Effects of direct-injection fuel types and proportion on late-injection reactivity controlled compression ignition

Combustion and Flame 211 (2020) 445–455

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Effects of direct-injection fuel types and proportion on late-injection reactivity controlled compression ignition Qinglong Tang, Haifeng Liu∗, Xingwang Ran, Mingkun Li, Mingfa Yao State Key Laboratory of Engines, Tianjin University, Tianjin 300072, PR China

a r t i c l e

i n f o

Article history: Received 1 April 2019 Revised 1 October 2019 Accepted 9 October 2019

Keywords: Late-injection strategy OH/formaldehyde Planar laser-induced fluorescence (PLIF) Polyoxymethylene dimethyl ethers (PODE) Reactivity controlled compression ignition (RCCI)

a b s t r a c t The late-injection strategy can decrease peak pressure rise rate (PRR) in reactivity controlled compression ignition (RCCI) and enables RCCI operation to high engine load. However, excessive soot emission is a limiting factor in this high engine load extension. Recently, polyoxymethylene dimethyl ethers (PODE) emerges as a promising direct-injection fuel for RCCI load extension due to the properties of high reactivity and high oxygen content. The late-injection RCCI strategy features a two-stage high-temperature heat release (HTHR) that has not been well understood. In this study, we investigated the effects of direct-injection fuel properties types and proportion on late-injection RCCI strategy on a light-duty optical engine using multiple optical diagnostic techniques. Iso-octane served as the premixed fuel and the in-cylinder direct-injection timing was set at the crank angle of −10° after the top dead center. Firstly, the combustion characteristics of RCCI with direct-injection fuels of n-heptane, PODE and cetane were compared. The NFL images prove that the PODE case shows less tendency of soot formation. The OH planar laser-induced fluorescence (PLIF) imaging indicates that the OH radical is widely distributed in the combustion chamber after the HTHR including the central part of the combustion chamber near the injector nozzle for all three cases. Secondly, the effects of direct-injection fuel proportion on the combustion characteristics of the late-injection RCCI strategy was evaluated using PODE as the direct-injection fuel. The NFL images show that the combustion regime in the low reactivity region of RCCI tends to change from auto-ignition to flame front propagation with the decreasing of the direct-injection fuel proportion from 30% to 6%. The introduction of the flame front propagation in the second-stage HTHR reduces the peak pressure rise rate of RCCI. We conclude that the peak PRR of RCCI at high engine load can be controlled by modulating the ratio between the auto-ignition and flame front propagation through tuning the proportion of the high-reactivity fuel in the direct injection. © 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction Homogenous charge compression ignition (HCCI) shows great potential in reducing NOx and soot emissions while maintaining high engine efficiency like the diesel engine. However, suffering from excessive pressure rise rate (PRR) at high engine load, the operation range of HCCI is limited to low engine load. Besides, it is difficult to control the combustion phasing of HCCI dominated by chemical kinetics [1,2]. One effective way of extending the HCCI operating load is to increase the fuel stratification by retarding the fuel injection timing [3]. High fuel stratification results in non-simultaneous auto-ignition events for the different in-cylinder charge regions with varied local equivalence ratio, producing lower peak PRR and enabling higher engine load operation. A certain

∗

Corresponding author. E-mail address: [email protected] (H. Liu).

amount of exhaust gas recirculation is introduced to prolong the ignition delay and to achieve the partially premixed combustion (PPC). PPC using gasoline fuel can be operated at higher engine load without excessive EGR to gain sufficient premixing time. However, the combustion stability of gasoline PPC at low engine load becomes unacceptable due to the low reactivity of gasoline fuel [4,5]. Researches in the literature indicate that there is an optimum fuel reactivity that can facilitate the partially premixed combustion while achieving high engine efficiency at a specified engine load, i.e., high reactivity (high cetane number) fuel at low load and low reactivity fuel at high engine load [6,7]. To realize variable overall fuel reactivity and fuel stratification degree, the reactivity controlled compression ignition (RCCI) strategy was proposed [8,9]. A large part of low-reactivity fuel is delivered in the port-injection to form a premixed charge, and a small part of high-reactivity fuel is delivered in the in-cylinder direct-injection to trigger combustion. Various combinations of

https://doi.org/10.1016/j.combustflame.2019.10.018 0010-2180/© 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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low- and high-reactivity fuels can be used in RCCI strategy [10–13]. Extensive experiments have been conducted to study the engine performance and emissions of RCCI. Results indicated that gasoline/diesel RCCI could achieve ultra-low NOx /soot emissions over a wide engine operation range from 2 bar indicated mean effective pressure (IMEP) to 14.6 bar IMEP, and the peak gross indicated thermal efficiency was higher than 50% [14]. Early direct injection strategy was adopted in these studies, for example, the injection timing of diesel in Refs. [8] and [12] was earlier than −30 °CA ATDC. Kokjohn et al. [15,16] demonstrated that the flame development of RCCI with early-injection strategy was dominated by sequential auto-ignition. Combining chemical kinetic analysis with fuel-tracer planar laser-induced fluorescence (PLIF) technique, they discovered that the charge reactivity stratification played a major role in the ignition process of RCCI, followed by the charge concentration stratification and the effect of temperature stratification was minor. When RCCI is operated at high or full engine load, the early-injection strategy results in excessive PRR [17]. Ma et al. [18] studied the diesel injection strategy of gasoline/diesel RCCI and found that late-injection strategy could reduce PRR and extend RCCI operation range to higher engine load, however, the upper load limit was restricted by the increasing soot emissions. It is widely accepted that oxygenated fuel is effective in reducing soot emissions during engine combustion [19,20]. Thus, oxygenated, high-reactivity fuel seems to be the best choice for direct injection in RCCI strategy. In recent years, polyoxymethylene dimethyl ethers (PODE), which has properties of high reactivity, high oxygen content and no C–C bond, becomes an emerging alternative fuel for diesel engines [21–23]. Tong et al. [24] investigated the engine performance of gasoline/PODE RCCI on a heavy-duty diesel engine. They found that the maximum load of gasoline/PODE RCCI could be extended to 17.6 bar IMEP with late-injection strategy, while still maintaining ultra-low soot and comparable engine efficiency and peak PRR. Wang et al. [25] found that gasoline/PODE RCCI with late direct injection timing and late intake valve closing strategy could provide a pathway to achieve clean and high-efficiency combustion over full engine load. However, compared with that of the early-injection RCCI strategy, there are few optical diagnostics on the late-injection RCCI and the in-cylinder process of late-injection RCCI operated with direct-injection fuels of different reactivity and oxygen content is not well understood. Besides, the above engine experiment studies show that the late-injection RCCI strategy presents a two-stage, high-temperature heat release (HTHR). Similar heat release features were reported by Bilcan et al. [26] and Boehman et al. [27] using LPG or syngas as the premixed fuel while diesel as the direct-injection fuel. The first-stage HTHR is due to the auto-ignition of the high-reactivity DI fuel and the second-stage HTHR is the combustion of the premixed low-reactivity fuel. The separation of the first- and the second-stage HTHR disperses the overall heat release in time, resulting in a lower peak PRR than that of the early-injection RCCI that only has single-stage HTHR. However, the combustion and flame development (flame front propagation or auto-ignition) in the transition from the first- to the second-stage HTHR of the late-injection RCCI has not been well interpreted [28–30]. To gain more detailed information about this combustion phenomenon, laser diagnostics on the late-injection RCCI is necessary. Recently, we studied the effect of the premixed equivalence ratio on late-injection RCCI using high-speed natural flame luminosity (NFL) imaging and formaldehyde PLIF imaging [31]. However, the effect of the direct-injection fuel proportion on late injection RCCI has not been explored. In the present work, the late-injection RCCI strategy was studied in a light-duty optical engine using multiple optical diagnostic techniques. Firstly, we chose n-heptane, PODE and cetane

Table 1 Optical engine specifications. Bore

92 mm

Stroke Displacement Connecting rod length Compression ratio Combustion chamber diameter Combustion chamber height Valve number

100 mm 0.66 L 155 mm 11 63 mm 9.4 mm 2

Fig. 1. Structure of the combustion chamber and the field of view for optical diagnostics [32,33].

(n-hexadecane) as the direction-injection fuels and the effects of fuel types on RCCI combustion were investigated using high-speed natural flame luminosity (NFL) imaging and OH PLIF imaging. Secondly, the effects of direct-injection fuel proportion on the combustion characteristics of the late-injection RCCI strategy were evaluated using PODE as the direct injection fuel. Finally, the low-temperature heat release (LTHR) characteristics were analyzed by comparing the formaldehyde PLIF imaging results with and without the port injection of isooctane. The natural flame luminosity and emission spectra during the combustion of these two cases were also compared. The motivation of this study is to gain insight into the in-cylinder combustion characteristics of late-injection RCCI strategy and explore the control principle for the RCCI operation at high engine load. 2. Experimental setup and optical diagnostic techniques 2.1. Optical engine A naturally aspirated, single-cylinder optical engine was used in this study. The main engine specifications are listed in Table 1. The engine was equipped with a dual-fuel injection system composed of a common-rail injector and a commercial low-pressure port injector. To allow optical access through the bottom of the extended piston in the optical engine, a flat piston crown window was utilized, forming a cylindrical combustion chamber with a diameter of 63 mm and a height of 9.4 mm. A quartz ring window with a height of 36 mm, mounted on the top part of the cylinder wall, allowed the entrance of horizontal laser sheets into the cylinder. A cut-out of 40 mm is set on the right side of the combustion chamber, as shown in Fig. 1, so that the laser sheet can pass into the combustion chamber around crank angles near top dead center. The above optical modifications of the combustion chamber reduce the compression ratio to about 11. Due to the two-valve

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355 nm 532 nm

Laser Mirrors

Dye Laser

282.95 nm

ECU Cylindrical Lens

Fuel Injection Control System

ICCD Camera

Filters Pulse Delay Generator

355 or 282.95 nm

Nd:YAG Laser

Image Acquisition System

Optical Engine

Fig. 2. Schematic of the laser diagnostic systems (355 nm and 282.95 nm lasers were used non-simultaneously to stimulate the fluorescence of formaldehyde and OH, respectively.) [32].

engine configuration, the common-rail injector nozzle, which has 6 holes with a diameter of 0.15 mm and a spray-included angle of 150°, deviates a little from the center of the combustion chamber. Figure 2 illustrates the schematic of the laser diagnostic systems for the optical engine. Solid lines with arrows show the electric signal flow direction, and dashed lines with arrows indicate the light signal flow direction. The electronic control unit (ECU) reads the crank signal from the engine motored at a speed of 1200 revolution per minute (rpm) by a dynamometer, producing a 10 Hz pulse to trigger the Nd:YAG laser (Pro-250, Spectra-Physics) through the pulse delay generator (DG535, Stanford Research). When the fuel injection order is confirmed, ECU energizes the injectors and triggers the intensified charge-coupled device (ICCD) camera (DH734i-18F-03, Andor) through the pulse delay generator. The timing sequence of signals among laser, ECU and camera are synchronized by the pulse delay generator. Two laser beams of 355 nm and 282.95 nm were used for formaldehyde and OH PLIF excitation, respectively. The wavelength of 355 nm was from the third harmonic of the Nd:YAG laser. To obtain 282.95 nm laser, the second harmonic (532 nm) of the Nd:YAG pumped a dye laser (Sirah CSTR-G-18) run with Rhodamine to produce 565.9 nm laser. The 565.9 nm laser then passed through a frequency doubling crystal to form a 282.95 nm laser. The 355 nm or 282.95 nm beam was formed into a 30 mm wide and less than 1 mm thick horizontal laser sheet by three cylindrical lenses and is directed horizontally into the cylinder through the quartz ring window. The relative position of the cylinder head, extended piston and laser diagnostics region are shown in Fig. 2. The horizontal laser sheet was maintained at a plane 10 mm below the cylinder head. The distance between the cylinder head and laser sheet was carefully chosen so that the laser sheet just passes through the main combustion region in the combustion chamber. The PLIF signals were reflected by an UV-mirror to an ICCD camera coupled with different fluorescence filters. More details of the experimental setups can be found in Ref. [32]. 2.2. Optical diagnostic techniques 2.2.1. Natural flame luminosity (NFL) imaging and spectrometry A high-speed camera (Photron SA1) was used to record the natural flame luminosity of RCCI combustion. A BK7-glass lens of

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50 mm focal length was coupled to the high-speed camera with an aperture number of F1.4. The high-speed camera was only sensitive to the light signals in the visible range. The frame rate and exposure time of the camera were set to 10,000 frames per second and 100 μs, respectively. Thus, the temporal resolution of the acquired images was 0.72 °CA at the engine speed of 1200 rpm. The natural flame emission spectra in RCCI combustion process at specific engine crank angles were captured by a fiber spectrometer (Bruker 250is). Five detecting fiber probes were horizontally placed on the opposite of the combustion chamber cut-out on the right-hand side of the quartz ring. The signals from these detecting fiber probes were accumulated to give the overall flame emission spectra. The detailed experimental setup for natural flame spectra acquisition can be found in Ref. [34]. A 150 lines/mm grating with a spectral resolution of 3 nm was chosen for the spectrometer. The gate width and the gain level of the spectrometer were set to 139 μs (about 1 °CA) and 50, respectively. When the natural flame images or emission spectra were measured, the lasers in Fig. 2 were shut down and the ICCD camera was replaced by the high-speed camera or spectrometer, respectively. 2.2.2. Formaldehyde and oh plif imaging As mentioned in Section 2.1, the third harmonic (355 nm, 70 mJ/pulse) of the Nd:YAG laser was adopted for formaldehyde fluorescence excitation. An edge filter with a transmission range of 390–480 nm collected the formaldehyde fluorescence. Although PAHs (polycyclic aromatic hydrocarbons) can also be stimulated by 355 nm laser, results from the literature indicate that PAHs fluorescence is produced in the HTHR stage and is mainly located in the downstream regions of the combustion chamber where soot arises [35–37]. Furthermore, the fluorescence intensity of PAHs is much higher than that of formaldehyde [38,39]. In this study, the formaldehyde PLIF imaging was applied to the RCCI cases with direct injection of PODE, for which no such PAHs fluorescence signal was found during the HTHR phase. An additional PLIF experiment was carried out using 266 nm laser for excitation, and no PAHs fluorescence was witnessed either. This indicated that the soot production for the RCCI cases fueled with PODE in this study was relatively low and PAHs were under the detection limits of the present PLIF system. Thus, the PLIF signal excited by 355 nm laser was considered to be mainly from formaldehyde. The laser beam of 282.95 nm (26 mJ/pulse), corresponding to the Q1 (6) line of the (1, 0) band in the A-X transition for OH radical, excited the OH fluorescence. An edge filter with a transmission range of 308–325 nm was used to collect the OH fluorescence signal. More details of formaldehyde and OH PLIF in the present study can be found in [33]. For both formaldehyde and OH PLIF imaging, the laser pulses were temporally bracketed by the ICCD gate width of 50 ns and the beginning of the gate width aligns with the onset of the laser pulse in time sequence. The gain level of the ICCD camera was set to 200. Both the laser frequency and the ICCD image acquisition rate were so low that only one frame PLIF image could be acquired in one engine cycle. 2.3. Fuels and engine operating conditions The fuel properties of isooctane, n-heptane, cetane (nhexadecane) and PODE are summarized in Table 2. The molecular structure of PODE (CH3 O–(CH2 O)n –CH3 ) is shown in Fig. 3a. It is a mixture of four short oligomers with n ranging from 3 to 6. The component of PODE is illustrated in Fig. 3b. N-heptane has similar fuel reactivity to diesel fuel considering their similar cetane number. The cetane number of cetane is 100 and thus it represents much higher fuel reactivity compared with that of n-heptane. The cetane number of PODE is 75.5 and the fuel reactivity of PODE is

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Table 2 Physical properties of iso-octane, n-heptane, cetane (n-hexadecane) and PODE [21,23,24]. Iso-octane N-heptane Cetane PODE Molecular formula Density (kg/L) at 20 °C Cetane number Oxygen content (wt%) Boiling point (°C) Kinetic viscosity (mPa·s) Vapor pressure (kPa at 20 °C) Low heat value (MJ/kg) Stoichiometric air–fuel ratio

C8 H18 0.69 – 0 99.2 0.50 5.5 44.8 15.14

C7 H16 0.68 56 0 98.4 0.57 5.3 44.9 15.09

C16 H34 0.77 100 0 287 3.94 < 0.1 44.1 14.92

C2 H6 O(CH2 O)3–6 1.05 75.5 47.95 156–280 1.11 – 21.8 5.86

Table 3 Fuel injection strategy 1.

Direct injection timing (°CA ATDC) Fuel mass in direct-injection (mg)a Proportion of the direct-injectionb Overall equivalence ratio in direction injection Port-injection timing (°CA ATDC) Mass of iso-octane in port-injection (mg) Premixed equivalence ratio a b c

Heptane-10c

PODE-10

Cetane-10

−10 9 30% 0.23

−10 9 30% 0.19

−10 9 30% 0.23

−360 29

−360 29

−360 29

0.54

0.54

0.54

Equivalent weight of n-heptane base on heat value. Calculated using the equivalent weight of n-heptane by heat value. This case is adapted from our previous work [32].

Table 4 Fuel injection strategy 2.

Direct injection timing (°CA ATDC) PODE mass of in direct injection (mg) Proportion of direct-injection fuelb Overall equivalence ratio in direct injection Port injection timing (°CA ATDC) Premixed equivalence ratio a b

mDI = 4 mga

mDI = 2 mg

−10 8.2 12% 0.1 −360 0.75

−10 4.1 6% 0.05 −360 0.75

Equivalent weight of n-heptane base on heat value. Using the equivalent weight of n-heptane by heat value.

respectively, to study the effects of the direct-injection fuel proportion. The mass of isooctane in the port injection was fixed at 29 mg, forming a premixed equivalence ratio of 0.75. The PODE cases with equivalent n-heptane weight of 4 mg and 2 mg, were denoted as mDI =4 mg and mDI =2 mg, respectively. Compared to the PODE-10 case in Table 3, for which the proportion of directinjection fuel is 30%, the proportion of direct-injection fuel in cases of mDI =4 mg and mDI =2 mg was decreased to 12% and 6%. One case that only had a direct injection of 8.2 mg PODE served as a control group to compare the combustion characteristics with and without port injection. The intake air temperature was preheated to 373 K to keep the CA50 of the mDI =4 mg and mDI =2 mg cases in between 5° and 10 °CA ATDC, similar to that of the PODE-10 case in the experiment of Section 3.1. The fuel injection strategy in this part is summarized in Table 4. 3. Results and discussions 3.1. Effects of direct-injection fuel properties on the late-injection RCCI combustion

Fig. 3. Molecular structure of PODE (a) and mass fraction of each component (n: 3–6) in PODE (b).

in between that of n-heptane and cetane. The oxygen content of PODE approaches 50%, which could be quite helpful to reduce the soot emissions. The engine was naturally aspirated (1 bar intake pressure) and run at 1200 rpm without exhaust gas recirculation. It was fired in every other 20 cycles to avoid thermal loading problems and to extend the experiment period limited by window fouling. The pressure of port-injection was set at 3 bar and direct-injection at 600 bar. The fuel injection timings of port injection and direct injection were at −360 °CA ATDC and −10 °CA ATDC, respectively. The temperature of cooling water was kept at 95 °C. In the experiment of Section 3.1, the mass of isooctane delivered in portion injection was fixed at 21 mg, and the direct injection fuel was varied while keeping the heat value of direct injection fuel equivalent to 9 mg n-heptane by heat value. The intake air temperature was preheated to 348 K. The cases fueled with n-heptane, PODE and cetane are denoted as heptane-10, PODE-10 and cetane-10, respectively. The fuel injection strategy in this part is listed in Table 3. In the experiment of Section 3.2, the mass of PODE was reduced to 8.2 mg and 4.1 mg, equivalent to 4 mg and 2 mg n-heptane

Figure 4 shows the liquid-phase fuel injection process indicated by Mie scattering in typical cycles recorded by the high-speed camera. A tungsten halogen lamp served as the light source for spray illumination. The first obvious spray images that can be observed are at −7.1 °CA. The difference in the low heat value of the direct-injection fuels results in different fuel injection durations when the total amount of heat input for the direct injection fuel is the same. Consequently, the PODE-10 presents the longest fuel injection duration (7.2 °CA), followed by the N-heptane case (5.7 °CA) and the Cetane-10 case (5 °CA). The injection duration indicated by the Mie scattering light signal for different fuels is presented in Fig. 5. The liquid-phase penetration length of the sprays reaches the maximum at −4.2 °CA for all cases. As the fuel varied from heptane to PODE and cetane, we can observe that the liquid-phase spray angle increase by about two degrees at this time. This is because the volatility of n-heptane is the best among all three fuels while cetane is the worst, as shown in Table 2. Figure 5 shows the cylinder pressures and heat release rates averaged from twenty engine cycles. Ignition takes place after the end of injection for all three cases. Although the differences in volatility and viscosity for heptane, cetane and PODE are distinct, the ignition timing of the late-injection RCCI is dominated by the cetane number of the direct injection fuel. Higher cetane number results in shorter ignition delay. Former studies show that volatility and viscosity only play a minor effect on engine combustion and emissions compared with the effects of cetane number and oxygen content [40]. The cetane10 case shows the shortest ignition delay and lowest pressure rise rate, followed by the PODE-10 case and heptane-10 case. 3.1.1. Natural flame luminosity high-speed imaging The natural flame luminosity from the typical engine cycle is recorded by the high-speed camera and the accumulated NFL intensity curves are shown in Fig. 5. The NFL intensity of the cy-

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Fig. 4. The liquid-phase fuel injection process illuminated by a background light source.

Fig. 5. The averaged cylinder pressure (P) and apparent heat release rate (AHRR), accumulated heat release and the typical natural flame luminosity (NFL) intensity of the late-injection RCCI cases with different direct-injection fuels. The fuel injection durations are shown by the Mie scattering light of the liquid-phase fuel jets. The crank angles marked on the heat release curves are chosen for OH PLIF imaging. The Heptane-10 case is adapted from the authors’ previous work [32].

cle that has the closest pressure tracer to the averaged pressure trace is chosen as a representative. The corresponding NFL images of this cycle are presented in Fig. 6. In the first three images for all cases before CA50, ignition first happens in the downstream region around the combustion chamber wall. Take the heptane-10 case, for example, the charge concentration, reactivity and temperature distribution at 5 °CA in our previous study [41] are shown in Fig. s1 in the supplemental material. The test region is the same to that of the PLIF measurement in the present study, which is 10 mm below the cylinder head. Fuel reactivity is represented by PRF number, which is the volume fraction of isooctane in the overall fuels of isooctane and n-heptane. By comparing the NFL images of the heptane-10 case before CA50 and the fuel stratification results, it indicates that the auto-ignition first emerges in the high-reactivity (also high-concentration) region for the late-injection RCCI strat-

egy, although the local temperature in this region is relatively low. Same to the founding in the early-injection RCCI strategy in [16], the kinetic analysis in our study showed that charge reactivity stratification plays a major role in the ignition of late-injection RCCI strategy, followed by the charge concentration stratification, and the effect of temperature stratification is minor [41]. For the cetane-10 case, ignition takes place just after the end of fuel injection so that the six combusting fuel jets, the upstream of which is close to the injector, can be clearly observed at 3.0 °CA with a brownish-red color. For the PODE-10 case, the flame regions corresponding to the six fuel jets move away from the injector nozzle compared with that of the cetane-10 case, but the six spray jets can still be recognized at 6.6 °CA. Bright blue flames at this time show that the natural flame luminosity for the PODE-10 case mainly comes from the chemiluminescence of the combustion intermediate species. The heptane-10 case has the longest ignition delay, the flames formed around the combustion chamber seem to be continuous and the fuel jets can no longer be recognized through the shape of flame by 8.0 °CA. At this time, the color of the flames is mixed with bright blue and yellowish red. As is mentioned in the work of Mueller et al. [42], the peak SINL values fall within the same range and show the same trends as the engine-out soot emissions. Thus, the peak accumulated NFL intensity in Fig. 5 can reflect the soot emissions to some extent. However, what we care more is the tendency of soot formation indicated by the true-color NFL images because the soot radiation and chemiluminescence can be differentiated spectrographically. The chemiluminescence from fuel decomposition is mainly located in the UV–visible range showing bluish flame (band spectra of C2 , CH, CHO and CHO2 and continuous spectra of CO oxidation). Soot radiation spectra are in the visible-infrared range showing mainly yellowish-red color. Take the NFL images at CA50, for example, the PODE-10 case almost shows pure bluish flame. In comparison, the Heptane-10 case shows mixed bluish and reddish flames indicating the increase of soot formation. For the Cetane-10 case, more soot radiation results in intense signal saturation by the yellowish-red flames. This indicates that the tendency of soot formation for the PODE-10 case is least and the heptane-10 case is between that of the PODE-10 case and cetane-10 case. In the following three images for all three cases in Fig. 6, the NFL intensity in the local high-reactivity regions gets so high that image saturation happens. Especially, for the cetane-10 case, it shows bright yellow soot radiation spots both around the combustion chamber wall and near the injector nozzle. Actually, in this period after the ignition of high-reactivity regions, combustion

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Fig. 6. The true-color natural flame luminosity of late-injection RCCI strategies with different direct-injection fuels. The crank angles are shown in the upper left of each image. The CA10, CA50 and CA90 are marked at the proximal images for each case. The Heptane-10 case is adapted in the authors’ previous work [32].

Fig. 7. Typical single-shot, false-color OH PLIF images of the late-injection RCCI strategies with different direct-injection fuels. The crank angles are shown in the upper left of each image. White dashed lines with arrows show the laser boundary.

happens in the low-reactivity region near the central part of the combustion chamber, which shows pale blue flames of quite low intensity, such as the images at 9.4 °CA for the heptane-10 case and at 8.7 °CA for PODE-10 case. For better observation of these pale blue flames, brightness-adjusted images of these two cases are shown in Fig. s2 of the supplemental material. As for the cetane-10 case, the soot radiation in the combustion chamber is so high that it further hinders the observation of the pale blue flames in the low-reactivity region. From this point of view, direct-injection fuel like PODE that has less soot formation is beneficial for a better observation of the flame development in the second-stage HTHR of the late-injection RCCI strategy. Thus, in the following section, we select PODE as the direct-injection fuel for further study. In the last two images after 20 °CA for all three cases in Fig. 6, the natural flame luminosity mainly comes from soot radiation because the main heat releases in Fig. 5 for all three cases have

largely finished. The CA10 (crank angle corresponding to 10% of the accumulated heat release), CA50 and CA90 are marked out at the proximal images (difference less than 0.5 °CA) for each case. It shows that the combustion duration (the interval between CA90 and CA10) of the PODE-10 case is the shortest (17.5 °CA), followed by that of the heptane-10 case (18 °CA) and cetane-10 case (26 °CA). 3.1.2. OH PLIF imaging Figure 7 shows the typical single-shot OH PLIF images in the initial phase and late phase of the RCCI combustion. The corresponding crank angles are marked out in Fig. 5. The natural flame luminosity in the middle phase shows intensive soot radiation, especially for cases of heptane-10 and cetane-10. Even if a small part of the broadband soot radiation passes the bandpass fluorescence filter, it causes severe interference to the OH PLIF signal and

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makes the data in the middle phase unusable. It shows that the single-shot OH PLIF images in the initial phase of RCCI combustion in Fig. 7 correspond well with the natural flame luminosity in Fig. 6. This is because the natural flame luminosity in the initial phase of RCCI combustion mainly comes from the chemiluminescence of species including OH. Similarly, shorter ignition delay in the cetane-10 case makes the combustion regions closer to the injector nozzle, the shape of which can be easily recognized at the positions of the six fuel jets. However, with longer ignition delay, the initial combustion region of heptane-10 gets continuous on the whole around the combustion chamber. Overall, the OH first forms in the high-reactivity region during the initial phase of RCCI combustion. Subsequently, the premixed charge in the low-reactivity region around the central part of the combustion chamber ignites during the middle phase of RCCI combustion, as indicated by the weak OH PLIF intensity in the central part of the combustion chamber at 20 °CA for all three cases. This is consistent with the low blue flame intensity in that area of the NFL images shown in Fig. s2. Finally, in the late phase of RCCI combustion, the OH radical is widespread in the combustion chamber and persists late in the power stroke. This characteristic of wide OH distribution must contribute to the low soot emission in RCCI combustion regime as OH radical plays an important role in soot oxidation. Based on the above comparison, PODE proves to be an ideal direct-injection fuel for the optical diagnostic of the late-injection RCCI strategy. This is because it produces quite less soot radiation in combustion, which favors the observation of the blue flame distribution and development in the second-stage HTHR. Besides, the volume fraction of iso-octane in the port injection is relatively low in this section so that the peak of second-stage HTHR is much lower than that of the first-stage HTHR. This makes the pale blue flame in the second-stage HTHR difficult to observe. So in the next section, the amount of PODE in the direct injection is cut down while the amount of the premixed iso-octane is increased to study the effects of the proportion of the direct injection fuel. 3.2. Effects of direct-injection fuel proportion on the combustion characteristics of the late-injection RCCI strategy Figure 8a shows that the peak cylinder pressure and heat release rate get lower when the fuel proportion in the direct injection (RDI ) decreases from 30% to 6%. Furthermore, the two-stage HTHR feature becomes more remarkable and the combustion duration longer. Note that the ignition delay gets longer with higher φ PI for the cases with mDI =4 mg and 2 mg because of the possible cooling effect of iso-octane evaporation that lowers the temperature of the overall premixed charge. The NFL intensity curves reproduce the heat release features very well. Especially, one can recognize the two-stage HTHR feature indicated by the NFL intensity trend for the cases with RDI of 12% and 6%. Figure 8b shows that the gross IMEP of the PODE-10 case is lower than the cases of RDI =12% and 6%. However, the peak pressure rise rate is more than two times higher than in the other two cases. This proves the great potential of using low fuel proportion for direct injection in reducing the peak PRR at high RCCI load. Figure 9a and b present the NFL images for the cases of RDI =12% and 6%. A background light source illuminates the PODE fuel injection process. The crank angles of “A”, “B” and “C” corresponding to the first peak, the transition between the first and the second peak and the second peak of HTHR are marked out in Fig. 8a and Fig. 9. Take the mDI =2 mg, φ PI =0.75 case in Fig. 9a for example, it shows that the combustion is dominated by the autoignition in the regions of PODE fuel jets that have high fuel reactivity before crank angle “B”. Subsequently, in the non-combustion region around the central part of the combustion chamber, we

Fig. 8. Late-injection RCCI cases with different fuel proportion of direct-injection (RDI ): (a) Cylinder pressure (P), apparent heat release rate (AHRR) and natural flame luminosity (NFL) intensity. (b) Peak pressure rise rate (PRR) and gross IMEP.

can see distinct blue flame development dominated by flame front propagation. By the time of CA50, the relatively week blue flames have occupied the whole combustion chamber. In comparison, for the mDI =4 mg, φ PI =0.75 case, the region dominated by autoignition before crank angle “B” is much larger because the highreactivity region is much wider due to the higher direct-injection fuel proportion and longer ignition delay time. Consequently, the flame front propagation after crank angle “B” is less pronounced but still can be observed. Similarly, the whole low-reactivity region is occupied by blue flames by the time of crank angle “C”. Comparing the NFL images of the PODE-10 case in Fig. 6b that shows no signs of evident flame propagation but auto-ignition in the low reactivity region (Fig. s2 in the supplementary material), we can conclude that the combustion regime in the low reactivity region tends to change from auto-ignition to flame front propagation with the decreasing of the direct-injection fuel proportion. The introduction of the flame front propagation in the second-stage HTHR reduces the peak pressure rise rate in the auto-ignition of the first-stage HTHR. In other words, one can control the heat release rate and peak PRR of RCCI by modulating the ratio between

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Fig. 9. The true-color natural flame luminosity of late-injection RCCI strategies with different proportion of direct injection fuel. The CA10, CA50 and the crank angles “A”, “B” and “C” corresponding to the first peak, the transition between the first and second peak and the second peak of HTHR are marked out.

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Fig. 10. Natural flame emission spectra averaged from ten engine cycles at specified crank angles. Distinct band spectra of OH and CH are marked out with arrows.

the auto-ignition and flame front propagation through tuning the proportion of fuel in the direct injection. 3.3. Natural flame emission spectra and LTHR of the late-injection RCCI strategy Figure 9a shows that the HTHR process is single-stage when there is no iso-octane in the port injection for the mDI =4 mg, φ PI =0 case. However, for the mDI =4 mg, φ PI =0.75 case, it presents a two-stage HTHR, for which the peak of the first-stage ignition is about two times higher than the φ PI =0 case. This indicates that the charge contributing to the first-stage HTHR includes certain iso-octane that is entrained into the PODE jets. The natural flame luminosity, which is mainly from blue chemiluminescence, is very weak for the case of φ PI =0 in Fig. 9c. However, it gets much higher when the φ PI increases to 0.75 presenting more signs of soot radiation, as shown in Fig. 9b. The natural flame emission spectra from 280 nm to 520 nm for the φ PI =0 case in Fig. 10a shows that the chemiluminescence mainly comes from the band spectra of OH, CHO and CH2 O and the continuum spectra of CO oxidation and there are little signs of soot radiation. Note that the spectrometer used in this study

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Fig. 11. Typical single-shot, false-color formaldehyde PLIF images of the lateinjection RCCI cases of mDI = 4 mg, φDI = 0 (a) and mDI = 4 mg, φDI = 0.75 (b). The crank angles are shown in the upper left of each image. White dashed lines with arrows show the laser boundary.

has a limited spectral resolution of 3 nm so that the band spectra peaks of CHO and CH2 O cannot be recognized clearly. Actually, only some peaks of CH2 O around 420 nm at −1 °CA in Fig. 10a can be observed. The high-speed camera is only sensitive to the visible light, so the blue flames captured by the camera come from chemiluminescence of CHO, CH2 O and CO oxidation. The natural flame spectra of φ PI =0.75 case before 3 °CA (crank angle “A”) in Fig. 10b, show higher overall spectra intensity and a distinct band spectra of CH at 431.5 nm, indicating a higher local charge concentration. After 5.1 °CA (about crank angle “B”), there is more soot radiation proved by the rising of the spectral intensity in the wavelength larger than 450 nm. By 15 °CA when the main heat release is coming to an end, the curve only shows band spectra of OH and continuum spectra of soot radiation, which peaks at around 500 nm. Figure 11 presents the single-shot formaldehyde PLIF images. For the φ PI =0 case, formaldehyde appears at −4.0 °CA in the position of the six PODE jets as soon as the fuel injection ends, and the initial formaldehyde parcels are detached from the combustion chamber wall. In the following two images, more formaldehyde is formed, the leading edge of which begins to touch the combustion chamber wall. Ignition kernels, which consume the local formaldehyde, emerge as shown by the arrow “a” at −2.5 °CA.

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More formaldehyde is consumed, showing an obvious wrinkled fractural structure in the residual formaldehyde at −2.0 °CA. There is little formaldehyde distribution in the chamber by −1.5 °CA. In a numerical simulation work of RCCI strategy, Kokjohn et al. [43] demonstrated that very little premixed iso-octane is consumed in the LTHR in which the direct-injection fuel decomposes and formaldehyde forms, and most of the iso-octane is consumed together with formaldehyde in the HTHR. Thus, the formaldehyde PLIF signal in the first-stage of HTHR for the φ PI =0.75 case in Fig. 11b should mainly come from the decomposition of PODE. Compared with that of φ PI =0 case, the initial regions of formaldehyde at −1.0 °CA and 0 °CA are larger, the leading edge of which has already touched the combustion chamber wall. This is because more premixing time results in a wider distribution of PODE in the in-cylinder charge. Consequently, the local concentration of PODE gets lower, as indicated by the lower maximum formaldehyde PLIF intensity. Ignition kernels appear at 0.5 °CA and formaldehyde formed by PODE is gradually consumed by 3.0 °CA. It is interesting to note that there is another formaldehyde formation process at 4.0° and 5.0 °CA around the central part of the combustion chamber where iso-octane resides and fuel reactivity is low. This time corresponds exactly to the transition between the first- and second-stage ignition as shown in Fig. 8a and Fig. 9b, indicating that formaldehyde derived from the decomposition of iso-octane forms before the rising of flame front propagation in the low-reactivity region. As mentioned in Section 3.2, flame front propagation dominates the second-stage HTHR. Consequently, the formaldehyde formed in the low-reactivity region is gradually consumed after 6.0 °CA by the propagating flame front. 3.4. Discussion on the principles of fuel injection strategies for full load RCCI operation We know that combustion stability is the main issue for engine operation at a very low load or cold start condition. Thus, it’s better to run the engine using the high-reactivity fuel only at these conditions. The fuel injection timing should be late so that the formed charge is rich enough for stable combustion. The combustion regime essentially falls into the well-known partially premixed combustion that is dominated by auto-ignition. When it comes to the medium-high engine load, the dual-fuel RCCI with early direct injection strategy should be used to form appropriate fuel reactivity stratification. Normally, the direct injection timing is early than −30 °CA ATDC. A moderate proportion of the direct injection fuel around 20% to 40% by mass is adopted. The HCCI-like, sequential auto-ignition dominates the dual-fuel combustion in RCCI, resulting in a fairly fast single-peak heat release rate with controlled peak pressure rise rate. The maximum RCCI engine efficiency is obtained around this engine load in the literature. For the case of high load operation suffering from excessive pressure rise rate, the fuel proportion in the direct injection should be further reduced to 10% or less. The late injection strategy is employed to increase the fuel reactivity stratification level and ultimately separate the dual-fuel heat release process, resulting in the two-stage HTHR feature. Based on the founding in Section 3.2, the second-stage HTHR is dominated by flame front propagation that normally has a slower heat release rate and thus longer combustion duration compared to that of auto-ignition. This is quite effective in reducing the peak pressure rise rate. From this sense, the lower flammability limit of the premixed charge is the limiting factor for this combustion regime. We can understand why the late-injection strategy is not preferred at medium-high engine load when the overall equivalence ratio is relatively low. There is a minimum premixed equivalence ratio below which the secondstage HTHR would not take place and the UHC emission could be high.

4. Conclusions The combustion characteristics of late-injection RCCI strategy were investigated in a light-duty optical engine using multiple optical diagnostic techniques. Iso-octane served as the premixed fuel in the intake port, and the direct injection timing was set at −10 °CA ATDC. Two key issues were resolved in the present study. One is how the direct-injection fuels of different reactivity and oxygen content affect the RCCI combustion under late directinjection condition. The other is how the direct-injection fuel proportion affects the role that flame front propagation plays in the HTHR of the late-injection RCCI strategy. Firstly, experimental cases with direct-injection fuel of nheptane (heptane-10 case), PODE (PODE-10 case) and cetane (cetane-10 case) were compared. Results indicate that the PODE-10 has the lowest tendency of soot formation and intermediate pressure rise rate compared with that of the heptane-10 and cetane10. Both natural flame luminosity and OH PLIF images show that the initial flame structure of the RCCI combustion is determined by the reactivity of direct injection fuel. The OH radical is widely distributed in the combustion chamber after the HTHR, including the central part of the combustion chamber near the injector nozzle, for all three RCCI cases with different direct-injection fuel types. This feature can explain why RCCI engine has lower soot emission compared to the traditional diesel engine. Secondly, the effects of direct-injection fuel proportion on the combustion characteristics of the late-injection RCCI strategy was evaluated using PODE as the direct-injection fuel. It indicates that the peak heat release rate and PRR decrease greatly when the direct-injection fuel proportion changed from 30% to 6% and the HTHR curve changes from single-stage to two-stage HTHR. The NFL images showed that the combustion regime in the low reactivity region of RCCI tends to change from auto-ignition to flame front propagation with the decreasing of the direct-injection fuel proportion. The introduction of the flame front propagation in the second-stage HTHR reduces the peak pressure rise rate of RCCI. Finally, using a small amount of PODE as the direct injection fuel, the low-temperature heat release (LTHR) characteristics of this two-stage HTHR was analyzed by formaldehyde PLIF imaging. Results indicate that the LTHR takes place in the unburned premixed iso-octane region during the transition between the firstand second-stage HTHR before the flame front propagation dominates. The natural flame spectra show distinct CH (431.5 nm) band spectra in the first-stage HTHR and increasing soot radiation in the second-stage HTHR when the premixed equivalence ratio gets higher. We discussed the principles of fuel injection strategies for full load RCCI operation and concluded that the peak pressure rise rate of RCCI at high engine load can be controlled by modulating the ratio between the auto-ignition and flame front propagation through tuning the proportion of the high-reactivity fuel in the direct injection.

Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC) through its project of 51922076 and 51921004.

Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.combustflame.2019.10. 018.

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