Humidity displaces oxygen, so you need to trim the fuel a little if the humidity is high and temperature and pressure are constant.

Knock sensors have nothing to do with A:F ratio. They work with ignition timing.

Oxygen sensors might cover the ground depending on how the data is used.

A humidity level sensor is predictive in that it measures inputs whereas an oxygen sensor is reactive as it measures output.

Regards

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Pat excellently summarizes the situation.
Ignition delay increases with increasing humidity, so ignition timing should be fine tuned as a function of humidity.
Flame speed & temperature decrease as a function of increasing humidity, so ideally this should be compensated also.  In a lean burn engine this can be done via enrichment with increasing humidity.  In a stoich engine perhaps this can be compensated via increased spark advance.

"Knock sensors have nothing to do with A/F ratio..."

WRONG, NOT!!!

Engines knock for reasons other than ignition timing, lugging for instance. These days with EFI, Electronic fuel injection the mixture is often enriched to prevent knocking.

Say if running regular fuel in a high compression engine.

beekeeper,

The purpose of humidity sensors, in the intake of a gasoline/diesel engine, are for NOx control: -

higher humidity will result in lower NOx  
lower humidity higher NOx

As has already been pointed out this is to do with combustion speed/heat release. A calibrated model, within the ECU, will have a humidity as a function and an output of spark retard or start of injection retard.

Humidity is NOT required for fuelling control since there is a direct correlation between air density and humidity and a Mass Air Flow sensor, which most modern engines use, is thus self correcting.


wwest

You are incorrect, modern engines do not enrich to prevent knocking only to reduce the temperatures of exhaust components.

MS

I must admit I was very doubtful if mixture enrichment would be used by an OEM as an antknock device as it would really hurt fuel economy. It is used in race cars and in high boost modified road cars, but in these cases fuel economy is a very low priority.

Humidity is only one factor that effects air density.

If you displace oxygen with moisture, then adjust pressure and temperature to correct density, I would strongly suspect you will end up with a different oxygen content at the same density. I don't have data on hand, so I am speculating.

Regards

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When I picked up my 2001 911/996 C4 at the factory I was told quite specifically that fueling it with regular, since I would NOT always be able to fuel with premum traveling in europe, would NOT be a problem since the ECU would automatically enrich the mixture to prevent engine knock.

Engine knock is NOT always the result of the ignition timing being too early.

And yes, enriching the mixture to prevent engine knock when using regular fuel in a high compression engine otherwise requiring premium does, ABSOLUTELY, result in lower FE.

So, why is that NEWS...??

So what's the fuel economy improvement?  That with humidity sensing you know to reduce EGR rate in high humidity climates?

In any control system (even if it is basically open loop), knowledge of disturbances, if significant, can be used to refine the control system.

During the combustion event, will water vapor (having already absorbed it's latent heat of evaporation) chemically combine with any of the HC chains present, flash into steam from the high temperature {(and thus add to the expansion pressure due to it's 1700X greater volume)(wouldn't the high pressure present prevent this?)}, or pass "neutrally" through to the exhaust tract?

Si Vis Pacem Para Bellum

water vapor (having already absorbed it's latent heat of evaporation)... flash into steam from the high temperature

so what's the difference between water vapor as described above and steam?

At 100% humidity and 80F sealevel, air holds 3.5% water as a vapor, so, there is 3.5% less O2 and the engine needs 3.5% less fuel and will produce 3.5% less work. Just to add numbers to patprimmers answer.

water decreases the air density!  Thats why planes have a harder time flying in high humidity.

wwest, a turbo engine is quite different. At the high pressures and high intake temperatures the engine would tend to run lean and at lean conditions there isn't enough mass to remove the heat and you would burn a hole in the piston, so the ECU makes the engine run rich high octane or not.  This is more noticable at high acceleration WOT, watch race cars for black smoke coming out of a turn and hitting it.

Here's what I've found from my reading.  Engine calibrators calculate spark timing to avoid detonation at worst case scenario, which is low humidity.  If humidity is higher than this, the spark can be advanced, which will result in more torque and better fuel economy.

Hi all, my first post,
Good point "beekeeper"  I have an old non-computerized 351c. When I lean the carb out to get misfire, then turn my  water injection on the misfire gos away. The humidity helps the initiation of combustion. By the way I do get more HP at the rear wheels  without changing timing.
Regards
Mark

maxc, not correct.  You inject water to get the heat of vaporization from it.  This energy lowers the air temperature and the density PLUS the temperature of the air fuel mixture, so that during the compression stroke the temperature in the cylinder is lower and therfore it does not pre detonate.

The water injection then allows you to add more fuel because the air is more dense (cooler) and more fuel = more HP.

dcasto

Although maxc does not say so, many people add alcohol to the water and this also enriches the mixture. If this is the case, it throws the assumptions made by maxc out the window.

Regards

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In chronological order, sorry for the length: -

wwest

Who cares what car it was that you picked up?

You (or more specifically the guy at Stuttgart) are incorrect. Your car runs Bosch ME7.x of which I have some experience and I can confirm that the statement....

"the ECU would automatically enrich the mixture to prevent engine knock"

...is 100% incorrect. The ECU changes the spark advance for knock control & changes the AFR for exhaust temp control.


Beekeeper

The sensor is for NOx control - not fuel economy. Higher humidity, better trade off between NOx & peak cylinder pressure (spark advance).

BTW generally in a throttled engine EGR often increases FE.

XXXLR8R

The water molecules/vapour/steam will pass through the combustion system and out of the exhaust pipe. Along with all the lovely oxidized Hydrocarbons (some of which will also be water).


Beekeeper

Almost right but this is not for Knock control, an engine has knock sensors for closed loop knock control. As above humidity sensors are for NOx control.

maxc

humidity does not, in anyway, help initiation of combustion. it slows combustion down, which then reduces peak cylinder pressure, detonation and thus - guess what - REDUCES NOx !!! It also allows more spark advance for the above reasons thus improving BSFC.

dcasto

the water injectoin moves you away from the knock limit of the trade offs of aircharge/spark advance/AFR allowing you to increase aircharge/increase the spark/reduce the afr - or play with all 3 depending on your target.

Anymore for anymore.....

MS

I was working a Cessna 182 once, and on a brutally hot,100 degree, 90% humidity day, the mag drop on both low time magnetos at 1700 RPM was 120 rpm. Just barely marginal rpm wise, but smooth as silk. Very slight leaning would improve the drop to 90 or so. The following weekend, a little front had come thru, dropping the temperature & humidity, and the full rich mag drop was 65/side. When you ground one magneto, your are in effect retarding the ignition.RPM down, MAP up slightly. The fact that high humidity exacerbated the mag drop certainly agrees with the previous poster's observations.

My AFR knows nothing about water, it sees mass of air only and is tweeked by the O2 sensor.  Injecting water cools the inlet and the mass sensor detects more mass and adds more fuel, but not correctly because the temperature compensation sensor doesn't see the cooler air.  

The AFR reads the knock detector, throttle position, temperatures and changes the spark advance.  If I put 88 Octane in, it slowly advances the timing and will eventually light the check engine along with make the then engine a little rich.

If I add NO in a dry system the same thing, the air cools, more mass goes in, fortunately, the NO carries fuel and Oxidizer so I would run a little lean (bad thing).

I could not add water to compensate for 88 octane, stll need 91 or more.

dcasto

What do you mean?

Your post makes no sense at all.

MS

NO = Nitrous Oxide

water injection adds cooling effect and yes moves away from the knock point, but not because its water but because it is colder.  

dcasto

I have no idea what your mother tongue is - but your post is very confusing. It is like looking trying to read the board of a game of 'Automotive Scrabble'.......

Complete nonsense - and a country mile away from the OP.

MS

humity does not change the  engines propencity to knock.  Adding liquid water to an intake cools the air off, the lower temperature of the air and fuel means that the temperture rise during the compression stroke is not high enough to cause pre-ignite or knock.

The only effect humity has on the whole process is that the water vapor takes up room for oxygen and therfore you should adjust the amount of fuel downward as the humity goes up.

dcasto

I am afraid that you are wrong on that one, old boy...

Take a peek at the extract shown in the link below, would remind you that Taylor is seen in equal standing to Heywood when it comes to all things engine........

http://books.google.com/books?id=mX1-OJBQ6ngC&pg=PA70&dq=humidity+detonation&sig=pjFpEvVaSkf0Pl8iNM34wbI51EM

Hope that clears the issue up for you.

MS

Charles is a fine chap - as are you, my good man, for linking to that excellent tome!

Si Vis Pacem Para Bellum

I'll back down, but the use of weaks words "increasing atmosphereic humity tends to reduce detonation.." which implies a weak connection or one that is not significantly measureable.  The author isn't sure how it works.

In the water alchol paragraph, its the temperature lowering qualities of the mixture, not a humity related connection.

He's merely employing the mother tongue to issue a nuance... obviously he had to much to say for just one volume as it was.

I have no reservations about Charles' ability to fathom the mysteries of the matter....

Si Vis Pacem Para Bellum

I use only distilled water. It's completely vaporized before I inject it in the intake. No evaporated cooling in intake. Some more details later.
Here's some basic combustion science.

From: Robert Harris <bob@bobthecomputerguy.com>
To: DIY_EFI@lists.diy-efi.org
Subject: Water and its effect on combustion.
Date: Mon, 10 Jul 2000 10:24:08 -0700
Message-ID: <9ptjms0uu4oe292mpk6a6vhm2hn8bu9h1j@4ax.com>

Let us take a quick look at ignition. Those who have a Heywood can look it up
- mines on loan so going by memory. The first thing that happens is a plasma
cloud is formed by the arc consisting of super heated electron stripped atoms.
When this cloud "explodes" a ball of high energy particles is shot outward.

The highest energy particles are the hydrogen atoms - and they penetrate the
charge about 5 times as far as the rest of the particles. As they lose energy
and return to normal temps - about 5000 k - they begin to react chemically
with any surrounding fuel and oxygen particles. The effectiveness of spark
ignition is directly related to the availability of free hydrogen. Molecules
containing tightly bound hydrogen such as methanol, nitromethane, and methane
are far more difficult to ignite than those with less bonds.

During combustion - water - H2O ( present and formed ) is extremely active in
the oxidation of the hydrocarbon. The predominate reaction is the following:

OH + H ==> H2O
H2O + O ==> H2O2
H2O2 ==> OH + OH
Loop to top and repeat.

The OH radical is the most effective at stripping hydrogen from the HC
molecule in most ranges of combustion temperature.

Another predominate process is the HOO radical. It is more active at lower
temperatures and is competitive with the H2O2 at higher temps.

OO + H ==> HOO
HOO + H ==> H2O2
H2O2 ==> OH + OH

This mechanism is very active at both stripping hydrogen from the HC and for
getting O2 into usable combustion reactions.

Next consider the combustion of CO. Virtually no C ==> CO2. Its a two step
process. C+O ==> CO. CO virtually drops out of early mid combustion as the O
H reactions are significantly faster and effectively compete for the available
oxygen.

Then consider that pure CO and pure O2 burns very slowly if at all. Virtually
the only mechanism to complete the oxidization ( Glassman - Combustion Third
Edition ) of CO ==> CO2 is the "water method".

CO + OH ==> CO2 + H
H + OH ==> H20
H2O + O ==> H2O2
H2O2 ==> OH + OH
goto to top and repeat.

This simple reaction accounts for 99% + of the conversion of CO to CO2. It is
important in that fully two thirds of the energy of carbon combustion is
released in the CO ==> CO2 process and that this process occurs slow and late
in the combustion of the fuel. Excess water can and does speed this
conversion - by actively entering into the conversion process thru the above
mechanism.

The peak flame temperature is determined by three factors alone - the energy
present and released, the total atomic mass, and the atomic ratio - commonly
called CHON for Carbon, Hydrogen, Oxygen, and Nitrogen. The chemical
reactions in combustion leading to peak temperature are supremely indifferent
to pressure. The temperatures and rates of normal IC combustion are
sufficient to cause most of the fuel and water present to be dissociated and
enter into the flame.

As can be seen above, water is most definitily not only not inert but is a
very active and important player in the combustion of hydrocarbon fuel.
Ricardo and others have documented that under certain conditions ( normally
supercharged ) water can replace fuel up to about 50% and develop the same
power output, or that the power output can be increased by up to 50% addition
of water. This conditions were investigated by NACA and others for piston
aircraft engines. It is important to note that these improvements came at the
upper end of the power range where sufficient fuel and air was available to
have an excess of energy that could not be converted to usable pressure in a
timely manner.

As a side note - Volvo recently released some SAE papers documenting the use
of cooled EGR to both reduce detonation and return to a stoic mixture under
boost in the 15 psi range - while maintaining approximately the same power
output. Notice - they reduced fuel and still get the same power output.

When you consider that EGR consists primarily of nitrogen, CO2, and water ( to
the tune of about two gallons formed from each gallon of fuel burned ), you
might draw the conclusion that it also was not "inert". They peaked their
tests at about 18% cooled EGR - which would work out to about 36% water
injection and got about the same results under similar conditions that the
early NACA research got.

quote" The effectiveness of spark
ignition is directly related to the availability of free hydrogen". Actually it's directly related to atomization.

humidity chart. http://www.aquamist.co.uk/rescr/faq/airmass/humid.GIF

Standard burn speed info. http://members.cox.net/dnaquin/burn%20rate.jpg I don't think this was posted here before.

Nice but oldish. I asume that experiments were conducted with carburator engines. Not to many corrections avaliabble.

I think:

higher humidity => less oxigen => rich mixture => less detonation

spraying wather => less oxigen + colder mixture => rich colder mixture => less detonation

spraying wather + alkohol => less oxigen + colder mixture => even richer colder mixture => less detonation

Some influence of methanol high octane value too.

Just enriching mixture is simplier with computer controled fuel injection. Some smoke and fire at the exhaust are a trade mark of turbo raceres.

I concur mishar.


maxc, you better explain your system to this company http://www.coolingmist.com/

"Ricardo and others have documented that under certain conditions ( normally supercharged ) water can replace fuel up to about 50% and develop the same power output, or that the power output can be increased by up to 50% addition
of water."

If this is true, the worlds energy problem is solved, just put turbocharged cars burning 50% water---right!!!

Humidity variation has an inevitable effect on maximum power in engines that are trying to react every last molecule of oxygen.  There will also be an effect on emissions, if not compensated for humidity.  It may be possible to mitigate one or both of these depending on the particular engine calibration.  
Engines that normally run lean, if they are adjusted for actual humidity, can  be compensated to maintain constant power and emissions, with varying humidity.

Water injection & humidity have similar effects. The available effect/benefit of water or water/methanol injection (compared to humidity), due to the ability to schedule with speed/load, makes power gains possible versus simply compensating for humdidity variation.

bobatbobthecomputerguy.

Sorry, I don't recognise the reference among noted experts on the subject.

I don't believe everything I read on the web unless backed up by known reliable sources, otherwise, I would have a huge penis, thousands of degrees, women clambering all over themselves to get at me, trillions of dollars for helping billionaires extract funds from third world countries.

Your 50% analogy is kinda half true (the most dangerous kind of lie in my opinion). If the engine is run very rich under boost so as to suppress detonation, then you find another way wo avoid detonation such as cooling the charge with water injection, you can remove half the fuel, however if you have an engine running at about stoich, and you add only water and remove half the fuel, you will get either a cut out, a miss fire or destroy the engine from detonation under lean conditions.

I think there is evidence that water dissociating into hydrogen and hydroxyl ions under ignition conditions can influence the nature and speed of the flame, but nowhere near the level you claim.

For instance, in my experience from purely subjective data, there is a small (as in not a huge) difference in engine performance in a normal car when driven in desert areas vs tropical areas when the atmospheric pressure and temperature are similar.

To the best of my knowledge, the laws of conservation of energy still hold true.

Thermal efficiency of an engine can be improved by having a quicker burn rate, thereby allowing later ignition while still reaching maximum pressure at but absolutely not before TDC. You will still have substantial losses to the cooling, and slightly lower losses to the exhaust system EMPHASIS ON SLIGHTLY. 50% is not slightly.

Injecting liquid water rather than vapour has several advantages:-

1) Per unit weight of water it has a lot less volume, and therefore displaces a lot less air.

2) As much of it evaporates in the cylinder during the compression stroke, the latent heat of evaporation cools the charge and suppresses detonation. This only helps if detonation was going to occur without the water.

3) Water evaporating due to heat of combustion creates steam and I believe releases more pressure than it absorbs in the process. A contributor called SBBlue (I think) did the thermo calcs in a previous thread some time ago. A site search with the google feature should find it.

Both vapour and liquid add water molecules to increase the potential for dissociated water molecules to act as an intermediary in the combustion process, however, satisfactory operation in desert areas indicates that not much water is required, or the assistance is minimal.

Regards

eng-tips, by professional engineers for professional engineers
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Nice to see some thinking outa the box for a change.

As I recall,the higher power recips of WWII vintage with pressure carbs that used ADI , (anti detonate injection), also included a "de richement" valve that activated with the ADI, but I don't think it cut anything like 50% of WF.

Pat,

when would the heat of compression - and/or then combustion - initiate the phase change of water into steam?
Even though the high ambient pressure kept the steam "saturated", wouldn't it's potential expansion energy still exist and add to the total available expansion pressure inside the combustion chamber?

Water injection is well-documented to increase the performance of certain Otto or Diesel cycle engines through the mechanisms previously posted, while water injection has been successfully used in jet turbine engines to provide emergency power due to the water molecules' expansion of 1700X it's volume when it flashes to steam in the exhaust cone.

The major difference appears to be the fixed (yes - dynamic - but in a finite sense) volume of the cylinder versus the non-constrained (open to atmospheric) - yet high pressure/high velocity turbine/exhaust nozzle region of a jet engine.

The "free" energy (not in a macro way) contained within the molucular bonds (I assume?) of liquid phase water is quite impressive if you've had the opportunity to observe or participate in a fuel-air explosion using water as the catalyst... and this same mechanism is useful in power production as noted above.   However, how would one calculate - even if roughly - the point of diminishing return?  In all of the above examples, too much water will quench combustion...

Si Vis Pacem Para Bellum

It's already been done to death. As previously suggested, do a site search for yourself.

I'm not doing it again and again every time this subject comes up and people don't do their homework first.

The site search will show up real and creditable data from NACA and calculations from someone who did the thermo calcs to the satisfaction of his peers in that field.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

LOL!

Done to death?

- not in my book...

If I thought a site search would've been sufficient, I'd have executed one.  Wasn't trying to make you do my homework, my good man - just asking for your analysis!
Wisdom requires that we stand on the shoulders of those who've gone before us...

Si Vis Pacem Para Bellum

If you look up the threads in engine & fuel where this guy participated you will find the thermo calcs.

You will find his member profile under the name SBBlue

Did I say done to death. Here are some links.

www.eng-tips.com/faqs.cfm?fid=811

www.eng-tips.com/viewthread.cfm?qid=72284

www.eng-tips.com/viewthread.cfm?qid=116462&amp;page=9

www.eng-tips.com/viewthread.cfm?qid=88490&amp;page=57

www.eng-tips.com/viewthread.cfm?qid=189819&amp;page=6

www.eng-tips.com/viewthread.cfm?qid=178103&amp;page=10

www.eng-tips.com/viewthread.cfm?qid=103672&amp;page=38

www.eng-tips.com/viewthread.cfm?qid=165071&amp;page=18

www.eng-tips.com/viewthread.cfm?qid=30437&amp;page=78

www.eng-tips.com/viewthread.cfm?qid=75887&amp;page=64

www.eng-tips.com/viewthread.cfm?qid=69488&amp;page=25

www.eng-tips.com/viewthread.cfm?qid=44564&amp;page=72

www.eng-tips.com/viewthread.cfm?qid=149981&amp;page=9

www.eng-tips.com/viewthread.cfm?qid=120134&amp;page=39

www.eng-tips.com/viewthread.cfm?qid=76628&amp;page=60

www.eng-tips.com/viewthread.cfm?qid=84413&amp;page=58

www.eng-tips.com/viewthread.cfm?qid=84647&amp;page=58

www.eng-tips.com/viewthread.cfm?qid=82878&amp;page=60

www.eng-tips.com/viewthread.cfm?qid=77013&amp;page=59

www.eng-tips.com/viewthread.cfm?qid=102741&amp;page=48

www.eng-tips.com/viewthread.cfm?qid=80554&amp;page=27

There are more, but this is enough for a while I think













Regards

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Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

I'm just the lowly ChemE and I run industrial engines and my own little engines.  If water could cut my $100,000/fuel bill I'd be on it. If I could tweek 100 HP more out of my 1500 HP spark engines, I'd do, but it's not there.

Thanks Pat....

NOW my cup runneth over...

thanks Pat!

I have yet to find a discussion or reference to injecting water into the pre-turbine exhaust tract of a turbocharged diesel engine, which is my point of interest.
Especially with heavy fueling, I believe this scheme would be useful to accelerate low-RPM turbo spool (improving throttle response and reducing smoke), and provide lower temperature/high TIPs under steady-state high loading of the engine.

Surely this has been done before?

Si Vis Pacem Para Bellum

XXXLR8R,

I don't see how injecting water pre-turbine is going to offer you any benefits.

If its throttle response/smoke reduction you want then you need more boost, more quickly - and VNT turbos are a fairly mature technology which work fairly well.

However, wont cooling the exhaust gas before it reaches the turbocharger rob you of the ability to actually produce this boost?

Besides, surely the turbine wheel would need to be fashioned from a pretty heroic material to cope with the erosion caused by the water droplets and also the thermal shock caused when the water injection begins (unless you propose that it always be running).


MS

i agree with mattsooty

i dont see how removing the exhaust energy before the exhaust gets to the turbine will help in spooling the turbo, which is your ultimate goal is it not?

if you want better spool up you need to match your turbos properly

The only thing I see that could work would be to inject water in the exhaust to make steam.  There would be more pounds per hour going across the turbine and more work avaiable for the compressor.  The thermodynamics say it will work, but the practicality and physical control would be difficult.

Things like, physically you'd condense water in the turbine, water saturated with CO2 and therfore very corrosive. If you over injected the water, you'd hit the turbo with liquid and the thing would self distruct.

Now that this thread has been successfully hijacked and we know exactly what you are getting at, a short shot of Nitrous Oxide in the inlet to compensate for extra fuel sure spools them up quick and removes smoke. A bit expensive to install and run though.

I have never seen your subject discussed before. It is different enough to the heading on this thread to deserve it's own heading.

I still recommend a site search as this site contains an enormous amount of information for those prepared to look, and no, I won't do it AGAIN for you.

Another way to overcome smoke, but not lag is to reduce fuel until boost builds or tie maximum fuel flow to boost.

In practical terms, as others have said, correct matching of components is the best way to reduce lag. Compounding a turbo and a positive displacement crank driven blower is another.

When you start your new thread, it will be best to cut and paste your question as asked here and the appropriate responses, then red flag your questions here and specific responses to it, BUT NOT responses relevant to other discussions. This course might attract the interest of several members with vast experience in the subject matter.

Regards

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Back to the original track (I hope).

I can see that water, evaporating in the intake manifold, LOWERS THE PRESSURE and increases intake air mass, than further evaporating in the cylinder RISES THE PRESSURE expanding to steam. Some smart water.

I think that neither will happen. Evaporating in the intake manifold it lowers temperature but adds volume of the steam, so that total volume and pressure will remain the same. Evaporating in the cylinder it would add pressure but lowers the temperature so pressure stays the same. Only effect, if I am right, is less oxygen in the process, mining less power. Off course if it would knock otherwise it’s good.

Trying to think this trough I was thinking how to simplify the problem in order to establish solid argument point. I came to the question: what will happen to the pressure of closed system if some water evaporates in it?

Some thermo calculations done previously do support the idea that as the steam evaporates you get out more than you put in.

The thermo calcs at the time were outside my sphere of knowledge, but were done by a member with good credibility, and not disputed by others with high levels of credibility.

Regards

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I think I found the link following the links you posted, but unfortunately there is no page there anymore. My thermodynamics is a bit rusty, but I wouldn’t mind polishing it a bit for the occasion.

Regarding credibility, experience teaches me always to save some skepticism, if possible. I’ve had quite a few perpetuum mobile discussions with very credible engineers. That’s so loveable idea. Nobody is immune.