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  • IV-6, 1

    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

  • Italian Section of the Combustion Institute

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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=110) 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, EG

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