Contents:
Updated December 5, 2007
Obsolete, plane now equipped with dual Lightspeed Plasma III's:
My airplane is equipped with Unison's LASAR ignition, which is an electronic ignition overlayed on magnetos. When the system has 12 volts supplied, the mags are disabled and the spark is electronically controlled. If ship electrical power is lost, the system reverts to magneto ignition. The system will vary the spark advance from between 25 (normal advance) and 37 degrees advance, depending upon conditions. I wanted to do an economy comparison between the electronic ignition and the magneto backup. Flight test data were gathered in the following manner:
1. Level flight was established at 5500' indicated altitude. Autopilot w/altitude hold engaged.
2. 15 minutes post take off were allowed, to enable Lasar full advance
3. Power was set at 2350 rpm, 21.9"Hg. This power setting is a bit arbitrary, but it is about my typical cruise.
4. Fuel flow was set to 8.4 gph with the mixture. (My fuel flow totalizer is always within +/-2% at fillups)
5. Ten seconds after mixture set, fuel flow, IAS and all four EGT's recorded. My fuel flow system is very stable, perhaps flickering +0.1/-0.0 gph.
6. Fuel flow is then leaned approx. 0.1 gph, and step 5 is repeated. As airspeed decreased, throttle was added to bring the MAP back to 21.9"Hg.
7. The entire process was repeated with the Lasar system shut down.
Of interest is what benefit is the Lasar system? One obvious question is speed vs fuel flow, and those results are presented here:
Discussion: The thin, jagged lines are the raw data plotted. The heavier lines are polynomial curves fit to the data thanks to Excel. One thing that is observed is that I didn't go rich enough to see the drop in airspeed which would occur, but that quite rich of peak area doesn't interest me. Another observation is the crossover in speed, albeit only 1 knot, which occurs at about 7.6 gph. This is interestingly where the EGT's peak, both for the Lasar and for the magneto test. The region of interest is in the 7.0 to 7.2 gph range, which is where I run this engine. In that range, the fuel savings due to the Lasar system is about 0.4 gph (to fly 152 kias with the Lasar is 7.0 gph, and about 7.4 gph with the magnetos). Importantly, it is noted that no benefit in terms of economy is obtained unless one runs lean of peak, indeed, the magnetos may have an advantage in the rich of peak area. Another noteworthy observation is that the engine continued to run smoothly down to 6.4 gph with the Lasar, while roughness started at about 7.0 gph with the magnetos.
With Average EGT's Added to Chart:
Discussion: In the above chart, I've supressed all the raw data, because of clutter concerns. It is interesting to note that the peak EGT's are basically identical, with respect to fuel flow, although their absolute magnitudes are different. The magneto fired EGT's are about 25C hotter than the Lasar fired EGT's. The upturn in the right ends of three of the polynomials are artifacts of the last data points. However, the upturns in the left ends of the EGT polynomials are real - this is where the mixture is so lean that the effective timing is changed.
Conclusions: It is important to keep in mind that all this data is at only one power setting. In the case of my aircraft and this typical cruise setting, I am saving about 0.4 gph versus standard magneto ignition. Is this worth the extra complexity? I don't know. Another way to look at it is that I'm cruising about 4 knots faster for the same fuel burn. This sounds a little better:>)