Researchers at the National Energy Technology Laboratory (NETL) recently demonstrated the latest version of their laser spark plug designed for use with lean burn gas (natural gas and hythane, an 80% CNG, 20% hydrogen mixture) reciprocating engines.
The push toward leaner air-fuel operating conditions to deliver higher engine efficiency combined with lower emissions requires increased spark energy to maintain stable combustion.
Unfortunately, increased spark energy negatively impacts spark plug durability and performance as an ignition source. An alternative is laser-spark ignition, where high-energy photons are focused to create a plasma breakdown at a point inside a compressor cylinder. One of the most promising aspects of laser-spark ignition is its ability to ignite lean mixtures that would be un-ignitable by a typical spark ignition system.
In March of 2007, NETL researchers demonstrated a prototype laser spark plug using a miniaturized diode-pumped solid-state laser. Those tests proved the concept but engine vibrations impacted laser performance. The new spark plug provides a more rugged design and uses a fiber optic delivery system to pump the laser. It also provides more consistent spark delivery enabling the engine to be started cold and run continuously for longer periods.
In December, the NETL team started and ran a single-cylinder Ricardo Proteus engine with an end-pumped, passively Q-switched, Nd:YAG laser with the pump light delivered by fiber optic.
The compact design produces a 6-9 millijoule laser pulse 2.5 nanoseconds long, which generates a spark through an NETL-designed lens. The lens is mounted in a spark plug housing and provides the pressure seal as well as focusing the plasma spark 6 mm inside the cylinder.
The engine was initially test fired on the laser plug on 5 December for ~15 minutes. During that time, the engine was run at up to 1,800 rpm, the target testing speed. A full-test plan to evaluate engine and laser spark performance was performed the following week involving operating the engine with laser spark approximately 8 hours over three days of testing. The laser spark plug was tested on natural gas and hythane, a mixture of 80 per cent natural gas and 20 per cent hydrogen by volume over a range of air-fuel ratios.
NETL backgrounder on laser spark plug