Enhancement of Combustion by Means of Squish Pistons

Transcription

1 Enhancement of Combstion by Means of Sqish Pistons Katshiko MIYAMOTO* Yoshiyki HOSHIBA* Kiyotaka HOSONO* Synichi HIRAO* Abstract Knowledge gained throgh stdies aimed at enhancing combstion by means of changes in iston shaes is resented in this aer. Attention was first focsed on tmble and sqish in the context of in-cylinder flows, and the characteristics of combstion atterns and effects on artial-load fel consmtion were investigated. Sqish is characterized by invigoration of combstion in the middle and late stages, which is desirable for combstion attern. For redction of fel consmtion, it was confirmed that imroved in thermal efficiency yielded by sqish-enabled increase in combstion seed is beneficial with low engine loading and that sqish-enabled sression of knock is beneficial with high engine loading. Dring fll load oeration, it was confirmed that not only knock sression bt also iston-shae-yielded imrovements in volmetric efficiency are beneficial. The benefits were evalated exerimentally and throgh the analysis of in-cylinder flows. These findings and articlarly inflential design factors are discssed in this aer. Key words: Gasoline Engine, Combstion, Knocking, Piston 1. Introdction Amid the growing roblem of global warming, abot 20 % of carbon-dioxide emissions are attribtable to transortation and abot 90 % of this amont is attribtable to atomobiles. Since imrovements in fel efficiency are vital, there is mch ongoing research into ways to enhance the thermal efficiency of engines. There are varios techniqes for imroving thermal efficiency. Some techniqes (exhast-gas recirclation and lean combstion, for examle) work rimarily by redcing ming losses. Others (sression of highload knock and otimization of in-cylinder flows, for examle) work rimarily by enhancing combstion. Efforts to enhance combstion by otimizing in-cylinder flows fall into two broad categories: 1 those that invigorate combstion by romoting tmble, swirl, and other in-cylinder flows throghot the engine s intake and comression strokes; and 2 those that se sqish to create trblence mainly in the vicinity of To Dead Center (TDC) on the comression stroke. At the time of writing, slanted sqish areas, which are sitable for entroof combstion chambers, are widely sed. The stdy described in this aer was focsed on comaring the effects of slanted-sqish istons with those of flat istons with resect to combstion and erformance. The effects of slanted-sqish istons on knock sression and intake air amonts dring high-load oeration and their effects on fel consmtion dring artial-load oeration were investigated. 2. Piston shaes and general characteristics Table 1 shows three main iston-crown shaes and their resective merits and demerits (1). A flat iston has a small crown srface area and conseqently has the merits of small heat losses and low weight. A horizontal-sqish iston is effective at romoting combstion, bt small valve diameters are necessary to rovide a sqish area. Dring high-seed oeration, therefore, there is a tendency for air volmes to decrease and for erformance to be concomitantly limited. A slantedsqish iston is well sited to a entroof combstion chamber in which ignition takes lace at a central oint between the for valves (the tyical combstion-chamber configration in recent engines), and it has the merit of roviding a sqish area withot reqiring any redction in valve diameter. However, it has the demerits of a relatively large iston-crown srface area and relatively high weight. It has been reorted that slanted-sqish istons sress knock by accelerating combstion toward the end of the combstion rocess (2). The shaes of the cylinder head and iston that form the combstion chamber are shown in Fig. 1. Sqish areas are formed between the entroof ortions, which rn in the intake-exhast direction, and the iston, bt no sqish areas are formed in the engine s front-rear direction. This combstion chamber was sed as the basis for varios tests. 3. In-cylinder flows and combstion characteristics 3.1 Comtational Flid Dynamics (CFD) comarison of flat iston and slanted-sqish iston The reslts of CFD calclation with a flat iston and a slanted-sqish iston are shown in Fig. 2. The software sed was STAR-CD and es-ice. In the tmble flow * Advanced Powertrain Develoment Det., Develoment Engineering Office 32

2 Enhancement of Combstion by Means of Sqish Pistons Table 1 Merits and demerits of iston shaes Combstion chamber with sqish areas Item Flat iston Slanted sqish Horizontal sqish Srface/ volme ratio Combstion dration Knock resistance Intake air resistance g g Shae Weight Piston Size Heat losses Fig. 1 Fig. 2 Shae of combstion chamber and shae of tyical slanted-sqish iston CFD comtation for flat iston and slanted-sqish iston (Flow velocity vector chart) on the comression stroke, the flow velocity in area A tends, with the slanted-sqish iston, to be slightly increased by the inflence of the sqish entrance of iston, bt there is no great difference from the reslts for the flat iston. At 10 BTDC on the comression stroke, there is an intense forward sqish flow on the slantedsqish iston. Owing to collisions between this flow and the tmble flow, slggishness in the flow occrs in the vicinity of the exhast-side sqish exit. Also, the flow in the vicinity of the sark lg is weaker than it is with the flat iston. Flctations in initial combstion are concomitantly limited(3). At 10 ATDC, a strong reverse sqish flow is formed on the slanted-sqish iston. The area where slggishness occrred before TDC (the area circled in the figre) is drawn into the reverse sqish flow and is simltaneosly shed by residal tmble, reslting in the generation of a flow there. To this extent, the reverse sqish flow is stronger on the exhast side than it is on the intake side. 3.2 CFD comarison of tmble orts and base orts Promoting in-cylinder flows is an effective means of invigorating combstion, bt CFD comtation was erformed with regard to varios combinations of tmble flow cased by the collase of the eddy in the vicinity of TDC from blk flow and sqish (slanted) that cases trblence only in the vicinity of TDC. The shae differences between tmble and base orts and the tmble characteristics of each of ort tye 33

3 Enhancement of Combstion by Means of Sqish Pistons Fig. 3 Fig. 4 Port shaes and their tmble characteristics CFD comtation for tmble ort and base ort (Flow velocity vector chart) in steady-flow tests are shown in Fig. 3. Comared with the base ort, the tmble ort is more bilt- (more restricted) above the intake valve and more ct-away (flatter) at the bottom. Its shae directs the main intake flow toward the sark lg, thereby intensifying the tmble. As shown in Fig. 3, the tmble ort makes flows more intense with a valve lift of 6 mm and more. As this ort was modified by hand, three-dimensional measrements were taken, the reslts were droed into CAD data, and a simlation model was made. CFD calclation reslts for the model and the base ort (with a flat iston crown) are shown in Fig. 4. It can be seen that the stream flow on the intake side with the tmble ort is relatively strong on the comression stroke (at 90 BTDC) in comarison with the flat iston, meaning that the ort modifications led to intensification of in-cylinder flows. At 10 BTDC, there is a strong flow from the intake side toward the center of the sark lg. In the vicinity of TDC, tmble is maintained with the high tmble ort. With the base ort, by contrast, tmble is weak; vertical flows cannot be maintained, so the flow rotates at a slant. (See the bottom-left art of 34 Fig. 4.) The right-hand arts of Fig. 4 corresond to a combination of a sqish iston and the tmble ort. With the sqish iston, the tmble flow is thrown ward by the slanted face of the sqish entrance on the intake side of the iston so that the tmble at middle of the liner is made stronger than it does with the flat iston. Conseqently, an area of high velocity extends as far as the vicinity of the sark lg. 3.3 Partial-load combstion with combination of sqish and tmble Heat release rate nder artial loading with varios combinations of iston and ort is shown in Fig. 5. With regard to the average high tmble ratio, it is 0.77 with the base ort and 1.08 with the high tmble ort. In Fig. 5, the broken red line corresonds to a combination of base ort and flat iston. It can be seen that only changing the ort to the tmble ort (see the solid red line) and only changing the iston to the sqish iston (see the broken ble line) both make combstion faster. With these two combstion-romoting arrangements

4 Enhancement of Combstion by Means of Sqish Pistons Fig. 5 Heat release nder artial loading with varios combinations of iston and ort Fig. 6 Trblence energy in vicinity of sark lg (see the solid red line and broken ble line), the lines corresonding to initial combstion areas in this stdy haen to more or less coincide. With the sqish iston sed, the rise in heat release is raid after TDC and the absolte vales are also high (evidence of the combstion-romoting arrangements of the reverse sqish flow that was confirmed by CFD calclation). The shar increase in combstion acceleration after TDC shows that the cylinder ressre increases dring the iston s descent and that the combstion energy is efficiently converted into work. With the tmble ort combined with the sqish iston (see the solid ble line), the late-stage combstion acceleration that characterizes sqish exists bt, in addition, early- and middle-stage combstion occrs earlier; the combstion attern shows a shift in the heat release toward the advance-angle side. This combstion attern has most of the heat release occrring too far toward the advance-angle side, which conceived as not desirable with resect to heat losses and cycle efficiency. Fig. 6 shows the trblence energy in the vicinity of the sark lg as shown by CFD comtation. With the combination of tmble ort and sqish iston, the trblence energy is high from BTDC and is the reason that combstion is fast in the early and middle stages. This henomenon is connected to the intensification of tmble by the sqish iston shae as shown by the CFD calclation reslts in Fig. 4. By contrast, combining the sqish iston with the base ort does not increase the trblence energy in the vicinity of the lg. The slanted-sqish iston tends to reserve tmble easily de to its sqish inlet shae (rise from the cavity). Therefore, care mst be taken to combine it with a ort which does not intensify trblence (3). 3.4 Partial-load fel consmtion with combination of sqish and tmble Fig. 7 shows artial-load fel consmtion with the aforementioned flow intensification imlemented. Average tmble ratios by steady-flow are lotted against the horizontal axis, and fel consmtion and combstion drations are shown against the vertical axes. As the tmble rate increases, the flow coefficient decreases. With the base ort, changing from the flat iston to the sqish iston enables a redction in fel consmtion withot a change in the tmble ratio, i.e., withot a decrease in the flow coefficient (see art 1 of Fig. 7). With the flat iston, changing from the base ort to the tmble ort enables a redction in fel consmtion (see art 2 of Fig. 7). If the tmble ort and sqish iston are combined, however, fel consmtion becomes higher than it is with the base ort and sqish iston. As stated in the discssion of combstion analysis, a combination of sqish iston and tmble ort makes combstion occr early and is ths nfavorable with regard to thermal efficiency and heat losses. Fig. 8 shows the calclation reslts for heat losses with the conditions sed for combstion analysis. (For the calclation eqations, reference was made to reference docment (4). It was not ossible to make reference to differences between in-cylinder flows cased by differences between heat transfer rates.) The faster the combstion, the greater the heat losses shown. It can be inferred that the heat losses are articlarly high with the sqish iston artly becase the combstionacceleration osition is close to TDC and the temeratre in the cylinder is concomitantly high and artly becase the srface area of the iston crown is relatively large. 4. Effects of sqish-area shae on knock and combstion 4.1 Effects of sqish-area clearance height and shae With regard to effects of clearance shae (a sqisharea design arameter), a stdy was made of artialload combstion and knock. Two tyes of clearance were evalated. Their heights were 0.8 mm and 2.0 mm. The other clearance was wedge shaed, having a height of 1.4 mm at the widest oint at the sqish entrance and a height of 0.8 mm at the narrowest oint of the liner side (Fig. 9). 35

5 Enhancement of Combstion by Means of Sqish Pistons Main combstion dration (θ 10 90) Fig. 8 Partial-load heat losses with flow-intensifying measres imlemented Fig. 7 Partial-load fel consmtion with flow-intensifying measres imlemented Fig. 10 shows test reslts for artial-load combstion and fll load knock characteristics. The testing for artial-load combstion was condcted at 1,250 min 1 with a constant fel amont. The higher the indicated mean effective ressre (Pmi), the more efficient the combstion. With the 0.8 mm clearance, the % combstion dration is short and the Pmi vales are high. With the wedge shae, by contrast, the Pmi vales are the lowest and the % combstion dration is the longest. Aarently, amle forward or reverse sqish flow is not generated with a shae that widens toward the sqish entrance like this. With regard to fll load knock, with the 0.8 mm clearance and the wedge shae the knock ignition timing is, relative to the knock ignition timing with the 2.0 mm clearance, advanced by 2 at 3,000 min 1 and by 1 at 1,000 min 1. With the 2.0 mm clearance, the knock ignition timing is almost the same as with the flat iston and no knock-sressing effect is obtained. Considering the common featres of the aforementioned test reslt and the reslts for the wedge shae and 0.8 mm clearance, it is sosed that a small clearance in the end gas area leads to knock sression. One reason for the knock sression is aarently that the 0.8 mm clearance and the wedge shae, with both of which the end gas area has a high srface-to-volme (S/V) ratio, yield large amonts of heat transfer to the wall sch that the end gases are cool. Fig. 9 Shaes of sqish-area clearances 4.2 Effects of sqish in cylinder-head front-rear direction This aer has ths far discssed evalation erformed with combstion chambers that have sqish areas aligned only in the engine s intake-exhast direction. Sbseqently, the effects on fll load erformance and combstion of sqish areas rovided in the engine s front-rear direction (on the cylinder head) in addition to sqish areas aligned in the intakeexhast direction were investigated. A hotograh of the cylinder head is shown in Fig. 11. The front-rearaligned sqish areas on the cylinder head are each aroximately 7 mm wide. The clearance between the iston and cylinder head is 1.4 mm in the sqish area on the exhast side and 1.2 mm in each other sqish area. Fig. 12 shows heat release rate with each of two sqish-area atterns and with a flat iston. Combstion is fastest with the for sqish areas (intake, exhast, front, and rear). The difference between combstion with the for sqish areas and combstion characteristics with sqish areas aligned in only the intake-exhast direction are small, bt there is a marked difference from combstion characteristics with the flat iston. Fll load oeration is fastest with the for sqish areas at any given ignition timing, meaning that torqe is concomitantly high and the knock ignition timing is retarded, meaning that ott of the knocking oint is serior to that with sqish areas aligned in only the intake-exhast direction (Fig. 13). A ossible reason is that cooling of the sqish areas aligned in the cylinder head s front-rear direction is inadeqate. 36

6 Enhancement of Combstion by Means of Sqish Pistons Fig. 12 Heat release rate with each of two sqish-area atterns and with flat iston Fig. 10 Effects of sqish-area shae on artial-load combstion and fll load knock Fig. 13 Fll load erformance with varios sqish-area istons Fig. 11 Cylinder head with sqish areas aligned in engine s front-rear direction 5. Intake erformance and iston shaes dring fll load oeration Fig. 14 shows the fll load erformance of the sqish iston and flat iston. At nearly all engine seeds, the sqish iston yields serior torqe and sresses knock sch that the ignition timing is advanced. In addition at engine seeds of 4,500 min 1 and lower, it can be seen that volmetric efficiency is better with the sqish iston than it is with the flat iston. In line with these benefits, the sqish iston realizes 1.4 % higher maximm torqe than the flat iston and 1.2 % higher maximm ott than the flat iston. Generally seaking, a sqish iston is inferior to a flat iston in terms of intake erformance since it does not readily ermit large valve srface areas. On the other hand, it has been reorted that a slanted-sqish iston is better than a horizontal-sqish iston in terms of intake erformance owing to a smaller masked area in the cylinder head (2). However, there are no stdy reslts indicating seriority of a slanted-sqish iston over a flat iston. A stdy was erformed based on two hyotheses regarding the seriority of a slanted-sqish iston in terms of volmetric efficiency: 1 Fast combstion and concomitantly low exhast temeratres and low exhast ressre redce the internal exhast recirclation amont. 2 Creating the slanted sqish areas 37

7 Enhancement of Combstion by Means of Sqish Pistons Fig. 14 Fll load erformance Fig. 16 Otline of test stand and reslts of steady-flow tests Fig. 15 Fll load volmetric efficiency relative to ignition timing at varios engine seeds necessitates thickening of the material at the erihery of the iston crown, so a cavity is rovided in the center to ensre amle volme; the cavity romotes intake flows. With regard to hyothesis 1, volmetric efficiency with varios ignition timing was stdied. If hyothesis 1 were correct, a change in the ignition timing wold change the combstion seed; retarding the ignition timing wold raise the exhast temeratre, and the volmetric efficiency wold decrease. As shown in Fig. 15, however, there is hardly any change in volmetric efficiency when the ignition timing is changed; volmetric efficiency does not decrease as the ignition timing is retarded. Next the effects of the elements mentioned in hyothesis 2 were stdied sing a test stand. An otline of the test stand and the reslts of steady-flow tests are shown in Fig. 16. In the test stand, one third of the iston is ct away for air sction. With each of two iston tyes (the flat iston and the slanted-sqish iston), tests were erformed with the ressre difference between the stream side of the valve and the liner area constant at 1.0 kpa and constant at 4.0 kpa. The airflow was comared. Differences between the two istons reslts are evident with valve-lift vales of 1.5 mm and greater. The greater the valve lift, the greater the difference in intake airflow between the two iston shaes. And the higher the airflow, the more marked the difference between the resective effects of the iston shaes. In actal engine oeration, large valve lift does not occr near TDC. Nevertheless, the reslts of the tests erformed with the test stand clearly show that the iston shae affects intake erformance. The effects with an actal engine were next stdied. Two test istons (both of the slanted-sqish tye) were sed. One of the istons (designated iston B) has large sqish areas arond its erihery. The other iston (designated iston A) is similar excet that its sqish area in the vicinity of the intake valve is smaller (Fig. 17). Fig. 18 shows volmetric efficiency dring fll load oeration. Althogh the shae difference is small, there is a clear difference in volmetric efficiency. It was ths verified with a real engine, too, that iston shae inflences volmetric efficiency. From these findings, it can be said that roviding a sace nder the intake valve (articlarly in the direction of the main 38

8 Enhancement of Combstion by Means of Sqish Pistons Fig. 17 Effect on fll load volmetric efficiency of redction in size of sqish area in vicinity of intake valve COV θ 0 10 (%) Fig. 18 Piston for evalation of cavity deth Fig. 19 Inflence of cavity deth on combstion intake flow) is a key factor of the iston design for imrovement of volmetric efficiency. 6. Effects on combstion of iston shae otside sqish areas To enable a slanted-sqish iston to be sed at a desired comression ratio, it is common ractice to form a cavity in the middle of the iston crown sch that the configration of the cavity yields the desired comression ratio. The inflence of the cavity on combstion was stdied. For the cavity itself, the cavity deth was sed as the main evalation arameter. For the ortion rnning from the cavity to each sqish area, the angle of the sloe was sed as the main evalation arameter. Fig. 18 shows the resective shaes of evalated istons with different cavity deths. The index indicating the deth was designated h and taken as the distance from the bottom of the thread on the sark lg to the center of the iston crown. The h/bore ratio is It has been reorted that the h/bore ratio controls the timing of the collase of tmble; the larger the h/bore ratio, the later the collase of tmble (5). Fig. 19 shows fll load heat release rate and initial combstion dration (0 10 %) variation (standard deviation / average vale). With the iston containing the deeest cavity, there is significant scattering in the data for initial combstion. And it can be seen from comarison with data for the base iston that mid-eriod combstion with the iston containing the deeest cavity is slggish. With a dee cavity and a concomitantly high h/bore ratio, the tmble-collase delay and the collase itself are insfficient, aarently meaning that a blk strong flow remains in the vicinity of the sark lg at the time of ignition sch that initial combstion is nstable. With regard to the slowness of mid-eriod combstion, it can be inferred that the dee cavity makes the blk flow-sqashing effect weak sch that the transformation into trblence is insfficient. Investigative comarisons of slanted-sqish istons and conventional-sqish (horizontal-sqish) istons have yielded the view that reverse sqish flow from the cavity searates on the sharly angled art of the iston at the sqish entrance, thereby intensifying trblence, with the reslt that knock is sressed by acceleration of late-stage combstion (6). In the stdy described in this aer, the inflence of a slantedsqish iston was stdied with three angles of the sloe 39

9 Enhancement of Combstion by Means of Sqish Pistons Fig. 20 Shae of sloe from cavity to sqish area Second half of main combstion (θ 50 90) Fig. 21 from the cavity. Fig. 20 shows the resective shaes of the three evalated istons. The figre shows only the sloe shaes in the intake-exhast direction, bt the sloe shaes are the same in the engine s front-rear direction. Fig. 21 shows the resective combstion drations of the three istons nder artial loading. With the iston whose sloe to the sqish entrance has the sharest angle, the first half of the main combstion (10 50 % combstion) and the second half of the main combstion (50 90 % combstion) are both shorter than they are with the other istons. Fll load heat release rate is shown in Fig. 22. The most notable characteristic is that acceleration of combstion can be seen ATDC at 1,000 min 1. (See the circled art of the figre.) This shows that it is ossible to inflence trblence with a given slanted-sqish iston by means of the design of the angle of the sloe from the cavity. At 3,000 min 1, however, in-cylinder trblence is increased sch that the difference in heat release between the evalated istons is smaller than at 1,000 min 1. It can be said that the sqish-area entrance angle is a crcial design factor. 7. Heat losses Partial-load combstion drations First half of main combstion (θ 10 50) increase the srface area and ths they are disadvantageos against heat loss. Since combstion is romoted nder artial loading with a slanted-sqish iston, however, the minimm advance for best torqe (MBT) is retarded sch that the eriod in which the iston crown receives heat is shortened. At this oint in the stdy, heat losses at MBT with a flat iston and with a slanted-sqish were verified. Fig. 23 shows indicated thermal efficiency against ignition timing. The ignition timing at which combstion is fastest and thermal efficiency is highest is later with the sqish iston than it is with the flat iston. For each iston, the measred cylinder ressre at the oint with maximm thermal efficiency was sed in calclation of heat losses (4). The reslts are shown in Fig. 24. With the flat iston, the ignition timing at which thermal efficiency is highest is relatively advanced so heat release starts relatively soon and heat losses are concomitantly high. For the sqish iston, the calclations indicate heat losses almost the same as those for the flat iston, notwithstanding the sqish iston s larger srface area. Even thogh a sqish iston is inferior to a flat iston in terms of geometric srface area, then, the demerits of a sqish iston in terms of heat losses can be minimized if amle consideration is given to combstion characteristics. 8. Smmary Fig. 22 Fll load heat release Sqish area was stdied mainly with resect to the inflence of iston shae on combstion and volmetric efficiency. An nderstanding of the effects of the heights and shaes of sqish areas and of the effects of the arts of a sqish iston s crown otside the sqish areas was gained. It was fond that the angle of the sloe from the cavity to the sqish area and the cavity deth are crcial design factors and that the sace formed by the iston below the intake valve inflences volmetric efficiency. Forming slanted sqish and roviding a cavity in the iston crown to achieve a sitable comression ratio 40

10 Enhancement of Combstion by Means of Sqish Pistons Indicated thermal efficiency (ηi) Fig. 23 Indicated thermal efficiency relative to ignition timing References (1) Yoshikaz Ishikawa: Key Factors in Gasoline Engine Design, Sankaido Pblishing Co., Ltd., 2002 (2) Takanori Ueda et al: Effects of Sqish Area Shae on Knocking, JSAE symosim roceedings, Vol. 982,. 99, 1998 (3) Michihiko Tabata, Masahiko Fjimoto, and Kohei Iwai: Imrovement of Knocking by Means of Tmble Flow and Reverse Sqish Flow, Mazda Giho, No. 21,. 192, 2003 (4) Toshio Shdo and Hiroyki Szki: Research into Wall Heat Transmission Formlae with Hydrogen Engine, JSME transactions, Vol. 68, No. 673,. 200, 2002 (5) Makoto Kaneko, Ssm Kato, and Yohei Saish: Control of Sark-Ignition Engine by Means of Control over Tmble Generation and Collase, Sbar Technical Review, Vol. 30,. 117, 2003 (6) Shinji Kojima: Elcidation of Knock-Sression Mechanism by Means of Combstion Simlation, Toyota Central R&D Labs R&D Review, Vol. 33, No. 3,. 114, 1998 Fig. 24 Heat losses (aroximate) with each iston tye Katshiko MIYAMOTO Yoshiyki HOSHIBA Kiyotaka HOSONO Synichi HIRAO 41