Injection strategies for reducing smoke and improving the performance of a butanol-diesel common rail dual fuel engine

Applied Energy 212 (2018) 1–12

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Injection strategies for reducing smoke and improving the performance of a butanol-diesel common rail dual fuel engine

T

⁎

Jaykumar Yadav , A. Ramesh Internal Combustion Engines Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai, India

H I G H L I G H T S study on multiple injection of diesel in a butanol-diesel dual fuel engine. • Detailed plus post injection enhances energy efficiency of butanol dual fuel engines. • Main reduction of smoke, NO and fuel consumption were achieved. • Simultaneous injection quantity of 1.5 mg and offset of 9.7° found to be optimal. • Post • Auto-ignition of butanol before diesel injection limits butanol energy share.

A R T I C L E I N F O

A B S T R A C T

Keywords: Butanol Dual fuel engines Post injection for soot reduction Injection strategies for dual fuel engines Engine emissions Alternative fuels for engines

In dual fuel engines auto-ignition of the inducted butanol creates a high temperature environment prior to the injection of diesel. This results in enhanced smoke emissions. This work was aimed at controlling the smoke level in a butanol diesel common rail turbocharged dual fuel engine through multiple fuel injections. Experiments were performed on a three cylinder turbocharged common rail diesel engine at a speed of 1800 rpm and BMEPs corresponding to 75% and 100% of full load (BMEP of 11.8 bar). Port fuel injectors along with dedicated circuitry were employed to control the quantity and timing of butanol introduction into the intake air. An open engine controller was used to vary the rail pressure, injection timing and number of pulses of the diesel that was directly injected into the combustion chamber. The injection timing of diesel was always set for best efficiency. First the effect of Main plus Post Injection (MPI) of diesel at a fixed butanol to diesel energy share (BDES) of 30% was evaluated at different post injection quantities and main to post offsets. Subsequently the influence of BDES was studied at a fixed post injection quantity and offset from the main injection. Finally Pilot plus Main Injection (PMI) of diesel, Main plus Post Injection (MPI) of diesel and Main plus Two Post Injections (MPTPI) of diesel were compared in the dual fuel mode. MPI resulted in improved brake thermal efficiency (BTE) and drastically reduced the smoke level because of enhanced mixing by the momentum of the post injected fuel. NO and CO2 were also reduced. Using high BDES values along with optimised post injection quantities and main to post offsets reduced the smoke level. PMI of diesel resulted in lower BTE and higher smoke, while the only advantage was reduced NO levels. MPI was better than MPTPI with respect to all the parameters. On the whole, in a dual fuel engine that uses butanol and diesel the main plus post strategy is effective in improving energy efficiency, reducing smoke and also in increasing the amount of butanol that can be utilized.

1. Introduction The increase in the demand for motive power, depletion of fossil

fuels and the need to reduce emissions including CO2 are motivating researchers to look for alternative engine fuels. On the other hand current electronically controlled fuel injection systems offer the

Abbreviations: BDES, butanol to diesel energy share; BMEP, brake mean effective pressure; BSFC, brake specific fuel consumption; BTE, brake thermal efficiency; CA50,, combustion phasing angle; CO, carbon monoxide; CO2, carbon dioxide; COV of IMEP, coefficient of variation of indicated mean effective pressure; deg. CA, degree crank angle; ECU, engine control unit; EGTs, exhaust gas temperatures; EVO, exhaust valve opening; FID, flame ionization detector; FSN, filter smoke number; FPGA, field-programmable gate array; HC, hydrocarbon; HRR, heat release rate; IMT, intake manifold temperature; MBT, maximum brake torque; MPI, main plus post injection; MPTPI, main plus two posts injection; NO, nitric oxide; MRPR, maximum rate of pressure rise; PMI, pilot plus main injection; PM, particulate matter; P1, Post-1; P2, Post-2; SPI, single pulse injection; TDC, top dead center ⁎ Corresponding author at: MS Scholar, Internal Combustion Engine Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600 036, Tamil Nadu, India. E-mail address: [email protected] (J. Yadav). https://doi.org/10.1016/j.apenergy.2017.12.027 Received 1 October 2017; Received in revised form 24 November 2017; Accepted 2 December 2017 0306-2619/ © 2017 Elsevier Ltd. All rights reserved.

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Similar observations were made by other researchers [24,25]. The common rail fuel injection system for diesel with electronic controls has brought in great flexibility and precision in control of fuel injection parameters. It has enabled injection in multiple pulses within the same cycle [26,27]. Multiple injection strategies in diesel engines generally consist of an early pilot injection of a small quantity followed by main injection and finally by post injection of a small quantity. In automotive engines it is common to use more than three injection pulses. The pilot injection helps in reducing combustion noise; however it can result in an increase in particulate matter (PM) because it reduces the ignition delay of the diesel that follows during the main injection [28]. Post injection that comes after the main injection enhances mixing due to its kinetic energy and increases the in-cylinder temperature which reduces soot by improved oxidation [29,30,31]. Reduced particulate emission without much effect on fuel consumption and HC emission has been observed in a diesel engine with post injection [32]. Another study has also indicated the effectiveness of post injection in lowering soot, with no effect on NOx emission [33]. Careful control of the timing of post injection was needed for reducing smoke [31]. Post injection too close (offset below 8°CA deg. CA) to the main injection resulted in an increase in smoke due to elevated temperatures during combustion [34]. This could also elevate the NOx levels [35]. However, in another work it was observed that too retarded post injection (above 13°CA) also did not help in lowering the smoke level. Further, with three injections (pilot-main-post) NOx emission was slightly decreased as compared to single injection [30]. Smoke was found to decrease when a fixed quantity of post injected diesel was used in a butanol dual fuel engine [36]. Butanol helped in reducing particulate matter and NOx emissions with no significant effect on brake thermal efficiency in an automotive dual fuel engine. However, it had a negative impact on CO and THC emissions [37]. Oxygen in butanol was found to be the main reason for reduction in soot. Prolonged ignition delay with butanol induction also had a positive effect [38]. Increase in the ethanol fraction reduced smoke and NOx emissions in a dual fuel engine. However, HC and CO levels were found to increase. Ethanol is more effective in reducing smoke and NOx emissions as compared to gasoline as it is an oxygenated fuel [39].When a butanol diesel blend (B20) was used in a compression ignition engine reduction in particle number and diameter were observed. The presence of the hydroxyl group in the butanol molecule led to a lower rate of PM formation and enhanced rate of PM oxidation as compared to neat diesel operation. Further, post injection enhanced the oxidation of soot and lowered the PM levels [40]. It has also been reported that increase in the butanol content in the blend with diesel reduces soot emissions due to the presence of oxygen in the fuel [41]. In the case of butanol diesel blends, the lower flame temperatures reached due to the higher latent heat of vaporization and lower energy content of butanol will hinder NOx formation. On the other hand butanol increases the amount of oxygen and also decreases the cetane number, both of which can enhance NO formation. Experiments indicated that NOx emissions with n-butanol/diesel blends reduced with increase in the butanol content as compared to reference diesel [42]. On the whole previous studies have indicated that at high outputs, butanol diesel dual fuel operation results in problems of enhanced smoke and high maximum rate of pressure rise. This is due to the autoignition of butanol prior to the injection of diesel. On the other hand injecting diesel in multiple pulses with in a combustion cycle has been effectively used to control smoke, NOx and rate of combustion in modern diesel engines. These injection strategies can also be employed in dual fuel engines in order to mitigate some of the drawbacks explained earlier. Since this aspect has not been explored in detail so far, experimental investigations which can assess the potential of multiple injection strategies for diesel in enhancing the energy efficiency and utilization of butanol in dual fuel engines have been conducted in this work.

possibility of utilizing alternative fuels like biogas, biodiesel, butanol, ethanol, methanol and hydrogen effectively in internal combustion engines. Biofuels will reduce greenhouse gas emissions [1,2]. Alcohols can be easily stored and distributed. They have oxygen in their molecule that will aid combustion. On account of their high octane numbers they are excellent fuels for SI engines. However, because they have low cetane numbers, they have mostly been used in the dual fuel mode in CI engines [3–6]. In the dual fuel mode a high octane number primary fuel like alcohol is inducted along with air and then compressed like in a CI engine. This compressed charge does not generally auto-ignite. Hence, it is ignited by injecting a small quantity of a high cetane number fuel (secondary fuel) like diesel. In this method the energy share of the inducted fuel can be varied. The premixed inducted charge helps in reducing smoke emissions. However, the amounts the primary and secondary fuels and their properties significantly influence the performance of dual fuel engines [7,8]. Neat alcohols have also been used directly in CI engines with ignition assistance in the HCCI mode [9,10]. Though ethanol and methanol have been widely investigated the use of butanol is gaining importance. Butanol is a viable renewable fuel as it can be produced by the fermentation of agricultural feed stock that are normally used for producing ethanol [11–14]. It has several properties that are closer to both gasoline and diesel as compared to methanol and ethanol. Properties of ethanol, butanol and diesel are compared in Table 1 [15,16]. Butanol has a higher calorific value and density compared to the other alcohols. Its low vapour pressure leads to difficulties in starting in SI engines [17]. It is less corrosive and less prone to water contamination than ethanol and so can be easily transported and distributed using existing fuel supply infrastructure. Its high octane number makes it a good SI engine fuel [18] while its cetane number which is much higher than other alcohols has enabled it to be used even as the sole fuel in compression ignition (CI) engines in some cases [19,20]. Since the cetane number of butanol is only 25, its use as the sole fuel is limited to low loads in CI engines. Thus butanol is generally used in the dual fuel mode or in the blended form along with diesel in CI engines. On the other hand it can also auto-ignite under certain conditions when premixed and compressed like in dual fuel engines [21]. This leads to problems related to combustion control and high smoke emissions. In dual fuel engines smoke levels fall when the quantity of diesel is reduced i.e. when the quantity of the inducted primary fuel is increased [22]. However, at high BMEPs (Brake mean effective pressures), when butanol was inducted along with air and diesel was used for ignition, high smoke levels were observed particularly when the butanol to diesel energy share (BDES) was high. This is because the inducted butanol auto-ignited during compression and thus the diesel that was injected entered into the high temperature environment that was created. This reduced the ignition delay of the injected diesel and left less time for its mixing with air and hence elevated the smoke level [23]. Table 1 Properties of fuels [15,16]. Property Molecular formula Lower heating value Cetane number Oxygen content Latent heating at 25 °C Stoichiometric A/F Motor octane number Auto-ignition temperature Density at 20 °C Boiling point Viscosity at 40 °C Flash point at closed cup Vapour pressure at 38 °C

Unit

MJ/kg % mass kJ/kg mass basis °C kg/m3 °C mm2/s °C kPa

Diesel

Butanol

Ethanol

C12–C25 42.5 40–55 – 270 14.3 20–30 ∼316 810–890 180–360 1.9–4.1 65–88 ∼1

C4H9OH 33.1 25 21.6 582 11.21 96 343 810 117 2.63 35 11.9

C2H5OH 26.8 8 34.8 904 9.02 108 423 789 78.5 1.08 8 15.9

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experiments. In Table 2 detailed specifications of the engine are given. Fig. 1 shows a schematic view of the experimental setup. The engine was coupled to an eddy current dynamometer with closed loop control of speed. An in-house developed Field-Programmable Gate Array (FPGA) based open engine controller along with commercially available driver modules was used to vary the diesel fuel rail pressure, injection timing and number of diesel injection pulses. The diesel fuel injection pulse diagram shown in Fig. 2 indicates the nomenclature followed with respect to the different injection strategies studied in this work. A specially developed port injection system was incorporated in the intake manifold after the compressor of the turbocharger for injecting butanol into the air stream. This system employed another in-house developed electronic controller for varying the quantity of the injected butanol. Both the controllers used crank and TDC signals from the angle encoder mounted on the crank shaft. Cam shaft position signals were also fed to the open controller for control purposes. Since this work focuses on reducing the problems arising because of the auto-ignition of butanol, experiments were limited to medium to high loads. Hence, all experiments were conducted at brake mean effective pressures (BMEP) of 11.8 bar (full load) and 8.8 bar (75% load) at varying butanol to diesel energy share (BDES) values. The speed of the engine was maintained at 1800 rpm while the coolant temperature was regulated at 60 ± 1 °C. The uncertainties of the measured parameters estimated based on the standard procedure are provided in Table 3 [43]. BDES was calculated as given below.

Table 2 Specifications of the engine. Type of engine

4 Stroke water cooled turbocharged three cylinder engine with common rail direct injection system

Bore × Stroke Compression ratio (geometric) Displacement volume Peak torque Rated power EGR% Coolant temp

80 mm × 98 mm 17.1:1 1478 cc 150 N-m @1600–2400 rpm 40.5 kW@3300 rpm Nil 60 ± 1 °C

1.1. Details of the work In this work various diesel multiple injection strategies were applied for assessing their potential to reduce smoke and improve energy efficiency of a butanol diesel dual fuel common rail automotive engine. Initially the effect of post injection was studied by varying its quantity and offset from the main injection at different BDESs (butanol diesel energy shares) and at a fixed BMEP. Subsequent studies were done with two other injection strategies of diesel (Pilot plus Main and Main plus two Post injections) while butanol was injected into the intake ports. All these results have also been compared with the previously published results of the authors on the same engine in the butanol diesel dual fuel mode when diesel was injected as a single pulse instead of multiple pulses [23].

Butanol to diesel energy share(BDES) =

EB EB + ED

(1)

Where EB is the energy input from butanol and ED is the energy input from diesel. The measurement of air flow into engine was done with a turbine flow meter. The flow rates of diesel and butanol were measured on the

2. Experimental setup and experiments A three-cylinder, 1.5 L turbocharged common rail automotive diesel engine run at a constant speed of 1800 rpm was used for the

Fig. 1. Schematic diagram of experimental setup.

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Fig. 2. The diesel fuel injection pulse diagram.

In the second phase of experiments the effect of BDES was studied at the best main to post offset and post injection quantity of diesel selected from the experiments in Phase-1. Thus these experiments were done at 100% load with a post quantity of 1.5 mg/cycle and main to post offsets of 9.7 and 13°CA both of which were found to be suitable in Phase-1. Again in all the cases both the pulses were moved with respect to crank angle and positioned for highest torque using the open engine controller. For studying the effect of post injection at medium load experiments were also conducted at BMEP of 8.8 bar (75% load) with the best main to post offset value of 9.7°CA and post injection quantity of 1.5 mg/cycle. In the third phase, three different injection strategies namely Pilot plus Main Injection (PMI) of diesel, Main plus Post Injection (MPI) of diesel and Main Plus Two Post Injections (MPTPI) of diesel along with port fuel injection of butanol were tried. The offset of the pilot and post injections from the main were kept at the best values obtained from the experiments in Phases 1 and 2. Comparison with different injection strategies were done at two BDES values namely 15% and 25% while the rail pressure was maintained at 700 bar. The main injection quantity was adjusted for attaining the BMEP of 11.8 bar. The injection pulses were moved with respect to crank position till the highest torque was attained. The Pilot, Post and Post-1 (P1) & Post-2 (P2) quantities were 2.5 mg/cycle, 1.5 mg/cycle and 1.5 mg/cycle & 1 mg/cycle respectively. A few results of SPI from earlier reported work by the authors [23] have been used for comparison here as mentioned earlier.

Table 3 Uncertainty in measurement. Measured parameter

Uncertainty

Torque (N m) Speed (rpm) Brake thermal efficiency (%) Smoke (FSN) NO (ppm) HC (ppm)

± 0.5% ± 0.1% ± 1.3% ± 5.2% ± 2.1% ± 4.7%

mass basis. Temperatures of intake air, coolant and exhaust gas were measured using K type thermocouples. HC, CO and NO emissions were measured using FID, NDIR and Chemiluminescence based analysers respectively. Smoke intensity was measured using an AVL filter paper based measuring system. A flush mounted piezo electric pressure transducer was used along with an optical crank angle encoder for cylinder pressure measurements. In-cylinder pressure data was acquired on the crank angle basis using a data acquisition system along with in-house developed software. Cylinder pressure data from 100 consecutive engine cycles was ensemble averaged. This was then post processed using in-house developed software for referencing and calculation of combustion parameters like ignition delay, peak pressure and heat release rate. Heat release rate was computed based on the method indicated in literature [44]. The Hohenberg’s heat transfer correlation was used for estimation of heat transfer [45]. All instruments were periodically calibrated as per the instructions of the manufacturers. In the first phase of experiments, diesel was directly injected as two pulses per cycle (Main plus Post Injection - MPI) and butanol was injected into the intake port at full load (BMEP of 11.8 bar at 1800 rpm) under a butanol to diesel energy share (BDES) of 30%. This is around the maximum BDES (28.5%) that could be used in the conventional dual fuel mode (single pulse injection of diesel) as restricted by high combustion rates which will be described later. The effects of quantity of the post injected diesel (1, 1.5 and 2.5 mg/cycle – amounting to 4%, 6% and 10% of the total amount of diesel respectively) and its offset in crank angle degrees from the main injection (6.5–32.5°CA) were also studied. The rail pressure of diesel was kept constant at 700 bar. This was found to be the most suitable based on the thermal efficiency and NO emission considerations in the dual fuel mode when diesel was injected as a single pulse (Single Pulse Injection – SPI). In these experiments while the offset between the main and post injections was fixed both the pulses were simultaneously moved with respect to crank position till the highest torque was achieved. It is also to be noted that some of the already published [23] experimental results (only brake thermal efficiency, NO, Smoke and HC emission at different BDES values at 75% and 100% loads) of the same authors on the same engine in the SPI mode alone have been used for comparison with the other modes evaluated here.

3. Results and discussion The results of experiments conducted in all the three phases are presented and discussed in this section. Comparisons have also been made with the results of Single Pulse Injection (SPI) of diesel taken from the earlier work of the authors which was also conducted on the same engine [23]. 3.1. Phase-1: Effect of quantity of post injection and offset between main and post injection The brake thermal efficiency (BTE) shown in Fig. 3 was not significantly affected by the main to post injection offset or the quantity of fuel in the post injection phase. There was a small increase in the BTE with Main plus Post Injection (MPI) as compared to SPI of diesel because of better combustion phasing. The BTE for the pure diesel case is also indicated on the same figure (shown as a black point on the primary Y axis at zero offset – Fig. 3). Fig. 4 indicates the Heat Release Rates (HRR) with different main to post offset values and also with SPI. The main and the post pulses (injector driving signals) for each offset are also indicated on the same figure. We see that in all the cases combustion started before the main diesel pulse i.e. before the diesel was injected. This indicates auto-ignition of the butanol that was injected into the ports before the diesel that was directly injected into the 4

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Table 4 In-cylinder conditions at EVO. Main to post offset

6.5°CA

9.7°CA

29.7°CA

Single pulse injection of diesel

In-cylinder Pressure at EVO (bar) Estimated temperature at EVO (K)

6.82

7.14

7.35

7.68

1313

1378

1412

1492

output of the turbine of the turbocharger and as a result the boost pressure and intake air temperature will go up. In addition the retained exhaust gas also will be at a higher temperature in such cases. These will favour auto-ignition of butanol and will raise the temperature after combustion which has a cascading effect. Table 4 shows the measured in-cylinder pressure and estimated averaged in-cylinder temperature at the time of exhaust valve opening (EVO) (pressure and temperature at the inlet of the turbine). In MPI strategy of diesel as the offset between the main and post was increased auto-ignition of butanol was also advanced (Fig. 4 – shown by the black ellipse). This was because pressure and temperature at the inlet of the turbine also increased as explained earlier (Table. 4). It is also noted that with the MPI strategy of diesel pressure and temperature at the time of EVO were always lower as compared to SPI of diesel. Because of this auto-ignition of butanol with the MPI strategy of diesel was delayed. It is also seen in Figs. 5 and 6 that advancing the injection timing of diesel in both MPI and SPI modes reduced the in-cylinder gas temperatures in the expansion stroke and thus retarded the auto-ignition of butanol. It however advanced the second heat release portion which was contributed by the combustion of diesel and the remaining butanol. This reduced the exhaust gas temperature and in-turn the intake temperature also was reduced as explained earlier. This retarded the first peak in the HRR which is due to auto-ignition of butanol. Fig. 7 indicates that with MPI of diesel NO emission decreased due to reduced and delayed peak HRR which lowered the maximum temperature in the cycle (Fig. 4). As the main to post offset was increased, the NO emission level decreased till an offset of 13°CA and after that there was no significant change. It is also reported in literature that increasing the offset of the post injection from the main injection in case of neat diesel engines could reduce the cooling effect the post injection produces after the main combustion and thus may lead to increase in

Fig. 3. Variation in BTE and IMT with post injection quantity and main to post offset.

Fig. 4. HRR and estimated in-cylinder temperature with different main to post offsets (Post quantity = 2.5 mg/cycle).

combustion chamber. In the case of MPI we see that this auto-ignition of butanol was delayed (which could lead to improved BTE) due to the reduced boost temperature that resulted in lower exhaust gas temperatures (EGTs). The estimated in-cylinder gas temperatures shown in Fig. 4 also indicate that lower values were attained in the case of MPI as compared to SPI (reduction of 70 K) particularly during the expansion stroke. The boost pressure was also lower with MPI at compared to the SPI strategy. Hence, due to the coupled effects of lower EGT, boost pressure and temperature, auto-ignition of butanol was delayed. With SPI the auto-ignition was quite early and this affected the BTE adversely. The advanced combustion with SPI [23] was found to be due to the higher boost temperature (intake manifold temperature- IMT) which is also indicated in Fig. 3 (shown as a point on the Y axis at zero offset). The advanced combustion of butanol by auto-ignition in the SPI mode also resulted in higher in-cylinder gas temperatures. Thus there was an influence of exhaust gas temperature on auto-ignition of butanol and vice versa. Higher exhaust gas temperatures will increase the

Fig. 5. HRR and estimated in-cylinder temperature at different SOIs of diesel in the MPI mode.

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Fig. 6. HRR and estimated in-cylinder temperature at different SOIs of diesel in the SPI mode.

Fig. 8. Variation in HC and CO emission with post injection quantity and main to post offsets.

low smoke. Small main to post offsets (post injected immediately after the main) did not reduce smoke as main and post combustion phases almost merged with each other. This lowered the effect of enhanced mixing by post injection. This was because under these conditions the influence of the small momentum brought in by the post becomes insignificant as compared to what is introduced by the main injection. We also see that when the offset was higher than about 13°CA the smoke level went up. This was because of the lower temperatures that were created by the late injection of the post which was influenced by the expansion of the gases in the cylinder. It has also been reported in literature that there is a possibility of the post injected fuel interfering with that part of the fuel injected during the main injection which rebounds from the combustion chamber [49]. Thus the benefits of the post injection can be realised only if the main to post offset is such that the post does not enhance the fuel available in the soot formation region created by the main injection but helps elevate the temperature for aiding oxidation of the soot particles. The smoke emission for the pure diesel case is also indicated on the same figure (shown as a black point on the secondary Y axis at 40 offset – Fig. 7). The HC and CO emissions (shown in Fig. 8) indicate that increased post injection quantities lead to elevated HC and CO levels. HC and CO emissions for the pure diesel case are also shown on same figure as a black point on Y axis. Fig. 9 indicates that the CO2 level with post injection was always below the level seen in the SPI mode [23] because of improved thermal efficiency. The CO2 emission for the pure diesel case is also indicated on the same figure (shown as a black point on the primary Y axis at zero offset – Fig.9). The coefficient of variation of indicated mean effective pressure (COV of IMEP) calculated over 100 successive cycles shown in Fig. 9 indicates higher values than the SPI mode only when the post was too close to the main injection. Under this condition, the influence of the post injected fuel on torque generation was significant because it was close to TDC. Thus any variation in the post quantity between cycles which is likely when the injected quantity is near the lower limit of the injector is influential when the main to post offset is low. However, the COV of IMEP was always below 3.5% which indicated stable combustion. Under the best offset the COV of IMEP was comparable to the SPI mode of operation. The COV of IMEP with the stock engine ECU was also around this value (Fig. 9).

Fig. 7. Variation in NO and SMOKE emission with post injection quantity and main to post offsets.

the NO levels [46]. In Fig. 7 in some cases it is seen that increasing the offset increased the NO level slightly. The quantity of the post injected fuel had only a small influence on NO emission. However, with SPI [23] (shown as a point on the Y axis at zero offset) the NO level was high since the auto-ignition of butanol was advanced which elevated the incylinder temperature (Fig. 4). NO emission for the pure diesel case is also indicated on the same figure (shown as a black point on the primary Y axis at zero offset – Fig. 7). We see in Fig. 7 that there was a drastic drop in smoke emission with post injection as the offset increased till 13°CA as compared to the SPI mode. This was because post injection which occurred after the diesel injected in the main pulse started burning, enhanced mixing within the cylinder due to its momentum which aided oxidation of soot [47]. As the post injected fuel burns it will elevate the temperature in the cylinder. This also has aided the oxidation of soot [48]. The quantity of fuel associated with post injection has to be optimised for 6

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Fig. 11. Variation in NO emission with butanol to diesel energy share at different main to post offsets at 75% and 100% loads.

Fig. 9. Variation in CO2 emission and COV of IMEP with post injection quantity and main to post offsets.

which is one of the reasons for the increase in BTE. Fig. 11 indicates that the NO emission level fell significantly with the introduction of butanol because of the charge cooling effect arising out of the vaporization of butanol whose latent heat of vaporization is significantly higher than conventional fuels. The NO level with post injection was lower than SPI [23] of diesel for both 75% and 100% loads which was due to lower peak temperatures as compared to SPI of diesel. In addition, combustion was also completed in a shorter duration with MPI [34]. Fig. 12 indicates that with post injection of diesel a significant reduction in smoke emission due to its effects on in cylinder mixing and temperature as discussed earlier was obtained. For the main to post offset of 9.7°CA smoke was lower as compared to the offset of 13°CA. This is because main to post offset influences the temperature and mixing as discussed earlier. Similar results were seen for different post injection durations as discussed in Phase-1 (Fig. 5). With SPI [23] of

3.2. Phase-2: Effect of post injection of diesel at 100% and 75% loads under different BDES Experiments were done in this phase with a post injection quantity of 1.5 mg/cycle and main to post offsets of 9.7°CA and 13°CA which were found to be the most suitable based on experiments in Phase-1 at 100% load while considering BTE, NO and smoke emissions. Effect of post injection at 75% load was also studied in this phase at different BDES values while the post injection parameters were kept at the same values (i.e. Post injection quantity 1.5 mg/cycle and a main to post offset of 9.7°CA). Fig. 10 compares the BTEs obtained with MPI and SPI of diesel at 75% and 100% loads. The BTE increased with post injection as compared to SPI [23] at all values of BDES. Under the set of injection parameters used, the heat release occurred with better combustion phasing and at a faster rate. With MPI of diesel the crank angle at 50% HRR (CA 50) was advanced as compared to SPI of diesel (by 3°CA)

Fig. 10. Variation in BTE with butanol to diesel energy share at different main to post offsets at 75% and 100% loads.

Fig. 12. Variation in smoke emission with butanol to diesel energy share at different main to post offsets at 75% and 100% loads.

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Fig. 14. Variation in CO emission with butanol to diesel energy share at different main to post offsets at 75% and 100% loads.

Fig. 13. Variation in HC emission with butanol to diesel energy share at different main to post offsets at 75% and 100% loads.

diesel, smoke levels decreased initially with increase in BDES due to the reduction in the injected diesel which contributes towards it. However, beyond a BDES of 20% the smoke level increased with SPI of diesel as the butanol air mixture became richer and started to auto ignite which created a high temperature environment when the diesel was injected. This is discussed further in the later paragraphs using HRR diagrams. Smoke emission was found to decrease with post injection at 75% load also (Fig. 12). With increase in BDES, HC levels (Fig. 13) increased due to quenching and crevice effects mainly related to the injected butanol. With post injection, HC levels were higher because of lower average gas temperatures in the expansion stroke which affected post oxidation. Similar trends were found at 75% load (Fig. 13). For higher main to post offsets, in-cylinder gas temperatures in the expansion stroke were higher as compared to smaller main to post offsets (Fig. 18). However HC levels with 75% load were higher as compared to 100% load. This is because of the lower temperatures attained at 75% load. CO emission shown in Fig. 14 was found to be decrease with increase in post injection quantity. Similarly CO2 emission (Fig. 15) was also found to decrease because of improved BTE. The COV of IMEP was always below 1.5% at all BDES values. Fig. 16 indicates that the maximum rate of pressure rise (MRPR) was low when low values of BDES were used due to reduced rate of combustion and increased ignition delay of diesel. However, with increasing BDES values when butanol started to auto-ignite the MRPR rapidly increased. The values of MRPR were lower with MPI as it created conditions that were less conducive for auto-ignition of butanol as discussed earlier. With SPI of diesel MRPR was always higher as compared to MPI (because of advanced and rapid heat release due to autoignition with SPI, Fig. 4). This is one of the reasons for higher NO and smoke emissions in the SPI mode. We see that with increase in the BDES the butanol air mixture tended to auto-ignite with significant high heat release rate values which led to knock. Thus the maximum BDES was limited to 32% at 100% load and 46% at 75% load. With MPI of diesel there was an increase of 4% in the BDES that could be used as compared to SPI [23] of diesel at both 75% and 100% loads. This is because lower MRPR values were observed with MPI of diesel. However, BDES was limited by knocking (MRPR) in both MPI and SPI of diesel. Fig. 17 shows the HRR with MPI (BDES = 28.3%) and SPI (BDES = 28.5%) of diesel cases at 100% load. There were two peaks in the HRR curves. With post injection the first peak of HRR which is due

Fig. 15. Variation in CO2 emission with butanol to diesel energy share at different main to post offsets at 75% and 100% loads.

to auto-ignition was lower and a little retarded. This is because boost pressure and temperature were lower (reason for auto-ignition retarded) and hence lesser amount of butanol was auto-igniting as compared to SPI of diesel. The second peak which was dominated by the burning of diesel was advanced and higher as compared to SPI of diesel. This is because more butanol was burning in this phase which favourably influenced BTE. Post injection also accelerated the final stage of combustion (Fig. 4) [31]. Further, with post injection, between the first and the second peaks the value of HRR did not drop off significantly as compared to SPI. This is also one of the reasons for the higher BTE with post injection. With SPI the first peak of HRR was advanced due to the reasons explained earlier. Fig. 18 shows that the estimated average in cylinder temperature reached higher values and stayed so for longer durations with SPI which led to increase NO levels. When the BDES was reduced as seen in Fig. 19 similar trends were observed but the first peak was not so dominant. 8

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Fig. 16. Variation in MRPR emission with butanol to diesel energy share at different main to post offsets at 100% load.

Fig. 18. Estimated In-cylinder temperature traces for main to post offset and SPI of diesel at 100% load.

Fig. 17. HRR and In-cylinder pressure traces at different main to post offsets at a BDES of 28.3% and 100% load.

Fig. 19. HRR and In-cylinder pressure traces at different main to post offsets at a BDES of 22% and 100% load.

Fig. 20 shows the HRR with MPI (BDES = 37.4%) and SPI (BDES = 36.5%) of diesel cases at 75% load. It may be noted that the actual start of injection of diesel will be slightly delayed as compared to the injection pulse (electrical signal) shown in Fig. 20. In both SPI and MPI cases auto-ignition of butanol occurs. Auto-ignition in the case of MPI is delayed as compared to SPI as a result of lower charge temperatures due to the reasons explained earlier. This trend was also observed at 100% load (Fig. 17). The initial peak in the HRR with MPI is higher than with SPI. This is because the start of combustion (autoignition) is more retarded as compared to the start of injection of diesel in the case of MPI. Hence, some of the injected diesel could also autoignite along with butanol leading to higher heat release rates.

diesel and the results are shown in Figs. 21 and 22. In this case there was an increase in smoke levels and reduction in BTE. This was because the pilot diesel resulted in very early auto-ignition of butanol and advanced the combustion process as seen in Figs. 23 and 24 particularly at high BDES values. Similar trends were also observed in the results presented in Phase-1. Smoke values were also higher as compared to SPI of diesel. Experiments with even pilot, main and post combination along with manifold injection of butanol were conducted. It was found that this mode was not beneficial in reducing smoke. Thus it is not discussed further. Main plus post injection (MPI) of diesel was the most suitable. Hence, experiments were also done with Main plus two post injections (MPTPI) of diesel along with port fuel injection of butanol at 100% load. The offset between main and post-1 (P1) was maintained at a value of 9.7°CA which was found to be the best from earlier results (Phase-2). The quantity of P1 was also kept at 1.5 mg/cycle which was obtained from the experimental results from Phase-1. The offset of Post-

3.3. Phase-3: Comparison of different injection strategies Experiments were also done with Pilot plus Main Injection (PMI) of 9

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Fig. 20. HRR and In-cylinder pressure traces with different injection strategies at a BDES of 37% and 75% load.

Fig. 22. BTE, MRPR, NO and smoke emission for different injection strategies of diesel at a BDES of 25% and 100% load.

Fig. 21. BTE, MRPR, NO and smoke emission for different injection strategies of diesel at a BDES of 15% and 100% load.

Fig. 23. HRR and In-cylinder pressure traces for different injection strategies of diesel at a BDES of 15% and 100% load.

2 (P2) from P1 and the quantity of P2 were varied and the best values (6.5°CA and 1 mg/cycle) were obtained. The quantity of fuel injected in the main diesel pulse and its timing were varied till the torque was maximum (MBT-Timing) while the main to P1 and P1 to P2 offsets and quantities of P1 and P2 were kept constant at the values indicated earlier. A comparison between all the methods is seen in Figs. 21 and 22 at two different BDES values namely 15% and 25%. From Figs. 23 and 24 we see that SPI resulted in a clear first peak in the heat release rate (HRR) even before injection of diesel and the second peak was influenced by diesel. In the case of PMI there were three peaks in the HRR (Fig. 23). The second peak (auto-ignition of butanol) was influenced by the pilot and BDES (richness of the inducted mixture). At higher BDES values the second and the first peaks merged together in the case of PMI (Fig. 24). When the auto-ignition of butanol was significant the main pulse of diesel had to be retarded in order to get the highest BTE with PMI. This effect was also seen in the HRR

curves. Between MPI and MPTPI there was little difference in heat release rates. From Figs. 21 and 22 we see that at full load the highest BTE and lowest smoke levels were reached with MPI. As compared to SPI the BTE was higher by 1.6% and the smoke was lower by 0.73 FSN. The maximum rate of pressure rise and NO level were also lower. With MPTPI the smoke levels were higher than MPI this is because the second post (P2) was injected in expansion stroke and that was contributing to smoke while the other parameters were similar. The only advantage with PMI was low NO levels. MPI is better than Main Plus Two Post Injections with respect to all the parameters evaluated. On the whole the Main plus Post Injection (MPI) strategy seems to be the best for dual fuel operation with butanol and diesel at medium to high loads as it can result in significant reductions in smoke, improved energy efficiency and no adverse effect on NO. However, HC levels were elevated as compared to SPI.

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In summary, Main plus Post (MPI) is the best amongst the different strategies evaluated in this work. It resulted in the highest thermal efficiency and lowest smoke and maximum rate of pressure rise levels. Post injection quantity of 1.5 mg/cycle and offset of 9.7°CA with the main injection were found to be generally suitable. A change in the injection strategy is needed when the engine shifts between neat diesel and butanol diesel dual fuel modes of operation. References [1] Nagy Karoly, Körmendi Krisztina. Use of renewable energy sources in light of the “New Energy Strategy for Europe 2011–2020”. Appl Energy 2012;96:393–9. [2] Koponen Kati, Tsupari Eemeli, Soimakallio Sampo, Thun Rabbe, Antikainen Riina. GHG emission performance of various liquid transportation biofuels in Finland in accordance with the EU sustainability criteria. Appl Energy 2013;102:440–8. [3] Karim GA, Amoozegar N. Determination of the performance of a dual fuel diesel engine with the addition of various liquid fuels to the intake charge (No. 830265). SAE Technical Paper 1983. [4] Abu-Qudais M, Haddad O, Qudaisat M. The effect of alcohol fumigation on diesel engine performance and emissions. Energy Convers Manage 2000;41(4):389–99. [5] Fang W, Huang B, Kittelson DB, Northrop WF. Dual-fuel diesel engine combustion with hydrogen, gasoline, and ethanol as fumigants: effect of diesel injection timing. J Eng Gas Turbines Power 2014;136(8):081502. [6] Kumar BR, Saravanan S. Use of higher alcohol biofuels in diesel engines: a review. Renew Sustain Energy Rev 2016;60:84–115. [7] Zhu Y, Chen Z, Liu J. Emission, efficiency, and influence in a diesel n-butanol dualinjection engine. Energy Convers Manage 2014;87:385–91. [8] Michikawauchi R, Tanno S, Ito Y, Kanda M. Combustion improvement of diesel engine by alcohol addition-investigation of port injection method and blended fuel method. 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Fig. 24. HRR and In-cylinder pressure traces for different injection strategies of diesel at a BDES of 25% and 100% load.

4. Conclusions Based on the experiments conducted at 75% and 100% loads in the butanol diesel dual fuel mode with different injection strategies for diesel the following conclusions are derived.

• Main plus Post Injection (MPI) of diesel resulted in a small increase •

• •

• • •

in brake thermal efficiency (BTE) as compared to the conventional Single Pulse Injection (SPI) of diesel both at 75% and 100% loads. At high BDES values, butanol always auto-ignited before the injection of diesel and two peaks were observed in the heat release rate (HRR). Retarding the start of injection of diesel advanced the autoignition of butanol (that is first peak in the HRR) and delayed the second peak of HRR which is the combined burning of diesel and remaining butanol. This is because of the increase in the exhaust gas temperature and resulting increase in boost pressure and temperature. The maximum rate of pressure rise also went up significantly with increase in BDES. By altering the injection strategy adopted for diesel auto-ignition of butanol that occurs at medium to high loads (at high BDES) cannot be prevented. However, smoke emission can be reduced and BTE can be improved. With MPI of diesel there was a drastic drop in smoke emission with post injection as the offset was increased till 13°CA because of enhanced mixing within the cylinder which helps oxidation of soot. Too retarded post injection was not beneficial as it resulted in lower gas temperatures. Higher BDES values along with optimised post injection quantity and main to post offset were better for reduction of smoke. NO emission decreased with post injection and was influenced significantly by the main to post offset. NO emission decreased with increase in BDES. Values lower than diesel could be reached with MPI of diesel. The CO2 level with post injection was also below the levels with SPI because of improved thermal efficiency. HC emission increased in the dual fuel mode due to quenching and crevice effects. MPI resulted in higher HC and CO than SPI because of lower temperatures during expansion. Pilot plus Main Injection (PMI) of diesel resulted in increased smoke levels (higher than SPI) and reduction in BTE because of very early auto-ignition of butanol which advanced the combustion process.

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