Fiber Optic Delivered Laser Ignition Systems

8/12/2019 Fiber Optic Delivered Laser Ignition Systems

1/4


2/4

multimode solid silica fibers for igniting engines have also been limited. Biruduganti et al. used a 1 mm

core silica fiber to deliver 35 mJ of 532 nm light for ignition of a single cylinder natural gas engine, but

could not reliably spark in atmospheric pressure air or start the engine without altering the ignition

timing *4+. Mullett et al. used 400 and 600 m solid core silica fibers to operate an engine *5+, but with

an ignition rate of only 35% for the 600 m core fiber and 8% for the 400 m core fiber. They attributed

the high misfire rate to poor beam quality at the fiber output.

Our group has shown spark delivery in atmospheric pressure air using cyclic olefin polymer-coated

silver hollow fibers. The experiments used 10 ns pulses of energy 32 mJ delivered via a 700 m hollow

core fiber having output beam quality of M2 = 15; however, practicality was limited by bending-induced

beam quality degradation [6]. Photonic crystal fibers (PCFs) and photonic bandgap fibers (PBGs) employ

periodic hole-structures within the (silica) fiber material to modify the refractive index and guide the

light pulses. Conventional PCFs has shown limited success for laser ignition but kagome PCFs have

recently shown delivery of pulse energies of ~10 mJ and ignition of simple butane flames. Also of

interest is high power delivery by recently developed polymer PBG fibers using one-dimensional Bragg

gratings [7].

3. Spark Delivery and Ignition Using Large Clad Multi-Mode Silica Fibers

Our group has demonstrated the use of large clad silica fibers for spark delivery and engine ignition

[8, 9]. Large clad refer to fibers with clad-to-core diameter ratios significantly greater than 1.1, for

example a 745 m clad on a 200 m core fiber or 720 m clad on a 400 m core fiber. These dimensions

are commercially available but are in contrast to widely used multimode fiber having clad to core

diameter ratios of ~1.1-1.2 (e.g., 440 m clad on a 400 m core, or 660 m clad on a 600 m core). The

benefits of the large-clad fiber can be seen in Fig. 1a) which shows beam profiles of the (unfocused) lightexiting the fibers for large clad (top) and regular clad (bottom) [3]. The speckle pattern in the profile

from the regular clad fiber is due to interference between multiple modes exiting the fiber, while the

profile from the large clad fiber is closer to that of single mode light. The improved output beam quality

(reduced M2) of the large clad fibers is attributed to increased mechanical rigidity of the large clad

fibers, which leads to reduced mode coupling at the core-clad interface. We have shown that sparks can

be formed at the output of the large-clad fibers with 100% reliability [8]. For pulse durations of 6 ns we

could deliver 8 mJ pulses and by increasing the duration to 50 ns we could deliver 25 mJ. Close attention

must be paid to optimize the fiber launch and to use low-stress mounting and positioning; more work is

needed in this area to increase the practicality of the fibers.

Ignition Images

Figure 1. Laser ignition using large clad fiber. a) Improved beam quality at exit of large clad fiber. Top

beam profile shows high spatial-quality at output of a large clad fiber while bottom is for regular clad

fiber of the same diameter [3]; b) Setup for on-engine testing (above and side views); c) Net Mean

Effective Pressure (NMEP) obtained in a single cylinder natural gas engine with laser and electrical

ignition.


3/4

We have also performed engine testing on a single cylinder engine (Waukesha co-operative fuel

research engine) converted to run on bottled methane [8, 10]. These tests used large clad fibers with

length of 2.85 m, core diameter of 400 m, and clad diameter of 720 m. In this configuration we could

deliver 11 mJ pulse energy with 25 ns pulse duration. The overall system is shown in Fig. 1b) and

included the laser and launch optics, fiber, and an optical spark plug. The optical spark plug had mating

connectors for the fiber optic and housed a diverging lens followed by a collimating lens and then a 10

mm focal length focusing lens (GradiumTM). (The role of the diverging lens was to shorten the length of

the overall plug by more strongly expanding the beam exiting the fiber). A 3 mm thick sapphire window

with copper gaskets sealed the optical spark plug from high pressure combustion gases in the engine

cylinder. The overall transmission of the system was 65-70% so that 11 mJ at the fiber input

corresponded to 7 mJ at the laser spark location. The engine was boosted with compressed air so that

various loads and air-fuel ratios could be achieved. Laser timing relative to engine crank angle was

controlled with a delay generator and custom circuitry. The final output beam (7 mJ) from the plug could

form sparks in pressures as low as 3.5 bar which guaranteed sparking at higher pressures and allowed

engine startup without changing the ignition timing. The amount of work available from the engine after

accounting for pumping losses is expressed as net mean effective pressure (NMEP). The observed trend

of laser ignition providing higher NMEP for given intake manifold pressure, as shown in Fig. 1c, is

consistent with the measured higher specific fuel efficiencies found from the engine output power and

fuel consumption. For higher pressure operation (8 and 12 bar NMEP), laser ignition provided an

average increase in fuel efficiency of ~1-2% relative to electrical ignition (relative increase of ~5-15%).

The laser also extended the lean limit of engine operation. The comparisons of NMEP and efficiency

were performed at the laser lean limit. The poorer efficiency for electrical ignition may be related to the

engine being operated overly lean for the electrical ignition.

References:

* [1] Phuoc, T.X., Laser spark ignition: experimental determination of laser-induced breakdown

thresholds of combustion gases. Optics Communications, 175(4-6): p. 419-423 (2000).

* [2] Kopecek, H., et al., Laser ignition of methane-air mixtures at high pressures. Exptl. Therm. and

Fluid Science, 27(4): p. 499-503 (2003).

* [3] El-Rabii, H. and G. Gaborel, Laser ignition of flammable mixtures via a solid core optical fiber.

Applied Physics B-Lasers and Optics, 87(1): p. 139-144 (2007)

* [4] Biruduganti, M., et al., Performance Analysis of a Natural Gas Generator using Laser Ignition,

ICEF2004-983, Fall Technical Conference ASME (2004).

* [5] Mullett, J.D., et al., A comparative study of optical fiber types for application in a laser-induced

ignition system. J. Opt. A: Pure Appl. Opt. 11 054007 10pp. (2009).


4/4

* [6] Yalin, A.P., et al., Use of hollow-core fibers to deliver nanosecond Nd : YAG laser pulses to form

sparks in gases. Optics Letters, 30(16): p. 2083-2085 (2005).

* [7] Ruff, Z., et al., Polymer-composite fibers for transmitting high peak power pulses at 1.55 microns.

Optics Express, 18(15): p. 15697-15703 (2010).

* [8] Joshi, S., N. Wilvert, and A.P. Yalin, Delivery of high intensity beams with large clad step-index

fibers for engine ignition. Applied Physics B-Lasers and Optics 108(4): p. 925-932 (2012).

* [9] Hurand, S., et al., Mode coupling and output beam quality of 100-400 mu m core silica fibers.

Applied Optics, 50(4): p. 492-499 (2011).

* [10] Wilvert, N., S. Joshi, and A. Yalin. On Comparative Engine Performance Testing With Fiber

Delivered Laser Ignition And Electrical Ignition. Proceedings of the ASME 2012 Internal Combustion

Engine

Documents

Fiber Optic Delivered Laser Ignition Systems