Mileage calcs

[Bob Valentine]

At 09:43 AM 6/18/2001 -0400, you wrote:
>That's very interesting, hadn't heard that.  So, high vacuum in the intake
>means less resistance to the fuel spray so more comes out. 

Yep.   

> HOWEVER, the regulator at high vacuum reduces the fuel pressure so less
> comes out, therefore compensating???  

Yep. 

>So, rating 22lb/hr injectors always at 22lb/hr is the right way???  

Yes, since the vac referenced FPR handles the effects of intake vacuum. 

-> Bob Valentine
-> bob at tecmark.com

>Steve
>
>-----Original Message-----
>From: Bob Valentine [mailto:bob at tecmark.com]
>Sent: Friday, June 15, 2001 10:13 PM
>To: gmecm at diy-efi.org
>Subject: RE: Mileage calcs
>
>
>Isn't the vacuum referenced FPR to account for the fact that the business
>end of the injector is exposed to intake vacuum, which would affect the
>flow rating if not compensated for?  That's my understanding as to why TBI
>units don't have a vacuum referenced FPR.
>
>-> Bob Valentine
>-> bob at tecmark.com
>
>At 12:13 PM 6/11/2001 -0400, you wrote:
>>Don't forget about vacuum controlled fuel pressure regulators.  One
>>typically sets pressure for WOT (engine off of course) but on the road fuel
>>pressures are ~5psi less.  Makes a >.5 change in my AFR calcs.
>>
>>Steve
>>
>>-----Original Message-----
>>From: David Papworth [mailto:papworth at ichips.intel.com]
>>Sent: Monday, June 11, 2001 11:10 AM
>>To: gmecm at diy-efi.org
>>Subject: Re: Mileage calcs
>>
>>
>>Gasoline is a mixture of a number of hydrocarbons without rigidly
>>fixed properties. One source gives an "as delivered" density range
>>from 640 to 780 kg/m(3), with a mean of 730.
>>
>>There are 2.2 pounds per kilogram and 264 gallons in a cubic meter.
>>So that gives us 5.3 to 6.5 pounds per gallon, with a mean of 6.08.
>>The same source:
>>http://www.users.qwest.net/~taaaz/AZgas.html
>>points out that retail gasoline pumps are only accurate to +/- 0.5 percent
>>and that the density of gasoline drops by 2% at 90 degrees vs. a
>>"standard delivery temperature" of 60 degrees.
>>
>>As a check, I know that 100LL aviation gasoline was historically figured at
>>5.83 lbs per gallon, with most flight instructors urging their students
>> to "use 6 lbs per gallon" when doing a weight-and-balance calculation.
>>
>> The density of auto gas has crept up over the last 6 years or so, as the
>>more volatile
>>components have been removed to help in the reduction of urban VOC. Ethanol
>>comes in at 789 kg/m(3) -- 6.6 lbs per gallon -- so 10% gasohol fuel would
>>have
>>a mean density of 6.13 lbs per gallon. MTBE, on the other hand, is 740
>>kg/m(3);
>>close enough to the mean of gasoline to have a negligible effect on
>density.
>>
>>So a figure of 6.0 to 6.1 lbs per gallon seems like a reasonable starting
>>point. It looks to me like getting +/- 0.5 "mile per gallon" accuracy
>>out of a calculation based on weight is about as good as it ever will get,
>>given the variable density, temperature issues, and gas pump inaccuracy.
>>
>>When you think about it, volume is simply not an accurate measure
>>for a chemical like gasoline. Weight would be much better --
>>and more reflective of the value (energy content)
>>of what you are buying.
>>
>>This brings up the question of the accuracy of taking the ECM's idea of
>>how much fuel is being used as the input to an MPG totalizer. The
>>ECM cannot know about evaporative losses not recovered
>>by the carbon cannister, or recovered during open-loop operation.
>>
>>If the 9th injector is used in your setup, the ECM does not know how much
>>fuel is used for starting. These two factors would tend to make an MPG
>>calculation read higher than reality.
>>
>>Open loop operation is accurate only to the extent that the fuel pump
>>pressure is accurate and constant. As the injectors wear, they probably
>>flow a little more at a given pulse width, so open loop operation gradually
>>enrichens over time -- the ECM doesn't really "know" how much fuel
>>the engine is getting without feedback. Again, your calculation will come
>>out too high.
>>
>>I would think you could get very accurate instantaneous MPG readouts
>>while the ECM is in closed-loop mode, assuming you take the BLM
>>counts into account. Just integrating the grams/second number off the MAF
>>and assuming stoichiometric operation might do it. But a long-term
>>total miles per gallon calculation, including the effects of starting,
>>open-loop,
>>and evaporative losses seems more challenging.
>>
>>It's an interesting project -- what kind of accuracy are you actually
>>seeing?
>>
>>
>>
>>----- Original Message -----
>>From: "Gabe" <gellett at earthlink.net>
>>To: <gmecm at diy-efi.org>
>>Sent: Sunday, June 10, 2001 3:10 PM
>>Subject: Mileage calcs
>>
>>
>>> Doing some MPG calcs..  Going to play around with calculating MPG from
>the
>>> output of my 165 ECM...  Need to know what the weight of a typical gallon
>>of
>>> pump gasoline is (or a lb/hr to GPH conversion), and any other tips
>anyone
>>> has...
>>>
>>> So far I've found 6.1 lbs/gal on the web - anyone have a more precise
>>> measurement or come up with different numbers?  My calcs with this factor
>>seem a
>>> bit liberal...
>>>
>>> Thanks
>>> Gabe
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