Effect of fuel injection parameters on combustion stability and emissions of a mineral diesel fueled partially premixed charge compression ignition (PCCI) engine

Applied Energy 190 (2017) 658–669

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Effect of fuel injection parameters on combustion stability and emissions of a mineral diesel fueled partially premixed charge compression ignition (PCCI) engine Ayush Jain, Akhilendra Pratap Singh, Avinash Kumar Agarwal ⇑ Engine Research Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India

h i g h l i g h t s
NOx and PM emissions were lowest at 700 bar fuel injection pressure (FIP).
PCCI showed lower knocking than compression ignition combustion mode.
Increasing FIP reduced emissions of nitrogen oxides and smoke opacity in PCCI mode.
Increasing FIP reduced nucleation mode particle concentration.
Increasing FIP with advanced main injection timings improved PCCI combustion.

a r t i c l e

i n f o

Article history: Received 22 August 2016 Received in revised form 29 December 2016 Accepted 30 December 2016

Keywords: Partially premixed charge compression ignition Combustion Heat release rate Exhaust gas recirculation Transmission electron microscopy

a b s t r a c t This experimental study focuses on developing new combustion concept for compression ignition (CI) engines by achieving partially homogeneous charge, leading to low temperature combustion (LTC). Partially premixed charge compression ignition (PCCI) combustion is a single-stage phenomenon, with combustion shifting towards increasingly premixed combustion phase, resulting in lower in-cylinder temperatures. PCCI leads to relatively lower emissions of oxides of nitrogen (NOx) and particulate matter (PM) simultaneously. To investigate combustion, performance and emission characteristics of the PCCI engine, experiments were performed in a mineral diesel fueled single cylinder research engine, which was equipped with flexible fuel injection equipment (FIE). Effects of fuel injection pressure (FIP) were investigated by changing the FIP from 400 bar to 1000 bar. Experiments were carried out by varying start of main injection (SoMI) timings (from 12° to 24° before top dead center (bTDC)), when using single pilot injection. This experimental study included detailed investigations of particulate characteristics such as particulate number-size distribution using engine exhaust particle sizer (EEPS), particulate bound trace metal analysis using inductively coupled plasma-optical emission spectrometer (ICP-OES), and soot morphology using transmission electron microscopy (TEM). PCCI combustion improved with increasing FIP (up to 700 bar) due to superior fuel atomization however further increasing FIP deteriorated PCCI combustion and engine performance due to intense knocking. NOx and PM emissions were also found to be lowest at 700 bar FIP. Moreover, count mean diameter (CMD) of particulate was maximum at 700 bar FIP. Therefore, medium FIP (700 bar) was the most suitable FIP for PCCI combustion. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction In the past few decades, the evolution and development of sophisticated technologies has enabled conventional energy resources to be extracted at an unexpectedly high rate. Therefore rate of depletion of fossil fuels has gone up exponentially. These

⇑ Corresponding author. E-mail address: [email protected] (A.K. Agarwal). http://dx.doi.org/10.1016/j.apenergy.2016.12.164 0306-2619/Ó 2016 Elsevier Ltd. All rights reserved.

non-renewable fossil fuels including mineral diesel, gasoline, natural gas, etc. are being consumed predominantly in automotive sector and their exhaust gas emissions are a prominent source of anthropogenic pollution in the ambient environment. Diesel engines are widely used in light-duty as well as heavy-duty vehicles, because of their higher brake thermal efficiency (BTE) compared to their gasoline counterparts. Other important features of diesel engines such as higher durability, higher reliability, lower pumping and throttling losses and superior fuel economy give them an edge

A. Jain et al. / Applied Energy 190 (2017) 658–669

over the gasoline engines. However, diesel engines suffer from the issues of relatively higher emission of particulate matter (PM) and oxides of nitrogen (NOx). Due to stringent emission norms being adopted globally, it has become top priority for the researchers and OEMs to control PM and NOx from diesel engines, either by using after-treatment devices or by using cleaner alternative fuels. Several after-treatment devices such as diesel oxidation catalysts (DOCs), diesel particulate filters (DPFs) and NOx traps are being used in diesel engines to reduce PM and NOx emissions. However, use of these devices has been limited due to their high initial cost and high operating costs. Alternative fuels have been investigated by several researchers and each alternative fuel is found to have its pros and cons. To comply with stricter emission legislations, a new combustion strategy, referred as homogeneous charge compression ignition (HCCI) is being investigated by researchers [1–3]. HCCI research was initiated by Onishi et al. [4] in 1979. These researchers first successfully applied HCCI concept to a twostroke gasoline engine. After successfully achieving gasoline HCCI combustion, their focus shifted towards development of diesel HCCI combustion. In 1990s, there was tremendous interest among researchers for developing understanding of HCCI combustion mechanism over a wide range of engine operating conditions. The underlying principle of HCCI combustion was in forming a homogeneous charge, which would not exhibit diffusion flames upon ignition. HCCI methodology was highly effective in bringing down PM and NOx emissions simultaneously however it suffered from the problem of lack of control over combustion phasing [5]. This led to development of a newer combustion concept i.e. ‘‘partially premixed charge compression ignition” (PCCI) combustion. This concept of modifying and altering the in-cylinder parameters was considered to be a more practical and economical method compared to HCCI combustion. PCCI combustion is an advanced combustion mode, in which premixed combustion dominates to a significantly greater degree compared to mixing controlled combustion. Particulate and NOx emissions from PCCI combustion are relatively slightly higher compared to HCCI combustion mode however PCCI still results in significantly lower particulate and NOx emissions compared to conventional CI combustion mode. Due to lower volatility of mineral diesel, diesel fueled PCCI combustion investigations focused on high pressure direct fuel injection strategies using exhaust gas recirculation (EGR) [6]. Effect of earlier start of injection (SoI) timing of mineral diesel fueled PCCI engine was studied by Kanda et al. [6]. They reported that optimized SoI timings together with retarded inlet valve closure (IVC) timings resulted in significant reduction in NOx emissions due to low temperature combustion (LTC). Jia et al. [7] investigated the effects of IVC timings and SoI timings on combustion and emission characteristics of a diesel fueled PCCI engine and verified the observations of Kanda et al. [6]. However they observed slight reduction in indicated specific fuel consumption (ISFC) with advancing start of main injection (SoMI) timings. Laguitton et al. [8] and Araki et al. [9] explored the effect of variation of compression ratio on low temperature diesel combustion. They reported that the operating range of PCCI diesel engine can be shifted from lower load range to higher load range by lowering the compression ratio. They observed significantly lower NOx and soot emissions compared to conventional diesel combustion however carbon monoxide (CO) and hydrocarbon (HC) emissions were relatively higher. Some researchers including Horibe et al. [10] focused on optimization of PCCI combustion using split injection. First, they performed PCCI experiments using advanced main injection (without pilot injection) and found lower NOx and PM emissions with higher BTE, however higher rate of pressure rise (RoPR) reduced the operating range of PCCI combustion. They resolved this issue by using split injection strategy. In their study, the experiments were carried out using different pilot injection

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strategies. The effects of FIP were studied on conventional as well as LTC methodologies by Kiplimo et al. [11] and Kokjohn and Reitz [12]. It was observed that increased FIP resulted in significantly higher in-cylinder pressure and improved BTE. Researchers [10–12] reported that higher FIP and enhanced swirl ratios improved fuel-air mixing, however, sufficient premixing time between end of injection and start of combustion (SoC) was necessary for efficient PCCI combustion. NOx emissions were relatively lower for lower FIPs whereas CO and HC emissions didn’t show any significant difference. Musculus et al. [13] summarized different aspects of PCCI combustion control techniques and described the importance of fuel injection parameters on spray characteristics, which directly influenced the combustion characteristics of PCCI combustion. The effects of SoMI timings, EGR, etc. were investigated to adequately control the combustion phasing of LTC [14]. High EGR rate was used to achieve a longer ignition delay and reduce combustion temperature, which led to lower soot and NOx emissions. Manente et al. [14] examined the physical and chemical effects of EGR on engine-out emissions in a PCCI engine. They reported that increasing EGR didn’t affect engine efficiency, however, CO and HC emissions decreased significantly. Higher EGR along with advanced SoMI timings were very effective in lowering emissions from diesel engines, and complying with stringent emission norms. However, the perception of negligible PM emissions from LTC was contradicted by Price et al. [15]. They reported that although PCCI combustion produced lesser PM compared to conventional CI combustion. PM from PCCI combustion mostly comprised of accumulation mode particles (AMPs) therefore they cannot be considered negligible. This was further reiterated by Kittleson and Franklin [16]. Desantes et al. [17] investigated the effect of boost pressure on particle number-size distribution in PCCI combustion mode. They were able to comply with EURO-VI emission norms for NOx and PM but could not significantly reduce CO and HC emissions. Moreover increasing boost pressure led to reduction in PM mass and number emissions. This was primarily due to reduced fuel deposition on the piston bowl surface and increased oxidation of particulate in later stages of combustion. Diesel engines emit trace metals such as phosphorus, calcium, zinc and silicon [18]. Springer [19] discovered that calcium was the most dominant trace metal in the particulate with concentrations as high as 0.29% (w/wparticulate). Phosphorus, zinc and silica were in relatively lower concentrations in diesel particulate, followed by small traces of sodium, barium, chromium and copper. Application of EGR also increased the trace metal concentration in PCCI combustion. Singh et al. [20] suggested that CI combustion in presence of EGR contributed significantly to lubricating oil degradation and trace metal emissions in particulate. Detailed analysis of trace metals in exhaust particulate using ICP-OES was done by Agarwal et al. [21]. They also reported significant trace metals in HCCI combustion generated particulate. These trace metals were in relatively lower concentration than that from conventional CI combustion generated particles. These trace metals from diesel particulates could possibly deposit in lower airways of human respiration system [22]. These trace metals generate hydrogen radicals and then trigger reactive oxygen species (ROS) generation, which can potentially cause acute and chronic lung injuries. To get better insights into particulate structure, morphological studies of particulate emitted from HCCI/PCCI combustion were carried out using scanning electron microscopy (SEM), and transmission electron microscopy (TEM) [21,23]. Agarwal et al. [21] carried out morphological studies of diesel fueled HCCI combustion particulate emitted at different engine loads and EGR rates. They reported that particulate mass deposited on the quartz filter paper reduced with decreasing engine load and increased with increasing EGR rate. Sun et al. [23] investigated the effect of fuel injection timings on particulate structure. They concluded that soot produced from late injection

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PCCI combustion generated larger primary soot particles. On the other hand, soot produced from early injection PCCI combustion generated smaller primary soot particles. The review of these studies gives an insight into investigations carried out by various researchers on LTC. However all above mentioned studies focused on some specific objectives and were not able to discuss different combined aspects of PCCI combustion. These studies didn’t explain the effect of fuel injection parameters, in order to develop a practically viable PCCI combustion engine. Therefore it is necessary to understand the effect of fuel injection parameters on PCCI combustion, performance and emission characteristics. This study aims to explore combined effects of different fuel injection parameters and EGR on PCCI combustion. Detailed investigations of particulate characteristics using different fuel injection strategies is another important aspect covered in this experimental study. This study deals with investigation of PCCI combustion by optimizing fuel injection parameters such as FIP and SoMI timings in an advanced single cylinder research engine.

2. Experimental setup and methodology To investigate the effect of fuel injection parameters on PCCI combustion, experiments were performed in a single cylinder research engine (AVL, 5402). An AC dynamometer (Wittur Electric Drives, 2SB 3) was coupled to the test engine in order to control its load and speed. The schematic of the experimental setup is shown in Fig. 1. This engine was equipped with lubricating oil and coolant conditioning systems so that the experiments were conducted under controlled conditions. During the experiment, lubricating oil and coolant temperatures were maintained at 90°C and 60°C respectively. Fuel temperature at the inlet to the high pressure common rail direct injection (CRDI) fuel pump was maintained at 25°C by employing a fuel conditioning system and chiller unit. INCA based engine management system (EMS) was used for controlling the FIP, start of pilot injection (SoPI) and SoMI timings. EMS has flexibility for user defined control of vital fuel injection parameters. Injector

solenoid opening current during fuel injection process was measured for detecting the SoPI timing, SoMI timing and injection duration. A piezoelectric pressure transducer (AVL; 6013) was mounted flush onto the cylinder head, which acquired the in-cylinder pressure data. Crank angle position was measured by an angle encoder (AVL; 365). This encoder had a resolution of 0.1 crank angle degree (CAD). For combustion chamber data acquisition and analysis, a high speed combustion data acquisition system (AVL; Indi-micro) was employed. In-cylinder pressure signals and crank angle signals were acquired as input by this data acquisition system. Test engine specifications are given in Table 1. To control the heat release rate (HRR) in PCCI combustion, fraction of exhaust gas was mixed with fresh intake air. Quantity of recirculated exhaust gas was controlled by two flow valves fitted in the EGR line. State-of-the-art inlet air measurement system (ABB Automation Products, Sensy-flow P) and Coriolis force based fuel flow-meter (AVL, 733S.18) were used for respective measurements and calculation of the fuel-air ratio. For PCCI engine performance analysis, brake specific fuel consumption (BSFC) and BTE were calculated using measured data of fuel consumption rate, torque and engine speed. For measurement of gaseous emissions, part of the exhaust gas was sampled through the exhaust gas emission analyzer (Horiba; MEXA 584L), which detected concentrations of gaseous species in the exhaust gas such as NO, HC, CO, and CO2. Brake specific mass emissions were calculated from measured species concentrations in the exhaust, intake air flow rate, fuel flow rate and engine power output [24]. Particulate were characterized by engine exhaust particle sizerTM (EEPS) spectrometer (TSI Inc., 3090), which was capable of measuring the particle sizes ranging from 5.6 to 560 nm with a concentration up to 108 #/cc of exhaust. EEPS performs the particle size classification based on differential electrical mobility of particulate of different sizes. For particulate bound trace metal analysis and particulate morphology, particulate sampling was carried out using a custom-built partial flow dilution tunnel. Particulate from the exhaust gas were sampled on quartz filter papers. These particulate laden filters were used for particulate bound trace metal detection and particulate

Fig. 1. Schematic of the experimental setup.

A. Jain et al. / Applied Energy 190 (2017) 658–669 Table 1 Details technical specifications of test engine. Engine parameters

Specifications

Engine make/model Number of cylinder/s Cylinder bore/stroke Swept volume Compression ratio Inlet ports Maximum power Rated speed High pressure system

AVL/5402 1 85 mm/90 mm 510.7 cc 17.5 Tangential & swirl ports 6.25 kW 4200 rpm Common rail direct injection BOSCH CP4.1 AVL-RPEMS + BOSCH ETK7 4 (2 inlet, 2 exhaust) DOHC cam follower Wet

Engine management system Valves per cylinder Valve train type Liner type/base

morphological investigations. Trace metal analysis was carried out using inductively coupled plasma-optical emission spectrometer (ICP-OES) (Thermo Fischer Scientific, iCAP DUO 6300). Morphological characteristics of particulate emitted from different fuel injection strategies were examined using transmission electron microscopy (TEM) (FEI, Tecnai G2 12 Twin TEM 120 kV). This TEM was capable of performing high contrast cryogenic microscopy. The particulate laden quartz filters were cut into pieces and then these particulate were suspended in benzene. Soot particles got detached from the filter paper and got transferred to benzene. The benzene drops containing particulate were then deposited on the TEM copper grid (300 mesh) and the solvent (Benzene) was allowed to evaporate, leaving the particulate on the grid for TEM analysis.

3. Results and discussion In this study, mineral diesel fueled PCCI experiments were performed at different SoMI timing and FIPs. Important properties of mineral diesel are given in Table 2. During the experiments, SoMI timings were selected in such a way that combustion could be achieved in both CI and PCCI combustion modes. This was done by varying SoMI timings (from 12° to 24° bTDC) since for advanced SoMI timings, premixed combustion phase dominated and resulted in PCCI combustion. To investigate the effect of FIP on PCCI combustion, experiments were performed at three different FIPs: 400, 700 and 1000 bar using constant SoPI timing (35° bTDC) and EGR (15%). For selecting SoPI timing and EGR, the experiments were carried out at different SoPI timings (30, 35 and 40° bTDC), in which 35° bTDC SoPI timing was found suitable for PCCI combustion. At advanced SoPI timing (30° bTDC), relatively lesser time availability resulted in slightly inferior in-cylinder conditioning of the fuel injected in the main injection, which led to relatively inferior combustion. At retarded SoPI timings (40° bTDC), slightly inferior in-cylinder conditions (temperature and pressure) resulted in poor fuel atomization, which affected fuel-air mixing. Similarly, the experiments were also carried out up to 30% EGR and it was found that PCCI combustion resulted in higher HC and CO emissions at higher EGR rate. To achieve a trade-off between higher

Table 2 Properties of mineral diesel. Property

Mineral diesel

Calorific value (MJ/kg) Density (g/cm3) @ 30 °C Viscosity (cSt) @ 40 °C Flash point (°C) (min)

43.54 0.831 2.82 54

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HRR and lower in-cylinder temperature, PCCI experiments were performed at 15% EGR. For selection of optimum FIP and SoMI timing, analysis was carried out in the following sub-sections namely combustion characteristics, performance and emission characteristics, particulate characteristics, particulate bound trace metals and particulate morphology. 3.1. Combustion characteristics Cylinder pressure – crank angle analysis is the most effective method to analyze engine combustion because in-cylinder pressure history directly influences the combustion characteristics, power output, and emission characteristics. In this study, cylinder pressure data was acquired w.r.t. crank angle position using high speed combustion data acquisition system. For eliminating cyclic combustion variability, average data set of 250 engine cycles was used to calculate various combustion parameters such as HRR, SoC, combustion phasing, combustion duration and knocking. HRR was calculated from the acquired cylinder pressure data using ‘‘zero dimensional heat release model” [25]. Fig. 2 shows variations in in-cylinder pressure and HRR at different crank angle positions for different SoMI timings and varying FIPs. Separation of in-cylinder pressure curve from the motoring curve (in-cylinder pressure curve without combustion) marks SoC. It is quite evident from in-cylinder pressure and HRR curves that SoC advanced with advancing SoMI timings. This was mainly due to availability of longer time for fuel-air mixing, which promoted relatively earlier SoC. Peak in-cylinder pressure and maximum HRR increased with advancing SoMI timings. Availability of longer time increased the premixed fuel-air mixture quantity, which led to dominant premixed phase combustion (fast combustion) compared to diffusion phase combustion (slow combustion). Due to dominant premixed combustion phase, peak of in-cylinder pressure and HRR increased resulting in relatively smoother combustion compared to retarded SoMI timings. HRR curves showed that SoC advanced with increasing FIP. At higher FIPs, improved fuel atomization resulted in finer fuel spray, which evaporated quickly and resulted in superior fuel-air mixing at a faster rate, compared to lower FIPs. Peak in-cylinder pressure and maximum HRR increased with increasing FIP. This was mainly due to improved fuel-air mixing at higher FIPs, which resulted in superior combustion. With increasing FIP, knocking was observed in the cylinder pressure and HRR curves. Retarded SoMI timings resulted in relatively higher knocking compared to advanced SoMI timings. Significantly higher RoPR (steeper in-cylinder pressure curves) at retarded SoMI timings were the main reason for this. Diffusion combustion phase was observed in HRR curves of retarded SoMI timings at lower FIP (400 bar). Variations in width of HRR curves also contain important information about the PCCI combustion. Advancing SoMI timings resulted in relatively narrower HRR curves, which depicted that main combustion duration (premixed combustion duration) decreased with advancing SoMI timings. At 1000 bar FIP, HRR curves at different SoMI timings showed slightly irregular behavior due to excessive knocking. Fig. 3 shows the variations in SoC, combustion phasing and combustion duration at different SoMI timings and FIPs. Mass fraction burned (MFB) is calculated using Rassweiler and Withrow method [26], which is a well-established method for estimating MFB. It was calculated as,

ðDpc Þ ¼ pi  pi1


c V i1 Vi

where Dpc is the increase in in-cylinder pressure due to combustion and c is the polytropic exponent.

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15% EGR SoPI= 35 CAD bTDC Motoring FIP= 400 Bar

100 80

40

260 195

SoMI= 16 bTDC SoMI= 20 bTDC SoMI= 24 bTDC

130

20 100

FIP= 700 Bar

80 60 40 20 100

FIP= 1000 Bar

Heat release rate (J/CAD)

60

In-Cylinder Pressure (Bar)

325

SoMI= 12 bTDC SoMI= 16 bTDC SoMI= 20 bTDC SoMI= 24 bTDC

15% EGR SoPI= 35 CAD bTDC SoMI= 12 bTDC FIP= 400 Bar

65 0 325

FIP= 700 Bar

260 195 130 65 0 325

80

260

60

195

40

130

FIP= 1000 Bar

65

20

0

0 -60

-40

-20

0

20

40

60

-20

0

20

CAD (aTDC)

CAD (aTDC)

Fig. 2. In-cylinder pressure and HRR variations w.r.t. crank angle at different FIPs.

Combustion Phasing (CAD)

Start of Combustion (CAD)

MFB can now be computed as,

15% EGR SoPI= 35 CAD bTDC FIP= 400 Bar FIP= 700 Bar FIP= 1000 Bar

0

-4

-8

0

-4

-8

Combustion Duration (CAD)

25 20 15 10 5 12

16

20

24

SoMI (CAD bTDC) Fig. 3. SoC, combustion phasing and combustion duration w.r.t. SoMI timings at varying FIPs.

Pi mbðiÞ j¼0 Dðpc Þj ¼ PN mbðtotalÞ j¼0 Dðpc Þj Here it is assumed that sample 0 is between inlet valve closing and the start of combustion, and sample N is after the combustion is completed. Crank angle position corresponding to 10% cumulative heat release (CHR) (CA10) was considered as SoC. SoC is mainly controlled by chemical kinetics of the fuel-air mixture, which depends on the pressure-temperature history of the combustion chamber. Results showed that advanced SoMI timing resulted in advancing SoC. Advancing SoMI timings formed premixed fuel-air mixture relatively earlier compared to retarded SoMI timings and resulted in advanced SoC. Advanced SoMI timings also resulted in slightly longer ignition delay. At advanced SoMI timings, slightly milder combustion chamber conditions (cylinder pressure and temperature) affected the fuel-air mixing, which led to relatively longer ignition delay compared to retarded SoMI timings. At all FIPs, SoC advanced to 7–9° bTDC when the SoMI timings were varied from 12° bTDC to 24° bTDC. SoC also advanced with increasing FIP. At higher FIPs, improved fuel spray atomization characteristics were mainly responsible for this trend. Rate of advancement in SoC was relatively higher at 1000 bar FIP compared to lower FIPs however no such noticeable difference in SoC timing were observed at 400 and 700 bar FIPs. This observation was similar to the findings of previous studies [27,28]. Crank angle position corresponding to 50% CHR (CA50) is termed as combustion phasing. Combustion phasing is a measure of overall combustion during an engine cycle. Results showed that combustion phasing advanced with advancing SoMI timings. Combustion phasing followed a similar pattern as that of SoC however combustion phasing was found to be slightly more sensitive to variation in SoMI timings compared to SoC. Similar to SoC, combustion phasing also advanced with increasing FIP. Combustion duration was found to be a strong function of SoMI

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timing and FIP. At all FIPs, combustion duration decreased with advancing SoMI timings. Advancing SoMI timings promoted premixed combustion (Fig. 2) therefore most fuel burned in premixed combustion phase rapidly, resulting in shorter combustion duration. Effect of improved fuel-atomization was also observed in combustion duration variation as combustion duration decreased with increasing FIP. However, an interesting behavior was observed at 12° bTDC SoMI timing, where combustion duration at 1000 bar FIP was higher than that at 700 bar. This might be due to spray impingement that took place when the spray plume strikes the piston bowl edges rather than its inner surfaces. This affected the fuelair mixing because after impingement, fraction of fuel spray burnt relatively later, resulting in slightly longer combustion duration [29]. Fig. 4 represents variations in knock integral, knock peak and cylinder noise w.r.t. SoMI timings at varying FIPs. Knock and noise analyses were performed for determining knock integral and knock peak. Knock integral is integral of superimposed rectified knock oscillations and knock peak is the absolute maxima of rectified knock oscillations superimposed on the cylinder pressure curve. For determining these parameters, cylinder pressure signal was filtered using a high pass filter and then rectified. Parameters such as integral or peak values of superimposed oscillations were determined from measured signals. Cylinder noise was also calculated from the measured cylinder pressure signals [30]. Fig. 4 shows that knock integral was significantly lower at all FIPs. With advanced SoMI timings, knock integral slightly reduced however relative magnitudes of knock integral were almost constant. This was mainly due to presence of pilot injection and

EGR, which led to smooth combustion. Variations in knock peak and cylinder noise with SoMI timings were negligible. However both these parameters were strong functions of FIP. A monotonous increase in knock peak and combustion noise was observed with increasing FIP. Knocking occurred mainly due to late shockwaves inside the combustion chamber, which produced noise. Increasing FIP resulted in higher RoPR (slope of in-cylinder pressure curves), which increased the knock peak. At higher FIP (1000 bar), knock peak reached up to 8 bar however at 400 bar FIP, knock was 2 bar. Cylinder noise increased with increasing FIP as well. At all FIPs, cylinder noise increased slightly with advanced SoMI timings. This was mainly due to dominance of premixed combustion phase, which led to higher HRR peak. At higher FIPs, effect of fuel spray impingement became more severe at advanced SoMI timings which resulted in relatively higher cylinder noise. At 1000 bar FIP, maximum cylinder noise reached 98 dB, which was way above human comfort level.

3.2. Performance and emission characteristics PCCI performance characteristics namely BTE, BSFC and exhaust gas temperature (EGT) were investigated at varying FIPs using constant SoPI timing (35° bTDC) and EGR (15% v/v). Fig. 5 shows that at all FIPs, BTE decreased with advancing SoMI timings. This was primarily due to dominant premixed combustion

15% EGR SoPI= 35 CAD bTDC 50

15% EGR SoPI= 35 CAD bTDC

6

BTE (%)

Knock Integral

40

FIP= 400 Bar FIP= 700 Bar FIP= 1000Bar

8

30 20

4

10 2

0.5

0

0.4

12

BSFC (kg/kWh)

Knock Peak (Bar)

FIP= 400 Bar FIP= 700 Bar FIP= 1000 Bar

8

4

0.3 0.2 0.1

0 100

280

95

EGT (oC)

Cylinder Noise (dB)

0.0

90 85

210 140 70

80 12

16

20

24

SoMI (CAD bTDC)

0 12

16

20

24

SoMI (CAD bTDC) Fig. 4. Knock integral, knock peak and cylinder noise variations w.r.t. SoMI timings at varying FIPs.

Fig. 5. BTE, BSFC and EGT variations w.r.t. SoMI timings for varying FIP.

A. Jain et al. / Applied Energy 190 (2017) 658–669

15% EGR SoPI= 35 CAD bTDC 25 FIP= 400 Bar FIP= 700 Bar

CO (g/kWh)

20

FIP= 1000 Bar 15 10

HC (g/kWh)

5

1.5

1.0

0.5

25

NOx (g/kWh)

phase at advanced SoMI timings. At advanced SoMI timings, relatively earlier SoC increased the negative piston work, which led to lower shaft power, resulting in lower BTE. This can also be observed in HRR curves that most heat was released before TDC. Increasing FIP also reduced BTE. This behavior was mainly controlled by spray characteristics. At higher FIPs, improved fuel atomization led to earlier SoC, which increased the negative piston work and reduced the BTE. Increased spray tip penetration at higher FIP was also responsible for lower BTE because spray impingement in the piston bowl resulted in excessive knocking. BSFC increased with advancing SoMI timings. BSFC followed inverse trend of BTE. With advancing SoMI timings, BSFC increased and 24° bTDC SoMI timing exhibited the maximum BSFC. BSFC can be also correlated with combustion phasing. BSFC increased at advanced combustion phasing, therefore 1000 bar FIP at 24° bTDC SoMI timing resulted in maximum BSFC. EGT was measured as a qualitative parameter, which contains information about the incylinder combustion behavior. EGT is mainly controlled by the injected fuel quantity and relative dominance of different combustion phases. In the present study, injected fuel quantity was constant therefore EGT mainly varied due to combustion behavior. PCCI combustion exhibited significantly lower EGT compared to conventional CI combustion [31]. Dominance of premixed combustion phase was the primary reason for this trend. Premixed combustion was further promoted by advanced SoMI timings and higher FIPs, which reduced the EGT slightly. Due to dominance of diffusion combustion phase, 400 bar FIP resulted in highest EGT (230 °C). It can be observed that increasing FIP resulted in slightly inferior performance parameters. This was attributed to relatively lower in-cylinder temperatures and dominance of premixed combustion phase, which improved emission characteristics of PCCI combustion. Fig. 6 shows the emission characteristics for CO, HC and NOx w. r.t. varying FIPs and SoMI timings. CO forms mainly due to incomplete oxidation of CO into CO2 and is attributed to two factors: lower in-cylinder temperatures, which prevent oxidation reactions; and lack of oxygen in the reaction zone. In PCCI combustion, lower in-cylinder temperature was the main reason leading to relatively higher CO formation. Results showed that variations in CO and HC emissions with SoMI timings were marginal. However for all SoMI timings, lowest CO emission were observed at 400 bar and highest CO emissions were obtained at 700 bar FIP. This was because of highest in-cylinder temperatures at 400 bar FIP, which promoted oxidation reactions for conversion of CO to CO2. At higher FIPs (700 and 1000 bar), improved fuel atomization resulted in homogeneous fuel-air mixing. Combustion of homogeneous fuel-air mixture resulted in lower in-cylinder temperatures, which prevented oxidation of CO to CO2. HC emissions increased with increasing FIP. This might be due to improper combustion as a result of fuel impingement in the piston bowl. Similar to CO, presence of low temperature caused flame quenching near the cylinder walls, resulting in higher HC emissions. NOx formation is very sensitive to peak in-cylinder temperature, which depends on combustion phasing. Retarded combustion phasing led to diffusion combustion, which resulted in higher peak in-cylinder temperature. In PCCI combustion, heat release occurred mainly in premixed combustion phase therefore in-cylinder temperature was significantly lower than conventional CI combustion. This is the main reason for ultra-low NOx emissions from PCCI combustion. Results showed that NOx emissions increased slightly with advancement in SoMI timings. Advancing SoMI timings resulted in superior combustion, which increased the in-cylinder temperature and subsequently the NOx formation. Highest NOx emissions were produced at 400 bar FIP. This was due to dominant diffusion combustion phase, as explained earlier. Due to the lowest peak in-

20 15 10 5 20

Smoke Opacity (%)

664

16 12 8 4 0 12

16

20

24

SoMI (CAD bTDC) Fig. 6. Brake specific CO, HC, and NOx emissions and smoke opacity variations w.r.t. SoMI timings at varying FIPs.

cylinder temperature at 700 bar FIP, NOx emissions were found to be lowest with 700 bar FIP in PCCI combustion mode. Smoke opacity emerged to be a strong function of FIP and SoMI timings. It decreased with advancing SoMI timing and increasing FIP. Advanced SoMI timings provided sufficient time for fuel-air mixing, resulting in more homogeneous fuel-air mixture. PCCI combustion in absence of fuel-rich and fuel-lean zones led to lower soot formation, thus resulting in lower smoke opacity. With increasing FIP, improved fuel atomization resulted in finer fuel spray droplets, which mixed easily with air and promoted homogeneous fuel-air mixture formation. At 1000 bar FIP, smoke opacity slightly increased compared to 700 bar FIP. This was due to dominance of certain spray characteristics (spray tip penetration and spray impingement), which hampered homogeneous fuel-air mixing, resulting in slightly higher smoke opacity.

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Comparison of emission characteristics showed that NOx emission and smoke opacity first reduced with increasing FIP (up to 700 bar) and then increased with further increase in FIP. However, HC and CO emissions increased continuously with increasing FIP. Therefore 700 bar FIP was chosen as the most suitable FIP for PCCI combustion. 3.3. Particulate characteristics Particulate formation in the engine combustion chamber is mainly affected by fuel-air mixing, which can be controlled by two parameters namely (i) time available for fuel-air mixing and (ii) fuel atomization characteristics. Variations in SoMI timings and FIP directly influence fuel-air mixing in the combustion chamber. Therefore particulate emitted from PCCI combustion were characterized at three different FIPs and four SoMI timings. Particulate characteristics were split into three categories namely particulate number-size distribution, particulate surface area-size distribution, and statistical analysis of particulate number-size distribution. Fig. 7 shows that particulate numbers decreased with advancing SoMI timings. Advancing SoMI timings provided longer time for fuel-air mixing thereby hampering the formation of particulate. Particulate number-size distribution also showed that particle size decreased with advancing SoMI timing. Higher FIP significantly affected particulate numbers as well as size in PCCI combustion. Fuel-air mixing improved at higher FIPs, therefore higher FIP reduced particulate number concentration. At higher FIPs, improved fuel-air mixing also contributed to lower emission of polycyclic aromatic hydrocarbons (PAHs) and other organic species in addition to smaller particulate. Maximum particulate concentration (8  107 particles/cm3 of exhaust gas) was observed at 400 bar FIP and minimum particle concentration (3  107 particles/cm3 of exhaust gas) was observed at 1000 bar FIP. Fig. 7 showed that PCCI combustion resulted in significantly lesser par-

ticulate number emissions compared to conventional CI combustion [32]. Particulate toxicity is directly related to particulate surface area. Higher surface area of particulate shows higher adsorption of toxic hydrocarbon species and condensation of PAHs on the particulate surface, making them more toxic. Moreover smaller particles have higher retention time in the atmosphere and deeper lung penetration compared to larger particles, which makes smaller particles more hazardous to the human health. In this study, particulate surface area-size distribution decreased with increasing FIP. This trend showed that increasing FIP reduced the particle surface area therefore its capacity to carry toxic species also reduced. These toxic species get adsorbed on the particulate surface, leading to adverse health impacts. Particle surface area-size distribution trend showed that the contribution of bigger particles (80– 125 nm) in the total particulate surface area was significantly higher compared to smaller particles. These trends were also similar to observations made by Agarwal et al. [32] in another study. Fig. 8 shows statistical analysis of particulate emitted in PCCI combustion at different FIPs and SoMI timings. Statistical analysis included four parameters namely: nucleation mode particles (NMP) (Dp < 50 nm), AMP (50 nm < Dp < 1000 nm), total particulate numbers (TPN) and count mean diameter (CMD) of particulate. At 400 and 1000 bar FIPs, NMP concentration remained almost constant at all SoMI timings however at 700 bar FIP, NMP concentration decreased with advancing SoMI timings. This showed the effectiveness of 700 bar FIP. Reduction in NMP concentration was attributed to trade-off between in-cylinder conditions and fuel spray characteristics. With increasing FIP, NMP concentration decreased (up to 700 bar FIP) however at 1000 bar FIP, NMP concentration increased due to spray impingement inside the piston bowl, which resulted in formation of fuel rich zones inside the combustion chamber, leading to higher particulate formation. AMP concentration decreased with advancing SoMI timings. At advanced SoMI timings, improved fuel-air mixing was the main

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3.4. Particulate bound trace metals

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factor for this trend. Increasing FIP also reduced AMP concentration. Maximum AMP concentration (4  108–6  108 particles/ cm3 of exhaust gas) was found at 400 bar FIP however 700 bar and 1000 bar FIP showed almost similar AMP concentration (3  108 particles/cm3 of exhaust gas). TPN concentration (Fig. 8) clearly shows the ineffectiveness of PCCI combustion at lower FIP. With increasing FIP, TPN concentration from PCCI combustion reduced significantly (from 400 bar to 700 bar) however 1000 bar FIP showed relatively higher TPN concentration compared to 700 bar FIP. This depicted another important observation for PCCI combustion. At 1000 bar FIP, PCCI combustion in presence of EGR resulted in formation of higher numbers of NMP compared to 700 bar FIP, which increased the TPN concentration. Similar to previous cases, for all FIPs, TPN concentration decreased for advanced SoMI timings. CMD results showed that average size of particles emitted at 1000 bar FIP were much smaller compared lower FIPs. At 1000 bar FIP, lower TPN concentration and dominance of NMP were the main reason for lower CMD. CMD increased for advanced SoMI timings (at 700 bar FIP) however it was almost constant for 400 bar and 1000 bar FIPs. Statistical analysis of particulate number-size distribution results also showed that 700 bar FIP was better compared to 1000 bar FIP. Lesser TPN compared to 400 bar and relatively bigger CMD compared to 1000 bar FIP made them relatively less harmful to the human health as well.

For detailed analysis, particulate samples were analyzed for trace metals. The experiments were carried out in two steps. In the first step, effect of SoMI timings was investigated by analyzing three particulate samples collected at 12, 16 and 20° bTDC SoMI timings (Fig. 9a). In the second step, effect of FIP was investigated by analyzing three particulate samples collected at 400, 700 and 1000 bar FIPs (Fig. 9b). During these experiments, all other parameters such as SoPI timing, EGR rate, etc. were maintained constant. Based on the source, harmful effects and relative concentration, all trace metals were divided into five groups. First group contained Al, Cu, Fe and Zn. These trace metals are harmful for human health due to their ROS generation potential, which promotes cancer. These trace metals originate from debris of engine components generated due to friction and wear and they eventually find their way into the engine exhaust. Results showed that the concentrations of these trace metals decreased with increasing FIP. Advancing SoMI timing did not show any regular pattern in the variation of concentration of these trace metals. With advanced SoMI timing, Al decreased however Cu, Fe and Zn increased slightly. Second group of trace metals contained Ca, K, Na and Mg. These trace metals originate from the pyrolysis of lubricating oil. Although the concentrations of these trace metals were slightly higher, they do not affect human health significantly. Results showed that concentration of these trace metals were slightly higher at 400 bar FIP compared to higher FIPs. This was attributed to higher peak cylinder temperature, which promoted pyrolysis of lubricating oil that resulted in relatively higher concentration of these trace metals in the exhaust particulate. With advanced SoMI timings, concentrations of Ca, Na and Mg decreased slightly, which also supported previous observations. Third group of trace metals contain Ni, Cr, Cd and As. Main sources of these metals are metallic engine components and lubricating oil additives. These trace metals can easily combine with other species and form highly toxic compounds. Results showed the effect of lower in-cylinder temperature on PCCI combustion, which reduced the pyrolysis of lubricating oil. These metals enter exhaust from engine wear and lubricating oil. Lower concentrations of Ni, Cd and As also showed lower in-cylinder temperature during PCCI combustion. Fourth group of trace metals contain Pb, Mo, Sr and Ba. These trace metals originate from the fuel, lubricating oil and sometimes from wear of engine components such as gaskets, piston rings etc. Pb, Mo, Sr and Ba are also harmful for human health. These trace metals did not show any specific trend of variations. However, Pb, Sr and Ba were minimum at 700 bar FIP. Last group of trace metals include Mn, Bi, In and V. Mn, Bi, In and V are generally used in engine components to enhance their properties. Therefore wear of engine components was the main source of these trace metals. Concentration of these trace metals were found to be very low, which slightly increased with increasing FIP. 3.5. Particulate morphology To investigate the effect of FIP on particulate structure, TEM imaging was carried out. The morphological investigations were carried out at two resolutions: 100 nm and 200 nm. Fig. 10 shows the TEM images of soot for single pilot PCCI combustion at three different FIPs with two different magnifications. Size of primary particles were the smallest for 1000 bar FIP. Primary particles became smaller in size with an increase in FIP because of relatively superior fuel spray atomization at higher FIP. At 400 bar FIP, large number of primary particles with lesser secondary particles were observed in the TEM images. Moreover, the particles were highly fused into one another therefore individual particle boundaries were not distinguishable [33–38]. The

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degree of agglomeration was also higher at 400 bar FIP compared to 700 and 1000 bar FIPs. Further, in case of 700 bar and 1000 bar FIPs, individual primary particle boundaries were easily identifiable. Each primary particle was surrounded by semitransparent secondary particles. The amount of organic fraction also increased with increasing FIP. This was mainly because of the lower EGT obtained at 1000 bar FIP. Lastly, the size of the overall cluster decreased but the particulate number density increased with increasing FIP [39,40].

3.6. Statistical analysis Fig. 11 shows the variation of particulate mass w.r.t. NOx emissions from mineral diesel fueled PCCI engine at different FIPs and SoMI timings. This analysis gives a direct comparison of combustion effectiveness at different fuel injection strategies. Height of rectangle shows particulate mass and width of the rectangle represents NOx emissions. Area of the rectangle represents the com-

bined emissions of particulate mass and NOx. Overall objective of this study was to reduce the area under the curve. At higher FIP, advancing SoMI timing showed increased NOx emissions however PM mass emission decreased significantly. This was attributed to superior air-fuel mixture at advanced SoMI timings due to more time available for mixing. This resulted in superior and more complete combustion, leading to higher incylinder temperatures, which subsequently increased NOx formation in the combustion chamber. Effect of FIP was clearly seen on both NOx and PM mass emissions. Increasing FIP resulted in simultaneous reduction in NOx and PM emissions. However, due to relatively inferior engine performance at 1000 bar FIP, 700 bar FIP was found to be more suitable for PCCI combustion. 4. Conclusions In this study, mineral diesel fueled PCCI combustion was achieved by applying pilot injection (at 35° bTDC) and 15% (v/v) EGR in a single cylinder research engine. To investigate the effect

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Fig. 10. TEM images of PCCI particulate generated at different FIPs.

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of FIP and SoMI timings on PCCI combustion, experiments were performed at three different FIPs (400, 700 and 1000 bar) and four different SoMI timings (12, 16, 20 and 24° bTDC). Advanced SoMI

timings showed superior premixed combustion, resulting in advanced combustion phasing and shorter combustion duration. Increasing FIP also resulted in superior PCCI combustion due to improved fuel atomization, however 1000 bar FIP showed slightly inferior combustion due to spray impingement, leading to knocking. Due to pilot injection and EGR, PCCI combustion showed relatively lower knocking compared to conventional CI engine combustion. Improved premixed combustion at advanced SoMI timings degraded PCCI engine performance slightly due to higher negative piston work. This resulted in slightly lower BTE. Increasing FIP also resulted in slightly lower BTE. PCCI combustion showed significantly lower EGT compared to CI combustion, which was also the reason for lower NOx emissions. HC and NOx emissions increased slightly with advanced SoMI timing however smoke opacity decreased with advanced SoMI timings. Increasing FIP reduced NOx emissions and smoke opacity of the exhaust gas from the PCCI engine. Results showed that particulate number concentration decreased upon advancing SoMI timing, as well as upon increasing FIP. Increasing FIP reduced nucleation mode particle concentration however 1000 bar FIP resulted in slightly higher total particle number concentration compared to 700 bar FIP. CMD of particulate emitted at 700 bar FIP was largest among all three test FIPs. Particulate bound trace metals were slightly higher at 400 bar FIP, however increasing FIP slightly reduced particulate bound trace metals. Particulate morphology investigations exhibited that primary particle concentration increased with increasing FIP. Effect of improved fuel atomization at higher FIPs was also reflected in these morphology experiments. Overall, it can be concluded that increasing FIP at advanced SoMI timings improved PCCI combustion however too high FIP (1000 bar) resulted in slightly inferior combustion, performance and emission characteristics of the PCCI engine.

Acknowledgements Authors are grateful to Technology Systems Group, Department of Science and Technology (DST), Government of India for providing financial support (Grant no. DST/TSG/AF/2011/144-G dated 1401-2013) for carrying out this study. Financial support from Coun-

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