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Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

# Performance of a Laser Ignited Multicylinder Lean Burn Natural Gas EngineOPEN ACCESS

[+] Author and Article Information

Department of Mechanical and
Aerospace Engineering,
University of Central Florida,
4000 Central Florida Boulevard,
Orlando, FL 32816

Subith Vasu

Department of Mechanical and
Aerospace Engineering,
University of Central Florida,
4000 Central Florida Boulevard,
Orlando, FL 32816
e-mail: subith@ucf.edu

Sreenath B. Gupta

Argonne National Laboratory,
362-G212, 9700 South Cass Avenue,
Lemont, IL 60439
e-mail: sgupta@anl.gov

Qing Wang

Princeton Optronics, Inc.,
1 Electronics Drive,
Mercerville, NJ 08619
e-mail: qwang@princetonoptronics.com

Robert Van Leeuwen

Princeton Optronics, Inc.,
1 Electronics Drive,
Mercerville, NJ 08619
e-mail: rleeuwen@princetonoptronics.com

Chuni Ghosh

Princeton Optronics, Inc.,
1 Electronics Drive,
Mercerville, NJ 08619
e-mail: cghosh@princetonoptronics.com

1Corresponding author.

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received October 3, 2016; final manuscript received April 19, 2017; published online June 6, 2017. Assoc. Editor: Eric Petersen.The United States Government retains, and by accepting the article for publication, the publisher acknowledges that the United States Government retains, a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States government purposes.

J. Eng. Gas Turbines Power 139(11), 111501 (Jun 06, 2017) (7 pages) Paper No: GTP-16-1480; doi: 10.1115/1.4036621 History: Received October 03, 2016; Revised April 19, 2017

## Abstract

Market demands for lower fueling costs and higher specific powers in stationary natural gas engines have engine designs trending toward higher in-cylinder pressures and leaner combustion operation. However, ignition remains as the main limiting factor in achieving further performance improvements in these engines. Addressing this concern, while incorporating various recent advances in optics and laser technologies, laser igniters were designed and developed through numerous iterations. Final designs incorporated water-cooled, passively Q-switched, Nd:YAG microlasers that were optimized for stable operation under harsh engine conditions. Subsequently, the microlasers were installed in the individual cylinders of a lean-burn, 350 kW, inline six-cylinder, open-chamber, spark ignited engine, and tests were conducted. The engine was operated at high-load (298 kW) and rated speed (1800 rpm) conditions. Ignition timing (IT) sweeps and excess-air ratio (λ) sweeps were performed while keeping the NOx emissions below the United States Environmental Protection Agency (USEPA) regulated value (brake-specific NOx (BSNOx) < 1.34 g/kW h), and while maintaining ignition stability at industry acceptable values (coefficient of variation of integrated mean effective pressure (COV_IMEP) < 5%). Through such engine tests, the relative merits of (i) standard electrical ignition system and (ii) laser ignition system were determined. A rigorous combustion data analysis was performed and the main reasons leading to improved performance in the case of laser ignition were identified.

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## Introduction

Market demand for higher engine efficiency and higher engine-specific powers with a concomitant requirement for compliance with USEPA emission regulations has engine designers evaluating various low-temperature combustion strategies. Out of these, turbocharged lean-burn operation is the most preferable ones as it offers all of the desirable performance characteristics without the need for an aftertreatment system. Further performance improvements are limited by the ability of ignition systems (i) to ignite very lean fuel–air mixtures and (ii) to reduce the combustion durations under lean burn conditions. Addressing these issues, various organizations are evaluating alternative ignition systems [13]. Among them, laser ignition offers promise as it

• (i)enables ignition under higher pressures,
• (ii)extends the lean ignition limit,
• (iii)enables multipulse ignition, i.e., use of consecutive multiple pulses,
• (iv)enables multipoint ignition, i.e., multiple spatially separated ignition locations and, further,
• (v)allows optimal placement of the ignition kernel in the combustion chamber.

###### Physical Processes Behind Laser Ignition.

Though laser ignition has been researched since 1960s, a clear understanding for improved ignition performance has evaded our understanding. However, recent studies have brought new insights into the processes associated with laser ignition [49] and the picture that emerges can be summarized as follows.

When a high power laser pulse is focused in a gaseous medium, high field gradients are introduced at the focal point, which result in stripping of the electrons from the local molecules via a multiphoton ionization process. These free electrons, in turn, absorb laser energy via inverse bremsstrahlung absorption, accelerate, and result in the release of more electrons when they collide with other gas molecules. This results in a cascade breakdown leading to the electron concentration increasing almost exponentially with time. The resulting plasma kernel is relatively opaque and absorbs the remaining incoming photons to increase in energy density. This results in a tear-drop shaped plasma kernel that extends toward the incoming laser. As the resulting high temperatures (∼50,000 K) and high pressures (∼100 atm) are localized close to the focal spot position, a blast wave develops and propagates into the surrounding gas. This shock wave, though tear-drop shaped initially, expands into a spherical wave as it expands into the surrounding gas. In parallel, the plasma core collapses and fluid elements move from the front of the plasma kernel in a direction opposite to laser propagation. This results in a roll-up leading to an expanding toroid.

Following the shock wave propagation, energy is transferred to the surrounding fuel–air mixture primarily through advection. In the immediate vicinity where temperatures reach 2000–3000 K, combustion reactions occur and a flame front develops. The energy released in this thin flame region helps sustain an expanding flame that results in “successful ignition.”

With the above sequence in mind, Endo et al. [7] following a rigorous experimental study and analysis concluded that the improved ignitability of a laser-induced spark is not only due to the avoidance of heat loss to the electrodes, but also due to the initial flame kernel enhancement as its apparent energy is augmented by the rapid heat release from the combustible mixture drawn into the kernel by a nonspherical inward flow which is created by the laser-induced spark.

###### Previous Attempts for Developing Laser Ignition Systems.

In parallel to the above efforts to improve our understanding of the physical processes behind laser ignition, numerous attempts have been made to reduce laser ignition to practice in reciprocating engines. Transmission of the high power laser output using optical fibers has been attempted by Bihari et al. [10] and by Yalin et al. [11,12], however, proved extremely challenging considering the fact that the required peak laser powers were close to the material breakdown thresholds. Alternately, researchers in Japan primarily led by efforts at the Institute of Molecular Science have devised a scheme wherein the long pulse from a pump laser is transmitted via conventional optical fibers and temporally compressed closer to the spark plug well [13]. Gupta et al. [14] have even evaluated the use of free-space transmission and mechanically multiplexing the pulsed laser output among various cylinders of the engine. However, variation in the time-response of electromechanical actuators limited the use of this approach to engine speeds lower than 1200 rpm.

On another note, recent advancements in laser materials and laser pumping schemes have resulted in microlasers having sufficient laser pulse energies to be able to create sparks when focused [13,1517]. While such designs are still evolving, we report here the results from ignition tests conducted using one such state-of-the art microlaser design.

## The Laser Igniters

Schematic representation of the microlaser along with a photograph is shown in Fig. 1. It consists of a water-cooled high power vertical cavity surface emitting laser (VCSEL) pump whose output is focused using a single positive lens to end-pump a Nd:YAG rod having appropriate antireflection coatings. At the other end of the Nd:YAG rod is a $Cr4+$:YAG saturable absorber that acts as a passive Q-switch. The pump VCSEL is operated up to 500 μs resulting in an output comprising of two 1064 nm pulses, approximately 190 μs apart, each with 14.5–16.5 mJ and 4.72 ns pulse width at Full Width at Half Maximum (FWHM). On account of various advanced design features of this microlaser, it had a wall-plug efficiency of 2.22%, which amounts to the requirement of a mere 1.35 J of electrical energy per combustion cycle for laser operation. Considering the fact that these are passively Q-switched lasers (as opposed to actively Q-switched lasers), they are also known as PQLs in the literature [8].

The microlaser of the above design was placed at the distal end of a hollow tube. The 1064 nm pulsed output was focused into the combustion chamber using a single sapphire lens having a back focal length of 8.6 mm. While the beam quality was not measured, it was sufficient to successfully create sparks in 1 atm air when the laser beam of 2 mm diameter was focused using the sapphire lens. Copper seals placed beneath the sapphire lens prevent the leakage of hot combustion gases into the hollow tube. Overall, the arrangement shown in Fig. 1 had the same footprint as a standard spark plug and would fit in the spark plug well without any interference.

Initial tests performed on the engine by placing the laser igniter in the cylinder with the hottest environment showed that the microlasers on these igniters would attain temperatures as high as 72 °C under full load conditions. Furthermore, a thermal analysis performed using temperatures measured at various points on the igniter showed an overall heat flux of 25 W, which could be removed by circulating cooled water. With the passive Q-switching used here, a pump laser operating at a slightly higher temperature results in delayed emission of the laser pulses from the rising edge of the trigger pulse. This, in turn, translates to delayed ignition timing in the engine. To improve the accuracy of ignition timing, the cooling water temperature was tightly controlled to be 22 ± 1 °C, and further the timing was monitored using a photodiode installed on the walls of the laser igniter (see Fig. 1).

## The Engine Test Platform

A Cummins QSK19G, turbocharged, inline six-cylinder, 350 kW, lean-burn natural gas fueled stationary engine was used as the test platform. Further specifications of the test engine are provided in Table 1. This engine was coupled to a 465 kW AC dynamometer to facilitate engine tests. Individual cylinders were instrumented with pressure transducers (Kistler 6067 C) and in-cylinder pressure data were recorded using an AVL indicom system after signal amplification using Kistler 5010B charge amplifiers. Cummins supplied calterm-II software enabled control of various engine operational parameters.

The standard capacitance discharge ignition (CDI) system on the engine was replaced with six laser igniters to perform the laser ignition tests (see Fig. 2). The engine control system used a UEGO sensor in conjunction with a Woodward throttle actuator to allow variation of the λ value. A National Instruments Field Programmable Gated Array (FPGA)-based control system that used signals from an encoder mounted on the crankshaft enabled changing the ignition timing (IT). The intake airflow and the fuel flow were measured using an appropriately sized laminar flow meter and a Coriolis flow meter. Emission measurements were performed using a Horiba 7100D emissions bench. Subsequently, the measured emission values were processed per the procedure given in the SAE J1003 standard.

## Test Matrix

According to tests conducted at Argonne and elsewhere [18], ignition stability could be significantly improved by operating the microlasers for multipulse operation. However, considering the power limitations of the laser diode drivers used here, the microlasers were operated for two consecutive pulses per combustion cycle, which were roughly separated by 190 μs (i.e., approximately 2 CAD).

The engine was operated at the rated speed of 1800 rpm and the throttle was adjusted to operate at 298 kW. The excess air ratio, λ, was varied from 1.6 to an ignition limited value, and the ignition timing was varied from 13 deg BTDC to a value limited by NOx emissions. Overall, the test matrix was fixed to steady-state test conditions closely confirming to

• (i)COV_IMEP < 5% (an industry accepted standard) and
• (ii)Brake specific NOx < 1.34 g/kW h (a US-EPA emission regulation).

For the test conditions used here, engine knocking was never encountered.

The above test matrix was repeated evaluating the performance of both standard spark ignition system (SI) and two-pulse laser ignition system (2 P-LI). SI was imparted using a capacitance discharge ignition (CDI) system that deposits 25–35 mJ per combustion cycle, whereas 2 P-LI was imparted using laser igniters operating at 15 mJ/pulse.

## Results and Discussion

For an ideal Otto cycle, one can represent the cycle efficiency as Display Formula

(1)$ηcycle=1−1CR(γ−1)$

where CR is the compression ratio and γ = (Cp/Cv) is the ratio of specific heats. In lean fuel–air mixtures, γ value increases and as a result efficiency increases (see Eq. (1)). However, in leaner mixtures, the flame speed decreases leading to lower efficiencies. In addition to these two counteracting phenomena, efficiency strongly varies with ignition timing. All of these manifest themselves in the trends shown in Fig. 3.

For the remainder of this paper, we will use the term efficiency, η, to represent brake thermal efficiency = (dyno power/fuel energy rate).

In these sets of tests, for a given excess air ratio, λ, the ignition timing (IT) was varied between 13 and 40 CAD BTDC. However, the actual test points closely confirmed to the two limitations corresponding to BSNOx < 1.34 g/kW h, and COV_IMEP < 5%, that are marked by horizontal red lines in Figs. 3(b), 3(c), 3(e), and 3(f) (see figure online for color). As shown in the case of λ = 1.68 in Fig. 3(b), COV_IMEP decreases with initial IT advance, however, it starts increasing at larger IT advance as the number of partial burns and misfires increases. Such a convex-up trend is not shown in some of the curves in Figs. 3(b) and 3(e), especially those corresponding to richer mixtures at advanced ignition timing, as the test points with BSNOx significantly higher than 1.34 g/kW h were excluded from the test matrix.

In a typical Otto cycle engine, with IT advance, efficiency increases but eventually decreases after the Maximum Brake Torque (MBT) timing is exceeded. Within the range of IT variation used here (see Figs. 3(a) and 3(d)), the engine efficiency monotonically increases with ignition timing advancement. Figures 3(c) and 3(f) show a similar trend for BSNOx variation with ignition timing.

Additionally, noticing the trends for 2P-LI in Figs. 3(e) and 3(d), one notices that for a given ignition timing (say, 30 CAD BTDC), COV_IMEP increases with the increasing λ, whereas efficiency shows the opposite trend.

In the case of 2P-LI, extension of the lean ignition limit up to λ = 1.7 is possible; however, the reduction in apparent flame speeds under lean-burn conditions appears to offset any efficiency gains due to increased γ values. As a result, identification of the optimal engine operating condition having high efficiency, low NOx emissions, and low COV_IMEP requires variation of both IT and λ.

The measured values of COV_IMEP, BSNOx for the above two ignition strategies are shown as a function of engine efficiency in Fig. 4. The current EPA emission regulation for BSNOx of 1.34 g/kW h is marked by a horizontal red line (see figure online for color). The industry accepted value for ignition stability, i.e., COV_IMEP = 5%, is shown similarly marked by a horizontal red line. Confirming to these two limitations, one notices by taking the baseline SI point to correspond to λ = 1.6 and IT = 24 CAD BTDC and the 2P-LI optimal operational point as that corresponding to λ = 1.68 and IT = 36 CAD BTDC, an efficiency gain of 2.6% points is possible. This efficiency gain may not be attributable only to the fact that an ignition advance of 12 CAD is facilitated by improved ignition with the use of 2P-LI. As one shifts from λ = 1.6 to λ = 1.68, the transition of ignition kernel into a sustained flame front and the subsequent combustion of in-cylinder charge are slower necessitating optimal ignition phasing. To gain further insight into the associated combustion processes, in-cylinder pressure data were processed to compare ignition delays and combustion durations.

Due to the design of the cooling passages in the inline six-cylinder engine used here, cyl#1 is the coolest and cyl#6 is the hottest in operation. To capture the trends midway between these extremes, cylinder pressure data corresponding to cyl#4 were processed and shown in Figs. 5 and 6. In actuality, the trends presented in these figures are observable in all six cylinders.

Figure 5(a) shows the rate of heat release (ROHR) ensemble averaged over 500 cycles for λ = 1.68 and IT = 29 CA BTDC. The corresponding ROHR plots for individual combustion cycles are shown in light color in the background. In spite of the cyclic variations that are typical of lean burn combustion, the ensemble averaged curve is representative of the overall combustion process. Similar plots for 2P-LI, as shown shown in Fig. 5(b), show a narrower spread about the ensemble average indicating smaller cyclic variation. Both of these plots were merged in Fig. 5(c) to compare combustion performance in SI and 2P-LI. As noticed, the ensemble averaged ROHR for 2P-LI shows earlier ignition and further accelerated combustion, which is representative of the trends in individual cycles.

For the remainder of the discussion, values derived from individual, as well as, ensemble-averaged ROHR curves were used: Ignition delay was defined as the time intervel between ignition event and that corresponding to 10% of mass fraction burned (MFB 0–10%); similarly, the time delay between 10% and 90% mass fraction burned is defined as the combustion duration (MFB 10–90%). These values corresponding to cylinder#4 with a spread of ± standard deviation are shown in Fig. 6. As noticed, 2P-LI always results in shorter ignition delays and shorter combustion durations, with these effects more pronounced under leaner conditions.

Additionally, reduction in cyclical variations with laser ignition, especially under lean mixtures conditions, is clearly illustrated. For example, by comparing values of MFB50% of SI and 2P-LI at λ = 1.68, one notices considerably less spread in the case of 2P-LI. Similarly, a closer observation of the standard deviation values of MFB 0–10%, MFB 10–90%, and MFB50% at the optimal points with baseline SI (λ = 1.6 and IT = 24 CAD BTDC) and 2P-LI (λ = 1.68 and IT = 36 CAD BTDC), shown with continuous line circles and dashed line circles in Fig. 6, reveals that combustion not only occurs earlier and faster but is also more stable with 2P-LI.

An important point to note is that for the baseline SI case, an ignition delay of 29 CAD leads to a combustion start at 5.5 CAD ATDC, whereas for the 2P-LI optimal point an ignition delay of 37 CAD leads to combustion start at 0.8 CAD ATDC. With combustion durations being similar (see Fig. 6(b)), this results in combustion phasing, i.e., MFB 50%, in the case of 2P-LI to precede that of baseline SI by 5 CAD (see Fig. 6(c)).

Recently, Sjöberg and Zeng [19] in their tests using a gasoline engine have reported a mixed mode of combustion, wherein the end gas autoignites following a brief period of deflagration. For the 2P_LI optimal point, a graph similar to the ones shown in Fig. 5 is plotted in Fig. 7. Due to the absence of a bimodal distribution in the ROHR curves, both in individual as well as ensemble averaged ones, we are able to rule out the possibility of such a mixed mode of combustion. Indeed, it is difficult to autoignite natural gas which has an Octane number ≥ 110 as compared to gasoline which has octane number in the range 87–93.

From the above discussion, the observed net benefit of Δη = 2.6% points for 2P-LI optimal point over SI baseline case is attributable to two effects: first, laser ignition improves ignition stability to enable highly advanced ignition timing; second, laser ignition enables ignition of leaner fuel–air mixtures. Improved efficiency from the former effect overcomes the loss in efficiency resulting from the latter to result in a net benefit of Δη = 2.6% points.

Measured values of brake specific unburnt hydrocarbons and carbon monoxide emissions are shown in Fig. 8 for the optimal operating conditions under SI and 2P-LI. As noticed, the difference in these emission levels is very small for optimal conditions using either ignition system to have a noticeable impact on combustion efficiency, ηc.

## Conclusions

A six-cylinder natural gas engine was successfully operated at rated speed and high load conditions with microlasers igniting all six cylinders. To the best of the our knowledge, this is the first time that this has ever been performed in the world. Ignition timing (IT) sweeps and excess air ratio (λ) sweeps were performed and the engine performance was compared for the cases of standard spark ignition (SI) and two-pulse laser ignition (2P-LI). A brake thermal efficiency improvement of 2.6% points was observed with the use of 2P-LI. A detailed analysis shows that laser ignition leads to significantly improved ignition under advanced ignition timings, which, in turn, enables optimal phasing to result in efficiency improvements. Further efficiency gains can be envisioned as strategies promoting multipoint ignition or “volumetric ignition” can be devised using laser ignition. These will be pursued in our future efforts.

## Acknowledgements

The authors gratefully acknowledge the help and support of various staff at Argonne and Princeton Optronics, Inc.

This work was performed under financial support from U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC02-06CH11357. Additionally, we acknowledge the partial support of UCF researchers by Public Authority for Applied Education and Training and Argonne National Lab (Contract No. 6F-30762).

## Nomenclature

• BMEP =

brake mean effective pressure (bar)

• BSCO =

brake-specific carbon monoxide (g/kW h)

• BSHC =

brake-specific hydrocarbon (g/kW h)

• BSNOx =

brake-specific NOx (g/kW h)

• COV_IMEP =

coefficient of variation of integrated mean effective pressure, %= 100* (mean of IMEP/standard deviation of IMEP)

• IMEP =

integrated mean effective pressure (bar)

• IT =

ignition timing (CAD BTDC)

• MFB =

mass fraction burned

• MFB 0–10% =

ignition delay = time duration (in CAD) between start of ignition and 10% mass fraction burned

• MFB 10–90% =

combustion duration = time duration (in CAD) between MFB 10% and MFB 90%

• MFB 50% =

combustion phasing = angle corresponding to MFB 50%

• ROHR =

rate of heat release (kJ/m3 deg)

• η =

brake thermal efficiency = (measured dyno power/fuel energy rate) $=(P/m˙f⋅LHV)$

• ηc =

combustion efficiency $=1−(m˙CO⋅LHVCO+m˙UHC⋅LHVUHC)/m˙f⋅LHV$

• λ =

excess air ratio = (1/equivalence ratio)

## References

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Lorenz, S. , Bärwinkel, M. , Heinz, P. , Lehmann, S. , Mühlbauer, W. , and Brüggemann, D. , 2015, “ Characterization of Energy Transfer for Passively Q-Switched Laser Ignition,” Opt. Express, 23(3), pp. 2647–2659. [PubMed]
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View article in PDF format.

## References

Ikeda, Y. , Nishiyama, A. , Katano, H. , Kaneko, M. , and Jeong, H., 2009, “ Research and Development of Microwave Plasma Combustion Engine (Part II: Engine Performance of Plasma Combustion Engine),” SAE Technical Paper No. 2009-01-1049.
Pineda, D. I. , Wolk, B. , Sennott, T. , Chen, J.-Y. , and Dibble, R. W. , 2015, “ Nanosecond Pulsed Discharge in a Lean Methane-Air Mixture,” Third Laser Ignition Conference (LIC), Argonne, IL, Apr. 27–30, Paper No. T5A.2.
Schenk, A. , Rixecker, D. G. , and Bohne, S. , 2015, “ Results From Gasoline and CNG Engine Tests With the Corona Ignition System EcoFlash,” Third Laser Ignition Conference (LIC), Argonne, IL, Apr. 27–30, Paper No. W4A.4.
Grey Morgan, C. , 1978, “ Laser-Induced Breakdown of Gases,” Rep. Prog. Phys., 38(5), pp. 621–665.
Bradley, D. , Sheppard, C. G. W. , Suradjaja, I . M. , and Woolley, R. , 2004, “ Fundamentals of High-Energy Spark Ignition With Lasers,” Combust. Flame, 138(1), pp. 55–77.
Lorenz, S. , Bärwinkel, M. , Heinz, P. , Lehmann, S. , Mühlbauer, W. , and Brüggemann, D. , 2015, “ Characterization of Energy Transfer for Passively Q-Switched Laser Ignition,” Opt. Express, 23(3), pp. 2647–2659. [PubMed]
Endo, T. , Takenaka, Y. , Sako, Y. , Johzaki, T. , Namba, S.-I. , and Shimokuri, D. , 2017, “ An Experimental Study on the Ignition Ability of a Laser-Induced Gaseous Breakdown,” Combust. Flame, 178, pp. 1–6.
Ghosh, S. , and Mahesh, K. , 2008, “ Numerical Simulation of the Fluid Dynamic Effects of Laser Energy Deposition in Air,” J. Fluid Mech., 605, pp. 329–354.
Phuoc, T. X. , 2006, “ Laser-Induced Spark Ignition Fundamental and Applications,” Opt. Lasers Eng., 44(5), pp. 351–397.
Bihari, B. , Gupta, S. B. , Sekar, R. R. , Gingrich, J. , and Smith, J. , 2005, “ Development of Advanced Laser Ignition System for Stationary Natural Gas Reciprocating Engines,” ASME Paper No. ICEF2005-1325.
Joshi, S. , Yalin, A. P. , and Galvanauskas, A. , 2007, “ Use of Hollow Core Fibers, Fiber Lasers, and Photonic Crystal Fibers for Spark Delivery and Laser Ignition in Gases,” Appl. Opt., 46(19), pp. 4057–4064. [PubMed]
Yalin, A. P. , 2013 “ High Power Fiber Delivery for Laser Ignition Applications,” Opt. Express, 21(S6), pp. A1102–A1112. [PubMed]
Tsunekane, M. , Inohara, T. , Ando, A. , Kido, N. , Kanehara, K. , and Taira, T. , 2010, “ High Peak Power, Passively Q-Switched Microlaser for Ignition of Engines,” IEEE J. Quantum Electron., 46(2), pp. 277–284.
Gupta, S. B. , Bihari, B. , and Sekar, R. , 2014, “ Performance of a 6-Cylinder Natural Gas Engine on Laser Ignition,” Second Laser Ignition Conference, Yokohama, Japan, Apr. 22–25, Paper No. LIC6–3.
Schwarz, J. , Wörner, P. , Stoppel, K. , Nübel, K.-H. , and Engelhardt, J. , 2014, “ Pumping Concepts for Laser Spark Plugs—Requirements, Options, Solutions,” Second Laser Ignition Conference, Yokohama, Japan, Apr. 22–25, Paper No. LIC3-3.
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Biruduganti, M. , Gupta, S. B. , Bihari, B. , Kanehara, K. , Polcyn, N. , and Hwang, J. , 2015, “ Performance Evaluation of a DENSO Developed Micro-Laser Ignition System on a Natural Gas Research Engine,” Third Laser Ignition Conference (LIC), Argonne, IL, Apr. 27–30, Paper No. T5A.4.
Tsunekane, M. , Inohara, T. , Kanehara, K. , and Taira, T. , 2010, “ Micro-Solid-State-Laser for Ignition of Automobile Engines,” Advances in Solid State Lasers: Development and Applications, M. Grishin , ed., INTECH, Croatia, pp. 195–212.
Sjöberg, M. , and Zeng, W. , 2016, “ Combined Effects of Fuel and Dilution Type on Efficiency Gains of Lean Well-Mixed DISI Engine Operation With Enhanced Ignition and Intake Heating for Enabling Mixed-Mode Combustion,” SAE Int. J. Engines, 9(2), pp. 750–767.

## Figures

Fig. 1

(a) Schematic and (b) photograph of the laser igniter equipped with water-cooled VCSEL pumped microlaser

Fig. 2

View of (left) standard spark ignition systems, (right) laser ignition system as seen from cylinder#1 of the engine

Fig. 3

Efficiency, COV_IMEP and BSNOx variation with ignition timing

Fig. 4

(left) BSNOx versus brake thermal efficiency tradeoff, (right) COV_IMEP versus brake thermal efficiency. Allowable limits for NOx emissions and ignition stability are marked with horizontal red arrows (see figure online for color).

Fig. 5

Rate of heat release in cylinder#4 for SI and 2P-LI

Fig. 6

(a) Ignition delay, (b) combustion duration, and (c) MFB50% in cylinder#4 for SI and 2P-LI

Fig. 7

ROHR plots for the optimal operational point with the use of 2P-LI

Fig. 8

Brake-specific carbon monoxide (BSCO) and brake-specific hydrocarbon (BSHC) emissions for SI (λ = 1.6) and 2P-LI (λ = 1.68). Circles mark the ideal conditions for either ignition system.

## Tables

Table 1 Specifications of the test engine

## Discussions

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