general tuning (with WI) questions

[cabe]

i understand the concepts of water injection, and will be setting up a direct port water injection system on my turbo 4cyl with a map for a secondary set of injectors that my ems provides me with. i believe that this will provide me with the most flexability. still, there are a few things that i do not know the answer to. here goes.

the one thing i still cannot understand is why the 12-12.5 number keeps coming up when people say "richer than 12-12.5 and there will be no more o2 to burn fuel" when i am under the impression that 14.7:1 is stoich.. i have a feeling that this somehow has to do with the fact that fuel does not react with just the oxygen in the air at high combustion temps. clarification greatly welcome. :wink:

anyway, the way i understand it, the tuning process should consist of a few things..

1) tuning the afr at each load/speed point so that there is no excess fuel left over after combustion (only a system with a seperate injector driver would be able to tune like this due to differing ammounts of water needing to be injected at different load/speed points) however i have no idea how i would determine the correct afr to shoot for.. anyone have any suggestions?
2) a balancing act between injecting water and advancing timing until peak torque is reached at each load/speed point (which would obviously have to be done at a load holding dyno)

^-- now there is nothing in there about keeping the water to fuel mix below any ratio (25% etc) because i could not find any reason in my research why there should be a cap on how much water to fuel there is. (maybe this is because not many people are doing it with a secondary bank of injectors dedicated to water injection, and thus do not have the flexability to tune at all load/speed points?)

sorry for the long post and thanks for taking the time to read this far :smile: . (constructive) comments on this are greatly desired!

[hotrod]

In order for a fuel molecule to burn it must be both hot enough and have oxygen molecues available at the right time. In a stoichimetric mixture its like you have exactly enough men and women at a party to pair everyone up. If you were to suddenly yell -- everyone choose a partner, by random chance some guys would be able to choose from 3 or 4 girls and some guys would not be near any girls. If there is not enough time to mix some guys and some girls will go unpaired. The closer you get to everyone finding a partner the harder it will be for the last few unattached guys to find the last few unattached girls.

A 12.5 :1 fuel air mixture is sort of like a party where the guys outnumber the girls. If your goal was to be sure every girl found a partner this would be the arraingement you want. When you shout eveyone find a partner (fire the spark plug) it would be easy for every girl to find a compatible guy. Some of the guys would go home single but all the girls would be happy.

Nearly all internal combustion engines make their max power rich at about 12.5:1. This is 12.5 lbs of air to each lb of fuel. This is a rich mixture that intentionally provides more fuel than can be burned by the available air (sort of like the party with too many guys). This ensures that "ALL" the oxygen (or as close as you can get ) is used up as it is the oxygen used in combustion that limits power. You are willing to leave some fuel unburned to ensure all your oxygen is consumed.

The 14.7:1 ratio or ( stoich ) mixture is 14.7 lbs of air to 1 lb of fuel, and provides enough air to burn "all the fuel". If time and condtions allow it.

In a stoichiometric mixture you should have nearly no fuel and no oxygen left after combustion. The problem with that, matching up all the oxygen with all the fuel takes some time. The last few couples take a while to find each other.

Mixtures that lean also burn too hot for most engines to make maximum power with. More of the burning process still continues in the exhaust manifold and as the exhaust passes the valve. This cooks the valve and causes very high EGT's.

Most engines can continue to make more power as you lean them out past 12.5:1 until you get to about 13.2:1 or so, but max power lean is very dangerous and very prone to detonation due to the high combustion temperatures. By running WI you can tune into that 13:1 max power range without melting the engine and without breaking it due to detonation.

This is highly simplified as other issues are involved, but engine temps, detonation and using all the available oxygen are the primary factors in producing maximum power.

Larry

[Richard L]

hodrod,

I really like your explaination. I think you will need to do that again and agian for the many years to come.

[JohnA]

Larry, that is a nice analogy mate.

Hope you won't mind if I use it at some point on my 'fuelling' page, it tallies with the style of the site. :wink:

[janis]

Indeed a great way to explain such a complicated concept, even I understand it. Thanks :!:

Is it possible to mix Air + Fuel better, or is there just not enough time?

There are some "snake-oil" products available, like a "cyclone" that creates a whirl-wind after the airfilter N/A engine or before the trottle body.
Those products claim that the whirl-wind causes the Air and Fuel to mix better. Probably a load of cr@p, especially with turbo charged engines.

http://www.dalhems.com/luftfiltersidor/cyclone.html

[Richard L]

Audi's GT racing team took 1, 2 and 4th position at Le Mans a few years ago running af/ratio between 13.6-16:1 on their turbocharged FSI engine.

The special piston top make the incoming charge tumble vertically, the need to run rich a/f ratio is no longer needed. Needless to say, it was fast and economical - also win races :razz:

If anyone is interested, I will dig out the article and scan it to the board.

[PuntoRex]

Richard,

Is this what you are saying?

:smile:

[hotrod]

Hope you won't mind if I use it at some point on my 'fuelling' page, it tallies with the style of the site.

Sure feel free, I like analogies to discribe complex issues, and tend to collect them. I probably adapted someone elses analogy I saw some place else.

Is it possible to mix Air + Fuel better, or is there just not enough time?

There are some "snake-oil" products available, like a "cyclone" that creates a whirl-wind after the airfilter N/A engine or before the trottle body.

Yes it is desirable to improve mixing, that is one of the ways they get ultra lean mixtures to burn. They intentionally create a turbulence pattern in the cylinder that creates a slighly richer mixture near the sparkplug to get reliable early ignition then the flame front progresses into a mixture that would normally be too lean to ignite easily.

High turbulence in the combustion chamber also helps create a quick burn and reduces the tendency to detonate.

Unfortunately creating turbulence / swirl in front of the carburator / throttle body does not translate to increased swirl/turbulence in the combustion chamber. The intake manifold and valves completely dominate the process of creating organized motion in the combustion chamber during the intake stroke. The shape of the combustion chamber,cylinder head and piston crown completely dominate turbulence/swirl or tumble created in the combustion gases during the power stroke.

The air swirl type products are an interesting example of something that actually works slightly in a very small set of circumstances. But only on certain carburated cars. The improvement is so small as to be difficult to measure. A professional race engine builder tested one, and found some interesting things.

On a dyno he could see very small hp gains at certain rpm points, but got no increase in gas milage in road tests. The device created tremendous swirl when tested by itself on a flow bench, but the intake swirl was completely killed by the carburator and was not measurable beyond the carburator throttle plates, when the entire intake tract was measured. The fact that it made small gains is probably more a testament to the fact the carburator was not working at its best, than a testament to any revolutionary developement in engineering. The tremendous swirl apparently helped the carburator give slightly better fuel atomization at key rpm's and the small improvement the driver detects is more a case of drivability rather than performance, ie eliminating slight stumbles or flat spots.

You can actually make a car feel faster to a driver while making it run slower, by changing the way the power comes in. A car with very smooth uniform power increase does not feel as fast as a car that has moderate power up to midrange rpms then suddenly improves to a higher power level. The driver will interpret the sudden surge of acceleration to more power when in fact it may only be a case of lower power production at low rpm.

Larry

[Richard L]

Originally Posted by PuntoRex

Richard,

Is this what you are saying?

:smile:

That's the one :lol:

[JohnA]

Ah, the old Squish, Tumble and Swirl sisters.

Interesting article this LeMans V8. Things move on, gone are the days of being proud of running pig-rich :lol: