马自达创驰蓝天发动机Mazda_sky_2.0L

MAZDA SKYACTIV-G 2.0L Gasoline EngineIchiro Hirose, Hidetoshi Kudo, Tatuhiro Kihara, Masanao Yamakawa,Mitsuo HitomiMazda Motor Corporation, Hiroshima, JapanSummaryThe SKYACTIV-G is the first gasoline engine which was developed based on Mazda's long-term vision for technology and the search of internal combustion engineers who are pursuing technologies to develop the ultimate internal combustion engine. This is done with Mazda's aim in mind to achieve a balance between enjoyable driving and environmental performance at the supreme level.The challenging target of 15% torque improvement compared with that of conventional engine was set. This was achieved by solving technological issues towards introduction of extremely high combustion ratio and a 4-2-1 exhaust system.1 IntroductionMazda is aiming at developing the ideal internal combustion engine before we see the complete EV age. We are sure we can contribute to environmental protection primarily by improving the internal combustion engine which will remain mainstream for the automotive powertrain in the mean time. We also believe that the system cost can be reduced by combining the internal combustion engine with the electric devices, which leads to marketability improvement, such as purchasing cost reduction and maintenance cost reduction. Therefore, as you can see from Figure 1, our plan is to upgrade the efficiency of base technologies for the internal combustion engine and others first, combine such technologies with the electric devices as a next step, and then introduce the combined technologies into markets one after another. We call this plan the "Building-Block Strategy".Figure 2 shows our approach toward the ideal internal combustion engine development. We consider that the approach is the same for both gasoline and diesel engines. To be more specific, bringing control factors, such as compression ratio, combustion duration, etc. to the ideal conditions means that the gasoline engine adopts advantages of the diesel engine and vice versa. The SKYACTIV-G, which has been developed based on the above-mentioned long-term technological development strategy, is the new 2.0 liter gasoline engine which will offer driving pleasure to our customers more than ever and also achieves superb environmentally-friendly performance. It is planned that the SKYACTIV-G will be gradually introduced into various markets in 2011 and thereafter.Fig. 1: MAZDA Building-Block StrategyFig. 2: Roadmap for the Ultimate Internal Combustion Engine2 Technological targetAs the first step to bring the gasoline engine closer to the ideal internal combustion engine, the technological targets were set: 15% improvement in the NEDC mode fuel economy and the full load performance respectively, compared with Mazda's existinggasoline engine (Figure 3). The idea behind these was to develop the world-best engine.Fig. 3: Functional TargetIn order to achieve the technological targets, all the controlling factors except for specific heat ratio were upgraded functionally. Considering the cost performance and assuming the natural-aspirated direct-injection engine, it was aimed to improve the compression ratio and pumping loss which were inferior to those of the diesel engine and reduce the mechanical loss further which was superior to that of the diesel engine.In particular, the functional targets were compression ratio increase to world highest 14:1 (@95RON) while keeping the same combustion period, 20% pumping loss reduction, 30% mechanical loss reduction and 10% charging efficiency improvement. Table 1 shows the main specification comparison of the current PFI engine (hereinafter referred to as "base engine") and the SKYACTIV-G.Table 1: Main Specifications3 Fuel Economy3.1 Compression ratioThe world-highest compression ratio of 14:1 was targeted. It is generally known that with only raising the compression ratio strongly, a high fuel consumption improvement cannot be gained. It was identified at the beginning of the development that primary sources for this were long combustion duration and high cooling loss due to quenching of initial flame kernel core, damping of tumble air motion at the top of the piston (Figure 4 & 5).Fig. 4: Effect of compression ratio on Constant Volume=11Fig. 5: Effect of Compression Ratio on In-cylinder FlowIn order to solve this issue, it was a must to balance two conflicting requirements: to form a large cavity while maintaining a high compression ratio. Therefore, basic designs and dimensions such as the bore diameter and the valve angle for intake and exhaust were carefully reviewed.Fig. 6: Requirements for Bore/Stroke SelectionFirst of all, as shown in Figure 6 it was decided to secure the compression ratio of 15 or more for the future. Then the bore diameter was determined to satisfy the peak power requirement with careful consideration of the small bore requirement to reduce surface-volume ratio and the valve diameter requirement. Consequently, the bore diameter was reduced from 87.5mm to 83.5mm. The valve angle was expanded from intake 19 deg./exhaust 20 deg. to intake 22 deg./exhaust 23 deg. The cavity was designed so that the flame should not touch the piston until the flame level reaches 5% MFB from the perspective of cooling loss reduction.Fig. 7: Effect of Cavity Piston on Tumble Ration and Cooling LossAs you can see from Figure 7, this cavity is effective in reducing the cooling loss and increasing tumbling flow. The cooling loss was reduced by 9% while the combustionspeed was increased as shown in Figure 8. As a result (see Figure 9), the new piston improved specific fuel consumption by 2-3% from the original piston. Further the thermal efficiency improvement effect of compression ratio came closer to thetheoretical expectation.Fig. 8: Effect of Piston Shape on Heat Release0%1%2%3%1500rpm/100kPa 1500rpm/262kPa2500rpm/100kPachanging the port angle to be gentler. With this, the combustion speed was increasedas described in Figure 11.Fig. 10:Intake Port Configuration Fig. 11:Effect of Tumble ration on Heat Release Fig. 12:Effect of Tumble ration on Combustion Duration36384042444648501 1.2 1.4 1.6 1.8Tum ble ratio [-]Target 87.5mmbore 83.5mmboreAs a result, a wider overlapping than ever before and a significantly late intake valve close timing were achieved. To be more precise, a dual S-VT (sequential valve timing system) was incorporated. The intake valve timing was set at IVC=110degATDC/EVC=50ATDC (@2000rpm/200kPa). Figure 13-15 show the pumping loss comparison between the base engine and the SKYACTIV-G. The base engine has TSCV (Tumble Swirl Control Valve) and an external EGR. In contrast, the SKYACTIV-G does not have such devices. However, the SKYACTIV-G increased the internal EGR ratio (11% to 18%), achieved intake valve close timing retarded up to 110ATDC and reduced the pumping loss by 20% without any combustion instability.2018161412105060708090100110Intake Valve Close(IVC) Timing [deg CA]40506070809010005101520Exhaust Gas Recirculation ratio [%]3.3.1 Crank/Piston/ConnrodThe diameter of the crank main journal was reduced from =47mm,while the required rigidity was maintained. In addition, the LOC (lubricant oil consumption) performance was enhanced. These improvements made it possible to use the piston ring with the tension lower than that of the current piston ring by 38%.3.3.2 Valve systemThe valve spring weight was reduced by introducing a roller follower and optimizing the cam profiles. For the chain system, the mechanical resistance was reduced due to chain behavior stabilization by reducing the wearing resistance between the high-rigid straight guide and the chain, and dividing and equalizing the load on the levers.3.3.3 Lubrication systemFirst the oil pump capacity was reduced by decreasing the pressure loss in the hydraulic pressure channel and by minimizing the hydraulic pressure requirements for hydraulic pressure-related devices. Then the hydraulic pressure during the partial load operation was reduced by adopting the electrically-controlled variable hydraulic pressure oil pump. The more effects of the variable hydraulic pressure on the fuel economy are seen under the cold condition when the viscosity and resistance are higher.3.3.4 Cooling systemThe water pump unit efficiency was upgraded by a highly effective plastic impeller and reducing the resistance in the cooling channels.Figure 19 indicates the positioning of the SKYACTIV-G in terms of friction loss by comparing with the base engine friction loss measured by a third party (under the condition of 2000rpm and all accessories included) and using the improvement rate which Mazda confirmed. It demonstrates that the SKYACTIV-G is the world's best in terms of the control factor of the friction loss as well.Fig. 17:Examples of Friction Loss ReductionFig. 18:Typical Components for Friction Loss Reduction050100150200100020003000400050006000Engine speed (rpm)F3.4 Fuel economy in vehicle levelThe vehicle fuel economy was measured by using the C/D-class vehicles equipped with the SKYACTIV-G. Figure 20 shows its breakdown. It was verified that the SKYACTIV-G improved the fuel economy in the NEDC by 15%, compared with the current Mazda engine. This is resulted from the fuel economy improvement effects under the hot and steady conditions, such as the above-mentioned high compression ratio with pumping loss reduction (8%) and friction loss reduction (4%). There are other contributors including effects of variable hydraulic pressure under the warm-up condition where hydraulic pressure difference becomes larger, idle engine speed reduction and so on.0%2%4%6%8%10%12%14%16%Compression Ratio and Pumping Loss EffectIdle-Speed Reduction140150160170180190200210TargetBaseInitial statusFull Load Performance improvement0%15%0%15%Current EngineTargetBreakthrough by combustionFig. 22: Toque Recovery Target by Combustion Improvement4.1 Functional target for full load performance improvementRoot cause of the torque decrease are exactly deterioration in knocking resistance and efficiency drop, which resulted from an increase in the pressure and temperature in the cylinder due to the high compression ratio. Therefore, it was focused to reduce the unburned gas temperature in the cylinder at TDC and to improve the combustion duration, referring to characteristics of the DI engine with low compression ratio of 11:1. As shown in Figure 23, the functional target is equivalent to 50 degree C reduction in the gas temperature in the cylinder and reduction in combustion duration by 5 deg.CA (equivalence of approx. 20%) from the initial status of the SKYACTIV-G. In order to meet this functional target, it was focused to improve the combustion system and the exhaust system.302826242220670680690700710720730740750 Unburned gas temperature at TDC (K)Flat piston20BTDC10BTDCTDC 10ATDCCavity pistonFig. 24:Effect of Piston Shape on Flame PropagationFig. 25:Effect of Piston Shape on Heat ReleaseFig. 26:Effect of Piston Shape on Torque4.2.2 Air motion enhancementProgresses were made in the following elements, a) the bore diameter reduction to diminish the cooling loss, and b) the tumble motion enhancement to raise the combustion speed, with a view to knocking resistance enhancement. The long stroke resulting from the bore diameter reduction effectively enhanced the tumble motion. As shown in Figure 27-28, the combined technologies, the tumble motion enhancement and the bore diameter reduction, reduced the combustion duration by 2 deg.CA and improved the torque by 4%.-20020406080100120140-10103050Crank Angle (deg)87.5mm Bore Original int.port83.5mmBore Enhanced int.port4%Fig. 28: Effect of Tumble Ratio on Torque4.2.3 Air-Fuel mixture formation improvementIt was challenged to maximize the charging effect as the combustion system approach toward the gas temperature reduction in the cylinder.In order to efficiently cool down the gas temperature in the cylinder, the latent heat of fuel and multi-hole injector with 6 holes and an excellent mixing characteristic was introduced. Some elements including the fuel spray angle of each cylinder, the spray penetration and the injection ratio for split injection were optimized by utilizing CAE in order to meet various requirements, such as the mixture homogenization for getting charging effect, and the high stratification for catalyst heating up at warming up, which will be touched on later, the oil dilution and the smoke so on.In general, the spray pattern is designed to inject the fuel homogeneously into the cylinder targeting a homogeneous mixture. However,penetration of #1 spray was controlled more to reduce the oil dilution. The spray angle in the horizontal direction for the #1, #2 and #3 sprays in the first and second rows, which play an important role in stratified mixture formation, is designed so that sprayed fuel is captured by the cavity. The spray angle and penetration for the #6 spray in the bottom row is designed to enhance tumble air motion (Figure 29-30).Fig. 29: Basic Spray ConfigurationFig. 30: Visualization Results of In-Cylinder FlowFigure 31-32 indicate differentials in homogenization among three spray patterns. The Layout-A spray pattern, which the SKYACTIV-G adopted, achieved a better homogenization than the other spray patterns and much better than the traditional swirl-type spray patterns.X sprayY spray 060mm20mm 40mm 0 60mm20mm 40mm (i) Layout A (ii) Layout B (iii) Layout CX sprayY spraysFig. 33: Comparison of Mixture HomogeneityLayout B Layout AFig. 36: Comparison of gas temperature distributionThe combustion duration reduction at 4 deg. CA and 10% torque improvement from the initial status were achieved by incorporating these combustion-related technologies. The SKYACTIV-G with high compression ratio of 14 realized the same torque at low engine speed as the current DI with the compression ratio of 11.4.3 Exhaust system upgradeAlthough the torque was improved by 10% by upgrading the combustion system, as indicated in Figure 37, further 8% improvements were required to meet the torque target at low engine speed. To achieve this, it was aimed at improving the exhaustsystem. Full Load Performance improvement 0%15%0%15%Current EngineTargetBreakthroughby exh. systemFig. 37:Target for Exhaust System DevelopmentFigure 38 indicates the relation between the charging efficiency for the residual gas ratio and the gas temperature in the cylinder at TDC based on the CAE analysis. The residual gas ratio for the 4-1 exhaust system, which is the base exhaust system, isapprox. 7%. The charging efficiency is 84%. The temperature in the cylinder is 720K.6080100120140]39[K]4-1CCstatusScavengingFig. 38: Functional Target for Exhaust System DevelopmentIt is required to secure approximately 160kPa boosting pressure @ IVC in order to gain the charging efficiency sufficiently enough to meet the low-end torque target while keeping the same residual gas ratio. However, this was not an option for the SKYACTIV-G because its design concept was naturally aspiration. Therefore, an approach taken was to reduce the residual gas and the temperature in the cylinder by optimizing the use of scavenging effects. As shown in Figure 38, with scavenging effects, the residual gas was reduced by approx. 45% and the temperature in thecylinder was reduced by 39K. As a result, the charging efficiency increased by approx. 9%, which gave a positive outlook toward the low-end torque target achievement.Fig. 39: Basic Concept of Exhaust System DesignIn order to maximize scavenging effects, it was started to develop the 4-2-1 long exhaust system which is almost disappearing from the industry because of emission reasons.First the exhaust manifold basic specification was decided from the aspect of the pressure wave timing control. In order to prevent the exhaust gas from getting into another cylinder during valve overlap, in other words, in order to prevent the residual gas increase in all the engine speed ranges due to the reversal exhaust gas flow, the runner length to the exhaust manifold collector position was set to approx. 600 mm. This contributed to shifting the resonance point of the reverse negative pressure wave at the exhaust manifold collector during valve overlap to the relatively wide ranges of the engine speed (2000/3000/5000 rpm).Then the pipe diameter and shape of exhaust manifold were optimized to maximize the reverse negative pressure. The exhaust manifold pipe inner diameter was set to) of the exhaust pipe.Fig. 40: Basic Concept of Exhaust System DesignSince it was difficult to predict the above-mentioned 3-dimensionnal effect of exhaust manifold on pressure wave characteristics by using CAE, as you can see from Figure 40, the characteristics were optimized through the rig test with which reflection/damping of the impulse acoustic wave was acoustically analyzed. There are high correlation between the rig test result and the actual engine test result.In addition, the blow down timing and the timing of reversed negative wave coming are carefully adjusted in the engine speed range where scavenging effects are expectable by controlling the EVO timing and the IVO timing optimally.The exhaust manifold was designed to be compact and loop-shaped as shown in Figure 41 so that it was installable on 4WD vehicles and small-sized vehicles. As mentioned earlier, the exhaust manifold was developed by optimizing the tube bending radius through the acoustic rig test. It was confirmed that by doing so, the performance equal to that of the straight 4-2-1 exhaust manifold was secured without deteriorating the pressure wave transmission function, which was the basic function for this exhaust manifold.Fig. 41: Final Design of 4-2-1 Exhaust ManifoldOn the other hand, it was clarified that if the piston protruded too much, scavenging effects were spoiled even when the optimized exhaust system was used. Therefore, the piston design was changed as shown in Figure 42 as a result of the bore diameter reduction. The bore diameter decrease made it possible to reduce the piston protrusion height, maintaining the compression ratio. With this, the torque was improved as expected through the use of the scavenging effects.Thanks to the above-mentioned efforts, the charging efficiency was improved by 9% and the torque at the low engine speed was improved by 8%.Fig. 42: Final Design of Piston ShapeThe improvements in the combustion duration and the gas temperature in the cylinder, and the torque improvement effects are put together and shown in Figure 43 & 44. The temperature in the cylinder and the combustion duration which are almost equal to those of the current DI engine (compression ratio of 11:1) were finally achieved. In addition, the charging efficiency was improved as planned. With these technologies, the low-end torque of SKYACTIV-G was finally upgraded by approx. 15 % from the current DI engine, although its compression ratio was 14:1.202224262830670680690700710720730740750Unburned gas temperature at TDC (K)140150160170180190200210IntialStatusTarget2%pistonoptimisation4%Small Bore andAlirMotion opt.3%mixtureoptimization8%ScavengingIntial Status TargetFig. 44: Roadmap to the Low-End Torque TargetThe scavenging effects gained by improving the exhaust manifold design are also gained from the reverse negative pressure wave from the other exhaust components. It is designed to get scavenging effects in the wide range of the engine speed by adjusting resonance point of the reverse negative wave from the pre-silencer to 2500 rpm and that from main silencer to 1500 rpm (Figure 39).The charging efficiency was improved by adjusting the resonance point of intake system is utilized for 2250rpm where no scavenging effects are expected because the reverse positive wave resonances to valve overlap. By this effort, the flat torque curve was developed in all over engine speed range.It was also verified that the torque drop sensitivity to noise factors, such as intake air temperature, hydraulic temperature, fuel octane value, etc., was equivalent to that of existing engines.As the vehicle models which the SKYACTIV-G will be installed on will commonize the locations and functions of all the intake/exhaust system components which have impacts on torque, it is expected that all the vehicle models will exhibit almost the same torque characteristics.Through the above-mentioned efforts, the technical targets were met: 15% improvements in the fuel economy and the power performance respectively, compared with those of the current engine.The improvements of the SKYACTIV-G fuel economy and power performance under full load operation are shown in Figure 45. The specific fuel economy achieves the same level as existing Mazda diesel engines. Figure 45also shows the positioning of BSFC of SKYACTIV-G by using the base engine's specific fuel consumption measured by the third party and BSFC improvement ratio in the SKYACTIV-Gmeasured in Mazda (absolute BSFC value is just reference). The SKYACTIV-G demonstrates far better fuel consumption than other stoichiometric combustion engines. It can be said that the SKYACTIV-G stands at the world-best level. Further more, the SKYACTIV-G is foremost level in the low-end torque among the existing engines. The max. torque and max. power also stand at the top level. Full Load Performance improvement 0%15%0%15%Current Engine30032034036038040042044046048050050010001500200025003000Engine diplacement (cc)Base engine (measured by 3rd party)15%Positioning of SKYACTIV-G SKYA CTIV-GenginScatter band measured by 3rd PartyFig. 45:Positioning of SKYACTIV-G 2.0L5 Other technical issues to be overcomeAs mentioned earlier, the higher compression ratio and the 4-2-1 exhaust system were incorporated as enablers to improve the fuel economy and low-end torque. There were various technical issues that to be overcome to achieve the development goals. One was the pre-ignition control technology development for high compression ratio. The other was the emission reduction technology development for the 4-2-1 exhaust system. How they were solved will be described next.5.1 Pre-ignition controlThe pre-ignition referred here is auto-ignition of the mixture caused by high pressure and temperature in the cylinder, not the pre-ignition caused by heat spot such as high-temperature spark plug. The heat-spot pre-ignition tends to occur in the high engine speed range easily. While the auto-ignition tends to occur at the low speed where the mixture is compressed for a longer time.As existing engines with the low compression ratio have enough margins from the pre-ignition limit, so far, it is not needed to worry about the robustness against the pre-ignition so much. However, the high compression ratio introduction surely raises possible risks of pre-ignition because the temperature and the pressure in the cylinder significantly increase.In order to prevent abnormal combustions including pre-ignition even under the conditions with considering the effect of multiple noise factors, such as compression ratio raised by carbon deposit or various environmental and driving conditions on pre-ignition limit, the development was proceeded by taking three approaches below.5.1.1 Sensitivity to pre-ignitionSensitivity of various noise factors to pre-ignition was examined. Figure 46 indicate that pre-ignition tends to occur when the engine speed is lower and the intake air temp./water temp. are higher and that it tends to take place more frequently when relative air-fuel ratio(AFR) is aroundFig. 46: Pre-ignition Sensitivity of Several Noise FactorsFigure 47 illustrates the effects of split injection on pre-ignition under the conditions offull load andFig. 47: Effect of Split Injection on Pre-ignitionFigure 48 demonstrates the pre-ignition robustness of SKYACTIV-G under multiple noise conditions. There are sufficient margins even with cam timing fully optimized for best torque under the nominal specifications and standard environmental conditions. However, considering the compression ratio increase due to carbon deposits and the upper limits of the temperature and the octane values so on, there are no more margins left. This indicates that the pre-ignition could occur under worst case.CompressionRationominal (14.0)nominal (14.0)nominal (14.0)nominal (14.0)worst (15.4)worst (15.4)FuelNominal 95RON 95RON 95RON 95RON 93RON worst 91RON Intake air temp.Normal(255858100100100)Normal(90100110110110Preignition MarginePreignition Limit Calibrated IVC for Best TorqueIVC controllable rangeAFR controllable rangeFig. 48:Robustness for Pre-ignitionOn the other hand, there is the pre-ignition controllable range by late IVC controlling and rich AFR operation. Please look at the right side of Figure 48.As this controllable range makes it possible to achieve the effective compression ratio which is below the pre-ignition limit even under the worst condition, pre-ignition can be prevented under any conditions as long as the pre-ignition limit is predicted in response to environment changes. The pre-ignition prediction model was developed for this reason.5.1.2 Pre-ignition prediction technologyFigure 49 shows the relation between the initial combustion position (corresponding MBF 10%) and the heat release under various engine operational conditions. The stronger the pre-ignition is, the earlier the initial combustion is. As a result, there is tendency that more heat is released. If it is possible to predict the combustion cycle where the MBF 10% is positioned at 25deg.BTDC, early signs of pre-ignition can be grasped without fail.Fig. 49: Correlation Between Predicted and Measured Pre-ignition Limit Therefore, the mean effective compression ratio where MBF10% was positioned at 25deg.BTDC was defined as the pre-ignition limit. As shown in Figure 50, Liven-Wood integral formula was used to clarify the relation between the temperature and the pressure in the cylinder which reaches the condition of pre-ignition limit. Based on this study, pre-ignition limit was described as the mean effective compression ratio under the ambient temperature condition (temperature, ambient temperature, coolant temperature etc. which can be measured by sensors existing in the engine.Fig. 50: Correlation Between Initial Combustion Position and Heat ReleaseFigure 51 indicates the correlation between the pre-ignition limit predicted based on the above formula and the measured pre-ignition limit under various conditions. They are in line with each other within +/- 0.2 of the mean effective compression ratio.Fig. 51: Ionization Current Sensor System for Pre-Ignition DetectionAs shown in Figure 47, the engine normally has enough safety against the pre-ignition limit and operates based on the IVC timing which was determined by the fuel economy/power performance requirements. However, when the intake air or hydraulic oil temperature increases extraordinary, IVC is retarded based on the pre-ignition limit predication calculated according to the above formula. With the help of。