WAI vs CAI test results

[MetroMPG]

WAI update:

i'm still trying to understand why the test showed no significant improvement. to do that i need to know the role of the IAT sensor in fuel metering or timing on my car.

another guy on the teamswift site sent me scans from the factory service manual for this car & year, and IAT is not even mentioned in the sections on fuel metering and ignition timing. so the mystery deepens.

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another guy on that forum set up WAI on his older geo (like jangeo's). he reported a 6.5% decrease in hwy mpg in 2 or 3 fill-ups - though he's doing tank-to-tank comparisons, so i'm not relying on his claims as experimentally valid.

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all in all, it didn't work and i've spent a lot of time trying to figure out why with no luck yet. i keep intending to abandon this tack, cool off, and wait for warmer weather when i can do aero mods to the car which are NOT "possible" improvements, but actual improvements.

[JanGeo]

Ok maybe the reason it didn't work is because the air gets to that temperature by the time it reaches the cylinder anyway. You don't have a low temperature plastic intake manifold right? my xB has a plastic intake so the air charge stays colder. Now if it was really cold outside then the wai would probably help but you are heating the air into the cylinder which makes it harder to compress and reduces the thermal expansion ratio between pre and post combustion. BTY the 1994 Geo gets its air from the right fender - have to watch deep puddles. If you had fuel vaporization issues then the warmer air would help but you probably are running acetone right? You need an air temp sensor right at the valve and see if you are really heating the air significantly.

[rh77]

I think you have to understand the dyanmics of your ECU. I know for a fact that the warmer the incoming air into my engine, the leaner it runs -- I can post charts and graphs straight from the OBD-2 port if necessary.

On this tank of gas, I bottomed-out on a visit to a friend's house -- it always happens when I visit there -- the curb is at a steep incline into the driveway -- basically it ripped the WAI in two pieces as usual, and I started sucking-in cold night air (30-40 degrees, which translated to 60-degree air at the temp sensor, and the fuel trim was closer to zero). Completely killed my mileage on the first half of the tank. The next day, I re-routed the intake duct through a tight angle, directly into the airbox. Air temps were up, fuel trim went negative (lean), and mileage increased. Honda/Acura fuel maps like hot air to lean the mix. I can't vouch for other ECUs, though...

RH77

[MetroMPG]

Originally Posted by JanGeo

Ok maybe the reason it didn't work is because the air gets to that temperature by the time it reaches the cylinder anyway.

i'm not so sure about that. try telling the millions of tuner car owners with CAI's bolted on that it makes no difference...

i'm pretty sure the air is moving too quickly through the intake tract to change temperature very much between the inlet and the intake valve.

you are heating the air into the cylinder which makes it harder to compress

actually, warmer air is easier to compress because it's less dense. expanding air on the other hand is harder to compress, but that's not what we're dealing with here.

Now if it was really cold outside then the wai would probably help

that's something i've considered. depending on the ECU, the WAI may help most below a certain temp threshold. the day i tested it, it was pretty mild out (39F).

<a href="http://www.metrompg.com/posts/wai-test.htm"><img src="http://metrompg.com/posts/photos/WAI-illust-s.jpg" align="right"></a>

and rh77, you're right: the key here is i need to know the role of my IAT sensor / ECU in all this. i need the factory service manual.

and i need to get a proper datalogger.

nevertheless, i've posted a page on my site with a full summary of my WAI experiment to date, with pics, for anyone interested: http://www.metrompg.com/posts/wai-test.htm

[rh77]

Originally Posted by MetroMPG

and i need to get a proper datalogger.

nevertheless, i've posted a page on my site with a full summary of my WAI experiment to date, with pics, for anyone interested: http://www.metrompg.com/posts/wai-test.htm

[/quote]

By the way, excellent detail on the subject of warm air at your site, I completely agree with your assumptions. But yes, I also agree -- the datalogger is an invaluable tool in using real data to analyze modifications. Due to the high integrity of your Scientific testing, I think you would definitely benefit from one (and for data nerds like yours truly, it's fun to gather data about your car). If you have a laptop, some freeware is available to run real-time logging -- all you need is a cable that plugs directly into your OBD port and PC (usually serial-to-USB). Otherwise, a flash-ROM chip can be used (which is what I have -- it stores up to 6 selected variables [2-mandatory: speed and if G-Forces on accel and stop have exceeded a preset limit] -- this is downloaded into the supplied program for charting on a PC. In addition, it pulls and clears Check-Engine codes).

RH77

[JanGeo]

It would depend upon the operating power level as too how long the air was in the intake manifold - low power levels would mean smaller slower air flow and more time for it to heat up. Also leaner fuel trim may be happening with warmer air to adjust to the fact that there are less molecules of oxygen in less dense air. Also too warm air means less density so to maintain the same power output you have to let more air in thus running at a lower intake vacuum higher volume which automatically would inject more fuel at a leaner trim net result is the SAME amount of fuel burned to get the same power out. REMEMBER it tries to burn the gas cleanly with the correct air fuel ratio. What water injection does it add some h2o which when heated by the ignition of the fuel makes steam which expands 1600 time adding more pressure to the cylinder. I used to run it in my 65 Rambler flathead 6 to break up the carbon deposits in the head squish zone to keep the pistons from hitting them. Helped with the knocking with the high compression I was running too.

Only way to get more mpg out is to improve combustion efficiency/completeness, reduce friction, reduce pumping losses, reduce drag.

[SVOboy]

Also leaner fuel trim may be happening with warmer air to adjust to the fact that there are less molecules of oxygen in less dense air.

I thought this was the hole point, and the fact that you are killing your HP a little bit.

[JanGeo]

But killing your HP a little means you would go slower but you tep on the gas more and burn more fuel and allow more of the warmer air into the engine to get the required HP to go the speed you want and the net savings is ZERO! Unless you effect combustion efficency you don't gain anything and could reduce efficency if timing is retarded due to excessive air temps. Developing more MAX HP requires getting more air and fuel into the cylinder and cold air, cold fuel and a less restrictive air filter is the way to do that. Getting the engine to operate more efficently at light loads requires changing valve timing to reduce intake stroke loading on the piston and maximizing power stroke duration and exhost stroke loading as well as back pressure exhost pulse tuning. Think about the forces on the piston drawing air into the cylinder under vacuum, the compression stroke, the power stroke, and pushing the exhost gasses out after combustion. The ideal engine could be a mixed internal combustion - Sterling cycle configuration.

[joke]
How do you top a car . . . you tep on the brake.

[MetroMPG]

Originally Posted by JanGeo

But killing your HP a little means you would go slower but you tep on the gas more and burn more fuel and allow more of the warmer air into the engine to get the required HP to go the speed you want and the net savings is ZERO!

and what about the argument that hot/less dense air reduces throttling losses? by stepping on the gas more, you're actually INcreasing the efficiency of the engine at a given RPM:

"The air is less dense, so you get less horsepower at the same throttle opening, thus, you have to open the throttle wider to let in more air and get the horsepower that you need. That increases the efficiency because one of the primary causes of the well-known part load inefficiency of gasoline engines is the throttle loss." - (url=http://lists.osourcery.com/pipermail/bricks/2004-October/008698.html]source[/url])

heating the air is analagous to reducing the displacement of the engine, forcing it to run at a wider, more efficient throttle opening (at light loads).

Getting the engine to operate more efficently at light loads requires changing valve timing to reduce intake stroke loading

doesn't reducing the density of the intake charge also effectively reduce intake stroke loading (a.k.a. energy required to pump air)?

[rh77]

I discovered a site that discusses air temperature and density with relation to fuel. Basically it is a lenghty discussion of Gasoline, but it did mention a few interesting points.

Taken from the site:

7.6 What is the effect of temperature and load?

Increasing the engine temperature, particularly the air-fuel charge
temperature, increases the tendency to knock. The Sensitivity of a fuel can
indicate how it is affected by charge temperature variations. Increasing
load increases both the engine temperature, and the end-gas pressure, thus
the likelihood of knock increases as load increases. Increasing the water
jacket temperature from 71C to 82C, increases the (R+M)/2 ONR by two [111].

Since I've increased the intake air temperature and water jacket temp, the vehicle may be compensating with timing adjustments to prevent knock (I have noted that at full-throttle, high-RPM runs, a slight knock is noted). I may have to increase the Octane rating to achieve optimal economy (my datalogger shows timing advance as a rapidly variable series of spikes, so I can't diagnose with that respect).

An increase in ambient air temperature of 5.6C increases the octane
requirement of an engine by 0.44 - 0.54 MON [27,38].

Hmm, more evidence to suggest that air temp effects power/consumption and, I guess, Octane -- which needs to be considered more thoroughly now.

Basically the site goes on to discuss air density (altitude) and temperature. Your answers lies here:

7.11 What is the effect of altitude?

The effect of increasing altitude may be nonlinear, with one study reporting
a decrease of the octane requirement of 1.4 RON/300m from sea level to 1800m
and 2.5 RON/300m from 1800m to 3600m [27]. Other studies report the octane
number requirement decreased by 1.0 - 1.9 RON/300m without specifying
altitude [38]. Modern engine management systems can accommodate this
adjustment, and in some recent studies, the octane number requirement was
reduced by 0.2 - 0.5 (R+M)/2 per 300m increase in altitude.
The larger reduction on older engines was due to:-
- reduced air density provides lower combustion temperature and pressure.
- fuel is metered according to air volume, consequently as density decreases
the stoichiometry moves to rich, with a lower octane number requirement.
- manifold vacuum controlled spark advance, and reduced manifold vacuum
results in less spark advance.

There you have it -- basic physics (as noted with older engines). Fuel consumption was based on air volume and temperature. Higher density and temperature = less fuel required for Stoichiometric fuel enrichment. So, with new engine management systems, the oxygen sensor is going to account for this and pump more air with a leaner mixture, equalling less fuel consumption.

So, yes, I have noticed that it takes more throttle input to get up to speed as compared with the CAI -- but it has trained me to use the decrease in power as a forced "slow-down". From a standing start, I accelerate to about 3000 RPM and lift my foot to get the car to shift to a higher gear, and use whatever pedal remains to get up to speed, more slowly, without pushing so far down as to downshift -- this is repeated until top gear with T/C lockup occurs. Generally with this procedure, the torque-converter locks up sooner, and acceleration commences easier with the added torque, despite the RPMS being in the 2000 RPM range, with is low for my little DOHC 1.8L, 4-cyl.

I hope this helps -- at least I now realize that I might have to go up a grade in octane to optimize efficiency with the added heat, especially this summer when humidity increases.

On a side note, water injection is noted to actually increase fuel consumption as it cools the charge and results in more fuel required to achieve Stoich.

RH77