Intake Air Humidity 4

[beekeeper6]

I've read recently about newer engines that will include a humidity sensor so the ECU knows the humidity of the intake air. Now I understand how the humidity level of intake air affects combustion, but don't knock sensors and other existing technology already essentially cover this ground?

 

[patprimmer]

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

 

[Lou Scannon]

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.

 

[wwest]

"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.

 

[mattsooty]

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

 

[patprimmer]

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

 

[wwest]

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...??

 

[beekeeper6]

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

 

[sreid]

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

 

[XXXLR8R]

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

 

[ivymike]

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?

 

[dcasto]

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.

 

[beekeeper6]

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.

 

[maxc]

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

 

[dcasto]

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.

 

[patprimmer]

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

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

 

[mattsooty]

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

 

[thruthefence]

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.

 

[dcasto]

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.

 

[mattsooty]

dcasto

What do you mean?

Your post makes no sense at all.

MS