Soot Observations and Exhaust Soot Comparisons from ... · port-fuel injection (PFI) system. These desirable features are attained with direct injection through increased volumetric

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ARTICLE INFOArticle ID: 04-11-02-0008Copyright © 2018 SAE Internationaldoi:10.4271/04-11-02-0008

HistoryReceived: 21 Nov 2017Revised: 08 Jan 2018Accepted: 28 Jan 2018e-Available: 07 May 2018

KeywordsEthanol-blended gasoline, Methanol-blended gasoline, Direct injection, In-cylinder soot imaging, Exhaust soot concentration, Spark ignition engine

CitationVedula, R.T., Men, Y., Atis, C., Stuecken, T. et al., “Soot Observations and Exhaust Soot Comparisons from Ethanol-Blended and Methanol-Blended Gasoline Combustion in a Direct-Injected Engine,” SAE Int. J. Fuels Lubr. 11(2):2018,doi:10.4271/04-11-02-0008.

ISSN: 1946-3952e-ISSN: 1946-3960

Soot Observations and Exhaust Soot Comparisons from Ethanol-Blended and Methanol-Blended Gasoline Combustion in a Direct-Injected EngineRavi Teja Vedula, Link Engineering Company

Yifan Men, Cyrus Atis, Tom Stuecken, Guoming Zhu, and Harold Schock, Michigan State University

Steven Wooldridge, Ford Motor Company

AbstractParticulate formation was studied under homogeneous-intent stoichiometric operating conditions when ethanol-blended (E10) or methanol-blended (M20) gasoline fuel was injected during intake stroke of a 4-stroke direct-injected engine. The engine was tested at wide open throttle under naturally aspirated conditions for a speed-load of 1500 rev/min and 9.8 bar indicated mean effective pressure. In-cylinder soot observations and exhaust soot measurements were completed for different fuel rail pressures, injection timings, coolant and piston temperatures of the optical engine. Fuel delivery settings were tested with both single and split injections during intake stroke. The target piston temperature of the optical engine was attained using pre-determined number of methane port fuel injection firing cycles. Overall, the in-cylinder soot observations correlated well with the engine-out soot measurements. A warmer cylinder head favored soot reduction for both fuels. A hot piston resulted in more soot than a warm piston, with a high fuel rail pressure. The two alcohol blends showed contrasting differences in their inclination for particulate formation. At the tested injection timing, a smaller and larger first split percent of fuel injection favored soot reduction for E10 and M20 respectively, compared to single injection operation.

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2 Vedula et al. / SAE Int. J. Fuels Lubr. / Volume 11, Issue 2 (May 2018)

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Introduction

Gasoline direct injection (GDI) offers several benefits such as better fuel economy, less CO2 emissions and higher thermal efficiency, compared to a conventional

port-fuel injection (PFI) system. These desirable features are attained with direct injection through increased volumetric efficiency, reduced heat losses, and high- compression-ratio operation without engine knock [1, 2, 3]. A long-time chal-lenge for GDI engines is their likeliness of forming more engine-out particulate matter compared to that produced by the PFI engines. Extensive studies conducted by the California Air Resources Board and others on several light-duty gasoline vehicles revealed that the fleet-averaged PM values for GDI vehicles were significantly higher than for PFI vehicles [4, 5]. Stringent regulations on particulate number and size (EURO VI and beyond) and particulate mass (US and EU) standards necessitate the urge to seek for strategies to reduce PM emis-sions from GDI engines. Soot morphology properties are dependent on the mode of GDI engine operation. Zelenyuk et al. [6] showed that a lean stratified operation resulted in large fractal agglomerates of elemental carbon and stoichio-metric operation yielded a large fraction of ash particles. Lee et al. [7] noticed that injecting gasoline or a blended fuel at an advanced timing resulted in large number of nucleation-mode particles (smaller than 15 nm). Piock et al. [8] noted that the short mixing time seen in GDI engines makes it difficult for them to meet the emission targets. The authors in that tech-nical review, however, concluded with a positive note that GDI engines can reach PFI-equivalent or lower PM emissions through optimization of fuel delivery strategies and air-fuel mixture formation.

Split injection is an optimized way of delivering fuel into the cylinder of a GDI engine. The split injection strategy is shown to improve engine power output and reduce engine-out emissions, mainly in lean-stratified mode [9]. Costa et al. [10] performed an optimization analysis for a split injec-tion mode with two injections of gasoline fuel occurring in the intake and compression strokes. They showed that timing the second injection pulse just after the intake valve closed and advancing the spark with respect to an equivalent single injection condition resulted in 3% more work output and 5% less soot. Kim et al. [11] attained a 10-28% improvement in brake-specific fuel consumption and a 72-84% reduction in NOx emissions by switching from a single injection of lique-fied petroleum gas to a split injection mode. Their lean-stratified split injection conditions, however, resulted in high combustion instabilities compared to the single-intake injection condition.

Usability of alternative fuels for transportation has attracted great attention in recent years. Detailed studies have been reported on fuel spray evolution, combustion and partic-ulate matter of alcohol-gasoline blends in direct-injection, spark-ignition engines (DISI). These studies mainly employed ethanol blends [12, 13, 14]. Some studies involved butanol blends [15] or multi-oxygenate blends [16]. In stratified

combustion mode, bio-ethanol leads to faster burn rates, high combustion stability, and reduced PM emissions compared to those obtained with gasoline [17]. Also, Gong and Rutland [18] noted that having very high ethanol content in the fuel (E85 vs. E20) resulted in significantly low particulate number concentration from a DISI engine. At cold start conditions, however, high ethanol content resulted in high PM emissions due to poor spray vaporization [19]. Khalek et al. [20] measured particle emissions from a GDI engine using three commer-cially available gasoline fuels, and results showed that the fuel having intermediate aromatic and olefin content and high vapor pressure resulted in the least PM emissions. Yinhui et al. [21] found that a fuel composition having lower aromatics and olefin content resulted in reduced particulate emissions for selected engine loads.

Methanol is another alternative fuel that can be directly burned in DISI engines or can be blended with gasoline. Engines that operate with pure methanol can run at much higher compression ratios, with increased volumetric effi-ciency due to high latent heat of evaporation, and with low CO and HC emissions; compared to gasoline [22]. Balki et al. [23] demonstrated that methanol application resulted in high combustion efficiency and lower NOx, CO, and HC emissions, compared to ethanol or gasoline. Wang et al. [24] determined that methanol-blended gasoline offered better emission control; but excess methanol substitution ratio phased out its usage due to high NOx emissions and due to its propensity to damage the engine.

In the current work, particulate formation in a DISI engine was studied under homogeneous-intent stoichio-metric operation using gasoline fuels blended with ethanol (LEV III E10 to represent United States market fuel) or methanol (M20 blend to represent a China market fuel). The cylinder head of this engine was based on a 2017 MY (model year) production turbocharged double overhead cam engine. The combustion system is based on a side-mounted direct injector configuration with the injector mounted in the cylinder head and positioned between the intake valves. All the testing was done for an engine speed-load of 1500 rev/min and an indicated mean effective pressure (IMEP) of 9.8 bar (±2%) at wide open throttle and under naturally aspi-rated conditions. This was the maximum attainable load for this engine at this speed using a naturally aspirated intake configuration. The 1500 rev/min engine speed is relevant to a variety of powertrain-vehicle combinations evaluated on various test cycles such as Federal Test Procedure Metro-Highway, New European Drive Cycle, and Worldwide harmo-nized Light vehicles Test Procedures (WLTP). In these standard test cycles, the speed-load cycle residency is heavily populated in the 1500-2000 rev/min range. For example, the averaged vehicle speed during WLTP cycle is 46.5 km/h which reflects to a nominal engine speed of 1500 rev/min. In-cylinder soot observations and exhaust soot measure-ments were completed for various operating parameters including injection timings, split injection conditions, piston temperatures, and wall thermal gradients.


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Optical Engine Test Setup

Engine SetupThe side-mounted injector direct-injection optical engine consists of a full metal liner and a flat-top sapphire window fitted in the piston. The engine cylinder was instrumented with a flush-mounted piezoelectric pressure transducer, and the intake manifold was instrumented with a TMAP sensor (Bosch 0-3 bar pressure range). A hydraulic actuated cylinder lift mechanism was employed to reduce the downtime of cleaning the piston window. A solution made of 50% water and 50% glycol was used as the coolant and was brought up to a temperature of 90 °C by a proportional-integral-deriva-tive-controlled heater. The hot coolant that returned from the cylinder head passed through a heat exchanger, and the condi-tioned coolant was recirculated back into the head. The engine was coupled to a direct current dynamometer and was controlled using Wineman’s INERTIA software. The engine dimensions and operating specifications are listed in Table 1. Unless otherwise specified, piston position mentioned here and throughout this work refers to crank angle (°CA) with respect to compression top dead center (TDC). The exhaust and intake camshafts were set to fixed angles as listed here for all the experiments studied (MOP - maximum opening point; BTDC/ATDC - before/after TDC).

Figure 1 shows the schematic of fuel delivery systems to the optical engine. Piston warm-up strategy was employed using methane PFI firing cycles. Methane was supplied to the intake port using a compressed natural gas injector and the injection pressure was regulated to approximately 5 bar. The direct-injected fuels tested in the current work are E10 and M20 fuels. The low-volatile E10 fuel has a high-octane rating of RON 98 and a Reid vapor pressure (RVP) of 7.1 psi. The high-volatile M20 fuel has an octane rating of RON 95 and a RVP of about 10.9 psi. The E10 fuel contains 10% ethanol by volume and the M20 fuel contains about 20% of methanol by volume. The direct-injected fuel of interest (E10 or M20) was filled in a piston accumulator. Compressed nitrogen was used to drive this piston pneumatically, and the pressurized fuel was thus supplied to the fuel rail (upstream of the DI injector) as shown in Figure 1. A six-hole DI injector designed for 20 MPa maximum fuel pressure from the production engine was employed. The six injector spray plumes target to optimize fuel-air mixing.

Simultaneously, the spray plumes need to manage the boundary constraints to avoid impingement on combustion chamber surfaces (valves, chamber walls, etc.) during the injection event. The exhaust soot concentrations were measured using a micro-soot sensor (AVL MSSplus).

The engine control tasks were carried out using dSpace’s real-time system MicroAutoBox (MABX) and RapidPro. The MABX provided the spark command and camera trigger signals. RapidPro consists of several configurable modules which, upon receiving command signals from the MABX, were used for actuating the intake throttle and the DI injector. The RapidPro DI injector module was used to perform both single injection conditions and split injection conditions. The PFI injector for warm-up sequence was actuated using an injector driver board built in house. The injection timings, pulse durations and all other command signals were controlled using the dSpace ControlDesk software.

Combustion ImagingOptical access to the combustion chamber was attained by viewing through the piston window and the 45-degree mirror. Figure 2 shows the piston window circumference and loca-tions of intake and exhaust values, spark plug and the DI injector. As seen here, the DI injector was side-mounted on the cylinder head. The natural luminosity of combustion was captured by a PHOTRON APX-RS high-speed visible video camera to which a Nikon AF Nikor 105 mm lens was attached. Combustion events of 200 DI firing cycles were captured at an imaging frame rate of 10 kHz with a resolution of 512×512 pixels. This frame rate corresponds to a temporal resolution of about one frame for every crank angle at an engine speed of 1500 rev/min; thus capturing crank angle-resolved spark and combustion event. The camera was externally triggered with a 5 V transistor-transistor logic pulse, and the command signal was synchronized with the spark timing using dSpace Control Desk.

TABLE 1 Engine geometry and operating point.

Bore 86 mm

Stroke 95 mm

Compression Ratio 10.0

Intake/Exhaust MOP 251 °CA BTDC/ 256 °CA ATDC

Engine Speed-Load 1500 rev/min - 9.8 bar IMEP

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FIGURE 1 DI optical engine test setup.

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MethodologyThe engine was motored and set at the desired speed by the direct current dynamometer. The methane PFI firing sequence was then carried out until the target piston temperature was reached. The length of warm-up sequence for the optical piston window to reach the desired temperature, was chosen based on infrared calibration experiments as discussed in [25]. In this methodology, infrared radiance of a chromium spot deposited on the sapphire window is correlated to its surface temperature using bench-top calibration experiments. Then the sapphire window is inserted in the engine piston and a warm-up sequence is carried out using methane PFI combus-tion. Thus, the number of methane PFI firing cycles was deter-mined to be 100 cycles and 400 cycles in order to attain piston temperatures of 110 °C (shorter warm-up) and 140 °C (longer warm-up), respectively. After attaining the target piston temperature, the control system switched off the PFI injection command and turned on the DI injection command. The engine continued to operate for at least another 200 DI firing cycles (continuously, no skip-firing) until the coefficient of variation in IMEP was below 2%. The piston was cleaned as needed to eliminate piston fouling.

Two DI injection configurations were tested in this work. A single injection configuration was tested for five start of injection (SOI) timings ranging from 330 to 240 °CA BTDC under different fuel rail pressures and warm-up durations (and hence, piston temperatures) as listed in Table 2. In addition, a split injection configuration was tested with the SOI of the first pulse at 310 °CA BTDC. One of three fuel mass fractions was selected for first injection pulse: 70%, 50%, or 30%.

The SOI timing of the second injection pulse depended on the amount of fuel mass percent chosen for SOI-1, while main-taining a constant pulse-to-pulse duration of 2 msec between the end of first injection and SOI-2. The split injection condi-tions tested in this work are listed in Table 3. In either injec-tion configuration, the fuel pulse width had to be adjusted for each SOI timing to maintain an average lambda value of 1.00 ± 0.01 over the span of 200 DI cycles recorded. In general, a constant fuel pulse width resulted in lower lambda value as the injection timing was advanced. Spark timing for all the conditions was adjusted close to the knock advance limit, and the CA50 or 50% burn angle was between 16 to 18 °CA ATDC.

In a separate set of experiments, the effect of wall temper-atures on particulate formation was studied. Related tests for a selected single injection condition with a lower rail pressure of 100 bar were designed as follows.

i. Coolant at 50°C and piston crown at 110°C - Effect of colder cylinder head and warmer piston

ii. Coolant at 50°C and piston crown at 140°C - Effect of colder head and hot piston

iii. Coolant at 90°C and piston crown at 90°C - Effect of warm head and warm piston

A high-speed combustion analysis system (CAS; A&D Tech) was employed for data acquisition. With this system, cycle-by-cycle analysis of engine combustion was made in real time by logging signals of in-cylinder and peak pressures, injection and spark current profiles, and camera trigger. The signals from the crank position sensor/engine-mounted encoder were logged to identify the TDC position. Data acqui-sition began after CAS synchronized with compression TDC. The IMEP values were noted from the CAS system, which post-processed the recorded pressure data.

In-Cylinder and Exhaust Soot

In-Cylinder Soot ObservationsRadiance of soot particulates in visible wavelengths is orders of magnitude higher than the chemiluminescence of other combustion gases (e.g. O2, CO, CO2, and H2O). This property was used to characterize soot. Soot probability maps were created from the captured images for each operating point and analyzed accordingly. Performing probability maps is beneficial as the cycle-to-cycle variations are retained to a

TABLE 3 Split injection operating conditions.

Fuel SOI-1 timing (°CA BTDC)

Fuel SOI-2 timing (°CA BTDC)

Fuel rail pressure (bar)

Piston temperature

310 286-272 100, 150 110°C, 140°C © S

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TABLE 2 Single injection operating conditions.

Fuel SOI timing (°CA BTDC) Fuel rail pressure (bar) Piston temperature330 (Early Intake) 100, 150 110°C, 140°C

310 100, 150 110°C, 140°C

290 100, 150 110°C, 140°C

270 100, 150 110°C, 140°C

240 (Late Intake) 100, 150 110°C, 140°C © S

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large extent, unlike the results from ensemble averaging. A thresholding approach was employed to identify soot. According to this approach, a lower limit and an upper limit were imposed to the mathematical expression 2*red - 1.5*green - 0.5*blue to identify pixels that are more inclined towards orange color (more likely soot) and less inclined towards purple or blue, which are usually the premixed f lame indicators. Red, green, and blue correspond to elements of the three dimensional matrix that represents the TrueColor image recorded by the camera. A pixel having these three intensity values (red, green, blue) that resulted in the above expression to exceed the set limits was filtered to zero value. The soot indicative pixels were observed to be in discrete pixel locations for almost all of the homoge-neous-intent operating conditions. As such, probability maps created from cycle-to-cycle visualizations were low. As a result of this, the probability map scale was adjusted to a range of 0 to 0.4. Also, to improve the visibility of in-cylinder soot locations with such discrete pixels, the algorithm for soot maps picked both the threshold-filtered pixel as well as its neighboring pixels.

Figure 3 illustrates the thresholding approach for soot detection. The resulting probability maps can range from 0 to 1, where a value of 1 implies that probability of soot presence in that pixel at that crank position when tested in all the cycles captured is 100%. To have a reliable comparison, the same pixel intensity limits were chosen for all the tested operating conditions.

Exhaust Soot MeasurementsThe engine exhaust soot was measured concurrently using a micro-soot sensor (AVL MSSPlus; measuring range 0.001-50 mg/m3). The working principle of this device is the photoacoustic phenomenon. When irradiated by light of constant intensity and a particular wavelength, molecules emit heat which in turn leads to local pressure changes. If the light is applied as a wave of changing intensity using a modu-lated laser beam, a pressure wave is formed. This acoustic signal coming from the photoacoustics of the soot molecules in the diluted exhaust was detected using a microphone.

Before reaching the measuring cell of the soot sensor, the engine-out exhaust was mixed with dilution air based on a chosen dilution ratio (DR). For the current set of experiments, a constant DR of 4 was chosen during exhaust soot measure-ments. It was later realized that a change in dilution ratio results in a different averaged-soot value and that a higher DR results in a directly proportional soot value. A lower DR might condense the sample and result in low soot values compared to those with a high DR. Due to this uncertainty in the DR selection, it is to be noted that the following results on exhaust soot concentrations were mainly used for comparing soot formation among different engine operating conditions. Results do not necessarily indicate the actual magnitudes. The reported soot comparisons are believed to be correct in relation to each other, because the soot measurements corre-lated well with in-cylinder soot observations. Additional details on the dilution of exhaust mixture are included in the Appendix attached.

Soot from Combustion of E10: Single InjectionThe primary results are presented here and in the subsequent sections in three categories. For each independent experiment at a specific piston temperature,

i. Combustion images acquired at the CA50 point, i.e. at 50% burn angle, are shown for different injection timings.

ii. Probability maps of in-cylinder soot are shown at approximately every 10 °CAs for the first 50 DI cycles. The labels listed above these images are crank angles after compression TDC. The locations of intake and exhaust valves, spark plug, and the DI injector are shown on the lower left of each figure.

iii. Measured exhaust soot concentrations are included under the probability maps for better demonstration of particulate formation during a given test.

Soot with Shorter Warm-Up (TPiston = 110°C)With a shorter warm-up period, a significant amount of fuel film burning was evident with E10 fuel at the most advanced injection timing of 330 °CA BTDC. Soot deposition on the piston can be noticed from the black marks on top of the piston window as shown in Figure 4. Brightness of these images is enhanced to make the soot deposition marks more visible. The soot deposition extended more towards the cylinder liner at 12 o’clock position and was closer to the piston center at 10 and 2 o’clock positions. This parabolic shape of soot deposition indicates the possible occurrence of longer penetration of the central spray plume compared to those of the right-side and left-side plumes as highlighted with a

FIGURE 3 Filtering a high-speed recorded instantaneous image for soot detection.

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hypothetical spray pattern; recall that the DI injector was located at the bottom in these images. Spray pattern diagrams are currently not available for the studied operating conditions.

Combustion images acquired in every 20 cycles (Cycle 1, 21, 41… 161 and 181) at the CA50 burn point for shorter warm-up sequence and different injection timings are shown in Figure 5. During combustion recordings, local diffusion flames were noticed inside the chamber. Due to their burning droplet-like small-scale appearance, these diffusion flames are instead called hereafter “non-premixed burn regions.” The non-premixed burn regions (yellow/orange color) found in these images are highlighted using solid circles. No addi-tional highlighting was employed for visibly long-length diffusion flames.

The soot probability maps for the five injection timings (row-wise) are shown in Figure 6. It should be noted that the probability maps throughout this work were created for 50 continuous DI firing cycles. As such, a probability value of 0.3 indicates that the likelihood of soot occurrence at that partic-ular pixel location was seen in 15 cycles (0.3*50 = 15) among the 50 cycles analyzed. The time-averaged micro-soot sensor readings for each injection timing are shown below their corre-sponding probability maps. The likelihood of in-cylinder particulate formation in the first 50 cycles, as indicated by the high soot probability in these images (see Figure 6), was dominant for the most advanced injection timing due to piston wall impingement. Accordingly, this injection timing condition resulted in high exhaust soot value due to pool fires there. Minimum exhaust soot was noted for an SOI at 310 °CA BTDC among all the injection timings tested. At this injection timing, the soot likelihood was relatively higher on the exhaust side of the chamber in the first 50 DI cycles. A further delay in injection timing showed more soot likelihood on the intake side and a higher exhaust soot value. Also, the combustion images in Figure 5 showed low luminous flames in several cycles for an SOI at 310 °CA BTDC. These results possibly indicate that, with a shorter warm-up, an SOI at 310 °CA BTDC led to minimal wall impingement. Also, a further delay in injection timing resulted in fuel-rich burning due to shorter mixing times.

When compared with combustion images recorded while using a low rail pressure of 100 bar (see Row #1 in Figure 7), the high rail pressure of 150 bar resulted in fewer pool fires and shorter soot deposit marks on the piston for an SOI at 330 °CA BTDC (see Row #1 of Figures 4 and 5). Also, the low rail pressure conditions resulted in more occurrence of non-premixed burn regions, especially close to the spark plug (Row #2 Cycle #1 and

Row #3 Cycle #41 in Figure 7). These observations indicate that the higher rail pressure improved spray vaporization.

Unless otherwise mentioned, the subsequent results are discussed for a DI rail pressure of 150 bar. To control the length of the article, only those CA50 burn point images which either contained significant diffusion flames or any non-premixed burn regions will be shown hereafter. Cycle numbers corresponding to these randomly selected images are desig-nated with arbitrary values of a, b, c, and d. Also, the soot probability maps of only the first 50 DI cycles will be shown by default to improve the visibility of the images. Probability maps of both first and last 50 DI cycles will be shown for coolant and piston temperature studies to observe the effect of wall temperatures on soot formation tendency.

FIGURE 4 Soot deposition on the piston window for an SOI at 330 °CA BTDC and shorter warm-up duration. (Image brightness enhanced to show soot marks).

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FIGURE 5 Cycle-to-cycle recordings of combustion at the CA50 burn point for single injection E10-150 bar-110 °C conditions.

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Soot with Longer Warm-Up (TPiston = 140°C)Combustion images at the CA50 burn point for longer warm-up conditions are shown in Figure 8. A longer warm-up significantly reduced pool fires on the piston for the most advanced injection timing condition, compared to shorter warm-up. Overall, all SOI timing conditions showed greater occurrence of non-premixed burn regions close to the chamber periphery. Soot probability maps in Figure 9, also showed a consistent occurrence of soot on the left intake valve side (lower left side) for all injection timings. The exhaust soot values with the longer warm-up period were higher than those with the shorter warm-up period.

Factors that can be accounted for high soot values with a longer warm-up period are as follows. A high rail pressure would atomize the spray better, thereby resulting in less dense mixture due to enhanced vaporization. The subse-quent burning of this mixture is expected to result in high combustion temperatures. In addition to the enhanced vaporization, a longer warm-up period would heat up both piston and the chamber walls. Under these high thermal boundary conditions, any insufficiency of local oxygen or limited f lame propagation before reaching the circumfer-ence could result in soot formation at the hot walls. A less dense in-cylinder charge would allow the fuel spray to penetrate further, reaching the liner walls. These infer-ences are based on the observations that the soot was located near the outer edge of the piston window (see Figures 8 and 9). In the future, these speculations can be clarified by performing fuel spray visualizations and by


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FIGURE 6 Soot probability maps and exhaust soot values for single injection E10-150 bar-110 °C conditions.

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recording burnt gas distributions using an infrared camera or equivalent [26]. Also, combustion and heat transfer models can confirm the presence of high wall temperatures during combustion.

Soot from Combustion of M20: Single Injection

Soot with Shorter Warm-Up (TPiston = 110°C)Combustion images at the CA50 burn point for shorter warm-up sequence and different injection timings are shown in Figure 10. The smaller non-premixed burn regions are high-lighted with solid circles, as is done for E10 combustion images. The combustion of fuel M20 that contains about 20% methanol exhibited a distinct violet-purple color, compared to ‘blue-purple’ combustion as seen previously with E10 fuel.

Another feature observed with the M20 fuel is the frequent occurrence of white regions during combustion. The exact reason behind these saturated-looking pixels is uncertain and can be attributed to (i) high temperature soot

radiation or (ii) excessive fluorescence of water vapor or (iii) hot O2 fluorescence; all of which have emission wavelengths in the visible range. Drake et al. [27] attributed similarly looking white regions to hot soot formed due to high local temperatures and amount of soot present there. Due to its high volatility, the M20 combustion is expected to result in locally high temperatures. Based on natural luminosity, however, it might not be safe to conclude that these white regions are due to hot soot. A sophisticated technique such as the laser induced incandescence can address this context. Therefore, the raw images having white regions were still retained for referencing. These white regions, on the other hand, were filtered out from the probability maps so that the same pixel threshold can be applied for all test cases; thus while capturing only the soot-indicative yellow/orange flames.

Between the two fuels tested, the M20 fuel resulted in less pool fires than E10 for the most advanced injection timing with a shorter warm-up period (compare Row #1 of Figures 10 and 5). This could be accounted to M20 fuel’s higher RVP value compared to that of E10 fuel. For the same warm-up period that led to a piston temperature of 110 °C, a high rail pressure seemed to atomize the spray better than a low rail pressure. This observation was based on reduced occurrence of non-premixed burn regions with the high rail pressure compared to low rail pressure (not shown here). This observa-tion was also noted with E10 fuel, indicating that a high rail pressure utilized at an advanced injection timing might coun-teract soot formation under low temperature conditions, e.g. during cold start operation. For almost all injection timings, non-premixed burn regions were noticed at radially inward locations with respect to the chamber center. An exception to this observation is for an SOI at 290 °CA BTDC wherein no such regions occurred in the CA50 combustion images.

FIGURE 9 Soot probability maps and exhaust soot values for single injection E10-150 bar-140 °C conditions.

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FIGURE 10 Cycle-to-cycle recordings of combustion at the CA50 burn point for single injection M20-150 bar-110 °C conditions.

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The soot probability maps and exhaust soot measure-ments for three of the five injection timings are shown in Figure 11. The exhaust soot values for SOIs at 270 and 240 °CA BTDC were measured with a different dilution ratio of 9, unlike the default dilution ratio of 4. To be consistent in regards to soot comparisons among different operating condi-tions, the probability maps and related exhaust soot values for these two injection timings were not included here. For the three injection timings shown in Figure 11, the M20 fuel under this operating regime (high rail pressure with lower temperatures) resulted in low soot probabilities.

Soot with Longer Warm-Up (TPiston = 140°C)The M20 combustion cycles following a longer warm-up period resulted in purple-magenta color images, as shown in Figure 12. The number of non-premixed burn occurrences were similar to those seen with the shorter warm-up period for all SOIs. Combustion images for an SOI timing at 310 °CA BTDC showed less-luminous flames in several cycles. Diffusion flames are usually more luminous than pre-mixed flames [28]. Thus, the operating condition that resulted in less-luminous flame is expected to have less chances of soot production.

The exhaust soot values with longer warm-up sequence (see Figure 13) were greater than those obtained with a shorter warm-up sequence (Figure 11). As in the discussion of E10 fuel, high combustion temperatures due to enhanced spray vaporization and hotter walls might have increased the soot formation with longer warm-up period.

FIGURE 12 Cycle-to-cycle recordings of combustion at the CA50 burn point for single injection M20-150 bar-140 °C conditions.

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FIGURE 11 Soot probability maps and exhaust soot values for single injection M20-150 bar-110 °C conditions.

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FIGURE 13 Soot probability maps and exhaust soot values for single injection M20-150 bar-140 °C conditions.

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Also, spray breakup of the high volatile M20 fuel might be aggravated by the cavitation effects in the injector nozzle at high wall temperatures [29]. An over enhanced spray breakup with shorter spray penetration can lead to poor air-fuel mixing and thus create soot-inducing fuel-rich regions. The possibility of M20 undergoing enhanced breakup can be reflected from the spatially scattered soot maps in Figure 13 compared to concentrated soot maps along the piston window edges for E10 fuel as in Figure 9. With M20 fuel in Figure 13, SOI at 310 °CA BTDC resulted in lower in-cylinder soot likelihood and exhaust soot value compared to other injection timings. This observation might support the speculation that hotter walls or fuel impingement on liner caused higher soot, because the soot probabilities showed soot likelihood close to the chamber periphery for all except an SOI at 310 °CA BTDC. Although soot was not near the chamber periphery for SOI 240 °CA BTDC during the first 50 cycles in Figure 13, soot was located closer to the liner wall during last 50 cycles (not shown here).

Soot from Combustion of E10: Split Injection

Soot with Shorter Warm-Up (TPiston = 110°C)Combustion images at the CA50 burn point for split injection conditions using E10 fuel are shown in Figure 14. The single injection results are included as well in Row #1 for better comparison of in-cylinder and exhaust soot obtained with the two injection configurations. Combustion with the single

injection condition showed a purple color with non-premixed burn regions at the top during the first cycle. In the subsequent combustion events, several cycles have showed less-luminous blue flames. The split injection condition with 70% mass in first pulse resulted in non-premixed burn regions near the spark plug after 50 cycles. Combustion under split injection with 50% mass in the first pulse exhibited a golden-brown region as highlighted with an arrow (Row #3, cycle #c). Such simultaneous occurrence of purple/brown and blue color could indicate combustion flame of a non-homogeneous mixture. Split injection with 30% mass in the first pulse resulted in less-luminous blue-white combustion flames.

The corresponding soot probability maps and exhaust soot concentrations are shown in Figure 15. The single injection condition showed higher in-cylinder soot formation and lower exhaust soot value compared to the split injection condition with 70% mass or 50% mass in first pulse. These observations might indicate that a longer mixing time with the single injec-tion advanced SOI timing allowed the in-cylinder soot to undergo oxidation during expansion or exhaust stroke, before reaching the sensor probe. It is worthwhile to note that the soot location was more on the intake valve side (lower half of chamber) for the 50%-50% split injection condition. The single injection condition which resulted in low exhaust soot showed soot occurrence on the exhaust valve side (upper half of the chamber). This could indicate the influence of split injection i.e. the influence of second spray on entrainment of first spray, thereby determining fuel-rich region formations from air-fuel mixing characteristics. The condition with shortest first pulse resulted in less soot formation, both in-cylinder and exhaust.

FIGURE 14 Cycle-to-cycle recordings of combustion at the CA50 burn point for single & split injection E10-150 bar-110 °C conditions.

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FIGURE 15 Soot probability maps and exhaust soot values for single & split injection E10-150 bar-110 °C conditions.

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Combining this result with previous observations on flame colors, a blue-white flame (less-luminous) can be a good indi-cator of soot-inhibiting combustion with E10 fuel. The reason for such low-luminous low-soot combustion with 30% split percent case is uncertain. The exhaust soot value is in agree-ment to a proposed corollary (discussed later) about expecting low exhaust soot when in-cylinder soot was seen on the exhaust side of the chamber. The reason behind soot being on the exhaust side, however, is currently not known. Spray visualiza-tions or computational spray patterns at these injection timings can provide a better insight to this end.

Soot with Longer Warm-Up (TPiston = 140°C)Combustion images at the CA50 burn point for split injection conditions with longer warm-up sequence are shown in Figure 16. The mixture homogeneity seemed to improve with fuel mass split percentages of 70% or 50%, as a more uniform blue-white burnt mixture was observed. Split injection with 30% mass in the first injection pulse showed some signs of inhomogeneous burning. The non-premixed burn regions occurred close to the chamber periphery for both injection configurations.

Compared to single injection configuration, soot prob-abilities and exhaust soot values decreased by switching to a split injection configuration with 70% or 50% mass in the first pulse (see Figure 17). The condition with shortest first pulse showed high soot probabilities on the left intake valve side, similar to the single injection condition. The exhaust soot values increased with the amount of fuel mass injected in the second pulse among the split injection conditions. This is a contrasting result from that obtained with shorter warm-up conditions; soot values decreased with larger fuel mass in the

first injection pulse (see Figure 15). The soot probability maps with shorter warm-ups show the disapperance of soot on the exhaust side as the split percent decreased in the first pulse. In contrast, the soot probabilities with longer warm-ups (Figure 17) show appearance of soot on the intake side as the split percent decreased in the first pulse. These observations imply the sufficiency of shorter warm-up period to reduce 1st pulse fuel impingement on the piston and vaporize (and enhance mixing) the 2nd pulse that was injected at a retarded injection timing. On the other hand, longer warm-up could have resulted in premature spray breakup with shorter pene-tration and/or hotter walls that can stimulate soot formation there.

Soot from Combustion of M20: Split Injection

Soot with Shorter Warm-Up (TPiston = 110°C)Combustion images at the CA50 burn point for single and split injection conditions using M20 fuel are shown in Figure 18. The split injection conditions showed reduced occurrence of non-premixed burn regions later on during the test run, compared to single injection condition. The single

FIGURE 17 Soot probability maps and exhaust soot values for single & split injection E10-150 bar-140 °C conditions.

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FIGURE 16 Cycle-to-cycle recordings of combustion at the CA50 burn point for single & split injection E10-150 bar-140 °C conditions.

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injection condition, however, showed less-luminous flames in several engine cycles.

The corresponding soot probability maps and exhaust soot values are shown in Figure 19. A decrease of fuel mass (70% to 30%) in the first injection pulse resulted in an increase in exhaust soot. The condition with 70% mass in the first injection showed higher soot probability than the single

injection condition. The exhaust soot value was, however, lower with this split injection condition. This seemingly anomalous result can be attributed to two possible factors: (i) A combination of longer mixing time and reduced wall impingement with 70% mass at the advanced injection timing of 310 °CA BTDC could result in locally less-rich regions before ignition and (ii) any soot formed on the exhaust side of the chamber might be more likely to partici-pate in the oxidation reactions, due to in-cylinder air f low structures. While the latter inference cannot be proved from the current testing, this observation of having higher exhaust soot with soot occurring on the intake valve side was consistently noted for several other tested conditions as discussed in the next section.

Soot with Longer Warm-Up (TPiston = 140°C)Combustion images at the CA50 burn point for split injection conditions with longer warm-up sequence are shown in Figure 20. The single injection condition and the split injection with 70% mass in the first pulse showed less-luminous flames in several cycles. The other two split injection conditions with 50% or 30% mass in the first pulse showed more non-premixed burn regions. The soot probabilities as shown in Figure 21 resulted in extensive soot formation on the right intake valve side for the condition with 30% mass in first pulse. This result is attributed to mixture inhomogeneity as majority of the fuel mass was injected at SOI of 285 °CA BTDC during the second pulse. Overall for the M20 fuel, the split injection configura-tion with high rail pressure and high wall temperatures does not have a significant advantage on particulate reduction compared to a single injection for a SOI at 310 °CA BTDC. It is important to note that the soot values for M20 fuel increased

FIGURE 18 Cycle-to-cycle recordings of combustion at the CA50 burn point for single & split injection M20-150 bar-110 °C conditions.

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FIGURE 19 Soot probability maps and exhaust soot values for single & split injection M20-150 bar-110 °C conditions.

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FIGURE 20 Cycle-to-cycle recordings of combustion at the CA50 burn point for single & split injection M20-150 bar-140 °C.

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with a decrease in 1st pulse’s fuel mass, when tested with both shorter and longer warm-up conditions. This is a contrasting result when compared to those obtained with E10 fuel. These observations indicate that the M20 fuel mixing is more sensi-tive to injection timing than to in-cylinder temperature due to its high vapor pressure.

Soot Comparisons from E10 and M20 CombustionThe in-cylinder soot observations correlated well with the engine-out soot measurements for operating conditions of both fuels. All soot measurement values obtained for both fuels under different piston temperatures and with single injection mode are summarized in Figure 22. The measured soot values are shown on the ordinate axis and the injection timings are shown on abscissa. The operating condition that resulted in least soot measurement for a given fuel and piston temperature was highlighted on top of the corresponding histogram bar. For a given piston temperature and injection timing, the methanol-blended M20 fuel resulted in lesser exhaust soot than with E10 fuel at almost all injection timings tested. It is to be recalled that soot values for M20 fuel with lower piston temperature and SOI at 270 or 240 °CA BTDC are not included here as the corresponding measurements were made with a different dilution ratio of the soot sensor.

For both fuels, less soot occurred at early-to-mid injection timing during intake stroke.

As mentioned in the previous sections, a correlation was noticed between in-cylinder soot location and the exhaust soot values. Figure 23 provides a pictorial summary of soot locations along with a qualitative scale of soot formed inside the combustion chamber for the tested single injection condi-tions. In this figure, black shade corresponds to high soot probability, grained shade to medium soot probability, and light grey shade to low soot probability. Furthermore, a corre-lation was noticed between warm-up duration and in-cylinder soot location.

i. In eight of the 10 cases (five SOI timings and two fuels; see Figure 23), high soot likelihood was noted on the intake side of the chamber by switching from a shorter warm-up to a longer warm-up period. Exceptions to these were M20-SOI@310 and M20-SOI@240.

ii. In seven of the eight measured cases (third row showing arrows in Figure 23), exhaust soot was more when soot was seen on the intake side of the chamber.

FIGURE 22 Exhaust soot values for E10 and M20 fuels with single injection configuration.

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FIGURE 23 Depictions of correlations among in-cylinder soot location, piston temperature, and exhaust soot measurements.

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FIGURE 21 Soot probability maps and exhaust soot values for single & split injection M20-150 bar-140 °C.

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This pictorial summary of observations (Figure 23) provides a useful insight about the influence of in-cylinder charge motion and wall temperatures on spray evolution and air-fuel mixing. There seems to be an optimal value of operation for piston temperature for a selected fuel rail pressure, in regards to partic-ulate reduction. A warm-up sequence would also determine the density of the in-cylinder residual mixture before starting the DI firing cycles. Under naturally aspirated conditions, the mixture density could have an effect on volumetric efficiency of the engine, apart from the spray penetration and evaporation. While care was taken to maintain stoichiometric proportion of air-fuel mixture for all the tests, details about in-cylinder swirl and tumble motions and their reaction to longer warm-ups are unknown. Hence, it would be worthwhile to perform spray visu-alizations as well as analyze the effect of in-cylinder flow motion on soot reduction in a potential future work.

Similarly, all soot measurement values obtained for both fuels under various operating conditions and with split injec-tion configuration are shown in Figure 24. The measured soot values are shown on the ordinate axis and the fraction of fuel injected in the first pulse is shown on abscissa. The results obtained from a lower rail pressure of 100 bar are included here. This additional set of data is included to further validate a contrasting behavior seen with the two fuels tested. With E10 fuel, the shortest first pulse at an SOI of 310 °CA BTDC

resulted in minimal soot. In contrast, the longest first pulse at this injection timing resulted in minimal soot with M20 fuel. These observations clearly indicate that the highly volatile M20 fuel benefitted from early injection and longer mixing times without noticeable impingement on the piston, espe-cially with high rail pressure. In contrast, the low-volatile E10 fuel favored low soot production due to reduced wall impinge-ment by delaying the fuel injection timing, especially with low rail pressure.

Effect of Wall Temperature on Soot ProductionThe soot probability maps and exhaust soot concentrations measured during E10 combustion under various thermal boundary conditions and 100 bar rail pressure are shown for an SOI at 290 °CA BTDC in Figure 25. Soot probabilities for

FIGURE 24 Exhaust soot values for E10 and M20 fuels with split injection configuration.

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FIGURE 25 Soot probability maps and exhaust soot values for E10-100 bar and different wall temperatures.

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both first 50 and last 50 cycles are shown to see the effect of DI warm-up as well. Also, the color bar scaling extends from a value of 0 to 1. The coolant temperature was designated as TCt and the piston crown temperature as TP. The condition with low coolant temperature and warm piston (row R1 in Figure 25) showed high soot probability close to the injector which was located near the bottom of the image. This likeli-hood of soot occurrence shifted to the left during the last 50 cycles, while still showing some signs of soot close to the injector. This shift in in-cylinder soot location later during the test signifies the importance of in-cylinder flow motion that changes depending on the in-cylinder charge tempera-ture. With a longer warm-up (R2), some of the soot appeared near the exhaust side while the majority of the soot was still close to the injector. Soot formation during the last 50 cycles was lower compared to that produced with a shorter warm-up period.

During the engine test with coolant at 90 °C without piston warm-up (R3), the amount of fuel injected was realized to be slightly short of stoichiometric proportion. The corre-sponding soot probabilities with a 200- cycle-average lambda of 1.01 are shown in Figure 25. The exhaust soot value is noted in ‘red’ ink. In a subsequent test, the amount of fuel was adjusted to attain stoichiometry and the exhaust soot value was measured to be 156 μg/m3. Although, this is a limited result based on the current scope of work, it would be worth-while to investigate the soot sensitivity to a slight change in lambda around one. The E10 operating conditions with coolant at 90 °C and piston temperature at 110 °C (R4) and 140 °C (R5), which were discussed previously, are included in the bottom rows of Figure 25 to assemble all relevant data on thermal gradient conditions. With high coolant temperature, an increase in the warm-up duration reduced soot formation (R3 to R5). As noticed here with the low rail pressure, having the cylinder head at regular coolant temperature and a hot piston/chamber resulted in the least amount of soot for E10 fuel. Also, the effect of having a high piston temperature on soot reduction (both in-cylinder and exhaust) was dominant at high coolant temperature; compare R2 vs R5.

Using identical operating conditions, soot probabilities and exhaust soot measurement results were conducted for fuel M20 and are shown in Figure 26. Compared to E10, M20 showed lower soot probability close to the injector during the first 50 cycles under colder wall conditions (row R1 in Figure 26). The last 50 cycles of M20 combustion, however, showed significant amounts of soot on the left intake valve side and in the central region of the chamber. A contrasting difference between the combustion behaviors of the two fuels is that a hot piston/chamber favored particulate reduction with E10 fuel, whereas a hot piston/chamber increased particulate formation with M20 fuel. The low-volatile E10 fuel, in high temperatures, showed enhanced evaporation and minimized chances of wall impingement and/or locally fuel-rich burning. At this SOI timing of 290 °CA BTDC, the temperature of the high-volatile M20 fuel surrounded by hot walls might have increased, thereby creating favorable temperatures for soot formation during combustion of this hot air-fuel mixture. This inference does not support a completely cold system,

because using a regular coolant temperature and moderately warmed-up piston resulted in lowest soot measured (R4).

Thus, it is understood that a high piston temperature does not necessarily result in soot reduction and the discussions above indicate the importance of accounting for the combined effect of coolant temperature and piston temperature on soot reduction.

Summary and ConclusionsParticulate formation of ethanol and methanol blends of gasoline was studied in a direct-injected engine running at 1500 rev/min and 9.8 bar indicated mean effective pressure. Soot luminosity recordings and simultaneous measurements of exhaust soot concentrations were made to evaluate details of local soot formation and soot emissions from the start of combustion until the end of the engine cycle. These homoge-neous-intent stoichiometric combustion investigations were completed for a series of start of injection (SOI) timings, piston

FIGURE 26 Soot probability maps and exhaust soot values for M20-100 bar and different wall temperatures.

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& chamber warm-up durations, fuel rail pressures, and injec-tion configurations. Fuel mass split percentages tested were 70%-30%, 50%-50%, and 30%-70% with first injection pulse at 310 °CA BTDC during intake stroke. The effect of wall temperatures on soot formation was studied for a “mid” injec-tion timing of 290 °CA BTDC. Soot probability maps were created to capture the location of in-cylinder soot formation on a cycle-to-cycle basis. Major findings from this work are summarized as follows.

• Single injection (rail pressure and piston temperature): For both fuels, increasing the fuel rail pressure from 100 bar to 150 bar mitigated particulate formation under low piston/chamber temperature conditions. An increase in piston temperature, in general, increased soot formation.

• Single injection (E10 vs. M20 soot): For 13 out of the 18 single injection conditions, the methanol-blended fuel M20 resulted in less soot than E10 fuel. The E10 fuel was more sensitive than M20 fuel, in terms of exhaust soot, to a change in warm-up duration.

• Split injection (E10): A split percent of 30% fuel mass in the first pulse resulted in minimal soot for both rail pressures and shorter warm-up. This could indicate that reducing E10 pool fires due to wall impingement was the key for particulate reduction at this advanced injection timing. Soot trends for split injection cases were opposite with shorter and longer warm-ups.

• Split injection (M20): A split percent of 70% fuel mass in the first pulse resulted in minimal soot for both warm-up durations. Thus, particulate formation tendency of M20 fuel was more dependent on longer mixing time and less on piston impingement.

• Wall temperature effect: At this mid injection timing, a colder cylinder head or a colder piston both resulted in significant amounts of soot close to the fuel injector (tested with low rail pressure). For both fuels, having a high coolant temperature favored soot reduction. The E10 resulted in less soot with high piston temperatures whereas M20 resulted in less soot with moderate piston temperatures.

AcknowledgmentsThe authors thank Kevin Moran for making the methane injector drive box and for assisting in the data acquisition setup. Also, the authors would like to extend their sincere grat itude to a l l t he rev iewers for prov id ing encouraging feedback.

Definitions/Abbreviations°CA - Crank angle degreeCA50 - 50% burn point of combustionTCt - Coolant temperature

TP or TPiston - Piston crown temperaturerev/min - Revolutions per minuteATDC - After top dead centerBTDC - Before top dead centerCAS - Combustion analysis systemDI - Direct injectionDR - Dilution ratioIMEP - Indicated mean effective pressureLEV III - Low emission vehicle programMABX - MicroAutoBoxMOP - Maximum opening pointPFI - Port fuel injectionRON - Research octane numberRVP - Reid vapor pressureSOI - Start of injectionTDC - Top dead center

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Appendix: Micro Soot Sensor OperationThe engine-out soot concentrations were measured using an AVL Micro Soot Sensor Plus. The inlet conditions at the measuring cell of the sensor unit are limited to 60°C and 60 mbar (rel. to ambient). In order to measure the soot concen-trations in the high-temperature engine-out gases, a Dilution Control Unit is employed. The setup used for measuring the exhaust soot concentrations is shown in Figure A1. The exhaust mixture is drawn from the exhaust pipe into a dilution cell. There the exhaust gases mix with a set amount of dilution air or shop air mass flow (MFC). The total mass flow (sum of sampled exhaust gas and dilution air) was determined with a thermal mass flow meter (MFM). The dilution ratio (DR) of the exhaust conditioning unit is defined as follows,

DRMFM

Total volume flow MFC=

− Eq. (1)


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All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE International.

Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE International. The author is solely responsible for the content of the paper.

The mass of the dilution air is controlled so that the DR value remains constant. The DR value is passed to the soot sensor that outputs the dilution-corrected soot concentration. The dilution-corrected concentration is determined as the product of DR and measurement value of the soot sensor. For an optimum determination of the dilution ratio, the mass flow

meters are instrumented with thermostats. In addition, the CO2 concentration of the diluted exhaust gas that affects the mass flow value MFM is taken into consideration by the hardware for its internal calculations of the total mass flow.

DILUTION RATIO EFFECT: During engine experi-ments, selecting a smaller DR value resulted in low measured value of exhaust soot concentration. An increase in the DR value resulted in an approximately linear increase in the measured soot value. This dependency of soot sensor measurements on the dilution ratio is unusual. The dilution control unit reduces the sample humidity to avoid water condensation on the measuring cell window. Selecting a DR value below a certain threshold can result in water condensa-tion, thereby giving low exhaust soot values. There is no standard threshold value for DR as it varies for different engine operation and fuels. It was learned later that a suggested range for DR is 7 to 10 for this micro-soot sensor under the current exhaust temperatures and pressures. As the selected DR of 4 was less than this recommended range, the soot values reported in this work may not represent the actual values. Nevertheless, the soot comparisons made among different test points can still be valid, as the same DR value was used for all the experiments.

FIGURE A1 Functional diagram of micro-soot sensor with the dilution control unit.

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