Twin plug question

[longhornchris04]

Paul - after reading your follow-up, I think I found part of my confusion...

For a given load condition, there's lean (below stoich), stoich (14.7 to 1), and rich. That said, you don't really want to run stoich at higher loads, so even though its technically still rich, its refered to the engine as "running lean" as in "its leaner than I'd prefer." I know going lean causes all kinds of issues with turbo engines (much moreso than NAs because of intake temperatures).

Also, doesn't the cooling effect of fuel come into play - the heat absorbed by fuel vaporization helping to reduce the chance of detonation? Perhaps that effect is adding to the confusion - all things being equal lean burns slower, but lean and hot vs. rich and cool one effect offsets the other, at least with respect to detonation? Just a though.

[eapcpa]

Very interesting. Looks like I need to go back to the books. Thanks for the info Paul.

edit:
Got it. Lean mixtures burn slower which lengthen the combustion time which causes more heat throughout the combustion process thereby raising EGT (exhaust gas temps). My thinking has been flawed on this for years.

[psalt]

Paul - after reading your follow-up, I think I found part of my confusion...

For a given load condition, there's lean (below stoich), stoich (14.7 to 1), and rich. That said, you don't really want to run stoich at higher loads, so even though its technically still rich, its refered to the engine as "running lean" as in "its leaner than I'd prefer." I know going lean causes all kinds of issues with turbo engines (much moreso than NAs because of intake temperatures).

Also, doesn't the cooling effect of fuel come into play - the heat absorbed by fuel vaporization helping to reduce the chance of detonation? Perhaps that effect is adding to the confusion - all things being equal lean burns slower, but lean and hot vs. rich and cool one effect offsets the other, at least with respect to detonation? Just a though.



Hello Chris,

You make these statements with such confidence, you should really consider running for public office..... You are still confusing the basics and there is no offset of effects in regard to detonation. Detonation by definition is the spontaneous combustion of "unburnt fuel". It is precisely because your previous statement (burn rate) is false that a rich mixture is less likely to cause detonation than a lean mixture. The rich mixture burns much faster and therefore there is no "unburnt fuel" to spontaneously combust to cause detonation. The lean mixture is burning slowly, raising temperature and pressure, while the "unburnt fuel" hangs around waiting to be burned or explode.

When discussing fuel mixture, it can be expressed many ways, AFR, lambda, fuel air ratio, by weight and volume. When using AFR, like 14.7:1, lean is above, not...lean (below stoich). If you are not afraid of textbooks, Stone's IC Engines has a good section on combustion science and for lighter reading Hartman did most of his homework on this subject for his books.

[longhornchris04]

Paul - "public office" - uh... not a chance in hell. I work in/for the government, military actually, and the politics here have conviced me that I don't ever want to be a politician. I'm only renting my soul to Uncle Sam, not selling it.

Sorry if I come off as overly confident, I don't intend to. For whatever reason text, without tone of voice & all the nonverbals, doesn't do a good conveying anything but the literal words (maybe I should use more s). Part of it is probably the military thing, they teach us to write/speak with confidence.

Back on topic...

I'm not afraid of textbooks... I just don't necessarily have access to them. I have more "free time" lately (hence the more frequent posting) because I'm getting ready to move and work isn't loading me down with long term work. If I can find a copy, I'll take a look.

So, to make sure I understand - because the rich mix burns faster, the flame progression "outruns" the rise in temperature and pressure and is able to consume all the mix. In a lean burn, the flame front travels slower, allowing the rising temperature and pressure to destabilize the remaining mix, which causes detonation.

This effect is amplified in high compression motors because the higher compression (that sounds redundant) causes increased temp/pressure even pre-spark. Additionally, the hemi-style combustion chambers Porsche uses (used?) because the offset spark plug leads to a long flame travel (single plug) and the high-dome piston isolates the mix on the far side.

Have I got this right now, or am I still somewhere off in left field?

[psalt]

Ed wrote,

Got it. Lean mixtures burn slower which lengthen the combustion time which causes more heat throughout the combustion process thereby raising EGT (exhaust gas temps). My thinking has been flawed on this for years

Hello Ed,

Almost there. The statement you made is only true within a certain range, in this case, rich of stoich (14.7:1 AFR). EGT vs mixture is a curve, it peaks around stoich and goes the other way (drops) as you lean. Leaner is cooler, lean of stoich. One reason absolute EGT alone is not a good reference and not a good indication of abnormal combustion or detonation.

[psalt]

Hello Chris,

Just kidding. Yes, rich burns faster. The slow combustion problem can be clarified somewhat by considering the diesel engine. The cylinder temperature is so high from the compression that the injected fuel auto ignities immediately and is burned very quickly, so there is no end gas waiting to be burned. Turbo diesels can be boosted up without regard to pressure spikes from abnormal combustion and will continue making power as long as more fuel is supplied. A chart that compares the actual in cylinder pressure under normal combustion and under detonation is eye openning. The Hemi head with a wide VIA has an interesting history. Pent roof 4 valve engine were used in racing as early as 1912, but FIAT introduce a GP 2 valve hemi with a supercharger in 1923. Racing is a sport of copying what works and the FIAT hemi was copied by everyone and the supercharger masked the superiority of the 4 valve pent roof until Honda did the science in the early 60's, then everyone copied them. The basic problem with the hemi is that you have to use a wide VIA to fit larger enough valves, this raises the combustion chamber and increases volume, which requires domed pistons to get a decent N/A compression ratio. The Italians used VIA's over 90 degrees, Jaguar brought it down to 70 in the 40's, the 911 is 59, Lotus down to 54 . The shrouding of the spark plug is less of an issue when you are running a low compression supercharged engine, especially on alcohol based fuels. The N/A hemis of the 50's ran twin plugs, the Maserati 250F ran as high as 15:1 compression and it's power output was largely a chemical experiment. Twin plugging does not "benefit from more aggresive timing", it needs less to get the job done and more timing is not desireable.

There are some good articles on the web for pilots, who actually adjust mixture under load, rather than turn idle screws. Try 43, 18, 63 and 64 here:

http://www.avweb.com/cgi-bin/texis/scripts/avweb-search/search.html?query=detonation+myths&x=29&y=9

Part of Jeff Hartman's EMS book is here but some of the best stuff you have to pay for.....

http://books.google.com/books?id=CaAP6B4TspcC&pg=PA34&lpg=PA34&dq=hartman+mixture+egine+management&source=bl&ots=DzzgjRyftW&sig=GHyDvWznOw1j6mQSLngZZrkzmh0&hl=en&ei=IWpCSoXMLpWltgfO7fHRCQ&sa=X&oi=book_result&ct=result&resnum=1

[longhornchris04]

Many thanks.

Off topic, but as I understand, in diesels the main problem is getting the fuel actually exposed to the air - the outside of the droplets combusts but the inside is deprived of air exposure. Thus, the limiting factor is usually fuel atomization or structural integrity, which is why the modern common-rail TDIs have such high injection pressure. Once you hit that fuel-atomization limit you get the black smoke, but even then the overall combustion is lean.

Back on subject - I figured alcohol (or other octane boosters) would reduce the need for twin plugging. I think I understand what you are saying. Optimum timing is when you get peak pressure right after TDC. Depending on conditions, you may need more or less advance, but you will always need more single plug compared to dual plug because the volumes involved. Dual plug provides a more rapid rise in pressure causing less negative work from rising pressures before TDC. Because this rise is faster, and there's less shrouding of the spark plug, it reduces the chance of unburnt pockets of mixture that can detonate.

More "agressive" timing then would only be desireable only if it can drive peak pressure closer to (but not before) TDC to extract as much energy as possible? The increase in cylinder pressure & temperature from the earlier spark can cause problems with lower quality fuel. I ask because in my other car (also NA), I can purchase a remapped chip that requires premium fuel, and would buy me a few extra horses by adding more timing advance and remapping the fuel. This of course decreased fuel efficiency. Of course, this could just be marketing BS.

Thanks for the "light" reading I'll add it to the list of stuff I'll have to start reading for my MS in Astro Engineering.

[psalt]

Chris wrote,

Optimum timing is when you get peak pressure right after TDC.... More "agressive" timing then would only be desireable only if it can drive peak pressure closer to (but not before) TDC to extract as much energy as possible?

Hello Chris,

No, the ideal time for peak pressure is around 14 ATDC for mechanical advantages, not before. Twin plugs start the burn from both sides, and the peak arrives earlier, so you need less advance to get the peak at 14 ATDC. The shape and size of the combustion chamber effects the time it takes to reach the peak and you adjust the timing advance to arrive at the same place. Peak output with less advance means a better combustion chamber.

One reason for 24,000 psi high pressure diesel injection, is that once you go over 4000 rpm, the window of time is very small to inject all the fuel. Air cooled engines love alcohol fuels, it is a very effective way to get heat out of the cylinder head. There are some interesting E85 turbos.

[longhornchris04]

Ahh, the small turbo-diesels with their "high" redlines. My Dad has a 7.0L F350, its redline is mighty low, but its got torque .

14 deg ATDC -- got it.

I was going to ask if anybody had built an E85 930 but figured it was off-topic. I do know the Koenigsegg has CCX variant running on E85 that generates more power. SAAB also has a turbo (concept I think) that increases the boost via electronic wastegate when the ECU detects E85. Both are interesting concepts as "they" keep increasing the ethanol content in fuel.

[psalt]

Once you get by the corn politics, ethanol becomes much more interesting. MIT has a paper that indicates that a small amount of ethanol injected directly into the cylinder allows much high boost pressures and advance with much lower detonation. Ethanol also allows a much higher EGR dilution than gasoline, which is effectively a reduction in displacement on the fly. You will soon see something like a 5 liter V-8 , with cylinder deactivation and high (+25%) EGR dilution that can have the efficiency of a 1.8 liter 4, and the power of +1 bar boosted 5 liter. All with a single plug........