Research History of High-Speed, Direct-Injection Diesel Engine Combustion Systems for Passenger Cars Kiyomi Nakakita

R&D Review of Toyota CRDL, Vol.45 No.3 (2014) 43-56 43

Special Review

Review Research History of High-speed, Direct-injection Diesel Engine Combustion Systems for Passenger Cars Kiyomi Nakakita

Report received on Jun. 15, 2014

High-speed direct-injection diesel engines applied to passenger cars since the second half of 1980’s have been making great strides through several stages in all of power, exhaust emissions, and noise, vibration and harshness under keeping high thermal efficiency, especially since appearances of common-rail (CR) injection system and high-boost-pressure supercharging systems in 1990’s. At each stage, our laboratories played some key roles in clarifying in-cylinder phenomena and indicating directions and measures for further improvement, such as clarifying high-pressure fuel injection effect, multiple-injection effect and its optimization, development direction of the modern combustion system consisting of the CR injection system and shallow-dish-type combustion chamber, effect of combining high-boost-pressure, high exhaust-gas-recirculation rate and extremely high injection pressure, alternative combustion systems of “dual-fuel stratified PCCI” and “quiescent” combustions, and so on. In this review, the evolution history of diesel-engine combustion systems for passenger cars is outlined, and then results of the above-listed subjects together with a useful numerical-simulation tool for engine planning and control-parameters adjusting are all introduced.

Direct-injection Diesel Engine, Combustion System, Passenger Car, In-cylinder Analysis, Optically Accessible Single-cylinder Engine, Numerical Simulation

1. Introduction various originally-developed analysis methods, and in indicating the direction and measures for further Diesel engines originally have the highest thermal improvement. Hereinafter, the evolution history of efficiency among internal combustion engines and diesel engine combustion systems for passenger cars is also high torque at medium and low engine speeds, outlined and then the major research results obtained and therefore have been widely used in trucks and in our laboratories are introduced. buses worldwide since 1940’s. On the other hand, the number of diesel-powered passenger cars was limited 2. Evolution History of Diesel Engine Combustion until the middle of 1990’s, mainly due to both exhaust Systems for Passenger Cars emissions, especially black smoke, and large noise, vibration and harshness (NVH). 2. 1 First-stage Diesel Engines for Passenger Cars Since the second half of 1990’s, however, not only the performance but also the exhaust emissions and The indirect injection (IDI) combustion system NVH of high-speed diesel engines for passenger cars with a sub combustion chamber was originally used have been greatly improved through several stages, for passenger-car diesel engines, because good due primarily to the appearance of common-rail fuel-air mixing was achieved by very high-speed injection systems, high-efficiency aftertreatment swirl flow generated in the sub chamber so that systems, and the remarkable advance of turbochargers high-speed operation was possible even with a simple and electronic control systems. At each stage of the fuel injection equipment, and also somewhat low level above-mentioned evolution of high-speed diesel of exhaust emission was realized. On the other hand, engines, our laboratories played some important key heat loss from in-cylinder gas to combustion chamber roles both in clarifying in-cylinder phenomena on wall was large, leading to relatively low thermal the exhaust emissions, NVH and fuel economy with efficiency and limitation in increase of power density

© Toyota Central R&D Labs., Inc. 2014 http://www.tytlabs.com/review/ 44 R&D Review of Toyota CRDL, Vol.45 No.3 (2014) 43-56 due to the high thermal load. In addition, further under actual driving conditions was excellent. Thus, reduction of exhaust emissions corresponding to the at this stage, diesel-powered passenger cars widely next generation, stringent emission regulation was spread in all the classes including premium cars. essentially difficult in the IDI diesel engine. 2. 4 Remarkable Increase in Power and Torque 2. 2 Shift in Combustion Systems from IDI to DI Densities

In order to solve the above-mentioned problems, in In the middle of 2000’s, performance of CR-HSDI the second half of 1980’s, shift in combustion systems diesel engines was remarkably improved mainly from the IDI type to a direct injection (DI) type began owing to advanced supercharging technology such in passenger-car diesel engines, which opened a door as a two-stage turbocharger system. In these engines, of the age of high-performance and good fuel-economy power and torque densities reached 65 kW/L and diesel engines. At the first stage, however, the 185 Nm/L, respectively. In addition, peak-torque high-speed DI (HSDI) diesel engines were equipped range was furthermore expanded from 1,500 rpm to with a jerk-type injection system and a large about 3,000 rpm, and also torque under higher engine hole-diameter nozzle, leading to large NVH and speeds were kept to be high value such as 70 to 80% of insufficiently low exhaust emissions. Therefore, at this peak torque at 4,500 rpm and still 60% at 5,000 rpm. stage, diesel-powered passenger cars did not spread These characteristics were sufficient for emotionally widely. sporty driving. In addition to the power increase, good fuel economy 2. 3 Appearance of DI Diesel Engines with was kept by reducing losses of friction and heat flux to Common-rail Injection System walls, and NVH level was further improved mainly by the evolution of CR FIE such as increased number of In the same age, the second half of 1980’s, a multiple injections and higher peak injection pressure common-rail (CR) injection system was first developed up to 180 MPa. Exhaust emission levels were also by Denso Corporation. In the conventional jerk-type further reduced with the evolved CR FIE, thermal injection system, fuel can be injected only at about management and high efficient aftertreatment systems compression top dead center (TDC) and injection of diesel particulate filter (DPF) and advanced catalysts pressure depends on engine speed and load (i.e. injected such as a nitrogen oxides (NOx) storage reduction fuel quantity). On the other hand, in the CR injection (NSR) catalyst, a selective catalytic reduction catalyst system, multiple fuel injections at any timing during with reducing agent of urea (Urea-SCR), and so on. compression and expansion strokes are possible and injection pressure can be set independently of engine 2. 5 Two Main Streams of Diesel Engine speed and load. In addition, the CR injection system Development had higher peak injection pressure of 145 MPa. Responding to this innovation of fuel injection Since the last stage of 2000’s, diesel engines newly equipment (FIE), in the second half of 1990’s, HSDI developed were classified into two categories. One diesel engines with the CR injection system (CR-HSDI is the downsized diesel engine with remarkably diesel engines) were developed and used for passenger high power and torque densities, and the other is cars. These engines, assisted by sophisticated the cost-effective diesel engine with both necessary turbochargers such as a variable-nozzle-turbine (VNT) performance for practical use and good fuel economy. turbocharger, had high power and torque densities of Both types are outlined as follows. about 50 kW/L and 145 Nm/L, respectively, which were sufficient for normal driving in passenger cars. 2. 5. 1 Engine Downsizing Based on Further In addition, with the multiple fuel injections, the NVH Increase in Power and Torque Densities levels decreased to levels comparable with those of gasoline-powered cars, and also exhaust emissions Since around 2010, power and torque densities of reduced drastically. Furthermore, the maximum brake CR-HSDI diesel engines have been increasing up thermal efficiency reached 42 to 43% and fuel economy to 93 kW/L and 247 Nm/L, respectively, owing to

© Toyota Central R&D Labs., Inc. 2014 http://www.tytlabs.com/review/ R&D Review of Toyota CRDL, Vol.45 No.3 (2014) 43-56 45 further advance in supercharging technology such described in Section 2. 2, usage of high pressure fuel as two-stage supercharging systems with double or injection was the most important key item for realizing triple turbochargers and decreased loss of air the HSDI diesel engines applicable to passenger cars. path systems, and also further evolution in CR FIE Concretely speaking, what should be understood were such as furthermore increased number of multiple fuel injection pressure effects on combustion and injections and peak injection pressure up to 250 MPa. exhaust emissions and also mechanisms of the effects, In addition, a new combustion technique, that and then desired specifications of the FIE and main is, combination of high boost-pressure charging, components of the combustion system. high exhaust-gas-recirculation (EGR) rate and the Responding to this requirement, researches on the very high injection pressure led very low engine-out effect of fuel injection pressure and also injection rate exhaust emissions even under medium and high load were conducted by using a conventional jerk-type conditions. This technique enabled drastic engine FIE and a pioneering prototype CR FIE supplied by downsizing such as a shift from 8-cylinder engines Denso. In these studies,(1-4) a special single-cylinder with displacement volume of 4.0 L to 6-cylinder with diesel engine with wide observation area shown in 3.0 L. The engine downsizing really contributed to Fig. 1 and diesel fuel with an additive of copper oleate reduction in engine weight and size and improvement for visualizing non-luminous flames were both used, in car-based fuel economy. and high-speed direct photography and two-color pyrometry method(5) were applied for analyzing the 2. 5. 2 Cost-effective and High-efficiency Diesel in-cylinder phenomena such as fuel spray behavior, Engines flame development process, flame temperature, and so on. As a result, the following points were clarified. The above-mentioned highly-downsized diesel (1) As shown in Fig. 2, increase in injection pressure engines require high-cost components such as the remarkably reduces smoke (i.e. particulate), but, in high-boost-pressure supercharging system, high-class this case, the effect is saturated at injection pressure of CR FIE, structural parts of high-class material about 100 MPa. for sustaining the high in-cylinder pressure and thermal load and so on, leading to increase in engine cost. Thus, there is the other trend Inj. Press.: 35 MPa to develop cost-effective diesel Inj. Timing: engines without downsizing which - 5 deg. ATDC have sufficient power and torque NOx: 530 ppm Smoke: 3.2 BSU 1.0 deg. ATDC 5.8 deg. ATDC 15.4 deg. ATDC for normal driving and good fuel Peak Temp.: economy under actual driving Flame Temperature K 2804 K conditions. These engines are 1470 2070 2670 suitable to popular-edition cars. 10.6 deg. ATDC

3. Researches on Combustion System for Each Generation Inj. Press.: Diesel Engine Conducted in 95 MPa 10.4 deg. ATDC 15.2 deg. ATDC 24.8 deg. ATDC Our Laboratories Inj. Timing: 5 deg. ATDC NOx: 530 ppm Peak Temp.: 2830 K 3. 1 Effects of High-pressure Smoke: 0.3 BSU Fuel Injection on deg. Combustion and Exhaust 15.2 ATDC Emissions Fig. 1 Effect of fuel injection pressure on flame development under the same exhaust NOx condition. At the stage of the shift in diesel Reprinted and modified from Proc. of JSAE Symp, New Aspects of Diesel Combustion combustion systems from IDI to DI (in Japanese), No. 9303 (1993), pp. 40-48, © 1993 JSAE.

(2) Increasing injection pressure leads to the following 3. 2 Optimization of Multiple Injection Patterns phenomena. and Its Effects on Combustion and Exhaust (a) Ignition delay is shortened (Fig. 2) and ignition Emissions position shifts from the vicinity of nozzle tip to that of piston cavity wall (Fig. 1). This shift prevents the At the stage of the development of CR-HSDI diesel spray from being wrapped by flame and thus ensures engines described in Section 2. 3, clarifying the usage air entrainment into the spray. of multiple fuel injections was the most important key (b) Luminous flames (diffusion flames) are reduced item, because various multiple injection patterns tried and non-luminous flames (premixed flames) shown at the first stage led to very puzzled results such as as the green flame areas in Fig. 1 appear, which unexpected, worsened exhaust emissions, lubricating corresponds to the accelerated mixture formation. oil dilution, and so on. (c) Both combustion period and lifetime of the Responding to this problem, studies to clarify the luminous flame are shortened, but those effects are effects of multiple fuel injections and the mechanisms saturated also at about 100 MPa in this case (Fig. 2). were conducted by using two optically-accessible, (3) Heightening injection pressure up to about 100 MPa single-cylinder engines; one is the same engine as increases NOx emission in this case. The increase that used in Section 3. 1 and the other is a modern one in NOx is caused not by increase in peak flame with four valves and a centrally-located injector.(7) temperature but by enlargement of high-temperature In these studies,(6,7) a high-speed color shadowgraph (for example, over 2,170 K) flame region.(3) photography were additionally applied for analyzing This study led to the above-described results was the in-cylinder phenomena. Some of the concrete a pioneering work and indicated the development problems tackled in these studies and also obtained direction. results and solutions are as follows.

3. 2. 1. Close-pilot Injections

A small-quantity fuel injection just before main Φ0.26 mm x 4 hole injection is called “close-pilot Φ0.29 x 4 injection”, which effectively reduced combustion noise, fuel consumption and NOx emission, but was apt to remarkably increase exhaust smoke in complicated manners depending on pilot fuel quantity, interval between pilot and main injections, main-injection timing, and so on. Thus, it was very difficult to properly utilize the close- pilot injection. The analysis results and solution from our study(6) are the followings. (1) In the cases of high exhaust smoke level shown in the

1800 rpm, 35 mm3/st ( Φ = 0.42), Inj. Timing: TDC left- and right-columns of Fig. 3, pilot flames formed Fig. 2 Effect of injection pressure on exhaust emissions and combustion characteristics. prior to main injection are Reprinted and modified from JSAE Tech. Pap. Ser. (in Japanese), No. 901078 (1990), © 1990 JSAE. conveyed by swirl flow and

© Toyota Central R&D Labs., Inc. 2014 http://www.tytlabs.com/review/ R&D Review of Toyota CRDL, Vol.45 No.3 (2014) 43-56 47 located just ahead of downstream main-injection 3. 2. 2 Early-pilot Injections sprays so that the main sprays enter into the pilot flames and are fully wrapped by flames. This leads to A small-quantity fuel injection far before main insufficient air entrainment into the main spray and injection is called “early-pilot injection”, which also therefore formation of large amount of soot, resulting had the potential to effectively decrease combustion in the high smoke. This is caused by excessively long noise and NOx with scarcely increased smoke and to pilot-main interval and large fuel quantity of pilot additionally increase low-speed full torque, but was injection (the left-column case), and also by excessively apt to deteriorate hydrocarbon (HC) emission, fuel long ignition delay of main-injection fuel due to the consumption and reduction rate of combustion noise far-retarded injection timing (the right-column case). at light loads.(7) Lubricant oil dilution by fuel adhesion Additionally, soot formation is promoted by poor to cylinder wall was also a problem. In this study,(7) a atomization of pilot fuel due to throttling effect at the special apparatus for detecting the adhered fuel state nozzle seat. and quantity shown in Fig. 4 were used. The analysis (2) Excessive fuel quantity of pilot injection also causes results and related findings are as follows: increase in NOx, because excessively large pilot flame (1) Double early-pilot injection, that is, two injections is generated and thus becomes a source of NOx and with an interval of about 20 deg. CA and each fuel also accelerates the combustion of main-injection fuel. quantity of about half of that in the original (3) Consequently, shorter pilot-main interval and single-pilot injection, effectively suppressed the smaller quantity of pilot fuel, whose optimum values increases in HC and fuel consumption, and effectively depend on swirl level and ignition delay, and also use reduced combustion noise. These effects result from of small-orifice-diameter nozzles are all essential. With that the double-pilot injection increases burnt amount optimizing these factors, each pilot flame is adequately of the pilot fuel before main-injection start due to less small-scaled and is located slightly downstream of fuel adhesion to cylinder wall, which increases each main spray, which prevents the main sprays from in-cylinder gas temperature and thus shortens ignition being enveloped by flames, leading to low smoke as delay of the main-injection fuel. Through measuring shown in the middle column case of Fig. 3. weight of the adhered fuel with blotting paper, it

ΦΦ0.26mm x 4, Qp::33mmmm33/st/st ΦΦ0.180.18mm x 5, Qp:2mmmm33/st/st ITpilot : -12.5 ATDC Smoke ITpilot : 0.0 ATDC Smoke ITpilot : 5.0 ATDC Smoke ITmain : 0.0 ATDC 1.1BSUBSU ITmain : 5.0 ATDC 0.2BSUBSU ITmain : 10.0 ATDC 1.1BSUBSU

2.2 6.3 11.9 deg. deg. deg. ATDC ATDC ATDC

MainMain sprayssprays enterenter 1800rpm1800 rpm intointo pilotpilot flamesflames 3 into pilot flames 35mm35 mm3/st/st 8.8 13.1 Equi. Ratio : 0.42 deg. deg. Inj. Pressure : 9595MPaMPa Main sprays enter intointo pilotpilot flamesflames 4.6 10.0 14.3 deg. deg. deg.

7.0 11.2 16.8 deg. deg. deg.

NOx : 1000ppm1000 ppm NOx : 600ppm600 ppm NOx : 405ppm405 ppm

© Toyota Central R&D Labs., Inc. 2014 http://www.tytlabs.com/review/ 48 R&D Review of Toyota CRDL, Vol.45 No.3 (2014) 43-56 was quantitatively confirmed that the adhered fuel the raised temperature and enhanced mixing with quantities were reduced by about 50% or more with surrounding air. Excessive fuel quantity of after double early-pilot injections. injection generates soot by itself and excessively long (2) Fuel quantity adhered to cylinder wall had been main-after interval prevents the after-injection flame naturally thought to decrease with reduced fuel from catching the remaining soot. quantity and lowered injection pressure of the pilot (2) Another problem of NOx increase by after injections, due to decreased spray-tip penetration. injection is solved by combination of after injection However, in reality, the fuel adhesion to cylinder and EGR, and the trade-off curve regarding smoke, could be prevented only with moderate quantity and fuel consumption and NOx is really improved. injection pressure of the pilot fuel, as shown in Fig. 4. The pioneering clarification of the multiple-injection This is because excessively small fuel quantity effects and mechanisms and also solutions to the and/or low injection pressure cause excessively low problems described above enabled the adequate use of real injection pressure at nozzle exit due to promoted multiple injections. throttling effect at the nozzle seat, so that very large fuel droplets are formed and reach the cylinder wall. 3. 3 Combustion System to Realize Both Increase in Power Density and Severely Low Exhaust 3. 2. 3 After Injections Emissions

A small-quantity fuel injection just after main Since 2000’s, exhaust-emissions regulations have injection is called “after injection”, which effectively been becoming greatly stringent step by step. In the decreased smoke, HC and fuel consumption only with case of diesel engines, smoke level at medium and low adequate main-after interval of about 5 deg. CA and loads should be first reduced for decreasing exhaust after-injection quantity of about 2 mm3. Mechanism emissions in frequently used operating region. For of the dependence on the interval and quantity this purpose, decrease in nozzle-orifice diameter should have been clarified for widely applying the of injectors is most effective and essential, so that after-injection technique. The analysis results in this injectors with the decreased nozzle-orifice diameter study(7) are as follows: and increased number of nozzle orifices were widely (1) In-cylinder observation, picture processing and adopted, leading to remarkably reduced smoke and computational fluid dynamics (CFD) analysis clarified then NOx through increased EGR rate. However, that jet flame of the after injection catches and oxidizes maximum power, that is, maximum torque at high the soot previously formed in the squish area, due to speed was notably reduced. The analysis results of our studies(8-10) responding to this problem are as follows.

3. 3. 1 Shallow-dish-type Combustion Chamber Fine-Droplets Adhesion Little Adhesion Hard Impingement In conclusion, the solution to the above-mentioned problem was a Quartz Window 100 MPa shift in combustion chamber shape 80 MPa from a conventional deep-bowl type to a shallow-dish type. The Exchangeable 60 MPa cause and the related findings are the followings.(8-10) Blotting 40 MPa Paper (1) In a conventional combustion Metal Part system with the deep-bowl cavity 3 Pilot Injection Quantity mm /st and a large-orifice-diameter nozzle, (1200( 1200rpm,rpm, Vol. Vol. Efficiency Efficiency = =105%, 105%, T Tinin== 27 27deg.deg.C,C, IT PilotPilot = 40 deg.BTDCBTDC)) intense air-flow momentum generated by the cavity balances Fig. 4 Effect of injection quantity and pressure on fuel adhesion to cylinder wall.

© Toyota Central R&D Labs., Inc. 2014 http://www.tytlabs.com/review/ R&D Review of Toyota CRDL, Vol.45 No.3 (2014) 43-56 49 with strong spray momentum by the large-orifice speed and calculated with nozzle-orifice diameter and nozzle, leading to good fuel-air mixture distribution injection pressure.(9) The smoke-limited equivalence in the whole combustion chamber at high-speed and ratio becomes a function of Vs/Vsp independently of high-load conditions (the left-column case of Fig. 5). engine size and FIE type, and thus specifications of the On the other hand, in the case of a small-orifice nozzle piston cavity and injection system to realize required combined with the deep-bowl cavity, only spray maximum power can be determined. momentum becomes smaller and thus most mixture (4) In reality, a high value of smoke-limited equivalence is conveyed out of the cavity by the intense ratio, 0.88 was attained even at 4,000 rpm by keeping reverse-squish flow, as shown in the middle-column the Vs/Vsp value of 0.7 which is realized with very case of Fig. 5. Therefore, air in the cavity is not high injection pressure of 240 MPa generated by a utilized, leading to high smoke level (i.e. lowered prototype, amplified-pressure CR FIE,(10) as shown in maximum power). Fig. 6. This result predicted the possibility of nowadays (2) By combination of the small-orifice nozzle and high power-density level of 90 kW/L or more. the shallow-dish cavity, air flow momentum is also These pioneering results and findings clearly suppressed and thus good momentum balance of air indicated the development direction of modern HSDI flow and spray is realized again, resulting in good diesel combustion systems. mixture distribution (the right-column case in Fig. 5). With regard to emissions at part loads (i.e. low 3. 3. 2 Combination of High Boost Pressure, High speed), flexibly increased injection pressure with CR EGR Rate and High Injection Pressure FIE compensates insufficient mixing effect of the shallow-dish cavity due to the excessively weak air As described in Section 2. 5. 1, with increasing all of flow at low speed conditions. charging efficiency, EGR rate and fuel injection (3) The factor governing “smoke-limited equivalence pressure, NOx-Soot tradeoff is remarkably improved ratio” which determines maximum power was found to at medium and high load conditions. This technique be a ratio of Vs/Vsp, where Vs is representative squish was first proposed by New ACE for heavy-duty diesel speed and analytically obtained with cavity diameter engines.(11) Responding to the result of New ACE, it and engine speed, and Vsp is representative spray was evaluated in this study whether or not the technique

ΦΦ0.17-nozzle ΦΦ0.14-nozzle Deep-Bowl (f43) Shallow-Dish (f51)

Photograph View -2.8 -3.8 -2.9 deg. deg. deg. ATDC ATDC ATDC

1.2 1.8 -0.8 deg. deg. deg.

40004000rpm,rpm, Full Load 12.8 23.0 23.0 deg. deg. deg.

Spiral Vortex Flame Flows out Motion Flame Develops to Squish Area to Bottom of Cavity

Reverse Swirl Squish

Fig. 5 Effect of nozzle orifice diameter and cavity shape on formation of spiral vortex motion for promoting fuel-air mixture distribution.

© Toyota Central R&D Labs., Inc. 2014 http://www.tytlabs.com/review/ 50 R&D Review of Toyota CRDL, Vol.45 No.3 (2014) 43-56 was really effective also for HSDI diesel engines.(12) nowadays newly-developed diesel engines. (1) As shown in Fig. 7, the “high-boost-pressure (i.e. high charging efficiency) & high-EGR-rate” 4. Our Researches of Alternative Combustion combustion system with high pressure injection Systems drastically improved NOx-Soot tradeoff also for a HSDI diesel engine. 4. 1 Dual-fuel Stratified PCCI Combustion (2) As shown in Fig. 8, the analysis results of heat release rate, shadowgraph photograph and flame As an alternative combustion system, premixed-charge temperature distribution indicate that, by shifting compression ignition (PCCI) combustion systems in combustion system from conventional type to have been being developed since around 1995, because the “high-boost-pressure & high-EGR-rate” type, less luminous flame is generated, Pressure that is, formed soot amount Vs／Vsp Amplifier is reduced, and combustion 0.7 0.7 HP-FIE period becomes shorter. In 0.9 （240MPa240 MPa）） 0.8 110MPa110 MPa Qf ：62 mm3/st addition, flame temperature Comb. Period ：32 deg.CA 0.7 CR-FIE 16 is lowered in the whole flame （）） 120 180180MPaMPa 10mm 12 1.1 MPa × 0.6 80 J/deg region, more quantitatively 8 0.5 40 describing, lowered by 260 K Ratio Equivalence 4 Rate of Heat

0 Release 2.4 Pressure 0 in the largest flame area, which LiftNeedle -40 -20 TDC 20 40 60 reduces NOx by 84% in this 2.0 Crank Angle ATDC (12) Qf : 42 mm3/st MPa case. IMEP 1.6 Comb. Period : 36 deg.CA 16 (3) In conclusion, by the 120

1.2 10mm 12 MPa × shift, oxygen volume fraction 1000 2000 3000 4000 5000 6000 80 J/deg 8 Engine Speed rpm 40 decreases from 18.5% to 4 Rate of Heat

0 Release Pressure 0 15.8% in this case due to the (Smoke(Smoke < < 1.5 1.5FSN,FSN, PmaxP max<< 15MPa15 MPa ) ) LiftNeedle -40 -20 TDC 20 40 60 increased EGR rate, whereas Crank Angle ATDC oxygen quantity increases by Fig. 6 Increase in smoke-limited equivalence ratio (i.e. IMEP) with extremely high 1.46 times due to the increased injection pressure. (12) charging efficiency. The Reprinted and modified from JSAE Tech. Pap. Ser. (in Japanese), No. 20075716 (2007), former leads to lowered flame © 2007 JSAE. temperature and then notably low NOx, and the latter reduces soot (i.e. smoke). 180 MPa Higher injection pressure 223% 180 MPa 180 MPa 200 Inj. Press.= 100 MPa 35.0% 200% 160% required in the “high-boost- Charg. Eff.=160% 30.5% 18.8% pressure & high-EGR-rate” 150 EGR Rate = 16.2% system is essential to keep 215 MPa mg/kWh sufficient spray-tip penetration 285% 43.6% 100 against the increased in-cylinder 180 MPa 285 % 50 gas density. Soot 39.0 % This study first clarified 0 the cause of the emissions 0 1 2 3 4 5 reduction and the required 215 MPa NOx g/kWh conditions on the “high-boost- 250% pressure & high-EGR-rate” 37.7% ( 2000 rpm, Pme =1.4 MPa ) combustion method. This Fig. 7 Emissions reduction potential of “high-boost-press. & high-EGR-rate” method is widely applied to combustion system.

Higher 24 23 22 21 20 19 Lower Flame Temperature x 100K

Flame Temp. Flame Temp.

16 140 16 140 14 120 14 120 MPa MPa 12 Photo. 100 12 Photo. 100 10 80 10 80 8 60 8 60 6 40 6 40 4 20 4 20 熱発生率 [J/deg] 2 0 2 0 Cylinder Pressure Cylinder Pressure Heat Release Rate J/deg 0 -20 0 -20 Heat Release Rate J/deg -40 -20 0 20 40 60 -40 -20 0 20 40 60 Crank Angle [ATDC] Needle Lift Crank Angle [ATDC]

Inj. Press. : 100MPa100 MPa Inj. Press.Press. : 180MPa180 MPa Charg. Eff.: 160%160% Charg. Eff.: 250%250% EGR Rate : 16% EGR Rate : 38% Conventional “High-boost-press. & high-EGR-rate”

Fig. 8 Comparison of heat release rate and flame temperature distribution between conventional and “high-boost-press. & high-EGR-rate” combustion systems.

the PCCI combustion has the potential for reducing Concept both NOx and smoke to near-zero levels without -10 ATDC any after-treatment systems and also for increasing High Cetane No. Fuel (ex. Diesel Fuel) thermal efficiency. However, the PCCI combustion has problems of misfire at low loads and premature -30 ATDC Low Cetane No. Fuel 0 Fuel Vapor Fraction 0.05 ignition and excessive burning rate at high loads, (ex. iso-Octane) Distribution of Diesel Fuel which causes deteriorated thermal efficiency and severely increased noise, limiting the operating region. Fig. 9 Concept of fuel-air mixture distribution in The ignition timing and burning rate, governed by “dual-fuel stratified PCCI” combustion and a temperature, heat capacity and oxygen concentration practical example of diesel fuel distribution. Reprinted and modified with permission from SAE Tech. of the in-cylinder gas, are therefore controlled by EGR, Paper Ser., No. 2006-01-0028 © 2006 SAE International. supercharging, variable valve timing (VVT) and so on. But, unfortunately, PCCI combustion is very sensitive to the governing factors, and also those controlling n-heptane iso-octane (RON 0) (RON 100) factors of EGR and so on have a long response time. 150 Dual-fuel Therefore, control at transient operation is difficult, 9dB9 dB PCCI Average RON=60 Down which still prevents the PCCI combustion from putting 100 Φ=0.35 to practical use. EGR=0% Dual-fuel 50 Average stratified PCCI Responding to the problem, related researches were RON=60 iso-octane (13) 0 conducted and a solution was proposed as follows: Heat Release Rate J/deg. (RON 100) -25 -20 -15 -10 -5 0 5 10 15 diesel fuel (RON 16) (CN 10) (CN 55) Crank Angle deg.ATDC Common Rail (1) “Dual-fuel stratified PCCI” combustion, in which IT: - 40 deg.ATDC 50 two fuels with opposite ignitability are used and both 40 ppm 30 the concentration and ignitability of fuel vapor are 20 9ppm9 ppm Average RON=60 10 3ppm3 ppm

NOx 0 stratified as shown inFig. 9, is able to realize moderate PCCI Strat. PCCI combustion rate under keeping very low NOx level without EGR, leading to moderate noise level, as Fig. 10 Effect of stratification in fuel-vapor concentration and ignitability on burning rate and exhaust NOx shown in Fig. 10. As understood from a system setup emission. shown in Fig. 10, the “dual-fuel stratified PCCI” Reprinted and modified with permission from SAE Tech. have basically good controllability even at transient Paper Ser., No. 2006-01-0028 © 2006 SAE International.

© Toyota Central R&D Labs., Inc. 2014 http://www.tytlabs.com/review/ 52 R&D Review of Toyota CRDL, Vol.45 No.3 (2014) 43-56 operation. NOx level of stringent Tier2-Bin5 regulation without (2) CFD analyses reveal the followings. In the case any de-NOx catalysts mainly due to enlarged PCCI of ordinary PCCI combustion, ignition simultaneously combustion zone, and simultaneously reduces fuel occurs anywhere in the cylinder, leading to peaky consumption due to the decreased heat loss by the combustion rate. On the other hand, in the case of the “dual-fuel stratified PCCI”, ignition starts in Highly-Atomized, the outer region and then combustion Conventional Low Penetration zone gradually spreads into the (1) Multiple Small-Orifices Injector ((ΦΦ0.080.08mmmm xx 16,16, 140 deg.) Swirl Port inner region, leading to moderate Low NOx Premixed Combustion Conventional combustion rate.(13) X Low Output Low Penetration The pioneering solution described - Advance Inj. Timing above is still a rare method to put - Ensure Spray Penetration Straight Port PCCI combustion to practical use. Good Atomization New System Low Comp. Press. & Density Fast Vaporization Regrettably, this concept has not yet Enlarge PCCI Region been applied to passenger cars due Suppress Premature Ignition to the necessity of two fuels, but Lip-Less Shallow-Dish is thought to be really useful for (2) Low Comp. Ratio Increase Max. Output (3) Weak Gas Flow ( ε =14 ) heavy-duty vehicles and has been High Charging (Rs: 0.5, Shallow-Dish Cavity) succeeded to nowadays “RCCI” Ensure Max. Output Efficiency X Poor Mixing Low NOx (14) Low Temp. Increase Intake Flow Rate research works. X Poor Startability Decrease Heat Loss Increase Comp. Temp.

4. 2 Quiescent Combustion System Fig. 11 Cause enabling “quiescent combustion system” of high efficiency and low emissions. with Multiple Highly-atomized Reprinted and modified with permission from SAE Tech. Fuel Sprays Paper Ser., No. 2011-01-1393 © 2011 SAE International.

As decreasing exhaust emissions, fuel economy of diesel engines had been deteriorated due to injection High Heat-Flux Area timing retard, increase in EGR rate and so on. Therefore, a new 3000 Conventional Injector Conventional combustion concept with both high - Swirl Ratio: 1.3 Reduce thermal efficiency and low exhaust 2000 - Lip-Cavity - Φ 0.11×9 Piston emissions was desired. Responding - ε : 15～

1000 （Bin5） Euro6 ≈ to this request, an ingenious, new Loss Heat W 138 combustion concept described below Quiescent 0 136 (15) 【NOx=20 mg/km】 200 was created in this study. -30 -20 -10 0 10 20 30 40 Multiple injection 134 7 Crank Angle deg.ATDC g/km 150 (1) “Quiescent combustion system” Quiescent 6 132 100 5 1600rpm, Pme : 0.3MPa 2 - Swirl Ratio: 0.5 PCCI described in Fig. 11 consisting of Torque Nm 200 Noise < 87dB - 130 Lip-less Cavity 50 4

CO × 3 multiple highly-atomized sprays by 195 - Φ 0.08 16 mode1 Conventional 0 2 128 - ε : 14 500 1000 1500 2000 2500 190 Engine Speed rpm a multiple small-orifices injector, ≈ 185 126 weak in-cylinder gas flow by both Reduce 0 50 100 200 180 Quiescent 【NOx=50 mg/km】

ISFC g/kWh g/kWh ISFC 200 a low-swirl-ratio intake port and NOx mg/km Multiple injection 7 175 150 a lip-less shallow-dish cavity, and -40 -35 -30 -25 -20 PCCI 6 100 5 low compression ratio, effectively Injection Timing deg.ATDC Torque Nm 50 4 mode1 3 2 decreases heat loss and NOx under 0 500 1000 1500 2000 2500 Fig. 12 Improvement of efficiency-emissions trade-off by Engine speed rpm keeping the maximum output, as “quiescent combustion system”. shown in Fig. 12. The “quiescent” Reprinted and modified with permission from SAE Tech. system has the potential to clear the Paper Ser., No. 2011-01-1393 © 2011 SAE International.

© Toyota Central R&D Labs., Inc. 2014 http://www.tytlabs.com/review/ R&D Review of Toyota CRDL, Vol.45 No.3 (2014) 43-56 53 suppressed gas flow. calculated based on Arai’s model,(17) and distributions (2) In generality, each factor of the extremely- of various fuel concentrations in the combustion small-orifices injector, weak gas flow and low zones are represented with a probability density compression ratio leads to each problem of poor fuel function (PDF). With regard to combustion reaction vapor distribution due to the insufficient spray-tip rates, the chemical reactions before and after ignition penetration, poor fuel-air mixing and poor cold-start in the mixture zones are represented with the “shell characteristic, respectively. Thus, it had been quite model”(18) and the “LTCTC model”,(19) respectively. As difficult to apply these factors to a real engine.However, shown in Fig. 13, the “multi-zone PDF model” gives it was found in this study that these problems are able very good agreements between simulation results and to be overcome by combining all of these factors as experimental data. This model can also represent PCCI shown in Fig. 11. As indicated in this figure, lowered combustion and thus analyze switching behaviors compressed-gas density around compression TDC between ordinary diesel and PCCI combustions. due to the low compression ratio enhances spray-tip (2) The “UniDES” gives various kinds of information penetration, good atomization and fast vaporization on in-cylinder phenomena such as spray zones of injected fuel due to the extremely-small-orifices behavior, history of equivalence ratio and temperature injector ensures sufficient fuel-air mixing, and the of each gas package, history of nitrogen oxide (NO) decreased heat loss due to the weakened gas flow gives concentration of each zone, and so on.(20) Some of necessary cold-start ability. them are shown in Fig. 14. The information of spray The findings in this study are epoch-making and zones behavior enables to judge whether or not the are being utilized mainly for next - generation, smoke-forming interference between pilot- and popular - edition diesel engines described in main-injection fuels described in section 3. 2 occurs, Section 2. 5. 2. and then to roughly determine optimum pilot-injection timing. The histories of both the equivalence ratio and 5. Development of Analytical Tools temperature of each gas package and NO concentration of each zone indicates what gas zone is a source of As mentioned above, various kinds of analytical methods and tools including special apparatuses were Measured Calculated developed in the studies of ours. 200 200 3 Fuel Amount 3 150 2727mmmm 150 2727mmmm Among them, as one of the most 3636mmmm3 3636mmmm3 100 100 important tools, a universal diesel 4141mmmm3 41mm41 mm3 engine simulator called “UniDES”(16) 50 50 0 0 is introduced here. This simulator 200 200 -10.5 ATDC Injection -10.5 ATDC is very useful to plan a new engine 150 150 -7.5 Timing -7.5 system and roughly determine main 100 -4.5 100 -4.5 specifications of the combustion 50 -1.5 50 -1.5 Heat Release Rate J/deg system. Additionally, it is very 0 0 -10 0 10 20 30 -10 0 10 20 30 helpful to adjust control parameters Crank Angle deg. ATDC Crank Angle deg. ATDC on injection and combustion systems. Entrainment Path 60 Measured Features of the “UniDES” are as Calculated follows: Ambient 40 (Zone2) Injection

(1) The core of “UniDES” is a J/deg Period 20 “multi-zone PDF model”, in which Diffusive Premixed Pilot 2 ReleaseHeat Rate 0 zones corresponding to spray regions Pilot 1 (Zone1) (Zone3) (Zone5) (Zone4) -30 0 30 60 and ambient gas region are introduced Crank Angle deg. ATDC and each spray zone is further divided into premixed and diffusion Fig. 13 Potential of “multi-zone PDF model”: comparison in heat combustion zones. In-cylinder gas release rate between measured and calculated results. flow such as swirl and squish are Reprinted and modified with permission from SAE Tech. Paper Ser., No. 2008-01-0843 © 2008 SAE International.

© Toyota Central R&D Labs., Inc. 2014 http://www.tytlabs.com/review/ 54 R&D Review of Toyota CRDL, Vol.45 No.3 (2014) 43-56 exhaust emissions, leading to an exact countermeasure. The “UniDES” and the diesel engine system (3) The “UniDES”, an in-house turbocharger model(21) simulator are both actively being applied to planning and a combustion noise simulator (22) were all linked new engines and also adjusting the engine control to a commercial code “GT-Power”, leading to a diesel parameters, resulting in effectively saving development engine system simulator. As shown in an evaluation time and labor. example of transient behavior during acceleration in EC mode of Fig. 15, the simulator is able to give good 6. Prospect of Future Researches agreements between calculated and measured data.(23) In the near future, it is thought that increases in power and torque densities are continued for further engine Swirl downsizing, and simultaneously innovative increase in thermal efficiency of diesel engines is required sect θ under realizing ultra-low exhaust 10ATDC emissions. 6ATDC From the view point of developing Pilot Main spray diesel combustion systems, the former 2ATDC trend requires exact understanding of in-cylinder phenomena especially -2ATDC under high-load and high-speed conditions, and thus analysis Fig. 14 Example of information on in-cylinder phenomena given by methods and tools applicable to high “UniDES”: spray zones behavior and gas state plots on f–T map. pressure (i.e. high density) and high Reprinted and modified from Trans. of the JSAE (in Japanese), Vol. 43, No.6 temperature conditions are necessary. (2012), pp. 1221-1226, © 2012 JSAE, with permission from Society of Automotive Engineers of Japan. Responding to this, a next-generation, optically accessible single-cylinder engine which sustains high cylinder pressure of 20 MPa and high-speed Indicated operation of 6,000 rpm has been Acceleration in EC mode developed in our laboratories.(24) Speed Vehicle By using both this engine and more Intake Air Flow Rate Engine Torque sophisticated analysis methods, in-situ 40 120 Measured 100 analyses should be conducted. 30 80 With regard to the latter requirement,

20 Nm 60 g/sec

Torque thorough reduction of various losses 40 10

Air Flow Rate Calculated 20 is most important. Among the losses, 0 0 heat loss from high-temperature EGR Rate Combustion Noise 60 90 working gas to walls of combustion chamber and exhaust paths should 40 80 dBA

% be first reduced. Responding to this 20 70 issue, an innovative heat-insulation EGR Rate EGR

1sec Noise Combustion method named “Temperature swing” 0 60 Time sec Time sec has been created.(25) With this method, harmful influence such as worsened Fig. 15 Potential of diesel engine system simulator with “UniDES”: exhaust emissions and lowered comparison in acceleration behavior of an engine system between measured and calculated results. thermal efficiency of conventional Reprinted and modified from Trans. of the JSAE (in Japanese), Vol. 38, No. 5 (2007), heat-insulation methods is able to pp. 77-82, © 2007 JSAE, with permission from Society of Automotive Engineers of Japan. be eliminated. Hereafter, putting the

“Temperature swing” method into practical use should Study by Using High Boost and High Injection be speedily accomplished and simultaneously a new Pressure in Single Cylinder Engine”, Proc. of combustion concept suitable for this method is desired COMODIA (2004), pp. 119-126, JSME. (12) Wakisaka, Y. et al., “Emissions Reduction Potential to be clarified. In addition, research and development of Extremely High Boost and High EGR Rate for an of thermal management including control of heat flux HSDI Diesel Engine and the Reduction Mechanisms to the walls and effective heat-recovery techniques are of Exhaust Emissions”, SAE Tech. Pap. Ser., also important. No. 2008-01-1189 (2008). Also hereafter, our laboratories will play key roles (13) Inagaki, K. et al., “Dual-fuel Stratified PCI Combustion on the above-mentioned researches for realizing a Controlled by In-cylinder Stratification of Ignitability”, SAE Tech. Pap. Ser., No. 2006-01-0028 (2006). next-generation diesel engine with both sufficiently (14) Splitter, D. et al., “Reactivity Controlled Compression high power density and innovatively high thermal Ignition (RCCI) Heavy-duty Engine Operation at Mid- efficiency. and High-loads with Conventional and Alternative Fuels”, SAE Tech. Pap. Ser., No. 2011-01-0363 (2011). (15) Inagaki, K. et al., “Low Emissions and High-Efficiency References Diesel Combustion Using Highly Dispersed Spray (1) Nakakita, K. et al., “A Study on Diesel Combustion with Restricted In-cylinder Swirl and Squish Flows”, with High Pressure Fuel Injection”, JSAE Tech. Pap. SAE Tech. Pap. Ser., No. 2011-01-1393 (2011). Ser. (in Japanese), No. 901078 (1990). (16) Inagaki, K. et al., “Universal Diesel Engine Simulator (2) Nakakita, K. et al., “Effects of High Pressure Fuel (UniDES) 1st Report: Phenomenological Multi-zone Injection on the Combustion and Exhaust Emissions PDF Model for Predicting the Transient Behavior of of a High-Speed DI Diesel Engine”, JSAE Review, Diesel Engine Combustion”, SAE Tech. Pap. Ser., Vol. 12, No. 1 (1991), pp. 18-25. No. 2008-01-0843 (2008). (3) Nakakita, K. et al., “A Study on Mechanisms of (17) Murakami, A. et al., “Swirl Measurements and Diesel NOx Formation and Soot Reduction under Modeling in Direct Injection Diesel Engines”, SAE High-Pressure Fuel Injection”, Trans. of the JSAE Tech. Pap. Ser., No. 880385 (1988). (in Japanese), Vol. 24, No. 1 (1993), pp. 5-9. (18) Halstead, M. P. et al., “The Auto-ignition of (4) Nakakita, K. et al., “Improvement of Diesel Hydrocarbon Fuels at High Temperatures and Combustion and Exhaust Emissions with Pressures — Fitting of a Mathematical Model”, High-Pressure Fuel Injection”, Proc. of JSAE Combustion and Flame (1977), pp. 45-60. Symposium: New Aspects of Diesel Combustion (in (19) Kong, S. C. et al., “The Development and Application Japanese), No. 9303 (1993), pp. 40-48, JSAE. of a Diesel Ignition and Combustion Model for (5) Kawamura, K. et al., “Measurement of Flame Multidimensional Engine Simulation”, SAE Tech. Temperature Distribution in Engines by Using a Pap. Ser., No. 950278 (1995). Two-color High-speed Shutter TV Camera System”, (20) Inagaki, K. et al., “Diesel Combustion Prediction in SAE Tech. Pap. Ser., No. 890320 (1989). Transient Operation Using a New Cycle-simulation (6) Nakakita, K. et al., “Optimization of Pilot Injection (Fifth Report) Highly Robust Prediction of Combustion Pattern and Its Effect on Diesel Combustion with Characteristics Over All Operating Points for Virtual High-pressure Injection”, JSME Int. J., Ser. B, Vol. 37, Engine Calibration”, Trans. of the JSAE (in Japanese), No. 4 (1994), pp. 966-973. Vol. 43, No. 6 (2012), pp. 1221-1226. (7) Hotta, Y. et al., “Achieving Lower Exhaust Emissions (21) Uchida, H. et al., “Transient Performance Prediction and Better Performance in an HSDI Diesel Engine of the Turbocharging System with Variable Geometry with Multiple Injection”, SAE Tech. Pap. Ser., Turbochargers”, 8th Int. Conf. of Turbochargers and No. 2005-01-0928 (2005). Turbocharging (2006), C674/018, I. Mech. E. (8) Hotta, Y. et al., “Cause of Exhaust Smoke and Its (22) Taki, M. et al., “Effect of Combustion Period on Reduction Methods in an HSDI Diesel Engine under HCCI Combustion Noise”, JSAE Tech. Pap. Ser. High-speed and High-load Conditions”, SAE Tech. (in Japanese), No. 20065179 (2006). Pap. Ser., No. 2002-01-1160 (2002). (23) Ueda, M. et al., “Diesel Combustion Prediction in (9) Nakakita, K. et al., “Direct-injection Diesel Engine Transient Operation Using a New Cycle-simulation and Combustion Method for the Same”, (Second Report): Transient Performance Prediction Patent No. EP0945602, No. US006161518 (1999). Using the Combustion Model”, Trans. of the JSAE (10) Hotta, Y. et al., “Combustion Improvement of an (in Japanese), Vol. 38, No. 5 (2007), pp. 77-82. HSDI Diesel Engine with Increasing Injection (24) Fuyuto, T. et al., “A New Generation of Optically Pressure and Injection Rate Shaping”, JSAE Tech. Accessible Single-cylinder Engines for High-speed Pap. Ser. (in Japanese), No. 20075716 (2007). and High-load Combustion Analysis”, SAE Tech. Pap. (11) Aoyagi, Y. et al., “Diesel Combustion and Emission Ser., No. 2011-01-2050 (2011).

(25) Kosaka, H. et al., “Concept of “Temperature Swing Heat Insulation” in Combustion Chamber Walls, and Appropriate Thermo-physical Properties for Heat Insulation Coat”, SAE Tech. Pap. Ser., No. 2013-01-0274 (2013).

Kiyomi Nakakita Research Field: - Engine Combustion and Engine System Academic Degree: Dr.Eng. Academic Societies: - Society of Automotive Engineers of Japan - The Japan Society of Mechanical Engineers - Combustion Society of Japan - Institute for Liquid Atomization and Spray Systems - Japan Awards: - The Outstanding Technical Paper Award, Society of Automotive Engineers of Japan, 1992, 2000, 2005 and 2009 - Harry Levan Horning Memorial Award, SAE International, 2004