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Enhancement of Combustion by Means of Squish Pistons Katsuhiko MIYAMOTO* Yoshiyuki HOSHIBA* Kiyotaka HOSONO* Syunichi HIRAO* Abstract Knowledge gained through studies aimed at enhancing combustion by means of changes in piston shapes is presented in this paper. Attention was first focused on tumble and squish in the context of in-cylinder flows, and the characteristics of combustion patterns and effects on partial-load fuel consumption were investigated. Squish is characterized by invigoration
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  1.Introduction Amid the growing problem of global warming,about 20 % of carbon-dioxide emissions are attributableto transportation and about 90 % of this amount isattributable to automobiles. Since improvements infuel efficiency are vital, there is much ongoing researchinto ways to enhance the thermal efficiency of engines.There are various techniques for improving thermal effi-ciency. Some techniques (exhaust-gas recirculationand lean combustion, for example) work primarily byreducing pumping losses. Others (suppression of high-load knock and optimization of in-cylinder flows, forexample) work primarily by enhancing combustion.Efforts to enhance combustion by optimizing in-cylin-der flows fall into two broad categories: 1 those thatinvigorate combustion by promoting tumble, swirl, andother in-cylinder flows throughout the engine’s intakeand compression strokes; and 2 those that use squishto create turbulence mainly in the vicinity of Top DeadCenter (TDC) on the compression stroke. At the time ofwriting, slanted squish areas, which are suitable forpentroof combustion chambers, are widely used. Thestudy described in this paper was focused on compar-ing the effects of slanted-squish pistons with those offlat pistons with respect to combustion and perfor-mance. The effects of slanted-squish pistons on knocksuppression and intake air amounts during high-loadoperation and their effects on fuel consumption duringpartial-load operation were investigated. 2.Piston shapes and general characteristics Table 1 shows three main piston-crown shapes andtheir respective merits and demerits (1) . A flat piston hasa small crown surface area and consequently has themerits of small heat losses and low weight. A horizon-tal-squish piston is effective at promoting combustion,but small valve diameters are necessary to provide asquish area. During high-speed operation, therefore,there is a tendency for air volumes to decrease and forperformance to be concomitantly limited. A slanted-squish piston is well suited to a pentroof combustionchamber in which ignition takes place at a central pointbetween the four valves (the typical combustion-cham-ber configuration in recent engines), and it has the mer-it of providing a squish area without requiring anyreduction in valve diameter. However, it has the demer-its of a relatively large piston-crown surface area andrelatively high weight. It has been reported that slant-ed-squish pistons suppress knock by accelerating com-bustion toward the end of the combustion process (2) .The shapes of the cylinder head and piston that formthe combustion chamber are shown in Fig. 1 . Squishareas are formed between the pentroof portions, whichrun in the intake-exhaust direction, and the piston, butno squish areas are formed in the engine’s front-reardirection. This combustion chamber was used as thebasis for various tests. 3.In-cylinder flows and combustion charac-teristics 3.1Computational Fluid Dynamics (CFD) comparisonof flat piston and slanted-squish piston The results of CFD calculation with a flat piston anda slanted-squish piston are shown in Fig. 2 . The soft-ware used was STAR-CD and es-ice. In the tumble flow 32 Enhancement of Combustion by Means of Squish Pistons Katsuhiko MIYAMOTO* Yoshiyuki HOSHIBA*Kiyotaka HOSONO* Syunichi HIRAO* Abstract Knowledge gained through studies aimed at enhancing combustion by means of changes inpiston shapes is presented in this paper.Attention was first focused on tumble and squish in the context of in-cylinder flows, and thecharacteristics of combustion patterns and effects on partial-load fuel consumption were investi-gated. Squish is characterized by invigoration of combustion in the middle and late stages, whichis desirable for combustion pattern. For reduction of fuel consumption, it was confirmed thatimproved in thermal efficiency yielded by squish-enabled increase in combustion speed is benefi-cial with low engine loading and that squish-enabled suppression of knock is beneficial with highengine loading. During full load operation, it was confirmed that not only knock suppression butalso piston-shape-yielded improvements in volumetric efficiency are beneficial. The benefits wereevaluated experimentally and through the analysis of in-cylinder flows. These findings and particu-larly influential design factors are discussed in this paper. Key words:  Gasoline Engine, Combustion, Knocking, Piston *Advanced Powertrain Development Dept., Development Engineering Office  on the compression stroke, the flow velocity in area Atends, with the slanted-squish piston, to be slightlyincreased by the influence of the squish entrance of pis-ton, but there is no great difference from the results forthe flat piston. At 10 ˚ BTDC on the compression stroke,there is an intense forward squish flow on the slanted-squish piston. Owing to collisions between this flowand the tumble flow, sluggishness in the flow occurs inthe vicinity of the exhaust-side squish exit. Also, theflow in the vicinity of the spark plug is weaker than it iswith the flat piston. Fluctuations in initial combustionare concomitantly limited (3) .At 10 ˚ ATDC, a strong reverse squish flow is formedon the slanted-squish piston. The area where sluggish-ness occurred before TDC (the area circled in the figure)is drawn into the reverse squish flow and is simultane-ously pushed by residual tumble, resulting in the gen-eration of a flow there. To this extent, the reversesquish flow is stronger on the exhaust side than it is onthe intake side. 3.2CFD comparison of tumble ports and base ports Promoting in-cylinder flows is an effective means ofinvigorating combustion, but CFD computation was per-formed with regard to various combinations of tumbleflow caused by the collapse of the eddy in the vicinityof TDC from bulk flow and squish (slanted) that causesturbulence only in the vicinity of TDC.The shape differences between tumble and baseports and the tumble characteristics of each of port type 33 Enhancement of Combustion by Means of Squish Pistons Fig. 1 Shape of combustion chamber and shapeof typical slanted-squish pistonFig. 2 CFD computation for flat piston and slanted-squish piston (Flow velocity vector chart)Table 1 Merits and demerits of piston shapes Combustion chamber with squish areasFlat pistonItemSlanted squishHorizontal squishShapeSurface/ u u p volume ratioCombustion p p u durationKnock resistance p p u Intake air p u p resistanceWeight u g p PistonSize u u p Heat u g p losses  in steady-flow tests are shown in Fig. 3 . Compared withthe base port, the tumble port is more built-up (morerestricted) above the intake valve and more cut-away(flatter) at the bottom. Its shape directs the main intakeflow toward the spark plug, thereby intensifying thetumble. As shown in Fig. 3 , the tumble port makesflows more intense with a valve lift of 6 mm and more.As this port was modified by hand, three-dimensionalmeasurements were taken, the results were droppedinto CAD data, and a simulation model was made.CFD calculation results for the model and the baseport (with a flat piston crown) are shown in Fig. 4 . It canbe seen that the upstream flow on the intake side withthe tumble port is relatively strong on the compressionstroke (at 90 ˚ BTDC) in comparison with the flat piston,meaning that the port modifications led to intensifica-tion of in-cylinder flows. At 10 ˚ BTDC, there is a strongflow from the intake side toward the center of the sparkplug. In the vicinity of TDC, tumble is maintained withthe high tumble port. With the base port, by contrast,tumble is weak; vertical flows cannot be maintained, sothe flow rotates at a slant. (See the bottom-left part of Fig. 4 .)The right-hand parts of Fig. 4 correspond to a com-bination of a squish piston and the tumble port. Withthe squish piston, the tumble flow is thrown upward bythe slanted face of the squish entrance on the intakeside of the piston so that the tumble at middle of the lin-er is made stronger than it does with the flat piston.Consequently, an area of high velocity extends as far asthe vicinity of the spark plug. 3.3Partial-load combustion with combination ofsquish and tumble Heat release rate under partial loading with variouscombinations of piston and port is shown in Fig. 5 . Withregard to the average high tumble ratio, it is 0.77 withthe base port and 1.08 with the high tumble port. In Fig.5 , the broken red line corresponds to a combination ofbase port and flat piston. It can be seen that only chang-ing the port to the tumble port (see the solid red line)and only changing the piston to the squish piston (seethe broken blue line) both make combustion faster.With these two combustion-promoting arrangements 34 Enhancement of Combustion by Means of Squish Pistons Fig. 3 Port shapes and their tumble characteristicsFig. 4 CFD computation for tumble port and base port (Flow velocity vector chart)  (see the solid red line and broken blue line), the linescorresponding to initial combustion areas in this studyhappen to more or less coincide.With the squish piston used, the rise in heat releaseis rapid after TDC and the absolute values are also high(evidence of the combustion-promoting arrangementsof the reverse squish flow that was confirmed by CFDcalculation). The sharp increase in combustion acceler-ation after TDC shows that the cylinder pressureincreases during the piston’s descent and that the com-bustion energy is efficiently converted into work.With the tumble port combined with the squish pis-ton (see the solid blue line), the late-stage combustionacceleration that characterizes squish exists but, inaddition, early- and middle-stage combustion occursearlier; the combustion pattern shows a shift in the heatrelease toward the advance-angle side. This combus-tion pattern has most of the heat release occurring toofar toward the advance-angle side, which conceived asnot desirable with respect to heat losses and cycle effi-ciency. Fig. 6 shows the turbulence energy in the vicinity ofthe spark plug as shown by CFD computation. With thecombination of tumble port and squish piston, the tur-bulence energy is high from BTDC and is the reasonthat combustion is fast in the early and middle stages.This phenomenon is connected to the intensification oftumble by the squish piston shape as shown by the CFDcalculation results in Fig. 4 .By contrast, combining the squish piston with thebase port does not increase the turbulence energy in thevicinity of the plug.The slanted-squish piston tends to preserve tumbleeasily due to its squish inlet shape (rise from the cavi-ty). Therefore, care must be taken to combine it with aport which does not intensify turbulence (3) . 3.4Partial-load fuel consumption with combination ofsquish and tumbleFig. 7 shows partial-load fuel consumption with theaforementioned flow intensification implemented.Average tumble ratios by steady-flow are plottedagainst the horizontal axis, and fuel consumption andcombustion durations are shown against the verticalaxes. As the tumble rate increases, the flow coefficientdecreases. With the base port, changing from the flatpiston to the squish piston enables a reduction in fuelconsumption without a change in the tumble ratio, i.e.,without a decrease in the flow coefficient (see part 1 of Fig. 7 ). With the flat piston, changing from the base portto the tumble port enables a reduction in fuel consump-tion (see part 2 of Fig. 7 ). If the tumble port and squishpiston are combined, however, fuel consumptionbecomes higher than it is with the base port and squishpiston.As stated in the discussion of combustion analysis,a combination of squish piston and tumble port makescombustion occur early and is thus unfavorable withregard to thermal efficiency and heat losses. Fig. 8 shows the calculation results for heat losses with theconditions used for combustion analysis. (For the cal-culation equations, reference was made to referencedocument (4). It was not possible to make referenceto differences between in-cylinder flows caused by dif-ferences between heat transfer rates.) The faster thecombustion, the greater the heat losses shown. It canbe inferred that the heat losses are particularly highwith the squish piston partly because the combustion-acceleration position is close to TDC and the tempera-ture in the cylinder is concomitantly high and partlybecause the surface area of the piston crown is rela-tively large. 4.Effects of squish-area shape on knock andcombustion 4.1Effects of squish-area clearance height and shape With regard to effects of clearance shape (a squish-area design parameter), a study was made of partial-load combustion and knock. Two types of clearancewere evaluated. Their heights were 0.8 mm and 2.0mm. The other clearance was wedge shaped, having aheight of 1.4 mm at the widest point at the squishentrance and a height of 0.8 mm at the narrowest pointof the liner side ( Fig. 9 ). 35 Enhancement of Combustion by Means of Squish Pistons Fig. 5 Heat release under partial loading with variouscombinations of piston and portFig. 6 Turbulence energy in vicinity of spark plug
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