CNG Economy vs Unleaded????

[younkin]

I am not sure why but in relation to Unleaded vs CNG everyone is always always asking about fuel economy? The real question should be fuel price which is what got people thinking about CNG in the first place. I guess my mind looks at the economy question as a no brainer, even if you get 15 mpg instead of 20, cng is $.85 and gas is $4.15 lets see $.85 into $4.15 is a little less the a fifth the cost???? As for the power and economy question, some people get less or the same mileage, some get the same or less power, but at a fifth the price CNG is a good choice, Jim

[freedml]

Re: CNG Economy vs Unleaded????

people get worked up about a lot of issues that aren't relevant. but, also, only under $1 in UT and OK. everywhere else it's $2-$3 (still a lot less)

[maxman]

Re: CNG Economy vs Unleaded????

I think it is good to know fuel mileage for a couple reasons. Obviously, on CNG, you want to know when you'll run out. Also, keeping track of your mileage lets you keep an eye open for trouble.
I realize it's hard to keep track of mileage on CNG because of inconsistent fills. However, I'm trying to keep a notebook with miles driven and amount fo fuel so I can get a good idea on mileage.
When I drove my F150 reg cab 4x4 7700 series out to Utah from the bay area, I got around 15 mpg going averaging about 75 all the way. I had a trip on CNG a few weeks ago I got about 190 miles on a 10 gallon fill.
However, the mileage is important because a significant change could indicate a problem.
Max

[freedml]

Re: CNG Economy vs Unleaded????

I don't need a notebook. I reset the trip counter when I fill and I know how many miles I should have driven at 3/4, 1/2, 1/4 and E.

[siai47]

Re: CNG Economy vs Unleaded????

Because natural gas sold for use in vehicles is sold by GGE, the amount of energy obtained in a gallon of NG is the same as a gallon of unleaded. NG has a higher octane rating than gasoline but burns slower. In a vehicle that runs on dedicated NG, the engine is optimized for operation gasous fuel and can use compression ratios and ignition timing that take advantage of the higher octane. The fuel economy of a dedicated Honda GX, for example, with it's higher compresion ratio, lower horsepower, engine is very close, if not better then the standard Civic. In a bi-fuel vehicle, the engine must be able to tolerate running on 87 octane fuel, therefore compression ratios have to be low enough to run on that fuel. Only the timing is advanced when switching to NG on most bifuels causing the fuel economy to suffer when running on NG. Power is always lower on the same engine when run on NG vs gasoline. The addition of fuel in a gasous state into the engine displaces some of the air that would be going in the cylinder thereby reducing the total amount available to the engine and reducing it's power. Again, compression ratio or turbocharging helps this problem. In the end, when making comparisions, the best way to look at fuel economy (and all operating costs) is on a cents per mile basis.

[CraziFuzzy]

Re: CNG Economy vs Unleaded????

Power is always lower on the same engine when run on NG vs gasoline. The addition of fuel in a gasous state into the engine displaces some of the air that would be going in the cylinder thereby reducing the total amount available to the engine and reducing it's power. Again, compression ratio or turbocharging helps this problem.

This is not entireley accurate. Yes, the power is lower, but it is not because of the displacement of the fuel itself, it is that a BTU of methane requires more O2 to burn than a BTU of gasoline. Therefore, to obtain the right fuel-to-air mixture, less BTU's of fuel can be fed into the same sized engine. As you said, turbocharging/supercharging helps, because it gets more O2 into the same space, and as such, you can put more fuel in and keep the right mixtrure.
However, a turbocharged CNG engine will still make less power than a turbocharged gasoline engine, all else being equal.

[Highmarker]

Re: CNG Economy vs Unleaded????

younkin,

I get that question a lot as well. Why are people concerned about MPGs when you're paying 85 cents/gal.? My fuel economy is greatly affected by city/hwy driving (starting and stopping) and using the A/C. I typically get 27 MPG on the hwy, but I have seen as low as 21 MPG in the city with the A/C on.

If I average 27 MPG on CNG at 85 cents/gal., that is equivalent to 127 MPG on unleaded at $4/gal. So, when someone asks me about fuel economy, I usually throw that number at them. I usually get a confused look on their face, but hey, they asked the question.

[wanpakumono]

Re: CNG Economy vs Unleaded????

I think the best way to "keep track" of your fuel economy is to look at the average of your numbers. This helps to give a more accurate overall picture rather comared to what a one-fill snapshot gives. To that end, I created a very simple excel spreadsheet that I can input the date, PSI, miles traveled since last fill-up, the gallons used, and cost. I then calculate the average MPG based on the totals. I have attached it for any one who is interested in using it. I just have to take note of the information when I fill up, and enter the values on the next line. I have left a few lines as an example. Hope this helps some of you!

[mostlyharmless22]

Re: CNG Economy vs Unleaded????

In our Cavalier, we just look at the trip meter to see how many miles we got on CNG. We rarely run a tank of gasoline -- usually keep about 5 gallons for emergencies. The fill sites around us are only about 3,000 psi rather than our 3,600 psi capacity, so we never get a good fill. Usually somewhere around 4 gallons in a 5.9 gallon tank and we always get over 100 miles on it. When we travel to Okmulgee, we get a thrill because their fill site is actually 3,600 psi - talk about your cheap date - Then we get around 150 miles. When our tank expires in 2015, we will probably try for a bigger tank.

[John Mitton]

Same or better

We will certify our dedicated CNG Impala with EPA at 19 city / 30 highway vs. the GM did on gasoline certifying at 18/29 (or on E85 at 13/20)