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Power-User

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### Compression Ratio for CNG Engine

04/09/2012 11:01 PM

Did anyone know what is the optimum compression ratio for a CNG (compressed natural gas) engine?

What else to look out for if I want to modify from a gasoline engine already converted to CNG use (dual fuel use) using existing commercial conversion kit. The compression ratio of the engine is not modified.

Do those knocking sensor meant for gasoline engine also work for CNG combustion?

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Guru

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#1

### Re: Compression Ratio for CNG Engine

04/09/2012 11:09 PM

depending on camshaft profile and a few other details anything in the 12:1 - 15:1 ratios is a favorable range to shoot for.

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#2

### Re: Compression Ratio for CNG Engine

04/10/2012 12:31 AM

I also found that the RON value for CNG is as high as 120, ethanol is 108.

Is there a table that give optimum compression ratio vs RON value for typical car?

Aslo from Wiki, F1 engine CR is as high as 17, is it true? What RON petrol they use in F1 engine ?

Appreciate if someone can shed some light here.

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2
Commentator

Join Date: Apr 2012
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#3

### Re: Compression Ratio for CNG Engine

04/11/2012 12:41 AM

Improper question.

The "optimum" for the combustin process, or the overall ENGINE?

BIG difference.

The upper bound for the combustion process is the pressure at which auto-ignition ocurrs. (P-V-T)

The upper bound for the engine is where mechanical parts start breaking.

The ideal compression ratio is as high as possible for the best thermodynamic efficiency.

What else you want to look out for is THAT YOU DONT DIE in an explosion from trying to use CNG. EXTREMELY dangerous.

Understand, that said from someone experienced in vapor phase fuel system design. Vapor phase fuels are dangerous to play with. Its a "if you have to ask, youre not qualified to be playing fuel system designer."

Its not possible to carry enough NG for more than a few hundred yards travel without

a.) cryogenic storage (NG phase point is very cold)

b.) VERY high pressure.

Youre driving a BOMB.

Learn before you burn:

http://www.mvfri.org/Contracts/Final%20Reports/CNGandH2VehicleFuelTankPaper.pdf

(page 12)

There were photos on line from Canada showing NG tanks that went off and blew HOLES in the road.

Zero survivability.

I've had the misfortune of having to explain to more than one mechanic why their water electrolysis device under the hood of their car kept exploding. They did have to concede I was right...

Knock sensors are designed for a certain expected flame front travel, a different fuel will have different knock characteristics. they are frequency dependent. I dont know beyond that.

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#6

### Re: Compression Ratio for CNG Engine

04/11/2012 11:04 AM

I have limited knowledge based on research for CNG fork truck systems to know this is a very good answer.

CNG is very dangerous, more so than propane, gasoline, or diesel. You are driving with a tank of highly compressed, cyrogenic material. In addition to explosion risk, you have frostbite risk in case of a liquid leak.

If you go beyond the theoretical to actually running an engine on it, be very very careful!

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#7

### Re: Compression Ratio for CNG Engine

04/11/2012 4:40 PM

Are you sure you are not thinking of propane?

CNG is still a gas where as propane is a liquid and when the tank pressure is released it comes out and super cools itself to around -40F which can cause severe frost bite and the like.

As far as cryogenic goes well we apparently have cryogenic winter nights here being -40 F is not that uncommon in January and February!

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#10

### Re: Compression Ratio for CNG Engine

04/11/2012 11:52 PM

No, this would have been used to replace propane on the fork trucks inside the plant.

Maybe my memory isn't what it used to be. Or maybe I don't know what I'm talking about. Take your pick!

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#9

### Re: Compression Ratio for CNG Engine

04/11/2012 10:42 PM

Your concerns seems a bit of outdated. IN world today, there are already tens of millions CNG car being used for more than a decade.

Its not possible to carry enough NG for more than a few hundred yards travel without

a.) cryogenic storage (NG phase point is very cold)

CNG is by pressure not cryogenic, typical CNG car can go 150 km full tank of about 60 liter size.

Although the pressure in CNG tank is very high (200 bar) , they are made to meet very high safety standard. Country like China , Pakistan and many others have used CNG car for many many years, there seems to be no serious accidents from CNG tank explosion. The fatalities due to CNG tank is far far lower than other car accidents.

The current practise with CNG conversion is to maintain the compression ratio ( due to dual fuel use), the result is loss of power and efficeincy when using NG. My thinking is, we can increase the CR to say 15~16 (should be possible because NG octane is 130, F1 engine CR is as high as 17) . If the user is using gasoline, there should be some mechanism to only allow low power use to avoid detonation.

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#11

### Re: Compression Ratio for CNG Engine

04/12/2012 2:15 AM

yes, that tank looks like its safe.

Then tell these people about it:

http://www.mvfri.org/Contracts/Final%20Reports/CNGandH2VehicleFuelTankPaper.pdf

despite your arrogance, my comments were not to you. they are to someone clearly lacking knowledge to, in my observation, to be playing with such systems.

Id rather he not die in an accident

Id hope your interest would be more along the lines of his safety than making yourself look wise.

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#12

### Re: Compression Ratio for CNG Engine

04/12/2012 4:13 AM

I am sorry you felt somewhat offended by my post, it was never meant to be arrogant, but I merely point out to the fact CNG car has been proven to be safe. There is nothing in the world is absolutely safe. Air travel is one very good example. What would you say to someone who say that air travel is dangerous, if the engine fail, people will die.

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#4

### Re: Compression Ratio for CNG Engine

04/11/2012 5:14 AM

Optimum is an interesting word in engineering because for any multiple variable process/system such as the internal combustion engine, what's optimum for one part of the process may not be optimum for the overall system. The mechanical part of the engine is usually not modified during conversion, however the fuel control and spark control systems do require retuning. Depending on the design of the engine and the valve and valve seat material, there is a higher rate of valve wear due to the lack of lubricity in CNG.

There are many qualified conversion shops that do this type of work, and there are many fleets (usually owned by the natural gas utilities) that run on CNG. It should be pointed out that CNG is a single phase system (gaseous only) and is nowhere near as energy dense as LNG which will never find its way into our highway system. LPG systems (propane, two phase storage) have been around for quite awhile also. Just let a professional shop do the work to avoid drivability, safety, liability and insurance problems.

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#5

### Re: Compression Ratio for CNG Engine

04/11/2012 9:06 AM

OK another thing to look into with the higher compression ratios is the materials of the engine, Chrysler is now releasing a 5.7 Liter Ram Heavy duty Bi fuel in Canada, the pistons and rods and heads had to be hardened because of the 120 octane rating, and that is without increasing the compression ratio. I would look for a qualified shop to do the work as well, then you are not the one holding the bag when the ECU or other part of the engine decides it can't handle the extra octane. In 5 years when these things are used I plan on buying one, for now, they are a little costly unless you have a business to deduct the cost against. There are other vehicles for Bifuel already in the used market, these might even be in your price range instead of the cost of the conversion.

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#8

### Re: Compression Ratio for CNG Engine

04/11/2012 5:38 PM

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

It is not the octane rating that is making them use hardened parts. possible a higher compression ratio, but you wrote "that is without increasing the compression ratio".

there is more to that story than you heard.

The higher the octane rating the lower the chance of having detonation happen. Detonation is measure of flame front speed. higher octane is just slightly slower burning.

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#13

### Re: Compression Ratio for CNG Engine

04/12/2012 10:20 AM

Thanks for the link on the octane rating. the engine size is 5.7 liter, that is the same size as the standard engine they currently produce.

I agree I wish i knew all the story on why they changed things all that it would take for CNG and or LNG conversion, I would love for someone to come out with a universal conversion kit meaning, fuel handling, and an ECU module like the MSD ignition system that can be switched on the fly to which ever fuel you wish to run on. The only thing is that the manufacturers of the major vehicles need to standardize on 3 engine sizes and provide bifuel capability for a of these engine types as work to create after market turbo and or supercharger kits certified by the manufacturer to make more efficient use of the fuel. Good luck on the conversion.

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