Regional Conference on Automotive Research (ReCAR) 2011 th th 14 -15 December 2011, Kuala Lumpur, Malaysia
ReCAR2011-P021
Homogenous Charge Compression Ignition (HCCI) Technique: A Review for Application in Two-Stroke Gasoline Engines Amin Mahmoudzadeh Andwaria and Azhar Abdul Azizb Automotive Development Center (ADC), Faculty of Mechanical Engineering Universiti Teknologi Malaysia (UTM), 81310, Johor Bahru, Malaysia a
[email protected] [email protected]
Keywords: Homogeneous Charge Compression Ignition (HCCI), Controlled Auto-Ignition (CAI), two-stroke engine, Active Thermo-Atmosphere Combustion (ATAC), emission.
Abstract. Since Homogeneous Charge Compression Ignition (HCCI) has significantly low temperature combustion, NOx will be dramatically reduced while the mixture will be largely homogenous, thus soot formation will naturally be reduced too. The system can be operated under an ultra lean fuel condition thus able to achieve high efficiency and low emission. In addition to, two-stroke engine’s advantages i.e. light, simple construction, less components and cheap to manufacture, two-stroke engines have the potential to pack almost twice the power density than that of four-stroke engine with similar capacity. The problem of poor combustion efficiency and high white smoke emission, which is caused by burnt engine oil, can be addressed by the incorporation some features that will ultimately convert a typical two-stroke engine into an efficient HCCI engine demonstrating bulk combustion. This paper briefly described an attempt to modify two-stroke engine design to prove the claim. Such an engine with its conversion will be suitable for use as a prime mover for series Hybrid Electrical Vehicle (HEV) giving high power-to-weight ratio and improved efficiency of the overall vehicle powertrain system. Introduction Energy conservation and environmental protection are exerting rigorous demand on internal combustion engines to further improve fuel economy and emission reduction. A new combustion concept, which is increasingly accepted as a solution to this global issue, is Homogeneous Charge Compression Ignition (HCCI) or better known as Controlled Auto-Ignition (CAI) combustion. In fact, the most recognized original work on HCCI/CAI by Onishi et al. [1] and Noguchi et al. [2] was motivated by their desire to control the irregular combustion caused by the auto-ignition of cylinder charge to obtain stable lean-burn combustion in the conventional ported two-stroke gasoline engine. HCCI/CAI combustion is radically different from the conventional spark ignition (SI) and compression ignition (CI) combustion. In such an engine, a homogeneous mixture of air, fuel and residual gases is compressed until auto-ignition occurs. In an ideal case, there will be no occurrence of either the propagating flame front that characterizes spark ignition (SI) combustion, or the diffusion burning that characterizes compression ignition (CI) combustion [3]. The combination of a diluted and premixed fuel and air mixture with multiple ignition sites throughout the combustion chamber eliminates the high combustion temperature zones and prevents the production of soot particles, hence producing ultra-low NOx and particulate emissions [4]. Indeed, it has been demonstrated that an HCCI/CAI gasoline engine can achieve fuel economy levels comparable to those of a diesel engine, while producing engine-out NOx emissions that are as low as tail-pipe NOx emissions from a conventional SI engine equipped with a three-way catalyst [5]. The studies for hybrid electrical vehicle (HEV) have attracted considerable attention solution as they match good performances in terms of both consumption and reliability. The concept of utilizing two-stroke CAI combustion as an IC engine powertrain on series hybrid vehicles will be one of the promising option in the future. HCCI/CAI combustion abilities offer the potential to meet current and future emissions legislation, without the need for expensive, complex and inefficient exhaust gas after-treatment systems.
Regional Confe ference on Automotive Researcch (ReCAR) 201 11 th th 14 -15 Decem mber 2011, Kua ala Lumpur, Mallaysia
ReCAR2011-P021
Principle and a combu ustion charaacteristics of o HCCI/C CAI combusstion engine the fuel and airr are mixedd Similar to a convenntional SI engine e conccept, in an HCCI/CAI H together eiither in the intake systeem or in thhe cylinder with w direct injection. T The premix xed fuel andd air mixturee is then com mpressed. Towards T thee end of thee compressiion stroke, ccombustion n is initiatedd by auto-ignnition in a similar s way to the convventional CII engine [1]. The tempeerature of th he charge att the beginniing of the compression c n stroke hass to be increeased to reaach auto-ignnition condiitions at thee end of the compressioon stroke [66]. This can be done by y heating thhe intake airr or by keep ping part off the hot com mbustion prroducts in the t cylinderr [7]. Both strategies result r in a hhigher gas temperature t e throughoutt the compression proccess, which in turn speeeds up the chemical c reactions thatt lead to thee start of com mbustion off homogeneeously mixeed fuel and air mixtures [7]. The ccontribution n of the low w temperaturre energy release to obtaining o auuto-ignition n and heat release r ratee from the HCCI/CAII combustionn depends not only onn the uniquue chemicaal kinetics of o the fuel used and the t dilutionn strategy, buut also on thhe thermal conditions c o the tempeerature-presssure historyy that the mixture or m goess through duuring comprression [1]. The heat reelease charaacteristics of o the HCC CI/CAI comb bustion cann be comparred with thoose of SI combustion c using Fig. 1(a). In thhe case of SI combusttion, a thinn reaction zoone or flamee front sepaarates the cyylinder charrge into burnned and unbburned regions and thee heat release is confineed to the reaaction zone. The cumullative heat released r in a SI engine is thereforee he reaction zone and it can be exprressed as the sum off the heat relleased by a certain masss, dmi, in th N
Q = ∫ q.dmi 1
(1)
where q is the heatingg value per unit u mass off fuel and aiir mixture, N is the num mber of reacction zones
Figure 1: (a) Heat reelease charaacteristics off SI, CAI/H HCCI combuustion, (b) A ATAC/SI co ombustion regioons accordinng to engine conditions [1]. In an iddealized HC CCI/CAI com mbustion process, com mbustion reaactions takee place simultaneouslyy in the cyliinder and all a the mixtture particippates in thee heat releaase processs at any insstant of thee combustionn process. The T cumulaative heat reelease in succh an enginne is therefoore the sum of the heatt released froom each com mbustion reeaction, dqi, of the com mplete mixtuure in the cyylinder, m, i.e. i K
Q = ∫ m.dqi 1
(2))
Where K is the total number n of heat h releasee reactions, and qi is thhe heat releeased from the ith heatt f and air mixture. Whereas W the entire heatiing value off release reaction involvving per unit mass of fuel each minutte parcel off mixture muust be releaased during the finite duuration spennd in the reaction zonee in a SI enggine, heat reelease takes place unifoormly acrosss the entire charge in aan idealized HCCI/CAII combustionn. Howeveer, in practtice, due to t inhomog geneities inn the mixtture compo osition andd temperaturre distributions in a reaal engine, the t heat releease processs will not bbe uniform throughoutt the mixturre [1]. Fasster heat reelease can take placee in the leess diluted mixture and/or a highh temperaturre region, reesulting in a non-uniforrm heat releease patternn as indicateed by the daashed lines.. Fig. 1(b) shows the relationship between the averag ge charge temperature t e at the beeginning off
Regional Conference on Automotive Research (ReCAR) 2011 th th 14 -15 December 2011, Kuala Lumpur, Malaysia
ReCAR2011-P021
compression and the quantity of fresh charge and residual gas [3]. To get stable Active ThermoAtmosphere Combustion (ATAC), which is another name of CAI combustion, high average charge temperature is necessary for auto-ignition. At very light load, the average charge temperature drops due to the low amount of energy released per engine cycle and becomes too low to produce stable ATAC. As the load increases, the charge temperature initially increases making ATAC possible. However, higher engine loads require more fresh charge, reducing the amount of residual gas and thus the average charge temperature. This decreases the likelihood of auto-ignition, and after a transition in which both SI and ATAC occur interchangeably, the average charge temperature becomes too low to maintain the ATAC combustion [7]. Advantages and Difficulties of HCCI/CAI Improved combustion stability. With SI combustion, a lot of fluctuations of the maximum incylinder pressure can be observed in correlation with corresponding IMEP fluctuations. This deterioration of the combustion comes from cycles with poor or incomplete combustion followed by cycles with higher IMEP [2]. On the other hand, in the case of CAI it is not clear where combustion commences because of the gradual change in combustion pressure. This smooth combustion with little cycle-to-cycle variation is advantageous towards improving thermal efficiency [1, 2, 4]. Improved pollutant emissions and fuel efficiency. Thanks to a remarkably stable cycle to cycle combustion can be achieved with, the incomplete or poor combustion disappear and the theoretical ideal specific fuel consumption (SFC). For the same reason, the HC emissions can be significantly improved [6]. The light load limit of CAI range (irregular combustion and misfiring). There is a low load limit for the CAI combustion. When the load becomes too low, the amount of energy released per combustion cycle is too low to maintain a sufficiently high level of temperature of the internal EGR [4, 7]. Below such loads, 2000 rpm for engine loads as low as 0.5 bar BMEP, it is difficult to maintain the CAI auto-ignition. Poor combustion and misfiring will then start to occur. The high load limit (too strong uncontrolled combustion). When the engine load increases in CAI operation, the combustion starts progressively earlier and earlier. Even if the engine can continue to run in auto-ignition, the heat release rate becomes very high with more than 50% of the mass fraction burnt before TDC [1-4, 6, 7]. In these conditions, the combustion noise becomes very strong like severe knock and the NOx emissions quickly rise. There is no interest in keeping the auto-ignition mode (which is no longer ‘controlled’ as the CAI ‘controlled auto-ignition’ name would suggest) and it is generally better to switch to the SI combustion mode. The relevant control parameters of the two-stroke CAI combustion It is important to define what has to be controlled: First, the combustion timing and second, the combustion heat release rate. Internal EGR and mixing/stratification between fresh charge/EGR. Inherently in a twostroke engine, there is a high amount of internal EGR at part load. But if this EGR is well mixed with the fresh charge as is the case in a conventional two-stroke engine, especially at low engine speed, it has almost no effect on the combustion. What is efficient for getting CAI is to limit as much as possible the mixing of this internal EGR with the fresh mixture [3, 4, 6]. In such cases, it is possible to achieve a temperature gradient within the charge for the same overall amount of incylinder EGR, which is to say, the same in-cylinder heat content. The overall charge is then composed of zones of higher fresh charge concentrations/lower gas temperature and zones with higher internal EGR charge concentrations/higher gas temperatures. These zones or pockets of both high temperature and more reactive gases are responsible for initiating and generating the autoignition process. Fig. 2(a) presents the in-cylinder temperature fields obtained by 3-D computation in SI and CAI modes [3].
Regional Confe ference on Automotive Researcch (ReCAR) 201 11 th th 14 -15 Decem mber 2011, Kua ala Lumpur, Mallaysia
ReCAR2011-P021
Fiigure 2: (a) Comparisoon of in-cyliinder temperature fieldss at 10 deg. CA BTDC C in SI and CA AI combustion, (b) Effeect of the enngine speed on the heatt release (inn millisecond ds) in CAIAR R combustioon conditionns, (c) Effecct of the air//fuel ratio on the CAI ccombustion timing and duration [4, 5]. An ind direct effecct of the engine e speed. The en ngine speedd has an iindirect efffect on thee mixing/straatification between b thee fresh chaarge and th he EGR. As shown byy Fig. 2(b)), when thee engine speeed increasees, the CAI combustion starts earrlier and earrlier and also becomess faster andd faster. A coonsistent exxplanation for f this phennomenon is to considerr that the tim me for mixing betweenn the internaal EGR andd the fresh charge is shorter and d shorter when w the enngine speed d increases.. Therefore the t internall stratification and the temperaturee gradient inside i the trrapped charrge increasee when the engine e speed increases [4]. This has h the finall consequennce of advanncing and accelerating a g the CAI coombustion. A signifficant effecct of the airr/fuel ratio. As shown by Fig. 2(cc), the exhauust air/fuel ratio variess from 15.9 to t 24.1. Thee figure shoows that whhen the mixtture is ratheer rich, the auto-ignitio on starts tooo early and 95% 9 of the charge burrns before TDC T in abo out 20 degrrees CA. Ass the mixture becomess leaner, thee air dilutioon increasess which prooportionally y decreasess the dilutioon by the hot h internall EGR. It exxplains whyy, as shown by the figuure, the startt of combusstion is proggressively delayed d andd the combusstion becom mes smootheer with a lonnger duratio on and moree ideal phassing [3]. The efffect of the overall temperaturee (intake ch harge, cooolant). Heatting the inttake chargee increases the t overall gas temperature and has h the effect of advanncing the C CAI combusstion timingg and therefoore of extennding the CA AI combusttion range in i the low loads low sppeed region n. Similarly,, the CAI combustion is also sennsitive to thhe engine liquid l cooliing temperaature which h indirectlyy affects the overall gas temperaturre [4]. ntrol devicees for the control c of tw wo-stroke CAI C combu ustion Technologgies and con Most off the technoologies thatt have beenn developed d and applied to obtaiin CAI com mbustion onn two-stroke engines weere based onn the controol of these tw wo internal temperature and pressu ure effects. Elongatted transfeer duct. A very long transfer t ducct can be seeen in Fig. 3(a) startin ng from thee bottom of the t crankcaase and reachhing the traansfer port after a a duct length probbably around d five timess longer thann in a convventional tw wo-stroke engine. e Thiss technologgy is very eefficient to reduce thee velocity off the fresh charge introoduction. Itts smooth delivery d minnimizes intternal mixin ng and thenn allows ATA AC combusstion. The main m drawback of this solution is that t the trannsfer duct leength is nott variable [1]. Transfeer duct throottling. Thee interest off the transfeer throttlingg solution iss its flexibillity and fastt response tiime. It allow ws the samee engine to be able to run r in part load l and alsso in high lo oad SI. Fig.. 3(b) showss one exam mple proposeed by an Onnishi patentt. The mainn drawback of all the solutions s off transfer thrrottling in a loop scavenged enginee is the spacce necessaryy around the cylinder [1, [ 3, 5]. Exhaust port throottling. A sppecial exhauust throttling g valve alloowing simulltaneous varrying of thee exhaust poort timing annd the exhaaust port oppening area. This special exhaust tthrottling valve namedd ARC (for Activated A R Radicals Combustion) exhaust e con ntrol valve is i shown inn Fig. 3(c). It I is used too
Regional Confe ference on Automotive Researcch (ReCAR) 201 11 th th 14 -15 Decem mber 2011, Kua ala Lumpur, Mallaysia
ReCAR2011-P021
control botth the internnal EGR straatification and a the in-cylinder presssure at exhhaust port cllosing [3, 4,, 7].
Figure 3: Three T techniiques for coontrol of CA AI Combustion in two-sstroke enginne which arre elongatedd transfeer duct, trannsfer port thrrottling arraangement an nd exhaust valve v controolling respeectively [1, 3, 4]. Summary HCCI/CAII combustioon has emerrged as an effective e an nd viable tecchnology thhat has the potential p off simultaneoously reduccing pollutaant emissioons and fueel consumpptions from m internal combustion c n engines. It has been shown s that CAI combuustion can be b achievedd in a two-sstroke engin ne which iss largely due to its inhherent highh amount of o residual gases with applying some techn niques. Thee CAI/HCCII operationaal range needs to bee enlarged and the reeal-time cloosed-loop control c andd switching between b SI and CAI combustion c are also neecessary. Inn the mean w while, CAI two-strokee engines cann be suggessted as an innput power source for series HEV V. In the casse of series HEV, H sincee IC engine is used jusst as an inpput of the generator (it is not coonnected too drivetrain)), it can bee operated inn a preset looad and speeed. In otheer word, thee engine is operated onnly on part load whichh has been most m promiising and effective appplication off CAI combbustion. Siggnificant reesearch andd developmeent efforts are a now beeing underttaken to ex xplore HCC CI/CAI com mbustion en ngine to bee adopted forr automotivve applicatioons. References [1] S. Onishi, et al.,, "Active Thermo-Atm T mosphere Combustion C (ATAC) - A New Combustion C n Processs for Internnal Combusstion Enginees," 1979. [2] M. Nooguchi, et al., "A Studyy on Gasoliine Engine Combustion by Obserrvation of In ntermediatee Reactiive Products during Coombustion," 1979. [3] P. Duuret and S.. p. Ventuuri, "Autom motive Calib bration of the IAPAC Fluid Dynamically D y Controolled Two-S Stroke Com mbustion Proocess," 1996 6. [4] Y. Ø. Ishibashi, "Basic Undderstanding of Activatted Radical Combustioon and Its Two-Stroke T e Engine Applicatioon and Benefits," 20000. [5] J. Lavvy, et al., "Towards a Better Understandi U ing of Conntrolled Auuto-Ignition n (CAI‚Ñ¢)) Combustion Proccess From 2-Stroke Enggine Resultss Analyses,"" 2001. a Combusstion Completion of n-Butane in a [6] T. Takkei and N. Iida, "17 Stuudy on Autoo-Ignition and Two-sstroke Homoogeneous Charge C Com mpression Ig gnition (HCC CI) Engine,," 2002. [7] N. Iidaa, et al., "Sttudy on Autto-Ignition and a Combustion Mechanism of HC CCI Enginee," 2004.
