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  #41  
Old 03-10-2004, 05:52 AM
hotrod hotrod is offline
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Default alcohol and evaporative emissions

The other issue is the evaporative emissions. High alcohol fuel blends have about .5-1 psi higher Reid Vapor pressure than normal gasoline depending on the alcohol concentration. Highest RVP is actually at about 10% concentration and goes down some with higher percentage blends.

That puts high alcohol fuel blends outside the current specifications for RFG. The major problem in California is that not all fuel sold in the region has ethanol blended in. If you mix a fuel that was blended with MTBE to have the proper Reid Vapor pressure with a fuel that is blended to have proper Reid Vapor pressure with ethanol as the oxygenate, you get a fuel blend with too high of an evaporation rate. That causes problems with photochemical smog from evaporative emissions, which is a big concern in Calif.

When the ban MTBE out there, if they mandate that all fuel must have some ethanol in it, than as I understand it the problem of in tank mixing of different fuels will no longer be a problem.

http://www.arb.ca.gov/fuels/gasoline/oxy/updatedwvr.pdf
http://www.nrel.gov/docs/fy03osti/32206.pdf
http://www.afdc.doe.gov/pdfs/6968.pdf



Larry
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  #42  
Old 03-10-2004, 08:22 AM
SaabTuner SaabTuner is offline
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Default Back to WI Tuning ...

To get back on topic I thought I'd made a post that represents my oppinions on WI tuning ...

General Tuning, and Making the Switch to Water Injection

Mixture for a Fixed Injection Rate:

If stuck wtih a fixed relative injection rate, I would go with a 70/30 methanol/water mixture at a rate of 50% of your fuel flow on an intercooled engine. I'd have it setup to switch on 2 psi lower than you first start to see knock at. This would put the overal water injection rate at 15% of your fuel flow, and methanol at 35% of your fuel flow. Other people may say much less is advisable. This is just my oppinion.

I like that number because it keeps the water level down relatively low, yet the alcohol level high. Water and alcohol will cool the charge, and alcohol will raise the blended octane of your fuel right when you need it most.

On 91 AKI octane, the blended octane with 30% methanol would be 98 AKI octane (RON + MON)/2. That means you have the benefits of near race fuel + water injection when your WI is active.

If your engine doesn't knock at all at any PSI on this mixture, try reducing to a 40% injection rate, then 30% and so on so that you only use just barely more than you need.

Ideal Mixture and Setup:

A truly ideal WI setup would have a variable Meth/Water ratio. However, given the unnecessary complication of such a setup, I say that a 70meth/30water setup is a good place to start. The reason I chose this mixture is primarily that it allows brief trips into the severely lean section without knock.

My ideal controller would use zero WI whenever possible, and upon detection of light knocking begin adding WI while holding ignition timing constant if possible. (It'd need to pull timing for one or two rotations as WI cannot always be added quickly enough.) Then as the knock persisted continue to add more WI until a certain limit, at which point it would begin to lower boost pressure and slowly pull ignition timing until the knock abated.

That is essentially how my stock Saab's ECU works in the first place, with the exception that it first pulls timing, then adds fuel (interernal coolant), then slowly lowers boost as necessary. I think given enough programming skill, most ECU's could be altered to do the same. Instead of adding to the fuel curve, they just add WI from supplimentary injectors on the intake runners.

My Oppinion on Certain Encountered Problems:

Again I'm far from an expert, but these are just my thoughts to add to the discussion ...

Hesitation:

I think that when WI seems to be causing a loss in power this may be due to the over-quenching of the spark. Most cars use quench pads to create a faster burn. With WI this may be "blowing out" the flame Kernel.

I think one possible solution is to advance the timing several degrees. Remember that the quench pads only work near TDC, so the further you are from TDC the easier it should be to ignite the mixture. If timed properly the flame kernel should develop before quench takes place, then as the flame kernel is quenched the flame wave begins to propogate quickly. If you're experiencing hesitation, you're probably nowhere near the knock threshold, so advancing the timing a little to compensate for quench and a very low Flame Development Angle should be ok.

Another, slightly more obvious, solution is to lean the mixture. If you are running just straight water see how close you can get it to 14.7:1. As shown by the previous graphs, when running lots of water (IE enough to make th engine hesitate) 15:1 should be just as resistant to knock (or nearly) as 12:1.

If none of that works, either change the mixture to a mostly alcohol mixture with just a little water, or reduce the quantity of the mixture until there is no hesitation. (I think that should be a last resort, as clearly even a 50% mixture can work if setup right ... just perhaps not on all cars.)

Some Thermodyamics to Consider:

Since most engines are NOT like the engine in those graphs I posted, I thought I'd make list of things that can make your engine different. (Aside from the name of your engine's manufacturer.)

1. Higher compression. The NACA engine was 7:1 compression. More compression means more heat, which means more cooling is needed. It also means it's harder to ignite the mixture, especially if you're injecting lots of water. The further away from TDC the charge is ignited the easy it will be to ignite, but too far advanced and cyllinder pressure's skyrocket.

2. Large Bore X Short Stroke. The NACA engine had a long stroke, and thus a relatively small bore. This means it's easier for the piston to dissipate heat as there is less distance for that heat to go to make it to the coolant through the cyllinder walls. Large bore engines may need more internal cooling.

3. Single Spark Plug. The NACA engine was a twin plug engine. But, because the plugs were both on one side of the combustion chamber, the burn rate was still somewhat sluggish. A DOHC or Hemi chamber should burn just as quickly, but with a single plug igniting a heavily water laden mixture is harder.

4. Computerized Spark Control. Some cars, like my newer Saab, have a pre-set spark voltage which is designed to be JUST enough to ignite the mixture at various boost levels. A high water content can bog down the car as it's not expecting to require that much ignition power at that boost level.

5. Quench Pads. As discussed earlier, these can blow out your spark if you have enough water. Quenching is greatest near TDC, so sometimes a very advanced ignition can compensate to some degree. High compression engines usually have a great deal more quenching, so high compression turbocharged engines may need a disproportionately large spark energy, or high ignition advance ... or a little less water. ::lol: (Easiest solution last!)

Anyway ... those are my thoughts for now. Any correlation to the real world is purely coincidental. ops:

Adrian~
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  #43  
Old 04-10-2004, 11:09 PM
b_boy b_boy is offline
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An interesting article has just appeared in Modified Mag on tuning with WI. It's a good article, and good for WI in general, but as tends to happen too short on detail.

It is reassuring that the conclusions reached on these boards are the one's espoused in the article.

November Issue Modified Mag
http://www.modified.com

Currently no link to the article on the site, maybe later after it's off news stands.
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  #44  
Old 13-10-2004, 12:50 AM
Richard L Richard L is offline
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b_boy's summary is great - just which someone has car with that set up that will reflect the result of the theory - on a modern engine of course.
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  #45  
Old 28-05-2005, 03:09 PM
Prometeus Prometeus is offline
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Hi Richard, hi Larry Hi Adrian....

sorry to disturb you but it might be possible that an italian army of forum subscribers is coming up to the gasoline forced induction forum to recive some informations... (hope to be 2... may be 2.000 :shock: )
I'm deeply sorry but you know Richard I'm completely unable to manage GASOLINE engines... just DIESEL... :lol:

Regards

Daniele
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  #46  
Old 12-06-2005, 07:35 PM
Richard L Richard L is offline
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Hello Prometeus,

999 Italians (average of 2...2000)!!! I 'd better strart learning italian.

Diesel is very popular in Europe, I am a bit behind with the techanology. I am learn fast.

When is the invasion?
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  #47  
Old 28-09-2005, 05:41 PM
Richard L Richard L is offline
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I would like to discuss the effect of flame speed against power - ideally we want a very high knock resistant set up that can also burn very fast as well.

Almost all race fuels burn quite slowly except those specials that F1 car uses. If anyone could chime in on this topic, may be some information on various kind of fuel or additives that can increase up frame speed. Low Knock resistance fuel doesn't always mean fast-burning.

I have heard that if peroxide is disassociate into free radicals (the OH molecules that has temporary lost their links) will initial and speed up the chain reaction of hydocarbon with air (oxygen).

Don't forget Nitromethane.

Anyone?
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  #48  
Old 29-09-2005, 09:59 AM
hotrod hotrod is offline
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Alcohol fuels burn faster than straight gasoline but the burn speed depends on the mixture as well.

Gasoline has its highest burn speed near 11.3:1 AFR

Max burn speed for ethanol and methanol fuels are at even richer mixtures. The higher burn speed of ethanol is one of the reasons E85 is more effecient than gasoline (aside from octane and charge air cooling).
That is also one of the reasons 50:50 water methanol WI needs less ignition advance than plain water.

http://www.epa.gov/otaq/presentation...mjb-051303.pdf

Check out the chart on page 9


Larry
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  #49  
Old 29-09-2005, 05:49 PM
JohnA JohnA is offline
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Advanced topic this one

I've got somewhere a model of this I made a couple of years ago. The graph on my site has the incorrect shape, by the way, it is exaggerated bell shape to make a point.

Max speed for gasoline is at AFR 12:1 or thereabouts. Go leaner or richer and it slows down (effectively retarding the ignition)

It is a 3D graph, because mixture DENSITY also affects burn speed. More density (boost, nitrous) more speed. It is the *main* reason you need to retard the ignition under boost or nitrous, despite what most people think.

One example of misguided 'experts': Running lots of NOS and running pig-rich as well. They think that the extra fuel cools down the situation, when in reality it simply slows down the burn speed so that they don't need to retard the ignition. False logic ofcourse, because if they were to lean down and retard the ignition properly they'd be better off. The extra fuel just messes up their overwhelmed spark plugs. :wink:
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  #50  
Old 29-09-2005, 08:06 PM
hotrod hotrod is offline
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Different sources give different numbers for best burn speed, all seem to be in the mid 11:1 - low 12:1 range for gasoline --- far too many variables to give an absolute number I guess.

Ignition advance ( indirectly an indicator of burn speed ) also depends on combustion chamber design and squish etc. as we all know. Interesting chart on the change in required ignition advance in one of the NACA reports also.

Check out Figure 5 In NACA report E5E18 for a good example of both the change with afr and engine rpm. On the Aircraft engines they were working with they got minimum required advance at a fuel air ratio of .088 or approx 11.36:1.

In the book How to Tune and Modify Engine Managment Systems he plots a curve on page 127 that shows best power at fuel air ratio of about 0.083 (12.05:1) and fastest burn speed at 0.09 (11.1:1)


John you raise a very important point that people need to keep in mind --- every time they change fuel air mix or in our case WI rate or water/methanol mix you are in effect changing engine ignition timing by changing when peak cylinder pressure occurs in the cylinder.

At a given engine rpm and manifold pressure and AFR, there is only 1 ignition advance that will optimize cylinder peak pressure with the mechanical best crank angle for max engine effeciency (somewhere near 14 deg ATDC)

It is much better to be a bit late on ignition timing in a high power engine than it is to be a bit early. If your on the ragged edge and you reduce your WI spray rate or add alcohol to the mix, you are for all practical purposes advancing ignition timing.

The window of acceptable ignition advance for best power/effecincy is only a few degrees wide so you don't have a lot of room to play around with if your pushing the engine hard.


In figure 6 in the NACA report E5E18 on the next page following figure 5 you can see that at max power output (863 lb/hr air flow) the curves get a sharper peak and the max power nose of the curve is only about 8 deg wide, where at a lower boost pressure (437 lb/hr air flow) the same zone was almost 14 deg wide.

Larry
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