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.
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