Effects of increased spark discharge energy and enhanced in-cylinder turbulence level on lean limits and cycle-to-cycle variations of combustion for SI engine operation

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Effects of increased spark discharge energy and enhanced in-cylinder turbulence level on lean limits and cycle-to-cycle variations of combustion for SI engine operation ⁎

Dongwon Jung , Kosaku Sasaki, Norimasa Iida Graduate School of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan

H I G H L I G H T S efficiency of SI engine increases with the extension of lean-stability limit. • Thermal spark discharge energy is effective at shifting lean-stability limit. • Increased limit is further extended by enhanced in-cylinder turbulence level. • Lean-stability • Shift of lean limits is mainly attributed to shortened initial combustion duration.

A R T I C L E I N F O

A B S T R A C T

Keywords: SI combustion Lean operation Spark discharge energy Turbulence level Cycle-to-cycle variations

Improving the thermal efficiency of spark ignition (SI) engines is strongly required due to its widespread use but considerably less efficiency than that of compression ignition (CI) engines. Although lean SI engine operation can offer substantial improvements of the thermal efficiency relative to that of traditional stoichiometric SI operation, the cycle-to-cycle variations of combustion increase with the level of air dilution, and become unacceptable. For improving the thermal efficiency by extending the lean-stability limit, this study examines the effects of spark discharge energy and in-cylinder turbulence level on lean limits and cycle-to-cycle variations of combustion for SI engine operation. The spark discharge energy was increased by a high-energy inductive ignition system using ten spark coils and the in-cylinder turbulence level was enhanced by a custom adapter installed in the intake port. The results show that increased spark discharge energy by ten spark coils is effective at shifting the leanstability limit to leaner operation, compared to that of a single spark coil. With shift of the lean-stability limit, significant improvement of thermal efficiency is observed, relative to that of stoichiometric operation. Furthermore, a combination of increased spark discharge energy and enhanced in-cylinder turbulence level makes it possible to allow stable operation at more extended lean-stability limit. This is mainly attributed to shortening the durations of spark timing-to-CA5 and CA10-to-CA90 by both increased spark discharge energy and enhanced in-cylinder turbulence level. However, the cycle-to-cycle variations of SI combustion increase with increasing excess-air ratio even for operation by ten spark coils with the intake port adapter. Finally, the relationship between the spark discharge energy and the SI combustion is examined and compared for ultra-lean operation without and with the intake port adapter. Although indicated thermal efficiency is improved by increased spark discharge energy, the variations of the spark discharge energy do not relate to the variations of the combustion, since total spark discharge energy does not affect both durations of the spark timing-to-CA5 and the CA10-to-CA90, and eventually the heat-release efficiency.

1. Introduction In the light of constrained petroleum supply and anthropogenic climate change [1], increasing efficiency of internal combustion engine

⁎

is of interest. With this in mind, recent studies on spark ignition (SI) and compression ignition (CI) engines are largely oriented toward improving the thermal efficiency [2,3]. Compared to CI engines, SI engines have lower efficiency at both low and high loads, despite the

Corresponding author. E-mail address: [email protected] (D. Jung).

http://dx.doi.org/10.1016/j.apenergy.2017.08.043 Received 20 April 2017; Received in revised form 8 August 2017; Accepted 9 August 2017 0306-2619/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: Jung, D., Applied Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.08.043

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∫ Qhr ηth Pc Qfuel MBT IMEPn I1 SI ST t Σtd ΣEd ΣQhr CA5 CA10 CA90

Nomenclature ATDC after top dead center Qhr heat release COV of IMEPn coefficient of variation of IMEPn CI compression ignition θ crank angle Id discharge current Vd discharge voltage Ed discharge energy dEd/dt discharge energy release rate ΔθST-5 duration from spark timing to CA5 Δθ10-90 duration from CA10 to CA90 λ excess-air ratio ηhr heat-release efficiency dQhr/dθ heat-release rate ∫ Ed integrated spark discharge energy

integrated heat release indicated thermal efficiency in-cylinder gas pressure lower heating value of the fuel supplied per cycle minimum advance for best torque net indicated mean effective pressure primary current spark ignition spark timing time total spark discharge duration total spark discharge energy total heat release 5% burn point 10% burn point 90% burn point

is occurred is much later into the expansion stroke. Since the volume expansion of the in-cylinder charge by piston motion increases during the expansion stroke, the greater volume and volume-expansion rate with combustion-phasing retard more strongly counteracts the temperature rise. This causes unfavorably long combustion duration from CA10 to CA90 (CA10-to-CA90 duration) with the increase of cycle-tocycle variations. Finally, the combustion efficiency drops off quickly due to unacceptable cycle-to-cycle variations of SI combustion with the appearance of partial-burn and misfire cycles, resulting in the observed drop in the thermal efficiency. Ultimately, the causes of these combustion fluctuations can largely be attributed to the cycle-to-cycle variations of the fragile early flame kernel. One effective way to shorten the inflammation time for successful inflammation under lean conditions is to produce robust flame kernels by effective ignition [24,25]. To provide effective ignition, a variety of advanced ignition systems have been developed and studied many times previously [26–34]. In particular, for extending the lean-stability limit and the dilution tolerance of SI engines, much effort to develop advanced ignition systems has focused on increasing spark discharge energy. If fairly high spark discharge energy is deposited in the sparkplug gap, large early flame kernels can be formed even for very lean conditions due to significant heat supply from the spark plasma [35]. Then, these flame kernels have to be developed quickly into a turbulent flame front, and propagate throughout the fuel-containing regions [36]. This must shorten the inflammation time compared to that of the regular spark discharge, and suppress the cycle-to-cycle variations of early flame kernel formations. However, there is a high probability of these flame kernels being failed to transit to fully developed turbulent combustion when the in-cylinder turbulence level is low. Modeling work by Dahms et al. [37] shows that, during the main combustion, the combustion rate is governed by the turbulence intensity. Therefore, for stable and efficient lean SI operation, both increased spark discharge energy and enhanced in-cylinder turbulence level are necessary. Furthermore, a combination of the increased spark discharge energy and the enhanced in-cylinder turbulence level should have strong potential to extend the lean-stability limits further while maintaining high combustion stability. When the spark discharge energy is increased for the leaner operation under the conditions of enhanced in-cylinder turbulence level, the gas flow in the spark-plug gap is significantly increased during the spark discharge, which leads to strong stretching of the spark channel. As discussed in Ref. [38], this may be advantageous for much more stable ignition under lean conditions, since dramatic stretching of the spark channel can increase the chances of successful ignition by delivering the more spark discharge energy to the gas near the spark gap. In addition, local ignition along the stretched spark channel could generate multiple flame kernel that speeds up the

inherent efficiency advantages of the Otto-cycle over either the Dieselcycle or combined cycle [4]. Due to widespread use of SI engines, a variety of technologies have been demonstrated for improving a thermal efficiency of SI engine such as turbocharging, direct injection, downsizing and cooled exhaust gas recirculation (EGR) [5–8]. Ultimately, all of these technologies are the methods to overcome the limitations of well-mixed stoichiometric operation which is a dominating combustion mode for SI engines used for automotive applications. There are several reasons why stoichiometric operation limits the observed thermal efficiency of SI engine [9] even though it allows the application of a three-way catalyst for cost-effective reduction of nitrogen oxides (NOx) and oxidation of carbon monoxide (CO) and hydrocarbon (HC) [10]. First, the required intake throttling results in pumping losses [11–13]. Second, high combustion temperatures lead to both high heat-transfer losses and unfavorable thermodynamic properties of the combustion products [14,15]. The latter manifests itself as high specific heat capacity, reducing both the ratio of specific heats (γ) and the work-extraction efficiency of the expansion stroke [10]. A third factor is the inability of stoichiometric combustion to fully complete near TDC due to dissociation of CO2 in the hot O2-depleted gases [4,16]. Lean operation can improve the thermal efficiency of SI engine by mitigating all of these limitations of stoichiometric operation [9,17,18]. Furthermore, the beneficial effects of lean operation on the thermal efficiency are much more pronounced for the leaner operation [19]. However, flame speeds during and shortly after spark discharges are expected to be substantially lower for the leaner operation, and the flame speed becomes too low to create and develop the flame kernel as the in-cylinder charge is leaned out [20–23]. To ensure stable, complete and fast combustion, successful inflammation is critically important first, which refers to creating a flame kernel that survives its nascent laminar state and transitions into fully developed turbulent deflagration [17]. For successful inflammation, the time scale for inflammation (inflammation time) is an important factor that significantly influences on both the combustion phasing and the combustion duration. Usually, for quantitative comparisons, inflammation time is equated to a duration from the spark timing (ST) to the 5% burn point (CA5). Furthermore, the combustion duration is equated to a duration from the 10% burn point (CA10) to the 90% burn point (CA90), which refers to the main combustion phase. With the earlier of spark timing for the leaner operation to maintain the targeted combustion phasing, the inflammation time from ST to CA5 (ST-to-CA5 duration) becomes longer by the compounding effects of lower laminar flame speed at higher excess-air ratio (λ), and the lower compressed-gas temperature at earlier crank angles. However, very slow inflammation resulted from the longer ST-to-CA5 duration leads to excessive combustion-phasing retard despite the more advanced spark timing, and then the combustion 2

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iridium tipped fine wire for the center and ground electrodes. The diameters of tipped wire for the center and ground electrodes are 0.55 mm and 0.7 mm, respectively, with a gap width of 0.9 mm. As mentioned in Ref. [39], ignition, flame kernel growth and flame propagation are influenced by the flow direction, the flow structure size and their corresponding energy content. Because of this, the spark plug was mounted so as to position the ground electrode at right angles to the direction of in-cylinder flow, which could increase the degree of spark-channel stretch as much as possible. Fig. 2 shows a comparison of a single spark coil and ten spark coils (without the time offset between discharges, i.e. simultaneous spark discharge of ten spark coils) with regards to typical traces of (a) primary current, (b) discharge voltage, (c) discharge current, (d) discharge energy release rate and (e) integrated discharge energy in air at atmospheric pressure. For all the test reported in this study, the charging time of the primary coil was set to 5 ms. During the charging of the primary coils, the energy from battery is delivered to each spark coil simultaneously, which results in around 8.5 times higher primary current for ten spark coils than that for a single spark coil just before shutting off the primary voltage. The higher primary current leads to significant increase of the magnitude of peak discharge voltage (Fig. 2b) and discharge current (Fig. 2c), and eventually the magnitude of peak discharge energy release rate (Fig. 2d). It should be noted that the spark discharge duration (defined here as the time or crank angle range for which the discharge energy release rate is positive) from a single spark coil is longer than that from ten spark coils. Nevertheless, the integrated spark discharge energy of ten spark coils calculated by integration of discharge energy release rate during the spark discharge duration becomes around 250 mJ, which is 5 times higher than that from a single spark coil.

transition from ignition to main combustion. Although previous studies have provided useful information on the effects of the spark discharge energy or the in-cylinder turbulence level on SI combustion for lean operation, respectively, the combined effects of increased spark discharge energy and enhanced in-cylinder turbulence level should be examined to achieve stable ultra-lean operation. The main objective of this study is to experimentally investigate the effects of spark discharge energy and in-cylinder turbulence level on the extension of lean-stability limit and the cycle-to-cycle variations of SI combustion at lean limits. First, engine facility, high-energy inductive ignition system and intake port adapter are described. Then the experimental procedure and the data analysis are explained with the measurement uncertainty for spark discharge energy and duration. The results are divided into four main parts: (1) The effects of the increased spark discharge energy and the enhanced in-cylinder turbulence level on the extension of lean-stability limits are identified. (2) It is demonstrated that how the cycle-to-cycle variations of SI combustion increase with the increase of excess-air ratio. (3) The underlying mechanisms responsible for the cycle-to-cycle variations of combustion duration and heat-release efficiency that are observed for stoichiometric and lean operations are explained. (4) Finally, the relationship between the spark discharge energy and the SI combustion is examined and compared for ultra-lean operation without and with the intake port adapter. 2. Experimental setup 2.1. Research engine facility The engine used for this study is a single-cylinder long-stroke research engine equipped with a flat-top piston. Engine specifications are listed in Table 1. For all operation, the fueling was accomplished by a port fuel injection. In order to accurately determine the amount of fuel supplied to injector, a positive-displacement flow meter was used with a built-in Coriolis mass flow meter which can measure the fuel density in real time. The air flow was metered by a laminar flow meter with high accuracy manometer. The phasings of the cam shafts relative to the crank shaft were maintained constant. In this study, the crank angles (deg) are referenced as after top dead center of the combustion stroke, ATDC. The gasoline fuel is a research-grade gasoline with an antiknock index of 93.6. It is a high-octane gasoline and its specifications are given in Table 2.

2.3. Intake port adapter As a method to enhance the in-cylinder turbulence level for this study, a custom adapter presented in Fig. 3a is installed to the intake port, as shown in Fig. 3b. For better understanding of the effects of the intake port adapter on the in-cylinder turbulence level, Fig. 4 compares the development of spark-channel stretch between without (upper) and with (lower) the intake port adapter for motored operation at 2000 rpm when a single spark discharge was occurred by ten spark ignition coils at -30degATDC. For these imaging, an optical engine was used, which has a nearly identical optical piston and head configuration to all-metal engine in Table 1. Side-view imaging for high-speed plasma was accomplished using a Memrecam GX-8. The acquired image resolution was 1024 × 768 pixels with the frame rate of 60000fps. Clearly, the stronger stretching of the spark channel is observed for motored operation with the intake port adapter. As mentioned above, this happens because enhanced gas flow in the spark-plug gap by the intake port adapter stretches the spark channel significantly during spark discharge. In addition, it can be noted from the shape of spark-channel stretch that the gas in the spark-plug gap flows from intake side to exhaust side. However, the direction of gas flow in the spark-plug gap is reversed for motored operation without intake port adapter.

2.2. High-energy inductive ignition system A high-energy inductive ignition system was fabricated for this study, which consists of a single spark plug, ten spark coils and an ignition-timing controller. As Fig. 1 shows, two spark coils were connected in series as a set, and five sets of two spark coils were connected in parallel. Each set of two spark coils was connected to the ignitiontiming controller capable of transmitting 5 ignition signals. The timing for the first of the 5 ignition signals was controlled by the electronic control unit (ECU) coupled to a shaft encoder of the engine. After a first ignition signal was transmitted, the remaining 4 ignition signals were transmitted in the time domain, in accordance with the time offset. The primary current between battery and spark coils was measured by Tektronix TCP303 current probe with Tektronix TCPA300 amplifier. For measuring discharge voltage and discharge current, Tektronix P6015A high-voltage probe and Tektronix TCPA312 current probe were used at the high tension cord. The data for discharge voltage and current from each probe were monitored by Tektronix MDO3034 oscilloscope and acquired by Onosokki DS3000 data acquisition (DAQ) system. The high tension cord from the distributor was connected to a double fine wire electrode spark plug (DENSO) featuring only the

Table 1 Engine specifications. Displacement (single-cylinder) Stroke Bore Connecting Rod Compression Ratio Number of Valves Intake Valve Open Intake Valve Close Exhaust Valve Open Exhaust Valve Close

3

497.0 cm3 112.5 mm 75.0 mm 250.0 mm 13:1 4 21° BTDC @ 0.1 mm lift 78° ABDC @ 0.1 mm lift 63° BBDC @ 0.1 mm lift 22° ATDC @ 0.1 mm lift

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30 20 0

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Fig. 2. Comparison of typical traces of (a) primary current, (b) discharge voltage, (c) discharge current, (d) discharge energy release rate and (e) integrated spark discharge energy for single discharge from a single (black line) and ten spark coils (blue line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2.5. Measurement uncertainty for spark discharge energy and duration This section explains the measurement uncertainty for electrical characteristics of spark discharge. Three factors might contribute to uncertainty and inconsistency in obtaining spark discharge duration and energy.

+

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For each operating point, the engine was allowed to run for several minutes until all measured parameters were stable, at which point the data were acquired. Air flow and fuel flow rates, thermocouple readouts, and exhaust emissions data were averaged over one minute. A Horiba MEXA-1600DEGR emission analyzer was used to measure CO, CO2, HC and NOx. The in-cylinder gas pressure, intake and exhaust pressures, and discharge voltage and current were acquired for 200 consecutive cycles using 0.1°CA resolution. For the in-cylinder pressure, an uncooled AVL GH15D piezoelectric sensor was used in combination with a Kistler 5018A charge amplifier. Fig. 5a shows an example of trace of in-cylinder gas pressure. The heat-release rate (HRR) shown in Fig. 5b is computed from the in-cylinder pressure for each individual cycle. For computing combustion-phasing metrics like the 5%, 10% and 90% burn points (CA5, CA10 and CA90), the HRR is integrated over the crank-angle range for which HRR is positive, as shown in Fig. 5c. As mentioned in the introduction, for quantitative comparisons, the inflammation time and the combustion duration are equated to the ST-toCA5 duration and the CA10-to-CA90 duration, respectively, as the initial phase and main phase of combustion. As means of showing the degree of total heat release from the combustion, the heat-release efficiency (ηhr) is presented as a reference of completeness of combustion. Here, the heat-release efficiency used in this study is the ratio of the total heat release during the combustion (ΣQhr) to the lower heating value of the fuel supplied per cycle (Qfuel), as shown in Eq. (1).

∑

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2.4. Experimental procedure and data analysis

ηhr [%] =

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Discharge Energy Release Rate dEd /dt [J/s]

93.6 99.8 87.5 0.7476 g/cm3 87.1 wt.% 12.4 wt.% 0.40 wt.% 14.22 42.28 MJ/kg 43.8 vol.% 36.6 vol.% 19.6 vol.%

Discharge Current Id [mA]

Antiknock Index (R + M)/2 RON MON Density@15 ± °C Carbon Hydrogen Oxygen A/F Stoichiometric Lower Heating Value Saturates Aromatics Olefins

Primary Current I1 [A]

Table 2 Gasoline specifications.

Ignition-Timing Controller

Fig. 1. Schematic of high-energy inductive ignition system, which consists of a single spark plug, ten spark coils and an ignition-timing controller.

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(1) As explained in the section for high-energy inductive ignition system, the discharge voltage and the discharge current are measured by each prove, respectively, and acquired by Onosokki DS3000 DAQ system. However, both signals for discharge voltage and current are acquired for 200 consecutive cycles using only at 0.1deg resolution. Under this low resolution (120 kHz), the discharge voltage and current during spark discharges cannot be captured accurately, especially for the breakdown phase due to a very short time duration (ns). During the breakdown phase, the discharge current rises to a first current maximum of several hundred amperes, which offers the highest power levels of up to several megawatts [40]. (2) The spark discharge energy for this study is calculated by only using discharge voltage and current. Because the secondary circuit impedance of high-energy inductive ignition system, which accounts for breakdown and arc mode energy released within 1 us after breakdown, was not considered, it could lead to a measurement uncertainty for the spark discharge energy. (3) In this study, the spark discharge duration is defined as the time for which the discharge energy release rate is positive. However, the measurement noise for discharge voltage and current could make the discharge energy release rate be negative during spark discharge, which leads to the lower spark discharge energy with the shorter spark discharge duration.

1000 0

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Crank Angle θ [degATDC] Fig. 5. An example of trace of (a) in-cylinder gas pressure, (b) heat-release rate, and (c) integrated heat release with the definitions of CA5, CA10 and CA90 for explaining ST-toCA5 duration and CA10-to-CA90 duration.

spark coil, 14.7% improvement of ηth relative to that of stoichiometric operation is observed at λ = 1.51, but ηth drops off at the higher λ. This observed drop in ηth is caused by a deterioration of combustion stability. As a measure of the combustion stability, the coefficient of variation (COV) of net IMEP (IMEPn) is computed for all 200 cycles. The resulting changes in COV of IMEPn with λ are shown in Fig. 6e for the three data sets. For all data sets, COV of IMEPn increases with increasing λ to the point of being unacceptable. (The combustion stability is not considered unacceptable until COV of IMEPn > 5%.) Nonetheless, it can be noted that ηth does not drop off quite as rapidly at the higher λ for operation by ten spark coils. As might be expected from Fig. 2, the increased spark discharge energy by ten spark coils is effective at shifting the stability

3.1. Effects of spark discharge energy and in-cylinder turbulence level on lean-stability limits One of the objectives of this study is to investigate the effects of the spark discharge energy and the in-cylinder turbulence level on the leanstability limit. This section identifies how the stability limit is shifted to leaner operation by increased spark discharge energy and enhanced incylinder turbulence level. Fig. 6a shows an improvement of the indicated thermal efficiency (ηth) with an increase of excess-air ratio (λ) for operation by a single spark coil, by ten spark coils, and by ten spark coils with the intake port adapter. First, for operation with a single

without

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The engine speed was kept at 2000 rpm with the port fuel injection at an injection pressure of around 300 kPa throughout the study.

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3. Results and discussion

Spark Timing 0.0ms 0.0deg In Ex

In

5000 4000

Heat-Release Rate dQ hr /dθ [J/deg]

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Fig. 3. Rendering of (a) an intake port adapter and (b) a cross-section of cylinder head equipped with the intake port adapter.

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Fig. 4. Comparison of the development of spark-channel stretch for motored operation (2000 rpm) without (upper) and with (lower) the intake port adapter when using a single discharge of ten spark coils. Side-view images at the spark timing of −30 degATDC.

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of stoichiometric operation. However, ηth drops off quickly when cycleto-cycle variations of SI combustion become unacceptable. Therefore, cycle-to-cycle variations of SI combustion for lean operation need to be discussed in more detail before examining the data sets in Fig. 6. First, Fig. 7 shows how ηth varies with λ for operation by ten spark coils with the intake port adapter. With the average ηth, ηth for all 200 cycles are plotted against the average λ. It can be seen that cycle-to-cycle variations of ηth increase with increasing λ, despite the increase of average ηth. However, at the average λ of 1.94 and 1.97, an increased frequency of low ηth that would be partial burns and/or misfires leads to the decrease of average ηth. (Note that a partial-burn cycle is defined as having an IMEPn lower than 95% of the average IMEPn.) On the other hand, the higher ηth appears at the higher λ, which can be demonstrated by the maximum ηth plot. (It can be noted that for the maximum ηth, all cycles that are preceded by partial-burn and misfire cycles are excluded.) This indicates that the leaner operation has strong potential for ensuring the higher ηth if fast, stable and complete combustion can be realized. To illustrate the changes in phasings of the initial and the main combustion, the spark timing, CA5, CA10, CA50 and CA90 are presented in Fig. 8 for each average λ. It is evident that the CA10-to-CA90 duration is longer for the higher λ. However, much more distinct is the large increase of time required from spark timing to CA10. For λ = 1, it takes 11.9deg from start of spark discharge to CA5. However, at λ = 1.94, for example, the ST-to-CA5 duration is 30.2 deg. This can be explained by the compounding effects of the lower laminar flame speed at the higher λ, and the lower compressed-gas temperature at the earlier crank angles, as stated in the introduction. This is more clearly identified in Fig. 9a, which shows the CA10-to-CA90 duration plotted

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Fig. 7. Indicated thermal efficiency of 200 cycles against average λ, and these average for operation by ten spark coils with the intake port adapter.

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th

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Fig. 6. Effect of spark discharge energy and turbulence level on (a) improvement of indicated thermal efficiency, (b) spark timing, (c) ST-to-CA5 duration, (d) CA10-to-CA90 duration, and (e) COV of IMEPn.

limit to higher λ (=1.81). Furthermore, the combination of increased spark discharge energy and enhanced in-cylinder turbulence level by the intake port adapter can then further shift the lean-stability limit, and this achieves the leanest stable operation at λ = 1.9 with 16.5% improvement of ηth relative to that of stoichiometric operation. The ability to operate stably at such lean conditions can be attributed to a more stable and shorter ST-to-CA5 duration. As Fig. 6c shows, both increased spark discharge energy and enhanced turbulence level can shorten the ST-to-CA5 duration, which allows a later spark timing. The selected spark timings for minimum advance for best torque (MBT) are shown in Fig. 6b, which was determined by sweeping spark timing in small steps. However, it should be noted that neither of these has effect on the CA10-to-CA90 duration, as shown in Fig. 6d. Moreover, the longer CA10-to-CA90 duration is observed for operation by ten spark coils compared to operation by a single spark coil. These results in Fig. 6 motivate a detailed examination of the data sets to understand the physics responsible for these trends.

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20 10 0 -10 -20 -30 -40 1

3.2. Cycle-to-cycle variations of SI combustion for lean operation

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Fig. 8. Spark timing, CA5, CA10, CA50 and CA90 as a function of average λ, corresponding to the data point of average ηth in Fig. 7.

As discussed briefly above, and as can be observed in Fig. 6a, lean operation can offer considerable improvement of the ηth relative to that 6

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COV of CA10-to-CA90 Duration [%]

CA10-to-CA90 Duration 10-90 [deg]

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heat release traces showing substantially lower total heat release (ΣQhr) than the average ΣQhr or near zero. On the other hand, many high-ΣQhr integrated heat release traces are also observed, which are preceded by a low-ΣQhr integrated heat release trace. This happens because the fuel recirculated from one cycle to the next with the residuals enhances the combustion sufficiently to be much higher than the average ΣQhr. As can be clearly observed in the results presented below, for a given λ of 1.56, the increased spark discharge energy by ten spark coils reduces these excessive cycle-to-cycle variations substantially. In addition, the enhanced in-cylinder turbulence level with the increased spark discharge energy makes the slope of the average integrated heat release curve steeper. This may contribute as well to the additional reduction of COV of IMEP. The combined effects of the increased spark discharge energy and the enhanced in-cylinder turbulence level eventually allows stable operation up to λ = 1.9, which cannot be realized solely by ten spark coils for this operating condition. However, despite the combination of ten spark coils and intake port adapter, the cycle-to-cycle variations of combustion increase with the increase of λ, and become unacceptable again at λ = 1.97. To better understand the effects of the increased spark discharge energy and the enhanced in-cylinder turbulence level on cycle-to-cycle variations of SI combustion, the integrated heat release traces in Fig. 11 are examined in more detail in terms of initial and main combustion durations and heat-release efficiency for each λ. Fig. 12 compares the ST-to-CA5 duration for all 200 cycles for the three data sets at λ = 1.0, 1.56 and 1.9. The data are plotted against the CA10-to-CA90 duration to clarify the correlation between the ST-to-CA5 duration and the CA10to-CA90 duration. For operation at λ = 1.0, each data set overlaps in a fairly narrow range (8.8deg < Δθst-5 < 15.8deg and 10.8deg < Δθ10-90 < 21.4deg), as shown in Fig. 12a. As mentioned in Fig. 11 for λ = 1.0, both increased spark discharge energy and enhanced in-cylinder turbulence level do not offer any benefits in terms of shortening the combustion duration for stoichiometric operation. Rather, for operation by ten spark coils with the intake port adapter, cycles that have a relatively long CA10-to-CA90 duration are observed. On the other hand, Fig. 12b reveals that for lean operation with λ = 1.56, the increased spark discharge energy by ten spark coils can strongly shorten the ST-to-CA5 duration, although this ST-to-CA5 duration is longer than that of λ = 1.0. Furthermore, by the intake port adapter for the

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Heat-Release Efficiency hr [%]

against the ST-to-CA5 duration for each λ. As can be seen, there is a relatively good correlation between the ST-to-CA5 duration and the CA10-to-CA90 duration, despite large differences in sensitivity to λ. Because of this, a similar trend in Fig. 9a is expected for the cycle-tocycle variations between these durations. However, as Fig. 9b shows, COV of CA10-to-CA90 duration does not increase linearly with the increase of COV of ST-to-CA5, in particular for operation at λ = 1.0 and 1.56. Based on the results shown in Fig. 9, Fig. 10 presents the relationship between the CA10-to-CA90 duration and the heat-release efficiency (ηhr). As plotted in Fig. 10a, the correlation between the shorter CA10-to-CA90 duration and the higher ηhr is relatively clear, except for operation at λ = 1.0. In addition, the trend of the higher COV of ηhr with the higher COV of CA10-to-CA90 duration is observed in Fig. 10b. Consequently, for lean operation, shortening the ST-to-CA5 duration is critically important factor for lean operation to reduce the cycle-to-cycle variations of SI combustion for increasing heat-release efficiency, and eventually achieve large improvement of ηth. 3.3. Underlying mechanisms responsible for the cycle-to-cycle variations of combustion duration and heat-release efficiency

COV of Heat-Release Efficiency [%]

This section explains the underlying mechanisms responsible for the cycle-to-cycle variations of combustion duration and heat-release efficiency that are observed for stoichiometric and lean operations. Fig. 11 shows integrated heat release traces of 200 cycles (colored lines) and these average (white line) for operation by a single spark coil (top), ten spark coils (middle), and ten spark coils with the intake port adapter (bottom) at λ = 1.0, 1.56, 1.9 and 1.97. The corresponding discharge energy release rate (black line) is also plotted against the right-hand axis in Fig. 11. For stoichiometric operation with λ = 1.0, relatively small cycle-to-cycle variations of integrated heat release are observed for all cases. Rather, it would seem that the cycle-to-cycle variations for operation by ten spark coils with the intake port adapter is slightly larger compared to operation by a single spark coil. This indicates that both increased spark discharge energy and enhanced in-cylinder turbulence level have no effects on the cycle-to-cycle variations of SI combustion for stoichiometric operation. As can be expected from Fig. 6e, for the operation by a single spark coil at λ = 1.56, partial-burn and misfire cycles start to appear, which corresponds to the integrated

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COV of CA10-to-CA90 Duration [%] Fig. 10. Relationship between CA10-to-CA90 duration and heat-release efficiency for (a) average and (b) its COV.

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Fig. 11. Integrated heat release traces of 200 cycles (colored lines) and these average (white line) with discharge energy release rate (black line) for operation with a single spark coil (top), ten spark coils (middle), and ten spark coils with the intake port adapter (bottom) at λ = 1.0, 1.56, 1.9 and 1.97. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 12. Relationship between ST-to-CA5 duration and CA10-to-CA90 duration for λ = 1.0, 1.56, and 1.9, corresponding to the data in Fig. 11.

Fig. 13. Relationship between CA10-to-CA90 duration and heat-release efficiency for λ = 1.0, 1.56, and 1.9, corresponding to the data in Fig. 11.

enhanced in-cylinder turbulence level, the ST-to-CA5 duration is further shortened. However, not evident from Fig. 12b that there is a large change in the CA10-to-CA90 duration for both cases. Instead, only significant scatter of each duration is observed in the data for operation by a single coils. At λ = 1.9, this scatter eventually becomes too significant to not be presented in Fig. 12c. Overall, both the ST-to-CA5 and the CA10-to-CA90 durations are longer for the leaner operation with λ = 1.9, compared to those of operation at λ = 1.0 and 1.56. Comparing the ST-to-CA5 durations for operation without and with the

intake port adapter in Fig. 12c reveals that enhanced in-cylinder turbulence level by the intake port adapter has strong potential to shorten the ST-to-CA5 duration, even for lean operation. The shorter ST-to-CA5 duration for operation with the intake port adapter is consistent with findings in Fig. 12b. In addition, the shorter ST-to-CA5 duration is thought to be the reason for the shorter CA10-to-CA90 duration. The trend of shorter CA10-to-CA90 duration with shorter ST-to-CA5 duration is also observed for operation without the intake port adapter. However, the correlation between the ST-to-CA5 duration and the

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between the higher spark discharge energy and the shorter ST-to-CA5 duration is unclear for both cases. Instead, the ST-to-CA5 duration is shortened by enhanced in-cylinder turbulence level, as explained in Fig. 12. This trend is also observed for the CA10-to-CA90 duration, as shown in Fig. 18b. In addition, as shown in Fig. 18c, there is no relationship between the total spark discharge energy and ηhr. Eventually, enhanced in-cylinder turbulence level by intake port adapter shortens the ST-to-CA5 duration, and leads to higher ηhr with less cycle-to-cycle variations. To better understand the cycle-to-cycle variations of SI combustion for lean operation, there is a need to investigate the effects of spark discharge energy and duration on the initial and main combustion durations.

CA10-to-CA90 duration does not seem to be enough, especially for the longer ST-to-CA5 durations than 30deg. Based on the observation in Fig. 12, ηhr for all 200 cycles are plotted against the CA10-to-CA90 duration for the three data sets at λ = 1.0, 1.56 and 1.9. As Fig. 13a shows, there are no significant cycle-to-cycle variations of ηhr, as might be expected from both the ST-to-CA5 duration and the CA10-to-CA90 duration in Fig. 12a. For lean operation at λ = 1.56, operation by a single coil shows some cycles that have distinct ηhr. On the other hand, there is no significant difference in ηhr for operation by ten spark coils without and with the intake port adapter. For two cycles with a much longer CA10-to-CA90 duration, relatively lower ηhr is observed, which can be considered as partial-burn cycles. In addition, ηhr is essentially zero due to misfire cycles when the CA10-to-CA90 duration is near zero. In contrast, the cycles that have ηhr near 100% are observed, which are preceded by a low-ηhr cycle (i.e. partial-burn and/or misfire cycles), as discussed in conjunction with Fig. 11. This indicates that the residual unburned fuel is contributing to the high ηhr. Finally, for the longer CA10-to-CA90 duration than 20deg at λ = 1.9, ηhr is reduced significantly for both cases. This means that the CA10-to-CA90 duration should be shortened as much as possible to ensure high ηhr for stable lean operation, and eventually for high ηth.

4. Conclusion This study experimentally investigates the effects of spark discharge energy and in-cylinder turbulence level on the lean-stability limit, and identifies the changes of cycle-to-cycle variations of SI combustion with the extension of lean-stability limit. The following conclusions are offered:

As discussed above, the spark discharge energy is critically important for stable lean operation. This section examines and compares the relationship between the spark discharge energy and the SI combustion for ultra-lean operation without and with the intake port adapter. With ten spark coils, this experiment was conducted with a constant fueling rate at 24.0 mg/cycle to within ± 0.8%. Fig. 14a shows that there are large cycle-to-cycle variations of the integrated spark discharge energy even for operation without the intake port adapter. Moreover, by the intake port adapter, the level of cycle-to-cycle variations of integrated spark discharge energy becomes larger, as shown in Fig. 15a. These variations cause strong variations of the discharge, both in terms of the total spark discharge duration and the total spark discharge energy. First, Fig. 16 compares the start and the end of spark discharge timing for operation between without and with the intake tumble adapter. As can be seen, the start of spark discharge timing is almost constant for both operations. On the other hand, the end of spark discharge timing varies significantly. Furthermore, the degree of variations of end of spark discharge timing becomes much larger by the intake port adapter. Consequently, the cycle-to-cycle variations of spark discharge duration are mainly occurred by the variations of end of spark discharge. To provide quantitative comparisons regarding variations of the spark discharge for operation without and with intake port adapter, Fig. 17 presents the total spark discharge energy plotted against the total spark discharge duration for 200 cycles. As like in the case of spark discharge duration shown in Fig. 16, the larger cycle-tocycle variations of total spark discharge energy are observed for the operation with the intake port adapter. This may happen because stochastic variations of fuel concentration, temperature and turbulent flow in the spark-plug gap. However, a comparison of the data for integrated heat release in Fig. 14b and Fig. 15b shows that less cycle-to-cycle variations of combustion are observed for operation with the intake port adapter, despite the larger cycle-to-cycle variations of both the total spark discharge duration and the total spark discharge energy. This indicates that variations of the combustion may not be directly related to variations of the spark discharge. Nonetheless, it should be noted that the higher total spark discharge energy tends to have the shorter spark discharge duration. Lastly, to clarify the relationship between the spark discharge energy and the SI combustion for lean operation without and with the intake port adapter, Fig. 18 presents the combustion duration and the ηhr plotted against the total spark discharge energy for 200 cycles. As Fig. 18a shows, the relationship

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(1) The increased spark discharge energy by ten spark coils shifts the stability limit to λ = 1.81. Furthermore, the combination of increased spark discharge energy and enhanced in-cylinder turbulence level by the intake port adapter can then further shift the leanstability limit, and this achieves the leanest stable operation at λ = 1.9 with 16.5% improvement of indicated thermal efficiency (ηth) relative to that of stoichiometric operation. (2) The correlation between the ST-to-CA5 duration and the CA10-toCA90 duration is relatively good, despite large differences in sensitivity to λ. However, COV of CA10-to-CA90 duration does not increase linearly with increasing COV of ST-to-CA5, in particular for operation at λ = 1.0 and 1.56. The correlation between the shorter CA10-to-CA90 duration and the higher ηhr is relatively clear with the trend of the higher COV of ηhr with the lower COV of CA10-toCA90 duration, except for operation at λ = 1.0. (3) Both increased spark discharge energy and enhanced in-cylinder turbulence level are effective at shortening the ST-to-CA5 duration for lean operation. However, there is no large changes in the CA10to-CA90 duration by the changes spark discharge energy and incylinder turbulence level. To achieve stable lean operation with high heat-release efficiency (ηhr), the ST-to-CA5 duration and the CA10-to-CA90 duration should be shortened within 30 deg and 20 deg, respectively.

3.4. Relationship between spark discharge energy and SI combustion for ultra-lean operation

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(4) The relationship between the variations of the spark discharge energy and the variations of the combustion is unclear because the total spark discharge energy does not affect both the ST-to-CA5 duration and the CA10-to-CA90 duration. (5) To realize stable lean operation which shows a strong potential for improving the thermal efficiency of SI engine, the combination of advanced ignition system for increasing spark discharge energy and intake port adapter for enhancing in-cylinder turbulence level is a practical and effective way.

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References [1] IPCC. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. [Stocker TF, Qin D, Plattner GK, Tignor M, Allen SK, Boschung J, Nauels A, Xia Y, Bex V, Midgley PM (eds.)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA; 2013. [2] Nakata K, Nogawa S, Takahashi D, Yoshihara Y, et al. Engine technologies for achieving 45% thermal efficiency of S.I. engine. SAE Int J Eng 2016;9(1):179–92. http://dx.doi.org/10.4271/2015-01-1896. [3] Splitter D, Wissink M, DelVescovo D, Reitz R. RCCI engine operation towards 60% thermal efficiency. SAE technical paper 2013-01-0279; 2013. http://dx.doi.org/10. 4271/2013-01-0279. [4] Heywood JB. Internal combustion engine fundamentals. New York: McGraw-Hill;

Acknowledgements This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Innovative Combustion Technology” (Funding agency: Japan Science and Technology Agency (JST)).

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