Gas Tank

4315 Views 9 Replies 0 Participants Last post by Guest, Mar 16, 2002

Discussion Starter · Feb 27, 2002

I have found quite a difference in the amount of gas that the bladder will take based on temp, when I first got my car the temp's were in the 60's and 70's and when my fuel light started blinking I am able to get about 11 gallons into the tank, at my last fillup last week with the temp reading 8 on the screen I was only able to get in 7.2 gallons. With a gas tank size this variable it makes it difficult to get a reliable MPG reading based on calculations at the pump.

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Discussion Starter · #2 · Feb 27, 2002

Jeff you must be reading my mind. I was about to post a question about the benifits of the rubber gas tank. So here goes.
What are the benifits of a rubber gas bladder? I understand that bladder can contract as your car uses gas up thus saving gas vapor from escaping. The amount of vapor in a 11 gallon tank maybe comes from a few drops of gas (my perception, someone correct me if I'm wrong). Isn't this a shot gun aimed at a fly? I assume traditional cars just have a vent in the gas cap thus letting gas vapor escape 24 hours, 7 days a week. Wouldn't a metal tank and a gas cap with a one way valve be better? Just letting air in as gas is pumped out to the motor. The trade off being at fill up. You would release some gas vapor but then no more guess gauge, no more bladder contraction problems.

Any comments?

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Discussion Starter · #3 · Feb 27, 2002

The benefit of the resin bladder in the gas tank is that it is one of the mechanisms that allowed the Prius to meet the stringent California SULEV standards.

The amount of gasoline vapors that would escape from an untreated metal tank during fillup is quite significant and used to contribute greatly to the smog problems in California. Most cars now sold in CA (those meeting LEV or better standards), draw the gas vapors into a charcoal cannister where they are reconverted into liquid gasoline.

The Prius engine has a low intake manifold vacuum and doesn't run all the time. It was not able to operate the charcoal cannister. In the words of the great "Priusman," Toyota opted for the resin bladder because unlike other cars, the Prius doesn't suck enough.

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Discussion Starter · #4 · Feb 27, 2002

After calculating MPG manually for the first few tankfulls, and determining that the computer only seemed off by 1 mpg I decided that was good enough for me. I don't actually keep track of my gas mileage anymore. When people ask I refer them to John1701A's site <http://john1701a.com>
Besides great pictures John has a wealth of long term Prius mileage data.

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Discussion Starter · #5 · Feb 27, 2002

I also found this to be true with my tank, when the temp remained constant between fillups the screen reflected the calculatated MPG from the pump. Once the temp started to fluctuate the MPG figures from the pump were all over the place.

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Discussion Starter · #6 · Mar 1, 2002

I ordered my 2002 Prius on June 14, 2001 and took delivery on Dec 14th. My first two fill-ups on 12/22 and 1/10 were with mid-grade 89 octane. After reading messages I have since switched to regular 87 octane, and car seems to perform a lot better and milage has increased. I have re-filled with 87 on Jan 27, Feb 9, and Feb 16. For the 2,120 miles I have averaged 43.6744 miles per gallon. On two occasions when it was cold, the pump shut off at around 9 gallons, and I was down to the last bar. By waiting a minute or two and filling slowly I was able to get in 11.117 gallons. I really had to baby the nozzle to get it full but since I had over 500 miles on that tank I knew it couldn't be full at 9 gallons. Also, I have been very carefull to park at the same pump every time and also make sure the car is level.

regards,
retired

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Discussion Starter · #7 · Mar 1, 2002

There was at least one report on the Yahoo group of someone who "babied" in the last few gallons during cold weather and ended up having gasoline spew back out of his tank when he removed the pump nozzle. So you might not want to do this. Since you succeeded this time, I'm wondering if there's a check valve to prevent gasoline from spewing out, but that it failed in this other case.

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Discussion Starter · #8 · Mar 12, 2002

If I remember correctly, this incident occurred when fuel was forced into the bladder by holding the filler nozzle tight against the fuel inlet and letting the pump continue to run, so that the bladder was overinflated. When the nozzle was removed, the surplus fuel gushed back out in a spectacular manner. (Good thing nobody was smoking nearby)!

When I want to be sure that I have completely filled the tank, I withdraw the filler nozzle a little so that it is not tightly sealed against the fuel inlet. I pump fast until the nozzle clicks off, then slowly until it clicks off again, then very slowly until it clicks off for the last time. This method seems to produce reasonably repeatable fill-ups for MPG calculation purposes, with no risk of overinflating the bladder.

Regards,
Julian.

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Discussion Starter · #9 · Mar 12, 2002

> The amount of vapor in a 11 gallon tank maybe
> comes from a few drops of gas (my perception,
> someone correct me if I'm wrong).

I'm pretty sure the ratio is higher than that. From memory it's something like 1% (or, to put it another way, roughly 0.5 MPG improvement if the loss of vapour can be prevented).

Regards,
Julian.

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Discussion Starter · #10 · Mar 16, 2002

I think there is another benefit: With less (no) air getting sucked into the tank, you would be less likely to get waterlogged when you drive a lot on one of those warmish humid winter days (where you suck humid air into the tank on a conventional car) and then a cold snap (which condenses some of the humidity into water that sinks to the bottom of the tank.

My wife had this happen with her Corolla a few weeks ago, and I had to lend her Lux, which made me think about this Prius advantage.

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