Future European market trends favor system solutions with low fuel consumption and low raw emissions to reduce the amount of exhaust gas aftertreatment. On this market, the challenge is to deliver a system concept and demonstrate its technical advantage in the competition. The optimization of the combustion system within the engine boundaries of engine friction, turbo-charger and gas exchange for low emissions, noise and fuel consumption with a target of high power density is complex. Hence the engine testing becomes time- and cost intensive even though state-of-the art tools as Design of Experiments and Model Based Calibration methods are applied. Therefore the optimization of the piston bowl design and selection of nozzle parameters e.g. spray cone angle, no. of holes is evaluated by the usage of CFD simulation. Although this combined approach of simulation and testing has limited prediction, the new combustion system achieves the emission targets with the given fuel consumption penalty. Introduction pressure system with DENSO's Piezo injector The market demands for the next legislation G3P. limit of EU6 are quite challenging from system 2. Adaptation of the air path by an additional point of view. Low fuel consumption and low raw Low-Pressure-Loop (LPL) EGR system to demonemissions are a necessity to get customer accep- strate EU6 emission levels tance from environmental and system cost point of 3. Change to a high performance turboview. The key to control the combustion process is charger with adaptation of the bowl change to the injection system to phase the combustion in achieve EU6 emissions with an increased power time and space and the air-path management for density. intake temperature and oxygen content control [1]. The design of the combustion chamber is the In this study, an existing 2.0l, 4 cylinder EU4 major focus in this study. LIEF measurements engine, CR=16, is used to demonstrate the capa- indicate that the spray of the G3P injector has a bilities of DENSO's Engine Management System. leaner distribution than the baseline injector as The engine configuration can be viewed in Fig. 1. seen in Fig. 2. Moreover, the spray penetrates deeper into the piston bowl due to an increased rail pressure in comparison to the baseline. Both features have to be addressed by the design of the combustion bowl chamber. Thus a re-design of the bowl-chamber is necessary and will be supported by CFD simulations. The CFD code FIRE from AVL was used in this study. The spray model is the well known Discrete Droplet Model (DDM). Baseline engine data was used to calibrate the spray model parameters due to the limitations of this approach [3]. With regard to combustion, the ECFM-3Z model [4] is applied. In the following, a Fig. 1 Test-engine with replaced Engine Management new piston bowl was developed by a combination System (EMS) of CFD and engine testing.
The objective of this study is to demonstrate
EU6 emissions and to increase the power density
from 55 to >60 kW/l by downsizing: two engine
versions A and B are existing. The higher boost
pressure of version B compared to A is beneficial
to increase the engine power density [
1. Change of the baseline series 1600 bar Piezo to 3rd generation 2000 bar common rail Fig. 2 Equivalence ratio distribution between baseline series engine injector and G3P. Development of a New Piston Bowl
An initial bowl design denoted as piston 1 was proposed based on CFD calculations as shown in Fig. 3. The potential for this bowl is indicated by a better soot oxidation among the baseline and other bowl proposals.
In a second step, nozzle parameters as spray
cone angle and no. of holes were optimised by
CFD to define a nozzle matrix since the spray-bowl
interaction is one parameter to control the soot
formation [
The piston 1 design and nozzle samples were manufactured and evaluated by engine testing.
The testing procedure includes a calibration procedure in all emission mode points by Design of Experiments (DoE) and Model Based Calibration (MBC) methods as well as manual calibration at 0 5 10 15 20 25 30 35 40 45
NOx Emission [g/h] Fig. 7 Effects of intake charge cooling and rail pressure
variation on performance
The effect of intake charge cooling can be viewed in the combustion analysis from Fig. 8. The full load. Fig. 6 shows the Filter Smoke Number (FSN) at rated engine conditions for various nozzle tip protrusions, hydraulic Flow Rates (HFR) and no. of holes. The rated power is only limited by the turbine temperature. Furthermore, the nozzle tip protrusion (NTP) was fixed to 2.9 mm, the no. of holes to 8 and the HFR to 750 cm³/min which results in a hole size diameter of 121µm. Overall, a power density of 63 kW/l can be achieved.
NTP 1.9, HFR 750, 8-hole NTP 2.4, HFR 750, 8-hole NTP 2.9, HFR 750, 8-hole NTP 2.4, HFR 800, 8-hole
NTP 2.4, HFR 750, 7-hole 90 95 100 105 110 115 120 125 130
Effective Power [kW]
Fig. 6 Selection of NTP, HFR and no. of holes
For the high load emission points e.g. at engine speed of 2250 rpm and BMEP of 8 bar, only LPLEGR was used. The NOx-soot trade-off is influenced by the cooling efficiency (Fig. 7). Increasing the efficiency from 54% to 85% reduces tic from 65°C to 40°C.
DOE optimum, 65deg downstr. Intercooler 40deg downstr. Intercooler Railpressure variation
Target 1.6 1.4 1.2 1.0 ][N0.8 S F 0.6 0.4 0.2 )] 260 FC *hW SB /([kg250 240 94 92 []dB90 e iso88 N 86 84 heat release by the early double pilot injection is not changed but the ignition of the late main injection is retarded. The premixed combustion is increased as the higher peak in ROHR indicates and less diffusion controlled combustion of rich areas occurs so that soot emissions are reduced.
100 90 re 80 ssu 70 reP ]ra 6500 re [b 40 ilydn 3200 C 10
0 120 ]100
A80 HRRO /[eJgCD264000 67 °C 40 °C 0-50 -40 -30 -20
-10 0 10
Crank Angle [deg] Fig. 8 Effect of intake charge cooling
A major challenge is to reduce the NOx-soot trade-off under a penalty in BSFC and noise. The retarded combustion shows a higher noise and lower soot level. If the rail pressure is additionally reduced, the soot emission benefit from the cooled intake charge is converted into a combustion noise benefit.
The engine testing of piston 2 bowl design indicated that the SCA of 159° has to be decreased to 155° due to an increase in soot emissions. In order to address the penalty in noise caused by decreased intake charge temperature a second option is to advance the pilot injections closer to the main (Fig. 11) which follows a more effective pilot combustion.
Evaluation of Piston 2 Bowl Design
In a second step, the piston 1 design was slightly changed to address the robustness sensitivity on injector production tolerances on the Fig. 11 Pilot timing effect on the noise model spray-bowl intersection and as shown in Fig. 3 and to improve the thermal robustness. A more effective pilot injection will shorten the
The evaluation of piston 2 design included both, ignition delay of the main injection. Therefore less simulation and engine testing in a simultaneous time is available to homogenise the mixture and process. The CFD simulation of piston 2 bowl de- less premixed combustion will decrease the noise sign shows that the fuel vapor is pushed from the but vice versa more diffusive burning of rich mixpiston bowl into the squish area (Fig. 9). The mix- ture increases the soot emissions as it can be obture in the bowl becomes leaner (Fig. 10) but air- served off-line from the MBC in Fig. 12. The soot excess is still available in the squish area. advantage of the decreased intake charge temperature can be changed into a noise benefit at a constant rail pressure level.
The final engine performance is demonstrated in Fig. 13. The emissions as well as the BSFC target can be achieved. An additional measurement showed that any further noise reduction would violate the BSFC penalty. This limitation is inherent to the system boundaries of the engine configuration. The high performance turbo-charger of engine B requires a higher back-pressure at the end of the expansion stroke compared to engine A and increases the pumping losses.
Additional measurement Bowl 1, Calib. for 67deg, SCA 159deg Bowl 2, Calib. for 40deg, SCA 159deg Bowl 2, Calib. for 40deg, SCA 155deg 9 8 s 7 e 6 ilttrcau /[]gh345 aP 21
0 275 ]h270 /kW265 [g260 FC255 SB250 245 92 91 90 iseoN ][dB888987 86 85 84 0 5 10 15 20 25 30 35 40 45
NOx Emission [g/h] Fig. 13 Engine performance of piston 2 bowl design
Vehicle emissions are estimated for a NEDC in Fig. 14 from four emission mode points. Overall, EU6 emissions are achieved with the current configuration. If noise and BSFC shall be furthermore reduced, the system configuration has to be changed. Either the low performance turbo-charger can be used if a lower power density is accepted or a two-stage turbo-charger with a better performance at part-load conditions could be considered but increase the system costs.
Summary and Conclusion
Engine development to meet new legislation limits is to be considered as a system optimization process within the given boundaries. This process was accomplished on a series production engine to demonstrate EU6 emissions with a high power density target including the FIS and airmanagement system.
The combined usage of CFD and engine testing enables a pre-selection of nozzle parameters and definition of bowl shape geometry which must be adapted to the individual spray characteristics.
Real engine testing is still mandatory and cannot be omitted. The final calibration of the engine testing by DoE and MBC is including high rates of cooled EGR to shift the combustion towards lower temperatures and better homogenisation of the spray. The BSFC depends on the air-management 30 aw 25 rs ] 20
m tae /kg15 l u ic [m10 traP 50 system. Especially the performance of the turbocharger has to be selected carefully. The higher specific power at rated conditions requires a higher boost pressure but will violate the constraint in BSFC on part-load conditions which is transferred into a violation of the constraint in noise level. A higher fun-to-drive pays back immediately by an acceptance of a higher noise level or by usage of a two-stage turbo-charger and increased system cost.
Opt. System Bowl 2 Target Baseline (bowl&TC) 0 50 100 150 200 250 300 350 400 450
NOx +HC [mg/km]
Fig. 14 Vehicle estimation