERENT JOURNALS
Progress and recent trends in homogeneous charge compression ignition (HCCI)
engines, Mingfa Yao, ZhaoleiZheng, Haifeng Liu:-Typical generalized diesel-fuelled
HCCI combustion modes include: early direct injection HCCI, late direct injection HCCI,
premixed/direct-injected HCCI combustion and low temperature combustion. Mixture
control (mixture preparation), including charge components and temperature control in
the whole combustion history and high pre-ignition mixing rate, is the key issue to
achieve diesel HCCI combustion. There are two measures to improve mixture formation:
1) by improving the mixing rate of fuel and air by such means as high pressure/ultra-high
pressure fuel injection and small nozzle holes, high boost, design of combustion chamber
geometry and utilization of energy of spray wall impingement and multi-pulse fuel
injection based on modulating injection mode; and 2) by extending ignition delay by such
means as EGR and variable compression ratio/valve actuation technology.
Since the diesel fuel has low volatility, the port fuel introduction is not a practical
way without significant change of intake system. An early in-cylinder injection strategy,
to some extent, can result in a quite homogeneous charge before ignition. Due to lower
charge density, in-cylinder pressure, and temperature, the liquid fuel impingement on the
liner wall or piston wall is unavoidable, which leads to high HC and CO emissions.
Another issue for the early injection strategy is the ignition timing control. For early
injection HCCI combustion, the ignition is purely controlled by the chemical kinetics.
The ignition is often advanced due to early injection timing and other measures have to
be taken to delay the ignition by using heavy EGR, variable compression ratio, changing
fuel properties, etc. In practice, both the HCCI mode and conventional diesel combustion
will have to be used to cover the complete engine operational range.
For the LTC, the short times between the fuel injection event and the start of
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combustion preclude thorough premixing, and significant regions exist where 4 > 1 at the
start of combustion. Even though there is a locally rich region in the mixture of
this strategy, the soot formation can be suppressed. The main soot suppression
mechanism is that using large amounts of EGR reduces the temperature, and this
temperature reduction is sufficient to allow the combustion to avoid the soot formation
region. This is the major reason why smokeless combustion can be accomplished with no
adjustment required in the mixture formation by changing fuel spray system, combustion
chamber geometry, etc., under rich operating condition.
Simultaneously, the NOx emissions can also be avoided due to the high EGR rates
and thus low combustion temperature. Furthermore, the EGR rate influences the path not
only through changes in the flame temperature, but also in ignition delay and the amount
of ambient fluid that must be mixed with the fuel to attain a given equivalence ratio. In
addition, the injection strategies (including injection pressure, timing and multiple
injections) influence the temperature (and density) during the ignition delays period, the
peak flame temperature reached, and the premixing improvement. Finally, in order to
keep the power density and the combustion efficiency of the engine at high EGR rates,
high boost levels are required. Therefore, the control and optimize of EGR rate, injection
strategies and high boost are the key issue to the LTC. The LTC has more benefits, such
as high efficiency over broad load range, simple control of ignition timing, reduced
pressure rise rates, high load capability. So, this strategy will be more promising in the
future.
The high octane numbers of gasoline fuels means that such fuels need high ignition
temperatures, which highlights the difficulty of auto-ignition. The main challenge for
gasoline HCCI operation is focus on the obtaining sufficient thermal energy to trigger
auto- ignition of mixtures late in the compression stroke, extending the operational range,
and the transient control.
The most practical means to obtain sufficient thermal energy in a gasoline HCCI
engine is through the use of large levels of recirculate exhaust gases. There are two EGR
strategies with VVA: one is exhaust re-breathing and the other is exhaust recompression.
These dilution strategies have no significant differences in the cylinder pressure profile or
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combustion characteristics. From a practical perspective, however, the exhaust
recompression strategy appears to be easier to implement and has become the strategy
favored in the literature.
To reduce the fuel consumption and emissions over real-life drive cycles, theengine must operate in HCCI mode over the widest possible speed and load range. To
extend gasoline-fuelled HCCI operation to high loads without transition to knock, some
methods can be used, such as fuel modification, variable compression ratio, charge boost,
the temperature or charge stratification, and the multiple fuel direction injections. To
extend gasoline-fuelled HCCI operation to light loads, high in-cylinder temperatures are
necessary to promote compression ignition. Meantime, the post- combustion
temperatures need to be optimized between 1500 and 1800 K for low HC, CO and NO
emissions. Approaches include variable valve strategies, variable injection timings,
charge boost, and spark-assisted ignition.
Active closed-loop real-time dynamic control is essential to maintain the desired
ignition timing for any practical HCCI combustion system. Speed and load control within
the HCCI mode and transitions between HCCI and SI modes have been demon- started in
single cylinder research engines. However, additional complications in multi-cylinder
engines require individual cylinder control to ensure the same combustion phasing and
reach HCCI/SI transition for all cylinders. A cycle-resolved, closed-loop control system
with individual sensors and actuators for each cylinder allowed combustion phasing to be
matched for all cylinders, but any changes in the combustion phasing in one cylinder
resulted in changes in another cylinder due to exhaust-manifold coupling.[4]
Understanding the transition between conventional spark-ignited combustion and
HCCI in a gasoline engine, C. Stuart Daw, Robert M. Wagner, K. Dean Edwards, Johney
B. Green :-The experimental gasoline engine studied here exhibits a repeatable region of
low-dimensional deterministic combustion oscillations as internal EGR is increased to
drive the transition between PF and HCCI combustion. We hypothesize that the
oscillation behavior represents a type of non- linear map bifurcation that begins with
destabilization of the PF fixed point and ends with the stabilization of the HCCI fixed
point. The transition dynamics include complex regions of multi- periodicity and
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deterministic chaos. The general similarity of the PFHCCI transition to the lean-limit
transition suggests that both processes are driven by nonlinear feedback through
recirculated exhaust gas. The types of inter-mode transition experiments described here
need to be investigated using other engines. If the same or similar patterns can be con-
firmed to be general features of a range of engines, it would seem appropriate to utilize
nonlinear time series diagnostics and chaos control theory to expand the practical
implementation of HCCI. [5]
A new heat release rate (HRR) law for homogeneous charge compression ignition
(HCCI) combustion mode- Miguel Torres Garca, Francisco Jos Jimnez-Espadafor
Aguilar, Toms Snchez Lencero, Jos Antonio Becerra Villanueva:-In this work, an
experimental and simulation study has been carried out to compare the performance of a
new HRR law that de- fines a proportion of slower combustion for HCCI engine
modeling. The new HRR law was implemented in an engine model to evaluate
performance in comparison to the experimental data obtained in detailed tests.
The study showed that by describing a proportion of slower combustion with the
new HRR proposed, it was possible to achieve a very good match to experimental data.
The new HRR law allows predicting the cylinder pressure curve perfectly with minimum
error. As has been shown, the HRR law depends on four parameters that can be related to
any load condition. Research is in progress on the development of a predictive model of
the engine in HCCI combustion mode.[6]
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Chapter 3.
HOMOGENEOUS CHARGE COMPRESSION IGNITION
3.1 WHAT IS HCCI?
HCCI is an alternative piston-engine combustion process that can provide
efficiencies as high as compression-ignition, (CI) engines while, unlike SI engines,
producing ultra-low oxides of nitrogen (NOx) and particulate matter emissions. HCCI
engines operate on the principle of having a dilute, premixed charge that reacts and burns
volumetrically throughout the cylinder as it is compressed by the piston. In some regards,
HCCI incorporates the best features of both spark ignition (SI) and compression ignition
(CI). As in an SI engine, the charge is well mixed, which minimizes particulate
emissions, and as in a CI engine, the charge is compression ignited and has no throttling
losses, which leads to high efficiency. However, unlike either of these conventional
engines, the combustion occurs simultaneously throughout the volume rather than in a
flame front. This important attribute of HCCI allows combustion to occur at much lower
temperatures, dramatically reducing engine-out emissions of NOx. [2]
Most engines employing HCCI to date have dual mode combustion systems in
which traditional SI or CI combustion is used for operating conditions where HCCI
operation is more difficult. Typically, the engine is cold-started as an SI or CI engine, and
then switched to HCCI mode for idle and low- to mid-load operation to obtain the
benefits of HCCI in this regime, which comprises a large portion of typical automotive
driving cycles. For high-load operation, the engine would again be switched to SI or CI
operation. [7]
3.2 WORKING PRINCIPLE
In Homogeneous Charge Compression Ignition, homogeneous mixture of fuel and
air is taken in the cylinder. This mixture is then compressed inside the cylinder to a point
where auto ignition occurs. Once the conditions suitable for auto ignition are reached,
ignition occurs simultaneously at several places in combustion chamber. Thus the
combustion takes place. This is the basic principle used to drive Homogeneous Charge
Compression Ignition engine.
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3.3 WORKING
An HCCI engine ignites a mixture of fuel and air by compressing it in the
cylinder. Unlike a spark ignition gas engine or diesel engine, HCCI produces a low-
temperature, flameless release of energy throughout the entire combustion chamber. Allof the fuel in the chamber is burned simultaneously. This produces power similar to
today's conventional gas engines, but uses less fuel to do it. Heat is a necessary enabler
for the HCCI process, so a traditional spark ignition is used when the engine is started
cold to generate heat within the cylinders and quickly heat up the exhaust catalyst and
enableHCCI operation. During HCCI mode, the mixture's dilution is comparatively lean,
meaning there is a larger percentage of air in the mixture. The lean operation of HCCI
helps the engine approach the efficiency of a diesel, but it requires only a conventional
automotive exhaust after-treatment. Diesel engines require more elaborate and more
expensive after-treatment to reduce emissions.
HCCI builds on the integration of other advanced engine technologies some of
which are already in production and can be adapted to existing gas engines. The cylinder
compression ratio is similar to a conventional direct-injected gas engine and is
compatible with all commercially available gasoline and E85 fuels. [8]
In an HCCI engine (which is based on the four-stroke Otto cycle), fuel delivery
control is of paramount importance in controlling the combustion process. On the intake
stroke, fuel is injected into each cylinder's combustion chamber via fuel injectors
mounted directly in the cylinder head. This is achieved independently from air induction
which takes place through the intake plenum. By the end of the intake stroke, fuel and air
have been fully introduced and mixed in the cylinder's combustion chamber.
As the piston begins to move back up during the compression stroke, heat begins
to build in the combustion chamber. When the piston reaches the end of this stroke,
sufficient heat has accumulated to cause the fuel/air mixture to spontaneously combust
(no spark is necessary) and force the piston down for the power stroke. Unlike
conventional spark engines (and even diesels), the combustion process is a lean, low
temperature and flameless release of energy across the entire combustion chamber. The
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entire fuel mixture is burned simultaneously producing equivalent power, but using much
less fuel and releasing far fewer emissions in the process.
At the end of the power stroke, the piston reverses direction again and initiates the
exhaust stroke, but before all of the exhaust gases can be evacuated, the exhaust valvesclose early, trapping some of the latent combustion heat. This heat is preserved, and a
small quantity of fuel is injected into the combustion chamber for a pre-charge (to help
control combustion temperatures and emissions) before the next intake stroke begins. [2]
3.4 WHY HCCI?
The modern conventional SI engine fitted with a three-way catalyst can be seen as
a very clean engine. But it suffers from poor part load efficiency. As mentioned earlier
this is mainly due to the throttling. Engines in passenger cars operate most of the time at
light- and part load conditions. For some shorter periods of time, at overtaking and
acceleration, they run at high loads, but they seldom run at high loads for any longer
periods. This means that the overall efficiency at normal driving conditions becomes very
low.
The Diesel engine has much higher part load efficiency than the SI engine.
Instead the Diesel engine fights with great smoke and NOx problems. Soot is mainly
formed in the fuel rich regions and NOx in the hot stoichiometric regions. Due to thesemechanisms, it is difficult to reduce both smoke and NOx simultaneously through
combustion improvement. Today, there is no well working exhaust after treatment that
takes away both soot and NOx.
The HCCI engine has much higher part load efficiency than the SI engine and
comparable to the Diesel engine, and has no problem with NOx and soot formation like
the Diesel engine. In summary, the HCCI engine beats the SI engine regarding the
efficiency and the Diesel engine regarding the emissions.
3.5 METHODS
A mixture of fuel and air will ignite when the concentration and temperature of
reactants is sufficiently high. The concentration and/or temperature can be increased
several different ways:
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High compression ratio Pre-heat induction gases Forced induction Retain or reinduct exhaust
Once ignited, combustion occurs very quickly. When auto-ignition occurs too
early or with too much chemical energy, combustion is too fast and high in-cylinder
pressures can destroy an engine. For this reason, HCCI is typically operated at lean
overall fuel mixtures.
3.6 ADVANTAGES
HCCI is closer to the ideal Otto cycle than spark-ignited combustion. Lean operation leads to higher efficiency than in spark-ignited gasoline engines Homogeneous mixing of fuel and air leads to cleaner combustion and lower
emissions. In fact, due to the fact that peak temperatures are significantly lower
than in typical spark ignited engines, NOxlevels are almost negligible.
Since HCCI runs throttleless, it eliminates throttling losses
3.7 DISADVANTAGES
High peak pressures High heat release rates Difficulty of control Limited power range High carbon monoxide and hydrocarbon pre-catalyst emissions. [1]
3.8 CONTROL
Controlling HCCI is a major hurdle to more widespread commercialization. HCCI
is more difficult to control than other popular modern combustion methods.
In a typical gasoline engine, a spark is used to ignite the pre-mixed fuel and air. In
diesel engines, combustion begins when the fuel is injected into compressed air. In both
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cases, the timing of combustion is explicitly controlled. In an HCCI engine, however, the
homogeneous mixture of fuel and air is compressed, and combustion begins whenever
the appropriate conditions are reached. This means that there is no well-defined
combustion initiator that can be directly controlled. An engine can be designed so that the
ignition conditions occur at a desirable timing. However, this would only happen at one
operating point. The engine could not change the amount ofworkit produces. This could
work in a hybrid vehicle, but most engines must modulate their output to meet user
demands dynamically.
To achieve dynamic operation in an HCCI engine, the control system must
change the conditions that induce combustion. Thus, the engine must control either the
compression ratio, inducted gas temperature, inducted gas pressure, fuel-air ratio, or
quantity of retained or reinducted exhaust.
Several approaches have been suggested for control.
3.8.1 Variable Compression Ratio
There are several methods of modulating both the geometric and effective
compression ratio. The geometric compression ratio can be changed with a movable
plunger at the top of the cylinder head. The effective compression ratio can be reduced
from the geometric ratio by closing the intake valve either very late or very early withsome form of variable valve actuation (i.e. variable valve timing permitting Miller cycle).
Both of the approaches mentioned above require some amounts of energy to achieve fast
responses and are expensive (no more true for the 2nd solution, the variable valve timing
being now maitrized). A 3rd proposed solution is being developed by the MCE-5 society
(new rod). Miller cycle:
In engineering, the Miller cycle is a combustion process used in a type of four-
stroke internal combustion engine. Ralph Miller, an American engineer, patented theMiller cycle in the 1940s. This type of engine was first used in ships and stationary
power-generating plant, but has recently (late 1990s) been adapted by Mazda for use in
their Millenia large sedan. The traditional Otto cycle used four "strokes", of which two
can be considered "high power" the compression and power strokes. Much of the power
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lost in an engine is due to the energy needed to compress the charge during the
compression stroke, so systems to reduce this can lead to greater efficiency.
In the Miller cycle the intake valve is left open longer than it normally would be.
This is the "fifth" cycle that the Miller cycle introduces. As the piston moves back up inwhat is normally the compression stroke, the charge is being pushed back out the
normally closed valve. Typically this would lead to losing some of the needed charge, but
in the Miller cycle the piston in fact is over-fed with charge from a supercharger, so
blowing a bit back out is entirely planned. The supercharger typically will need to be of
the positive displacement kind (due its ability to produce boost at relatively low RPM)
otherwise low-rpm torque will suffer. The key is that the valve only closes, and
compression stroke actually starts, only when the piston has pushed out this "extra"
charge, say 20 to 30% of the overall motion of the piston. In other words the compression
stroke is only 70 to 80% as long as the physical motion of the piston. The piston gets all
the compression for 70% of the work.
The Miller cycle "works" as long as the supercharger can compress the charge for
less energy than the piston. In general this is not the case, at higher amounts of
compression the piston is much better at it. The key, however, is that at low amounts of
compression the supercharger is more efficient than the piston. Thus the Miller cycle uses
the supercharger for the portion of the compression where it is best, and the piston for the
portion where it is best. All in all this leads to a reduction in the power needed to run the
engine by 10 to 15%. To this end successful production versions of this cycle have
typically used variable valve timing to "switch on & off" the Miller cycle when efficiency
does not meet expectation. In a typical Spark Ignition Engine however the Miller cycle
yields another benefit. Compression of air by the supercharger and cooled by an
intercooler will yield a lower intake charge temperature than that obtained by a higher
compression. This allows ignition timing to be altered to beyond what is normallyallowed before the onset of detonation, thus increasing the overall efficiency still further.
A similar delayed valve closing is used in some modern versions of Atkinson cycle
engines, but without the supercharging.
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3.8.2 Variable induction temperature
This technique is also known as fast thermal management. It is accomplished by
rapidly varying the cycle-to-cycle intake charge temperature. It is also expensive to
implement and has limited bandwidth associated with actuator energy.
3.8.3 Variable Exhaust Gas Percentage
Exhaust gas can be very hot if retained or reinducted from the previous
combustion cycle or cool if recirculated through the intake as in conventional EGR
systems. The exhaust has dual effects on HCCI combustion. It dilutes the fresh charge,
delaying ignition and reducing the chemical energy and engine work. Hot combustion
products conversely will increase the temperature of the gases in the cylinder and
advance ignition.EGR in spark-ignited engines.
In a typical automotive spark-ignited (SI) engine, 5 to 15 percent of the exhaust
gas is routed back to the intake as EGR (thus comprising 5 to 15 percent of the mixture
entering the cylinders). The maximum quantity is limited by the requirement of the
mixture to sustain a contiguous flame front during the combustion event; excessive EGR
in an SI engine can cause misfires and partial burns. Although EGR does measurably
slow combustion, this can largely be compensated for by advancing spark timing. The
impact of EGR on engine efficiency largely depends on the specific engine design, andsometimes leads to a compromise between efficiency and NOx emissions. A properly
operating EGR can theoretically increase the efficiency of gasoline engines via several
mechanisms:
Reduced throttling losses. The addition of inert exhaust gas into the intake systemmeans that for a given power output, the throttle plate must be opened further,
resulting in increased inlet manifold pressure and reduced throttling losses.
Reduced heat rejection. Lowered peak combustion temperatures not only reducesNOx formation, it also reduces the loss of thermal energy to combustion chamber
surfaces, leaving more available for conversion to mechanical work during the
expansion stroke.
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Reduced chemical dissociation. The lower peak temperatures result in more of thereleased energy remaining as sensible energy near TDC, rather than being bound
up (early in the expansion stroke) in the dissociation of combustion products. This
effect is relatively minor compared to the first two.
It also decreases the efficiency of gasoline engines via at least one more
mechanism:
Reduced specific heat ratio. A lean intake charge has a higher specific heat ratiothan an EGR mixture. A reduction of specific heat ratio reduces the amount of
energy that can be extracted by the piston.
EGR is typically not employed at high loads because it would reduce peak power
output. This is because it reduces the intake charge density. EGR is also omitted
at idle (low-speed, zero load) because it would cause unstable combustion,
resulting in rough idle.
3.8.4 EGR Implementations
Recirculation is usually achieved by piping a route from the exhaust manifold to
the inlet manifold, which is called external EGR. A control valve (EGR Valve) within the
circuit regulates and times the gas flow. Some engine designs perform EGR by trapping
exhaust gas within the cylinder by not fully expelling it during the exhaust stroke, which
is called internal EGR. A form of internal EGR is used in the rotary Atkinson cycle
engine.
EGR can also be used by using a variable geometry turbocharger (VGT) which
uses variable inlet guide vanes to build sufficient backpressure in the exhaust manifold.
For EGR to flow, a pressure difference is required across the intake and exhaust manifold
and this is created by the VGT.
Other methods that have been experimented with are using a throttle in a turbocharged
diesel engine to decrease the intake pressure to initiate EGR flow.
Early (1970s) EGR systems were relatively unsophisticated, utilizing manifold
vacuum as the only input to an on/off EGR valve; reduced performance and/or drivability
were common side effects. Slightly later (mid 1970s to carbureted 1980s) systems
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included a coolant temperature sensor, which didn't enable the EGR system until the
engine had achieved normal operating temperature (presumably off the choke and
therefore less likely to block the EGR passages with carbon buildups, and a lot less likely
to stall due to a cold engine). Many added systems like "EGR timers" to disable EGR for
a few seconds after a full-throttle acceleration. Vacuum reservoirs and "vacuum
amplifiers" were sometimes used, adding to the maze of vacuum hoses under the hood.
All vacuum-operated systems, especially the EGR due to vacuum lines necessarily in
close proximity to the hot exhaust manifold, were highly prone to vacuum leaks caused
by cracked hoses; a condition which plagued early 1970s EGR-equipped cars with bizarre
reliability problems (stalling when warm, stalling when cold, stalling or misfiring under
partial throttle, etc.). Passing an unlit blowtorch over them should check hoses in these
vehicles: when the engine speeds up, the vacuum leak has been found.
Modern systems utilizing electronic engine control computers, multiple control
inputs, and servo-driven EGR valves typically improve performance/efficiency with no
impact on drivability.
In the past, a meaningful fraction of car owners disconnected their EGR systems
Some still do either because they believe EGR reduces power output, causes a build-up in
the intake manifold in diesel engines, or believe that the environmental impact of EGR
outweighs the NOx emission reductions. Disconnecting an EGR system is usually as
simple as unplugging an electrically operated valve or inserting a ball bearing into the
vacuum line in a vacuum-operated EGR valve. In most modern engines, disabling the
EGR system will cause the computer to display a check engine light. In almost all cases,
a disabled EGR system will cause the car to fail an emissions test, and may cause the
EGR passages in the cylinder head and intake manifold to become blocked with carbon
deposits, necessitating extensive engine disassembly for cleaning.[7]
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Fig. 4.1 HCCI accomplished with SI
Chapter 4.
HOW TO ACCOMPLISH THE HCCI
Because of the high compression ratios in a diesel, the engine must be more
robust to withstand the loads and the temperature of the combustion tends to be high
enough to cause the nitrogen in the air to react with the oxygen resulting in NOx. As the
name implies, homogeneous charge compression ignition (HCCI) relies on the high
temperatures generated by compressing the intake stream to cause the fuel to auto ignite
just like a diesel. The difference is that an HCCI
engine runs on gasoline (or ethanol) instead of
diesel fuel and has a significantly lower
compression ratio.
That lower compression ratio contributes
to a lower combustion temperature and helps
keep nitrogen oxide generation to a minimum. In
order for this work, very precise metering of the
fuel is required and that is now possible thanks
to the latest direct injection technology. The
fuel is injected directly into the cylinder and
mixed with the air. Since gasoline vary in different regions and different times of the
year, the timingoperation and concentration has to be adjusted in real time. Having this
capability built in also makes it easier to accommodate alternate fuel like ethanol.
In order to have smooth, consistent performance with varying fuels the engine
management system needs to be able to vary the valve timing and lift which allows the
compression ratio to be adjusted. Determining how to adjust the fuel and valve control
requires a pressure sensor in the combustion chamber as well as fuel sensor like the ones
already used on flex-fuel engines.
Because HCCI works best at relatively constant, partial-load conditions, the HCCI
engines being developed right now are actually combination engines that can run as
either spark ignition or HCCI. At higher speeds or loads, the engine runs as a normal SI
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type and then transitions to HCCI when the conditions warrant. The control software
required to reliably detect when to operate in either mode as well as transitioning between
modes is extremely complex and requires a lot of development. Most of the hardware
necessary required to produce HCCI/SI engines exists now and the main stumbling block
is getting reliable, cost effective cylinder pressure sensors.
All of this technology results in an engine that approaches the efficiency of diesel
engines at a significantly lower cost. An HCCI engine provides a fifteen percent boost in
fuel economy and reduced emissions compared to a conventional SI engine using pretty
much the same exhaust after-treatment systems.
For the first media sampling of HCCI, GM provided an automatic transmission-
equipped Saturn Aura and five-speed manual Opel Vectra. Both cars had the same 2.2L
Ecotec four cylinders modified to operate in HCCI mode at speeds up to 55 mph and
partial loads. A display mounted on top of the dashboard shows a map of engine speed
and fuel mass and indicates when the engine is in SI or HCCI mode.
On the test loop that we were able to
drive, the transitions between SI and HCCI
were largely transparent and far smoother
than any of the current production hybrids
when starting and stopping the engine.
Performance felt pretty much the same as a
regular Vectra or Aura. The only detectable
difference was a slight audible ticking when
the engine was in HCCI. The technology
definitely works, the main problem now will be making the control software robust
enough to deal with all real world weather, road and driver Conditions. It's critical to
make sure that the fuel injection and valve timing and lift are managed correctly. If the
fuel ignites too early, it can cause excessive noise or damage to the engine internals. If it
happens too late, the engine can misfire or stall so the software and the cylinder pressure
sensor have to be reliable. Currently GM is not giving a timeline for when HCCI engines
will go into production, but it will probably be sooner rather than later.[11]
Fig. 4.2 HCCI operating range
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Chapter 5.
CASE STUDY
After many years of development and research, General Motors has brought a
completely drivable and street-worthy HCCI (Homogeneous Charge Compression
Ignition) test mule (working concept vehicle) from the proving grounds of Detroit to the
streets of major metropolitan areas like Washington D.C. and greater New York City.
Finally, GM took on HCCI development in a serious way, and when it was out for test
drive, they gave the chance to many of the automotive journalists to drive the Saturn
Aura HCCI on the streets of New York. [8]
Fig 5.1 Saturn Aura
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5.1 THE HCCI CONCEPT
In a brief nutshell, HCCI is an engine design that falls somewhere between
a diesel and a spark ignition gasoline engine. Instead of a rich fuel mixture ignited by a
spark plug in an engine's combustion chamber (like almost every gasoline car out there),HCCI uses a super lean (high air-to-fuel ratio) homogeneous gasoline or E85 mixture
ignited by compression ignition (heat triggered much like a diesel, but without using
diesel fuel). So what's the big deal? Why would GM consider it worthy of investing many
millions of dollars of R&D money? Why not just stick with diesels? The answer, friends,
is fuel efficiency (up to a 15 percent gain) AND clean emissions--two of the most
difficult to achieve (simultaneously) parameters in all of engine design.
5.2 DRIVING IMPRESSIONS OF THE HCCI SATURN AURA
5.2.1 The Look
On the exterior, aside from the splashy graphics (GM really does want the
attention), the HCCI Saturn Aura looks every bit the part of the run-of-the-mill Aura
sedan. On the inside, it was pretty much the same except for the engineers' laptop
computer plugged into the engine's computer and the HCCI feedback display mounted on
the dash. (Don't look for these options when the car hits production).
5.2.2 Cold start
As with diesels, cold starts require a bit of special treatment for HCCI engines--
it's a function of heat. When cold, compression ignition engines need an initial heat
source. Diesels supply initial startup heat with glowplugs, whereas HCCIs use traditional
spark plugs for cold fire. It initially start-up in spark mode and then stays there for a
minute or so during idle. After that, it automatically switches to HCCI mode (as
evidenced by the operation mode display) . When that happens, we can notice a slight
change in the engine's timbre, an ever so faint diesel-like clack just after switchover.
5.2.3 Merge into traffic
Into traffic, the engine works fully in HCCI mode and it get accelerated quickly
and smoothly into the fold with other vehicles without one bit of spark ignition
assistance. The engine runs so smoothly and effortlessly.
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5.2.4 Cruise with traffic at moderate speed
While having operation display with laptop, sundry HCCI control adjustments
that happens nearly instantaneously. Throughout the cruise, fuel delivery pulses seems
fluctuating (more fuel, less fuel), the variable valve lift dimensions continuously changes(a little more valve lift, a little less valve lift) and electromechanical cam phases rotates
back and forth among all manner of early-open, late-close and late open-early close
modes to keep the engine's valves (and subsequent cylinder pressure) in perfect harmony
with whatever load and speed requirements prevailed at the moment. These continuous
micro adjustments really are the heart and soul of HCCI. Powering a highway-traveling
vehicle with its myriad and ever changing load, speed, temperature and atmospheric
condition parameters is perhaps the greatest challenge that can be presented to an engine.
That probably goes double or triple for the HCCI process. [9]
5.2.5 Stomp on the gas
Matthias, Vijay and the development team decided long ago that they'd engineer-
in dual mode capability to this package so that it could do diesel-like efficiency and
emissions, but still pound out spark ignition-like instant response. When I nailed the
Saturn's gas pedal, it took but a few brief Nano seconds for the onboard computer to
detect a change in engine dynamics and elevated cylinder pressure readings and kick the
2.2-liter 4-banger into spark ignition mode.
The engine management system disables HCCI and initiated spark mode to meet
instantaneous high load demands. Here's how Matthias put it in a GM press release:
"GM's HCCI development focuses the technology where it will deliver the most benefit
at the most reasonable cost for the consumer. An HCCI engine that uses HCCI in the
entire operating mode would be heavier, noisier, more costly and would not deliver the
performance experience people expect from a modern car." In effect, he's saying that they
could make it do HCCI from idle to top speed, but it would miss the bang-for-the-buck
threshold.
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5.2.6 Highway Speed
HCCI can sustain speeds of up to about 55 mph (somewhere around 3000 RPMs).
After that, the engine transitions into spark mode to keep torque and horsepower up
without detonating the engine block and heads from excessive compression. According toengineers from GM, the engine is not built to handle the intense cylinder pressure that
would develop at high RPMs and speeds.
5.2.7 Stop and idle
We find fun in this car--and want to go again. Idling in HCCI mode, ifwe wait to
hear or feel something different, but no, it felt like a regular ole engine. Actually we are
able to trackHCCI versus spark ignition time of operation, and on the first test drive, the
engineers were pretty eager to find out how it did. One of them punched a button or two
and the score displayed. Not too bad as it turns out: they spent 2.42 km out of 3.26 km in
HCCI mode.
5.2.8 Shutdown
Shutdown with an HCCI engine is no different than any other car.
The engineers said that challenges do still exist, and controlling the complicated
HCCI process over the long haul in a vehicle with many years and miles on the odometer
is as yet an unknown. This is what Dr. Uwe Grebe, executive director for GM Powertrain
Advanced Engineering has to say in a GM press release: "Our development costs for
HCCI are very expensive; however, we have made tremendous strides in bringing this
much awaited combustion technology out of the lab and onto the test track with the
Saturn Aura concept vehicle. More research and testing are required to ensure the
technology is ready for the great variety of driving conditions that customers experience."
[10]
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CONCLUSION
Therefore, it can be concluded that the SI/HCCI dual mode is the developmental
direction for the large-scale production of gasoline- fuelled HCCI engines in the future.
While the flexible valve actuation and direct multiple injection strategies are the keystone
to reach the combine HCCI combustion mode at low to medium loads with traditional SI
mode at high speed and high loads. However, to realize the practical HCCI combustion
system, active closed-loop real-time dynamic control is necessary for the gasoline-fuelled
HCCI engines.
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REFRENCES
[1] http://en.wikipedia.org/wiki/Homogeneous_charge_compression_ignition[2] http://alternativefuels.about.com/od/researchdevelopment/a/HCCIbasics.htm[3] http://alternativefuels.about.com/od/glossary/g/HomogeneousChg.hmt[4] Progress and recent trends in homogeneous charge compression ignition (HCCI)
engines Mingfa Yao, ZhaoleiZheng, Haifeng Liu, Progress in Energy and
Combustion Science, 428-432 (2009)
[5] Understanding the transition between conventional spark-ignited combustion andHCCI in a gasoline engineC. Stuart Daw, Robert M. Wagner , K. Dean Edwards,
Johney B. Green Jr,Proceedings of the Combustion Institute-2886-2894 (2007)
[6] A new heat release rate (HRR) law for homogeneous charge compression ignition(HCCI) combustion mode -Miguel Torres Garca , Francisco Jos Jimnez-
Espadafor Aguilar, Toms Snchez Lencero, Jos Antonio Becerra Villanueva,
Applied Thermal Engineering -36543662 (2009)
[7] The influence of Exhaust Gas Recirculation (EGR) on combustion and emissionsof n-heptane/natural gas fueled Homogeneous Charge Compression Ignition
(HCCI) engines, MortezaFathi, R. KhoshbakhtiSaray, M. David Checkel,Applied
Energy June 2011
[8] http://www.autoblog.com/2007/08/24/gm-shows-off-hcci-engines-in-working-prototypes
[9] http://alternativefuels.about.com/od/researchdevelopment/a/HCCISaturnAura.htm[10] http://alternativefuels.about.com/od/researchdevelopment/a/HCCISaturnAura_2.h
tm
[11] http://green.autoblog.com/2007/08/26/abg-tech-analysis-and-driving-impression-gms-hcci-engine.html
http://en.wikipedia.org/wiki/Homogeneous_charge_compression_ignitionhttp://alternativefuels.about.com/od/researchdevelopment/a/HCCISaturnAura.htmhttp://green.autoblog.com/2007/08/26/abg-tech-analysis-and-driving-impression-gms-hcci-engine.htmlhttp://green.autoblog.com/2007/08/26/abg-tech-analysis-and-driving-impression-gms-hcci-engine.htmlhttp://green.autoblog.com/2007/08/26/abg-tech-analysis-and-driving-impression-gms-hcci-engine.htmlhttp://green.autoblog.com/2007/08/26/abg-tech-analysis-and-driving-impression-gms-hcci-engine.htmlhttp://green.autoblog.com/2007/08/26/abg-tech-analysis-and-driving-impression-gms-hcci-engine.htmlhttp://green.autoblog.com/2007/08/26/abg-tech-analysis-and-driving-impression-gms-hcci-engine.htmlhttp://alternativefuels.about.com/od/researchdevelopment/a/HCCISaturnAura.htmhttp://en.wikipedia.org/wiki/Homogeneous_charge_compression_ignition8/3/2019 Homogeneous Charge Compression HCCI
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ABSTRACT
HCCI has characteristics of the two most popular forms of combustion used in IC
engines: homogeneous charge spark ignition (gasoline engines) and stratified charge
compression ignition (diesel engines). As in homogeneous charge spark ignition, the fuel
and oxidizer are mixed together. However, rather than using an electric discharge to
ignite a portion of the mixture, the concentration and temperature of the mixture are
raised by compression until the entire mixture reacts spontaneously. Stratified charge
compression ignition also relies on temperature increase and concentration resulting from
compression, but combustion occurs at the boundary of fuel-air mixing, caused by an
injection event, to initiate combustion.
The defining characteristic of HCCI is that the ignition occurs at several places at
a time which makes the fuel/air mixture burn nearly simultaneously. There is no direct
initiator of combustion. This makes the process inherently challenging to control.
However, with advances in microprocessors and a physical understanding of the ignition
process, HCCI can be controlled to achieve gasoline engine-like emissions along with
diesel engine-like efficiency. In fact, HCCI engines have been shown to achieve
extremely low levels of Nitrogen oxide emissions (NOx) without after treatment catalytic
converter. The unburned hydrocarbon and carbon monoxide emissions are still high (dueto lower peak temperatures), as in gasoline engines, and must still be treated to meet
automotive emission regulations.
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TABLE OF CONTENTS
SR.
NO.TITLE
PAGE
NO.
TITLE iCERTIFICATE ii
ACKNOWLEDGEMENT iii
ABSTRACT iv
TABLE OF CONTENTS v
LIST OF FIGURES vii
1. INTRODUCTION 1
1.1 Homogeneous Charge 1
1.2 What is HCCI Engine? 1
2. HISTORY AND LITERATURE SURVEY 3
2.1 Following are some summery points collected on HCCI from
different journals
4
3. HOMOGENEOUS CHARGE COMPRESSION IGNITION 8
3.1 What is HCCI? 8
3.2 Working Principle 8
3.3 Working 9
3.4 Why HCCI? 10
3.5 Methods 10
3.7 Disadvantages 11
3.8 Control 11
3.8.1 Variable Compression Ratio 12
3.8.2 Variable induction temperature 14
3.8.3 Variable Exhaust Gas Percentage 14
3.8.4 EGR Implementations 15
4. HOW TO ACCOMPLISH THE HCCI 17
5. CASE STUDY 19
5.1 The HCCI Concept 20
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5.2 Driving Impressions of the HCCI Saturn Aura 20
5.2.1 The Look 20
5.2.2 Cold start 20
5.2.3 Merge into traffic 20
5.2.4 Cruise with traffic at moderate speed 21
5.2.5 Stomp on the gas 21
5.2.6 Highway Speed
5.2.7 Stop and idle 22
5.2.8 Shutdown 22
CONCLUSION 23
REFRENCES 24
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LIST OF FIGURES
FIGURE NO. TITLE PAGE NO.
1.1 SI,CI and HCCI Engine 1
2.1 Some early results gave piston damage 3
4.1 HCCI accomplished with SI 17
4.2 HCCI operating range 18
5.1 Saturn Aura 19