Influence of flash boiling spray on the combustion characteristics of a spark-ignition direct-injection optical engine under cold start

Combustion and Flame 188 (2018) 66–76

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Influence of flash boiling spray on the combustion characteristics of a spark-ignition direct-injection optical engine under cold start Jie Yang, Xue Dong, Qiang Wu, Min Xu∗ a

School of Mechanical Engineering, National Engineering Laboratory for Automotive Electronic Control Technology, Shanghai Jiao Tong University, Shanghai 200240, China

a r t i c l e

i n f o

Article history: Received 11 March 2017 Revised 29 May 2017 Accepted 13 September 2017

Keywords: Flash boiling spray Cold start High-speed color imaging Flame propagation rate Soot formation Cycle to cycle variation

a b s t r a c t Flash boiling occurs when liquid fuel is injected into an ambient environment below its saturation pressure. Compared to non-flash-boiling (liquid) spray, flash-boiling spray features a two-phase flow that constantly generates vapor bubbles inside the liquid spray thus results in much smaller drop size and faster evaporation, which are favorable for direct-injection gasoline engine combustion. In this study, the combustion characteristics of flash boiling spray was investigated under cold start condition in a sparkignition direct-injection (SIDI) optical gasoline engine. Three spray conditions, including liquid, transitional flash boiling, and flare flash boiling spray were studied for comparison. Optical access into the combustion chamber was realized by a quartz insert on the piston. The crank angle resolved color flame images as well as in-cylinder pressure of 150 consecutive cycles were recorded simultaneously. From the color images, the blue flame generated by excited molecules and the yellow flame resulted from soot radiation was identified and analyzed separately alongside with the cylinder pressure. Results show an improvement of indicated mean effective pressure-gross (IMEPg ) and a reduction of soot formation with the introduction of flash boiling spray under cold start condition. The emission measurement shows that the formation of soot is positively related to particulate number (PN) emissions. Further study on the transient development of in-cylinder flames shows that flash boiling spray leads to higher propagation rate of the blue flame, and a subsequent statistical analysis shows a positive correlation between IMEP and the propagation rate of the blue flame. © 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction Spark-ignition direct-injection (SIDI) has become the mainstream technology for the gasoline engine due to its improved fuel economy, higher thermal efficiency, more precise air-fuel ratio control and improved transient response [1,2]. However, in the current generation of production SIDI engines, several issues such as wall wetting, soot formation, output cycleto-cycle variation remain as challenges. Specifically, even with a high injection pressure which increases atomization and evaporation, the non-evaporated spray still impinges onto the cylinder wall and/or piston surface. Liquid fuel impinged on the combustion chamber wall can easily leak into the crankcase, diluting the lubricant oil. Oil dilution could lead to excessive oil consumption, deterioration of engine friction and is also considered as a source of super knock [3]. Moreover, evaporation for the fuel film is deteriorated and any liquid fuel remaining on the combustion chamber

∗

Corresponding author. E-mail address: [email protected] (M. Xu).

surface will lead to pool fires, which is a significant source for high levels of unburned hydrocarbon (UHC) and soot emission [4–6]. Finally, it is well known that combustion process does not behave repeatedly for consecutive engine cycles, even under the carefully maintained engine condition due to the turbulent nature of incylinder air and spray motion [7]. This leads to substantial cycleto-cycle (CCV) of in-cylinder pressure. Variations of engine output such as the indicated mean effective pressure (IMEP) not only results in fluctuations of the engine torque, but also prohibits the engine from operating at its optimal condition, which degrades the drivability and reduces the engine power even more [8]. Also, the non-optimal cycles resulted from partial or unstable combustion will increase the hydrocarbon (HC) emission and fuel consumption. Flash boiling, as an alternative technique to high pressure injection, has shown great potential to solve critical issues caused by high pressure liquid spray system in today’s SIDI engines [9,10]. Flash boiling occurs when the ambient pressure is below the saturation pressure of the injected fuel. As the fuel temperature increases to the flash-boiling regime, fuel plumes expand significantly, prompting faster fuel evaporation [11,12] and more uniform fuel distribution, enhancing plume-to-plume interaction [13,14] as

https://doi.org/10.1016/j.combustflame.2017.09.019 0010-2180/© 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

J. Yang et al. / Combustion and Flame 188 (2018) 66–76

well as plume-air interaction [15,16]. Furthermore, experimental study in constant volume chamber on both liquid and vapor fuel show that more stable spray structure could be achieved by increasing the superheat degree in a 6-hole injector [17]. Although better fuel air mixing quality and combustion by flash boiling spray is expected, direct investigation of combustion performance of a SIDI engine under flash boiling condition is rarely reported. Hence the primary aim of this paper is to fill in this gap. One of the most concerned engine operating conditions is cold start in which the efficiency of the internal combustion (IC) engine is significantly low due to the unfavorable heat transfer and insufficient lubrication [18,19]. Also, at cold start condition, the low engine temperature contributes to poor atomization and evaporation, which results in heterogeneous fuel air mixture before ignition. It is reported that for a New European Driving Cycle (NEDC), over 30% of particulate mass (PM) and particulate number (PN) is emitted in the early phase due to the insufficient fuel air mixing [20]. Also, the three-way catalyst (TWC) system is not fully functional for CO, HC, NOx conversion under cold start [8] as the working temperature of the converter is not reached yet. Therefore, homogeneous mixture and stable combustion under cold start is desirable. Features of flash boiling spray including prompt atomization and smaller fuel droplet size promises a more homogenous fuel mixture, a more stable subsequent combustion and lower PN emission which is ideal for cold start. Therefore, this paper intends to investigate the combustion performance of flash boiling spray under cold start conditions. In recent years, optical engines have been widely applied in engine research which allows for direct observation of the flow and combustion inside the cylinder. High-speed imaging of incylinder combustion can record crack angle-resolved flame propagation, flame structure, and CCV of the engine [21–23]. However, most of the previous research used monochromatic cameras to capture the in-cylinder combustion, which are not able to provide spectral information of the flame luminance. Usually, in a flame of hydrocarbon fuels, there is broadband incandescence signals emitted from soot particles, as well as chemiluminescent signal emitted from excited molecules/radicals [24]. The spectrum measurement in the combustion process can reflect the generation of intermediate chemical species in the flame [25]. However, no spatial information could be acquired by spectrometer. In contrast, color camera can identify the spectrum of flame radiation at a large area, which is critical to analyze the formation and propagation of different intermediate species inside the flame [26,27]. Gaydon reported that the radiation in the near-ultraviolet regime (blue flame) mainly comes from fluorescence emission by four excited radicals, including: OH∗ (306.4 nm), CH∗ (431.5 nm), C2 ∗ (516.5 nm) and CO2 ∗ (340–650 nm) [28]. Among these, OH∗ , CH∗ and CO2 ∗ have been demonstrated to be effectively related with heat release rate [29]. Zeng reported that the blue flame image can be used to well represent the OH∗ chemiluminescence [27]. The yellow flame generally comes from the broadband radiation/emission from PAH and soot. However, the spectral radiation of most PAH molecules ranges from 350 to 550 nm. Although the wavelength shifts to around 600 nm when carbon number increases to 16, the amount of such species is considered small [30]. So yellow flame, spectrally ranging from 550 nm onwards, is considered to correlate with the soot formation [31]. Therefore, the high-speed chromatics imaging of the in-cylinder combustion can effectively trace the representative species inside the flame, thus assessing the flame propagation rate and the soot formation at the same time. In this study, influence of flash boiling spray on the combustion performance was investigated in an optically accessible single cylinder SIDI engine under simulated cold start condition [32]. The influence of flash boiling on flame development, engine out-

67

put performance and soot formation for both transient and stable stages was assessed. Also, CCV of flame development and engine output work was evaluated quantitatively. In the end, the correlation between flame area derived information and IMEP of the cycle was established. 2. Methodology 2.1. Engine configuration The experiments were conducted in a single-cylinder fourstroke SIDI optical engine based on a prototype from General Motors (Fig. 1). In this engine, the optical access to the combustion chamber could be achieved through two locations: (1) two pent– roof windows, which allow the view through the clearance between the cylinder head and the linear, as is shown in Fig. 1a; (2) a quartz-insert piston combined with a 45° mirror, which provides a bottom view of the cylinder through Bowditch extended middle piston [33]. The quartz piston insert has a diameter of 62 mm, which is shown in Fig. 1c and also illustrated as the dashed yellow circle in Fig. 1e. It can be seen in Fig. 1e that an eight-hole fuel injector and a spark plug were centrally installed in close proximity of each other. Eight red arrows in Fig. 1e illustrate the directions of eight spray plumes. For the combustion test, a metal liner equipped with water jacket (Fig. 1d) was used to condition the engine boundary temperature, although for non-combustion test, a geometrically identical quartz liner will be used as an alternative to provide full visualization of the entire engine stroke, Fig. 1b. More details of the engine layout can be found elsewhere [34]. The engine was motored at 1200 rpm by an AVL alternating current (AC) dynamometer. An AVL coolant and lubricant oil supply conditioning unit was used to control the engine coolant and oil temperature to 30 °C with an uncertainty of ± 0.2 °C. This is to simulate a dynamometer-based cold start test in order to characterize the performance of flash boiling spray at catalyst warmup phase [32,35]. The fuel temperature was conditioned by a water bath along with the heating tape. Specifically, as long as 5 m fuel pipe was conditioned by the water bath to keep the fuel temperature constant. The temperature of fuel between the outlet of the water bath and the injector (about 30 cm long) was monitored by a thermal couple and compensated by the heating tape using PID controller (proportional-integral-derivative controller) to make sure that the fuel temperature was conditioned within an error of ±3 °C. The temperature of fuel supply system was stabilized for 20 min before operation. The fuel temperature was conditioned to generate three typical types of fuel condition, i.e. liquid spray (30 °C), transitional flash boiling (60 °C) and flare flash boiling spray (90 °C) [36]. The spray types were estimated by the superheat degree of n-hexane under the test condition [10]. A separate experiment capturing the spray structure images has been conducted using quartz liner to confirm the conditions of the spray, the results are shown in Fig. 1f. The specific engine parameters and operating conditions are summarized in Table 1. The engine was operated at premixed mode with an overall stoichiometric equivalent ratio. The injection and ignition timing was optimized in a prior test through spark and injection sweep [23]. The injection timing was selected to be 300 bTDC as a compromise between more impingement amount and longer evaporation period under such operating condition. The engine was equipped with a swirl control valve (SCV) in one of the intake ports to regulate the swirl ratio of the intake air. The intake port without the control valve was designated as the ‘primary intake port’; the one with the control valve was called the ‘secondary intake port’ (Fig. 2a). In this study, the primary intake port was always open while the secondary intake port was closed to generate high swirl flow [37,38]. A Kistler pressure transducer

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J. Yang et al. / Combustion and Flame 188 (2018) 66–76

Liquid spray

(a)pent-roof window Transitional flash boiling spray

(d) (b) quartz liner Flare flash boiling spray

62 mm

(c) optical piston (e)

(f) Side-view

Bottom-view

Fig. 1. Single-cylinder engine with optical access through: (a) pent–roof window, (b) quartz linear and (c) optical piston; (d) overall layout of the engine; (e) bottom view of engine head; (f) typical side-view and bottom-view spray images under three superheat degrees. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

HORIBA

Spark plug

Exhaust

Fuel supply system Injector

Intake

Thermocouple

Fuel Metal liner

Heat exchanger

Swirl motion control valve

Heating tape

Injector 45º mirror

a)

b)

High-speed color camera

Fuel supply system

Fig. 2. Schematic of experimental setup of high-speed imaging system.

(6125 A) was mounted to the cylinder head to trace the in-cylinder pressure. Pressure data were post-processed to calculate the gross indicated mean effective pressure (denoted as IMEPg ), amplitude and timing of peak pressure (PP and LPP). The coefficients of variation of IMEP (COV = RMS/Mean) was used to evaluate the variation of combustion. Apparent heat release rate (AHRR) was calculated using a constant ratio of specific heats following [39]; also the mass fraction burned (MFB) and combustion phasing (CA10, CA90 et.al, which is crank angle when engine achieves 10% or 90% of heat release, respectively) were derived to evaluate engine performance. A MEXA 10 0 0 SPCS from Horiba was used to measure the solid particle number concentration in engine exhaust gas based on condensation particle countering method [36]. The emission tests were conducted in identical conditions as those in optical tests using all

metal piston to avoid piston failure in long time operation. Continuous combustion experiment was undertaken for three minutes at each superheat degree condition and the average PN number of the last 75 seconds was recorded since it took around 1 minute to purge the sampling line with exhaust gas before stabilization. A 12-bit high-speed complementary metal-oxide-semiconductor transistor (CMOS) RGB camera from NAC combined with a Nikon 50 mm f/1.8D lens was utilized to capture the natural luminosity of flame. As is shown in Fig. 2, the camera recorded the flame images reflected by the 45° mirror installed inside the elongated middle piston. The recording frequency was 13,0 0 0 Hz, corresponding to 1 frame per 0.554 crank angle degree (CAD), starting from the discharge of the spark plug. 100 images (corresponding to −15 to 42.05 CAD aTDC) were recorded in each cycle for 150 consecutive engine running cycles, starting from the first fuel injection.

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Fig. 3. Spectral response of the RGB camera.

Table 1 Summary of the specific engine parameters and operating conditions. Parameter

Specification

Bore Stroke Con-rod length Displacement Compression ratio Fuel type Intake timing (IVO/IVC) Exhaust timing (EVO/EVC) Coolant/oil temperature Engine speed Injection pressure Intake pressure (MAP) Start of injection (SOI) Spark timing Injection duration Equivalent ratio Fuel temperature Representative superheated degree (by n-hexane) Swirl ratio

86 mm 94.6 mm 160 mm 549.51 cm3 11:01 95 Gasoline 366 / 114 o bTDC Firing 131 / 372 o aTDC Firing 30 °C 1200 rpm 10 MPa 40 kPa 300 o bTDC 15 o bTDC 1300 μs Stoichiometric 30 °C, 60 °C, 90 °C −10, 20, 50 °C High (5.68)

The spatial resolution of the image was 547 × 607 pixels. The in-cylinder pressure trace was acquired simultaneously with the Kistler pressure transducer at a temporal resolution of 0.1 CAD. 2.2. Data processing To obtain quantitative information of the combustion process, high-speed color flame images were processed in order to identify the boundary and structure of the flame. The flame images recorded were in the RGB format so that it was possible to extract the flame characteristics from the three matrices corresponding to red (R), green (G) and blue (B) channels. The spectral response of R, G and B channels for the high-speed color camera is provided in Fig. 3. It could be observed that the red and blue channel have negligible crosstalk so that the signal of flame propagation and soot formation could be analyzed separately. Also, the influence of PAH on ‘R’ channel is trivial as the spectral response

of this channel before 550 nm is very low. A Matlab code was developed in house for image processing, the procedure of which is illustrated in Fig. 4. Figure 4 shows an example of flame image from Tfuel = 30 °C. In Fig. 4a, the white dashed circle shows the boundary of the piston quartz insert. To illustrate the position and size of the flame with respect to the combustion chamber, the cylinder head including four valves (two intakes on the top and two exhausts on the bottom), injector tip and spark plug were overlaid to the flame image. The left intake valve was deactivated during the engine operation so that a clockwise swirl flow was generated, as is shown by the green arrow. Even though the color flame image was the 2D projection of a 3D flame inside the cylinder, useful information on flame propagation could still be acquired. Specifically, two types of flames can be identified in the image, i.e. blue flame (representing excited radicals such as CH∗ , C2 ∗ and CO2 ∗ ) and yellow flame (mainly represents soot) [24]. Within the flame temperature (10 0 0–190 0 K), all the soot radiation remains within the red channel. As the yellow flame from soot radiation is much brighter than that of the blue flame, statistical analysis on those two types of flames was conducted separately to avoid the interference of yellow flame incandescence on blue flame. While the flame area detected by G was close to that from B but with higher noise level, G channel could sometimes also detect yellow flame. Therefore, G matrix was not used in this study due to its ambiguity in boundary definition. User-defined thresholds were decided for R and B matrices based on method developed in prior study [23]. As a result of this definition, the boundary for blue flame and yellow flame can be determined as is shown in Fig. 4b and Fig. 4c, respectively, and area enclosed by each boundary can be calculated accordingly. 3. Results and discussions A standard test procedure was used for all engine tests to ensure consistent engine performance: the single cylinder engine was first motored to a target speed, 1200 rpm in this study, and stabilized for 20 s; secondly, the spark plug discharging was activated while the fuel injection was kept disabled; next, the injection was enabled by a trigger signal from engine controller; the injection

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Fig. 4. Example of flame image (a); extracted boundary of blue flame (b) and yellow flame (c).

4

3.5

IM EP (bar)

3 2.5

stable stage (cycle 51-150)

2

1.5

transient stage

1 0.5

oC) Liquid spray (Tfuel=30T_fuel=30°C Transitional flash boiling (Tfuel=60 oC) T_fuel=60°C =90 oC) Flare flash boiling (Tfuel T_fuel=90°C

0 0

30

60 90 Engine cycle #

120

150

Fig. 5. IMEP of 150 consecutive cycles starting from the first injection.

trigger also enabled the recording of high-speed camera and combustion analyzer system (CAS). In this way, the high-speed camera and cylinder pressure sensor was synchronized from the very first injection cycle. The combustion information from 150 consecutive engine cycles was recorded by the high-speed color camera and CAS simultaneously. Figure 5 shows the IMEP of 150 consecutive cycles under three spray superheat conditions (fuel temperature 30, 60 and 90 °C correspond to liquid, transitional flash boiling and flare flash boiling spray, respectively). IMEPg is derived from the cylinder pressure of every single cycle, thus representing the global performance of each cycle. It can be seen that for all three fuel temperatures, the IMEP first increased with cycle number and then stabilized at a certain level. Also, the lower the fuel temperature, the more cycles it takes for the engine to stabilize. For the ease of discussion, the 150 consecutive cycles can be divided into ‘transient’ and ‘stable’ stages, with Cycle 51 being the cut-off point. This division leaves a relatively large margin for the transient stage to make sure the analysis of the stable stage is on a much fair basis. The characteristics of transient and stable stages will be discussed separately in the following sections. 3.1. Transient stage It can be seen in Fig. 5 that, under typical cold start condition (Tfuel = 30 °C), the first 2 cycles failed to ignite with an IMEP being 0 bar (misfire cycles); after that the IMEP gradually increased and it took almost 30 cycles to stabilize. For Tfuel = 60 °C, after one initial partially burned cycle with an IMEP of 0.85 bar, the IMEP of the following cycles quickly increased, and it took only 5 cycles to stabilize. When the fuel temperature increased to 90 °C, only one partially burned cycle was found with the IMEP being 2.45 bar, and the engine switched to stable condition from the second cycle. Figure 6a and b show the heat release rate and flame development of the initial cycles of three fuel conditions, respectively. In Fig. 6b, flame propagation for Tfuel = 30 °C was shown from Cy-

cle 4 since no obvious flame could be observed in the first three cycles as a result of failed combustion. The flame boundary was indicated by red line since for several cycles at transient stage the flame intensity was too low to observe. No positive heat release rate was recorded for Cycle 1 at Tfuel = 30 °C in spite of the fact that clear spark plasm could be observed, confirming a misfired cycle. Even in Cycle 4, very low peak heat release was found, which is consistent with its slow flame development shown in Fig. 6b. This can mostly be attributed to the poor atomization and evaporation of fuel under low temperature. In the next few cycles, the IMEP gradually increased to 1.8 bar before another misfired cycle [11] occurred, as is highlighted by the red circle in Fig. 5. In this cycle, only small and dim blue flame was found. Following this, Cycle 12 ended up with an IMEP of as high as 2.41 bar and very yellow (sooty) flame. The sudden increase of IMEP and the large amount of yellow flame is assumed to be related to the unburned fuel trapped in the cylinder from Cycle 11. The fluctuation lasted for about 30 cycles, after which smooth engine operation were achieved. Cycle 17 shows a ‘typical’ flame propagation process with the blue flame gradually filled the whole observing window although the combustion phasing was retarded. Meanwhile, yellow flame was formed due to the existence of rich fuel mixture. When the fuel temperature increased to 60°C, teat release curve raised greatly from Cycle 1 to Cycle 2, as is shown in Fig. 6a, indicating a much shorter transient stage compared with that under Tfuel = 30 °C. With regards to flame propagation under Tfuel = 60 °C, a vague blue flame can be observed since 6.05 CAD from the 1st cycle, but the flame didn’t sustain too long and the flame intensity was weak and the flame failed to propagate to the whole quartz window. In the next cycle, a complete combustion was observed with an earlier combustion phasing than Cycle 17 under Tfuel = 30 °C although the IMEP of the two cycles were found the same. As for the case with Tfuel = 90 °C, a flame kernel appeared at a similar CAD as that in Tfuel = 60 °C, but the blue flames expanded to a much wider area and occupied the whole piston window at 27.65 CAD. Accordingly, the heat release rate for Cycle 1 at Tfuel = 90 °C is much higher than its counterparts in the rest of the conditions. Nevertheless, the IMEP of Cycle 1 under Tfuel = 90 °C is still lower than its following cycles in the stable stage, this can be attributed to the cold surface of the combustion chamber. It is noteworthy that under Tfuel = 90 °C, yellow flame was observed at 20.45 and 27.65 CAD in the middle of the view field indicating soot formation. As was pointed out by Akihama, et,al, soot was generated when local rich region (equivalence ratio higher than 2.0) reached appropriate high temperature between 1700 K and 2600 K [40]. As no yellow flame can be observed in the initial cycle of Tfuel = 30 °C and 60 °C conditions whose mixtures were considered to be worse, the combustion temperature for Cycle 1 of Tfuel = 90 °C was expected to be higher. This is also considered to be a contributor of an improved combustion efficiency.

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Fig. 6. (a) Heat release of the early cycles in transient stage under different fuel temperature conditions and (b) flame development of early cycles in transient stage under different fuel temperature conditions.

Results in Fig. 6 show that at transient stage, the cold engine initial condition was not favorable for engine to proceed successful combustion at the very first few cycles. But the engine combustion performance improves gradually in the subsequent cycles. This was achieved by the heat transfer between the burned gas and the cylinder liner. However, flash boiling prompted atomization and enhanced evaporation of liquid fuel

under low engine temperature which was otherwise deteriorated for fuel mixture formation. Moreover, the enhanced fuel air interaction induced by the strong entrainment of the flash boiling also improved the mixture formation [15,16]. As a result of these multiple advantages of flash boiling spray, a successful combustion in the first cycle was realized under cold start condition.

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Fig. 7. (a) In-cylinder pressure curves, (b) normalized cumulative heat release and (c) both blue and yellow flame propagating area under different conditions for three fuel temperatures, average of 100 consecutive stable cycles.

3.2. Stable stage After the transient stage of cold start, the thermodynamic condition of the cylinder head as well as liner was stabilized as a result of the heat transfer from the burned gas. As was discussed in the previous section, Cycle 51 to Cycle 150 for all the conditions was considered as the stable stage in this study. Figure 7 presents the in-cylinder pressure, normalized cumulative heat release rate and the flame area development of the stable stage, all averaged from 100 stable cycles. The maximum flame area observed was limited by the quartz piston, which was 3019 mm2 in this case. The key thermodynamic parameters concerning peak pressure and combustion phasing are also listed in Table 2. Figure 6a shows that for Tfuel = 30 °C, a bimodal pressure curve can be observed indicating a poor early combustion development under typical stable stage of cold start condition. For Tfuel = 60 °C (transient flash boiling), an advanced combustion phasing (LPP = 24.6 CAD) with higher peak pressure (12.06 bar) was observed. When fuel was heated to flare flash boiling spray, the average combustion performance was further improved with higher

peak pressure and earlier combustion phasing of LPP = 22.6 CAD aTDC. Figure 7b shows the heat release curve. It can be seen that the average CA10 appears as late as 12.5 CAD aTDC for Tfuel = 30 °C, also the combustion duration (defined as the duration between 10% and 90% of heat release) was 38.4 CAD. The combustion duration decreased by 7.4 CAD at Tfuel = 60 °C, this was further decreased by 2.6 CAD under Tfuel = 90 °C. The thermodynamic results show that, with the improvement of superheat degree, higher peak pressure, earlier and faster combustion was achieved. As is shown in Fig. 5 and Table 2, the COVimep (coefficient of variation) is also lower under flare flash boiling condition. Higher output performance along with reduced COV reflect an improved stability of combustion which is of significant importance for engine at cold start condition. Lower COV is especially desirable for cold start condition which allows for further delay of the combustion phasing so that elevated exhaust temperature and faster lightoff could be realized with acceptable combustion stability [8]. Figure 8 shows the typical flame images at the stable stage under three fuel temperatures. Under Tfuel = 30 °C, a small blue flame

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Table 2 Parameters of average pressure curve and heat release. Spray condition

Liquid spray (30 °C)

Transitional flash boiling (60 °C)

Flare flash boiling (90 °C)

Peak pressure (bar) Location of PP (CAD aTDC) CA10 (CAD aTDC) Burn Duration (CAD) IMEP (bar) COVimep (%)

7.94 31.4 12.5 38.4 2.796 7.78

12.06 24.6 6.4 31 3.11 2.73

13.23 22.6 4.7 28.4 3.299 1.49

Fig. 9. Normalized yellow flame intensity at peak HRR averaged from 100 cycles and PN emission for each running condition.

Fig. 8. Typical flame development under different fuel temperature conditions.

can be found at 1.15 CAD bTDC from Cycle 75. At the same crank angle, the flame areas of Tfuel = 60 °C and 90 °C were twice the size of that at Tfuel = 30 °C, indicating an advanced flame development under higher superheat degree, which agrees with the average heat release results. As the crank angle evolves, the blue flame gradually occupied the observing window. In the meantime, yellow flame, representing the formation of soot generated at rich mixture zone, also appeared. Different from the blue flame that always originates from the spark plug, the occurrence of yellow flame was random and dispersive, due to the influence of turbulent air flow. Until the end of the image recording, corresponding to 80% of average MFB, large area of yellow flame could still be observed when the spray temperature was 30 °C, suggesting a very late and sooty combustion. As for Cycle 119 from Tfuel = 60 °C, the yellow flame only takes up one sixth of the quartz window with comparable

flame intensity to that at Tfuel = 30 °C, indicating much less soot generation. For Cycle 88 of Tfuel = 90 °C, the yellow flame was barely noticeable until the end of combustion, which suggests a complete combustion with little soot generation as a result of the improved mixture formation and less impingement. To directly study the relationship between the cold start PN emission and the in-cylinder combustion performance, the PN emission was plotted as a function of normalized yellow flame luminance, as is shown in Fig. 9. For the ease of comparison, average yellow flame luminance of 100 cycles in optical test was normalized by the case of 30 °C fuel temperature. It was reported that the in cylinder soot concentration could be semi-quantitatively calculated by yellow flame luminance at the peak HRR for the corresponding cycle [26]. Even though the luminance of yellow flame only represents soot formation, it is still found to be positively related with engine out PN emission, as is shown in Fig. 9. Also, both yellow flame luminance and PN emission decrease as the superheat degree of fuel spray increases. This is because when fuel experienced flash boiling, large amount of fuel evaporated promptly after injection while the remaining liquid fuel was found to have smaller drop size [11,13]. Despite the fact that the fuel spray ‘collapsed’ into single plume with longer penetration and longer period of impingement, the smaller droplet and enhanced evaporation is helpful for the reduction of total adhered fuel amount which is a main source of soot formation in the tested operating condition. As a result, little soot can be found in the chamber and the exhaust pipe when flash boiling took place.

3.3. Cycle-to-cycle variation In the previous section, the combustion characteristics of three operating conditions were discussed based on the cycle-averaged in cylinder pressure and flame images of typical cycles. However, these cannot reflect the cycle-to-cycle variation of the engine combustion. Therefore, in this section, the cyclic variation of in-cylinder combustion will be discussed.

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Fig. 11. Blue flame area at 8.25 CAD aTDC for three spray temperature conditions.

Fig. 10. Probability maps of blue flame sampled over 100 cycles for three superheat degrees.

To obtain a general idea of the combustion characteristics at stable stage under different spray conditions, the presence probability image (PPI) of 100 blue flame images are plotted in Fig. 10 using pseudo color scheme [41]. Each blue channel image was binaried as described in Section 2.2. Then, the PPI of each condition was calculated in the following equation: N 

PPI(x,y) =

i=1

I(i x,y ) N

(1)

where x and y are the pixel index of the image, I is the intensity of the blue flame image after binarization, N is total cycle number (100 in this study) and i is the cycle number index (1 to 100). Lower PPI (blue) area represents locations where flame rarely appears while higher PPI (red) area denotes regions where flame appears in most cycles. Firstly, it could be observed that for all the conditions, the flame kernel initiated from the left of the spark plug due to the local high fuel concentration under high swirl ratio [34]. The low PPI area can be observed to evenly distribute at the periphery of the PPI images before the flame reached the observing boundary, indicating that the flame suffers from large variation at the flame front. This also shows that even though the bulk flow of spray and air determines the average flame propagation direction [34], the microscopic development of the flame is still stochastic. However, the stability of early flame varied with the superheat

degree. Take −5.58 CAD aTDC for example, under 30 °C, the PPI occupied a smaller area with lower value, while that for 60 °C and 90 °C takes up larger area with higher value. As the crank angle evolved, the PPIs grew for all conditions. However, lower value could still be observed for 30 °C, suggesting a lower repeatability of flame presence under this condition. To quantify the repeatability of the flame using PPI, we calculated the area with a presence probability of more than 0.9 ( APPI > 0.9), and also that more than 0 (APPI > 0). The ratio of the two areas (R90 ) was then obtained to evaluate the repeatability of the flame development. For ease of illustration, the APPI > 0.9 as well as R90 is shown on the top left and top right of each image in Fig. 10. The R90 increased from 0 to 0.57 for 30 °C fuel temperature. A R90 of 0 indicates an extremely unstable early kernel development. This is attributed to the poor mixture preparation under cold start condition when fuel endured insufficient evaporation. As the blue flame area got larger, the R90 increased accordingly. However, when the fuel superheat degree increased, R90 developed from 0.14 to 0.71 and 0.15 to 0.71, respectively, for 60 and 90 °C, suggesting that the repeatability of the flame development is greatly improved. At 8.25 CAD aTDC, the R90 for the two flash boiling conditions was the same since the flame filled up the observing window. Figure 11 shows the blue flame area of each cycle at 8.25 CAD for three conditions. This crank angle represents early phase of combustion when the piston just began to move downward, a larger flame area represents a faster combustion cycle. From Tfuel = 30 °C to 60 and 90 °C, the blue flame area increased by 80% and 99%, respectively, indicating a faster flame development under flash boiling condition. Also the variation decreased by 25.2% and 35.3%, suggesting a higher repeatability of flame propagation. Figure 12 plotted the probability density function (PDF) of the critical crank angle when blue flame area reached 10 0 0 mm2 . The PDF first calculated the occurrence of cycles at each crank angle with an increment of 1 CAD under each operating condition. Then the occurrence was normalized by the total cycle number (100 in this study). This critical flame area was selected because the blue flame experiences stable growth rate after this value (Fig. 7). It is clear that for Tfuel = 30 °C, the flame grew to the critical area very late after TDC, and the distribution of the crank angles reaching such area was much wider (95% cycles fall in was 7.3 CAD), showing high level cycle-to-cycle variation of flame development process. When the fuel temperature was increased to 90 °C, the average PDF advanced by 5-6 CAD while the distribution became narrower showing more repeatable flame development (95% cycles within 3.6 CAD). Therefore, it can be concluded that a faster and

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Fig. 12. Probability density function of critical crank angle when the blue flame area reaching 10 0 0 mm2 . (The total counts are 100 cycles for each condition.)

Fig. 14. Correlation between IMEP and CA when blue flame area reached 10 0 0 mm2 .

Fig. 13. Correlation between IMEP and blue flame area at 8.25 CAD aTDC.

more repeatable blue flame development can be achieved when flash boiling spray was used. 3.4. Correlation between flame image and engine performance It is noteworthy from Figs. 5 and 11 that, the IMEP of each cycle and the blue flame area at 8.25 CAD showed similar trend, implying the propagation of blue flame (embodied by the flame area at a certain crank angle) is closely related with the engine output performance. To study this relationship, Fig. 13 plots the correlation between IMEP and the flame area of the corresponding cycle at 8.25 CAD from both transient and stable stage. Notice that cycles failed to form a noticeable flame were removed. Figure 13 shows that after 2.5 bar and 500 mm2 , the engine output IMEP increases linearly with the propagation rate of the blue flame while no obvious correlation could be found between the two when IMEP is lower than 2.5 bar. It could be observed that the dots from Tfuel = 30 °C lied in a wider range while the dots from Tfuel = 90 °C concentrate on the top right, meaning the latter case has higher IMEP and earlier flame development. However, the analysis was limited by the size of the observing window (3019 mm2 in total) as the flame propagates to the edge of the window such that the calculated flame area becomes less reliable. After careful observation of the PPI maps in Fig. 10, it was found that the flame generally does not touch the quartz boundary under 10 0 0 mm2 . Therefore, we next correlate the IMEP with the crank angle it requires for the blue flame to reach 10 0 0 mm2 , in order to better characterize the relationship between engine performance

and the early flame development, as is shown in Fig. 14. Different from the two-stage correlation found in Fig. 13, uniform linear relation regardless of the IMEP level was found for all cycles from both transient and stable stage. This suggests that the engine output performance can be well evaluated by the development rate of blue flame. Specifically, the earlier the blue flame area reached 10 0 0 mm2 , the higher the IMEP. It is noteworthy the timing when flame reached such an area corresponds to less than 5% MFB, when the in cylinder pressure detection is considered not sufficiently accurate, whereas the flame images at such early stage was reported to be more reliable to assess the combustion performance at the initial phase of the expansion stroke [27]. The data under flare flash boiling condition is concentrated on the top left of the plot, indicating a faster blue flame propagation and higher IMEP as a results of enhanced atomization and evaporation. Of course the final output of the engine is also influenced by other factors including air fuel mixture gradient in unburned zone and the heat transfer through the combustion process, all of which are stochastic and can result in the variations of the output work. But those factors should be considered as secondary compared to spray condition. The error in this correlation mainly results from the frequency resolution of the high-speed camera which is 0.277 CAD. 4. Conclusion In this study, the combustion characteristics of flash boiling spray under cold start condition was examined using high-speed color camera and pressure transducer in a SIDI single cylinder optical engine. Various analysis methods were conducted to evaluate the combustion process of flash boiling spray, including the flame development, CCV and soot formation, under both transient and stable stage. Several conclusions could be drawn as follows: 1. Under typical cold start condition, the engine took as long as 30 cycles to stabilize due to the insufficient evaporation and unfavorable mixture preparation, with misfire and partial burn cycles found in the early stage; 2. The transient stage was reduced from 30 cycles to 2 cycles by flash boiling spray with no partial burn and misfired cycles observed; 3. At stable stage, an advanced combustion phase as well as faster blue flame propagation rate was achieved by flash boiling spray; 4. The engine out PN emission is positively related with the luminosity of the in-cylinder yellow flame. Flash boiling spray

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results in both lower yellow flame luminosity and lower PN emission. 5. From liquid spray to flare flash boiling spray, the variation of the blue flame area decreases by 35.3% indicating a more repeatable combustion; 6. The engine IMEP increases linearly with the propagation rate of blue flame, and the flare flash boiling condition presents fastest flame propagation and highest IMEP. It is worth noting that this study only focuses on the influence of flash boiling spray on combustion characteristics under cold start condition when the benefits of prompt evaporation and improved atomization could be maximized. The effect of flash boiling on engine performance under high load when knock could occur is not considered in this study and still a direction of ongoing work. Acknowledgments This research is sponsored by General Motors Company (USA), and National Natural Science Foundation of China (NSFC), under grant No. 51376119/E060502. It was carried out at the National Engineering Laboratory for Automotive Electronic Control Technology of Shanghai Jiao Tong University. Reference [1] F. Zhao, M.C. Lai, D.L. Harrington, Automotive spark-ignited direct-injection gasoline engines, Progr. Energy Combust. Sci. 25 (5) (1999) 437–562, doi:10. 1016/S0360-1285(99)0 0 0 04-0. [2] M.C. Drake, T.D. Fansler, A.M. Lippert, Stratified-charge combustion: modeling and imaging of a spray-guided direct-injection spark-ignition engine, Proc. Combust. Inst. 30 (2) (2005) 2683–2691, doi:10.1016/j.proci.2004.07.028. [3] E. Chapman, R. Davis, W. Studzinski, P. Geng, Fuel octane and volatility effects on the stochastic preignition behavior of a 2.0L gasoline turbocharged DI engine, SAE Int. J. Fuels Lubr. 7 (2) (2014), doi:10.4271/2014- 01- 1226. [4] J. Serras-Pereira, P.G. Aleiferis, D. Richardson, Imaging and heat flux measurements of wall impinging sprays of hydrocarbons and alcohols in a directinjection spark-ignition engine, Fuel 91 (1) (2012) 264–297, doi:10.1016/j.fuel. 2011.07.037. [5] S.T. Chin, C.-F.F. Lee, Numerical investigation of the effect of wall wetting on hydrocarbon emissions in engines, Proc. Combust. Inst. 29 (1) (2002) 767–773, doi:10.1016/S1540- 7489(02)80098- 0. [6] Sagawa, T., Fujimoto, H., and Nakamura, K., “Study of fuel dilution in directinjection and multipoint injection gasoline engines,” 2002-01-1647, 2002, doi:10.4271/2002-01-1647. [7] H. Chen, D.L. Reuss, D.L.S. Hung, V. Sick, A Practical guide for using proper orthogonal decomposition in engine research, Int. J. Engine Res. 14 (4) (2013) 307–319, doi:10.1177/1468087412455748. [8] G.G. Zhu, I. Haskara, J. Winkelman, Closed-loop ignition timing control for si engines using ionization current feedback, IEEE Trans. Control Syst. Technol. 15 (3) (2007) 416–427, doi:10.1109/TCST.2007.894634. [9] W. Zeng, M. Xu, M. Zhang, Y. Zhang, et al., Macroscopic characteristics for direct-injection multi-hole sprays using dimensionless analysis, Exper. Thermal Fluid Sci. 40 (0) (2012) 81–92, doi:10.1016/j.expthermflusci.2012.02.003. [10] W. Zeng, M. Xu, G. Zhang, Y. Zhang, et al., Atomization and vaporization for flash-boiling multi-hole sprays with alcohol fuels, Fuel 95 (0) (2012) 287–297, doi:10.1016/j.fuel.2011.08.048. [11] G. Zhang, M. Xu, Y. Zhang, M. Zhang, et al., Macroscopic characterization of flash-boiling multi-hole sprays using planar laser induced exciplex fluorescence technique. Part I. on-axis spray structure, Atom. Spray 22 (10) (2012) 861–878, doi:10.1615/AtomizSpr.2013006760. [12] G. Zhang, D.L.S. Hung, Temporal investigations of transient fuel spray characteristics from a multi-hole injector using dimensionless analysis, Exper. Thermal Fluid Sci. 66 (2015) 150–159, doi:10.1016/j.expthermflusci.2015.03.011. [13] M. Xu, Y. Zhang, W. Zeng, G. Zhang, et al., Flash boiling: easy and better way to generate ideal sprays than the high injection pressure, SAE Int. J. Fuels Lubr. 6 (1) (2013) 137–148, doi:10.4271/2013-01-1614. [14] G. Zhang, M. Xu, Y. Zhang, M. Zhang, et al., Macroscopic characterization of flash-boiling multihole sprays using planar laser-induced exciplex fluorescence. Part Ii: Cross-sectional spray structure, Atom. Spray 23 (3) (2013) 265–278, doi:10.1615/AtomizSpr.2013007450. [15] M. Zhang, M. Xu, Y. Zhang, G. Zhang, et al., Flow-field investigation of multihole superheated sprays using high-speed piv. part i.cross-sectional direction, Atomiz. Spray 22 (11) (2012) 983–995, doi:10.1615/AtomizSpr.2013006775. [16] M. Zhang, M. Xu, Y. Zhang, G. Zhang, et al., Flow-field investigation of multihole superheated sprays using high-speed piv. Part II. axial direction, Atom. Spray 23 (2) (2013) 119–140, doi:10.1615/AtomizSpr.2013007454.

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