|
All this talk about EI spark plugs may be slightly off topic for a Lancair list, but I find it useful, especially since I am about to replace the plugs in my Lycoming (parallel-valve, 10:1 compression. First, I'm confused about the term "automotive plugs" and the idea that they somehow "can't handle the pressure". What pressure are we talking about? Looking at the construction of both automotive and aircraft plugs I see that they are both built essentially the same, with the automotive plugs probably more substantial with most using metal seals while some or most of the aircraft plugs use a composition seal. The only problem there is you might get a trace of leakage, not really a major issue. Whether the plug is designed for a moped or a Ferrari the plugs are essentially identical in construction and quality with the biggest difference being things like heat range and center electrode extension. The key is to use the "right" heat range and I see the plugs that came with my LSE III are very cold, similar to what we used to use in turbocharged race engines. It might be tempting to use a hotter plug to keep the insulator cleaner, but the plug "guru" at Autolite once told me that is playing with fire in an aircraft engine. He said that lead deposits are conductive when hot, just before they burn off. Ground-based lead-burning vehicles work well with hot plugs as the lead is continually burned off. However, aircraft engines can run for a long time cold (on the ground) and then are suddenly run very hot, which can get the lead hot enough to conduct before it can burn off. He had seen cases of total engine failure caused by all the plugs shorting from lead fouling at the same time. Using cold plugs will let the lead build up over time, but won't suffer from the shorting problem Perhaps the difficulties related had something to do with that. By the way, the Denso WEMR27-C plugs supplied with my EI system look about right regarding the heat range after 100 hours, but the electrodes are significantly eroded.
As I understood it, the problem with using "aircraft" plugs with automotive coils is coming up with an ignition wire that connects to both. Is there a wire that has an "automotive" connection at the coil end and an "aircraft" connection at the plug end? Apparently one "Lancair group" stopped using "automotive plugs in their EI systems. What "aircraft" plugs do they use now? If that were readily available why not use aircraft plugs?
The other discussion centered on fine center wires. As the referenced articles said, the advantages are reduced center electrode erosion (longer life) and reduced voltage requirement. They also imply that there is a slight improvement in "ignitability." Longer life is a no-brainer, but in an aircraft application we probably want to replace the plugs on account of lead buildup anyway, so we won't see longer life. Reduced voltage requirement is a good thing, especially if the rest of the system is on the weak side and is not capable of producing the higher voltages. I don't see that being an issue as long as the plug gaps are reasonable (.035 or less, maybe and they could be set at .025). Ignitability is an issue if you are trying to eliminate misfires under cold, idle or very lean conditions. The ignitability of the mixture in an aircraft engine is never as difficult as in a car engine (we don't have to meet emissions, don't care about an occasionaly miss at idle or when the throttle is closed on short final and we never run really lean. We put in lots of fuel for a cold start). There is another issue that might have a slight effect, and that is that apparently the flame front initiation is a little faster with fine wires - the assumption is that there is less electrode to shroud the flame kernel and therefore the flame builds to a true flame front faster. I would guess that this is probably true, but the effect is ignition timing that is "advanced" slightly on account of that the flame front builds more quickly. Studies I did a long, long time ago showed that simply advancing the ignition time a degree or two did the same thing. So the improvement in BSFC measure by RAM was likely just that effect.
Bottom line is the choice between massive wire or fine-wire electrodes in an EI system - which is it to be? Certainly the fine-wire plugs are "better", but are they worth the cost? I'm not sure - the difference is small enough to make it a good question.
Sorry about the long opinion. Gary Casey On May 27, 2008, at 3:00 AM, Lancair Mailing List wrote: I thought maybe I should add to the anecdotal data base here (even though I fly a crummy RV8). I have a parallel valve 360 with 10:1 pistons, forward facing sump/bendix injection, and 4 pipe exhaust. Like Scott, I run dual Light Speed Plasma IIIs, and the IK27 spark plugs. I change them once a year at annual. I also run many of the races, Airventure Cup and some of the Sport Air Racing League events. These are all low level, wide open races (for 2 hours in the AirVenture Cup). I have never seen any damage to any of the plugs, or had any problem with the LS system. John Huft RV spy Race 18, “Nuisance” What you said is, of course, contrary to my experience and the recommendations made by Light Speed Engineering. Which plugs had "the tips burns down fast or the ceramic crack under the pressures" and what were the pressures? Were the plugs ever dropped? Are you sure the coils shorted internally because the auto plug gaps were getting larger or were the coils placed in too hot of an environment? What plugs were used? It is hard to imagine internal coil arcing in the absence of any plug gap or even the absence of a plug altogether. Which actual auto plugs cracked under full power and/or had the electrodes burned away? What were the other engine conditions? I have run my 9:1 CR injected 320 at takeoff power (racing is like that) for hours on end with no difficulty - no difficulty with either the EI or the Denso IK27's - where the timing and A/F ratios have been appropriate for the power and the coils are located in an area behind the engine that usually reaches 70C. You may wish to poke around the web for info on the differences betwixt Champion and Unison (Autolite) iridium plugs. BTW, I noticed that you re-gapped your aviation iridium plugs (out of the box the gaps are too small for EIs, .016-.018.). This is a very difficult task with Champion plugs (they are happy to re-gap them for you). Finally, I didn't even try the LSE recommended W27EBR (27 heat rating for higher compression engines) since they seemed too cheap. I did try the W27EMR-C plugs (triple massive electrode), but didn't like them because I could not control all three gaps (well, that was one reason). Scott Krueger AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL (KARR)
Pilot not TSO'd, Certificated score only > 70%. Not even FAA tested for Repairman Certification. In my 360 I use Iridium plugs with my Jeff Rose EI. They work great. Set to .035. I wouldn't use car plugs in an aviation engine. They don't work well. I've seen the tips burns down fast or the ceramic crack under the pressures. One multiple failure I've seen is on a friends very low time Legacy that has had 3 shorted coils on his LSI system. This was from the Denso auto plug gaps getting larger from wear then eventually arcing inside the coil. Another failure in a 360 I've seen from auto plugs is the ceramic cracks under full power and completely burns away the electrodes, damaging the cylinder. Aviation plugs are made for aviation use and can take the abuse. They last much longer and rarely fail. All the Lancair's in my group stopped using auto plugs. Uppers are Massives with a mag and lowers are Iridium with an EI.
|