(HCCI Engines) Homogeneous charge compression ignition (HCCI Engines)
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Homogeneous charge compression ignition (HCCI) is a form of internal combustion in which well-mixed fuel and oxidizer (typically air) are compressed to the point of auto-ignition. Homogeneous charge compression ignition (HCCI) engines have
the potential for high efficiencies and low pollutant emissions.By comparison with spark ignition (SI) engines, for example,HCCI engines can yield a 15-20% increase in fuel economy while emitting lower levels of oxides of nitrogen (NOx).HCCI engines can also achieve thermal efficiencies comparable to diesel engines yet maintain near zero levels of
particulate emission,As in other forms of combustion, this exothermic reaction releases chemical energy into a sensible form that can be translated by an engine into work and heat
http://web.mit.edu/greengp/project and implimentations/HCCIeng.htm
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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 aftertreatment catalytic converter. The unburned hydrocarbon and carbon monoxide emissions are still high (due to lower peak temperatures), as in gasoline engines, and must still be treated to meet automotive emission regulations.
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passwordeminar and presentationproject and implimentations
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 simultaneously. 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 (due to lower peak temperatures), as in gasoline engines, and must still be treated to meet automotive emission regulations. The homogeneous charge compression ignition (HCCI) engine has caught the attention of automotive and diesel engine manufacturers worldwide because of its potential to rival the high efficiency of diesel engines while keeping NOx and particulate emissions extremely low. However, researchers must overcome several technical barriers, such as controlling ignition timing, reducing unburned hydrocarbon and carbon monoxide emissions, extending operation to higher loads, and maintaining combustion stability through rapid transients.
HCCI engines can operate using a variety of fuels. In the near term, the application of HCCI to automotive engines will likely involve mixed-mode combustion in which HCCI is used at low-to-moderate loads and standard spark-ignition (SI) combustion is used at higher loads. This type of operation using standard gasoline-type fuels requires a moderate compression ratio of 10:1 to 14:1 for SI operation and significant intake heating for HCCI operation.
1.1 Automotive HCCI Engine Laboratory.
The CRF's Automotive HCCI Engine Laboratory houses a versatile light-duty engine designed to allow investigations of a wide variety of issues for this type of HCCI application. The automotive-sized engine (0.63 liters/cylinder) has a 3-valve pent-roof head and is equipped with extensive optical access for the application of advanced laser-based diagnostics, including an extended cylinder with a piston-crown window and a full transparent quartz cylinder liner. The intake air system provides intake pressures up to 2 bar and heating to 250 °C. These high intake temperatures allow investigations of HCCI operation with lower compression ratios (10:1 to 12:1). Alternatively, hot residuals can be used to induce HCCI combustion using cam shafts designed to retain large amounts of combustion products. The engine is equipped with a centrally mounted gasoline-type direct injector, a port fuel injection capability, and a fully premixed fueling system, allowing investigations of both well-mixed and stratified HCCI operation Researchers are currently using laser elastic scatter and laser-induced fluorescence imaging to study the distribution of both liquid- and vapor-phase fuel in the cylinder from the time of injection to the time of ignition. Images recorded during direct injection provide details of spray morphology, interactions between the spray and the intake air flow, and wall wetting. Images recorded during the compression stroke capture the evolution of the fuel vapor/air mixture, and provide a measure of mixture homogeneity at the time of ignition. Correlations are sought between these fuel-distribution data and simultaneously recorded combustion performance and emission data
Researchers are currently using laser elastic scatter and laser-induced fluorescence imaging to study the distribution of both liquid- and vapor-phase fuel in the cylinder from the time of injection to the time of ignition. Images recorded during direct injection provide details of spray morphology, interactions between the spray and the intake air flow, and wall wetting. Images recorded during the compression stroke capture the evolution of the fuel vapor/air mixture, and provide a measure of mixture homogeneity at the time of ignition. Correlations are sought between these fuel-distribution data and simultaneously recorded combustion performance and emission data.
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. Homogeneous charge compression ignition (HCCI) engines have the potential to provide high, diesel-like efficiencies and very low emissions. In an HCCI engine, a dilute, premixed fuel/air charge autoignites and burns volumetrically as a result of being compressed by the piston. The charge is made dilute either by being very lean, or by mixing with recycled exhaustgases
Several technical barriers must be overcome before HCCI can be implemented in production engines. Variations of HCCI in which the charge mixture and/or temperature are partially stratified (stratified charge compression ignition or SCCI) have the potential for overcoming many of these barriers.
Because of HCCI's strong potential, most diesel engine and automobile manufacturers have established HCCI/SCCI development efforts.
The Sandia HCCI Engine Combustion Fundamentals Laboratory supports this industrial effort. The laboratory is equipped with two Cummins B-series engines mounted at either end of a double-ended dynamometer. These production engines have been converted for single-cylinder HCCI/SCCI operation.
One engine (the so-called all-metal engine) is used to establish operating points, test various fuel types, develop combustion-control strategies, and investigate emissions.
The second engine has extensive optical access for the application of advanced laser diagnostics for investigations of in-cylinder processes. The design includes an extended piston with piston-crown window, three large windows near the top of the cylinder wall, and a drop-down cylinder for rapid cleaning of fouled windows.
The engines are designed to provide a versatile facility for investigations of a wide range of operating conditions and various fuel-injection, fuel/air/residual mixing, and control strategies that have the potential of overcoming the technical barriers to HCCI. The size of these engines (0.98 liters/cylinder) was selected so that results are applicable to both automotive and heavy-duty applications. They are equipped with the following features:
Variable in-cylinder swirl: swirl ratios of 0.9 to 3.2, convertible to swirl ratios up to 7.6
Multiple fuel systems: fully premixed, port fuel injection, gasoline-type direct-injection, and diesel-type direct-injection
Complete intake charge conditioning: simulated or real EGR, intake pressures to 6 atmospheres, and intake temperatures to 220°C
Speeds up to 3600 rpm (metal engine) and 1800 rpm (optical engine)
Variable compression ratio variable through interchangeable pistons (compression ratios from 12:1 to 21:1 are currently available)
Custom HCCI piston design
Full emissions measurements: CO2, CO, O2, HC, NOx, and smoke
Mechanical valves with a conversion to fully flexible variable valve actuation (VVA) under development
Investigations are addressing several issues, including:
Stratification of the fuel/air mixture as a means of improving emissions and combustion efficiency during part-load operation
The effects of fuel-type on performance and emissions over a range of speeds and loads
Intake pressure boosting for increased power, heat transfer effects, combustion-phasing control, and extending operation to higher loads
Because fuel characteristics are central to HCCI engine design, a variety of fuels are being examined including gasoline, diesel fuel, and a number of representative constituents of real distillate fuels.
3.1 WHY HCCI
The modern conventional SI engine fitted with a three-way catalyst can be seen as an very clean engine. But it suffer from poor partload efficiency. As mentioned earlier this is mainly due to the throttling. Engines in passenger cars operates most of the time at light- and partload 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 a 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 these mechanisms, 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.
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:
High compression ratio
Pre-heat induction gases
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.
• 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, NOx levels are almost negligible.
• Since HCCI runs throttleless, it eliminates throttling losses
• High peak pressures
• High heat release rates
• Difficulty of control
• Limited power range
• High carbon monoxide and hydrocarbon pre-catalyst emission
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 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 of work it 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
7.1 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 with some 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. The Miller cycle was patented by Ralph Miller, an American engineer, 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 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 in what 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 normally allowed 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.
8.1VARIABLE 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.
8.2VARIABLE 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, and sometimes 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 system means 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 reduces NOx 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.
• Reduced chemical dissociation. The lower peak temperatures result in more of the released 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 ratio than 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.
8.2a EGR IN DIESEL ENGINES
In modern diesel engines, the EGR gas is cooled through a heat exchanger to allow the introduction of a greater mass of recirculated gas. Unlike SI engines, diesels are not limited by the need for a contiguous flamefront; furthermore, since diesels always operate with excess air, they benefit from EGR rates as high as 50% (at idle, where there is otherwise a very large amount of excess air) in controlling NOx emissions.
Since diesel engines are unthrottled, EGR does not lower throttling losses in the way that it does for SI engines (see above). However, exhaust gas (largely carbon dioxide and water vapor) has a higher specific heat than air, and so it still serves to lower peak combustion temperatures; this aids the diesel engine's efficiency by reduced heat rejection and dissociation. There are trade offs however. Adding EGR to a diesel reduces the specific heat ratio of the combustion gases in the power stroke. This reduces the amount of power that can be extracted by the piston. EGR also tends to reduce the amount of fuel burned in the power stroke. This is evident by the increase in particulate emissions that corresponds to an increase in EGR. Particulate matter (mainly carbon) that is not burned in the power stroke is wasted energy. Stricter regulations on particulate matter(PM) call for further emission controls to be introduced to compensate for the PM emissions introduced by EGR. The most common is particulate filters in the exhaust system that result in reduce fuel efficiency. Since EGR increases the amount of PM that must be dealt with and reduces the exhaust gas temperatures and available oxygen these filters need to function properly to burn off soot, automakers have had to consider injecting fuel and air directly into the exhaust system to keep these filters from plugging up.
8.2b 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 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.). Hoses in these vehicles should be checked by passing an unlit blowtorch over them: 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.
9.1VARIABLE VALVE ACTUATION
Variable valve actuation (VVA) has been proven to extend the HCCI operating region by giving finer control over the temperature-pressure-time history within the combustion chamber. VVA can achieve this via two distinct methods:
1. Controlling the effective compression ratio: A variable duration VVA system on intake can control the point at which the intake valve closes. If this is retarded past bottom dead center (BDC), then the compression ratio will change, altering the in-cylinder pressure-time history prior to combustion.
2. Controlling the amount of hot exhaust gas retained in the combustion chamber: A VVA system can be used to control the amount of hot internal exhaust gas recirculation (EGR) within the combustion chamber. This can be achieved with several methods, including valve re-opening and changes in valve overlap. By balancing the percentage of cooled external EGR with the hot internal EGR generated by a VVA system, it may be possible to control the in-cylinder temperature.
Whilst electro-hydraulic and camless VVA systems can be used to give a great deal of control over the valve event, the componentry for such systems is currently complicated and expensive.
Mechanical variable lift and duration systems, however, whilst still being more complex than a standard valvetrain, are far cheaper and less complicated. If the desired VVA characteristic is known, then it is relatively simple to configure such systems to achieve the necessary control over the. valve lift curve
FIGURE 1. The i-VTEC system found in the Honda K20Z3.
Piston engines normally use poppet valves for intake and exhaust. These are driven (directly or indirectly) by cams on a camshaft. The cams open the valves (lift) for a certain amount of time (duration) during each intake and exhaust cycle. The timing of the valve opening and closing is also important. The camshaft is driven by the crankshaft through timing belts, gears or chains.
The profile, or position and shape of the cam lobes on the shaft, is optimized for a certain engine rpm, and this tradeoff normally limits low-end torque or high-end power. VVT allows the cam profile to change, which results in greater efficiency and power.
At high engine speeds, an engine requires large amounts of air. However, the intake valves may close before all the air has been given a chance to flow in, reducing performance.
On the other hand, if the cam keeps the valves open for longer periods of time, as with a racing cam, problems start to occur at the lower engine speeds. This will cause unburnt fuel to exit the engine since the valves are still open. This leads to lower engine performance and increased emissions. For this reason, pure racing engines cannot idle at the low speeds (around 800rpm) expected of a road car, and idle speeds of 2000 rpm are not unusual.
Pressure to meet environmental goals and fuel efficiency standards is forcing car manufacturers to turn to VVT as a solution. Most simple VVT systems (like Mazda's S-VT) advance or retard the timing of the intake or exhaust valves. Others (like Honda's VTEC) switch between two sets of cam lobes at a certain engine RPM. Still others can alter timing and lift continuously, which is called Continuous variable valve timing or CVVT.
10.1HIGH PEAK PRESSURES AND HEAT RELEASE RATES
In a typical gasoline or diesel engine, combustion occurs via a flame. Hence at any point in time, only a fraction of the total fuel is burning. This results in low peak pressures and low energy release. In HCCI, however, the entire fuel/air mixture ignites and burns nearly simultaneously resulting in high peak pressures and high energy release rates. To withstand the higher pressures, the engine has to be structurally stronger and therefore heavier.
Several strategies have been proposed to lower the rate of combustion. Two different blends of fuel can be used, that will ignite at different times, resulting in lower combustion speed. The problem with this is the requirement to set up an infrastructure to supply the blended fuel. Alternatively, dilution, for example with exhaust, reduces the pressure and combustion rate at the cost of work production.
In a gasoline engine, power can be increased by increasing the fuel/air charge. In a diesel engine, power can be increased by increasing the amount of fuel injected. The engines can withstand a boost in power because the heat release rate in these engines is slow. In HCCI however, the entire mixture burns nearly simultaneously. Increasing the fuel/air ratio will result in even higher peak pressures and heat release rates. Also, increasing the fuel/air ratio (also called the equivalence ratio) increases the danger of knock. In addition, many of the viable control strategies for HCCI require thermal preheating of the charge which reduces the density and hence the mass of the air/fuel charge in the combustion chamber, reducing power. These factors makes increasing the power in HCCI inherently challenging.
One way to increase power is to use different blends of fuel. This will lower the heat release rate and peak pressures and will make it possible to increase the equivalence ratio. Another way is to thermally stratify the charge so that different points in the compressed charge will have different temperatures and will burn at different times lowering the heat release rate making it possible to increase power. A third way is to run the engine in HCCI mode only at part load conditions and run it as a diesel or spark ignition engine at full or near full load conditions. Since much more research is required to successfully implement thermal stratification in the compressed charge, the last approach is being studied more intensively.
10.3CARBON MONOXIDE AND HYDROCARBON EMISSIONS
Since HCCI operates on lean mixtures, the peak temperatures are lower in comparison to spark ignition and diesel engines. The low peak temperatures prevent the formation of NOx. However they also lead to incomplete burning of fuel especially near the walls of the combustion chamber. This leads to high carbon monoxide and hydrocarbon emissions. An oxidizing catalyst would be effective at removing the regulated species since the exhaust is still oxygen rich.
10.4. DIFFERENCE FROM KNOCK
Engine knock or pinging occurs when some of the unburnt gases ahead of the flame in a spark ignited engine spontaneously ignite. The unburnt gas ahead of the flame is compressed as the flame propagates and the pressure in the combustion chamber rises. The high pressure and corresponding high temperature of unburnt reactants can cause them to spontaneously ignite. This causes a shock wave to traverse from the end gas region and an expansion wave to traverse into the end gas region. The two waves reflect off the boundaries of the combustion chamber and interact to produce high amplitude standing waves.
A similar ignition process occurs in HCCI. However, rather than part of the reactant mixture being ignited by compression ahead of a flame front, ignition in HCCI engines occurs due to piston compression. In HCCI, the entire reactant mixture ignites (nearly) simultaneaously. Since there are very little or no pressure differences between the different regions of the gas, there is no shock wave propagation and hence no knocking. However at high loads (i.e. high fuel/air ratios), knocking is a possibility even in HCCI.
As of August 2007 there are no HCCI engines being produced in commercial scale. However several car manufacturers have fully functioning HCCI prototypes.
• General Motors has demonstrated Opel Vectra and Saturn Aura with modified HCCI engines.[
11.1 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 compressionratio.
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 gasolines vary in different regions and different times of the year, the timing FIGURE2.hcci operation
and concentration has to be adjusted in real time. Having this capability built in also makes it easier to accommodate alternatefuel 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 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 cylinderpressuresensors.
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 cylinder 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 driverConditions.
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.
Studies of intake valve actuation for combustion phasing, variable spray geometries for fuel and air mixing and spray fumigation
Characterization and techniques for achieving homogeneous charge compression ignition for reduced emissions
Transient control strategies for variable engine speed/loads and different combustion regimes
Mechanisms of pollutant formation and destruction and extension of combustion limits for application of after treatment systems
Characteristics of soot emissions and the regeneration of diesel particulate filters
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12-01-2011, 09:03 PM
Homogeneous Charge Compression Ignition
The Homogeneous Charge Compression Ignition Engine, HCCI, has the potential to combine the best of the Spark Ignition and Compression Ignition Engines. With high octane number fuel the engine operates with high compression ratio and lean mixtures giving CI engine equivalent fuel consumption or better. Due to premixed charge without rich or stoichiometric zones, the production of soot and NOx can be avoided. This paper presents some results from advanced laser diagnostics showing the fundamental behaviour of the process from a close to homogeneous combustion onset towards a very stratified process at around 20-50% heat released. The need for active combustion control is shown and
possible means of control are discussed. Results with multi-cylinder engines using negative valve overlap, variable compression ratio, fast inlet temperature control as well as dual fuel are given.
HCCI ENGINE.doc (Size: 1.3 MB / Downloads: 219)
The internal combustion engine is the key to the modern society. Without he transportation performed by the millions of vehicles on road and at sea we would not have reached the living standard of today. We have two types of internal combustion engines, the spark ignition, SI, and the compression ignition, CI. Both have their merits. The SI engine is a rather simple
product and hence has a lower first cost. This engine type can also be made very clean as the three-way catalyst, TWC, is effective for exhaust aftertreatment. The problem with the SI engine is the poor part load
efficiency due to large losses during gas exchange and low combustion and thermodynamical efficiency. The CI engine is much more fuel efficient and hence the natural choice in applications where fuel cost is more important than first cost. The problem with the CI engine is the emissions of nitrogen oxides, NOx, and particulates, PM. Aftertreatment to reduce NOx and particulates is expensive and still not generally available on the market. The obvious ideal combination would be to find an engine type with the high efficiency of the CI engine and the very low emissions of the SI engine with TWC. One such candidate is named Homogeneous Charge Compression Ignition, HCCI. The fuel efficiency of HCCI has been compared to
that of normal SI operation by Stockinger et al. . Figure 1 shows that they noted an improvement of fuel efficiency from 15% to 30% at 1.5 bar BMEP. This is an improvement of 100% equivalent to a reduction of fuel onsumption with 50%. More recently Yang et al. presented a comparison between HCCI, denoted OKP, and normal SI and direct injected SI concepts, DISI. He found a much higher fuel consumption benefit for HCCI than for DISI concepts. The major benefit of HCCI compared to CI is the
low emissions of NOx and PM. The CI engine normally has a trade-off between particulates and NOx. If the engine operates at conditions with higher in-cylinder peak temperature, the oxidization of soot will be good
but the thermal production of NO will increase. If on the other hand the engine is operated with lower temperature NO can be suppressed but PM will be high due to bad oxidation. Figure 3 shows this trade-off and also the allowed emissions in EU and US today and in the near future. Clearly the CI engine must use exhaust aftertreatment of NOx and/or PM. In the CI engine, NO is formed in the very hot zones with close to stoichiometric conditions and the soot is formed in the fuel rich spray core. The incylinder average air/fuel ratio is always lean but the combustion process is not. This means that we have a large potential to reduce emissions of NOx and PM by
simply mixing fuel and air before combustion. In Figure 3 the normal emission level from an HCCI engine is also displayed. The NOx is normally less than 1/500 of the CI level and no PM is generated by combustion.
THE HCCI PRINCIPLE – HCCI means that the fuel and air should be efore combustion starts and that the mixture is auto ignited due to the increase in temperature from the compression stroke. Thus HCCI is similar to SI in the sense that both engines use a premixed charge and HCCI is similar to CI as both rely on auto ignition for combustion initiation. However, the combustion process is totally different for the three types. Figure 4 shows the difference between (a) SI combustion and (b) HCCI. In the SI engine we have three zones, a burnt zone, an unburned zone and between them a thin reaction zone where the chemistry takes place. This reaction zone propagates through the combustion chamber and thus we have flame propagation. Even though the reactions are fast in the reaction zone, the combustion process will take some time as the zone must propagate from spark plug (zero mass) to the far liner wall (mass wi ). With the HCCI process the entire mass in the cylinder will react at once. The right part of Figure 4 shows HCCI, or as Onishi called it Active Thermo-Atmosphere Combustion, ATAC. We see that the entire mass is active but the reaction rate is low both locally and globally. This means that the combustion process will take some time even if all the charge is active. The total amount of heat released, Q, will be the same for both processes. It could be noted that the combustion
process can have the same duration even though HCCI normally has a faster burn rate. Initial tests in Lund on a two stroke engine revealed the fundamental difference between these two types of engines. Figure 5 shows
normal flame propagation from two spark plugs at the rated speed of 9000 rpm. We see two well defined flames and a sharp border between burned and unburned zones. Figure 6 shows the same engine when HCCI combustion was triggered by using regular gasoline (RON 95) instead of iso-octane. The engine speed was increased up to 17000 rpm and a more
distributed chemiluminescense image resulted.
REQUIREMENTS FOR HCCI – The HCCI
combustion process puts two major requirements on the conditions in the cylinder: (a) The temperature after compression stroke should equal the autoignition temperature of the fuel/air mixture. (b) The mixture should be diluted enough to give reasonable burn rate. Figure 7 shows the autoignition temperature for a few fuel as a function of . The autoignition temperature has some correlation with the fuels’ resistance of knock in SI engines and thus the octane number. For iso-octane, the autoignition temperature is
roughly 1000K. This means that the temperature in the cylinder should be 1000 K at the end of the compression stroke where the reactions should start.
This temperature can be reached in two ways, either the temperature in the cylinder at the start of compression is controlled or the increase in temperature due to compression i.e. compression ratio is controlled. It
could be interesting to note that the autoignition temperature is a very weak function of air/fuel ratio. The change in autoignition temperature for iso-octane is only 50K with a factor 2 change in . Figure 7 also shows the normal rich and lean limits found with HCCI. With a too rich mixture the reactivity of the charge is too high. This means that the burn rate becomes
extremely high with richer mixtures. If an HCCI engine is run too rich the entire charge can be consumed within a fraction of a crank angle. This gives rise to extreme pressure rise rates and hence mechanical stress and noise. With a high autoignition temperature like that of natural gas, it is also possible that formation of NOx can be the load limiting factor. Figure 8 shows the NO formation as a function of maximum temperature. Very
low emission levels are measured with ethanol. If the combustion starts at a higher temperature like with natural gas, the temperature after combustion will also be higher for a given amount of heat released. On the lean side, the temperature increase from the combustion is too low to have complete combustion. Partial oxidation of fuel to CO can occur at extremely lean mixtures; above 14 has been tested. However, the oxidation of CO to CO2 requires a temperature of 1400-1500 K. As a summary, HCCI is governed by three temperatures. We need to reach the autoignition temperature to get things started; the combustion should then increase the temperature to at least 1400 K to have good combustion efficiency but it should not be
increased to more that 1800 K to prevent NO formation.
HCCI COMBUSTION PROCESS IN DETAIL
The above description of HCCI gives just a rough idea about the requirements and conditions of the combustion process. It is also of greatest interest to acquire detailed knowledge of the process. In order to get such information, laser based diagnostics is of crucial importance.
The first experiments with laser based diagnostics were performed to analyze the difference in combustion between a perfectly homogeneous fuel/air mixture and one with small gradients. Laser induced fluorescence of fuel tracer or OH was used to mark the combustion process. Figure 9 shows the system setup with a laser generating a vertical laser sheet. Figure 10 shows the fuel distribution for the two cases with an inhomogeneity of approximately 5% in the case of port fuel injection and homogeneity within the detection limit for the case with a mixing tank and fuel injection far upstream. Figure 11 and Figure 12 shows the fuel concentration with half the heat released. We can from these images conclude that the combustion is far from homogeneous. There are islands with much fuel remaining and close to them regions with very little fuel left. Figure 13 shows the same behavior for the concentration of OH. Zones with much OH are close to zones with no OH and the gradients are steep. Each individual cycle was also found to be unique. The four cycles displayed are randomly picked samples. No preferred type of structure could be detected.
SINGLE CYCLE INFORMATION
A major limitation with the information from Figure 11 to Figure 13 is that only one image can be captured from each cycle. Due to the very large cycle to cycle variation in the process, it is impossible to extract information on possible expansion of zones with intense reactions i.e. flame propagation. To overcome this problem a unique laser system was used. Four individual lasers which can generate eight laser pulses were combined with a framing camera using eight individual CCD chips. This system was used in an
optical Scania engine with transparent liner and a window in the extended piston. The setup can be seen in Figure 14. The measured area was 95x 55 mm thus enabling distinction between local and global effects.
Figure 15 shows a sequence of fuel LIF images captured with 0.5 CAD time separations at 1200 rpm. The images are from 20% to 50% heat release. From these and numerous similar mini-movies it was possible to conclude that the combustion changed behavior during the process. In the initial phase a slow but stable decrease in the fuel LIF signal was detected. This was
interpreted as a slow and rather homogeneous start of the process. At around 20-30% heat released the fuel LIF image changed. Then even the smallest structures found before were amplified to give an image with more intense gradients. The gradients were found to be amplified even more as the process evolved and at approx. 50% heat release the structures found earlier
during the single shot experiments were clear. From 50% heat release and onwards the structures were stable and the fuel signal disappeared not long after that. This single cycle observation of the process leads to a phenomenological description of the HCCI combustion process.
THE PHENOMENOLOGICAL MODEL OF HCCI
The HCCI combustion process is assumed to start with a gradual decomposition of the fuel with well distributed reactions. The reactions will
become significantly exothermic when a critical temperature is approached. At this critical condition the reaction rate will be very sensitive to the temperature of the charge. Even the smallest variations in temperature
will thus influence the reaction rates. As we will have random variation in temperature in the cylinder, some locations will have more favorable conditions. In those locations, sometimes denoted “hot spots” the reactions
thus will start a bit earlier. As the exothermic reactions start the temperature is increased and thus reactions become even faster. We thus have a local positive feedback in temperature. Figure 16 shows an attempt to illustrate this. As the local positive feedback is fast, there will not be sufficient time to distribute all the heat to the surrounding cold bulk. Thus we have a gradual
amplification of small inhomogeneities generating the very large structures seen in the experiments. The size of the hot spots was found to be of the same order as the integral length scale of turbulence in the cylinder. In
the Scania engine, this was 4-6 mm.
Flame propagation? –
It could be argued that the “hot-spots” grow as a function of time and this growth Te could be translated to a reaction zone propagation
or in other words flame front. However, after studying numerous individual cycles it was concluded that the concept of flame propagation in HCCI could not be supported. There will be a time lag between combustion starting point at different zones but new “hot-spots” show up randomly and the structures seen in the images are rather fixed i.e. do not move from image to image. If we would use the term flame speed for a case where two hot spots show up at exactly the same time we would also have a problem as the flame
speed then would be infinity.
Valve Timing Events For HCCI Engine
The valve timing events for a HCCI engine making use of internal EGR are different: Residuals are trapped by closing the exhaust valves early (e.g. 90 crank angle degrees before TDC). As a result, the HCCI process becomes a kind of six-stroke process with two extra strokes during which the combustion products in the cylinder are first compressed and then expanded. After this, the intake valves open, although this happens later than for a SI engine.
Fig.7 Cylinder pressure as a function of crank angle degree for a SI engine. The intervals during which the intake and exhaust valves are opened are shown. Note the interval during which both valves are open.
Fig.8 Cylinder pressure as a function of crank angle degree for a HCCI engine. The intervals during which the intake and exhaust valves are opened are again shown. In this case, no valve overlap is present (it is often called the negative valve overlap). The numbers in the figure explain why it is sometimes referred to as a six-stroke process.
Figure 8 shows the following six events that take place during the HCCI engine cycle:
1. The compression stroke. Auto-ignition will occur just before 0 CAD, after which combustion takes place.
2. The expansion stroke.
3. The interval during which the exhaust valves are open.
4. The exhaust valves were closed early (normally the exhaust valves are closed just after 360 CAD: See Fig.7 for the SI cycle.) and the residuals that were left in the cylinder are compressed until 360 CAD.
5. The residuals are expanded again from 360 CAD until approximately 450 CAD.
6. The intake valves are open to take in the fresh mixture of air and fuel.
THE NEED FOR CONTROL
For better understanding of the combustion process, laser diagnostics is needed and this knowledge can be used to optimize the system. However, the HCCI process is very sensitive to disturbances. It can be sufficient to change the inlet temperature 2°C to move from a very good operating point to a total misfire. This sensitivity makes the HCCI engine require closed loop combustion control, CLCC. Closed loop control requires as always a sensor, control algorithm and control means. The main parameter to control for HCCI is the combustion timing i.e. when in the cycle combustion
takes place. Figure 17 shows the rate of heat release for a range of timings. With early phasing the rate of heat release is higher and as it is phased later the burn rate goes down. With combustion before top dead center, TDC, the temperature will be increased both by the chemical reactions and the compression due to piston motion. Thus for a given auto ignition temperature, combustion onset before TDC will result in faster reactions. With the conditions changed to give combustion onset close to TDC, the temperature will not be increased by piston motion, the only temperature
driver would be the chemical reactions. This gives a more sensitive system and the later the combustion phasing the more sensitive the system is. This is the underlying problem with HCCI combustion control. We want a late combustion phasing to reduce burn rate and hence pressure rise rate and peak pressure but on the other hand we can not accept too much variations in
the combustion process. How late we can go depends on the quality of the control system. With a fast and accurate control system we can go later and hence reduce the noise and mechanical loads of the engine.
COMBUSTION SENSOR –
The most accurate and reliable signal for combustion is the in-cylinder
pressure. With the standard heat release equation it is very easy to extract the combustion onset etc. The most usable parameter for combustion phasing is the crank angle of 50% of the heat released. Figure 18 shows the procedure to extract this 50% heat released point denoted CA50. The cylinder pressure is a very stable and robust signal but the cost of such sensors is still too high for production engines. One alternative could be an ion current measurement system. The ion current can be measured by applying a voltage on the electrodes of a normal spark plug. The technique has been used by SAAB Automobile in production since 1993 for the detection of knock and misfire in SI engines, but the application on HCCI is not straight forward. The signal
intensity is very sensitive to the temperature in cylinder and thus lean burn HCCI give low signal. Figure 19 shows a typical ion current measurement
system and Figure 20 shows the typical signal obtained in HCCI mode. The best representation of combustion phasing was found by extracting the crank angle which 50% of the maximum amplitude was detected. This gave good correlation to the crank angle of 50% heat released, CA50 as shown in Figure 21. Two individual operating points are shown, one with relatively early timing and hence less cycle to cycle variations and one with late timing. For both cases, small phase difference was detected between the crank angle at 50% of maximum ion signal and CA50 but this can easily be compensated by the controller.
CONTROL MEANS –
The HCCI combustion control can be considered as a balance in temperature. With low temperature at TDC the combustion will be
late and with high temperature at TDC the combustion will start early. To control temperature, three major parameters can be used. Inlet temperature and compression ratio will directly change the TDC temperature. The third parameter is the amount of residual gas retained in the cylinder from the previous cycle. A fourth possible way of controlling the process is to change the required autoignition temperature by adjusting the fuel quality. Figure 22 shows possible combinations of inlet temperature, compression ratio
and fuel octane number for combustion onset at TDC for a 1.6 liter single cylinder Volvo Truck engine. The figure shows that a higher octane fuel needs higher inlet temperature or higher compression ratio to reach
autoignition at TDC. Figure 23 shows similar combinations but here the two fuels are regular gasoline and diesel oil instead of the primary reference fuels nheptane and iso-octane. A very popular concept for achieving HCCI in SI engines at part load is the use of negative valve overlap. With this concept the exhaust valves close early and thus hot burnt gas is trapped in the cylinder. After a short compression and expansion the inlet valve
is opened late. This type of process often denoted Controlled Autoignition, CAI, gives good performance but in a limited operating range. Figure 24 shows the operating range of a 6-cylinder 3 litre Volvo Cars engine. A better way of controlling the process is by applying variable compression ratio or fast inlet air temperature control. With this concept it is possible to run at idle at all engine speeds between 600 and 5000 rpm. Maximum load is the same as for CAI but it can be maintained also for higher engine speeds. Figure 26 shows the operating range for a SAAB 1.6 liter 5- cylinder variable compression engine using fast thermal management as shown in Figure 25. It should be noted that the BMEP is presented in contrast to the IMEP for CAI in Figure 24. A possible way of HCCI combustion control can also be the use of dual fuels. Using two fuel tanks could cause some problems with costumer acceptance but it is possible to generate two fuels from one using a reformer. Experiments with dual fuel in Lund have
shown that it is a very powerful control means. Figure 27 shows the operating range possible with a Scania 12-liter 6-cylinder truck engine running on a mixture of ethanol and n-heptane.
In order to achieve the high loads reported for the SAAB and Scania multi cylinder engines, it is absolutely necessary to use closed loop control with a well tuned controller. To make the controller usable over the entire speed and load range, the gain of the controller must be changed in accordance with the change of gain of the process. Figure 28 shows the combustion phasing, CA50, as a function of octane number for the Scania dual fuel
engine at different operating conditions. With early combustion timing and conditions requiring low octane number, the slope of the curves are low. This means that a large change of octane number is needed to change the combustion timing one crank angle. Thus we should have a large gain of the controller in these operating conditions. If we then look at conditions with
high octane number and late combustion phasing, the required change in octane number to change phasing a crank angle is much less. With this higher gain of the process we must reduce the gain of the controller;
otherwise the system will become unstable. Tuning the gain of the controller to compensate for changes in the process can be done by using gain scheduling. With this it is possible achieve close to optimal performance
for all operating conditions. In fact it is even possible to operate an HCCI engine at unstable operating points with the closed loop combustion control active. Figure 29 shows one such case.
The Homogeneous Charge Compression Ignition, HCCI, combustion process is an interesting alternative to the conventional Spark Ignition and Compression Ignition processes. The potential benefit of HCCI is high with simultaneous ultra low emissions of NOx and PM and low fuel consumption. Thus it can combine the best features of the SI (with TWC) and CI engines. To better understand the process, laser based techniques
must be used. Such measurements in Lund have revealed that the combustion process is rather homogeneous in the initial stage but it gradually transfers into a highly inhomogeneous process with steep gradients between reacting and non-reacting zones. The HCCI engine requires active control of the combustion process. Such closed loop combustion control has been demonstrated in a number of multicylinder HCCI engines in Lund. Use of negative overlap is possible but often generates a limited operating range. The use of variable compression ratio is a very powerful control means but can have some problems to reach production for cost reasons. Fast Thermal Management can perhaps be the key technology to be used for HCCI combustion control. The maximum engine speed for HCCI in Lund is
17000 rpm and the maximum load is 20.4 bar IMEP/ 16 bar BMEP. This indicates that most interesting speeds and loads can be reached with HCCI.
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12-01-2011, 09:44 PM
HOMOGENOUS CHARGE COMPRESSION IGNITION ENGINE
Lijo p joseph
HOMOGENOUS CHARGE COMPRESSION IGNITION ENGINE.ppt (Size: 1.08 MB / Downloads: 278)
PROBLEMS FACED BY I.C ENGINES
1.Problem with S.I engine-poor part load
efficiency due to large losses during gas exchange, low combustion and thermo dynamical efficiency
2.Problem with C.I engine- Emissions of nitrogen oxides and particulates
HCCI ENGINE -AN IDEAL COMBINATION
Ideal combination of an engine type with efficiency of CI engine and very low emissions of that of SI engine with TWC
HCCI ENGINE PRINCIPLE
HCCI means that the fuel and air should be mixed before combustion starts and that the mixture is auto ignited due to the increase in temperature from the compression stroke.
HCCI is similar to SI in the sense that both engines use a premixed charge
HCCI is similar to CI engine both rely on auto ignition for combustion initiation
DIFFERENCE BETWEEN HCCI AND SI COMBUSTION
In S.I .Engine we have three zones
1 Burnt zone
2 Unburnt zone
3 Thin reaction zone
In hcci combustion entire mass in the cylinder reacts at once
There are two well defined flames and a sharp border between burned and unburned zone
same engine when HCCI combustion was triggered by using regular gasoline A distributed chemiluminescence's image resulted
REQUIREMENTS OF HCCI
a) The temperature after compression stroke should equal the auto ignition temperature of the fuel/air mixture.
(b) The mixture should be diluted enough to give reasonable burn rate
AUTO IGNITION TEMPERATURE OF FUELS AS A FUNCTION OF DILUTION
The auto ignition temperature has some correlation with the fuels’ resistance of knock in SI engines and thus the octane number.
With a too rich mixture the reactivity of the charge is too high. This means that the burn rate becomes Extremely high with richer mixtures. If an HCCI engine is run too rich the entire charge can be consumed within a fraction of a crank angle.
This gives rise to extreme pressure rise rates and hence mechanical stress and noise. With a high auto ignition temperature like that of natural gas, it is also possible that formation of NOx can be the load limiting factor.
NOx AS FUNCTION OF MAXIMUM TEMPERATURE
Very low emission levels are measured with ethanol.
If the combustion starts at a higher temperature like with natural gas, the temperature after combustion will also be higher for a given amount of heat released.
Partial oxidation of fuel to CO can occur at extremely lean mixtures
The oxidation of CO to CO2 requires a temperature of 1400-1500 K.
TEMPERATURE GOVERNANCE OF HCCI
Governed by 3 temperatures
Need to reach auto ignition temperature to get things started
Combustion should then increase the temperature to at least 1400 K
Temperature should not be increased to more than 1800 K to prevent NO formation
Laser based diagnostics were performed to analyze the difference in combustion between a perfectly homogeneous fuel/air mixture and one with small gradients
system setup with a laser generating a vertical laser sheet
The fuel distribution for the twocases
The fuel concentration with half the heat released with port fuel injection
Fuel distribution of at 50% heat released with mixing tank
LOCAL POSITIVE FEEDBACK IN TEMPERATURE
No concept of Flame propagationin HCCI
After studying numerous individual cycles it was concluded that the concept of flame propagation in HCCI could not be supported.
There will be a time lag between combustion starting point at different zones but new “hot-spots” show up randomly and the structures seen in the images are rather fixed i.e. do not move from image to image
Valve timing diagram for S.I engine
CYLINDER PRESSURE AS A FUNCTION OF CRANK ANGLE IN HCCI ENGINE
COMBUSTION CONTROLL IN HCCI
The HCCI engine require closed loop combustion control, CLCC. Closed loop control requires as always a sensor, control algorithm and control means.
The main parameter to control for HCCI is the combustion timing i.e. when in the cycle combustion
The rate of heat release for a range of timings
SIGNAL OBTAINED IN HCCI MODE
ADVANTAGES OF HCCI ENGINE
High compression ratio is used compression is fast, this gives high efficiency at low and part load as compared to S.I that has low efficiency at part load
Fuel consumption is very low. As very lean mixture is inducted into the cylinder, fuel economy is high
It produces very low amounts of NOR. the formation of NO strongly dependent on the combustion temperature
: It does not produce same levels of soot as the diesel engine because the charge is very well mixed and does not contain excess hid
The control combustion is more difficult in HCCI engines than in spark ignition or compression ignition engines.
The second problem with HCCI engines revolves around the factor that there is no triggering event, like a spark in SI or fuel injection in CI.
The fuel air mixture has to be lean to obtain the emission benefits, so HCCI is suitable for light and medium loads and speeds. To handle higher loads and speeds, more fuels need to be added to the mixture, but doing this would raise the combustion temperature and eliminates much of the environmental benefits
The Homogeneous Charge Compression Ignition, HCCI, combustion process is an interesting alternative to the conventional Spark Ignition and Compression Ignition processes.
The HCCI engine requires active control of the combustion process. Use of negative overlap is possible but often generates a limited operating range.
Faced with increasingly stringent government pollution standards as well as the realization that practical and affordably fuel cell technology is still many years’ away, researchers at the world’s major automakers and diesel engine manufacturers are working to determine whether HCCI technology will be technically and economically feasible.
. If so power plants based on this new combustion mode would serve as a potential bridge technology between today’s high emission diesel and gasoline fuelled piston engines and tomorrow’s ultra clean fuel cells
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Joseph Simon Clark
Homogeneous Charge Compression Ignition _HCCI_ Engines.ppt (Size: 1.19 MB / Downloads: 192)
Homogeneous Charge Compression Ignition (HCCI) Engines
What is an HCCI Engine?
• HCCI is a form of internal combustion in which the fuel and air are compressed to the point of auto ignition.
• That means no spark is required to ignite the fuel/air mixture
• Creates the same amount of power as a traditional engine, but uses less fuel.
How Does It Work?
• A given concentration of fuel and air will spontaneously ignite when it reaches its auto-ignition temperature.
• The concentration/temperature can be controlled several ways:
– High compression ratio
– Preheating of induction gases
– Forced induction
– Retaining or reintroducing exhaust gases
• Can achieve up to 15% fuel savings
• Lower peak temperature leads to cleaner combustion/lower emissions
• Can use gasoline, diesel, or most alternative fuels
• Higher cylinder peak pressures may damage the engine
• Auto-ignition is difficult to control
• HCCI Engines have a smaller power range
The Future of HCCI
• The future of HCCI looks promising
• Major companies such as GM, Mercedes-Benz, Honda, and Volkswagen have invested in HCCI research.
• Preliminary prototype figures show that HCCI cars can achieve in the area of 43 mpg
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