Nd:YAG Replaces Spark Plug in Lean-Burn Engine

Daniel S. Burgess

Forget about putting a tiger in your tank; to really get your motor running, put a laser under the hood. Research scientists at the US Department of Energy's National Energy Technology Laboratory in Morgantown, W.Va., have demonstrated that substituting a laser for the spark plugs in a natural-gas-fueled lean-burn engine offers higher durability and performance. The effort has shown significantly greater engine performance and improved durability, which is critical to lean-burn natural-gas engines. In principle, the same should be true for gasoline power plants.

Running an engine on a leaner mixture of air and fuel increases the combustion efficiency and lowers heat loss, boosting fuel economy. It also supports higher compression ratios and can reduce knock and waste-gas emissions. Unfortunately, lean mixes do not ignite as easily as stoichiometric ones and so require high spark energies that tend to quickly burn out even the precious-metal plugs used in the stationary engines for power generation.

The researchers turned to a Q-switched, diode-pumped Nd:YAG laser to supply the ignition spark. They focused the 10-ns-long pulses of 1064-nm radiation from a Spectra-Physics Quanta-Ray DCR-2 through the spark plug port and into the cylinder of a Ricardo Proteus engine. They used energies of 90 and 180 mJ per pulse in the experiments, although 50 mJ per pulse should be sufficient to induce ignition.

The engine displayed improved combustion stability and lower hydrocarbon emissions during 10 hours of testing compared with a conventional spark plug system. No issues with the laser system were apparent, either with vibration or with combustion products fouling the sapphire window into the cylinder. "We were happy to see how easy it was to start the engine in the first place," said Steven D. Woodruff, one of the scientists on the project. "We hooked it up, and off it went."

The work, which the group presented at the Natural Gas Technologies Conference in Orlando, Fla., in September, is the first to investigate the laser approach on an engine in more than two decades. Woodruff said that he had discussed the subject with some of the researchers who had last investigated laser-spark ignition in 1979. "They thought it was really neat, but the cost wasn't practical."

Today, high-power solid-state lasers are much less expensive, and the move from flashlamps to diode pump sources has greatly reduced downtime. The precious-metal spark plugs, such as iridium, that many stationary engines require cost up to $500 each and have a lifetime of only 300 hours, leading to downtime costs, so the researchers believe that lasers can compete. "We're taking advantage of where the market is going," Woodruff said. "People are interested in it because it looks like it will be economical now."

Potential for application

Mike McMillian, the project leader, said that the next step is to thoroughly model what is happening in the engine and to quantify the benefits of single-point ignition. The group also is designing access ports for multiple-port ignition. "What we've been doing so far is just a beginning," he said.

McMillian said that the researchers would like to discuss the technology with interested laser and optics companies to determine how they may optimize the setup. They envision using fiber to deliver the laser pulses, but they acknowledge that the energy densities may be too high and are investigating pulse-stretching techniques.

Although the application of laser ignition in lean-burn engines may be two to three years distant for power generation, the scientists are optimistic that the technique has potential, if they can attract the attention of prospective users.

"If we can get the technology going, there will be great interest in it," McMillian said. "We're establishing a baseline to understand the potential of the technology."