homogeneous charge compression ignition (hcci) engine · the homogeneous charge compression...

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Int. J. Mech. Eng. & Rob. Res. 2014 Anku Kumar Singh and M K Paswan, 2014

HOMOGENEOUS CHARGE COMPRESSIONIGNITION (HCCI) ENGINE

Anku Kumar Singh1* and M K Paswan1

*Corresponding Author: Anku Kumar Singh,[email protected]

Homogeneous Charge Compression Ignition (HCCI) is a new combustion technology that maydevelop as an alternative to diesel engines with high efficiency and low NOx and particulatematter emissions. HCCI engines can operate on gasoline, diesel fuel and most alternative fuels.The Homogenous Charge Compression Ignition (HCCI) is a promising new engine technologythat combines elements of the diesel and gasoline engine operating cycles. Like an SI engine,the charge is well mixed which minimizes particulate emissions, and like a CIDI engine it iscompression ignited and has no throttling losses, which leads to high efficiency. However, unlikeeither of these conventional engines, combustion occurs simultaneously throughout the cylindervolume rather than in a flame front. With the advantages there are some mechanical limitationsto the operation of the HCCI engine. The main drawback of HCCI is the absence of directcombustion timing control. This paper reviews the technology involved in HCCI engine, and itsmerits and demerits. The challenges encountered and recent developments in HCCI engine arealso discussed in this paper.

Keywords: HCCI, Ignition timing, Valve timing, Exhaust gas recirculation, Emission

INTRODUCTIONThe internal combustion engine is the key tothe modern society. Without the transportationperformed by the millions of vehicles on roadand at sea we would not have reached theliving standard of today. Internal CombustionEngines (ICEs) play a dominant role in theautomotive industry owing to their simplicity,robustness and high thermal efficiency. The

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Int. J. Mech. Eng. & Rob. Res. 2014

1 Department of Mechanical Engineering, National Institute of Technology, Jamshedpur 831014.

Internal Combustion (IC) Engine is perhapsthe most wide-spread apparatus fortransforming liquid and gaseous fuel to usefulmechanical work. The reason why it’s so wellaccepted can be explained by its overallappearance regarding properties likeperformance, economy, durability,controllability but also the lack of othercompetitive alternatives.

Research Paper

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There are two kinds of internal combustionengines: the Petrol and the Diesel. Thecombustion processes of them are verydifferent. In the Diesel engine the combustionis initiated because of some specialconditions of pressure and temperature.However, in the Petrol engine the combustionis caused by a spark that ignites a mixture thathas been premixed before.

Due to these different kinds of combustion,the two engines have different characteristics.The CI has a high efficiency, but it is verycontaminating. Contrarily, the SI is not veryefficient but it has low emissions.

The obvious ideal combination would be tofind an engine type with the high efficiency ofthe CI engine and the very low emissions ofthe SI engine with TWC. One such candidateis named Homogeneous ChargeCompression Ignition, HCCI.

With the advent of increasingly stringent fuelconsumption and emissions standards, enginemanufacturers face the challenging task ofdelivering conventional vehicles that abide bythese regulations. HCCI combustion has thepotential to be highly efficient and to producelow emissions. HCCI engines can operate ongasoline, diesel fuel, and most alternative fuels.While HCCI has been demonstrated andknown for quite some time, only the recentadvent of electronic sensors and controls hasmade HCCI engines a potential practicalreality. HCCI represents the next major stepbeyond high efficiency CIDI and Spark-Ignition,Direct-Injection (SIDI) engines for use intransportation vehicles. In some regards,HCCI engines incorporate the best featuresof both Spark Ignition (SI) gasoline engines

and CIDI engines. Like an SI engine, thecharge is well mixed which minimizesparticulate emissions, and like a CIDI engineit is compression ignited and has no throttlinglosses, which leads to high efficiency.However, unlike either of these conventionalengines, combustion occurs simultaneouslythroughout the cylinder volume rather than in aflame front. HCCI engines have the potentialto be lower cost than CIDI engines becausethey would likely use a lower pressure fuel-injection system. The emission control systemsfor HCCI engines have the potential to be lesscostly and less dependent on scarce preciousmetals than either SI or CIDI engines.

HCCI is an idea whose time has come withnearly all of the parts and pieces of technologyand know-how in place to make a real go of it.

HOMOGENEOUS CHARGECOMPRESSION IGNITION(HCCI)The Homogeneous Charge CompressionIgnition (HCCI) engine is often described as ahybrid between the spark ignition engine andthe diesel engine. The blending of these twodesigns offers diesel-like high efficiencywithout the difficult—and expensive—to dealwith NOx and particulate matter emissions. Inits most basic form, it simply means that fuelis homogeneously (thoroughly and completely)mixed with air in the combustion chamber verysimilar to a regular spark ignited gasolineengine, but with a very high proportion of air tofuel, i.e., lean mixture. As the engine’s pistonreaches its highest point (top dead center) onthe compression stroke, the air/fuel mixtureauto-ignites from compression heat, much likea diesel engine. The result is the best of both

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worlds: low fuel usage and low emissions. Asin a diesel engine, the fuel is exposed tosufficiently high temperature for auto-ignitionto occur, but for HCCI a homogeneous fuel/airmixture is used. The homogeneous mixture iscreated in the intake system as in a SI engine,using a low pressure injection system or bydirect injection with very early injection timing.The Homogeneous Charge CompressionIgnition (HCCI) engine is a promisingalternative to the existing Spark Ignition (SI)engines and Compression Ignition (CI)engines. To limit the rate of combustion, muchdiluted mixtures have to be used.

Compared to the diesel engines the HCCIhas a nearly homogeneous charge and virtuallyno problems with soot and NOX formation. Onthe other hand HC and CO levels are higherthan in conventional SI engines. Overall, theHCCI engine shows high efficiency and feweremissions than conventional internalcombustion engines.

Homogeneous Charge CompressionIgnition (HCCI) uses a lean premixed air-fuelmixture that is compressed with a highcompression ratio. During the end of thecompression stroke, ignition occurs throughself-ignition in the whole combustion chamberat once.

Engine efficiency rises with increases in theleanness of the premixed gas (the gascomposed of premixed fuel and air), in thecombustion rate, and in the compression ratio.In the HCCI engine, ultra-lean premixed gas ishighly compressed by the piston and ignitedat the self-ignition temperature. Then,combustion occurs in the entire combustionchamber, and the combustion rate is very high.As a result of ultra-lean combustion with a highcompression ratio, high thermal efficiency andextremely low NOx emission are achieved ascompared to the conventional spark ignitionengines. Figure 2 shows photos of the sparkignition and HCCI combustion processes

Figure 1: HCCI (as Most-Typically Envisioned) Would Use Low-Pressure Fuel InjectionOutside the Cylinder, and No Ignition System. If Charge Stratification is Desired, it May

be Necessary to Use in Cylinder Injection

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using engines affording visualization. In thisfigure, the portions shining white indicatecombustion. It can be seen that, unlike thespark ignition combustion, the HCCIcombustion occurs rapidly and throughout thecombustion chamber.

Figure 3 shows the difference between (a)SI combustion, and (b) HCCI. In the SI enginewe have three zones, a burnt zone, and anunburned zone and between them a thinreaction zone where the chemistry takes place.This reaction zone propagates through thecombustion chamber and thus we have flame

propagation. Even though the reactions arefast in the reaction zone, the combustionprocess will take some time as the zone mustpropagate from spark plug (zero mass) to thefar liner wall (mass wi). With the HCCI processthe entire mass in the cylinder will react at once.The right part of Figure 3 shows HCCI, or asOnishi called it Active Thermo-AtmosphereCombustion, ATAC. We see that the entiremass is active but the reaction rate is low bothlocally and globally. This means that thecombustion process will take some time evenif all the charge is active. The total amount ofheat released, Q, will be the same for both

Figure 2: Comparison of HCCI (Upper Row) and Spark Ignition (Lower Row) CombustionProcesses

Figure 3: The Difference Between SI and HCCI Combustion Process, Q = Total Amountof Heat, q = Heat per Mass Unit, w = Mass

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processes. It could be noted that thecombustion process can have the sameduration even though HCCI normally has afaster burn rate.

Since the mixture is lean, the maximumtemperature, both locally and overall, becomeslow compared to other engines, whicheffectively reduces NOx formation. However,at richer mixtures the combustion becomes toofast and knocking, or ringing, occurs. Therefore,if a higher load is desired, supercharging, orturbo charging is necessary. The load limit(without supercharging) is said to be either theengine structure capabilities (knocking limit)or NOx emissions. The problem with the HCCIengines is related to the lean mixtures, the fastcombustion, and the high compression ratio(high engine efficiency) that causes theexhaust temperature to become quite low. Thiscan make it difficult to get both turbo chargingand oxidizing catalysts to work.

Despite advantages, HCCI enginesproduce high HC and CO emissions as theignition timing and combustion duration isdifficult to control. Therefore, the HCCIoperating zone is limited between misfire andknocking. Lack of direct control over ignitioninitiation is one of the obstacles that need tobe addressed.

In self-ignition combustion, the ignitiontiming varies with the temperature of the intakeair, cooling water, and lubricating oil. Becausethe efficiency changes along with this variation,the ignition timing must be precisely controlled.Studies on HCCI engines have revealed thatthe load range over which HCCI engines canoperate is limited by the risk of misfire,combustion instability, and engine knock.Furthermore, the higher pressure of gas within

cylinders due to the higher compression ratioand more rapid combustion creates a needfor engines with a higher withstand pressure.

REQUIREMENTS FOR HCCIThe HCCI combustion process puts two majorrequirements on the conditions in the cylinder:

• The temperature after compression strokeshould equal the auto-ignition temperatureof the fuel/air mixture.

• The mixture should be diluted enough to givereasonable burn rate.

Figure 4 shows the auto-ignitiontemperature for a few fuels as a function of .The auto-ignition temperature has somecorrelation with the fuels’ resistance of knockin SI engines and thus the octane number. Foriso-octane, the auto-ignition temperature isroughly 1000 K. This means that thetemperature in the cylinder should be 1000 Kat the end of the compression stroke wherethe reactions should start. This temperaturecan be reached in two ways, either thetemperature in the cylinder at the start of

Figure 4: Ignition Temperature for a FewFuels as a Function of Dilution

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compression is controlled or the increase intemperature due to compression, i.e.,compression ratio is controlled. It could beinteresting to note that the auto-ignitiontemperature is a very weak function of air/fuelratio. The change in auto-ignition temperaturefor iso-octane is only 50 K with a factor 2change in . Figure 4 also shows the normalrich and lean limits found with HCCI. With atoo rich mixture the reactivity of the charge istoo high. This means that the burn ratebecomes extremely high with richer mixtures.If an HCCI engine is run too rich the entirecharge can be consumed within a fraction of acrank angle. This gives rise to extremepressure rise rates and hence mechanicalstress and noise. With a high auto-ignitiontemperature like that of natural gas, it is alsopossible that formation of NOx can be the loadlimiting factor.

Figure 5 shows the NO formation as afunction of maximum temperature. Very lowemission levels are measured with ethanol. Ifthe combustion starts at a higher temperature

like with natural gas, the temperature aftercombustion will also be higher for a givenamount of heat released. On the lean side, thetemperature increase from the combustion istoo low to have complete combustion. Partialoxidation of fuel to CO can occur at extremelylean mixtures; above 14 has been tested.However, the oxidation of CO to CO2 requiresa temperature of 1400-1500 K. As a summary,HCCI is governed by three temperatures. Weneed to reach the auto-ignition temperature toget things started; the combustion should thenincrease the temperature to at least 1400 K tohave good combustion efficiency but it shouldnot be increased to more than 1800 K toprevent NO formation.

EMISSIONS BEHAVIOURThe main motivation for studying HCCI is itspotential for significant reductions in exhaustemissions in comparison to conventionalcompression or spark-ignition combustion. Abrief overview of the emissionscharacteristics from HCCI engines isprovided in this section.

Oxides of Nitrogen (NOx)Perhaps the single largest attraction of HCCIcombustion is that it can reduce NOxemissions by 90-98% in comparison toconventional Diesel combustion (Gray & Ryan1997; Hashizume et al 1998). The underlyingmechanism responsible for this reduction inNOx emissions is the absence of hightemperature regions within the combustionchamber. HCCI combustions occur at theglobal air-fuel ratio, which is typically quite lean,and at a temperature significantly below thoseencountered within the reaction zone in Dieselor Spark-ignition engines.

Figure 5: NOx as a Function MaximumTemperature Evaluated from the

Pressure-Trace

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Figure 6 shows the predicted NOxemissions from HCCI combustion of Dieselfuel compared to a direct-injected Dieselengine (DI Diesel) and DI-Diesel withaggressive EGR levels. The compression ratioin all cases was 16:1.

Hydrocarbons (HC) and CarbonMonoxide (CO)In contrast to NOx and PM emissions, HCCIcombustion typically results in higher HC andCO emissions than conventional Dieselcombustion (Suzuki et al., 1997; andChristensen and Johamsson, 1998). Onefactor that contributes to these observed levelsof HC and CO emissions is the low in-cylindertemperature due to the lean mixtures and/orhigh levels of EGR which are necessary forsatisfactory HCCI operation. It is well knownthat reduced burned gas temperatures leadto decreased post-combustion oxidation rateswithin the cylinder (Roberts and Matthews,1996) and increased levels of HC and CO inthe exhaust. Mixture preparation is of greatimportance to HC emissions for HCCIcombustion of liquid fuels; for it is well knownthat liquid fuel deposition on combustionchamber surfaces can result in dramaticincreases in HC emissions (Stangimater et al.,1999). This problem is exacerbated for heavyfuels such as Diesel.

EFFICIENCYHCCI combustion is generally characterizedby high heat-release rates, which canapproximate the ideal Otto cycle whenproperly phased in relation to the engine cycle.The distributed low-temperature reactions andnon-luminous combustion result in reducedheat rejection to the engine. Hence HCCIcombustion is, in itself, conducive to highthermodynamic cycle efficiencies. HCCI fueleff iciencies comparable to those ofconventional Diesel combustion at part loadhave been reported by several researchers(Aoyama et al., 1996; Suzuki et al., 1997; andChristensen and Johamsson, 1998).

Figure 6: Predicted NOx Emissions vs.Engine Load for Typical HCCI

and DI Diesel Combustion

These results show that HCCI combustioncan result in large NOx reductions at partengine load, but that the potential NOxadvantage of HCCI combustion vs. DI-Dieseldiminishes at higher equivalence ratios.

Particulate MatterHCCI combustion has also been reported toproduce low levels of smoke and PMemissions (Suzuki et al., 1997; and Maseet al., 1998). The mechanisms for these smokereductions are not as well documented but itis thought that the absence of diffusion-limitedcombustion and localized fuel-rich regionsdiscourages the formation of soot. Oneexception to this can occur when poor mixturepreparation leads to liquid fuel deposition onthe combustion chamber and localized fuel-rich regions of combustion.

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CHALLENGESHCCI combustion is achieved by controllingthe temperature, pressure and composition ofthe air/fuel mixture so that it auto ignites nearTop Dead Centre (TDC) as it is compressedby the piston. This mode of ignition isfundamentally more challenging than using adirect control mechanism such as a spark plugor fuel injector to dictate ignition timing as inSI and CIDI engines, respectively. While HCCIhas been known for some twenty years, it isonly with the recent advent of electronic enginecontrols that HCCI combustion can beconsidered for application to commercialengines.

There are a number of obstacles that mustbe overcome before the potential benefits ofHCCI combustion can be fully realized inproduction applications. This sectiondescribes the main difficulties with thistechnology.

Controlling Ignition Timing Over aRange of Speeds and LoadsExpanding the controlled operation of an HCCIengine over a wide range of speeds and loadsis probably the most difficult hurdle facingHCCI engines. Unlike in spark-ignition orconventional diesel engines, a direct methodfor controlling the start of combustion is notavailable. HCCI ignition is determined by thecharge mixture composition and itstemperature history (and to a lesser extent, itspressure history). Changing the power outputof an HCCI engine requires a change in thefuelling rate and, hence, the charge mixture.As a result, the temperature history must beadjusted to maintain proper combustion timing.Similarly, changing the engine speed changes

the amount of time for the auto ignitionchemistry to occur relative to the piston motion.Again, the temperature history of the mixturemust be adjusted to compensate. Thesecontrol issues become particularly challengingduring rapid transients.

Hence, combustion phasing of HCCIengines is affected by:

• Auto-ignition properties of fuel

• Fuel concentration

• Compression Ratio

• Heat transfer to the engine

• Intake temperature, Engine temperatureand latent heat of vaporization of fuel

• Mixture homogeneity

Several approaches for controlling thecombustion phasing have been attempted, buta fundamental distinction can be madebetween those methods attempting to controlht time-temperature history to which themixture is exposed, and methods aimed ataltering the propensity for auto-ignition of themixture as illustrated in Figure 7.

Figure 7: Methods for Controlling HCCICombustion Phasing

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system can act as a more direct methodof EGR by controlling the amount oftrapped residual gases thus allowingtemperature and mixture control. As VVTallow the variation of compression ratio notthrough geometric means but throughtiming of valve opening hence by changingthe timing of the intake/exhaust valvechanges the amount of combustible air inthe cylinder thus controlling combustionstrength and timing. This could be used tochange the cylinder performancesindividually if a good control method isfound. It should be noted that typical VVTschemes run cylinders with set timing,whereas these could need flexibility notonly in timing but per cylinder.

• Fuel Mixtures and Additives: By using twofuels with varying combustion properties,combustion timing could be improved overa wide operating range. However a two fuelsystem brings about practical andinfrastructural issues that could preventcommercialization. Experiments haveshown that ozone as a fuel additive cangreatly improve ignition even at very lowconcentrations add some more effort we willfocus on three dimensional simulations ofHCCI engine analysis and effects on fuelperformance at part load and full loadconditions. Mixing an additive into the fuelcould change the combustioncharacteristics of a cylinder. Mixing acombustion retardant such as water wouldcause a delay in combustion, while mixinga combustion accelerant such as hydrogengas would increase the speed ofcombustion. This could be controlled foreach cylinder individually.

• Exhaust Gas Recirculation (EGR): It is theprocess of recycling exhaust gases andadding them to the intake air. With EGR itis possible to control temperature, mixture,pressure, and composition. In comparisonto the other control methods, EGR isrelatively simple, which is a great benefit.EGR can produce more power in an enginebecause more fuel could be pumped intothe cylinder without spontaneous ignitiondue to the relative inertness of theemissions gas compared to air. It also couldbe used to control individual cylinderperformance.

• Variable Compression Ratio (VCR): Thiscould be achieved through a couple ofdifferent methods. One method would be toplace a plunger within the cylinder head thatcould vary the compression ratio. Anotheroption would be to have an opposed-pistondesign which would include variable phaseshifting between the two crankshafts. Otherpossibilities exist as well but the key is todevelop these in order to have excellentresponse time to handle transient situations.In order to study the Variable CompressionRatio (VCR) effect on the engineperformance we could change the amountof compression for each cylinder and canstudy the effect. This could change theengine characteristics. By incorporating adevice that could change cylinder volumerapidly, individual control of each cylindercould be conceivably achieved

• Variable Valve Timing (VVT): VVT allowsvariation of the compression ratio notthrough geometric means but throughtiming of the opening and closing of theintake and exhaust valves. In addition, this

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Some of the most promising include varyingthe amount of hot exhaust gas recirculationintroduced into the incoming charge, using aVCR mechanism to alter TDC temperatures,and using VVT to change the effectivecompression ratio and/or the amount of hotresidual retained in the cylinder. VCR and VVTare particularly attractive because their timeresponse could be made sufficiently fast tohandle rapid transients. Although thesetechniques have shown strong potential, theyare not yet fully proven, and cost and reliabilityissues must be addressed.

Power OutputA current drawback of HCCI combustion is thatit is presently limited in power output. AlthoughHCCI engines have been demonstrated tooperate well at low-to-medium loads,difficulties have been encountered at high-loads. Combustion can become very rapid andintense, causing unacceptable noise, potentialengine damage, and eventually unacceptablelevels of NOx emissions (Christensen et al.,1997; and Gray and Ryan, 1997). Fuels withinherently lower heat release rates likemethane, can be combusted at lower A/F ratiosand achieve higher specific engine outputs(Christensen et al., 1997).

Given this apparent limitation in A/F ratiofor HCCI combustion, power increases can beobtained by augmenting the air flow throughthe engine. Supercharging has been provento be effective in this respect, and it also has abeneficial influence on reducing the heatrelease rate (Christensen et al., 1998).Increasing the engine speed may also be aneffective method of increasing HCCI poweroutput but this approach is not welldocumented.

Another approach to overcome thelimitations in power output has been to pursuethe development of “dual-mode” engines thatemploy HCCI combustion at low loads andDiesel combustion or spark-ignition at highloads.

Cold-Start CapabilityAt cold start, the compressed-gas temperaturein an HCCI engine will be reduced becausethe charge receives no preheating from theintake manifold and the compressed chargeis rapidly cooled by heat transferred to the coldcombustion chamber walls. Without somecompensating mechanism, the lowcompressed-charge temperatures couldprevent an HCCI engine from firing. Variousmechanisms for cold-starting in HCCI modehave been proposed, such as using glowplugs, using a different fuel or fuel additive, andincreasing the compression ratio using VCRor VVT.

Perhaps the most practical approach wouldbe to start the engine in spark-ignition modeand transition to HCCI mode after warm-up.For engines equipped with VVT, it may bepossible to make this warm-up period as shortas a few fired cycles, since high levels of hotresidual gases could be retained fromprevious spark-ignited cycles to induce HCCIcombustion. Although solutions appearfeasible, significant R&D will be required toadvance these concepts and prepare them forproduction engines.

Hydrocarbon and CarbonMonoxide EmissionsHCCI engines have inherently low emissionsof NOx and PM, but relatively high emissionsof hydrocarbons (HC) and carbon monoxide

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(CO). Some potential exists to mitigate theseemissions at light load by using direct in-cylinder fuel injection to achieve appropriatepartial-charge stratification. However, in mostcases, controlling HC and CO emissions fromHCCI engines will require exhaust emissioncontrol devices. Catalyst technology for HCand CO removal is well understood and hasbeen standard equipment on automobiles formany years. However, the cooler exhausttemperatures of HCCI engines may increasecatalyst light-off time and decrease averageeffectiveness. As a result, meeting futureemission standards for HC and CO will likelyrequire further development of oxidationcatalysts for low-temperature exhaust streams.However, HC and CO emission control devicesare simpler, more durable, and less dependenton scarce, expensive precious metals than areNOx and PM emission control devices. Thus,simultaneous chemical oxidation of HC andCO (in an HCCI engine) is much easier thansimultaneous chemical reduction of NOx andoxidation of PM (in a CIDI engine).

RECENT DEVELOPMENTS INHCCIRecent developments in the HCCI technologyhave given very positive results to overcomethe limitations of this technology. Thetechnology has huge scope of use and it isused in a wide range of industries, whichmakes it a promising technology for the cominggenerations. Automobile giants like GM, Fordand Cummins have been exploring thepossibilities in the HCCI technology for morethan 15 years. General Motors has startededucational programs in various Universitiesto promote the research work in thistechnology. HCCI has also enabled engineers

to experiment with different blends of fuelmixture so that performance and efficiency ofHCCI engines can be tested with differentcombinations of non conventional fuels. As thedemand of conventional fuels is increasing,scope of research and experimentation inHCCI technology will increase only with time.

Of late, many companies have launchedHCCI based automobiles in the market amongwhich Mercedes F700, Volkswagen Touran,Opel Vectra, and Saturn Aura the prominentnames.

CONCLUSIONThe Homogeneous Charge CompressionIgnition, HCCI, combustion process is aninteresting alternative to the conventional SparkIgnition and Compression Ignition processes.A high-efficiency, gasoline-fueled HCCI enginerepresents a major step beyond SIDI enginesfor light-duty vehicles. HCCI engines have thepotential to match or exceed the efficiency ofdiesel-fueled CIDI engines without the majorchallenge of NOx and PM emission control ora major impact on fuel-refining capability. Also,HCCI engines would probably cost less thanCIDI engines because HCCI engines wouldlikely use lower-pressure fuel-injectionequipment and the combustion characteristicsof HCCI would potentially enable the use ofemission control devices that depend less onscarce and expensive precious metals. Inaddition, for heavy-duty vehicles, successfuldevelopment of the diesel-fueled HCCI engineis an important alternative strategy in the eventthat CIDI engines cannot achieve future NOxand PM emissions standards.

HCCI engines are a promising technologythat can help reduce some of our energy

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problems in the near term. However, controlremains a challenge because HCCI enginesdo not have a direct means to control thecombustion timing. Two fundamentally differentapproaches to controlling HCCI combustionphasing are possible.

• Altering the mixture propensity for auto-ignition.

• Altering the time-temperature history towhich the mixture is exposed.

A viable method of controlling thecombustion phasing in production applicationshas not yet been identified.

REFERENCES1. Christensen M and Johansson B (2000),

“Supercharged Homogeneous ChargeCompression Ignition (HCCI) withExhaust Gas Recirculation and PilotFuel”, SAE Paper 2000-01-1835.

2. Christensen M, Hultqvist A and JohanssonB (1999), “Demonstrating the Multi-FuelCapability of Homogeneous ChargeCompression Ignition Engine withVariable Compression Ratio”, SAE Paper1999-01-3679.

3. Christensen M, Johansson B, Amn´eus Pand Mauss F (1998), “Supercharged

Homogeneous Charge CompressionIgnition”, SAE Paper 980787.

4. Gray III A W and Ryan III T W (1997),“Homogeneous Charge CompressionIgnition (HCCI) of Diesel Fuel”, SAEPaper 971676.

5. Hisakazu S, Koike N and Odaka M(1998), “Combustion Control Method ofHomogeneous Charge Diesel Engines”,SAE Paper 980509.

6. Johansson B (2007), “HomogeneousCharge Compression Ignition-the Futureof IC Engines?”.

7. Johansson B, Einewall P and ChristensenM (1997), “Homogeneous ChargeCompression Ignition (HCCI) UsingIsooctane, Ethanol and Natural Gas—AComparison with Spark-IgnitionOperation”, SAE Paper 972874.

8. Najt P M and Foster D E (1983),“Compression-Ignited HomogeneousCharge Combustion”, SAE Paper830264.

9. Stanglmaier R H and Roberts C E (1999),“Homogeneous Charge CompressionIgnition (HCCI): Benefits, Compromises,and Future Applications”, SAE Paper199-01-3682.

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