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Combustion Analysis in PCCI Diesel Engines by Endoscopic and Pressure-Based Techniques

A.E Catania1, E. Spessa1, G. Cipolla2, A. Vassallo2

1. IC Engines Advanced Laboratory – Politecnico di Torino 2. General Motors Powertrain – Europe

1. Abstract

Endoscopic and pressure-based techniques were applied to the combustion diagnostics in a PCCI (Premixed Charge Compression Ignition) diesel engine featuring a low compression ratio (15.5:1). The pressure-based technique is based on an innovative premixed-diffusive multizone approach for heat release and emission formation analysis developed at Politecnico di Torino (PT). The combustion chamber is split into homogeneous zones (liquid fuel, unburned-gas/vapor-fuel rich mixture, unburned gas, premixed burned gas, diffusive burned gas zones), to which mass and energy conservation equations are applied. The diagnostic tool includes submodels for estimating NO, CO and PM formation. The two approaches were compared at a single engine operating point for different EGR rates. Endoscopic and pressure-based technique detected virtually the same combustion temperatures and similar trends of PM emissions. However, in addition to the endoscopic approach and thanks to the multizone diagnostic tool capability to take into account the burned gas expansion, NOx and CO levels were also evaluated, whereas PM trace could be extended to crank-angles at which extremely low combustion luminosity does not provide any more possibility for optical soot detection.

2. Engine specification and experimental setup

The baseline engine is derived from the GM Powertrain 1.9l 4-cylinder in-line 4-valves-per-cylinder EU4 engine, whose features are listed in Table 1. A new combustion bowl prototype was manufactured ([1]) so as to obtain a compression ratio (CR) target of 15.5:1. The bowl prototype features a central-dome shape and is characterized by both a high K-factor for improving the air utilization at full load and a lower aspect ratio in order to tolerate advanced injection timings at partial load. The choice of the target compression ratio value has been done on the basis of previous investigations ([1]) looking at mid-term post EU 5 timeframe that assessed the possibility to still have a robust low-temperature combustion ignition with such a CR value. Furthermore, the analyzed engine has been fitted with a glowing system with high protrusion metallic glow-plugs that offer an increased contact surface for igniting the diesel spray. All of the experimental tests on the reference engine as well as on piston and injector prototypes are carried out on a new AVL high-dynamic test bed (Fig. 1) at ICE Advanced Laboratory of Politecnico di Torino. The test rig is equipped with an ‘ELIN AVL APA 100’ cradle-mounted AC dynamometer, capable of

Engine type 1.9l EURO4

Displacement 1910 cm3

Bore x stroke 82.0mm x 90.4mm

Stroke-to-bore ratio 1.10

Compression ratio 17.5:1

Valves per cylinder 4

Turbocharger Single-stage with VGT

Fuel injection system

Common Rail 2nd gen. CRI2.2 – 1600bar

Maximum power and torque

110kW @ 4000rpm 320Nm @ 2000rpm

Specific power and torque 57.6kW – 167Nm/l

Table 1: Main engine specifications

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realizing full four-quadrant operation with high speed and torque dynamics, including simulation of zero torque and gear shifting oscillations in the drivetrain. The main measuring systems includes the ‘AVL KMA 4000’ for continuously metering the engine fuel consumption, the ‘Pierburg AVL AMA 4000’ raw exhaust-gas analyzer (2 trains for THC, NOx, CO, CO2, O2 measurements upstream of and downstream form DOC; 1 train for CO2 measurement in inlet manifold), and the heated AVL 715S smokemeter. A high-frequency KISTLER 6053 CCSP piezo-electric transducer is installed on the engine cylinder head for taking the pressure time-history of the gases in the combustion chamber of cylinder 4, whereas a high-frequency KISTLER 4075A10 piezo-

resistive transducer is used to detect pressure level in the inlet runner of cylinder 4 for referencing in-cylinder pressure. An AVL 365C crank-shaft driven encoder generates the time base for an automatic 14-bit data-acquisition system based on AVL IndiModul 620 system, which can acquire up to 8 channels with a maximum frequency of 800 kHz per channel. The engine was equipped with AVL Visioscope system for fuel spray and combustion imaging.

3. PT Multizone Premixed-Diffusion Combustion Model

The cylinder content is split into homogenous zones. Between the injection (SOI) and combustion start (SOC), three zones are present: the liquid fuel zone containing the injected fuel (designated by the index f,l), the unburned gas zone (u), filled with a mixture of fresh air, EGR and residual gas, and the mixture zone (m), i.e., a rich mixture of fuel vapor and unburned gas. The model accounts for the fuel evaporation and transfer from liquid zone to mixture zone. At SOC an additional zone (bp) is formed, made up of burned gas from the premixed combustion in the mixture zone. The premixed combustion products complete their oxidation through a diffusion flame around the jet periphery, where the required oxygen is available. At the diffusive combustion start (SOCd), a diffusive burned gas region is generated, which in turn is split into zones. These are generated at specific crank angles (�), which are selected so as to divide the fuel mass that undergoes diffusive burning into a user-defined suitable number of equal portions. The last-generated zone (bd,n) is fed by the premixed burned-gas zone and by the unburned-gas zone, so as to reach stoichiometric conditions, whereas the other diffusive zones (bd,j) evolve at constant mass. From SOCd onward, combustion proceeds as a two-stage quasi-steady process, i.e., for each � heat release is due to both premixed and diffusive contributions. The model performs measured in-cylinder pressure heat-release analysis applying the conservation principles of thermodynamics to each zone, the perfect gas law to each gaseous zone and a proper fuel evaporation model to the liquid zone. The new pressure-based combustion model is thoroughly described in [1]. The computed thermodynamic and thermo-chemical properties in the burned gas zones allowed the post-processing analysis of nitric

Fig. 1: AVL high-dynamic test bed

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oxide (NO), particulate matter (PM) and carbon monoxide (CO) formation.

4. AVL Endoscopic technique

The AVL VisioScope (Fig. 2) system has been applied for combustion analysis in the combustion chamber of the GMPT-E multicylinder engine. It features a cooled endoscope (Fig. 2a) which provides optical access to the interior area of the engine through the glow-plug seat (Fig. 2b). A digital CCD camera transfers digital data straight to a PC, whereas synchronization with the engine is achieved via an AVL 365C encoder at a resolution of 1 crank-angle (CA) deg. A strobe connected to the Light Unit is used to facilitate the correction angle adjustment. In addition to fuel spray and combustion imaging, the system has been applied to the determination of instantaneous combustion temperatures and soot levels of the diesel flames based on the spectral flame temperature measurement technique using the AVL ThermoVision two colour method (for its detailed explanation, please refer to [2]).

5. Results and discussion

Figures 3 and 4 show the images taken with AVL VisioScope at the four different CA indicated on the correspondent in-cylinder pressure (“Pressure”, black solid line) and HRR time-histories reported to the bottom-right of each figure. The engine was operated at N = 1500 rpm and bmep = 2 bar. Fig. 3 and 4 refers to EGR rate = 0% and 32.5%, respectively. HRR distributions were worked out by means of the PT multizone premixed-diffusion combustion model and show the contribution of the premixed burning (“HRR, Premixed”, dash-dotted line) in addition to the global heat release rate time history (“HRR”, red solid line). The four images in each figure show the pilot pulse (1), the main pulse (2) and the flame images for the crank angle at which maximum HRR (3) and maximum luminous emissions (4) occur. Figure 5 shows the diesel flame temperature (Fig. 5a) and the soot concentration (Fig. 5b) maps obtained by AVL ThermoVision two-colour method at the CA of Fig. 4.4. For temperature images, a frequency distribution of the temperature values in the image region was also generated for each CA. At each CA the temperature values of an image are sorted and divided into 10 ranges (j=1÷10) and for each range the mean temperature values Tj are worked out so that T1(CA) represents the mean of the highest 10% of the temperature values, T2(CA) the mean of the next highest 10% of the temperature values and T10(CA) the mean of the lowest 10% of the temperature values. Fig. 6 plots the time-histories of each Tj (labeled as “TV j” in the legend) for the test case at EGR rate = 32.5% along with the correspondent ensemble-averaged mean temperature Tm (labeled as “TV Tm”), which is a characteristic temperature of the diffusive flame and of the sooting regions encircled by the

(a) (b) Fig. 2: AVL VisioScope setup and schematics of in-cylinder angle of view.

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diffusive flame. Figure 7 shows a comparison amongst ThermoVision Tm (labeled as “TV Tm” in Fig. 7) and burned gas temperatures of premixed (Tbp: labeled as “MZ, Premixed” in Fig. 7) and diffusive (Tbd,j: labeled as “MZ j” in Fig. 7) zones evaluated by means of multizone premixed-diffusion

Fig. 3: VisioScope flame images along with correspondent experimental in-cylinder pressure

and multizone HRR time-histories. N = 1500 rpm, bmep = 2 bar, EGR = 0%.

Fig. 4: VisioScope flame images along with correspondent experimental in-cylinder pressure

and multizone HRR time-histories. N = 1500 rpm, bmep = 2 bar, EGR = 32.5%.

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combustion model for the test case with EGR = 0% (Fig. 7a) and 32.5% (Fig. 7b). Multizone diagnostic model generates each zone bd,j at the temperature of diffusive flame, therefore Tbd,j peaks are virtually equal to ThermoVision Tm at the corresponding CA. Then, as CA increases the temporal evolution of each zone bd,j takes the burned gas expansion into account, thus Tbd,j decreases with respect to ThermoVision Tm. Figure 8 compares the soot levels worked out by multizone program (“MZ Soot [mg]”) to the corresponding trends evaluated by AVL ThermoVision (“TV Soot [a.u.]) for the test cases at EGR = 0% (Fig. 8a) and at EGR = 32.5% (Fig. 8b). The global trends evaluated by the two methods show a good agreement. It is worthwhile pointing out that no quantitative statement can be made about the correctness of the soot levels worked out by the two colour method, therefore “TV Soot” is expressed in arbitrary units (a.u.). On the other hand, based on a specifically developed calibration procedure ([1]) multizone approach is able to calculate quantitative soot levels by coupling Hiroyasu soot formation model and Nagle and Strickland-Constable soot oxidation model to the computed thermodynamic and thermo-chemical properties in the burned gas zones. Such quantitative soot levels are in line with the experimental outcomes, as is supported by Fig. 9 where experimental and multizone-computed global soot levels are compared as a function of EGR rate at N = 1500 rpm and bmep = 2 bar. Figures 10 and 11 reports the NO (Fig. 10) and CO (Fig. 11) time-histories calculated with the multizone diagnostic tool for the EGR = 0% (Figs. 10a, 11a) and EGR =

(a) (b) Fig. 5: Diesel flame temperature (a) and soot concentration (b) maps obtained by AVL

ThermoVision two-colour method. N = 1500 rpm, bmep = 2 bar, EGR = 32.5%. CA = 373 deg (Fig. 4.4).

350 360 370 380 390 400 410CA [deg]

2000

2200

2400

2600

2800

3000

T [K

]

TV 1TV 2TV 3TV 4TV 5TV 6TV 7TV 8TV 9TV 10TV Tm

Fig. 6: Frequency distribution and mean value of temperature values at: N = 1500 rpm, bmep = 2 bar, EGR = 32.5%.

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32.5% (Figs. 10b, 11b) test cases. NO and CO time-histories are determined by applying the SEZM (Super Extended Zeldovich Model) and Bowman submodels, respectively, to the burned gas zones according to the procedure detailed in [1]. The thin lines refer to the NO and CO formation and decom position in diffusive (“Zone j”) and premixed (“Premixed”) zones, whereas the thick line indicates the resulting global NO and CO levels in the combustion chamber throughout the engine cycle. The black arrow to the right-hand side indicates the value at exhaust valve opening worked out based on NO and CO engine-out measurements. Fig. 12 compares experimental and multizone-computed global NO (Fig. 12a) and CO (Fig. 12b) levels as a function of EGR rate at N = 1500 rpm and bmep = 2 bar. An excellent agreement between experimental and diagnostic-based results is apparent, thus supporting the capability of multizone approach to capture the trends and the levels of CO and NO emissions, with a generally

0 10 20 30 40 50EGR [%]

20

25

30

35

SOO

T [m

g/kW

h]

SOOTSOOT exp

Fig. 9: Experimental and multizone-computed

global soot levels at different EGR rates. N = 1500 rpm, bmep = 2 bar.

TV TmMZ 1MZ 2

MZ 3MZ 4MZ 5

MZ 6MZ 7MZ, Premixed

HRR, PremixedHRR

350 360 370 380 390 400 410CA [deg]

0

600

1200

1800

2400

3000

T [K

]

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30

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R [J/deg]

350 360 370 380 390 400 410CA [deg]

0

600

1200

1800

2400

3000

T [K

]

0

10

20

30

40

50

HR

R [J/deg]

(a) (b) Fig. 7: Comparison amongst ThermoVision flame temperature (TV Tm) and PT multizone

burned gas temperatures of premixed and diffusive regions for EGR = 0% (a) and 32.5% (b). N = 1500 rpm, bmep = 2 bar.

350 360 370 380 390 400 410CA [deg]

0

40

80

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TV, S

OO

T [a

.u.]

0

0.002

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0.01M

Z, SOO

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TV, SOOT [a.u.]MZ, SOOT [mg]

350 360 370 380 390 400 410CA [deg]

0

40

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200TV

, SO

OT

[a.u

.]

0

0.002

0.004

0.006

0.008

0.01

MZ, SO

OT [m

g]

TV, SOOT [a.u.]MZ, SOOT [mg]

(a) (b) Fig. 8: Comparison amongst ThermoVision (TV Soot) and PT multizone (MZ Soot) soot levels

for EGR = 0% (a) and 32.5% (b). N = 1500 rpm, bmep = 2 bar.

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satisfactory accuracy degree at a very low computational cost.

6. Conclusions

Pressure-based and endoscopic techniques showed to be powerful means to investigate cause and effect relations between injection rate, heat release, in-cylinder temperatures and pollutant emissions. In particular, AVL VisioScope system are very effective for spray and combustion imaging when a comparison amongst different hardware configuration (intake ports, injector nozzle, piston bowl, …) is required. By applying AVL ThermoVision to endoscopic measurement, diesel flame temperature and soot levels distributions can also be calculated.

350 360 370 380 390 400 410CA [deg]

0

300

600

900

1200

1500

1800N

Ox

[ppm

]zone 1zone 2zone 3zone 4zone 5zone 6zone 7Premixed Global

195 ppm

350 360 370 380 390 400 410CA [deg]

0

200

400

600

800

1000

NO

x [p

pm]

zone 1zone 2zone 3zone 4zone 5zone 6zone 7Premixed Global

65 ppm

(a) (b) Fig. 10: NO time-histories calculated with the multizone diagnostic tool for the EGR = 0%

(a) and EGR = 32.5% (b) test cases. N = 1500 rpm, bmep = 2 bar.

350 360 370 380 390 400 410CA [deg]

0

20000

40000

60000

80000

100000

120000

CO

[ppm

]

zone 1zone 2zone 3zone 4zone 5zone 6zone 7Premixed Global

713 ppm

350 360 370 380 390 400 410CA [deg]

0

20000

40000

60000

80000

100000

CO

[ppm

]

zone 1zone 2zone 3zone 4zone 5zone 6zone 7Premixed Global

954 ppm

(a) (b) Fig. 11: CO time-histories calculated with the multizone diagnostic tool for the EGR = 0%

(a) and EGR = 32.5% (b) test cases. N = 1500 rpm, bmep = 2 bar.

0 10 20 30 40 50EGR [%]

50

100

150

200

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x [p

pm]

NOx NOx exp

0 10 20 30 40 50EGR [%]

600

700

800

900

1000

CO

[ppm

]

COCO exp

(a) (b) Fig. 12: Experimental and multizone-computed global NO (a) and CO (b) levels at different

EGR rates. N = 1500 rpm, bmep = 2 bar.

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PT multizone premixed-diffusion combustion model can be used to separate the premixed and diffusive burning effects on combustion parameters and emissions, based on in-cylinder pressure measurements. A very good accuracy degree can be achieved at a very low computational cost and by using the measuring instruments that are usually available for base engine development activities. The flame temperatures calculated by AVL VisioScope/ThermoVision and PT multizone diagnostic models shows a very good agreement. However, PT approach is also capable of evaluating burned gas temperatures during the expansion, thus allowing quantitative evaluation of NO and CO levels. With reference to soot level, both AVL VisioScope/ThermoVision and PT multizone techniques give virtually the same trends during combustion. However, soot levels worked out by the two-colour method are only expressed in arbitrary units, whereas PT multizone technique can calculate quantitative soot levels, which are in line with the experimental outcomes.

7. References

1. Baratta, M., Catania, A.E., Ferrari, A., Finesso, R., Spessa, E.: “Innovative Multizone Premixed-Diffusion Combustion Model for Performance and Emission Analysis in Conventional and PCCI Diesel Engines” Comodia 2008, Japan, July, (2008).

2. AVL Product Guide AT1650E, “Thermovision advanced”, (2004).


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