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© Denso
Diesel Combustion Potentials by Further Injector Improvement The further development of modern common rail injectors is focused on reducing the CO2 emissions of future diesel engines. Denso has succeeded in achieving fuel economy benefits by increasing the needle opening speed while at the same time reducing soot emissions. The new injectors from Denso with a solenoid or piezo valve also offer further degrees of freedom in the calibration of accumulated pilot quantities. On the basis of a tripple pilot injection pattern, further potential for reducing fuel consumption and soot emissions has been shown, both for passenger car and commercial vehicle applications.
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AUTHORS
Dr.-Ing. Jürgen Hagen is Director Powertrain Engineering at the Aachen Engineering Center of the Denso Automotive Deutschland GmbH in Wegberg (Germany).
Dr.-Ing. Olaf Erik Herrmann is Head of the Department Advanced Diesel Systems at the Aachen Engineering Center of the Denso Automotive Deutschland GmbH in Wegberg (Germany).
Dr.-Ing. Jost Weber is Team Leader FIE System and Diesel Combustion at the Aachen Engineering Center of the Denso Automotive Deutschland GmbH in Wegberg (Germany).
Dipl.-Ing. (FH) Dirk Queck is Senior Engineer Diesel EMS at the Aachen Engineering Center of the Denso Automotive Deutschland GmbH in Wegberg (Germany).
INCREASING DEMANDS ON INJECTION SYSTEMS
Stricter CO2 limits in combination with the coming real driving emissions (RDE) legislation impact the current developments for passenger car diesel engines of all segments. The conflict remains despite the increased effort on after treatment technology due to the restrictions in the DeNOx efficiency caused by lower exhaust gas temperatures corresponding to vehicles with lower CO2 emissions. Due to these limitations both the engine out NOx/soot and the NOx/BSFC trade-off need to be further improved. Finally the end user’s comforts has to be considered by improvement of combustion noise and avoiding inconvenience e.g. urea re-filling. The first common rail system was launched in December 1995 by Denso – at that time for a commercial vehicle application, FIGURE 1. This first common rail injector realised 1200 bar injection pressure with a pilot injection dwell time of 0.7 ms. For Euro 3 emissions legislation Denso launched the first 1800 bar common rail system supported by a solenoid injector G2S with 0.4 ms dwell time and a piezo G2P with already 0.1 ms dwell time between the pre injection and main injection. In 2013 the Denso 2500 bar 4th generation system was launched first only with solenoid technology keeping the 0.2 ms dwell time but with focus on pilot stability and leakage reduction [1]. Thanks to its structure with less moving masses, reduced leakage and the possibility to combine it with the so-called i-ART [2] closed loop injection control option the G4S principle has still further potential to improve the hydraulic performance to the need of the combustion concept for each application. Already some projects apply e.g. triple pilot strategies using i-ART technology to control accurate the pilot quantities. However there is further room to expose some benefits and utilise the hydraulic potential for combustion improvement as it will be explained in the following. INJECTOR IMPROVEMENT
For commercial diesel engines the solenoid technology remains the most promising solution for robustness and simplicity. The structure of the G4S injector uses a so-called control plate as shown 04I2016
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FIGURE 1 Denso common rail injector history (© Denso)
in FIGURE 2, which keeps the inlet orifice closed during the injection [1]. This reduces the injector leakage to only the amount which is needed as energy to actuate the needle (dynamic leakage) and the static leakage becomes zero. Thanks to this the drive power of the high pressure pump and thus the fuel consumption can be reduced as less fuel needs to be delivered to the injector. As shown in FIGURE 2 the speed of needle opening can be influenced by the out orifice flow rate. Denso is working on an improved solenoid injector G4.5S, which has the possibility of a significant
increased out orifice flow rate and allows for faster needle opening. This allows to inject the same injection quantity in a shorter time at a given rail pressure. While the G4.5S injector will be developed for both heavy duty and passenger car applications Denso has started also the development of the 4th generation piezo injector as it is shown in FIGURE 3 [3] for passenger car application only. The combination of the existing piezo stack and piezo three-way valve technology of the 3rd generation piezo injector with the control plate structure of the G4S injector allows to further
increase the outlet orifice flow rate. A faster needle opening speed at the same needle closing speed is achieved as it is required for high power diesel engines. Therefore the injection duration at same rail pressure and same nozzle flow rate is significantly reduced as shown in FIGURE 3 (right). Regarding the multiple injection performance the G4P achieves a short hydraulic dwell time of an injection interval <100 µs as shown in FIGURE 4. To separate the effect of fast needle actuation and short interval pilot injection for pre-development a concept G4S injector with reduced in-orifice is prepared, which also can achieve pilot dwell times approximately of 100 µs [4]. ENGINE RESULTS FOR PASSENGER CAR APPLICATION
FIGURE 2 Current G4S injector versus new G4.5S injector with larger out-orifice to achieve shorter injection duration, heavy duty injector with 1550 cm3/min flow rate [6] (© Denso)
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As mentioned beforehand the G4P injector enables for new, triple pilot injection strategies with short intervals. This pattern is investigated on a 2.2-l four-cylinder passenger car engine (HP-EGR, two-stage turbocharger). In addition a concept injector of G4S with reduced in-orifice to achieve a hydraulic interval of 100 µs is used on this engine and compared with the G4P injector for a conventional double pilot
FIGURE 3 Comparison of G4P injector versus G4S injector at a rail pressure of 80 MPa and quantity of 20 mm3/stroke [3, 4] (© Denso)
injection strategy both with conventional hydraulic intervals, as shown in FIGURE 5. Both solenoid injectors show the same performance. In comparison to these solenoid injectors, the G4P injector with the increased outlet orifice has a faster needle opening. Thus the seat throttle losses at the nozzles are reduced. In this concept, the rail pressure momentum can be used effectively to entrain the air sufficiently and create a good mixture formation. Therefore the premixed combustion is enhanced and soot emissions of G4P are slightly reduced com-
pared to the G4S injectors. Thanks to the shorter injection duration, the combustion duration is shortened and a benefit in BSFC reduction of 0.9 % could be achieved. Secondly, the advanced injection pattern with a triple pilot injection and after injection has been applied. The hydraulic intervals have been selected at the limit of each concept to demonstrate the full potential, as presented in FIGURE 6. Here the steep injection rates in combination with very short intervals of the G4P injector offers a smoke reduc-
tion of 50 % and BSFC advantage of 2.2 % (NOx = 1 g/kWh) while combustion noise is increased by 1 dB(A) compared to the G4S injector. On the other hand the concept G4S injector with reduced in-orifice could already reduce the BSFC by 1.2 % at the same soot emissions level. Clearly the advantage of steep injection rates offers the potential to reduce the soot emissions furthermore. This advantage of soot reduction potential allows for even further BSFC reduction by recalibration of combustion timing and rail pressure.
FIGURE 4 Short interval pilot at a rail pressure of 120 MPa [6] (© Denso) 04I2016
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FIGURE 5 Engine result with conventional fuel injection system (FIS) calibration, n = 2000 rpm and BMEP = 6 bar [4] (© Denso)
ENGINE RESULTS FOR HEAV Y DUT Y APPLICATION
For the heavy duty G4S and G4.5S evaluation a 13-l six-cylinder engine with single stage turbocharger has been equipped with the Denso 4th generation injection system. While EGR rates up to 50 % have been investigated with injection pressures up to 3000 bar in the past [5] today a diversification of combustion and after treatment concepts with various amount of EGR can be found – for the European market even truck engines without EGR have been launched. When optimising the common rail injector calibration without EGR late injection timings with quite low rail pressures have to be calibrated. This causes actually higher soot numbers than known with EGR condition for such engine. As shown in FIGURE 7 for the G4S injector at mid engine load a rail pressure of only 840 bar was found as optimum (with EGR
approximately 1500 bar is used). To achieve a NOx level in the range 8 g/kWh (95 % SCR efficiency) the start of injection has to be retarded causing that a later centre of combustion leads to non-optimum BSFC. The target of the combustion optimisation under this condition is to reduce NOx at given BSFC without sacrificing the soot emission. In order to achieve a better NOx/BSFC trade-off the in FIGURE 7 shown triple pilot approach was found by Design of Experiments (DoE) whilst keeping the injection interval within the 200 µs hydraulic interval limitation of the G4S injector. As with higher pilot injection quantity the soot emissions increase, therefore a higher rail pressure was as well required to achieve the same soot level as the block injection. Adequately the rail pressure had to be increased from 84 to 120 MPa for the triple pilot injection pattern. However even with higher rail pressure the NOx/BSFC trade-
FIGURE 6 Engine result with advanced triple pilot calibration, n = 2000 rpm and BMEP = 6 bar [4] (© Denso)
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off could be slightly reduced. This confirms that the triple pilot approach has the potential to improve the combustion. As the injection duration of the G4S injector with the triple pilot injection is increased, additional benefit for soot and fuel consumption could be expected by a faster needle opening. This is enabled by the G4.5S injector. The result in FIGURE 7 proves that by applying a triple pilot injection in combination with a quicker needle opening the soot emissions can be reduced by 50 %. Comparing the G4S with block injection with the triple pilot approach supported by faster needle opening at same NOx the BSFC can be reduced by 1 g/kWh (approximately 0.5 %). SUMMARY
The new Denso common rail injectors G4P (servo piezo) and G4.5S (solenoid) have been investigated on a passenger car engine and a heavy duty engine. Two
FIGURE 7 Variation of start of injection at constant boost condition, triple pilot approach on heavy duty engine without EGR for NOx reduction, n = 1294 rpm, T = 625 Nm, heavy duty injector with 1550 cm3/min flow rate, variation of needle opening speed for soot reduction [6] (© Denso)
features contribute for BSFC improvement and emissions benefit: On the one hand faster actuation of the needle in the opening direction and on the other hand closer pilot injection and more flexible injection patterns. The reduction of needle throttling effects by an increased needle opening speed has been investigated on a 2.2-l passenger car engine – BSFC could be improved by 0.9 % and soot reduced at the same time. Denso’s new injectors G4P and G4.5S also provide more flexibility for closer pilot interval. By applying triple pilot injection strategy on a 2.2-l engine a further 1.2 % BSFC improvement for G4S with reduced in-orifice flow rate could be achieved. Here the G4P advanced injection pattern of a triple pilot can improve the BSFC by 2.2 % and soot emissions are improved by 50 %. For the heavy duty engine a triple pilot injection pattern without EGR has been applied. At same NOx emissions the BSFC could be reduced by 0.5 %. Under this condition the soot emissions are reduced by 50 % thanks to a fast opening needle.
[3] Takeuchi, K.; Shinohara, Y.; Kojima, A.; Ishizuka, K.; Uchiyama, K.; Nakagawa, M.; Herrmann, O. E.: Further Innovations for the Diesel Engine Management and Aftertreatment System. International Vienna Motor Symposium, 2015 [4] Weber, J.; Ruwe, J.; Kiyanni, J.; Sashima, N.; Hagen, J.; Ueda, D.; Sugawara, S.; Uchiyama, K.; Ishizuka, K.; Suzuki, M.: The Next Generation of Common Rail Injector – Steps to Control Diesel Combustion in Time. Symposium for Combustion Control, Aachen, 2015 [5] Ruhkamp, L.; Kind, M.; Laumen, H.-J.; Maassen, F.; Mashida, M.; Takeuchi, K.; Shinohara, Y.; Herrmann, O. E.; Kudo, T.; Nakagawa, M.; Rajamani, V.: Options for Diesel Engine Improvements by Increased Injection Pressure up to 3000 bar. Aachen Colloquium Automobile and Engine Technology, 2011 [6] Hagen, J.; Nakagawa, M.; Herrmann, O.; Weber, J.; Queck, D.; Uchiyama, K.; Ishizuka, K.; Kawamura, J.; Tomida, Y.; Mashida, M.: Diesel Combustion Potentials by further Injector Improvement in case of different engine applications. Aachen Colloquium, 2015
REFERENCES [1] Matsumoto, S.: Date, K.; Taguchi, T.; Herrmann, O. E.: The New Denso Common Rail Solenoid Injector. In: MTZworldwide 74 (2013), No. 2, pp. 44-48 [2] Shinohara, Y.; Takeuchi, K.; Herrmann, O. E.; Laumen, H.-J.: New 3000 bar Common Rail System with Closed-Loop Injection Control. In: MTZworldwide 72 (2011), No. 1, pp. 4-8
THANKS
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The authors are very grateful to Ken Uchiyama, Koji Ishizuka, Masayoshi Ito and Kenji Date, all Denso Corporation in Kariya (Japan), for their valuable contribution to this publication.
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