homogeneous charge compression hcci

8/3/2019 Homogeneous Charge Compression HCCI

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Homogenous Charge Compression Ignition

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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

understanding 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 nonlinear 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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18

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.
http://alternativefuels.about.com/od/dieselbiodieselvehicles/a/dieselvehicle.htmhttp://alternativefuels.about.com/od/glossary/g/sparkignition.htmhttp://alternativefuels.about.com/od/researchdevelopment/a/HCCIbasics.htmhttp://alternativefuels.about.com/od/glossary/g/E85.htmhttp://alternativefuels.about.com/od/glossary/g/compignition.htmhttp://0.tqn.com/d/alternativefuels/1/0/a/H/-/-/HCCI_Saturn_HCCIgraphics.jpghttp://0.tqn.com/d/alternativefuels/1/0/b/H/-/-/HCCI_Saturn_HCCImode.jpghttp://0.tqn.com/d/alternativefuels/1/0/b/H/-/-/HCCI_Saturn_HCCImode.jpghttp://0.tqn.com/d/alternativefuels/1/0/a/H/-/-/HCCI_Saturn_HCCIgraphics.jpghttp://alternativefuels.about.com/od/glossary/g/compignition.htmhttp://alternativefuels.about.com/od/glossary/g/E85.htmhttp://alternativefuels.about.com/od/researchdevelopment/a/HCCIbasics.htmhttp://alternativefuels.about.com/od/glossary/g/sparkignition.htmhttp://alternativefuels.about.com/od/dieselbiodieselvehicles/a/dieselvehicle.htm


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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]
http://0.tqn.com/d/alternativefuels/1/0/_/H/-/-/HCCI_Saturn_Drivestatistics.jpghttp://0.tqn.com/d/alternativefuels/1/0/_/H/-/-/HCCI_Saturn_Drivestatistics.jpg


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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_ignition


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iv

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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v

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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vi

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

homogeneous charge compression hcci

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