Analysis of the Influence of Diesel Spray Injection on the Ignition and Soot Formation in Multiple Injection Strategy
Abstract
:1. Introduction
2. Materials and Methods
2.1. Facility and Injection System
2.2. Methodology
2.3. Experimental Techniques
2.3.1. Diffused Back-Illumination (DBI)
2.3.2. Single-Pass Schlieren
2.3.3. Optical Configuration
2.4. Image Processing Method
- Image masking: This consists of providing the processing routine with a region where the spray is located, with the purpose that undesired elements present in the frame are discarded for the segmentation between the background and the rest of the image where the spray exists.
- Background substraction: For DBI images, the background is considered static and is obtained with a simple average of the first few images. Nevertheless, new problems related to this step for the schlieren images and the decoupling of the contour of the pilot or post injections from the main come along with the multiple injection strategies. For instance, some areas in schlieren images belonging to the spray might present pixels with the same intensity levels as the background, that might appear throughout all the injection event. Therefore, other criteria different to the classic was applied, described by Payri et al. [26]. For the first issue, the procedure consists of combining a dynamic-background-composition subtraction that constantly updates the background information [15,27], with an image temporal-derivative approach that works very well for capturing the contour of a spray in the diluted regions [28]. Regarding the decoupling of the schlieren contour, given that the second injection occurs in conditions where density gradients were induced by the previous one, the same dynamic background subtraction strategy was applied to the second pulse, as the non-spray pixels of the composed background contain the image with the first injection, so it is substracted, and the remnant pixels, identified as the spray in the previous frame, were filled with the original background before injection. Additionally, higher binarization threshold was used for the second injection to minimize the probability of over-predicting its boundaries. This procedure is more detailed in [14].
- Contour detection: For DBI movies, a fixed value of optical thickness was used as a threshold of the spray image [25]. In contrast, for schlieren images, the threshold was calculated as a fixed factor multiplied by the dynamic range of the frame being analyzed. Lastly, a filtering process is carried out [24,26,29].
- Contour analysis: The data gathered with the image processing was analyzed with a moving average to assess shot-to-shot dispersion, and obtain a global value for each variable per time step [8,26]. Additionally, the start of injection (SOI) was obtained to phase the time domain of the results and adequately compare test points [8,23].
2.4.1. Soot DBI
3. Results and Discussion
3.1. Novel Method for Decoupling the Start of Combustion of the Second Pulse
3.2. Ignition Delay for Multiple Injection Strategies
- Mixing interaction mode: when the second injection takes place during the mixing phase of the first pulse. It was seen only for pilot-main strategies, mostly at the lowest chamber density and/or temperature, and for some intermediate conditions where the pilot did not ignite before the main caught up. The SOC of the main pulse is driven by the ignition of the pilot.
- Premixed interaction mode: the second injection takes place during the premixed combustion of the first pulse and independent combustion events for each injection are visualized. This was observed for most points in the pilot-main strategy at the highest chamber density and/or temperature, and also for combinations of either low chamber temperature and high density, or vice versa. The reach of the trade-off between the premixed and diffusive combustion is dependent on the boundary conditions and also on the DT and pilot quantity, as they determine the temperature of the local gases entrained by the main injection.
- Diffusive interaction mode: the second injection befalls during the diffusive combustion of the first pulse. It was observed for all conditions in main-post strategies due to the time allowed between the SOC of the main injection and the relatively short DT to the post-pulse. It was also noted for some of the 3 mg pilots at higher chamber temperature and density, and low rail pressure.
- Completed combustion interaction mode: the second injection occurs after the combustion of the first pulse is completed. This interaction mode did not appear for the conditions studied because of the short dwell times used in this investigation.
3.3. Ignition Location
3.4. Soot Distribution for Multiple Injections
3.4.1. Effect of Boundary Conditions in Soot Distribution
3.4.2. Soot Distribution for Pilot-Main Strategies
3.4.3. Soot Distribution for Main-Post Strategies
4. Conclusions
- A novel image processing methodology based on the absolute pixel-wise frame-to-frame difference was developed to decouple the start of combustion of each spray in multiple injection strategies. The methodology was successfully validated against a different experimental campaign results, although validation with more experimental data is recommended.
- For all test points, the pilot injection enhanced conditions that promote a faster ignition of the main pulse. Increasing the dwell time promoted higher ID for the pilot injection. On the contrary, increasing the pilot quantity provided a higher momentum, which increased the ID of the pilot. On average, the ignition delay of the main pulse was reduced around 30% to 40% compared to its single injection case.
- At lower chamber temperatures, the pilot and main pulses ignited simultaneously, and increasing the dwell time negatively influenced the ignition process. On the contrary, at higher temperatures, all conditions presented a separate combustion event for each injection, with no effect from neither the pilot quantity nor the dwell time on the main. Ignition delay values seemed to decrease slightly when lowering dwell time, and no clear trend was observed for pilot quantities.
- For the main-post strategy, all conditions manifested a separate combustion event for each injection. In fact, higher localized temperature promoted an even faster ignition of the second pulse compared to pilot-main strategies. Overall, the ID of the post injection was reduced around 40% to 50% compared to its reference case.
- From soot measurements, boundary conditions affected soot formation in single injections as expected from literature. On the other hand, for pilot-main strategies, more soot was observed compared to its reference case. Overall, it was observed that conditions that promote faster premixed combustion enhance soot formation, as most of the fuel is burned in a diffusion flame. In this context, when increasing the pilot quantity a diminution in the cross-sectional KL near the SOC was observed, and no clear trend of the dependence of soot on the dwell time was observed, although, for fixed boundary conditions differences are noticed.
- In contrast to what is reported in the literature, main-post strategies depicted slightly higher or similar soot formation than a single injection. The reason for it is that for this type of large volume vessels, the post injection behaves like a main and the actual main like a pilot, and there is no re-circulation of the combustion products. In this sense, different experimental works also showed that the soot oxidation processes are very dependent on the interaction between the injections and the confined engine bowl-shaped combustion chamber.
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
Abbreviations
DBI | Diffused back-illumination |
ECN | Engine Combustion Network |
ECU | Engine Control Unit |
EOI | End of injection |
ID | Ignition delay |
LED | Light-emitting diode |
LOL | Lift-off length |
ROD | Rate of difference |
ROI | Rate of injection |
SOC | Start of combustion |
SOE | Start of energizing |
SOI | Start of injection |
DT | Dwell time |
PCR | Piezo common-rail |
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Parameter | Value | Units |
---|---|---|
Holes | 6 | - |
Avg. outlet diameter | 90.1 | m |
Outlet diameter (Do) | 91.7 * | m |
Avg. k factor | 5.3 | - |
Nominal flow rate | 313 ** | mL min |
Avg. heigth angle | 74.8 | degrees |
Degree of hydro-erosion | 7.7 | % |
Parameter | Value | Units |
---|---|---|
Rail pressure () | 100–200 | |
Chamber density () | 15.2–22.8 | |
Chamber temperature (T) | 800–900 | |
Oxygen concentration | 21 | %vol |
Injector operating temperature | 363 | |
Pilot/post dwell times | 200–350–500–650 | μs |
Pilot/post injected quantity | 1–3 | |
Total mass per injection | 30 | |
Injection frequency | 1 | |
Recorded injection cycles | 10–20 | - |
Injection | ID ROD | ID OH Chem. | Units |
---|---|---|---|
Pilot | 0.71 ± 0.02 | 0.68 ± 0.02 | after SOE |
Main | 1.24 ± 0.03 | 1.23 ± 0.08 | after SOE |
KL Sum × 10 | Ref | 1–29 | 3–27 | T (K) |
---|---|---|---|---|
DT = 350 | 0.0366 | 0.1512 | 0.0795 | 800 |
DT = 500 | 0.0366 | 0.1670 | 0.0874 | 800 |
DT = 650 | 0.0366 | 0.2351 | 0.0907 | 800 |
DT = 350 | 2.09 | 2.37 | 2.46 | 900 |
DT = 500 | 2.09 | 2.37 | 2.34 | 900 |
DT = 650 | 2.09 | 2.23 | 2.29 | 900 |
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Payri, R.; García-Oliver, J.M.; Mendoza, V.; Viera, A. Analysis of the Influence of Diesel Spray Injection on the Ignition and Soot Formation in Multiple Injection Strategy. Energies 2020, 13, 3505. https://doi.org/10.3390/en13133505
Payri R, García-Oliver JM, Mendoza V, Viera A. Analysis of the Influence of Diesel Spray Injection on the Ignition and Soot Formation in Multiple Injection Strategy. Energies. 2020; 13(13):3505. https://doi.org/10.3390/en13133505
Chicago/Turabian StylePayri, Raul, José M. García-Oliver, Victor Mendoza, and Alberto Viera. 2020. "Analysis of the Influence of Diesel Spray Injection on the Ignition and Soot Formation in Multiple Injection Strategy" Energies 13, no. 13: 3505. https://doi.org/10.3390/en13133505