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Fuel Economy Calculations


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#1 Bob_Sheaves

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Posted November 30, 2007 at 05:13 pm

(Note: Fixed 2 typos)

Assuming an example of a Chrysler 413-3 truck engine spec of:

Fuel consumption, brake specific: .45pounds of gasoline/ horsepower/ hour of run time
Running time: 1 hour at constant speed
Max hp developed at most effecient RPM: 65 road load horsepower at 2175 engine RPM and 65mph road speed

You will burn:

.45*65=29.25 pounds of fuel per hour

...or:
29.25/6.39 (pounds of fuel per gallon)=4.577 gallons of fuel per hour

...and get:
65/4.577=14.20 miles per gallon

Now, of that 65hp road load, your fuel burn rate of 29.25 pounds of fuel per hour means you are releasing (125,000 BTU/ pound of gasoline or 36,633.883771528 Watts of energy - this figure assumes 91RON at 60 degrees F):

4.577*125,000= 572,125 BTU/hour or 167,561.11 Watts of energy

....while you are only developing 65hp or 48,470.49 Watts of energy. This means you are wasting:

167561.11-48,470.49=119,090.62 Watts of energy or, said another way:

119090.62/167561.11=.7107*100 (to get percent)=71.01 percent of the fuel is wasted through engine friction losses, heat rejection to the cooling system, and exhaust heat.

Edited by Bob_Sheaves, December 1, 2007 at 01:49 am.


#2 Bob Lincoln

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Posted November 30, 2007 at 09:51 pm

That's exactly in line with everything we've heard about how only 30% of the energy content of fossil fuels actually is used to propel the vehicle. Thanks for showing us the math behind it. This is really useful for planning optimum engine size and power for a given application (which we all wish we could customize for our own cars).

#3 Gibons

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Posted November 30, 2007 at 10:17 pm

How efficient would a fuel cell be by comparison?

#4 namvet67a1f

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Posted December 1, 2007 at 12:26 am

Assuming an example of a Chrysler 413-3 truck engine spec of:


Max hp developed at most effecient RPM: 65 road load horsepower at 2175 engine RPM and 65mph road speed



And this motor would be in which vehicle ?

#5 Bob_Sheaves

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Posted December 1, 2007 at 01:58 am

How efficient would a fuel cell be by comparison?


It depends on the type of fuel cell you mean-I assume you mean the "hydrogen fuel cell" that has been touted for the past couple of years, as opposed to something like the old Allis Chalmers hydrogen peroxide and methanol fuel cells used in the early space program.

Experimental H2 fueled PEM cells in development vary from a low of around 20% to high of about 50%. See:

http://en.wikipedia.org/wiki/Fuel_cell

http://www.un.org/esa/sustdev/csd/csd14/lc...n/hydrogen4.pdf

http://inventors.about.com/library/invento...lfuelcells2.htm

And this motor would be in which vehicle ?



It was just an example-I was only calculating the engine fuel losses. The road load HP number comes from SAE J688 calculations. Just because an engine revs 2150 RPM, the road load hp the engine puts out will vary with configuration. Road load HP is defined as the amount of hp required to move a given weight along a certain road surface, on certain tires, with a certain aerodynamic effeciency, up a certain grade. See: http://eogld.ecomm.g...t...n&year=2004 for specifics- it is too long to go into here. Ask any question about something you may not understand and I will try to help to the best of my ability.

Edited by Bob_Sheaves, December 1, 2007 at 04:01 am.


#6 Bob Lincoln

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Posted December 1, 2007 at 08:34 am

Right. For instance, I've heard that a typical number for a car the size and shape of my Daytona might be 12-15 road HP required to maintain a highway speed. The car is rated 100 HP at 4000 RPM. I travel at 2600 RPM, and of course, only need a fraction of the rating under most cruising conditions.

It can also be used to demonstrate the hideous wastefulness of the average commuter who drives an empty SUV at 80 mph with an engine rated for 260-300 HP and never really needs it, and pays the fuel penalty. (But since we're a credit society, they just willingly ring up the debt.)

#7 Bob_Sheaves

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Posted December 1, 2007 at 11:33 am

You number is in the ballpark. I have not run the J688 calcs on a Daytona, but your number is going to be close. One sidebar note- a turbocharger or supercharger only alters this consumption slightly (depending on volumetric effeciency) by something less than 3 percent either way. The power gains or losses from each will seem to increase or decrease the gross fuel economy, but this is due to the apparent hp increase at lower RPM capability. Don't use it (meaning do not increase the road load hp) and you will not see a decrease in economy with ridiculous power increases-not because turbos or superchargers are somehow magic...hehehe.

ADDENDUM: One thing to remember with any charge air increase...if you USE that "new" hp, your fuel economy will go DOWN generally as EACH HP burns at least that .45 pound of fuel (to use this example). Gasoline engines generally also have a wider variance in the fuel burned from the most effecient point to the worst. In the case of the old RB used here, that range was .45 to .59 in the old Max Wedge engines, IIRC. By comparison, the Cummins B5.9 used in the Dodge pickups burns from .36 #/hp/hr to .41 #/hp/hr at worst. The effeciency point is the rpm where the torque developed crosses the HP curve, usually somewhere near (within a couple hundred RPM) of the torque peak.

Edited by Bob_Sheaves, December 1, 2007 at 12:53 pm.


#8 Bob Lincoln

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Posted December 1, 2007 at 02:15 pm

Bob, do you know of any online graphs of HP and torque vs RPM for the 2.2L or 2.5L TBI engines, and for the Magnum 3.9L V-6? Just curious as to how far into the power band I am, the way I normally drive.

#9 Bob_Sheaves

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Posted December 1, 2007 at 05:42 pm

Bob, do you know of any online graphs of HP and torque vs RPM for the 2.2L or 2.5L TBI engines, and for the Magnum 3.9L V-6? Just curious as to how far into the power band I am, the way I normally drive.


None that I know of, as the engines are too old for the industrial group's info to be available (the 2.2 and 2.5 were available from the industrial group when they were in production). However, it is not a complicated process to generate your own curves. All that is required is a fuel flowmeter (about 250.00 for a good one), a long, flat road (at least 5 miles), and time. What you do is install the instrumentation, and drive at a set rpm in the range you would want to measure. A chassis dyno can also be used, as well as an engine dyno, but these tests require stabilized flow to be accurate and statistically significant.

Sidebar- this is one of the reasions I laugh at the hp claims from the aftermarket-they do not use stabilized hp as a valid criteria, and as such, have no idea how to really run a curve accurately.

The SAE procedure will take you anywhere from a day (8 hours) to a week (8 hours a day) to run. Given your interest, I would recommend you do it yourself as you are smart enough and meticulous enough to get good and valid data.

#10 Bob Lincoln

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Posted December 2, 2007 at 10:32 am

Thanks, Bob, but I don't have the time or the money to take it to that level. I'm being pulled in all directions - executor of my dad's estate, husband, father to 2-yr old girl, mechanic for 3 family cars, lots of woodworking projects (kitchen remodel, toy box, play room) and house addition planned, genealogy project administrator, running a side business designing and hosting web sites - it's overwhelming at times.

My modest project now is adding preheated air to my Dakota's V-6. With the advent of the Magnum, Chrysler removed this feature. I think it could still use it, as it does chug a little when cold, and gets far better gas mileage when warm out. During the winter I get 18-19 mpg. During summer on long trips I can attain 22 mpg, and even hit 25 mpg coming back from Carlisle last year. If you look at the fact that 10% of my miles are cold operation when commuting, and a Carlisle trip means only 1% are cold miles, it's obvious how important it is to get the engine to warm up faster and atomize fuel better.

I could not find a heat stove at a junkyard, so I'll be fashioning my own from sheet metal, and securing it to the manifold with jam nuts over two studs. I bought a 318 air cleaner and cut the snorkel off, then cobbled it onto my existing air cleaner, so I have two snorkels. I have to bring over the vacuum temperature switch, build the heat stove and hook up a vacuum source, then run a fresh air intake hose from grille to snorkel.

It was pointed out to me that the original snorkel has no preheater, so it's mixed hot air. But if this works out, I'll cannibalize another flapper valve assembly onto the original snorkel, build a heat stove for that side and have dual preheaters.

Next spring and summer will be spent trying to find time to fix the body panel rust on my Daytona, now that I have the frame members and floorpan repaired.

Edited by Bob Lincoln, December 2, 2007 at 10:34 am.


#11 mpgmike

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Posted December 11, 2007 at 11:27 am

Bob, let us know your results.

Some food for thought, at any given efficiency rating, you have certain fixed losses. You will only be driving one camshaft, one oil pump, the same number of pistons, the same tranny, etc, etc. If you increase the engine's actual efficiency rate by a mere 1%, you may be able to realize as much as a 5% increase in mileage. Increase the engine's efficiency rate by 5% and you may see a realistic 25% or more at the pump. The losses stay the same.

Mike

#12 Bob Lincoln

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Posted December 11, 2007 at 03:11 pm

I finished installing the homemade preheater on the Dakota. It works well. Will post pictures. I took it for a 4-mile test drive while cold. I don't really notice any difference in power, but it does feel much more smooth - about as smooth as warm operation, whereas before there was just the slightest chug.

This weekend I'm leaving for FL, so I'll get a chance to evaluate better its impact on driveability, and also on fuel economy. I have a previous winter trip to compare it to, and about the same load in the truck. Round trip will have more than 10 tankfuls, so I'll be able to average any differences and see how I do. The only thing I haven't done yet is install a fresh air hose from grill to the new snorkel; so it will suck underhood air once the flapper opens. But the original snorkel is drawing cold air, so the effect will be mitigated.

I was also able to rebuild/replace the rotted preheater system on my Daytona, so now it runs better while cold, too.

Edited by Bob Lincoln, December 11, 2007 at 03:12 pm.


#13 dusty_duster

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Posted December 12, 2007 at 11:27 am

You number is in the ballpark. I have not run the J688 calcs on a Daytona, but your number is going to be close. One sidebar note- a turbocharger or supercharger only alters this consumption slightly (depending on volumetric effeciency) by something less than 3 percent either way. The power gains or losses from each will seem to increase or decrease the gross fuel economy, but this is due to the apparent hp increase at lower RPM capability. Don't use it (meaning do not increase the road load hp) and you will not see a decrease in economy with ridiculous power increases-not because turbos or superchargers are somehow magic...hehehe.

ADDENDUM: One thing to remember with any charge air increase...if you USE that "new" hp, your fuel economy will go DOWN generally as EACH HP burns at least that .45 pound of fuel (to use this example). Gasoline engines generally also have a wider variance in the fuel burned from the most effecient point to the worst. In the case of the old RB used here, that range was .45 to .59 in the old Max Wedge engines, IIRC. By comparison, the Cummins B5.9 used in the Dodge pickups burns from .36 #/hp/hr to .41 #/hp/hr at worst. The effeciency point is the rpm where the torque developed crosses the HP curve, usually somewhere near (within a couple hundred RPM) of the torque peak.


Bob-

I thought I heard a while ago that turbochargers slightly increase the efficiency of the engine while under boost, because the boost pressure better atomizes the fuel and promotes more of the fuel being burned. Is this true?

#14 Bob_Sheaves

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Posted December 12, 2007 at 12:07 pm

Bob-

I thought I heard a while ago that turbochargers slightly increase the efficiency of the engine while under boost, because the boost pressure better atomizes the fuel and promotes more of the fuel being burned. Is this true?

A turbocharger decreases the flow effeciency of the exhaust port due to flow restriction, but the expanding exhaust gas due to the heat of the exhaust provides the effort (just like a jet turbine engine) to compress more air into the cylinder than it can naturally draw itself, allowing more fuel to be burned, but the additional air itself will not cause more of the existing fuel charge to be burned (that is why there is a number of pounds per hp hour of fuel an engine consumes-it makes the turbos, etc. irrelevent, and focuses on how much fuel is consumed for each hp generated, not how it is generated).

Now, if you keep your foot in the throttle, causing the turbo to spool up, and add additional fuel, you are going to keep (within the 3% limits spoken of earlier) the fuel burn rate the same, but the overall fuel consumption overall will go up, solely due to more hp being generated at a given RPM.

To the second part-atomization- that is purly a function of the size of the fuel droplets and heat of vaporization.

The upshot to your question is, no the statement is not true (within the parameters I just mentioned)

#15 dusty_duster

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Posted December 12, 2007 at 02:36 pm

Why is it, then, that you can get slightly better fuel economy by turbocharging an engine (at low boost levels, of course)?

Also, does the efficiency of the engine increase if you're able to run more "lean"?

#16 Bob Lincoln

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Posted December 12, 2007 at 02:39 pm

Why is it, then, that you can get slightly better fuel economy by turbocharging an engine (at low boost levels, of course)?

Wow. I never read that. In fact, every EPA number I've seen shows a few mpg worse for a turbo version of an engine.

#17 Bob_Sheaves

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Posted December 12, 2007 at 03:15 pm

Why is it, then, that you can get slightly better fuel economy by turbocharging an engine (at low boost levels, of course)?

Also, does the efficiency of the engine increase if you're able to run more "lean"?

You don't "always" get better. Again, you are looking at plus or minus 3%. The level of boost is not important, but the "adapting" of the turbo (actually, optimizing the engine for the turbocharger-called "normalizing" for an aircraft engine) will allow the same (+/-3%) amount of fuel to be burned to it's theoretical maximum effeciency. If you do a bad job of matching the turbo and ALL it's parts to a specific engine, you MAY see even worse effeciency-outside the +/-3% range. This is why most of the aftermarket kits are a joke-and the OEM "kits" are so dang expensive-the tuning and integration into a base engine. Mismatch and you can have a hand grenade, and that is not even at high boost-take a look at the Corvair Spyders and Olds F85's that were turbocharged in the early 1960's from GM and all the work that went into the 2.2/2.5 CHryslers. The difference in components, sizing, timing, strength, etc. is like night and day.

In normalizing an engine, you are turning all the parameters to operate at peak effecieny from sealevel up through (generally) 10,000 feet ASL. You are not tuning for maximum power, but rather, ensuring you have a constant torque and hp level through a greater range of ambient conditions.

ADDENDUM: The slant 6 that Mike Holler works on is a good example of an old, very ineffecient design that slightly exceeds the +/-3% rule of thumb. A 4.0L Jeep engine comes nowhere near the same level at the end, because it is a more effecient engine to start with. The USER end result is similar however-and is the reason the 4.0 was never turbocharged from the factory (there was not enough gain for the cost involved). The 225 was a different story however. I would bet (this is PERSONAL OPINION ONLY) that, had the Dakota not had such a short front engine compartment, a 225 turbo WOULD have appeared instead of the 3.9L v6.

Edited by Bob_Sheaves, December 12, 2007 at 03:15 pm.


#18 Bob Lincoln

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Posted December 12, 2007 at 03:48 pm

Dare to dream - Bob, build me one with a 5-speed manual and I'll buy it. :P I really miss my slant-6.

#19 mpgmike

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Posted December 18, 2007 at 09:54 am

I did 2 /6 turbos, both A-bodies. The first was a '70 Duster that went from about 18 to about 22 mpg. The second was a '73 Dart that had about the same increase. This would be normal commuting without the heavy turbo whine drowning out the radio.

Parasitic losses factor into the eqation. Even though exhaust is restricted with the addition of the turbo, you can cruise at 65 mph with a lower vacuum reading because the turbo is helping to force the charge into the engine. The net difference between the exhaust restriction at low flow levels associated with cruise, and the decreased pumping losses due to the addition of the turbo yields somewhat better fuel economy. With the /6 conversions, the turbo was intentionally sized small. The /6 screams silly murder when your tach approaches 4k. Without modification, it is not a high winding engine. The undersized turbo is well suited for the engine.

An engine that uses a larger turbo for the nice efficient high rev range probably won't see any benefit from the added turbo.

Mike

#20 Bob_Sheaves

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Posted December 18, 2007 at 03:26 pm

I did 2 /6 turbos, both A-bodies. The first was a '70 Duster that went from about 18 to about 22 mpg. The second was a '73 Dart that had about the same increase. This would be normal commuting without the heavy turbo whine drowning out the radio.

Parasitic losses factor into the eqation. Even though exhaust is restricted with the addition of the turbo, you can cruise at 65 mph with a lower vacuum reading because the turbo is helping to force the charge into the engine. The net difference between the exhaust restriction at low flow levels associated with cruise, and the decreased pumping losses due to the addition of the turbo yields somewhat better fuel economy. With the /6 conversions, the turbo was intentionally sized small. The /6 screams silly murder when your tach approaches 4k. Without modification, it is not a high winding engine. The undersized turbo is well suited for the engine.

An engine that uses a larger turbo for the nice efficient high rev range probably won't see any benefit from the added turbo.

Mike


Parasitic losses do NOT enter into the original calculation as I was discussing only the engine fuel consumption during operation. They are integral with the BSFC (brake specific fuel consumption) number, and would change with the overall volumetric effeciency. The fuel burned would not change. Your increase observed comes from the engine power and turque curve shifting due to the turbo. The actual amount of fuel burned BY THE ENGINE does not change, except as the BSFC changes.

Road load hp (which is a function of torque over time) determines the overall fuel economy, which, by solely adding a turbo, does not change a statistically significant amount. In your case, the apparent increase in economy comes from the shift of both the torque and hp curves in the engine's rpm range (which would be shown by a proper stabilized hp dyno analysis), giving an apparent "decrease" of the load applied to the engine by the road and static factors. In other words, for the same road speed, the engine is developing a higher torque at the same RPM, without burning any more fuel, but the load is such that only the original torque and hp are required, so the engine burns less fuel, or perhaps more clearly, an engine that spins "X" rpm, is only going to make the power needed to stabilize the fuel burn, NOT the maximum the oxygen will allow to be burned. This is demonstratable by a dyno test of constant, steady engine RPM, and applying a variable load to the engine. In this case, the fuel consumption will change with the loading demand of "Y" hp x the BSFC.

This is why the BSFC is used for fuel economy calculations, not some arbitrary vehicle speed.


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