MFD accuracy over long distances (many tanks)

<div class='quotetop'>QUOTE(JimboJones @ Oct 19 2007, 01:03 PM) [snapback]527801[/snapback]</div>

I was trying to keep the explanation simple enough for the less technically inclined. Obviously I should have aimed a little lower.

Tom

 

<div class='quotetop'>QUOTE(qbee42 @ Oct 19 2007, 04:02 PM) [snapback]527896[/snapback]</div>

giggle

 

<div class='quotetop'>QUOTE(Presto @ Oct 19 2007, 03:59 PM) [snapback]527871[/snapback]</div>

Presto, if you've been following the thread so far, you'll note that other posters have been making assumptions about fuel metering and fuel indications in the Prius, based on what hey think they know from other cars. I'm trying to find out which is what.

Do you know if the Prius has a fuel metering system or not? If you have any technical contributions to the discussion, feel free to post what you know.

<div class='quotetop'>QUOTE(qbee42 @ Oct 19 2007, 05:02 PM) [snapback]527896[/snapback]</div>

Sorry Tom, but you've been caught out.

You've first indicated there is a fuel flow meter in the Prius, then you turn around and say the Prius does not have nor need a fuel flow meter.

It's surprising that though you implied you're an engineer who designs fuel flow meters, it's obvious you don't know what a fuel flow meter is, the difference between fuel flow metering and fuel injection, and you're just trying to cover up your mistake.

In any case, why take offense? Just admit it and let's move on.

 

The definition of a fuel flow meter is a device which measures the flow going through a calibrated orifice or passage. Tom said that the MFD totals fuel going through the injectors. Now it may not seem like it to you, but "totaling fuel" can easily mean the same as measuring it (after all, how do you total fuel if you don't know how much of it there is?). Therefore he has implied a fuel flow RATE meter for the Prius. If he did not intend that meaning, a simple "no" will have cleared it up.

He is wrong in implying there is a fuel flow meter in the Prius. If he did not mean this, then say so.

You're doing the very same thing right now. Read the posts again.

Who said anything about a trap?

Who said anything about him making assumptions?

And his explanation is contrary to his previous assertions, with respect to fuel flow meter and fuel flow indicating.

Show me where I was rude and insulting to Tom, prior to his being so first.

I'm not afraid nor ashamed of being shown to be wrong, as long as the explanation is technically credible. What about you, efusco?

Unfortunately, your lectures are inappropriate in this thread. You have a flawed understanding of the exchange that took place, and your perceptions of the intents and underlying motivations of the statements made between Tom and I are incorrect. You're entitled to your observations, but don't let your personal animosity towards me cloud your observations of other exchanges.

Fine, it's "clear to you". That really doesn't mean clear to everyone else. Not to engineers whose words and terminologies need to be precise and unambiguous. Oversimplification in this case resulted in Tom being wrong. No big deal, he's not all of a sudden a leper in the community.

This doesn't make sense, and isn't what Tom wrote. Yes, the fuel is metered by the fuel pump in the Prius - "metering" as you used here means the distribution of fuel to the injectors. The fuel flow being totaled is a different concept, which has to do with fuel flow RATE.

Sure the injector cycle use for fuel flow rate determination may well be very accurate, sure it may be that fuel flow metering is unnecessary in the Prius because of it - great. What I'm after is the appropriate technical definition and description of the fuel system component, which can only help everyone's understanding.

efusco, this thread of exchanges is not a big deal, people can be shown to be wrong or right: I'm happy to be either, but what baffles me is the hostility and personal stake that people like you have in responding to my posts. Do you really only want posts from people that agree with you and everyone else?

 

<div class='quotetop'>QUOTE(qbee42 @ Oct 18 2007, 09:17 PM) [snapback]527574[/snapback]</div>

This would seem to be the case, more or less. I know the first mile it won't show anything or be quite a bit off, but after a mile it's pretty close. (I thought the calculated mileage reset itself if more than 3 gallons were put in, but I haven't noticed it in my experience, and from the comments here it seems not to be the case...maybe that was in the original Prius?).

It would surprise me if the Prius has a suspension system and weight measuring system for the tank and bladder and use that for the calculated mileage. It would need to be quite accurate, especially since it starts producing reasonable results in a mile or so, when 1/50th of a gallon has been consumed (about 2.5 ounces). We all know how accurate the gas gauge (guess gauge) is, so if it could weigh the tank to the ounce, that should be really accurate as well. But Jimbo might be correct that a fluid flow meter might not be used, and I think Evan did get something backwards in his post about who said what (although he's right in that Jimbo is showing more frustration than might be warranted).

I'm thinking the computer gets its number by extrapolation - it must know how much gas goes thru the injectors per millisecond (or whatever time unit, but that seems likely), based on multiple test runs at the factory. It obviously knows how many milliseconds it keeps the injectors open for each pulse since the computer controls it based on accelerator position and other variables. So to me (as an embedded SW engineer) the easiest answer is just to approximate gasoline used based on those two numbers. No additional equipment needed - no sensitive weight-measuring device that averages out road bumps or an uneven load, not even a physical fluid flow meter. And yes, it won't be the most accurate method available, but it's essentially free for the company, so call it good enough and ship it. Only the real nerds will even notice or care about the difference.

 

Tom was right 100% when he said this: -

Jonesy, you are dead wrong, you couldn't be more wrong in this post: -

In fact, the only really accurate way of calculating mpg is through the use of a fuel flow meter as you alluded - unfortunately, these are pretty expensive (I know, I looked into it).[/quote]
You are making stuff up as you go along Jonsey!

The rate fuel flows through the injectors is a fixed and know figure because the pressure across the injectors is a fixed value and the size of the injector nozzle is a fixed value. The only variable is the amount of time the injector spends open. Because the computer that displays the consumption figure knows how long the injectors spend opened it also knows how much fuel can get through the injectors in that time. By using the amount of fuel and dividing that by the distance the vehicle covers on the road the computer can calculate the fuel consumption.

Yes this is over simple just so Jonesy can follow it.

 

Hehe.. somehow, I think we've seen the last of "Jimbo", for awhile, after getting spanked in a couple forums. Looks like he's lurking, for now.

 

I didn't read "who said what" in this whole thread, but I may have something to offer here -- after reconstituting about 20,000 miles, my 2005 MFD reads about 4% high, or about +1.9 MPG.

 

<div class='quotetop'>QUOTE(Presto @ Oct 26 2007, 08:44 PM) [snapback]531003[/snapback]</div>

He made a cameo encore appearance posting as puppeteer.

 

Just filled my 50th tank and MFD has been high by average of 1.22 mpg.
I am impressed that accurate.
MFD average is 65.52 and actual is 64.30

 

Can View

Where can I buy a Can View unit?