The MPG of an NGV

For three months now I’ve been cruising around Oklahoma City in the official CNGnow Tahoe. It’s an awesome ride – cool, comfortable, big enough to haul stuff around in and cheap to fill up. It’s wrapped in graphics that show off some of the benefits of compressed natural gas (CNG) and, for anyone I share the road with, there is no doubt that my truck is powered by natural gas.



It’s not uncommon for me to be approached about the virtues of CNG. At stop lights, people have me roll down my window and in parking lots, they box me in. It’s an interesting pattern, but I’m always happy to chat natural gas.

What fascinates me the most is the number one question I’m asked: “what kind of gas mileage do you get?” It’s an interesting question because CNG is a gas and therefore there’s no such thing as a gallon of it. That’s hard to explain while screaming out a car window.

Though the question is slightly flawed, the answer is fundamentally easy. Compressed natural gas is measured in gasoline gallon equivalents (GGE), which means exactly what it sounds like. One GGE of natural gas produces an equal amount of energy as a gallon of gasoline. Therefore it gets the exact same gas mileage. However, a GGE of natural gas costs significantly less (I pay $1.39).

For example, the gas mileage for my 2011 CNG Tahoe is the same as its gasoline counterpart: 21 MPG on the highway. However, if I spend $2.90 (the cost of gasoline in Oklahoma) on CNG, I’ll get over two GGEs and be able to travel around 50 miles.

This is true in all cases. So, the next time you’re asked what kind of MPG an NGV gets, you can say: “The same as I would with gasoline, it’s just cheaper to fill up. And cleaner burning, too.”  Or I might just yell out my window, “Visit cngnow.com/blog!”

Questions? Ask them below.
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Comments

Avatar
crawbot
Have you experienced any differences in power between the two fuels?
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chadosko
If you're looking for it, you'll notice a slight loss in horsepower - but you really have to be looking for it.
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FenleyNicol
Chad are there any Dyno numbers as to what its making at the crank or to the wheels?
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Grdnokc
If the CNG injectors and computer are working properly there should NOT be any difference between gasoline and CNG.  If you are using a "fogger" system then you will notice from 12% to 18% loss of power and mpg because the gas is not acurately metered.  Actually there is a gas measurement for CNG.  It's called the gasoline gallon equivalent or gge.  It's value was established by the IRS and it is 126,250 BTUs of CNG equal one gasoline gallon.  The US DOT has a slightly
Avatar
chadosko
Thanks Grd, for hopping in with a more technical answer!
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DerekLebsack
I have 2 bi-fuel CNG conversions, both pickups, and the only major difference I have noticed is a higher tendency to downshift while cruising and encountering a stiff headwind or hill. While I guess this may indicate a loss of horsepower, I notice no difference at all when accelerating, so maybe the downshifting is just a quirk of the transmission control module interacting with the CNG system.
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burnusgas
Expecting my CNG Tahoe any day now.  120 miles on $1.39/gge CNG and all the gasoline is still there if I get in a bind.  The CNG tanks are under the Tahoe where the spare tire was (just like the Tahoe Hybrid). 
Avatar
chadosko
Derek - you're exactly right. That's what I notice also.
Burnusgas - congrats on the new ride!
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DerekLebsack
To clarifiy Grd's point further, the GGE is not actually a measurement of volume of CNG, it's a standard measurement of the energy content of the gas. Natural gas is billed to consumers in decatherms (1,000,000 BTU) based on the average energy content of a volume of natural gas from your utility. This is how GGE is calculated. However, since this is based on "average" specific energy content in your area, you may see significant swings in how many miles you get on a CNG fillup if you happen to get lower energy content gas for whatever reason. This is because your tank has a set volume with no regard for the energy content of the gas, and you may be getting less than a gallon of gasoline equivalent's worth of energy. Energy content of the gas will also vary month-to-month and regionally, as well as daily within a month.
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chadosko
Wow! Thank you, Derek, for digging in and explaining it all in better detail. Truly fascinating stuff here.
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EricRehagen
I have noticed a loss in power in my Bi-fuel 2500hd but mainly while towing. It will actually switch over to gasoline if it held wide open and dose not go above 4000 rpm within a few seconds. I only seen this once while pulling 10k trailer on a long up hill grade. My Bi-fuel Cavalier will downshift while using cruise and a hill where if was running on gasoline it wouldn't. I the engine was ment to run on CNG only the compression ratio should be raised to take advantage of the higher octain.
Avatar
plugger
I had a thought. just for fun.
I understand there is a small loss of power with CNG.
If it's  120 octane could you bump the compression up
to, say,  15 or 18 to 1 & make up for the loss
with out fear of ping
I can't post a new post
I'll try to post as an edit
Kevin K loos like your post got cut off at "un"
I had a 2 part post & part 2 got cut off too.
Exactly what certification do installers need?
Who preps you for what test?
I'm in Florida.
Avatar
KevinKoloff
Actually the BTU of nat gas usually is around 1000-1100 btu. Depending upon the gasoline blend it is still higher than nat gas. If there is a government eq. for IRS or whatever it would still "equate" to EPA on emissions where everything (and I still cannot un
Avatar
BCouch
Thought you all might find this informative if not outright useful!
We are in the process of putting this into a mobile app if anyone is interested in haveing a copy, just let me know!
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Gasoline gallon equivalentFrom Wikipedia, the free encyclopedia

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

Gasoline gallon equivalent (GGE) or gasoline-equivalent gallon (GEG) is the amount of alternative fuel it takes to equal the energy content of one liquid gallon of gasoline. In 1994, the U.S. National Institute of Standards and Technology or NIST defined "gasoline gallon equivalent (GGE) means 5.660 pounds of natural gas."[1]

GGE allows consumers to compare the energy content of competing fuels against a commonly known fuel—gasoline. Compressed natural gas (CNG), for example, is a gas rather than a liquid. It can be measured by its volume in cubic feet (ft³), by its weight in pounds (lb) or by its energy content in joules (J) or British thermal units (BTU) or kilowatt-hours (kW·h). It is difficult to compare the cost of gasoline with other fuels if they are sold in different units. GGE solves this. A GGE of CNG and a GGE of electricity all have the same energy content as one gallon of gasoline. CNG sold at filling stations is priced in dollars per GGE.

One important point that somewhat clouds the practical utility of a GGE for comparing different fuels to each other is that machines which run on them produce usable energy from different fuels at different efficiencies.

GGE - Gasoline Gallon Equivalent (US Gallons)

Tables

Fuel - Liquid, US Gallons GGE GGE % BTU/Gal kWh/Gal GGE Calculated for

Gasoline in US Gallons at 114,000 BTU per Gallon Gasoline (base)[2] 1.0000 100.00% 114,000 33.41
Gasoline (conventional, summer)[2] 0.9960 100.40% 114,500 33.56
Gasoline (conventional, winter)[2] 1.0130 98.72% 112,500 32.97
Gasoline (reformulated gasoline, ethanol)[2] 1.0190 98.14% 111,836 32.78
Gasoline (reformulated gasoline, ETBE)[2] 1.0190 98.14% 111,811 32.77
Gasoline (reformulated gasoline, MTBE)[2] 1.0200 98.04% 111,745 32.75
Gasoline (10% MBTE)[3] 1.0200 98.04% 112,000 32.83
Gasoline (regular unleaded)[4] 1.0000 100.00% 114,100 33.44
Diesel #2[4] 0.8800 113.64% 129,500 37.95
Biodiesel (B100)[4] 0.9600 104.17% 118,300 34.80
Bio Diesel (B20)[4] 0.9000 111.11% 127,250 37.12
Liquid natural gas (LNG)[4] 1.5362 65.10% 75,000 21.75
Liquefied petroleum gas (propane) (LPG)[4] 1.3500 74.04% 84,300 24.75
Methanol fuel (M100)[4] 2.0100 49.75% 56,800 16.62
Ethanol fuel (E100)[4] 1.5000 66.67% 76,100 22.27
Ethanol (E85)[4] 1.3900 71.94% 81,800 24.04
Jet fuel (naphtha)[5] 0.9700 103.09% 118,700 34.44
Jet fuel (kerosene)[5] 0.9000 111.11% 128,100 37.12
GGE calculated on Non-Liquid Fuels Fuel - Non Liquid GGE GGE % BTU/unit kWh/Unit
Gasoline (base)[2] 1.0000 100.00% 114,000 BTU/gal 33.41
Compressed natural gas (CNG)[4] 126.67 cu ft (3.587 m3)  900 BTU/cu ft
Hydrogen at 101.325 kPa 357.37 cu ft  319 BTU/cu ft[6]
Hydrogen by weight 0.997 kg (2.198 lb)[7]  119.9 MJ/kg (51,500 BTU/lb)[8]
Nitromethane ~2.3 41.23% ~47,000 BTU/gal
Electricity 33.40 kilowatt-hours  3,413 BTU/(kW·h) [9][10] 33.40
Electricity Costs

for 1 GGE 1 GGE = 33.40 KWH
For Local Rate
Per KWH $/Gallon
Equivalent
$0.07 $2.338
$0.08 $2.670
$0.09 $3.006
$0.10 $3.340
$0.11 $3.674
$0.12 $4.000
$0.13 $4.342
$0.14 $4.670
$0.15 $5.010
$0.16 $5.344
$0.17 $5.678
$0.18 $6.012
$0.19 $6.346
$0.20 $6.680
$0.25 $8.350
$0.27 $9.018
$0.28 $9.352
$0.29 $9.686
$0.30 $10.020

Rates per KWH for Residential Electricity in the USA Range From $0.0728 (Idaho) to $0.166 (Alaska) and $0.2783 (Hawaii) [11] [12]

[edit] Compressed natural gasOne GGE of natural gas is 126.67 cubic feet (3.587 m3) at standard conditions. This volume of natural gas has the same energy content as one US gallon of gasoline (based on lower heating values: 900 BTU/cu ft of natural gas and 115,000 BTU/gal of gasoline).[13]

One GGE of CNG pressurized at 2,400 psi (17 MPa) is 0.77 cubic foot (21.8 liters). This volume of CNG at 2,400 psi has the same energy content as one US gallon of gasoline (based on lower heating values: 148,144 BTU/cu ft of CNG and 115,000 BTU/gal of gasoline.[13] Using Boyle's Law, the equivalent GGE at 3,600 psi (25 MPa) is 0.51 cubic foot (14.4 L or 3.82 actual US gal).

The National Conference of Weights & Measurements (NCWM) has developed a standard unit of measurement for compressed natural gas, defined in the NIST Handbook 44 Appendix D as follows: "1 Gasoline [US] gallon equivalent (GGE) means 2.567 kg (5.660 lb) of natural gas."[14]

When consumers refuel their CNG vehicles in the USA, the CNG is usually measured and sold in GGE units. This is fairly helpful as a comparison
Avatar
EnergyMan
You can make a valid comparison based on energy equivalence, because energy is what moves a car, whether measured in ergs, joules, kilowatt-hours, or BTU

Here is a cost-per-energy comparison, in cents per mile, among natural gas (at various distributed prices), gasoline and electricity: http://jaydiatribe.blogspot.com/2012/03/comparative-energy-costs-of-driving.html

Natural gas' advantages are not limited to price.  It also promotes energy independence and economic growth and competitiveness and even has environmental benefits.  You can read about these advantages here: http://jaydiatribe.blogspot.com/2012/03/natural-gas-best-transitional-fuel.html
Avatar
TurtleNick
127 cf of CNG = 1 GGE = Same MPG as Gasoline
1,000 / 127 = 7.87 GGE's
Current Well Head Price is $2.40/MF = 1.000 CF's
$2.40 / 7.87 = 30 cents per GGE

Government has 100's of Private Stations, provide Public Access & we have
good supply CNG.
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LavonnaKitz
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LavonnaKitz
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