AltFuel Volume of CNG.. Check my math please (1 Viewer)

[onefsatchevy]

SO I am looking a putting a Natural Gas conversion on a Diesel HDJ81. And I want to put in a home filling system but I cant figure out how to calculate the price of gas at my house. I Know how much it is per Gigajoule but how do I convert that into something useful???

Here is what I have found..

According to my local supplier 1 gig is 38.2599 liters at atmosphere

according to boyles law

pa Va = pc Vc

where
pa = atmospheric pressure (14.7 psia, 101.325 kPa)
Va = volume of the gas at atmospheric pressure (cubic feet, cubic meter)
pc = compressed pressure (psi, kPa)
Vc = volume of the gas at compressed pressure (cubic feet, cubic meter)


​

atmosphere is 14.7 PSI, CNG for cars is 3600 PSI

so 38.2599 x 14.7 / 3600 = 0.15622L

Im obviously getting something wrong here, because that would make CNG like $30 / L

What am I doing wrong??

Thanks

 

[John Galt]

Diesel fuel is about 0.04 GJ per liter

with that datum you can calculate the relative cost.

I don't know how to calculate the cost of compressing the gas to a liquid.

 

[onefsatchevy]

1 gig would then give you about 25L. Making the cost around $0.16 per L. (i pay just under $4 per gig average) That sounds closer to accurate, but I'm still not 100% sure...

Any one else have thoughts???

 

[45Kevin]

Will CNG self-ignite in a diesel combustion chamber?

 

[onefsatchevy]

Apparently it will. But you don't run straight CNG, from the research I have been doing apparently a 60% CNG 40% diesel mix is optimum. My guess is the diesel ignites, in turn igniting the gas.

The bonus is if you run out of CNG you run straight diesel. So range is not affected (even increased due to more fuel on board), better HP, cheaper and cleaner fuel, and if you can't find a CNG filling station your not screwed. The systems don't look too expensive ($2k with tanks). And it looks like there will be home filling stations on the market soon for under a grand. Cost of the system would be paid for in under a year I would think.

Still lots of research to do before I bite though...

 

[45Kevin]

Apparently it will. But you don't run straight CNG, from the research I have been doing apparently a 60% CNG 40% diesel mix is optimum. My guess is the diesel ignites, in turn igniting the gas.

...

Aah yes. Similar to propane injection maybe.

I would be concerned about the lubrication I would loose by introducing that much CNG. You'll need a big honking compressor to get the CNG from the low psi in your house supply to the 2000 psi or so in your on-board tank.

I also wonder how long it will take for someone to whistleblow to the authorities that you are running untaxed fuel. I remember back in the 70's when chevy came out with the old 6.2l diesel that folks would fill them with furnace oil from their house tank. If you got caught, big fines.

 

[onefsatchevy]

There are home fueling stations out there, but they are pricey at the moment. They take 6 hours or so (overnight) to fill your tank...

There are CNG filling stations that are taxed though.... Hard to prove which you would have in the tank.

I was wondering about the lubricant side of things too. But then gas is a solvent.... More questions...

 

[John Galt]

They take 6 hours or so (overnight) to fill your tank...

find out how much electricity they take to do that. It could change the economics considerably.

 

[onefsatchevy]

Oh ya, that's all part of the equation...

 

[bugsnbikes]

The home refilling systems are pricey to start- and then you should really drill down at the cost to rebuild them. From what i saw- the Phil systems were over a $1k to rebuild and the run/hours on them before service werent long enough to really make it economically a "savings". Apparently the Phil system that is 1 step up from the budget system has a much longer service interval---but that system is in the $4k-6k range with rebuilds still in the $1k+ nut. Those systems also refill at twice the rate of the budget phil systems and cut it to 4hours or so.

 

[aressiont27]

I have done the conversion professionally but now, i came to know how easy it was. Great thread.
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