Laser sparks that were built with high-peak power passively Q-switched Nd:YAG/Cr4+:YAG lasers have been used to operate a Renault automobile engine. The design of such a laser spark igniter is discussed. The Nd:YAG/Cr4+:YAG laser delivered pulses with energy of 4 mJ and 0.8-ns duration, corresponding to pulse peak power of 5 MW. The coefficients of variability of maximum pressure (COVPmax) and of indicated mean effective pressure (COVIMEP) and specific emissions like hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and carbon dioxide (CO2) were measured at various engine speeds and high loads. Improved engine stability in terms of COVPmax and COVPmax and decreased emissions of CO and HC were obtained for the engine that was run by laser sparks in comparison with classical ignition by electrical spark plugs.
© 2015 Optical Society of America
Laser ignition was investigated extensively in recent years and it was seen as a possible answer to human concern on environment impact of the automobiles that are powered by internal combustion engines. Such ignition, which is applicable to gasoline engines, can lower fuel consumption and decrease gas emission, but it still improves the automobile engine performances and efficiency. In comparison with classical ignition by an electrical spark plug laser ignition offers several advantages [1–3]. Thus, due to the absence of spark plug electrode there is no quenching effect of the developing flame kernel; furthermore, the position of the ignition point inside the combustion chamber can be chosen, whereas multiple-point ignition could provide better and more uniform combustion; moreover, laser ignition offers the possibility to ignite leaner air-fuel mixtures.
A rapid development of such a laser device was not possible due to technical or price-related problems. Thus, when the first air breakdown phenomenon was reported in 1963 by focusing the third harmonic of a Q-switch ruby laser, the authors have characterized their experiments as “the most expensive spark plug in automotive history” . Still, motivated by the attractiveness and importance of this subject, commercially available lasers that delivered pulses of tens of mJ and several-ns duration experiments were used to determine the laser-induced breakdown threshold ignition or to ignite various gases (oxygen, argon, helium, or methane) [5–8]. The first laser ignition of an engine was made in 1978 with a CO2 laser, using a single-cylinder engine [9,10]; moreover, Q-switched Nd:YAG lasers were used to ignite a four-cylinder engine in 2008 . In these experiments the laser beams were directed to and then focused into the engine cylinders by common optics (lenses and mirrors).
An important step toward realization of a compact laser-spark device was made in 2007, when a Nd:YAG laser that was passively Q-switched by Cr4+:YAG saturable absorbed (SA) was proposed by H. Kofler et al. . The laser (which was built of discrete elements) was end-pumped by a fiber-coupled diode laser and delivered pulses with energy up to 6.0 mJ and 1.5-ns duration. Furthermore, based on the same combination of active medium and SA crystal, a side-pumped laser that yielded pulses with long 3.0-ns duration but high energy of 25 mJ was reported in 2009 by G. Kroupa et al. . Further performance optimization of an end-pumped Nd:YAG-Cr4+:YAG laser  has enabled realization by Tsunekane et al. of the first spark-like micro-laser device ; the laser oscillator (made also of discrete components) delivered pulses with 2.7-mJ energy and short duration of 0.6 ns, corresponding to pulse peak-power of 4.5 MW. Successful ignition of stoichiometric C3H8/air mixture fuel was achieved with this laser in a constant-volume chamber at room temperature and atmospheric pressure.
A multi-beam laser spark that was built, for the first time, with a monolithic, all poly-crystalline ceramic diffusion-bonded Nd:YAG/Cr4+:YAG media was reported in 2011 . Such a laser possessed robustness, compactness and resistance to vibrations, suitable for direct use on an engine. Consequently, the first report of laser ignition of an automobile gasoline engine was made in 2013, by Taira et al. . The laser medium was a square-shaped Nd:YAG/Cr4+:YAG ceramic that delivered pulses with 2.4-mJ energy and 0.7-ns duration; in addition, a train of four-pulses was used for ignition of each engine cylinder. It is also worth to mention that data released recently by Bosch Co. showed this company interest in the field of laser ignition . Thus, based on research that started around 2000, Bosch Co. has developed laser-spark igniters with monolithic diffusion-bonded Nd:YAG/Cr4+:YAG single-crystals media, yielding pulses with high 12.3-mJ energy at long 2.4-ns duration, or shorter pulses of 0.9-ns duration and 8.1-mJ energy . The laser ignition was also used for thrusters control and orbital maneuvering  or in natural gas engines [21,22]. Furthermore, recent published papers that reported on realization of side-pumped miniaturized Nd:YAG-Cr4+:YAG or of end-pumped multiple-beam Nd:YAG-Cr4+:YAG lasers suitable for ignition are proving the importance of this research subject [23,24].
The performances of an automobile engine that is ignited only by laser sparks are still to be investigated. Thus in , Taira et al., the coefficient of variance of the indicated mean effective pressure (COVIMEP) was determined depending on the air-fuel ratio at 1.200 rpm engine speed and 73 N·m torque; comparable engine operation for both classical ignition and ignition by laser sparks was obtained. Recently we have reported laser ignition of a Renault car engine ; the coefficient of variance of maximum pressure (COVPmax) was measured at various engine speeds (1.200 rpm to 2.800 rpm) and light loads (330 mbar and 440 mbar, the intake manifold pressure); better engine stability was observed for the ignition by laser. In this work we are presenting new data regarding operation of this engine that was ignited only by laser sparks. The laser-spark prototype is described in section 2. A four laser-spark system that was controlled by the automobile electronic control unit was built. The system was used to ignite the Renault car engine; the in-cylinder pressure as well as HC, CO, NOx and CO2 specific emissions were measured at various speeds of the engine (1.500 rpm to 2.000 rpm) and high loads (770 mbar to 920 mbar). The results are given in section 3; improved engine stability, decreased values of CO and HC, but also slight increases of NOx and CO2 emissions have been obtained in comparison with classical ignition by electrical spark plugs.
2. The Nd:YAG/Cr4+:YAG laser spark
It is worthwhile to mention that in previous research we studied the influence of temperature on the laser performances of a Nd:YAG-Cr4+:YAG laser . Advantages of laser ignition in comparison with ignition by classical spark plugs were investigated in a static chamber filled with methane-air mixtures . Based on these results, a first laser-spark prototype was built in 2011. The device, shown in Fig. 1, consisted of a diffusion-bonded Nd:YAG/Cr4+:YAG ceramic medium that was end-pumped by a fiber-coupled diode and yielded laser pulses with energy up to 3 mJ and 1.0-ns duration. Also, a new configuration made of a diffusion-bonded Nd:YAG/Cr4+:YAG medium that is pumped laterally through a prism was proposed recently by our group as a solution for a laser spark .
The laser-spark device used in this work, which is an improved version of the first prototype, is presented in Fig. 2(a) in comparison with a classical spark plug; a cross-sectional view of this laser spark is shown in Fig. 2(b). The laser medium was a diffusion-bonded Nd:YAG/Cr4+:YAG structure. The monolithic resonator was obtained by coating the high reflectivity HR (R> 0.999) mirror at lasing wavelength, λem = 1.06 μm on the free Nd:YAG side (i.e. toward the pump line, Fig. 2(b)) and the outcoupling mirror (OCM) with reflectivity ROCM at λem on the Cr4+:YAG opposite surface (toward the focusing line); also, the Nd:YAG side was coated for high transmission (T> 0.98) at the pump wavelength, λp = 807 nm.
The Nd:YAG/Cr4+:YAG media that were investigated in the experiments consisted of either of all-polycrystalline media, i.e. ceramic media (Baikowski Co., Japan) or of single crystals (China supplier). The Nd:YAG characteristics (1.0-at.% Nd, length of 8 mm) were chosen such to obtain absorption efficiency higher than 90% at λp. The optical pump (at λp) was performed with fiber-coupled diode lasers (JOLD-120-QPXF-2P, Jenoptik, Germany) that were operated in quasi continuous-wave mode at repetition rate up to 100 Hz; the pump pulse duration was 250 μs and maximum energy of the pump pulse was nearly 50 mJ.
For the pump optics line [Fig. 2(b)] two configurations were used. The first one consisted of only one lens (L) of focal length f; the distances between the fiber end and the lens and between the lens and Nd:YAG are denoted by d1 and d2, respectively. The second pump optics scheme was made of a collimating lens L1 of focal length f1 (for collimation, L1 was positioned at the working distance, as indicated by the manufacturer) and a focusing lens L2 of focal length f2; the distance between L2 and Nd:YAG is d. The following experimental data were obtained with a Nd:YAG/Cr4+:YAG ceramic with SA initial transmission Ti = 0.40 and an OCM with ROCM = 0.60; the optical fiber had a 600-μm diameter.
Figure 3(a) presents the laser pulse energy Ep as a function of distances d1 and d2 for two pump lines, each made of a single lens L. Laser pulses with 5.5-mJ maximum energy were obtained by positioning a lens L with f = 4.0 mm at d1 = 3.35 mm and the laser medium at d2 = 10.4 mm; the corresponding pump pulse energy Epump [Fig. 3(b)] was 47.5 mJ. The ratio Ep/Epump, which can be seen as the laser optical-to-optical efficiency ηo, was 0.115. A maximum energy of 5.9 mJ was obtained when L had f = 6.2 mm and it was placed at distances d1 = 4.85 mm and d2 = 18.5 mm; the pump pulse energy was Epump = 47.3 mJ (at ηo = 0.12).
The laser performances obtained with several pump optics lines that were built with two lenses are given in Fig. 4. When the collimating lens L1 had a focal length f1 = 4.0 mm, pulses with energy Ep = 3.6 mJ [Fig. 4(a)] were obtained by placing the Nd:YAG/Cr4+:YAG ceramic at d = 6.3 mm from a focusing lens L2 with focal length f2 = 4.0 mm; the pump energy was Epump = 44 mJ [Fig. 4(b)], corresponding to efficiency η0 = 0.08. Changing L2 to a lens with f2 = 7.5 mm and increasing d at 12.8 mm improved the energy Ep to 4 mJ (at Epump = 47.5 mJ, η0 = 0.08). For this collimating lens (f1 = 4.0 mm), the highest energy Ep = 4.3 mJ was achieved with L2 of f2 = 6.2 mm at d = 8.9 mm; the pump energy amounted at Epump = 46.9 mJ (at η0 = 0.09). Similar pulse characteristics, with Ep = 4.3 mJ at Epump = 49 mJ, were obtained with a lens L1 of f1 = 6.2 mm and a lens L2 with f2 = 7.5 mm, the laser medium being positioned at distance d = 10.3 mm. The optical efficiency η0 was also plotted in Fig. 4(b) for combination of lenses (f1 = 4.0 mm, f2 = 6.2 mm) and (f1 = 6.2 mm, f2 = 7.5 mm).
Simulations on the laser pulse energy Ep were performed on a rate equation model [29,30], in which the spatial overlap between the laser beam and the pump beam was considered by the ratio a = wp/wg, where wp and wg denotes the pump-beam radius and the laser-beam radius, respectively. For better understanding, we remember that the laser pulse energy can be written by [30,16]:
In modeling various points were chosen for all the pump lines used in the experiments. Furthermore, knife-edge method (10%-90% level) was used to determine, for each of these points, the pump-beam propagation after lens L or L2, the radius of the laser beam near the OCM and the laser beam M2 factor. It was found out that M2 was in the range of 1.5 to 2 for laser pulses with energy Ep below 2 mJ and increased up to 5 for laser pulses with Ep higher than 3 mJ. Therefore, in the simulations the pump beam was taken as having uniform (like top hat) distribution, whereas the laser beam distribution was considered Gaussian but also top-hat like. The Nd:YAG emission cross section was taken as σg = 2.63 × 10−19 cm2 and absorption cross section and excited-state absorption cross section of Cr4+:YAG were σSA = 4.3 × 10−18 cm2 and σESA = 8.2 × 10−19 cm2, respectively. Figure 5 shows results of modeling by continuous and by dashed lines for laser beam of Gaussian and uniform (top-hat like) distribution, respectively. The parameter used in simulations was the double-pass residual losses (Li) of the monolithic medium. We found out that a value Li~0.05 (that should account for Nd:YAG losses as well as for the final transmission of Cr4+:YAG) described well the experimental data. Moreover, in our pump conditions with a = wp/wg< 1.0, differences between experiments and simulations were small whatever the laser beam was Gaussian or top-hat like in the modeling.
We performed further experiments and concluded that with an OCM of ROCM = 0.60 and Cr4+:YAG having the initial transmission Ti ranging from 0.30 or 0.50, each pump line and Nd:YAG/Cr4+:YAG medium could be arranged such to deliver pulses with energy Ep higher than 3 mJ. Then, a diffusion-bonded Nd:YAG/Cr4+:YAG medium with wedged Cr4+:YAG SA (and thus with variable initial transmission of Cr4+:YAG), similar to that recently proposed in  by Cho et al., can be used to realize a laser spark.
The diffusion-bonded Nd:YAG/Cr4+:YAG made of single crystals delivered pulses with energy close to those yielded by the ceramic counterpart, but at increased (by up to 20%) Epump. The increased pump pulse energy could come from higher losses at the bonding interface between Nd:YAG and Cr4+:YAG SA single crystals, in comparison with a ceramic medium. Finally, for realizing the laser sparks we used diffusion-bonded Nd:YAG/Cr4+:YAG ceramic media; both kinds of pump optics lines, consisting of one or two lenses were used. Typically, the laser was designed to deliver pulses with energy Ep = 4.0 mJ and duration of 0.8 ns, corresponding to a pulse peak power of 5.0 MW.
The focusing line [Fig. 2(b)] assured collimation and then focusing of the laser beam. Position of ignition inside the engine cylinder can be varied by changing the focusing lens. In the preliminary experiments (before testing on the engine) lenses with focal length between 11 mm and 18 mm were used to obtain air breakdown, indicating the set-up usability for laser ignition. As interface between laser spark and the engine chamber a sapphire window was used [Fig. 2(b)]. The windows thickness was around 2.0 mm, chosen such to withstand static pressures higher than 20 MPa. The optical components were fixed with an epoxy adhesive, having high shear and peel strength and a service temperature between −70°C and 170°C.
3. Ignition of the Renault automobile engine
The laser ignition experiments were performed on a K7M 812 k, 1.6-litter gasoline Renault engine with a multi-point injection system (indirect fuel injection, outside the combustion chamber); the engine was mounted on a test bench. An integrated four laser-sparks system was built, tested and then installed on the engine; the ignition triggering was realized by the electronically control unit of the car. The in-cylinder pressure was measured with an AVL GU-21D piezoelectric transducer. The exhaust gases were sampled from the valve gate with a Horiba Mexa 7100 analyzer. The acquisitions were made on 500 consecutive cycles for engine speeds between 1.500 rpm and 2.000 rpm and high loads of 770 mbar, 880 mbar and 920 mbar (the load was the absolute pressure from the intake manifold). The engine was running steady near the stoichiometric air-fuel ratio (λ = 1) for all investigated engine modes. This is normal value for a classic gasoline engine, which means that the mass ratio of air to fuel is ~14.7 and it is calculated so each carbon atom from the fuel to react with one oxygen atom from the air and to result just CO2 in a theoretical combustion. Figure 6(a) presents the laser system during preliminary testing (before being installed on the engine). A comparison between the plasma generated in air by a laser spark and the discharge of a classical spark plug is given in Fig. 6(b). The engine is shown in Fig. 6(c) during running with the laser ignition system.
We mention that after the first ignition experiments  a temperature test of one of our laser spark was performed. Thus, a slit was cut in the laser spark body and a FLIR T620 thermal camera (−40°C to + 150°C range, ± 2°C accuracy) was used to measure the Nd:YAG/Cr4+:YAG temperature at both Nd:YAG/Cr4+:YAG medium ends. When operating at room temperature (24°C) and 50 Hz repetition rate for more than 30 min., the temperature of Nd:YAG (near the input surface, toward the pump line) and that of Cr4+:YAG (near the exit side, toward the focusing line) reached 37°C and 29°C, respectively. The laser spark was then mounted on a metallic block that was heated at various temperatures. For example, an increase of this temperature up to 250°C (i.e. the temperature of the laser spark around the sapphire window) resulted in an increase of Nd:YAG temperature to 75°C and of Cr4+:YAG to ~55°C. Consequently, the pump-pulse energy has to be raised in order to maintain laser operation. Next, cooling of the laser spark was made with a thin copper jacket (that was also cooled with water at its free end). In similar conditions of operation (50-Hz repetition rate, metallic base at 250°C temperature) the Nd:YAG and the Cr4+:YAG temperature (at the same points, as explained before) increased up to 55°C and 40°C, respectively. Little adjustment of the pump-pulse energy was necessary in order to maintain laser operation. These measurements are not absolute (thus, additional heat comes from the engine body that surrounds the laser spark, or maximum temperature is reached on the central axis of Nd:YAG/Cr4+:YAG); however, the results indicate that cooling of the laser sparks could be beneficial for operation on the automobile engine. Considering some technical issues, in the ignition experiments cooling was done by a device (not shown in Fig. 6(c)) that blew air toward each laser spark. A short movie of the engine while operating with laser sparks was associated to Fig. 6(c).
An example of maximum pressures recorded in the engine cylinder (cylinder 1) is shown in Fig. 7. The engine stability was characterized by the coefficient of variability of maximum pressure COVPmax (defined as the ratio between standard deviation and the average peak pressure) and by the coefficient of variability of mean effective pressure, COVIMEP (defined as the ratio between standard deviation and the average of mean effective pressure).
Table 1 summarizes comparative results regarding operation of the engine that was ignited by classical spark plugs and by laser sparks. Improvements of the coefficients of variability were measured at medium speed of the engine ignited by laser sparks. For example, when the engine speed was 1.500 rpm the reduction of COVPmax was about 15%, whereas the COVIMEP improvement was in the range of 18.5% (at 920-mbar load) to 22.6% (at 880-mbar load). On the other hand, it is known that cyclic variability of an engine is better at both high speed and load; therefore, less influence of laser ignition on the coefficients of variability was expected in these conditions. Indeed, it was observed that differences for COVPmax and COVIMEP between the two types of ignition were small at 2.000 rpm and high 920-mbar load. These results indicate a better stability of the car engine that was operated at medium speeds by laser sparks, resulting in reduced noise, vibrations and mechanical stress. It is worthwhile to comment that improvements of COVIMEP were also observed in the first car engine that was ignited only by laser sparks  or in experiments of ignition that were performed with laser beams that were directed into a four-cylinder car engine ; an improvement of COVIMEP in the range of 20% to 10% was measured from a single-cylinder gasoline direct injected engine that was ignited by multiple pulses .
It is known that the regulated emissions are CO, HC, NOx and particulate matter, as well as the greenhouse gas CO2 with limits becoming more stringent. Lower CO and HC emissions were measured for the engine ignited by laser sparks. Thus, the decrease of CO was in the range of ~18% to ~25% for all measurements. HC emissions were by ~14% to ~17% lower at 1.500 rpm speed, whereas a decrease of ~3% was observed at higher 2.000-rpm speed. These improvements can be associated with a more efficient combustion under ignition by laser, meaning a better oxidation of the carbon and hydrogen atoms in the fuel. On the other hand, an increase of NOx, up to nearly 8% at 1.500 rpm speed and around 2% at 2.000 rpm speed, was measured for laser ignition in comparison with ignition by classical spark plugs. This, however, is a compromise between unburned fuel and NOx for the internal combustion engine calibration . The increase of NOx can be explained by a higher flame temperature in the first part of combustion, when much NOx is produced. A solution to reduce NOx could be, for example, an increased re-circulating rate of the exhaust gases. On the other hand, the amount of carbon entering into and resulting from the combustion reaction is constant; this explains the increase of CO2 under laser ignition. Measurements concluded that for the range of loads used in these investigations, the power of the engine ignited by laser increased by ~1.7% at 1.500-rpm speed and by ~3% at 2.000-rpm speed in comparison with classical ignition.
Regarding the laser spark operation, one issue was the damage of the optical element coatings used to build the focussing line and seldom damage of the lenses from the pump line. However, as all lenses were purchased from market they had no special coatings. This problem is expected to be solved by coating the lenses with high-damage threshold layers, or even using uncoated lenses at critical (high intensity laser beam) points in the laser beam; this solution was already used. Combustion deposits on the sapphire window were also observed. A solution proposed and investigated by H. Ranner et al.  for this problem is the window cleaning by the laser beam itself (or self-cleaning). We have considered this method in several ways. First, the laser pulse energy was high (Ep = 4 mJ) and thus the initial part of it was supposed to clean partially the window. Secondly, the pump pulse duration was lengthened such to obtain two laser pulses; in this way the first pulse is used for cleaning, a more efficient method than the first approach. On the other hand, however, at this stage of the experiments we cannot rule out that the first laser pulse is not contributing to the ignition, similar to the pulse-train ignition by laser spark plugs [32,35]. Thirdly, as the engine allowed twice triggering per cycle we made use of this feature by applying laser pulses in cylinder 4 (on the exhaust stoke) while ignition was realized in cylinder 1; thus, the window of cylinder 4 was cleaned before a new ignition. The same procedure was applied for cylinders 2 and 3. Furthermore, efficient cooling of each laser spark was realized by a compact cooling system with re-circulating water. Based on these approaches, the car engine could be continuously operated for few hours, without noticing coatings problems of the optics and maintaining clean the window. We comment, however, that additional research and work are needed before such a laser system could meet requirements for integration in an automobile engine and commercial application.
In summary, a Renault car engine was operated only by laser sparks that were built with high-peak power passively Q-switched Nd:YAG/Cr4+:YAG lasers. Several engine parameters, like coefficients of pressure variance and HC, CO, NOx and CO2 specific emissions were determined for engine speeds ranging from 1.500 rpm to 2.000 rpm and high (up to 920 mbar) loads. Improved engine stability at medium (below 2.000 rpm) speed was observed for the engine that was ignited by laser sparks. Furthermore, decreases of CO and HC emissions and a slight increase of NOx and CO2 were determined for laser ignition in comparison with ignition by classical spark plugs. In recent experiments, the optimum spark advance was determined for various speeds and loads of the engine and the influence of air-fuel combustion on the engine operation was investigated; results are to be reported. Although hindered by various technical issues and still uncompetitive price, laser ignition is considered an attractive research subject that could lead to further improvement and optimization of gasoline engines.
This work was financed by a grant of the Romanian National Authority for Scientific Research and Innovation (ANCSI), CNCS - UEFISCDI and co-financed by Renault Technology Roumanie, both through the project PN-II-PT-PCCA-2011-3.2-1040 (58/2012); also, it was partially supported by project LAPLAS PN 09.39.01.01 of ANCSI, CNCS – UEFISCDI, Romania.
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