MEDIA BACKGROUNDER:

Laser spark ignition for lean-burn natural gas reciprocating engines

CONTEXT

Engines would run much cleaner if they could run leaner: in other words, use less fuel and more air. When a typical internal combustion engine runs, it burns fuel and air. The heat of combustion turns the nitrogen in that air into nitrogen oxides. These oxides are the precursors to what we call urban smog (www.howstuffworks.com/ozone-pollution.htm). Running lean (lean-burn) lowers the combustion temperature and significantly reduces the amount of NOx produced. As the demand for higher engine efficiencies and lower emissions drive reciprocating engine combustion to leaner air/fuel operating conditions, increased spark energy is required to maintain stable combustion and low emissions.

PROBLEM

Delivering increased spark energy negatively impacts spark plug durability and the effectiveness of the spark plug in transmitting adequate energy as an ignition source. The lack of spark plugs with the required durability is quickly becoming the limiting factor for developing ultra low emission, high-efficiency natural gas engines. Laser ignition delivers the high energy required to ignite ultra-lean mixtures and eliminates the need for spark plugs.

PROCESS

NETL Researchers designed a laser ignition system and coupled it with a fully-instrumented internal combustion engine. Focusing a 10 nanosecond, 1.06 micron, laser pulse into the cylinder through the spark plug port generates a laser ignition spark. The laser pulse comes from a Quanta Ray DCR-2 Nd:YAG laser directed to the cylinder with high energy laser mirrors. A two-meter focal length lens is placed 1.7 meters from the final focusing lens to reduce the diameter of the laser beam before entering the lens tube. The laser is focused into the cylinder with a 6 millimeter diameter, 36 millimeter focal length lens through a 3.2mm thick sapphire window. The lens is positioned on a 20 cm long, 7.9 mm diameter lens tube aligned radially to the crankshaft axis of the engine. The final mirror directing the laser beam to the lens is positioned directly above the tube and 45 degrees to the tube axis such that the beam incident on to the mirror is perpendicular to the laser plug axis and tangent to an arc centered on the crankshaft. A schematic of the laser ignition system experimental setup is shown, followed by a photo of the actual setup in operation.

A comparison of engine emissions and combustion using a Ricardo Proteous, single-cylinder, 4-stroke, spark ignited natural gas engine using both a conventional spark system and a laser spark system was conducted. The engine was operated at a constant speed of 1200 rpm and at moderate load conditions. The emissions and combustion performance data for each ignition system at three equivalence ratios and three timing conditions were compared.

The NETL research provides the first lean-burn natural gas engine data using a laser-spark ignition source and the first laser-spark ignition engine operation since the effort of Dale et al*, in which gasoline was used as the test fuel

* (Dale, , J.D., Smy, P.R. and Clements, R.M., "Laser Ignited Internal Combustion Engine – An Experimental Study," SAE Paper 780329, 1978)