CNG Vehicle conversion to Gasoline 2

[mer250c]

I am about to embark on a conversion of my dedicated CNG Dodge 5.2L full size van to gasoline due to the closing of CNG refueling locations in the Midwest. The Engine has a 10.5:1 compression ratio. I am going to attempt to run this engine on Premium gasoline with 10% ethonal. Question, will the injectors that were used for CNG be adaquate for use with gasoline given the correct fuel map? Can you guys help me out with this one?? Yes, there may also be a question concerning the standard ignition timing map due to high compression knocking with 90 octane!!!

Thanks,
mer250c

 

[franzh]

The injectors are NOT compatible with gasoline. The orifices are too large. You will need to swap out the complete fuel system with a gasoline system, including lines, pressure regulator, pump, tank, etc.

As for the CNG computer, you will also need to swap it out for a gasoline unit too, since the CNG fuel mapping is different than gasoline.

The 10.5 CR engine will have a tough time with todays gasoline. The timing is not adjustable and is controlled by the PCM. Changing the PCM should take care of that.

Franz

 

[mer250c]

Thanks Franz for the info. Would it make more sense to convert the CNG vehicle to E85 (Ethanol) and use the injectors that are installed. The existing fuel lines are Stainless Steel therefore with the addition of a Stock polypropolene 35 gal tank and a Ethonal safe fuel pump and regulator it seems to be more appropriate for the high compression engine to run on 106 octane E85. Is the CNG orfice size adaquate for E85 since an additional 30% more fuel is needed when using E85 vs Gasoline.

The following is a chart that I found online compression and octane. Do you agree with these numbers?

10:1 compression requires 96 light load and 98 heavy load
10.5:1 compression needs 89 to 100 octane
11:1 compression needs 102 to 110 octane
12:1 to 13.5:1.....110 to 114

mer250c

 

[gbent]

Have you thought of selling it as a complete CNG vehicle?

If the fueling station is closing, can you buy one of their compressors? My only exposure to CNG was a long time ago. At that time, home compressors were about $2,500USD. Does anyone have any current prices for compressors, and compressor suppliers?

 

[patprimmer]

Have you thought of converting to LPG or propane gas.
It will handle CRs of over 10:1, and is it is now possible to buy evaporators that regulate the gas pressure, it could quite probably be injected through your CNG injectors.
I think (but don't know for sure) that the LPG regulators are set at about 30 to 35 psi. Injection nozzels to suit can be bought if the CNG ones don't work at that pressure.
I don't know fuel air ratios for either, but I think LPG nozzels that are adjustable might be avaliable.
I guess CNG is mainly methane and ethane whereas LPG is mainly propane and butane. I think propane is about 105 octane, whereas butane is somewhat less, but I think that LPG is always over 100 octane, but that might change from region to region or supplier to supplier, or even over time from one supplier.
All this is of the top of my head, and the information was gathered in Australia. Regards
pat

 

[mer250c]

Thanks gbent and Pat,
Yes, I thought about selling it but it only has 20,000 miles on a 5.2L engine. After buring CNG for 20,000 miles the engine is all but new.

The LPG conversion sounds reasonable but I have little experience with LPG vs Ethonal.

Is there an LPG expert out there that can help with a few numbers such as fuel-air mixture, ignition timing, etc. Is the ignition timing for the CNG and LPG close to the same? Will the PCM need to be changed for LPG. Will the 3600 PSI tanks that I have for CNG work with LPG? Is the fuel map close to the same for LPG?

 

[franzh]

I have some experience here;

LPG vapor is considerably more dense than CNG vapor, requiring a remap of the computer. The injectors will work ok. LPG vapor will need to be in the range of about 60 psi. You will need to calculate the fuel delivery tables. LPG is around 104 octane, butane around 99, CNG (methane) is around 125.

The fuel tanks CANNOT be used for LPG! PLEASE DO NOT EVEN TRY IT!

All in all, its easier to trade the van in on a gasoline model, or find a complete donor van and trade the components on a one for one match. You might even find a pickup of a similar year to use.

Franz

 

[mer250c]

Franz, Thanks for the info but I am currently an unemployed Manufacturing Engineer with some time on my hands and thought it would be an interesting project to convert.

It is my understanding that the computer will relearn fuel maps and system changes up to 40%. According to my calculations it should only have to relearn 15% when converting to Propane. Is this an accurate statement? The only calculation that is giving me problems is how to maintain a constant 60 psi from the vaporizer to the fuel rail in cold weather. Some have this problem almost under control with a 4 stage vaporizer-regulator unit. Yes, I have received quotes from Manchester tank for replacement of the the CNG tanks to Propane tanks. I also am waiting for information from a gentleman in Canada who has converted an EFI engine to run on Propane and used the multiport injection system for fuel supply. He is the one that is using the 4 stage vaporizer-regulator to manage fuel vapor presure at the injectors. During the summer temperatures it is not a problem but during the winter months in the Midwest when the temp is 10 below I am not sure how I will maintain 60 PSI at the fuel rail. This has been the problem that I have no solution for todate. Can you help with this problem? The computer is the least of my worries, I am also a programmer and have engineering friends in the Motor city that can help with the remap.

It should be an interesting project.

Franz, by the way, I read your article on building the ideal propane engine, very good information. Feel free to give me advice where you feel it is needed.

Thanks,
Steve

 

[franzh]

Steve:
60 psi is an arbitrary number, you can use lower pressures if you have the ability to change the pulse width. The lower the pressure, the less reserve you have on WOT though.

I dont really think the stock PCM can adapt 40%, but it might given the circumstances. It gets its input primarily from the O2 sensor and MAP when adapting (changing from CNG to LPG).

Some properties factors:

Natural gas has a weight of .0457 lbs/cu ft. or about .5 air.
1000 btu's cu/ ft (average US)
LPG vapor weighs about 1.5 X air. (60 deg F)
LPG liquid is about .504 X water. (60 deg F)
LPG has 91,500 btu's liquid gallon
2500 btu's cu/ft.

Thanks for the comments, hope this helps.

Franz

 

[mer250c]

Thanks Franz for the data. I found some of the same data at

http://www.mipga.org/PhysProp.htm.

The Vapor Pressure LB/SQ IN at 60 Deg F is 92 psi. Question; Is this a linear equation? If so I can calculate vapor pressure under various weather conditions.

Also WOT is not a priority for this project due to the vehicle being a family vehicle with an overdrive transmission. Probably cruising at 2500 rpm 80% of the time if not less.

Interesting fact about Propane; Expansion rate: One gallon of Liquid = 270 gallons of Gas. This is a very interesting alternative fuel, just wish I had been interested in this fuel earlier in life....I guess it is never too late to learn.


Thanks,
Steve

 

[franzh]

LPG vapor pressure (also called the Vapor Pressure Index) is not linear.

mail me at my personal address and I will forward an Excel graph.

Franz