Spark Timing
 

here's another article from the Innovate site i found interesting... feedback invited


"Spark Timing Myths Debunked

A widely-held myth is that maximum advance always means maximum power. Here’s what’s wrong with this thinking:

The spark plug ignites the mixture and the fire starts burning. The speed of this flame front depends on the mixture, this means how many air and fuel molecules are packed together in the combustion chamber. The closer they are packed together in the same volume, the easier it is for the fire to jump from one set of molecules to the other. The burning speed is also dependent on the air-fuel-ratio. At about 12.5 to 13 air-fuel-ratio the mixture burns fastest. A leaner mixture than that burns slower. A richer mixture also burns slower. That's why the maximum power mixture is at the fastest burn speed. It takes some time for this flame front to consume all the fuel in the combustion chamber. As it burns, the pressure and temperature in the cylinder increases. This pressure peaks at some point after TDC. Many experiments have shown that the optimum position for this pressure peak is about 15 to 20 degrees after TDC. The exact location of the optimum pressure peak is actually independent of engine load or RPM, but dependent on engine geometry.

Typically all the mixture is burned before about 70 deg ATDC. But because the mixture density and AFR in the engine change all the time, the fire has to be ignited just at the right time to get the peak pressure at the optimal point. As the engine speed increases, you need to ignite the mixture in the combustion chamber earlier because there is less time between spark and optimum peak pressure angle. If the mixture density is changed due to for example boost or higher compression ratio, the spark has to be ignited later to hit the same optimal point.

If the mixture is ignited to early, the piston is still moving up towards TDC as the pressure from the burning mixture builds. This has several effects:

The pressure buildup before TDC tries to turn the engine backward, costing power.
The point where the pressure in the cylinder peaks is much closer to TDC, with the result of less mechanical leverage on the crankshaft (less power) and also causes MUCH higher pressure peaks and temperatures, leading to knock.
Many people with aftermarket turbos don't change the spark advance very much, believing that earlier spark creates more power. To combat knock they make the mixture richer. All that happens really then is that the mixture burns slower and therefore hits the peak pressure closer to the right point. This of course reaffirms the belief that the richer mixture creates more power. In reality the flame front speed was adjusted to get the right peak pressure point. The same result (with more power, less emissions and less fuel consumption) could be achieved by leaving the mixture at the leaner optimum and retarding the ignition more instead.

Turbo charging or increasing the compression ratio changes the mixture density (more air and fuel molecules are packed together). This increases the peak pressure and temperature. The pressure and temperature can get so high that the remaining unburned mixture ignites by itself at the hottest part in the combustion chamber. This self-ignition happens explosively and is called 'knock'. All engines knock somewhat. If there is very little unburned mixture remaining when it self-ignites, the explosion of that small amount does not cause any problems because it can't create a large, sharp pressure peak. Igniting the mixture later (retarding) causes the peak pressure to be much lower and cures the knock.

The advances in power of modern engines, despite the lower quality of gasoline today, comes partially from improvements in combustion chamber and spark plug location. Modern engines are optimized so that the flame front has the least distance to travel and consumes the mixture as fast as possible. An already burned mixture can no longer explode and therefore higher compression ratios are possible with lower octane fuel. Some race or high performance engines actually have 2 or three spark plugs to ignite the mixture from multiple points. This is done so that the actual burn time is faster with multiple flame fronts. Again, this is to consume the mixture faster without giving it a chance to self-ignite.

Higher octane fuel is more resistant to self-ignition. It takes a higher temperature and pressure to cause it to burn by itself. That's why race fuels are used for engines with high compression or boost. Lead additives have been used, and are still used to raise the self-ignition threshhold of gasoline, but lead is toxic and therefore no longer used for pump-gas. Of course a blown engine is toxic to your wallet."

This is why being too "consecutive" with timing maps can be just as bad as being too aggressive. We see cars all the time come through the shop with really conservative timing tables and the owners complaining of lack of power and detonation.

Also with our engine primarily having split timing it's really key to have that spot on.

I wonder if the author of that article has any experience with rotary engine and if he thinks the same methods should be used.

Timing should be always set to MBT - minimum spark advance which realizes maximum brake torque. Any more timing creates stress and very high peak pressures and temperatures. Rotary engines are very same in this regard.

If the engine can't be tuned for maximum brake torque timing on given fuel at given AFR mixture without detonation, then it means that you have exceeded fuel capabilities. But knock resistance is AFR dependent - stoichiometric AFR permits lowest levels of power without abnormal combustion, lean mixture - read lean with excess air, permits higher power but engine behavior is no good for automotive application. Rich mixture permits highest levels of power on given fuel. Knock limited power increases all the way and beyond 50% of excess fuel and this has been well known for at least 70 years....

This article didn't brought anything new and these basic things about air/fuel ratio and timing should be known to anyone who tunes anything:blush:

If you study the two traces for pressure (top) and location (bottom) you will notice that our optimum rotary pressure location appears to be about 45 degrees ATDC.
You will also notice how much deviation we have between ignition events.
Keeping the AFRs, temps and humidity constant (very difficult) helps verify data.
Barry


http://i287.photobucket.com/albums/l...location-3.jpg

might some of the variability come from timing of the data? how many CPS Hz are you logging?

one of the (many) reasons i am switching ECUs was i want alot more datapoints per second.

howard

^ Howard brings up a good point. What is the sample speed and resolution of the crank trigger wheel?

MBT spark timing can be calculated in simplified form with a certain differential equation... certain stock ECU's use an on-board MBT calculation and then set a baseline off of that, with knock learning applied at the end

We need 54,000 samples for 360 degrees X 9000 rpm. I believe the system is 88,000 CPS Hz.

One Thought... when we tune at say 10.8 AFR and the mixture leans to 12.5 on a hard run we are advancing the spark in essence because of the faster burn rate.
If we could tune to 12.5 AFR any variation would retard the ignition.

Notice the chart below the greater variation at about 6000 rpm. It could be the water/meth cutting in... or a leaner/richer cell... or a cell that needs better timing.
Many variables that need to be balanced.

http://i287.photobucket.com/albums/l...rdes/imep4.jpg

an interesting display.

can you edit the Y line so we have no deadspace and therefore greater data focus?

Yes, notice the light yellow Select Data button on the bottom of the program. It has already been selected to eliminate roll-in and back-down.

I forgot to add that the HKS Twin Power might be causing that change on the log.
Does anyone know when it switches from CDI to Transistor ignition?

The variation on the initial burn is striking! An analogy would be like lighting a candle in a hurricane.
If the wick can be heated enough by the initial flare of the match we would get the best result. Unfortunately that only happens about one in five.
Most of the time it will take longer to get it going.

And idle is much-much worst. It clears up appreciably as soon as a slight load is added.

This initial light-off where the kernel starts to grow is the basis of the great variation that we see in the rest of the burn.

My guess is that exhaust reversion is the biggest addressable problem.
This could be analyzed with sensors in the intake and exhaust.
Unfortunately fast sensors are about $1K each.

Raymond,
ECU's using on-board MBT calculations sounds simple and the way to go. How can we adapt that system to our Rotaries.

I used an InstaCal program to read the HET sensor. The program had to be adjusted slightly for the switching between seeing the tooth vs seeing the space.

An interesting side note. In calibrating the system a double check is done by doing a high RPM ignition cut and then opening the throttle wide open.
You are checking the centering of compression on the TDC mark.

It is like doing a dynamic compression check at 3000 RPM? Guess what the pressure was.....
about 180 psi.

Barry

Hey Barry, I may have missed it somewhere, what model pressure transducer are you using? Also, what kind of peak pressure rise rates(in bar or psi per degree) have you been seeing during combustion?

Max, it is a TFX unit and uses an Optrand sensor.
It shows where the pressure rise is at its max rate of increase but not the rate itself.
My latest tuning experiment has the IMEP at close to 325 psi and the Max pressure at almost 1200psi.
I believe that there are other rotary users but no one else has posted.
Barry

325psi is about 22 bar, that's along the lines I was thinking for IMEP. From what I've seen that's getting into or beyond the range of the most modern turbo direct injected gas and diesels engines. I'm curious what boost and timing (and on what turbo) that corresponds to.

So the crank sensor--does it use the stock trigger wheel? How does it work? I can say from personal experience that expensive lab-grade is ~720 teeth for a resolution of 1/2 a crank angle degree. Most of the better production crank triggers use ~60 teeth for a resolution of every 6 crank degrees.

Honda's onboard cylinder pressure-based knock control for the aborted V10 NSX was designed to operate accurately for every crank degree. It seems like that would mean a trigger wheel with ~360 teeth. The system was also implemented as part of the misfire detection to protect the catalytic converters. It used two dedicated 100mhz processors, one for each bank, with a sample rate of 50 kHZ. As Howard has run into, a Power FC samples at about 20 hz hopefully.

https://www.rx7club.com/attachment.p...1&d=1332385279

https://www.rx7club.com/attachment.p...1&d=1332385279

How exactly is IMEP calculated? Actual pressure trace can be measured, but mean effective pressure is calculated from engine torque and displacement. Or does the software somehow calculate extracted energy from given cycle based on pressure or temperature drop?

Otherwise such IMEP would indicate about 345 lbf·ft indicated torque and assuming it happens around 7500 rpms, about 490 IHP. From NASA enablement program it can be noted that two rotor wankel of similar geometry and displacement around this power level losses at least 70 HP as friction horsepower. So brake power should be around 420 BHP.

They also show heat release rate and pressure curves with peak at 20°ATDC (Direct injection with pilot injection). Maybe you should exploit very low boost setting and optimize mixture and timing for higher peak pressures in earlier crank degrees. Maybe even shut down water injection as uneven flow rate could be large part of the combustion scatter. But thats just my opinion:)

Measuring cylinder pressure traces has a lot to do with spark timing, but yes we are perhaps going on a tangent here about Barry's equipment.

Barry, you were asking how an OEM would calculate MBT spark timing in a way that makes sense for onboard use in a stock ECU. I don't know the exact specifics with a rotary. On a piston engine you can start at the 60% mass fraction burnt time in crank degrees and then calculate backwards from there.

"60% mass fraction burnt" comes from generally agreed ballpark figures. Normally peak combustion chamber pressure on a good cycle occurs between 12 and 15 degrees ATDC (-12 to -15 BTDC), and at this time about 60% of the mass inside the cylinder has been burnt.

https://www.rx7club.com/attachment.p...1&d=1332551548
We use a first-order differential equation to describe the basic relationship:

instantaneous range of change in combustion gas mass = the gas density * the flame area * the combustion rate

https://www.rx7club.com/attachment.p...1&d=1332551548

then set up a series of separate calculations that take this mathematical relationship and apply it to the time period from 0% fraction burnt through 60% burnt. You are working backwards to figure out how long the whole process should take to have peak pressure at the point in the cycle. A lot of it requires lookup tables or single values (constants) in the ECU to make the model work.

The basic things in the ECU that need to either be calculated through a model or looked up in tables:

1. Ignition delay--time between the plug getting the signal to fire and the beginning of the burn

2. Unburned gas density & combustion mass

3. Flame velocity--broken into a laminar (smooth-ish) component and turbulent component

4. EGR, AFR, and water temperature compensation

This is the kind of stuff you find in a modern stock ECU. It's far from perfect, but used in conjunction with other adjustment maps it's more accurate than having a different lookup table for every situation. It's also a lot cheaper than having a system that calculates MBT by directly measuring combustion chamber pressure (those do exist in experimental form but cost is way too high).



.

http://img651.imageshack.us/img651/8...stfueltest.png

i still am amazed at this series of data...

the top chart is a burn speed comparison between straight gasoline and E85. note the HUGE difference as boost (X line) is added!

gasoline around 11-12 degrees and E85 (alcohol) at over 30! note how linear alcohol behaves as boost increases and how very progressive gasoline changes, as well as how much it degrades burnspeed-wise under boost.

then importantly, there are EGTs... this is especially interesting as i assume these EGTs do result from proper timing. often people complain of too hot egts or too cold egts and the condition may be because of timing not being where it should be. (firing out the exhaust or too early).

here we have proper timing and look at the advantage of alcohol.

the first comparative plot boostwise shows 810 V 770 or 1490 V 1418.
the next similar boost plot is 870 V 800 or 1598 V 1472
finally the last (highest common boost plot 950 V 820 or 1742 V 1508!

the data supports my last dyno session:

my fuel mix was 93 octane pump gas as base fuel w 31% to total BTUs being provided my methanol (alcohol).


at 24 psi here are my EGTs:

N12
4800
1438-1452-1474-1502-1530-1517-1453-1487-1464

timing IGL w 11 split
11-----12---12------12-----13-----13-----14---14---14

going back to 2004, the start of my methanol, my egts would linearly climb to about 1780 at 8000.

the difference is the amount of alcohol being used. my new setup uses two EV14 1000 CC injectors overdriven to 123 psi rail pressure which adds 69% flow to the 1000. they were running at 74% duty.

i am thinking that i may be able to use more advance.

howard

Howard,

I know myself or someone else posted that in another thread somewhere. It's slightly out of context so I don't want to have any confusion about it--I can email you the source document if you'd like to see the whole study. Technically, none of the graphs show the exact length of the combustion period (say, the time from 10% mass fraction burned to 90%). You're completely on track in your overall interpretation though.

1. The top chart is commanded ignition advance. It is essentially the commanded spark timing (when the plug has been fired) punched into the engine dyno system. If you ever watch an engine dyno at work, you can do step-by-step adjustments of rpm, load, timing etc in a steady state rather than changing maps in a laptop and going WOT on a chassis dyno.

2. The middle chart is the degrees ATDC when 50% of the mass in the combustion chamber for this experimental engine was burned. A higher number is considered later combustion phasing, meaning that the engine is going to run less optimally overall and have higher EGT. If you look closely, it says on the chart that their target is 6 degrees ATDC for 50% mass fraction burnt. This isn't possible at higher loads on 98 octane RON fuel (roughly 93 octane pump gas in the USA).

3. The bottom is exhaust temperature measured before the turbo, which is straightforward enough.

As you were saying, the ethanol fuel allows more spark advance as load/boost increases (1st graph). Thus the combustion does not have to be delayed (2nd graph) and turbine inlet temperatures are significantly lower (3rd graph). I think you should consider switching to ethanol versus methanol for further improvement in knock resistance. I have a Honda study related to this--Honda engineers seemed to like Toluene a lot (despite its need to be heated) and methanol not so much. Let me know if you want to check it out.

"switching to ethanol versus methanol for further improvement in knock resistance"

i think i would need some selling on that idea.

Latent heat per gallon (cooling capability)

Methanol 3136 BTUs
Ethanol 2398
gasoline 952.... just thought i would throw that in BTW that includes racegas

perhaps that isn't the whole story. pls enlighten me.

i do know that most full tilt drag racers run meth rather than ethanol. currently various race sanctioning bodies are running ethanol but the reason is for, uh, another forum... political type

bottom line for me is taking a step back and looking at the data, the difference is profound.

howard

Evaporative cooling certainly isn't whole story. You are assuming that evaporation extract energy only from air itself. In reality, large part of heat used to evaporate fuel is drawn from intake manifold, not from the air, this results in higher charge temp. This is why your manifold is really cold to touch, but this doesn't indicate anything about what happens inside the engine. More the cooling happens in the manifold, higher will be final temperature - no good, plus condensation of fuel on these extremelly cold part, again, no good for anything.

Second point, methanol does preignite very violently. Flashpoints and evaporative cooling has nothing to do with it and when you hit the google search with right keywords, you will find many research papers which examines this phenomena.

I have seen different thing. Many racers converting from methanol to blends of ethanol and race gas. Reasons like much lower tendency to preignite, better fuel stability and lower wear of engine parts comes to mind.

Methanol due to its lower peak flame temperatures can be slightly more efficient than gasoline fuels on energy basis, but on the mass basis, its abortion. Many OEM manufactures are making studies of highly boosted small capacity spark ignited engines with direct injection of methanol or ethanol as mean of charge coolant (note:main fuel supply is usually port injected gasoline) which allows them to operate engine with lambda 1 and MBT timing ie extremely efficiently, but they are well aware of disadvantages of methanol and consequently, ethanol is number one choose.

autoignition temps

ethanol 793
methanol 878
gasoline 495

i understand that if you cool the manifold it is subtractive to cooling in the chamber. it is also indicative of the difference between gasoline and alcohol. clearly alcohol has alot of remaining cooling capacity as it (also) works in the chamber. i could never make over 500 rwhp SAE on 93 octane. i would have lots of knock and break things. most who do run 100% meth AI find the logged Power FC knock drops to under 15 at max torque and max power. meth is magic based on my dyno experience going back to 04.

that said, i am very interested and open to explore ethanol V meth as my AI injectant.

i am going to call a friend this weekend and ask him. he runs 100% meth on his 2 rotor 13 brew, makes over 1000 rwhp, and enjoys surprising longevity to his motors which he builds.

empiricism and theory together is a good formula.

howard

You must realize that this test is done in somewhat atmospheric conditions and doesn't reflect what happens in the chamber of engine. From research papers covering this phenomena in engines, it can be noted that methanol through certain catalyst process and also due to its volatility falls apart and creates unstable combustion species which spontaneously ignite at much lower temperature and pressure.

Also, just think about temperature of spark plug electrode tip. Its much higher than these values. Based on this, every engine should preignite to death even with methanol...



You should ask him, and maybe ask yourself, why methanol fueled engines are run so rich on lambda scale.

When you consider difference in fuel mass between gasoline fueled engine with certain lambda, and methanol fueled one with same lambda and huge cooling properties even increasing effect, you must ask yourself, why so much is needed? And why race gasoline could do same even without all the cooling. Answer is preignition of this very tricky fuel.

For what it's worth, I've heard a few people speculate that the burn rate of today's pump gasoline is significantly different than it was 10-20 years ago. Someone who has spent a while tuning Nissan engines claimed that common pump-gas SR20 setups today are making the same power with less ignition advance vs what he experienced 5-10 years ago. I'm not sure how well this translates over to rotary engines, but I thought it was interesting.

PS, E85 is great for piston engines, I've seen a turbo engine run 10-15 degrees more ignition advance after already having been tuned to the 'MBT' timing values (Minimum Best Torque timing, or whatever people like to call it) without a significant loss of power, or 'squiggle' in the dyno's power readout, or audible detonation. The tuner did it just to see if they could make the engine detonate on E85, apparently he was already planning to remove the engine and upgrade the internals soon.

I don't want to get too deep into this here, but to summarize the Honda study (Toshiyuki, "Pre-ignition Phenomena of Methanol Fuel (M85) by the Post-Ignition Technique," 1989) found that

1) Due to chemical reactions, methanol fuel preignites at lower temperatures and with greater severity inside an actual engine compared to premium pump fuel.

https://www.rx7club.com/attachment.p...1&d=1332727338

Note that in this study, the methanol-fueled engine runs a colder plug and still preignites at a lower temperature than premium gasoline

2) Methanol is particularly prone to preignition when the spark plug uses noble metals (platinum), such as what you find in typical race plugs and even regular spark plugs today

https://www.rx7club.com/attachment.p...1&d=1332727338

https://www.rx7club.com/attachment.p...1&d=1332727338

So In this study, the methanol-fueled engine with platinum plug (plug A) had extra sensitivity to preignition. This is because noble metals help set off the very chemical reactions that cause preignition with methanol fuel.

in the study they didn't test irridium (which is also a noble metal) plugs but I presume the overall trend holds.

3) higher rpm and richer mixtures mitigate the pre-ignition tendency of methanol, which would explain how people have gotten away with it

https://www.rx7club.com/attachment.p...1&d=1332727812

PM me if you are interested in reading the whole study. I haven't seen such issues arise in ethanol studies, although I won't claim to have read everything out there. Howard, if you want to we could take a discussion on this issue to a dedicated thread in the AI section.

This is what auto-ignition or pre-ignition looks like on a rotary.
Normally the pressure from the growing kernel would not start till after TDC if ignition was say 15 degrees BTDC.
This test is running 50/50 meth/water and 12 lbs boost.
Peak pressure has moved to 40 deg ATDC.
Interesting but scary when we are not in control of the timing.


http://i287.photobucket.com/albums/l...reignition.jpg

dam went to edit lost what i had said.

Nice!

I take it you aren't a methanol/water inj fan? Do you like lower ratios of water to methanol under 50/50?

I´m big fan of water injection. But I´m just proposing what I learned from various research studies. Theoretically, 100% methanol is extremely good charge coolant, but due to its preignition properties (examined above) it simply doesn´t work as someone wants to think. 50:50 water/methanol by mass is vastly superior, any more methanol in mix increases preignition chance and lowers permissible power level.

I asked very simple question, why methanol fueled engine, even with its huge cooling properties, has to be run so rich on lambda scale compared to gasoline? Assuming that both fuels are run at same lambda, methanol fueled one will have 8 times higher cooling effect due to evaporation. So why run it even richer? As chart above indicates, its done due to preignition tendencies of this very sensitive fuel.

"Due to chemical reactions, methanol fuel preignites at lower temperatures"

thanks all for the excellent content. it does have me reconsidering my AI injectant. i will say i am scratching my head re the "chemical reactions" aspect of meth in the combustion chamber. it certainly is opposite of what i would expect given methanol's approx 850 F autoignition temperature V gasoline around 450. of course chemical reactions can change things and i still have lots of gasoline in the chamber to start the burn process...

i am thinking i may consider going to a 50/50 meth distilled water mix and see where that leads. w a few of the turbos in my stable we will be reaching for 600 and it will be interesting to see if 93 pump can hold together. i need 600 for my Texas Mile run and was planning to switch over to 100% meth for that particular event. maybe it can be done w pump and a mix. it certainly would be easier than setting up about 12,000 CC/min (Gross) w straight meth.

i did speak briefly w my 100% meth drag race 1000 rwhp+ 2 rotor guy over the weekend. our conversation was unfortunately short due to our schedules.

he indicated that Lambda generally run by his meth burning competitors is generally between .76 and .81.

that is 11.17-11.91 gas and 4.88 to 5.21 meth.

which means by weight 5 parts air and one part meth.

while gasoline is 11.5 parts air and one part gasoline

2.3 times (weight) meth to gas.

you need a 2.02 ratio to get equal BTUs so they are running another 14% richer.

the only conclusion fits exactly in line w Liborek. they run that ratio because they have to.

very interesting.

hc

That's exactly like our drag or any all meth car. You have to run twice as much fuel as you would with pump but the advantages are still worth it to run it. Albeit it takes twice as much. Lol. I really wish I could tell you what Afr we run on it but the car is tuned solely off egt and sound(carlosmis super old school) but I will admit, it works flawlessly and has worked for many years for him. Even his NHRA sport compact car he won the championship with was set up exactly as this car and tuned the same way with a mechanical injection setup.

just a thought, but is the gas tank big enough to do the texas mile with that kind of fuel flow?

12,000cc/min = 3.17 gallons/min. tank is 18 gallons/3.17 = 5.67minutes of run time full out. @150mph (150/60=2.5 miles/min 2.5miles a minute X 5.67 minutes) that is 14 miles, should be enough.

14 miles ought to be less than one mile even in texas!

Our drag car has a 5gal cell on it. That is enough fuel for the burnout,staging,the run, and sometimes the trip back to the pits.

Howard,

I have had friends run denatured alcohol from the hardware store with some success, although not at the power levels you are talking about. I'm pretty sure that's just grain ethanol + something to make it unfit for human consumption.

I wish I had some data directly comparing methanol vs ethanol in terms of preignition tendencies (as a result of these chemical reactions). The sense I get is that whatever seems to make methanol preignite more than gasoline (on paper at least) is still present in ethanol, but to a significant degree less. So the autoignition temperatures and whatever is happening on the chemical level could be two separate but related things.

Theoretically then, in my mind it goes like this

1. premium gasoline --> lowest octane, least cooling effect, least tendency to preignite (?!)

2. ethanol --> higher octane, greater cooling effect than gasoline, higher autoignition temperature than gasoline, higher tendency to preignite (?!)

3. methanol --> highest octane, greatest cooling effect, highest autoignition temperature, highest tendency to preignite (?!)


Now to murky the waters further, I recently read a Bosch study where they converted a Pontiac Solstice turbo engine (GM LNF motor, 2.0 direct injected) to E85.

https://www.rx7club.com/attachment.p...1&d=1333059417

E85-->autoignition temperature is much higher than gasoline, but surface ignition temperature is lower ?? What the heck does that mean?

"Fueling levels change as stoichiometric ratio changes as
a function of the alcohol concentration. Spark timing
requirement changes due to the difference in flame
speed with alcohol concentration. There are additional
issues to consider with spark timing. With E85 there is a
much greater propensity for pre-ignition to occur due to
the lower surface ignition temperature. In some cases
spark advance has to be limited even though the spark
advance is not limited by knock. Spark plug fouling
during cold start is also an issue on E85 often requiring
compromise between spark plug heat range selection for
cold start and that required to avoid pre-ignition. "

Call me confused, or at least a little unsure. I suppose trying to glean a lot of information out of journals and lab tests can be just as confusing and contradictory as talking to race teams or people on the internet. I doubt there is much work being done about methanol, considering all the infrastructure is built around ethanol today.

For reference here are specs on the fuel Honda used in their study of the naturally aspirated M85 engine I was talking about earlier. It's not presented in the same way as the chart above.

https://www.rx7club.com/attachment.p...1&d=1333059417

I'm not positive on how all the effects relate to each other, but it still seems to me that ethanol/denatured alcohol could potentially be a better fuel than methanol for resisting all the various forms of knock.

So by being worried about pre-ignition but not worried about knock then that should just mean that, like it said, you have more to worry about before TDC than after. Essentially the residual heat left over in all your metals is your biggest concern since after the kernel is formed and the pressure and temperature start to rise, ethanol is able to handle it due to having a higher auto-ignition temperature for the fuel itself. Do you think this is mostly due to the evaprative nature of alcohols since they are far more likely to turn to vapor near a hot surface than gasoline? Would go along with what has been said by others about cooling the intake manifold itself more than the actual air since the alcohol is having the greatest heat rise by pulling heat from the engine itself before combustion ever starts.

^ there may be different perspectives on this, but I think we're reaching the limits of my chemistry knowledge and the limits of what I've been able to find so far in the literature. It would be nice if we had better resources to study how all these fuels work inside a rotary engine specifically. You really need an engine dyno for that, or even better an optical engine. Just having combustion analysis equipment on an engine dyno would help us understand combustion chamber pressure, heat release, combustion phasing, etc so much better. Then we could get a better idea of spark timing and fuel under controlled conditions.

What happens if we crank up the intake temps? Does methanol and ethanol give us more marginal safety then? Are either of them more sensitive to some other factor of preignition, like carbon deposits inside the engine? How about premix or lubrication in general? Are there interactions between that and whatever else you are injecting? There's been some studies of carbon deposits and oil mists in piston engines regarding how they affect preignition tendencies.

Clearly cooling is good for our purposes. But there's something else at work on a chemical level regarding the preignition tendencies mentioned here that I think perhaps only a few researchers would understand fully as it applies to an actual engine. And making the leap from this mostly academic and theoretical discussion to a practical setup meant to achieve real world goals is tricky.

It kind of makes you want to go with 100% water (except in winter maybe) not necessarily because it's "better" but just because it's less complicated in a way.

That's why the majority of people who have tested with diff injectants through the ages have either went with straight water or 50/50 by mass. its been proven for years to make the most power while detterring preignition.
http://www.aquamist.co.uk/vbulletin/index.php

endless amounts of info you are looking for can be found there.

When you do some simple math with ideal gas law and apply common sense, you will see that theoretically every engine, even naturally aspirated, should preignite to death just from final temperature after compression stroke, at least on gasoline. We know it doesn't happen. When you combine this fact with one of the charts above, which indicates that higher engine speed pushes surface preignition temperature higher, its obvious that this destructive phenomena takes certain amount of time to occur. So far it seems that autoignition temperature is meaningless parameter for internal combustion engines - IMO.

Surface ignition temperature is different story. Its obvious that alcohol fuels due to certain chemical reactions are very sensitive in this regard but as above, rich mixtures and higher engine speeds are somewhat helpful in this regard.

Forget about engine parts, actual cycle happens so fast, that it can be considered as adiabatic. My point was, that measuring temperature in intake manifold is meaningless when we doesn't know what happens inside the engine and also the fact, that condensed fuel doesn't burn, it just washes away oil film and contributes to irregularities of combustion.

You don't wanna cool engine parts, but whole charge itself as one of the main determining factors if the engine will or won't knock is charge temperature after compression - obviously lower initial T means lower final T.
Charge is composed from air, fuel and residual exhaust gas. Residual amount is very small but very hot and contains partialy burned HC which is very prone to detonation. What you can do with charge coolant (water, ethanol, methanol) is to cool whole charge to bring whole bulk temperature as low as possible.
In one study of direct injection of E85/Methanol, they go so far, that injection pattern is designed to inject towards residual gasses. Proof of concept is that they used less charge coolant for same permissible power. Minimizing residual gas content and temperature is very vital for power and reliability.

If 50/50 water/methanol has been known to work, does 50/50 water-ethanol actually work better in a test lab and in the real world? I know people who do that instead of methanol, as I mentioned before. They use denatured alcohol from the hardware store + distilled water.

methyl, ethyl and propyl alcohol
1, 2 and 3 carbon chains
,, denatured stuff from the chemist over here is iso-propanol

things to note that are going undiscussed but factor in the equation -

A-not all fuels have the same explosive range
hydrogen being the widest explosive range , and petrol being fairly narrow

the implications when you mix two differing fuels ( alcohol and hydrocarbon ) are at the moment largely unknown in the mainstream
they may be benificial in raising the preigintion point
,, but they also quite likely may lower the potential to resist pre-ignition to the lowest denominater
they may also under go chemical or catylitic reactions that may prove to lower the preigintion threshold ,, as is hinted with methanol

B-not all fuels stratify the same
and this has implications in judging the ideal no - det and no- ping mixture if fundemental mixture stratifcation is not consistant , or changes with rpm

C- charge dilution with EGR or unburned HC fractions
and their role in the mixture in relation to detonation and preignition is often discussed but very difficult to quantify

D interactions of all of the above points


in some other topics on this forum we can see the effect of the peripheral exhaust port and its charge dilution
compared to that of the side exhaust engine
and the charts show the ideal idle mixes to achieve consisitant and steady combustion at idle rpms
its demonstrated that the rx8 can do so at much closer to stoich due to less internal EGR

now,, i have the opposite ,, an engine with peripheral exhaust and slightly more than stock overlaps,, and late close ( 6p ) inlet timing
ie,, reasonable amounts of internal EGR ,, and some reversion and compression heating back into the inlet manifold from the late close inlet timing

( from the chart ) you would expect that i need to keep my idle mixes reasonably rich to keep idle stable
i idle at lambda 1 ,, 15.5:1 to be exact,, and the fuel is hydrocarbon LPG
the factor that makes all the changes here is purely the inherently good stratifcation of this particular fuel






also -
i think what people are missing when they see auto ignition point
is the explosive range of the air fuel mix under pressure and in proximity to the ( auto ) ignition source

IE
they are not seeing that the auto ignition number is for an ideal air fuel ratio at standard pressure
,, and this is not what is happening under compression and stratification
( which is possibly why we see a "surface ignition temp" number to reflect this a little better )

back in the 40's when the real work was done on this stuff they initially added the alcohol for antifreeze due to the altitudes they had to run
the germans had MW30 and MW50 and even EW30 and EW50
and never twigged that they didnt get duration in the inlet tract to cool like carbed systems the allies used
( the pre turbo V cylinder direct argument !! )

both of these fuels ( methyl and ethyl ) are below the LEL ( lower explosive limit ) when at 50 -50 mixes

-but may not be when above that concentration-

both germans and allies realised at some point that higher than 50 % meth / eth ratio had negative effect on detonation limits
the allies also found out that the ideal ratio for propyl was much lower,, below that required to have its necessary antifreeze property

case in point -
in 45 the US inadvertantly used propyl alochol at the normal ( meth ) 50-50 mix
( to prevent freezing at altitude )
and found the detonation threshold was significantly lowered for the worse, costing a spate of engines

- they had crossed over the LEL threshold and the propyl alcohol was preigniting before the ( very high octane , PN -130 ) fuel

http://enginehistory.org/Frank%20WalkerWeb1.pdf

That someone was probably me. I'm beginning to wonder if there's something unsuitable about it as during my hot-air experiment, over the course of about 2 years, I've cracked three plates. So some sort of knock happened although, according to the numbers, everything looked just fine. The setup was injecting about ~1600cc/min in at top load point. Highest boost achieved through a full 4th gear run up just past 120mph was 27psi. AFR's parked in the low 10's:1 with base fuel (93 octane pump) consisting of about 65% of total fuel delivery. The ambient temps out that day were in the high 70*F's if I recall correctly and from memory the highest air temp I datalogged was 127*F at the inlet just in front of the throttle body. I never could pinpoint exactly what's done it but something odd did. I've changed plugs, run leaner and richer, and seemingly no change. It would run terrific for days on end, beating the crap out of the car, then wham out of nowhere, a lower boost run and a broken plate; no misfire, no hint of anything else.

I can say however with a measure of confidence that I think using 100% methyl alcohol as a charge cooler does work and work very well. I've seen it on a number of vehicles including my own which has no intercooler. But past that, I wonder if it's not enough.

^^^ That's what I wonder too. Is there something going on in the chamber that's "outside" of all of the numbers that some of us (myself included) have assumed are what determine it to be of "acceptable" use as an auxiliary injectant?

B

Was talking to Howard about this last night. On the next go-round of my hot-air experiment, assuming I change the EFI system (my E6K ECU is finally going bad; temperature and MAP Sensor readings are going askew when engine is at rest), I'm considering trying a 50/50 water to methyl alcohol and even perhaps a 50/50 water to ethyl alcohol and see what happens. Probably will not change the total output volume of 1200-1600cc/min at higher loads. I'm curious to see what the introduction of water does in the current spots on the hot-air pipe (one M10 nozzle about 8" post turbo discharge and the other M10 nozzle about 10" past that) with respect to air temps. Also considering adding an additional nozzle pre-turbo or just moving the whole setup over there.

B

Have you considered purchasing Rice Racing's kit? He seems to know his shit on this topic.

^1

If I did it'd be in the future somewhere after I exhaust using the Alkycontrol setup. It's not mega-ultra precise like Howard's setup with the precision injector control but it seems to work. I'm not against changing the setup, though. Regardless of all the Internet forum bickering and arguments, I'm a pragmatist when it comes to car stuff and tend towards simply whatever works. If his works best, so be it!

Back to the methyl alcohol chemical reaction thing - Does anybody else have any other info on this?

B

^ shoot me a PM and I'll send you some literature. You can use it to form your own opinion.

I'm with him. Whatever works best and givem me the result that I want that works for me.