Engineering at its Peak
Blue Summit Technologies, with support from the Defense Advanced Research Projects Agency (DARPA), has developed methods and technologies to optimally ignite combustible fuel mixtures in any geometry chamber at multiple locations using a single laser.
Ignition in Long Chambers
Ignition of gaseous propellants or fuel-air mixtures in cylindrical chambers with large length-to-diameter ratios can be tricky. Single-point ignition at one end can lead to the creation detonation waves as the combustion travels down the tube, compressing the mixture. This can be avoided by having multiple ignition sites, however traditionally these are mounted in the chamber wall and produce non-uniform combustion fronts as shown at right. Blue is fuel, red is combustion.
Blue Summit Technologies has developed the ability to simultaneously ignite at multiple locations along the centerline, which optimizes the combustion front propagation and avoids detonation as shown right (bottom). This is accomplished without any protrusions in the chamber using a single laser.
Ignition in Short Chambers
Ignition of fuels in short chambers where the length is much smaller than the diameter can be accomplished with endpoint ignition. However, this is often far from optimum. An example of this is the internal combustion engine where the rapidly moving piston leaves little time for the combustion to be completed, particularly when ignited at one end only. This is currently done using technology which is over 100 years old: the spark plug. Blue Summit Technologies is developing the ability to burn the fuel more completely using single-laser, multiple-point ignition. In this case, the ignition sites are spread both longitudinally and radially as shown at right.
The figure at the left shows a top view of a single-point ignition combustion wave propagation in a cylinder. Item 1 is the ignition point. Item 2 is the propagation wave moving radially outward, while Item 3 is the cylinder wall. The combustion wave front expands as the square of the combustion wave radius times pi times the cylinder length.
Shown at right is an experimental pressure curve produced from a single-point spark plug ignition of a propane-air mixture in a chamber representing a single cylinder of an engine. As with any spark plug, the combustion starts at one end of the chamber only. The energy released is directly proportional to the area under the pressure curve. The total area under the curve can be used to indicate how complete the combustion process is.
Since the engine piston moves rapidly downward, only part of the energy produced from the combustion has time to do useful work. In the cases shown, the engine speed is roughly 2,000 rpm and the time available to work on the piston is 60 ms. Any energy produced after 60 ms (box overlay on graphs) is wasted in the form of exhaust gases, which also implies that any area under the curve that is not in the 60 ms box does no useful work. On the back side of the wave form is a slight ripple. This most likely indicates late burn of fuel obstructed by the physical intrusion of the electrode of the spark plug.
The next experimental pressure curve (at left) is produced from a single-point laser ignition in the same cylinder. The ignition point is projected into the center of the chamber. The total area under the curve is increased by 53% over the spark plug. This increased area is the result of ignition at the center of the cylinder and no physical obstruction of the combustion wave front. The pressure wave is similar to the spark plug, but the total area indicates a more complete burn with no ripples. The area in the power stroke box or usable energy with laser ignition is increased by 10 %.
The figure to the bottom right shows three-point ignition in a cylindrical container. Three points of ignition will propagate at the same speed as a single point until the wave front runs out of fuel or is obstructed. Therefore, three ignition points (4) ignited at the same time in the same container will release energy three times as fast as a single-point ignition until wave interaction or wall (6) collisions occur. Proper spacing of the ignition sites will maximize this potential as the waves expand (5).
The last figure at left shows a predicted pressure curve for three-point ignition-induced combustion using a laser. The prediction was made by multiplying the pressure curve by a factor of 2.3. The multiplier was developed considering the ignition wave quenching on sides of the cylinder and other ignition points as it expands outward. As the wave front impinges at walls and other wave fronts there is no longer fuel to burn. The total usable area under the curve is just slightly over that produced by the single-point laser ignition, however the usable area in the 60 ms stroke time is increased by 101%. This shows the enormous potential of multi-point, optimized-location ignition.
Side-wall ignition in long chamber
Centerline ignition in long chamber
Radial and longitudinal spaced multi-point ignition in short chamber