Multi Spark Plugs 1

[thundair]

What if any is the advantage of dual plugs in a combustion chamber?
Will it cause a flame front interferance?
If two plugs are better would three be ok?

I remember from my limited flying time that to check the mags you could switch back and forth and then to both and see the RPM change, but....

 

[rmw]

I remember as a kid, on some type of school or Boy Scout tour of the fire station, that they ran the engine and switched from one set of plugs to two, and there was a noticable difference in the smoothness and rpm of the engine. I must have been old enough to have been tuned in to engines (even if it was motor scooter engines) and remember it to this day.

Engines probably weren't very sophisticated in those days. We had just moved on from flat heads.

I have seen it as you have as pilots did their checks. I always wondered why they didn't just fly on both and start looking for plan B if one set went out.

But the experts will have to enlighten us. As Paul Harvey says, I'd like to know the 'rest of the story.'

rmw

 

[EngJW]

It is usually claimed that you have to have dual everything in an airplane and there might be some truth to that, but some aircraft engines have huge cylinders, 5.25 inches, and it takes a long time for the flame front to travel that distance. The flame will cool down and burn out before it reaches the end of the chamber. A single spark plug is ideally placed in the center of a hemi chamber, while dual spark plugs are across from each other. Dual spark plugs go back a long way. Three spark plugs have also been used, but I'm not sure if any production vehicles use them.

 

[swall]

Double plugging, via the aftermarket, has been used on BMW aircooled 2 cylinder motorcycle engines to prevent detonation. These engines are hemi head, running 9 or 9.5 to 1 compression ratios and need 94 octane (pump rating) to run without spark knock. Double plugging allowed the engines to run on 89 octane fuel, with the same compression ratio, without spark knock.

 

[thundair]

Is it possible to over plug an engine.

With the advent of 10mm plugs and now .5mm dia Iridium pins it seems possible to put 4 or more plugs in a chamber.

 

[NickE]

swall- huh? how does that work? (I just cant even figure it, but then agian I'm justa metallurgist.. (Who loved the BMW boxer down the stree when I was a kid))

 

[EngJW]

Twin plugs could prevent detonation by burning the fuel at the far end of the chamber before the flame front heats it up and it auto ignites.

I would think that the combustion process would determine whether more than one or two plugs is needed. There could also be an enormous packaging problem to get in more plugs along with the valves, ports, and head bolts in a small cylinder, and then get the cooling passages around them and finally have room for a mechanic to get to them.

 

[thundair]

I am sure the packaging of multi plugs cannot look like the existing configuration. I heard that Yamaha did a study of how many plugs were too many. I haven't found anything written but I am still looking.

I am sure they went through the same doubts when they asked how many valves are too many

 

[evelrod]

Well there is hemi combustion chambers and semi hemi chambers. In most true hemis the valves are not at a good angle to allow a small chamber. Bigger chamber leads to a long burn and, in cases like the Alfa DOHC engines and the Fiat 124 DOHC engines this really inhibits their use as racing engines because you cannot run a really high CR without detonation...the twinplug Alfa GTA's of the 60's corrected the problem with an extra sparkplug (the Fiat had other chamber related issues and needed a LOT more work to be equal in output) Other semihemi DOHC engines of the period, Lotus, Cosworth, etc. Made equal or greater power with only one plug. Small variations in chamber design can make BIG differences in power output. I watched a rather large drag race hemi being assembled and it used three plugs. Big chamber, nitro and, reallllly high rpm. Three seems to do the job for them. Oh yeah, I run a twincam Lotus with ~15:1 CR on 104 octane Sunoco with no detonation problems and a single spark plug.

One of the first engines I helped build was for a Cessna T-50 "Crane" in 1958. When we did the final runup on either engine, the rpm could not vary from left or right mag, or both begore delivery.

"Engines probably weren't very sophisticated in those days. We had just moved on from flat heads."

Actually, many engines in the beginning of the 20th century were OHV and OHC. David Dunbar Buick never built anything but OHV engines for the cars that bore his name, only the 29 Marquette used a flathead and it was really a "rebadged" Oldsmobile.

My dad retired from the fire dept.---the twinplug engines were also twin carbed, twin pumped, dual batts...you get the idea.

Keep in mind that "high octane" in 1930's was only 67 octane !!!! Oil changes were mandatory every 1000 miles, most cars did not have oil filters and many had no airfilter (valve jobs were common at 20k, too).
Lube jobs came EVERY oil change. Tires were good for, ummm, 12,000 miles if you were careful. AND...sparkplugs would last, maybe, 8 to 10 thousand miles at most! Factor in twinplugs and ??????

I learned to drive in a 1937 Nash Ambassador straight 8 with 16 plugs (8 on each side) and a giant 16 plug wire dual coil ign. I did not really appreaciate the significance then, it just looked cooooool!

So, twinplugs are used in modern engines to modify the flame propagation in a chamber that is a little too big(?), poorly engineered/designed (?)
to modify the exhaust emissions (?) What else?

Rod

 

[jrnsr]

Multiple plugs in a hemi or pent roof combustion chamber sounds good (better than wedge anyway), but probably doesn't 'really' reduce detonation any more than firing the plug 20 times in a row adds power.
Fire engines with gas engines had dual plugs and other duplicate backup systems. La France had a magnificent V12 that qualifies as overhead cam although it had a very tall/long combustion chamber. It was designed by Pierce for Pierce Arrow cars in the 1920s but they were built by Lycoming for American La France up to just about 1960, and yes, mine runs best when both ignitions are kicking in.
How many out there have heard about the PLASMA ignition? Hint- GM patented it in 175 for those curious enough to check it out. I told Jack Roush about it to squeak out more horsepower, but you don't see anyone running it in NASCAR (or anywhere else).
A guy in Ann Arbor, Michigan has probably the largest spark plug collection on earth (not just my opinion, Poplar Science's too). If my memory serves me correctly, he informed me the first tiny 10mm plugs showed up in the huge dirigible engines, and so Cadillac designed them into their large V12/16(?) engines to stay up with the latest technology.
I asked some engineers from Champion why on earth they put 3 or 4 ground prongs on aviation plugs when that would only promoted early fouling. Their response, "that's what the government specified!"

 

[jrnsr]

GM patented Plasma ignition in 1975, sorry for typo! It is a very detailed and very interesting read.

 

[MikeHalloran]

Perhaps in that time frame, Hot Rod magazine featured on its cover a rod with a Buick engine having all the spark plugs wired in parallel, describing it as a "Kaehni ignition".

Googling that phrase produced a 1975 patent:

http://freepatentsonline.com/4041922.html

... which describes a 'corona discharge' prechamber ignition system assigned to Tokai TRW, which references two Kaehni patents issued in 1958.

The cool part, I thought at the time, was that there wasn't even a discrete timing mechanism; all the plugs fired all the time, in prechamgers, and the fuel in the cylinders ignited automatically, when it was ready. I recall no fuel supply in the prechambers; the engine had carburetors.

I speculate that the prechambers acted as plasma reservoirs, and ignition occured when the gas dynamics in the cylinder blew a little plasma out of the prechambers.



Mike Halloran
Pembroke Pines, FL, USA

 

[kenre]

Most recent example of dual plugs i can remember would be the Nissan Pintara, 1990 ish? 4 cyl, plug either side of head.
Single rotor cap, but dual coils.

Ken

 

[jrnsr]

Mike,
I studied that bucket T in Hot Rod magazine for hours as a kid to try to understand the principles. Looks like it could possibly lend itself to HCCI technology.
Back in the 60's, a guy & his son replaced the distributor in a 62 Chevy, 327 with 12.5:1 CR, with a direct injection pump and substituted injectors with NI-chrome wire for the spark plugs. The engine could burn anything at suerlean ratios. They had it at GM Tech Center- 650 HP at 62 MPG on coal slurry (virtually pure carbon)!
No one, not even Petersen Publishing (Hot Rod Magazine), nor EPA, paid any interest in it. The son wound up with a job at GM Tech Center. I offered $5000 for the car, but he said they were so letdown by the whole project, he'd leave it in the barn and will it to his buddy.
Jim Northrup, Sr.

 

[hydrae]

Mid 80 nissan pickups with 4 cyliders had twin plugs, the combination of carberated lean burn hemi head with 18% egr allowed for a 2 part catylictic instead of a 3 part and no air pump.
Hydrae

 

[MikeHalloran]

I think I understand how the Kaehni works. I'd expect short plug life because of the continuous duty. I'd expect a glow plug to last a bit longer. Probably neither would last as long as today's periodically fired spark plugs with egr.

I wonder if the continuous presence of plasma would erode the prechambers and the small passages to the cylinders.

Q: Does anybody know if the igniters in jet engines are fired continuously, or just when you're trying to start the damn thing?



Mike Halloran
Pembroke Pines, FL, USA

 

[rmw]

In industrial jets it is just when they are lighting the things off, and not all burner cans have igniters. The jets we ride on, I can't say, but I wouldn't think it to be any different. Igniters off, afterburners on.

rmw

 

[franzh]

I too remember that old Buick. When he arrived at the auto show, he was banned becasue no one believed it would run (a requisite of the show, all show cars had to start and run) to which the owner started the engine and drove it in instead of pushing it.

Franz

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[dalcazar]

To answer part of the original question

the reason there are two plugs in aircraft engines is for redundancy and safety backup, but they run on independent magneto systems, something you don't need in cars even if you want to double plug a cylinder, if one mag fails then you have another one to run it on until you can land and get it fixed. But the design of the engine and the combustion chamber is made around the twin plugs, they didn't add the extra plug as and afterthought, that's why it becomes less efficient when running on just 1. There have been cases of both mags failing, especially when something breaks and overheats them.

I can think of at least one example of a 3-plug system. I read somewhere that mazda produces or produced a special racing version of the RENESIS rotary engine in the RX-8 with 3 plugs, the normal one having only 2

 

[thundair]

Thanks to all for the info I pulled the GM patent and will study that.

Regards