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Way back in Dec, Stephen Bentley suggested taping over the front bumper air intake to improve MPG during cold weather. I took his advice and have gathered some pretty convincing, detailed data:



Clearly there is a strong correlation between average temperature and MPG. The average temp is the average of the AVG daily temp reported by the NWS for all of the preceeding days before the fill up. The MPG is the odometer mileage added since the last fill up divided by the gas dispensed as reported by the pump. My average fill up is 6.5 gallons (typically 1/2 full on guess gauge). The circled point had 3 days of no tape and 6 days with the tape on, so it sort of belongs to both series. Next month I'll be removing the tape for the coming warm weather, so there will be more tapeless data soon.

I used the heater/defroster as necessary. I didn't run the AC at any point. You probably shouldn't run the AC with the intake taped. In my case, I didn't tape over the inlet tube on the far driver's side - this appears to
bring in the fresh air for the MD temp display. My MD temp display reads
fine with the tape over the bumper grill. Perhaps it's different for '01 vehicles, but this is my experience.

One more bonus: I took my car to the car wash the other day, and borrowed a piece of the tape to cover the battery vent temporarily.
That sure was handy!

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Re: Restricted Air ... more info

Here is a photo of the inlet part that I didn't cover. I think this may be why my MD temperature display reads normal; there is also a small region behind the front license plate that I didn't tape because I was too lazy to remove the plate :)


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Do keep in mind though that even though this idea may work to improve economy, I suspect you are actually increasing emissions by disturbing what Toyota designed to be very clean. I would worry about that possibility..

sorry for the downer.

steve d.
nys '02 super white - 12,000 miles.

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There is no basis to believe that the emissions are increased.
Restricting the cold air flow across the radiator/engine does nothing
different than what a hot day with no restriction does.

What really changes is the average air temperature under the hood,
and possibly a miniscule air pressure change. The Prius is designed to operate properly on a 110 degree day with no flow restriction which will have even higher average engine compartment temperatures than a 45 degree day with the restriction of the tape.

The THS design is capable of handling very large variabilities in the ambient environment - hot air, cold air, moist air, dry air, high altitude,
low altitude, variations in gasoline, air-condition on, air-condition off, cold engine, warm engine, etc. Throughout these variations it maintains low emissions. I am doing nothing that will fool any sensors - the heat transfer due to air flow is a very strong function of velocity (think wind), humidity and air density. If the heat transfer is critical (it isn't, component temperature is) then the prius needs to measure each of these components. I have created an effect which is extremely similar to having a very specific wind speed and direction.

Further the tape should only be present over a restricted set of environmental possibilities: NO air conditioning use, No high ambient temperatures.

There is no downer here.

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Interesting, But makes me very nervous. If I did such a thing. I just can't imagine what that does to the warranty. One thing I know about these all aluminum engines...heat kills, and anything that supresses the ability for the engine to rid itself of heat is plain scary to me. Also since there is no temp guage, how would one know how close to the max temp the engine is at, just short of the hi-temp light going on.

Doug
 
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Interesting, But makes me very nervous. If I did such a thing. I just can't imagine what that does to the warranty. One thing I know about these all aluminum engines...heat kills, and anything that supresses the ability for the engine to rid itself of heat is plain scary to me. Also since there is no temp guage, how would one know how close to the max temp the engine is at, just short of the hi-temp light going on.

Doug
 
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Harry,
I think it's a great idea, and thanks for sharing your results with us. As a native Minnesotan I have discovered over the years that some vehicles need to be slightly modified. Semis sometimes have grille covers with zippers. All of my diesel rabbits needed a sheet of cardboard in front of the radiator during the winter months. Without cardboard to snug up the engine compartment you could actually watch the temperature gauge fall while sitting at a traffic light. Here in the chilly North we lose 4 to 8 mpg in our Prius because of the cold. It would be nice to get a little back...

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Re: Restricted Air ... more info

Harry,

Thanks for posting this data. This sounds like a good idea for improving winter fuel economy. I tend to agree with you. The engine compartment is not an air tight environment. Freezing cold outside air knows how to find its way into even very tiny cracks and will keep the engine from overheating. And I would not call the cracks between the engine compartment and the outside world tiny.

The temperature under the hood with tape over the front on a typical 30 degree high temperature winter day will be much lower than it will be without tape on a typical 95 degree high temperature summer day. And the Prius is designed to run just fine on 95 degree summer days.

Engines run better when they are up to proper operating temperature. An engine that constantly runs cold wastes fuel. You get incomplete combustion and the spark plugs will foul if they are not hot enough. Too bad the Prius does not have an engine temperature gauge. I bet we would all be surprised just how cold the Prius engine runs when winter comes and weeks go by without ever getting above that freezing mark. Especially when the mercury gets mean and decides to spend some time in the negative numbers (we're talking fahrenheit). :)

As far as the comment about not using the AC ... I usually don't turn on the AC until the days start warming up above 80 degrees. I don't really see why AC is an issue. If it is warm enough outside to run the AC that means you should have already removed the tape weeks ago.

Regards,

Ed (Thinking about this for next winter in Idaho and enjoying pleasant spring weather while it lasts.)
 
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Ok, I've updated the graph - sadly, the message text remains
the same so now it's misleading. Some of the new no-tape
data did have the air conditioner running, but not much of it.

I've started a new series for 10W30 oil which sadly was put
into my car (against my orders) at the first service.

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>MPG w/<AirFlow: Only in Winter?

Hi Harry,
Delicious work.
On the mpg increase with the bumper air inlet blocked.

1. Jeff can edit your caption so that it accurately reflects the new data. I messed up an image file and he fixed it in 5 minutes! Charming host. Just send him the updated caption.

2. Are not you curious whether the tape would continue to increase the mileage in balmier weather? There is of yet no hint of crossover or degradation. Overheating in a traffic jam is not something that the Prius would normally do, so this should not be a worry. Who knows, maybe the tape would help people in Florida too. So much of our world is based on continuity with the past even though it is archaic. Maybe this is the case with the Prius cooling system, i.e. oversized for its new task. (Does anyone know if the Prius cooling system is common to the other lines in the Toyota line or if it was engineered specifically for the hybrid situation?)

3. This also raises the elemental question of what exactly is overheating and why is it bad? Maybe the Prius ecu can control for the greater ignition tendency of gas in a hot engine and get better efficiencies at most any temp. As for the high-temp thermal character of the battery (it is in the trunk afterall and am guessing that the battery coolant strategy is actually a winter heating strategy--does anyone know if the battery coolant is used for cooling?)???

4. Pondering further. Why do we have a water-cooled ice here anyway? Would not an air-cooled engine have been simpler, cheaper, and better?

5. So many questions. So little data.

cheers, nathan
MY02 Wakahiru-Me
 
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Discussion Starter #11
MPG Accuracy (was Re: Restricted Air Flow ... )

Harry,
As we all know, measuring m.p.g. using pump readings is subject to large errors. For what it is worth, here's how I measure m.p.g.
I have a spreadsheet on which I record the gas I put in, the odometer reading and the m.p.g. reading on the consumption display. I reset the average on every fill-up. As well as the m.p.g. = miles divided by gas method, the spreadsheet calculates how much gas the consumption display thinks was used by the car as miles divided by displayed m.p.g. The ratio of this to the actual gas gives me an error which I average over all tanks. The error average in my case is setting down to about 4.5%. In other words, the consumption display thinks I've put 4.5% less gas in the car than I actually have. So, I simply correct the displayed m.p.g. by this factor and use that as the m.p.g. for the tank. Now, this assumes that the displayed m.p.g. has a fairly constant error, but I think this is more defensible than assuming you achieve the same fullness of the tank at each fill-up. Just an idea. I also record miles-per-day.

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Re: MPG Accuracy (was Re: Restricted Air Flow ... )

>As we all know, measuring m.p.g. using pump readings is subject to large
>errors.

I don't reset my mpg gauge every fill-up. I do it about every 1500 miles.
The first three times there was excellent agreement between pump
and MD mpg values (less than 0.5 mpg error), this last time the MD
was reading about 4 mpg higher than pump mpg.

I don't think the errors are all that large since I typically fill up at 1/2
tank level. In any event, it only creates scatter in the data, as does
varying wind, trip length etc. Using the graph, you can do an
eyeball fit to the data which is pretty convincing, even with the
scatter.

If the 10W30 turns out to be as bad as it's looking, I may have to
change the oil early; it makes we wonder how much of the hybrid
civic's mileage can be attributed to its use of 0W30 oil. For that
matter, I may consider 0W30 oil for the Prius.

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Re: MPG Accuracy (was Re: Restricted Air Flow ... )

Funny, my first 2 or 3 tanks were also spot-on with the gauge, but then over the winter pump measurement started running lower than the MD's measurement. I thought it might relate to winter, but even with warmer weather and improving mpg, the MD MPG is staying lower than measured with the gas pump. Sounds similar to your situation, Harry.

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