Effects of Variable Valve Lift on In-Cylinder Air Motion

Transcription

1 Article Effects of Variable Valve Lift on In-Cylinder Air Motion Tianyou Wang 1, Daming Liu 2, Gangde Wang 1, Bingqian Tan 1 Zhijun Peng 1,3, * Received: 10 September 2015; Accepted: 11 November 2015; Published: 4 December 2015 Academic Editor: Paul Stewart 1 Stake Key Lab of Engines, Tianjin University, Tianjin , China; wangtianyou@tju.edu.cn (T.W.); wanggangde@tju.edu.cn (G.W.); tanbingqian@tju.edu.cn (B.T.) 2 School of Automotive Transportation, Tianjin University of Technology Education, Tianjin , China; ldam@tju.edu.cn 3 School of Engineering Technology, University of Hertfordshire, Hatfield AL10 9AB, UK * Correspondence: pengzj@tju.edu.cn; Tel.: Abstract: An investigation into in-cylinder swirl tumble flow characteristics with reduced maximum valve lifts (MVL) is presented. The experimental work conducted in modified four-valve optical spark-ignition (SI) test engine with three different MVL. Particle image velocimetry (PIV) employed for measuring in-cylinder air motion measurement results were analyzed for examining flow field, swirl tumble ratio variation distribution. Results of ensemble-averaged flow fields show that reduced MVL could produce strong swirl flow, n resulted in very regular swirl motion in late stage of intake process. The strong swirl flow can maintain very well until late compression stage. The reduction of MVL can also increase both high-frequency low-frequency swirl flow remarkably. Regarding tumble flow, results demonstrate that lower MVLs result in more horizontal intake flow vectors which can be easily detected under valve seat area. Although result of lower MVLs show a higher tumble ratio when piston is close to bottom dead centre (BDC), higher MVLs substantially produce higher tumble ratios which can be confirmed when most cylinder area lies in measuring range. Keywords: in-cylinder air motion; swirl flow; tumble flow; variable valve lift (VVL); particle image velocimetry (PIV) 1. Introduction In recent years, advanced combustion technologies like gasoline direct injection (GDI) [1], gasoline CAI (controlled auto-ignition) [2], etc. have made significant contributions to improving fuel economy of gasoline engines for reducing CO 2 emissions, in particular with support from variable valve actuation (VVA) [3,4] which has played a critical role in enhancing combustion performance of gasoline engines. From recent production, in particular combined with downsizing, it has been seen that new GDI engines have achieved significant fuel consumption improvements over conventional port fuel injection (PFI) gasoline engines [5]. With regard to application of VVA in gasoline engines, re are several possible control strategy options. Basically it can run as variable valve timing (VVT) /or variable valve lift (VVL), though some VVA provides some different operation options, such as two valve-open pulses etc. [6]. As most familiar VVA technology, VVT has been applied as main valve phasing technology practically since last decade for improving volumetric efficiency, for reducing pumping losses for reducing or harmful emissions. To achieve those benefits, early/late intake Energies 2015, 8, ; doi: /en

2 valve closing (EIVC/LIVC), late intake valve opening (LIVO), variable valve overlap (VVO) have been implemented. The main function of VVA is generally to replace throttle under suitable operating conditions for regulating intake air, thus achieving lower pumping losses than throttle operation. Sometime, a VVL system can be integrated into appropriate cams with different profiles corresponding maximum valve lift (MVL) for an optimal relationship between discharge coefficient crank angle (CA) [7]. Compared to throttled operations, VVL with general valve timing opening duration could help to maintain a higher in-cylinder pressure (or lower vacuum) during early intake stage. Only during late intake stage, lower valve lift will make in-cylinder pressure be similar to case with a throttle [8]. As demonstrated by Grimaldi et al. [9], pumping loss of a non-throttled VVL configuration could be decreased approximately by 50% compared to that of throttled conditions. In recent years, VVL has been successfully applied on several models of production engines. Mitsubishi Motors Corporation developed a unique multi-mode valve system called mitsubishi innovative valve timing lift electronic control (MIVEC) has utilized it on a 4-cylinder in-line engine in early 1990s [10]. Anor successful application of VVL on production engines is new generation of BMW s Valvetronic system which is one real fully VVL system through which advantage of non-throttle load control can be completely realized [11]. Following those practical utilizations, new research developments about flexible VVL are being continuously revealed, such as twin mechanical variable lift (TMVL) valvetrain technology innovated by Ricardo Engineering Consultant Company [12] 2-Step VVA developed by Delphi Corporation [13]. There are also or successful VVA technologies, such as those developed by Porsche, Audi, Fiat (UniAir MultriAir), etc. While VVL can reduce pumping losses for improving efficiency of gasoline engines, its influence on in-cylinder air motion has been noted [14]. While a number of researches have demonstrated that optimization of in-cylinder air motion is beneficial to combustion emission reduction, it is noted VVA might increase possibility of cycle-to-cycle variation [15,16]. Consequently to address this issue, relevant research using in-cylinder flow measurements have been carried out [17 19]. In an initial measurement on a steady flow test rig with same cylinder head as used in following investigation, results (as shown in Figure 1) have demonstrated that reduced MVL could significantly enhance in-cylinder swirl motion, but simultaneously reduce tumble motion strength. With those influences of VVL on in-cylinder flows, it will be necessary to investigate how air-fuel mixing combustion would be affected under those conditions. In present study, swirl tumble flow fields under reduced MVL have been examined with a particle image velocimetry (PIV) system necessary data processing technology. Results concerning swirl ratio tumble ratio, of swirl tumble flows ir high-frequency low-frequency fractions have been analyzed to investigate effects of VVL on air-fuel mixing processing combustion. As most of current GDI engines have been equipped with turbochargers or supervisors, it is understood that current research lacks necessary consideration of turbocharged condition. When intake pressure is increased with possible boosters, intake flow will increase. Effects of reduced MVL on swirl tumble flows might become more obvious. If this is correct, consequently influence of reduced MVL on air-fuel mixing combustion would be more important. One more factor which has been also ignored in current research is existence of bowl shaped piston used for most GDI engines. Although it would become complicated with a bowl shaped piston, it would be more possible that swirl flows would be slowed down more or less tumble flows could be accelerated if bowl shape could be designed for this purpose. Meanwhile, while reduced MVL reduces pumping losses improves air-fuel mixing flame propagation, reduced swirl-tumble interaction as a result of poor tumble flow might also affect air-fuel mixing behavior. It has been reported in some research on diesel engines that air-fuel 13779

3 significantly enhance in cylinder swirl motion, but simultaneously reduce tumble motion strength. With those influences of VVL on in cylinder flows, it will be necessary to investigate how air fuel mixing combustion would be affected under those conditions. In present study, swirl tumble flow Energies fields 2015, 8, under reduced MVL have been examined with a particle image velocimetry (PIV) system necessary data processing technology. Results concerning swirl ratio tumble ratio, mixing is highly dependent of swirl on interaction tumble offlows swirl tumble ir flows high frequency angle of spray low frequency [20]. fractions Considering have been at analyzed least se three to investigate influences, it will be effects worthwhile of VVL to undertake on air fuel a more detailed mixing study processing combustion. in future investigations to examine operating characteristics of GDI engines with reduced MVL Swirl ratio Tumble ratio Swirl ratio Tumble ratio Valve lift (mm) 0 Figure 1. Figure Influence 1. Influence of reduced of reduced maximum valve lifts (MVL) on on swirl swirl ratio ratio tumble tumble ratio under ratio under steady flow steady flow measurement. 2. Experimental Section 2.1. Test Engine, Particle Image Velocimetry System Experimental Conditions 2 The optical engine employed for those PIV experiments is a modified Cagiva FOX350 four-valve single cylinder engine whose main specifications can be found in Table 1. A schematic including optical engine system PIV system used for swirl tumble flow measurement can be found in Figure 2. For measuring swirl tumble flows, one extended transparent piston top with a 55 mm diameter quartz window one 27 mm long quartz ring that mounted between cylinder head cylinder block are installed. A mirror fixed at an angle of 45 on extended cylinder block for reflecting swirl flow images in horizontal plane through piston top window to charge coupled device (CCD) camera. Table 1. Main specifications of FOX350 optical engine operating condition. Before bottom dead centre: BBDC; after top dead centre: ATDC; before top dead centre: BTDC; after bottom dead centre: ABDC; CA: crank angle. Specification Unit Value Valve number 4 Bore ˆ Stroke mm ˆ mm 78.7 ˆ 66.0 Compression ratio 6.7:1 Combustion chamber Pent-roof shape Intake valve diameter mm 30.0 Exhaust valve diameter mm 26.0 Intake port Siamesed, tangent Exhaust valve open CA 74 BBDC Exhaust valve close CA 26 ATDC Intake valve open CA 22 BTDC Intake valve close CA 68 ABDC Engine speed rpm 960, motored Or engine operating conditions Wide open throttle, naturally aspired Most parts of original cylinder head were kept, except that camshafts were changed for different valve lifts. The engine has a general pent-roof type combustion chamber a siamesed 13780

4 but tangent intake port. For two intake valves, valve seating width is about 1.5 mm, which gives approximately 140 mm 2 of valve seating area. The MVL of original valvetrain 6.8 mm. For this research, two more camshafts with MVLs of 4.0 mm 1.7 mm were prepared used on engine. Considering this investigation is focused on influences of VVA on in-cylinder flows, Energies 2015, 8, page page those Energies different 2015, 8, camshafts page page still have same valve timings/phasing as that of original valve train. Compared to to those thosevva which whichhas hasboth bothvariable lift lift variable variable timing/phasing, timing/phasing, this this may may reduce reduce Compared possibility to to tothose reduce VVA which pumping has losses. both losses. variable lift variable timing/phasing, this may reduce possibility to reduce pumping losses. Figure 2. Optical engineset up set-up particle image velocimetry (PIV) (PIV) system system for swirl for swirl flow flow tumble tumble flow Figure measurements. 2. Optical engine set up particle image velocimetry (PIV) system for swirl flow tumble flow measurements. The valve profiles are presented in Figure 3, with 0 CA referring to top dead centre (TDC) The valve profiles are presented in Figure 3, with CA referring to top dead centre (TDC) at beginning The valve of profiles induction are presented stroke. in The Figure engine 3, speed with 0 is CA motored referring at 960 to rpm, top wide dead open centre throttle (TDC) at beginning of induction stroke. The engine speed is motored at 960 rpm, wide open throttle with at naturally beginning aspirated of intake induction during stroke. all The experiments engine speed for is different motored camshafts. at 960 rpm, wide open throttle with with naturally naturally aspirated aspirated intake intake during during all all experiments for different camshafts. camshafts. Figure 3. Valve lift profiles of variable valve lift (VVL). Figure 3. Valve lift profiles of variable valve lift (VVL). Figure 3. Valve lift profiles of variable valve lift (VVL). The FlowMaster PIV system developed by LaVison [21] employed for flow field measurements. The FlowMaster The basic PIV setup system has been developed presented by in LaVison Figure 2. [21] The light employed source is for supplied flow by field second harmonic measurements. The FlowMaster output The basic PIV (532 setup system nm) of has developed two been SOLO120 presented by LaVison Nd:YAG in Figure [21] lasers. 2. The Then, light employed source laser beams is for supplied flow were sent by field measurements. through second harmonic The a spherical lens output basic (1000 (532 setup mm nm) has focal of been two length), SOLO120 presented followed Nd:YAG in Figure by a concave lasers. 2. Then, light cylindrical laser source lens beams is supplied ( 20 mm were focal sent by length). through second-harmonic Finally a spherical laser lens output sheet (1000 (532 with mm nm) a focal of thickness length), two SOLO120 of followed Nd:YAG about 2 mm by a at concave lasers. beam cylindrical Then, waist lens laser formed. ( 20 beams mm The focal were sent lasers length). through have Finally approximately a spherical laser lens 3 5 sheet (1000 ns pulse with mma width focal thickness length), 120 of about mj followed pulse 2 mm. byat a concave The beam laser cylindrical waist pulses from formed. lens ( 20 two The mm focal Nd:YAG lasers length). have lasers approximately Finally were controlled laser 3 5 sheet by ns pulse with controller width a thickness (shown 120 of mj in about pulse Figure. 2 mm 2) to at fire The at laser beam right pulses CA waist from with formed. two a separation Nd:YAG of lasers 25 μs. were A trigger controlled signal by provided controller to (shown controller in Figure from 2) a to shaft fire at encoder right CA which has with a a 1 CA separation resolution. of 25 A μs. CCD A trigger camera signal (Sony ICX085 provided CCD to sensor, controller Nikon from AF Nikkor a shaft encoder 50 mm f/1.8 which lens, has a manufactured 1 CA resolution. by Sony, A CCD Japan) camera with an (Sony image ICX085 resolution CCD of sensor, Nikon AF 6.7 Nikkor μm pixel 50 spacing mm f/1.8 lens, used manufactured for image acquisition. by Sony, Japan) with an image resolution of μm pixel spacing used for image acquisition.

5 The lasers have approximately 3 5 ns pulse width 120 mj pulse. The laser pulses from two Nd:YAG lasers were controlled by controller (shown in Figure 2) to fire at right CA with a separation of 25 µs. A trigger signal provided to controller from a shaft encoder which has a 1 CA resolution. A CCD camera (Sony ICX085 CCD sensor, Nikon AF Nikkor 50 mm f/1.8 lens, manufactured by Sony, Japan) with an image resolution of 1300 ˆ µm pixel spacing Energies 2015, 8, page page used for image acquisition. Energies 2015, 8, page page Seeding Seeding added added into into intake intake air flow air by flow a Laskin by a sprayer Laskin (Filter sprayer Integrity (Filter Limited Integrity Company, Limited Durham, Company, Seeding UK) Durham, with added disoctyl UK) into with sebacate disoctyl intake as sebacate air solution. flow by as Air a solution. Laskin (3 5 bar) sprayer Air (3 5(Filter supplied bar) Integrity to supplied Limited sprayer to Company, by sprayer air compressor by Durham, an air compressor UK) in with accordance disoctyl in accordance with sebacate seeding with as solution. density seeding Air requirement. (3 5 density bar) requirement. The supplied particle The to mist particle sprayer had mist an by average had an air an 1 3 average compressor μm diameter. 1 3 µm in accordance diameter. with seeding density requirement. The particle mist had an average 1 3 μm When When diameter. in cylinder in-cylinder swirl swirl flows flows were were investigated, investigated, laser laser sheet sheet arranged arranged horizontally horizontally to to access ring window. Then images were obtained via piston top window 45 access When ring window. in cylinder Then swirl flows images were were investigated, obtained via laser piston sheet top window arranged horizontally 45 mirror, to as mirror, access shown as shown ring Figure window. Figure 2. As Then 2. As diameter images diameter of were of optical obtained optical window via window piston on top piston window piston top is top smaller is smaller 45 than mirror, than cylinder as shown cylinder bore, in bore, Figure as shown as2. shown As in Figure in diameter Figure 4a, as 4a, a of as result a result optical not not all window of all of cross on cross section section piston area area top of is ofsmaller cylinder cylinder than can can be measured. be cylinder measured. bore, Swirl Swirl as flow shown flow images in images Figure at at three 4a, three as measuring a measuring result not planes all planes of of of 5 cross 5mm, mm, section 10 10mm mmarea of mm mm cylinder horizontally horizontally can be under under measured. TDC TDC Swirl plane, plane, flow respectively, images at three were were measuring taken. taken. planes of 5 mm, 10 mm 15 mm horizontally under TDC plane, respectively, were taken. Figure4. 4. Optical measurement areafor investigating swirl flow tumble flow. Figure 4. Optical measurement area for investigating swirl flow tumble flow. For exploring tumble flows, laser sheet did still access ring window but in vertical For exploring tumble flows, laser sheet did still access ring window but in vertical direction. For exploring The images were tumble acquired flows, through laser sheet ring did window, still access by placing ring window CCD but camera in a toward vertical direction. The images were acquired through ring window, by placing CCD camera toward direction. laser sheet The perpendicularly images were acquired (as shown through in Figure ring 2). Two window, measuring by placing planes with CCD one camera as vertical toward laser sheet perpendicularly (as shown in Figure 2). Two measuring planes with one as vertical symmetric laser sheet plane perpendicularly in cylinder (as (Plane shown in Figure 5) 2). Two anor measuring through planes axes with of one same as side vertical intake symmetric plane in cylinder (Plane in Figure 5) anor through axes of same side intake symmetric exhaust plane valves in were cylinder selected (Plane for locating 1 Figure laser 5) sheets. anor The tumble through flow axes measurement of same side area intake is exhaust valves were selected for locating laser sheets. The tumble flow measurement area is shown exhaust in Figure valves 4b. were selected for locating laser sheets. The tumble flow measurement area is shown in Figure 4b. shown in Figure 4b. Figure 5. Optical measurement planes for investigating tumble flow. Figure 5. Optical measurement planes for investigating tumble flow. Figure 5. Optical measurement planes for investigating tumble flow. Synchronization with engine angular information laser operation frequencies allowed having Synchronization one measurement, with i.e., engine one instantaneous angular information field, laser at each operation engine cycle. frequencies 100 consecutive allowed cycles having are one performed measurement, at each i.e., given one instantaneous CA. field, at each engine cycle. 100 consecutive cycles are performed at each given CA Flow Parameters 2.2. Flow Parameters The in cylinder flow field is highly turbulent during individual cycles. Therefore, necessary

6 Synchronization with engine angular information laser operation frequencies allowed having one measurement, i.e., one instantaneous field, at each engine cycle. 100 consecutive cycles are performed at each given CA Flow Parameters The in-cylinder flow field is highly turbulent during individual cycles. Therefore, necessary analysis must be undertaken for identifying different flow parameters which might have different influence on flow performance. As most flow parameters analysis technology have very similar application on both swirl tumble flow, definitions introduced in following paragraphs will be based on swirl flow. When those are used on tumble flow analysis, one only needs to change coordinates from (x, y) (horizontal plane) to (x, z) (vertical plane), as shown in Figure 4. In order to identify basic main flows, cyclic variation in-cycle turbulence levels, instantaneous vector Ñ U px,y,θ,iq at flow field in horizontal planes with coordinates of (x, y) can be decomposed into an ensemble-averaged mean vector two components: Ñ Upx,y,θ,iq Ñ U EApx,y,θq ` Ñu LFpx,y,θ,iq ` Ñu HFpx,y,θ,iq (1) where, θ is CA at which measurement is performed i is cycle number. On right h side of Equation (1), first term is ensemble-average mean over a number of cycles, as defined by Equation (2): Ñ UEApx,y,θq 1 N Nÿ Ñ Upx.y,θ,iq (2) In present study, instantaneous results of 100 cycles were used for calculating ensemble-average mean. An example for one instantaneous flow field flow field of ensemble-average mean can be found in Figure 6a,b. In Equation (1), second Ñ u LFpx,y,θ,iq third terms Ñ u HFpx,y,θ,iq on right h side are related to low-frequency fluctuation high-frequency fluctuation. For measurement result of an instantaneous cycle, sum of low-frequency high-frequency fluctuations can be obtained by subtracting ensemble-average mean from instantaneous vector as defined in Equation (3). For in-cylinder flow in internal combustion engines, low-frequency fluctuation normally refers to cyclic variation but high-frequency to in-cycle turbulent fluctuation. As cyclic variation in-cycle fluctuation have totally different impacts on air-fuel mixing consequent combustion process, it is necessary to extract m from sum of fluctuations. In present study, a low-pass filtering scheme in spatial frequency domain which has been introduced by several researchers [22 24] employed for that purpose. In order to identify an appropriate spatial cut-off frequency, at first two-dimensional data were transformed into spatial frequency domain using a two-dimensional fast Fourier transform (FFT). Then from power spectral density (PSD) trace (as a function of spatial frequency), two higher PSD regions an apparent low flat PSD region between those two higher PSD regions could be found. Normally spatial cut-off frequency could be determined by selecting a frequency value in middle point of low PSD region. In this study, spatial cut-off frequency which used is between 70 m m 1, corresponding to m m (14 mm 8 mm) in spatial domain: Ñ u 1 Fpx,y,θ,iq Ñ U px,y,θ,iq Ñ U EApx,y,θq (3) i

7 R s M j 1 mru 2πn 60 j j EA( j, ) M j 1 mr 2 j j (7) where, mj is mass of unit j (at (x, y)); rj is distance from unit j to rotational axis of flow; Energies 2015, 8, UEA(j,θ) is tangential component of in j; n is engine speed in rpm. To be noted, coordinate system origin used here is swirl center in measured plane. Figure 6. Swirl flow with ensemble averaged field spatially filtered results for achieving Figure 6. Swirl flow with ensemble-averaged field spatially filtered results for achieving high low frequency fluctuations (for maximum valve lifts (MVL) 1.7 mm at horizontal plane high low of 5 frequency mm under dead fluctuations centre (TDC)). (for maximum Instantaneous valve lifts filed; (MVL) 1.7 ensemble averaged mm at horizontal mean plane of 5 mm under dead field; (c) centre low frequency (TDC)). Instantaneous field; (d) high frequency filed; ensemble-averaged field. mean field; (c) low-frequency field; (d) high-frequency field. 3. Results Discussion First of all, it needs to be noted that CA is defined as that seen in Figure 3 for results Figure 6c,d shows flow fields of low-frequency high-frequency velocities shown in following sections, The TDC when intake valve just opens has a CA of 0 CA estimated with compression/combustion above method. TDC After is at 360 CA. components (low- or high frequency) were obtained, ensemble-average root-mean-square (RMS) (for low frequency or high frequency) at CA of θ could be calculated with following formula: g 7 Ñ u 1 EApx,yq f e 1 Nÿ p Ñ u N 1 2 Fpx,y,iq q Similar to estimate of ensemble-average mean, 100 cycles were still used for computing ensemble-average RMS. Based on above analysis, of (for low-frequency or high-frequency) at (x, y) could be derived as follows: i 1 E EApx,yq 1 2 ppu1 EApx,yq q2 x ` pu1 EApx,yq q2 y q (5) By integrating all energies of at all spatial points of measured plane, total of at a measured plane could be estimated. Based on this, total non-dimensional RMS can be obtained with following definition: b v RMS 2E EApx,yq V meanpiston (6) Finally, swirl ratio (R s ) of a measured plane at CA of θ could be derived by using Equation (7): Mř m j r j U EApj,θq j 1 R s (7) 2πn Mř m 60 j r 2 j j 1 (4) 13784

8 where, m j is mass of unit j (at (x, y)); r j is distance from unit j to rotational axis of flow; U EA(j,θ) is tangential component of in j; n is engine speed in rpm. To be noted, Energies 2015, 8, page page coordinate system origin used here is swirl center in measured plane Effects of Maximum Valve Lifts on Swirl Flow Field Swirl Ratio 3. Results Discussion As shown in Figure 7a c, re are swirl motions at horizontal planes of 5 mm under TDC, with First of MVL all, 6.8 itmm needs to 1.7 be mm noted at that different CAs of is120 defined CA (intake as that valves seen fully inopened), Figure for CA results (intake valves totally closed) 300 CA (late stage of compression). Those flow fields are shown in following sections, The TDC when intake valve just opens has a CA of 0 CA ensemble averaged results of 100 cycles with compression/combustion TDC is at 360 CA. methods shows in Equation (2). Obviously it can be seen from Figure 7a that intake flow for MVL 1.7 mm had much higher vectors than that of MVL 6.8 mm when intake valves were nearly fully opened. This 3.1. Effectsdemonstrates of Maximumthat Valve re Lifts a on much Swirl higher Flow pressure Fielddifference Swirl across Ratio intake valves for MVL 1.7 mm due to a smaller curtain area for achieving adequate intake air amount. The high pressure difference As shown small in Figure valve lift 7a c, result in re flow arefields swirl much motions more irregular at horizontal planes of 5 under TDC, with MVL 6.8 mm 1.7 mm at different CAs of 120 CA for MVL 1.7 mm than 6.8 mm. For MVL 1.7 mm, re were two vortex centers observed under two (intake valves valves (positions fully of two opened), intake 240 CA (intake valves totally can be found closed) in Figure 4a). 300This CA suggested (late stage y should of be formed compression). from intake flows Those from flow two fields are intake valves. For MVL 6.8 mm, though two vortex centers could be also observed, y were very ensemble-averaged results of 100 cycles with methods shows in Equation (2). weak located near exhaust valves rar than intake valves. Figure 7. Swirl flow ensemble averaged flow fields at horizontal plane of 5 mm under TDC for Figure 7. Swirl MVL 6.8 flow mm ensemble-averaged MVL 1.7 mm. At flow 120 CA fields (close atto horizontal maximum valve plane open); of 5 mm at 240 under CA TDC for MVL 6.8 mm (close to MVL valve 1.7 close); mm. (c) at 300 At CA 120 (close CA to (close compression to maximum end). CA: crank valve angle. open); at 240 CA (close to valve close); (c) at 300 CA (close to 8 compression end). CA: crank angle. Obviously it can be seen from Figure 7a that intake flow for MVL 1.7 mm had much higher vectors than that of MVL 6.8 mm when intake valves were nearly fully opened. This demonstrates that re a much higher pressure difference across intake valves for 13785

9 Energies 2015, MVL 8, page page 1.7 mm due to a smaller curtain area for achieving adequate intake air amount. The high pressure difference small valve lift result in flow fields much more irregular for MVL 1.7 mm than 6.8 mm. For MVL 1.7 mm, re were two vortex centers observed under two intake valves (positions of two intake valves can be found in Figure 4a). This suggested y should be formed from intake flows from two intake valves. For MVL 6.8 mm, though two vortex centers could be also observed, y were very weak located near exhaust valves rar than intake valves. With progress of intake process, a very strong swirl flow with an apparent vortex center near cylinder axis could be seen for MVL 1.7 mm at 240 CA when intake valves would fully close (Figure 7b). For MVL 6.8 mm, flow field still dominated by irregular air motion with a high flow very, weak flowbut velocities. also form As ita had very been obvious timing swirl when motion. intake process would finish, flow strength shown in Figure 7b could be considered as results of intake process. Those differences of flow field between MVL 1.7 mm 6.8 mm suggests that reduced MVL would not only produce a high flow, but also form a very obvious swirl motion. Shown in Figure 7c are swirl motions at 300 CA when it very close to compression end. Although re is still a very apparent difference between MVL 1.7 mm MVL 6.8 mm as seen in Figure 7b, it can be seen that evolutions of two flow fields have totally different trends. For MVL 1.7 mm, from 240 CA to 300 CA, strength of swirl motion decreases a little but flow field becomes even vortex center is closer cylinder center. For MVL 6.8 mm, from very weak swirl motion at 240 CA, now at 300 CA a large-scale vortex appears in lower part of flow field strength increased somewhat though not very much. The increase may be because stronger swirl flows were moved upward by piston from cylinder bottom. As in-cylinder air motion at compression end is always critical for air-fuel mixing flame propagation, enough attention should be paid to effects of reduced MVL on those flow fields when relevant information is used for optimizing GDI combustion systems. when relevant information is used for optimizing GDI combustion systems. In Figure 8, variations of swirl ratio with CA for different MVLs at different measured planes are shown. The estimates of swirl ratio were based on those ensemble-averaged fields measured at different CAs Equation (7) used for those calculations. With progress of intake process, a very strong swirl flow with an apparent vortex center near cylinder axis could be seen for MVL 1.7 mm at 240 CA when intake valves would fully close (Figure 7b). For MVL 6.8 mm, flow field still dominated by irregular air motion with very weak flow velocities. As it had been timing when intake process would finish, flow strength shown in Figure 7b could be considered as results of intake process. Those differences of flow field between MVL 1.7 mm 6.8 mm suggests that reduced MVL would not only produce Shown in Figure 7c are swirl motions at 300 CA when it very close to compression end. Although re is still a very apparent difference between MVL 1.7 mm MVL 6.8 mm as seen in Figure 7b, it can be seen that evolutions of two flow fields have totally different trends. For MVL 1.7 mm, from 240 CA to 300 CA, strength of swirl motion decreases a little but flow field becomes even vortex center is closer cylinder center. For MVL 6.8 mm, from very weak swirl motion at 240 CA, now at 300 CA a large scale vortex appears in lower part of flow field strength increased somewhat though not very much. The increase may be because stronger swirl flows were moved upward by piston from cylinder bottom. As in cylinder air motion at compression end is always critical for air fuel mixing flame propagation, enough attention should be paid to effects of reduced MVL on those flow fields In Figure 8, variations of swirl ratio with CA for different MVLs at different measured planes are shown. The estimates of swirl ratio were based on those ensemble averaged fields measured at different CAs Equation (7) used for those calculations. Figure 8. Figure Comparison 8. Comparison of swirl of swirl ratio ratio variations variationswith CA CA at at different different measured measured planes planes between between different different MVLs MVLs (5 mm (5 mm measured plane of 55 mm mm under under TDC; TDC; 10 mm 10 mm measured measured plane of 10plane of 10 mm under mm under TDC; TDC; mm measured plane of 15 of mm 15 mm under under TDC). TDC). With regard to 5 mm plane result, it can be found that reduced MVLs could result in increase of swirl ratio at all different CAs. For MVL 6.8 mm, swirl ratio at this measured plane varies around zero re might be different rotating directions at different CAs. Before 180 CA, reductions of MVL from 4.0 mm to 1.7 mm regularly produce a double swirl ratio compared to MVL reduction from 6.8 mm to 4.0 mm. From 180 CA to 240 CA, increase rate of swirl ratio

10 Figure 10. Variations of total low frequency non dimensional RMS of swirl flow with CA for three MVLs (at 5 mm 15 mm plane). Energies 2015, 8, With regard to 5 mm plane result, it can be found that reduced MVLs could result in increase of swirl ratio at all different CAs. For MVL 6.8 mm, swirl ratio at this measured plane varies around zero re might be different rotating directions at different CAs. Before 180 CA, reductions of MVL from 4.0 mm to 1.7 mm regularly produce a double swirl ratio compared to MVL reduction from 6.8 mm to 4.0 mm. From 180 CA to 240 CA, increase rate of swirl ratio for MVL 1.7 has a significant growth. As se two CAs were at late intake stroke, sudden increase of swirl ratio might come from quick increase of intake flow speed due to a continuous increase of pressure difference over intake valves. These variations resulted in a much higher swirl ratio for MVL 1.7 mm at compression end stage than MVL 4.0 mm 6.8 mm. For results at 10 mm 15 mm planes, it can be seen increase of swirl ratio for reduced MVLs at lower measured planes is still very obvious, but increase rates are not as high. Similar to those findings shown in Figure 7, increase of swirl ratio due to reduced MVL is more obvious at near intake valve area or upper volume of cylinder. Energies 2015, 8, page page 3.2. Effects of Maximum Valve Lifts on Fluctuating Kinetic Energy Based on Swirl Flows 3.2. Effects As mentioned of Maximum in Valve Section Lifts 2, on Fluctuating high-frequency Kinetic Energy Based on Swirl Flows low-frequency As mentioned in are Section produced 2, by totally high frequency different sources. In this study, high-frequency low frequency are refers produced to in-cycle by totally turbulence different which sources. should In this be study, beneficial high frequency to air-fuel mixing flame refers propagation. to in cycle For turbulence extracting which different should frequency be beneficial to air fuel mixing as demonstrated flame propagation. in Section For 2, extracting it needs to different be noted frequency that variation of in-cylinder as demonstrated density not in Section taken into 2, it account needs to due be noted to that difficulty variation in obtaining of in cylinder density distribution density in not taken cylinder. into In account or due to h, difficulty actualin spatial obtaining variation density of in-cylinder distribution density in iscylinder. normallyin small or enough h, to be ignored, actual spatial in particular variation forof gasoline in cylinder engines. density Those is normally results shown small enough in Figures to be 9 ignored, 10 are in also particular based on for 100 gasoline measured engines. cycles. Those results shown in Figures 9 10 are also based on 100 measured cycles. Nondimensional RMS Velocity Valve Lift=1.7mm Valve Lift=4.0mm Valve Lift=6.8mm Nondimensional RMS Velocity Valve Lift=1.7mm Valve Lift=4.0mm Valve Lift=6.8mm Crank Angle( CA ATDC) Crank Angle( CA ATDC) Figure 9. Variations of total high frequency non dimensional root mean square (RMS) Figure 9. Variations of total high-frequency non-dimensional root-mean-square (RMS) of swirl flow with CA for three MVLs (at mm 15 mm planes). of swirl flow with CA for three MVLs (at 5 mm 15 mm planes) As shown in Figure 9, those are variations of total high-frequency non-dimensional RMS Valve Lift=1.7mm Valve Lift=1.7mm with CA for different Valve Lift=4.0mm MVLs. As non-dimensional RMS Valve Lift=4.0mm for Valve Lift=6.8mm Valve Lift=6.8mm which 2.5definition has been shown in Section 2 is based 2.5 on, each value Nondimensional RMS Velocity of total high-frequency non-dimensional RMS shown in figure actually achieved from total high-frequent which is integration of high frequent 1.5 for all spatial points 1.5 at measured plane. From those results, it can be seen that high-frequency non-dimensional RMS keeps decreasing with CA, due to gradually dissipated small- micro-scale vortexes. When MVL is reduced from 6.8 mm 0.5 to 4.0 mm, increase of high-frequency 0.5 non-dimensional RMS Nondimensional RMS Velocity Crank Angle( CA ATDC) Crank Angle( CA ATDC)

11 Nondimensional R Energies 2015, 8, Crank Angle( CA ATDC) Crank Angle( CA ATDC) is not so apparent, but from MVL 4.0 to 1.7 mm, re is a very remarkable increase at all CAs at different measured planes. Until 320 CA, MVL 1.7 mm s total high-frequency Figure 9. Variations of total high frequency non dimensional root mean square (RMS) still keeps of swirl very flow high with level CA for compared three MVLs to MVL (at mm mm mm mm. planes). Nondimensional R Nondimensional RMS Velocity Valve Lift=1.7mm Valve Lift=4.0mm Valve Lift=6.8mm Nondimensional RMS Velocity Valve Lift=1.7mm Valve Lift=4.0mm Valve Lift=6.8mm Crank Angle( CA ATDC) Crank Angle( CA ATDC) Figure 10. Variations of total low frequency non dimensional RMS of swirl flow Figure 10. Variations of total low-frequency non-dimensional RMS of swirl flow with CA for three MVLs (at mm 15 mm plane). with CA for three MVLs (at 5 mm 15 mm plane). As shown in Figure 9, those are variations of total high frequency non dimensional RMS While low-frequency with CA for different MVLs. As mainly non dimensional results from RMS cyclic variation, its impact for which on definition air-fuel mixing has been shown combustion in Section is 2 not is based as simple on as impact of high-frequency, each value of total high frequency. On non dimensional one h, it may RMS provide useful assistance shown to in mixing figure similar as actually from achieved high-frequency from total high frequent but at a different spatial which scale. is On integration or of h, high frequent it could possibly be main reason for for all causing spatial points cycle-to-cycle at measured variation plane. of combustion From those owing results, to it its can long be spatial seen that scale high frequency high strength non dimensional relatively. As shown RMS in Figure 10, variation keeps of decreasing total low-frequency with CA, non-dimensional due to gradually RMS dissipated smallwith CA micro scale has a similar vortexes. trend to When that of MVL total is high-frequency reduced from 6.8 non-dimensional mm to 4.0 mm, RMS increase, of but high frequency increase at non dimensional compression end RMS for reduced MVL is not is not so high, so apparent, as shown but in from Figure MVL to 1.7 mm, re is a very remarkable increase at all In brief, when reduced MVL is used to provide some benefits for gasoline engine combustion emissions by reducing pumping losses improving 10 air-fuel mixing flame propagation, special attention should be paid to possible drawback of increasing cycle-to-cycle variation due to increase of. In addition, only swirl flow details are presented in this paper for effects of VVL on in-cylinder air motion. From results achieved under steady flow tests, it is shown that reduced MVL has a reverse influence on tumble flow. Furr analysis should be considered for total effect of VVL on in-cylinder air motion by combining presentations of swirl tumble flows Effects of Maximum Valve Lifts on Tumble Flow Field Tumble Ratio Figure 11a,b, it show tumble flow fields under MVLs of 1.7 mm, 4.0 mm 6.8 mm at CA of 128 CA at which intake valves are fully opened. It needs to be noted that those flow fields are still ensemble-averaged from 100 continuous cycles. One of main features in figures is that maximum flow decreases obviously in both planes with increase of MVL. This may be because adequate intake air amount for higher MVL during early intake stage resulted in very small in-cylinder vacuum low intake flow. For flow field, it can be found in Figure 11a that with MVL of 1.7 mm, two oppositely rotating tumble structures exit, this is presented more clearly in Figure 11b. For MVL of 4.0 mm, two tumble structures can still be seen, but it becomes re is only an obvious tumble vortex for MVL of 6.8 mm. This suggests re is a strong interaction between intake air flow cylinder wall under MVLs of 1.7 mm 4.0 mm. When MVL increases to 6.8 mm, take air flows toward more to 13788

12 piston top direction. Then it results in formation of single rotation direction structure, with a large scale structure. Although whole range of in-cylinder tumble flow can t be imaged because of limited measurement area, result from current measurable region shows that a large scale with same rotating orientation is more easily formed under high MVL, while small scale counter-clockwise rotating flows exist because of interaction obstruction among flows moving from different sides of cylinder wall under low MVL conditions. Energies 2015, 8, page page 4m/s 4m/s Figure 11. Tumble flow ensemble averaged flow fields at 128 CA (during intake stroke) for MVL Figure mm, Tumble 4.0 mm flow 6.8 ensemble-averaged mm: at Plane 1 flow fields at Plane at 128 CA 2. (during intake stroke) for MVL 1.7 mm, 4.0 mm 6.8 mm: at Plane 1 at Plane 2. Figure 12a,b shows ensemble-averaged flow fields in two measurement planes with MVLs of 1.7 mm, 4.0 mm 6.8 mm when intake valves are completely closed (228 CA). It can be found in Figure 7a that distortion break happen in tumble structure owing to compression. For MVL of 1.7 mm, clockwise tumble structure on left side of cylinder has decayed. The flow motions at two planes present similar flow structure a clockwise large scale tumble structure is dominant in 4m/s flow field, maximum is approximately 7 m/s. For MVL of 6.8 mm, it can be found tumble structure has broken in measurement plane no obvious vortex can be found. At Plane 2 (directly under valves) in Figure 12b, trends have apparently become weak while still lower MVLs still have high vectors. When piston moves to near end of compression stroke, as shown in Figure 13 (at 300 CA), obvious big vortexes for MVLs of 1.7 mm 4 mm have disappeared in measurement area, but an obvious vortex exists for MVL of 6.8 mm. The reason may be that main tumble flow of MVL of 6.8 mm may exit in cylinder bottom area which can t be seen in observed area at 128 CA 228 CA. This should be due to stronger vertical intake flow of MVL of 6.8 mm. 4m/s Figure 12. Tumble flow ensemble averaged flow fields at 228 CA (during compression stroke) for MVL 1.7 mm, 4.0 mm 6.8 mm: at Plane 1 at Plane 2.

13 Energies 2015, Figure 8, Tumble flow ensemble averaged flow fields at 128 CA (during intake stroke) for MVL 1.7 mm, 4.0 mm 6.8 mm: at Plane 1 at Plane 2. 4m/s Energies 2015, 8, page page 4m/s When piston moves to near end of compression stroke, as shown in Figure 13 (at 300 CA), obvious big vortexes for MVLs of 1.7 mm 4 mm have disappeared in measurement area, but an obvious vortex exists for MVL of 6.8 mm. The reason may be that main tumble flow Figure of 12. MVL Tumble of 6.8 flow mm ensemble averaged may exit in cylinder flow fields bottom at 228 area CA (during compression stroke) for Figure 12. Tumble flow ensemble-averaged flow fields at 228 CA which can t be seen in observed (during compression stroke) for area MVL at CA mm, mm CA. 6.8 This mm: should at Plane be due 1 to stronger at Plane vertical 2. intake flow of MVL MVL 1.7 mm, 4.0 mm 6.8 mm: at Plane 1 at Plane 2. of 6.8 mm. 12 4m/s 4m/s Figure 13. Tumble flow ensemble averaged flow fields at 300 CA (near compression end) for Figure 13. Tumble flow ensemble-averaged flow fields at 300 CA (near compression end) for MVL 1.7 mm, 4.0 mm 6.8 mm: at Plane at Plane 2. MVL 1.7 mm, 4.0 mm 6.8 mm: at Plane 1 at Plane 2. Figure 14a,b shows variations of tumble ratio as function of CA. As is shown in Figure 14a, between CA of 128 CA 180 CA which is during intake stroke, counter clockwise rotating flow takes predominant place in measurement plane for MVL of 1.7 mm, n high positive tumble ratio values are produced. For MVLs of 4.0 mm 6.8 mm, lower tumble ratio with negative values are presented due to intake air flows domination at clockwise direction. Between 120 CA 210 CA, MVL of 4.0 mm MVL of 6.8 mm produced very similar tumble ratios (direction value), but during compression stroke, tumble ratio of MVL

14 Figure 14a,b shows variations of tumble ratio as function of CA. As is shown in Figure 14a, between CA of 128 CA 180 CA which is during intake stroke, counter-clockwise rotating flow takes predominant place in measurement plane for MVL of 1.7 mm, n high positive tumble ratio values are produced. For MVLs of 4.0 mm 6.8 mm, lower tumble ratio with negative values are presented due to intake air flows domination at clockwise direction. Between 120 CA 210 CA, MVL of 4.0 mm MVL of 6.8 mm produced very similar tumble ratios (direction value), but during compression stroke, tumble ratio of MVL of 4.0 mm decayed quickly, while tumble of 6.8 mm MVL kept direction strength very well. This should still come from reason as mentioned in last section more tumble flow of MVL of 6.8 mm stays just above piston top. Similarly tumble ratio of 1.7 mm MVL becomes weak very quickly during compression stroke due to unapparent vortex structure, though most of vectors are still big in measurement area. Figure 14b reveals that a counter-clockwise flow structure dominates flow for MVL of 1.7 mm at early intake stroke, as piston moves to bottom dead centre (BDC), clockwise tumble structure predominate in plane near BDC (150 CA to 180 CA) until end of compression of compression stroke. For MVL of 6.8 mm, as air intake in early stage of intake stroke has made in-cylinder vacuum so small, intake flow is very low this results in very low tumble ratio (in measurement area). Energies 2015, 8, page page Figure Figure Tumble Tumble ratio ratio varations varations as as function function of of CA: CA: at at Plane Plane 1 1 at at Plane Plane Figure 14a indicates at end of compression stoke, modulus of tumble ratio with Figure 14a indicates at end of compression stoke, modulus of tumble ratio with MVL of 6.8 mm can be up to 0.6 is much larger than that of or two MVLs. For GDI engines MVL of 6.8 mm can be up to 0.6 is much larger than that of or two MVLs. For GDI which have fuel injection around this time, this is especially beneficial for fuel air mixing. engines which have fuel injection around this time, this is especially beneficial for fuel-air mixing. This suggests propitious in cylinder tumble motion can be provided for GDI fuel air mixing when This suggests propitious in-cylinder tumble motion can be provided for GDI fuel-air mixing when higher valve lift is operated. higher valve lift is operated Effects of Maximum Valve Lifts on Fluctuating Kinetic Energy based on Tumble Flows 3.4. Effects of Maximum Valve Lifts on Fluctuating Kinetic Energy based on Tumble Flows As shown in Figure 15a,b, re are variations as a function of CA As shown in Figure 15a,b, re are variations as a function of for MVL of 1.7 mm 6.8 mm. For each MVL, results shown comprise total CA for MVL of 1.7 mm 6.8 mm. For each MVL, results shown comprise total, total fluctuation, high frequency fluctuation, total fluctuation, high-frequency fluctuation low frequency fluctuation. Compared to MVL of 1.7 mm, all those four low-frequency fluctuation. Compared to MVL of 1.7 mm, all those four parameters of 6.8 mm MVL have much smaller valves at all CA positions, though as demonstrated in parameters of 6.8 mm MVL have much smaller valves at all CA positions, though as demonstrated last section, MVL of 6.8 mm produced a higher tumble ratio than MVL of 1.7 mm at in last section, MVL of 6.8 mm produced a higher tumble ratio than MVL of 1.7 mm at compression end. compression end. nd fluctuation (m 2 /s 2 ) Low-frequency fluctuation High-frequency fluctuation Total fluctuation Total uctuation (m 2 /s 2 ) Low-frequency fluctuation High-frequency fluctuation Total fluctuation Total

15 for MVL of 1.7 mm 6.8 mm. For each MVL, results shown comprise total, total fluctuation, high frequency fluctuation low frequency fluctuation. Compared to MVL of 1.7 mm, all those four parameters of 6.8 mm MVL have much smaller valves at all CA positions, though as demonstrated in last Energies section, 2015, 8, MVL of 6.8 mm produced a higher tumble ratio than MVL of 1.7 mm at compression end. Total fluctuation (m 2 /s 2 ) Low-frequency fluctuation High-frequency fluctuation Total fluctuation Total Crank Angle ( CA) Total fluctuation (m 2 /s 2 ) Low-frequency fluctuation High-frequency fluctuation Total fluctuation Total Crank Angle ( CA) Figure 15. Averaged fluctuation of 100 cycles at Plane 1. MVL = 1.7 mm; Figure 15. Averaged fluctuation of 100 cycles at Plane 1. MVL = 1.7 mm; MVL = MVL = 6.8 mm. 6.8 mm. From results of both MVL of 1.7 mm 6.8 mm, it can be found that fluctuation From is dominant results of both in total MVL of 1.7 mm. 6.8 Furrmore, mm, it can be found low that frequency fluctuation fluctuation accounted is dominant for about in 90% total of total fluctuation. Furrmore,. The low proportion frequency of fluctuation fluctuation accounted to for total about 90% of total is higher fluctuation than that obtained. by The Druault proportion et al. [25], of fluctuation some of reason is to that total engine speed is higher set lower than in that obtained present study, by Druault which et may al. [25], enhance some of reason fluctuation is that [26]. Moreover, engine speed energies set lower of in flow fields present were study, enhanced which largely may enhance as MVL decreased, fluctuation which [26]. can Moreover, be seen by comparing energies of same flow kind fields of were enhanced curves at largely different as MVL MVL conditions. decreased, The which total can be at seen lower by comparing MVL conditions same kind about of four times curves more at than different those at MVL higher conditions. MVL The conditions total at CA lower at which MVL conditions peak reached about four advanced times by more about than 30 those CA. at higher MVL conditions CA at which peak reached advanced by about 30 CA. 14 Combining results of swirl flow tumble flow, demonstrates reduced MVL can more or less improve air-fuel mixing flame propagation due to enhanced high-frequency fluctuation, but simultaneously it may increase cycle-to-cycle variations due to increased low-frequency fluctuation. 4. Conclusions An investigation into in-cylinder swirl flow tumble flow under reduced MVL has been carried out on an optical engine. A PIV system including necessary data analysis has been applied for examining swirl tumble flow field, swirl tumble ratio variation distribution. The following conclusions have been drawn from present study: Initial experiments conducted on a steady flow test rig showed that reduced valve lift would significantly enhance total swirl strength, but reduce total tumble ratio. Under present measurement conditions, PIV investigation results showed that reduced MVL could enhance swirl flow, which resulted in a very regular swirl motion in late stage of intake process strong swirl flow can maintain very well until late compression stage. However, measurement results also showed that effects of reduced MVL on swirl motion have a stronger presence in upper part of cylinder volume than in bottom part. By comparing swirl ratio for different MVL values, it can be found that reduced MLV can significantly increase swirl ratio at all almost measured CAs on different measured planes. The reduction of MVL can increase both high-frequency low-frequency swirl flow thus remarkably. In particular at late stage of compression process, increase of high-frequency across all spatial points on measured planes will be beneficial to air-fuel mixing, but simultaneous increase of low-frequency will possibly result in high combustion cyclic variation

16 Regarding tumble flow, ensemble-averaged flow field results demonstrate that lower MVLs result in more horizontal intake flow vectors which can be easily detected under valve seat area, while higher MVLs can produce more vertical flows which turn more toward to piston top finally are more possible to form big scale tumble flow structure. Although result of lower MVLs show a higher tumble ratio when measuring range can t cover most cylinder area, higher MVLs substantially produce higher tumble ratios which can be confirmed when most of cylinder area lies in measuring range with pistons moving close to TDC. The big scale vortex structures produced by higher MVLs made main contribution for producing high tumble ratios of higher MVLs. In terms of tumble flow, lower MVLs result in higher values due to higher vectors, though most of time y did not form good big vortex structures. Based on higher total under lower MVL condition, higher including high-frequency low-frequency fractions are presented under lower MVL conditions. The result would on one h be helpful for better air-fuel mixing flame propagation, but on or h it would enhance cyclic variations. Acknowledgments: The financial supports from NSFC ( ) Ministry of Education of China via 985 Program are gratefully acknowledged. Author Contributions: The investigation leaded supervised by Tianyou Wang Zhijun Peng. Experimental works, data processing most illustrations were completed by Daming Liu, Gangde Wang Bingqian Tan. The manuscript finalized by Zhijun Peng. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations ABDC ATDC BBDC BDC BTDC CA CAI CCD EIVC EVC EVO FFT GDI IVC IVO LIVC LIVO MVL PFI PIV PSD RMS TDC TMVL VVA VVL VVO VVT After bottom dead centre After top dead centre Before bottom dead centre Bottom dead centre Before top dead centre Crank angle Controlled auto-ignition Charge coupled device Early intake valve close Exhaust valve close Exhaust valve open Fast Fourier transform Gasoline direct injection Intake valve close Intake valve open Late intake valve close Late intake valve open Maximum valve lift Port fuel injection Particle image velocimetry Power spectral density Root mean square Top dead centre Twin mechanical variable lift Variable valve actuation Variable valve lift Variable valve overlap Variable valve timing 13793

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