Are Laser-Based Spark Plugs a Viable Option for Igniting ICE Fuel?

I’ve always been interested in innovations that are pursued for years without wide adoption, yet proponents keep working on them. Sometimes, these advances just fade away. Other times, they contribute to progress in a more-general way and, in some cases, the necessary pieces—technologies, manufacturing, market needs—finally converge, and the years of laboring in semi-obscurity final pay off.

That’s one of the reasons I have been following the efforts to use laser-based spark plugs in place of the classic spark plug as the igniter of gasoline and other volatile fuels in the internal combustion engine (ICE). There have been innovations in the plug itself over the years, such as the pre-chamber, as well as in the entire battery-to-plug power-and-distribution arrangement. 

But why stay electrical/electronic? There’s been a lot of research over several decades on using laser technology in place of the spark, and it’s been demonstrated as doable. (A look at the dates of some of the publishers cited in References shows how long this idea has been an ongoing work in progress.)

Of course, “doable” is often not the same as “viable” or “practical.” In the classic spark-plug system, a lower voltage from the battery (or battery-free, in the self-powered magneto version) is stepped-up via a transformer (called a coil) and then directed to the plug at the appropriate time instant to “flash over” and ignite the air/fuel mixture.

This isn’t new at all, as it has been in use since the 1800s in early cars; a custom self-built design was used by the Wright brothers to power their 1903 Flyer. (Ironically, they’re the “exposed” version of the enclosed gas-discharge tube–GDT–used to protect circuits against overvoltage transients.) Since the days of those first magneto-based systems, the spark plug system has advanced from a single coil firing multiple cylinders via a mechanical distributor (Figure 1) to one coil per cylinder with distributor-free electronic control of the timing.

Despite the advances, the fundamental principle is the same: create and control high-voltage, short-duration pulse across a carefully shaped electrode pair, with the resultant flash-over spark igniting the fuel/air mixture. (If you need a brief refresher on the evolution of spark-plug ignition systems, check out “How Ignition Systems Work.”)

While the drive and distribution circuitry has gone electronic and the topology has changed, the spark plug itself remained largely unchanged over decades, although there have been modest improvements like the “prechamber.” Plugs work well, they’re inexpensive, and are easily replaced. Combine that with modern electronic-ignition systems, plugs used to last 10-20,000 miles but now are good for 5× that value—so, what’s not to like?

Enter the laser

That hasn’t stopped researchers from looking for alternative ways to ignite the fuel. Recently, I came across another story about the use of a laser in the cylinder to do the job, in a housing that is form-and-fit compatible with standard plugs. (I first saw reports on this effort in a 2019 Optics Express paper “On the improvement by laser ignition of the performances of a passenger car gasoline engine.”)

The operating simple isn’t obvious:

• A pulsed laser focuses high-energy photons into the combustion chamber.
• This intense energy creates a plasma breakdown, essentially an artificial spark, at the focal point.
• This plasma generates heat and radicals, igniting the surrounding fuel-air mixture—even very lean ones. 

Sounds very high tech and it is. In contrast to a standard spark plug that generates an electric arc between two electrodes, the laser can generate a plasma spark and pulse energy of several millijoules at a point several millimeters ahead of the laser, as defined by its focusing optics. This spark can be placed deeper in the prechamber since electrodes don’t hinder the movement of the flame front.

The potential advantages are:

• Leaner combustion: The laser spark can ignite mixtures that are too lean for conventional spark plugs, and using these lean mixtures can improve efficiency and reduce CO2. (“Lean” means that proportion of air to fuel is larger than that which is stoichiometrically needed for combustion.)
• Improved emissions: Lower combustion temperatures in lean conditions reduce nitrous oxide formation.
• No quenching: There are no electrodes to cool the flame kernel, improving reliability, especially for cold starts and alternative fuels like biofuels.
• Precise control: The ignition point can be precisely positioned for improved combustion stability.
• Fuel flexibility: Works well with various fuels, including biogas and biofuels. 

[Note: Do not confuse these experimental laser-based spark plugs with the “Laser Iridium” spark plugs from vendors like NGK (now known as Niterra North America Inc.). These are conventional spark plugs with added precious metals added to supposedly improve the spark action and provide longer life, but there is nothing “laser” about them except the marketing name, see here.]

One ongoing laser effort is a joint project led by the Fraunhofer Institute for Applied Optics and Precision Engineering IOF (Germany) and the National Institute for Laser, Plasma and Radiation Physics (INFLPR, Romania), along with others.

Their Nd:YAG ceramic laser medium was bonded to a piece of Cr⁴⁺:YAG saturable absorber ceramic (Figure 2). One side of the Nd:YAG is coated for high reflectivity of the laser radiation at 1,064 nm and high transmission of the pump radiation at 807 nm. The end facet of the Cr⁴⁺:YAG is used as an out-coupling mirror.

The researchers designed the laser-based spark plugs to allow use of lenses with different focal lengths in order to focus the laser beam, as well as for in-chamber and prechamber configurations (Figure 3). There are many complex issues related to focusing, ruggedness, and soot build-up on the lens assembly. These are in addition to the basics of assembling the unit from its constituent parts, a process which involved multiple sophisticated steps with advanced soldering and mounting technologies.

The researchers ran tests on stationary power-plant engines, such as those used in factories, at 200 kW, 250 kW, and 275 kW (approximately 270, 335, and 370 horsepower, respectively). In these tests, the energy and duration of the laser pulse reached 3.4 mJ and 0.9 ns, respectively, corresponding to a peak power of almost 3.8 MW. The researchers said that performance with the laser spark plugs was comparable to conventional plugs at the lower levels but was somewhat inferior at the highest level. Long-term performance is still under evaluation.

Will this advanced approach go mainstream? There are three customary possibilities:

1) The scheme will be abandoned as being too disruptive, complex, and costly for the benefits.

2) It will fade away, but some of its advances will be adopted for unrelated applications, as is so often the case in technology.

3) Eventually, the technique will be improved enough, and its benefits will be clear, so it will be adopted—although perhaps only in specialty, high-end applications.

It will certainly be interesting to watch its evolution and outcome. Let’s face it: boasting of “laser-sparked fuel ignition” does make a nice marketing boast for high-end cars!

AUTOMOTIVE (EV/AV)