Is the overhead mpg reading really less accurate than a hand calc??

[RedSled]

anyone know what size the tires are "assumed" to be?

 

[Kimber]

wouldn't the OH computer be using the same odometer and fuel level readings that we are when we hand calculate?

 

[RedSled]

not sure on the odometer portion, but the fuel level readings I think are different. I believe the computer uses some type of measurement taken at the actual injection events.

 

[klenger]

The overhead computer does not use the fuel level in the tank in it's computations. If it did, it would go wacko when you filled the tank.

 

[Kimber]

The overhead computer does not use the fuel level in the tank in it's computations. If it did, it would go wacko when you filled the tank.

DOH!!

Your right, I was thinking about the estimated miles till empty feature

 

[MacHaggis]

On the consistency matter . . .



IF you're pumping to as nearly the same level in the tank as you can, and IF we can assume that the temperature of the fuel being pumped doesn't vary by much (by virtue of being insulated underground from variations caused by weather) both the overhead and hand calculations should produce relatively consistent calculations (not to each other, but each relative to itself -- I'll come back to that at the end) , regardless of any temperature change in the fuel after pumping. The overhead, though, has more potential for variance.



Assume all underground fuel is at 60*. Pump 30 gallons that happen to weigh 225# at that temp. Then park the rig and go back to the trailer and sleep. While you're snoozing, old man winter cools the fuel that is now in your tank to 30*. Assume the 3% loss of volume that someone mentioned earlier is the correct factor for getting the density of your now cold fuel. You now have about 29 gallons, but it still weighs 225#. (My physics professor would kill me -- the correct way of stating it is to say that the fuel still has the same mass -- 102. 04 kilograms) All of the diesel molecules are still there, they're just closer together.



Then several things can happen. First, let's look at the overhead. Say your fuel heater at the filter increases the fuel temp back to 60*, just where you pumped it. If the overhead is truly and accurately measuring the VOLUME of fuel delivered, it will accurately calculate miles per GALLON based on volume of fuel pumped. If you have no fuel heater, you will pump the cold fuel to your injectors, but you will be lighter on the go pedal, because it takes less VOLUME of fuel (therefore, less pedal) to supply the same number of diesel molecules to the injectors, but the overhead will see less volume and show higher mpg. Conversely, if your fuel heated up to an ambient 85* in the truck's tank while you're sleeping, you'd have to go heavier on the pedal to get the same mass of fuel into the engine, but, since we don't have fuel chillers on or trucks, you haven't cooled the fuel back down to 60* and the overhead will have sensed more fuel flow and will show lower mpg. All of this assumes a constant temp for the air supplied for combustion, which we know doesn't happen.



Now the manual method. Remembering the assumptions that you are filling to a consistent level with fuel of a generally consistent temperature, you will, from tank to tank, be basing your calculations on fuel of a similar density and an odometer that reads consistently (discounting the miniscule tire tread wear between fill-ups). It will generally be a more accurate indicator of your efficiency because it is based on mass of the fuel rather than volume. Consistent fuel temp at the pump means consistent density of that fuel, so you will be calculating on an average that eliminates any variation of the volume of the fuel.



For all of that I agree with (I can't remember who ) up above, and pay little mind to minor variations from tank to tank. Given the potential for variability in my assumed constants (fill level and temp of pumped fuel), there are too many other variables. Above 50 mph or so, wind resistance becomes a bigger factor than rolling resistance, thus quartering in and out of a 15 mph head or tail wind as the road winds will have an effect that would require more than I can measure and more intricate calculations that I can (or am willing to) do. How much did I use the Jake? Did I hit the stop lights or not? When didi I last check the tire inflation? And on and on . . .



I calc by hand (in my head, actually), and would only be concerned by a significant variation in mpg from my last calc under a similar load.



It would be more accurate if we could figure efficiency on miles per pound (or, more accurately, kilogram) of fuel, but that ain't gonna happen.



Sorry for being so long-winded.



Perfesser Haggis

 

[Gary - K7GLD]

anyone know what size the tires are "assumed" to be?

That spec should be on a factory sticker on the driver side door frame...

 

[ilovetrains]

I am guessing that the DTE and MPG readings are calculated differently. By that I mean the MPG is taken by determingin fuel flow by looking at injector pulse over time. Injector pulse being a constant in terms of volume. However DTE must be taken by looking at the float position in the tank and using the MPG reading.



My point is that when my truck hits 1/4 tank I will typically set the OH display to DTE. However, the readings become wildly unpredicatable (I apparently burned 84 miles of fuel in about 20 this morning) yet the MPG seem to stay constant.



And for what it is worth, the OH MPG reading seams fairly accurate.

 

[RedSled]

MacHaggis,



good post. I had to read it twice to make sure I understood everything, I think I have it .



first assumption is that fuel comes into our trucks at the same temp 60. If that is so I would be ok with the fact that you can refill your the tank within a half gallon or so pretty consistantly. I know for a fact that fuel isnt always at 60, that is why fuel distributors compensate for temperature when they sell it. For some reason there is no regulation that the pumps you an I buy from have a temperature compensation.



I acutally filled up today with this in mind and I must say my tank was "full to the rim" for almost 2 gallons. I could fill it up until it was showing at the neck, then wait two minutes and it would go down. after 10 minutes of this I guess it was full, but honestly i didn't wait 2 more minutes to make sure. I added 2 gallons after it was "full to the rim" the first time



second point you made was that the sensor in our truck is based on volume and not mass. I am not sure that is the case. do you know is it a mass sensor or not? Intuitively it would not make sense it is a volume sensor base on the points you make. If the sensor was volume based it would perform erratically above and below 60 degrees. the mass of the fuel is what the engine needs to know not the volume. I will research this more



many gasssers have a mass flow sensor that responds to the amount of a fluid usually a gas flowing through a chamber containing the sensor. It is intended to be insensitive to the density of the fluid.

 

[Gary - K7GLD]

"second point you made was that the sensor in our truck is based on volume and not mass. I am not sure that is the case. do you know is it a mass sensor or not? Intuitively it would not make sense it is a volume sensor base on the points you make. If the sensor was volume based it would perform erratically above and below 60 degrees. the mass of the fuel is what the engine needs to know not the volume. I will research this more



many gasssers have a mass flow sensor that responds to the amount of a fluid usually a gas flowing through a chamber containing the sensor. It is intended to be insensitive to the density of the fluid. "



I would suspect the onboard MPG computer relies upon known and controlled VOLUME of flow metered to each cylinder, as applied by the injection pump on 2nd generation trucks, and the injector fueling impulses on the 3rd generation trucks.



From that fuel volume info, combined with distance traveled, the computer can determine MPG.



Mass fuel flow sensors can be expensive - and as far as I know, the only mass sensors used on our trucks are mass airflow, not fuel flow...

 

[RedSled]

what impact would the high pressure common rail have on the whole equation.

 

[rbattelle]

Strictly speaking, I'm not aware of any such thing as a "true" mass flow sensor. "Mass flow" sensors actually measure the velocity of the fluid passing through a fixed volume. This allows you to calculate the volume flow rate, defined as flow=velocity*area. To get mass flow rate you'd have to multiply by the density. Unless density is being measured, it must be assumed. If a temperature and pressure sensor is available, density can be calculated for a gas (air). I'm not sure if gassers with MAF sensors actually do so for air.



3rd gen trucks do not have a MAF sensor. Do we have a fuel temp sensor? I don't think we do, which would mean fuel volume flow through the injector would need to be calculated using an assumed density. I'd bet that the very small volumes encountered at the injector make the constant density assumption quite acceptable. [Actually, I'm not sure how you'd calculate fuel density even if you had a temperature value... ]



-Ryan

 

[MacHaggis]

Red Sled,



I'm with Gary and suspect that any metering done to feed the overhead is volume-based, which would still be independent of fuel density.



As for volume variation due to temperature, you're right, it may seem a little unfair not to compensate the station but not you and me at the pump. Only thought there is that there's a heck of a lot more difference in the potential for variance between 10,000 gallons and 35 or 40, but, if they're compensating to 60* and we get it at roughly that, the difference is even less. Any discrepancy will certainly be less than the cost of installing technology at the pump to pass the compensation along. i. e, what we'd have to cpompensate for the cost of fancy pumps would be more than we lose or gain in the discrepancy (even with the effect of double wall tanks that somebody mentioned).



I wrote the long piece just to clear up some confusion that was creeping into the thread. More to the point is my belief that, going down the road, there are just too many variables on a single vehicle or between individual drivers and their routes to try to resolve reasons for differences of 0. 5 mpg between vehicles or between tanks on one vehicle. We calcualte mileage to watch for deviations, not minor variances, that would indicate problems.

I'll just

 

[RedSled]

isnt there law of thermodynamics which is way outa my league, but isnt there a relationship between pressure, temp, mass and volume, that says if you know two (pressure and volume) you know the other two (mass and temp)



I may have butchered this, but chem. is not my thing

 

[MacHaggis]

You're gettin' it!



First my apologies. If I may edit the last post, especially the last paragraph. I (not we) calculate mileage with an eye to significant deviations from the norm.



As for your question, yes. It's called Boyle's law. You can pick a temperature and pressure for a given amount (mass) ofany substance -- say diesel fuel. Add the third factor of the volume, and any variation in any of the three will result in a change in the others.



Do a Google on Boyle's Law -- it will likely explain it more clearly than I can, but it's the very concept that makes a diesel fire. Fill a cylinder (on the intake stroke) with air. When you reduce the volume of the cylinder without changing the amount of air in the cylinder (compression stroke) , the temperature and pressure will increase. In the case of the diesel, it increases enough to ignite the fuel that is introduced at that point. When the fuel is ignited, we then get a further increase in temperature, and thus pressure, which in an expandable environment like an engine cylinder, results in increased volume (the "lower case" power stroke).



Now if we can just find a way to tap the exhaust stroke (a mere diplacement event, dammit) for propulsion. But then, I've grown used to having one of four strokes to make the Jake work.

 

[RedSled]

I am tired, but doesnt that remove the potential error with the overhead knowing volume not mass??



the pressure of the fuel is fixed (common rail) and we can measure the volume. so we acually know the mass of the fuel thanks to Mr Boyle ( probably should be Dr).



Why did I start this topic?

 

[MacHaggis]

Why did I start this topic?

It's a good topic! And you're spot on as it relates to Boyle. Though Boyle was dealing with gasses, the law holds with liquids. Because they're harder to compress, you see a larger change in temps than in volume with liquids under pressure (That's why hydraulic systems heat up like they do. ).



The common rail system has a fixed average volume (accounting for the cycling of the pump)and, assuming a constant average pressure, we can derive the mass of fuel in that part of the system. "Derive" is the key word. Because of Boyle, with the volume and pressure being relatively constant, the mass and volume are directly related.



I think we are agreed that the overhead is deriving mpg from measurements taken at some point in the pressurized fuel system. Trick is, to my thinking, that the fuel in that system, by being under some intense pressure, is hotter than the fuel in the tank. If the overhead is metering flow from the common rail system, then variations in readings of mpg from the overhead will suffer less variation from tank to tank if engine performance is constant.



But I've slipped into apples and oranges. The thread started with asking whether the overhead was more or less accurate than a hand calc. If the overhead does indeed measure miles from the relatively consistent rate of rotation of the transmission output shaft, and gallons from the relatively constant volume and pressure environment of the common rail system, then it will be less susceptible to variation than the hand calc.



But you and I are measuring gallons at the pump (where the nasty notion of dollars are introduced) and burning fuel that may heat up or cool off, thus changing volume, after it is pumped.



If the question is, "Is the overhead the more accurate indicator given a constant temperature and pressure?", I would concede that it is, but we (the owners/drivers) have no way of knowing how many gallons that is at those temps and pressures. At the same time, going back to the assumptions of consistent filling levels and temperature of the fuel pumped, and given its subsequent pressurization, etc. , as it flows toward combustion, mileage calculated by hand will fluctuate at pretty much the same rate as the overhead on a tank-to-tank level. They will differ, as others have noted, but the differences (many cite differences of 0. 5 mpg) are consistent with the change in volume that might be attributed to the increased pressurization of the fuel.



Bottom line is you and I are measuring gallons at the pump (manual calc) but have no idea what that translates into for gallons at the temps and pressures at the injectors(overhead calc). My argument is that the numbers may (and will) be different, but that any change in engine performance will reveal itself in pretty much the same proportion by either method. The only variables left are the accuracy of the meter on the pump and the temp/density of the fuel being pumped. Apart from rogue pumps, I believe one method is as accurate as the other for our purposes.

 

[rbattelle]

Though Boyle was dealing with gasses, the law holds with liquids.

I'm not a chemist, but I believe this is false. Boyle's work is predicated on the ideal gas assumption and is therefore not applicable to liquids.



-Ryan

 

[MacHaggis]

I'm not a chemist, but I believe this is false. Boyle's work is predicated on the ideal gas assumption and is therefore not applicable to liquids.



-Ryan

Hmmmm. I'm gonna have to look for a book around here and think about that one. I'm thinking it works for all matter, but the expansion/volume variation is much more pronounced in gasses, making the temp and pressure the big variables in liquids. It could be I meant to talk about a coefficient of expansion, though I'm more accustomed to using that in terms of solids than liquids.



Hmmmm. . .

 

[rbattelle]

Gettin' complex here!!

Hmmmm. I'm gonna have to look for a book around here and think about that one. I'm thinking it works for all matter,

How about this. We can back out Boyle's equation from the ideal gas equation of state:

pV=mRT

for pressure p, volume V, mass m, temperature T, and gas constant R. This equation is derived, in part, from Boyle's law.



First, we rearrange a bit:

pV/T=mR

Then, assuming constant mass, we have only constants on the right hand side. This becomes:

pV/T=C

where C is a constant. If we further assume constant temperature from state 1 to state 2, we get exactly Boyle's Law:

pV=C

This only works for idealized gasses. Liquids, on the other hand, are governed (in part) by the hydrostatic equation, which states:

grad(P)=rho*g

with density rho, gravity g, and pressure P. By "grad" I mean:

grad(P) = dP/dx+dP/dy+dP/dz

... but I think we're already WAY out of scope on this thread to continue! I'm not aware of a pressure/volume/temperature relationship for liquids that's as convenient as the ideal gas law is for gasses... what's known as the "Tds" equations might be the only way to work temperature and volume into the mix for liquids. Again, WAY out of scope!



Please, if I've made a mistake someone correct me.



-Ryan