Lets get this one started!

Natural Gas, CNG, LNG, NGV, CH4
Posts: 1197
Joined: Thu Oct 07, 2004 7:57 pm
Location: Central Texas

Lets get this one started!

Post by franz » Tue Oct 11, 2005 4:39 pm

This poor forum has been dormant way too long. Lets get it off to a couple of topics:

The drive for alternative fuels has yielded two prime candidates, LPG and CNG. Depending on where one lives and what access one has to these fuels, each has its significant advantage, and a significant disadvantage.

Topic 1: Why would one choose CNG over LPG, financial, logistical (proximity to refueling stations) performance, vehicle choices;

Topic 2: Why would one choose LPG over CNG, same criteria.

The wheels are off, no holds barred, this has the potential to be a very vocal topic.


Site Admin
Posts: 868
Joined: Wed Feb 25, 2004 2:17 pm
Location: Stevensville, ON

Post by Frank » Wed Oct 12, 2005 9:49 am

When I first started learning about alternative fuels, I considered both CNG and LPG. I think when I was pricing out the two conversions, the total costs were similar. The main consideration for me was simply the range between fillups. I was intending to convert a large vehicle and the equivalent of a few gallons of gasoline that a large CNG would hold would just not have been practical for me. I could easily find a LPG fuel tank that would give me a similar driving range as I had had on gasoline.

The secondary consideration for me was that there were relatively few CNG stations around at the time. The range might not have been as important a consideration if there was something like PHILL back then. PHILL is a vehicle refuelling appliance (VRA) which simply a small natural gas compressor in your home's garage that allows you to refill your car overnight.

I hadn't originally considered the price of the two fuels. Around here today, natural gas is sold in equivalent litres, and the formula is 1 L of gasoline = 0.65 kg NG = 0.93 m³ NG. In doing a quick survey of local stations, natural gas costs in the range of 60-70¢/L. Propane costs anywhere from 50-70¢/L. I haven't yet worked out the price of fuel if it were delivered by PHILL but I would expect that it would be a bit cheaper than the station price.

Posts: 1010
Joined: Fri Sep 09, 2005 11:23 pm
Location: Winnipeg, Canada


Post by C3H8 » Mon Oct 17, 2005 12:16 am

Not necessarily true on the self filling consideration. You have to take nto account that these stations usually need to be installed with some electrical upgrades in the garage or home location. Once they are in use they take 6 to 8 hours of compressing to fill a tank. I don't know the actual estimates on electrical usage but I suspect a compressor pumping 3000 psi would need signigant electricity to complete its job. In addition the range of your average CNG tank would mean filling the tank every night or every other night. Thats 1500 to 3000 hrs of pumping annually and the motor is quite large in these units I believe. Correct me if I'm wrong. On top of this you need to lease or purchase the VRA and I am told they need to be overhauled approximately every five years. You may save on the fuel costs but the added costs of owning the unit may offset the savings.

Posts: 83
Joined: Tue Oct 04, 2005 8:10 am
Location: Utah

Post by 86Chevy » Wed Oct 19, 2005 10:51 pm

I tried to find info on cng conversion but I found it extremely difficult to get any info. I am already commited to propane but I would be interested in cng information. Any links?

Posts: 8
Joined: Wed Feb 22, 2006 6:00 pm
Location: Argentina

Post by gabriel1 » Wed Feb 22, 2006 6:09 pm

Posts: 3
Joined: Mon Mar 13, 2006 10:44 pm
Location: Berwyn Heights, MD

Post by Tofuball » Mon Mar 13, 2006 10:48 pm

Any links in english? :)

Posts: 8
Joined: Wed Feb 22, 2006 6:00 pm
Location: Argentina

Post by gabriel1 » Sat Mar 18, 2006 1:40 pm


Posts: 1
Joined: Fri Apr 14, 2006 8:30 pm
Location: Prince George, BC


Post by cowboy » Fri Apr 14, 2006 8:53 pm

I converted my ford quite a few years ago-installed 2 tanks underslung, and 2 in the box (removeable). since this truck was dual fuel EFI it worked quite well. I also purchased a used fuelmaker in Vancouver at ECO fuel systems-it has worked out well too-sure is nice to fill at home. The only thing is cleaning the window, and checking the oil (never at the squeegee. I have since bought a dodge diesel that performs well-lately I have been running it on a small percentage of CNG sure helps the fuel mileage (less diesel purchases replaced by cheaper CNG).
I converted to CNG because of my work with Hydrogen-it uses the same storage system.

Posts: 5
Joined: Sun Dec 17, 2006 7:57 pm

Post by rancid » Tue Dec 26, 2006 9:36 pm

whats the benifits of natural gas vs. propane. how does natural gas power compare to regular gas power. im putting together a twin turbo 468 big block 67 cevy sortbed and would like to run something other than regular gas. and i have really been looking into propane. but i want to know about natural gas.

Posts: 393
Joined: Wed Sep 21, 2005 1:22 am
Location: BC Canada

Post by sleepybu » Mon Jan 01, 2007 12:34 am

It's alot ezer to efi then lpg and i'd say there would be more potentl to make big power over carbed cng

Posts: 1197
Joined: Thu Oct 07, 2004 7:57 pm
Location: Central Texas


Post by franz » Tue Jan 02, 2007 10:05 am

Natural Gas can be injected easier than with LPG vapor only because phase change from vapor to liquid will not happen. This eliminates one factor when calculating the injector flow tables.

Items to consider:
CNG vapor is about 40% as dense as propane vapor, thus the injectors must be that much larger, and again, they MUST be specialized injectors, gasoline injectors will not flow enough and will burn out quickly since they are not designed for dry-firing.

CNG pressures at the tank range up to 3600 psig and storage volume is low, about 1/5 that of propane for the same tank space.

CNG pressures at the injectors are usually in the 30 to 60 psig range, a good middle ground to allow for stable flow at pressures (regulator droop) while providing enough volume to run an engine.

CNG is an excellent engine fuel since it has a very good anti-knock property, but consider that CNG vapor displaces about 10% of the air by volume, the primary reason a CNG engine does not produce the same power as an equal gasoline engine.


Posts: 8
Joined: Mon Mar 05, 2007 3:36 pm


Post by utahguy » Mon Mar 05, 2007 6:11 pm

Here in Utah I think CNG gets the edge due to the cost difference. CNG is only $ .74 per gge. Propane is $ 2.17 and gasoline is $2.19 (e-85 if you can find it is also $ 2.19 - $ 2.25). Hence, gasoline and propane are almost 3 times as much as CNG. Also, I don't have to wait to fill up my CNG tank, but can do it myself at the local Flying J. On the propane, you have to wait for the attendant to come out to the tank and compressor. I do not understand the exact difference between the CNG gge figure and the gasoline cost per gallon, but as far as I know they were supposed to make it so you can compare the costs. Is my understanding not correct on this? I think Franz made some inference to this. :?:

User avatar
Posts: 1488
Joined: Fri Feb 02, 2007 8:32 pm
Location: JAFA , New Zealand

Post by Steptoe » Mon Mar 05, 2007 11:36 pm

About 20 odd yrs ago LPG and CNG where both common and freely available
I had the Camaro on LPG and The Holden Station Wagon on CNG
The CNG was way cheaper to run, but the weight of the tank and the range from a tank sucked big time
Running CNG on a petrol engine..even tuned and curved it was gutless, espec on hills and loaded...
I dont not believe that the lack of power or lack of tank range would be anywhere as bad IF the engine was built as a CNG engine.
I cant rem the figures..was a few yrs back now, but petrol engine designed for 91 or 96 octane runs LPG around the 100 to 115 octane ok
A petrol engine running on CNG up around the 135 octane..it looses a heap.

The CNG ran even cleaner than the LPG

I have designed the Camaro as a LPG engine 20 odd yrs ago, and wouldnt change
CNG althu on paper the potential is there, tank range etc...I would not and have not bothered to go down that route

As I said above CNG stations used to be a dime a dozen in NZ...now they are virtually non existent...even for buses, taxis, the range issue was too inconvenient to the extreme...the demand died while LPG ...nearly 1 in 3 stations have LPG in NZ now.

Posts: 1197
Joined: Thu Oct 07, 2004 7:57 pm
Location: Central Texas

CNG conversion

Post by franz » Tue Mar 06, 2007 8:53 am

A rough calculation for the GGE is as follows:

Generally speaking, and in the Southern US, pipeline natural gas has a BTU rating of 1025 BTU's per cubic foot (do the conversion for your measurement system of choice).

Gasoline (petrol) has a rough BTU rating of 125,000 BTU's per gallon. Depending on where you are and if the fuel has been modified by oxygenates (ethanol), it can vary, but I have never seen pump grade gasoline over 125,000 BTU's. I have seen it as low as 110,000. (Diesel is around 150,000).

Divide 125,000 by 1025 and we get 122 cubic feet of natural gas per gasoline gallon equivalent in energy. Most of the industry uses 117 cf/gallon but this number is arbitrary due to changes in BTU rating and other factors tossed in.

Now, add in the fact that CNG enters the engine in vapor form, or about 10% of the incoming air mixture, and we have an effective reduction in air available for combustion, or a resultant reduction in power of about 10%.

CNG has a relative octane of 125 meaning much of the potential energy available is pumped out of the tailpipe when the exhaust valve opens. Modern gasoline engines with 8.5 CR cannot take advantage of the higher octane and we have another lost 5 to 10% power. Yes, the engine can be optimized for CNG by increasing the CR and that is the proper way of doing it. Advancing the ignition timing helps a little, but there is only so much that can be done until an engine modification is necessary.

CNG is an excellent fuel (I got my start in the CNG industry) and it works well in large engines because of its inherent anti-knock characteristics, one reason there has been much more development of large engines on CNG than LPG.


Posts: 8
Joined: Mon Mar 05, 2007 3:36 pm

12:1 compression ratio?

Post by utahguy » Tue Mar 06, 2007 12:44 pm

I would echo Franz's comments with what we see in the real world. I'm getting roughly 10% worse mileage with my car running on CNG than when running on 85 octane gasoline - (that's the cheapest grade they sell here in Utah and we're at 4200 ft. altitude). On hills here in Utah I usually downshift my auto transmission to the next lower gear because of the less power issue. But, I don't find that annoying because its seldom and only on steep hills. Most of my driving is highway between cities. If you look at the CNG factory built Civic GX (which is the only factory-built NG vehicle sold in the U.S. anymore :cry:), it is dedicated CNG and has a compression ratio of 12:1!! You couldn't run that engine on today's gasoline unless you bought racing gasoline. It only gets slightly less fuel economy than the regular gasoline Civic and they claim its the cleanest fossil fuel burning auto on the planet, with an emissions rating of ILEV AT_PZEV. :!: I think Honda's engineers know what they're doing here. (I assume its not an aftermarket kit they're bolting on and is their own design.) Franz, you raise an interesting issue with the anti-knock characteristics and high octane rating. :idea: Wouldn't this make CNG a more suitable candidate than propane for dual-fuel use (partial replacement of the diesel fuel) in diesel engines? Why the big push for propane in that role considering the compression ratios and the advantages of vapor NG?

Post Reply