Best compression ratio for propane? 1

[vintageAP]

Hi all, new here - great site!

I'm investigating propane conversions for performance engines and understand that because of the higher octane with propane a higher mechanical compression ratio is desired for optimum performance.

Does anybody have this information handy or experience with this?

I'm sure that the same laws apply as with gasoline engines; too high and pre-ignition will come into play, but about where is that magic number? In gasoline engines there are factors that won't apply in a propane fuled engine such as quench area, puddling of liquid fuel, etc.. Am I still looking for the same aproimate A/F ratio - 14.7? Will cylinder temp affect flame travel and pre-ignition in the same way with propane? Any info and discussion is appreciated.

 

[patprimmer]

11:1

Why won't quench apply?

Of course cylinder temperature will affect flame travel and pre ignition. Burning hydrocarbons is a chemical reaction. Heat will accelerate that action. To much heat will cause a spontaneous reaction.

Regards

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[kenre]

Very hard to quote a compression ratio without knowing what duration the camshaft is.

My LPG 350 chev i have recently built, havent fired it up yet, is right on 10:1, the max i can go according to the manufacturers of the particular camshaft i am using. They develop and test all there cams on there own dyno.

Ken

 

[franzh]

Both Pat and Kenre have a point, there is too much information needed to make a recommendation, but again, what is good for us may not be good for you, and frankly, thats not the purpose of this forum.
If we were to say that ONLY 11.3:1 will work, you will have to build the engine around that CR, not the other way around.
Quench works to induce turbulence in the combustion chamber and it works with all fuels. The greater the quench action, the faster the turbulence and corresponding flame propogation action. Large open chambers have relatively low quench and need more ignition timing to develop the same combustion pressure than a small chamber and tight quench.
The tradeoff is that tight quench areas can bring in pockets of unburned or not fully burned Hydrocarbons. Vapor fuels have an inherent advantage in that they more easily mix with air and develop fewer pockets of unburned HC's.
Franz

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[vintageAP]

"The tradeoff is that tight quench areas can bring in pockets of unburned or not fully burned Hydrocarbons.  Vapor fuels have an inherent advantage in that they more easily mix with air and develop fewer pockets of unburned HC's."

- This is what I'm referring to. A tight quench will mix still liquified fuel much better before combustion. An already gaseous fuel will not benefit from that.

I'm not looking for an exact compression number, that will be for me to determine with experimentation. Let's face it though, changing the compression in an engine is time consuming and can be costly. I'd much rather start in the correct range. For example; a street driven gasoline engine is tunable up to about 11:1, an alcohol race engine likes 13:1 much better, a top fuel nitro-methane goes beyond that etc..

Of course the camshaft makes a big difference, but I can swap a cam in an hour.....

My goal here is to take advantage of the octane and gaseous fuel in an engine. To build a purpose-built engine, if you will, rather than using parameters in an engine designed for gasoline, and having those parameters compromise the efficiency of the propane engine.

 

[MikeHalloran]

So, mill the heads on a junkyard engine, and add gaskets until it stops knocking and pinging, then build a good engine to that ratio. I could swear I've read that propane is good to 15:1 or more.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

There is Propane and Propane.

If it is not pure and contains reasonable amounts of Butane the octane will suffer. Also I have been told there are different isomers with different octane ratings.

11:1 will be safe with any commercial Propane and any camshaft.

Quench does a lot more than evaporate fuel. It transports the flame front across the chamber, thereby increasing burn rate, and reducing required advance, while still allowing peak cylinder pressure to build at about TDC.

There is one theory (I am still to be convinced) that a faster burn rate suppresses detonation as it consumes the fuel and oxygen before the mixture has time to detonate. Also with the later optimum spark timing, the piston is moving up slower or building compression slower as the burning fuel is building it's pressure.

Regards

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[franzh]

Thre is also a factor called the critical compression ratio, where by compression, the air and fuel mixture can reach a combustion temperature as in a diesel engine.

US spec Propane (Commercial grade vs HD-5) has a CCR somewhere around 12.5:1, and again, this depends on the purity of the fuel, usually between 90 and 95% propane, the balance being propylene, ethane, butanes, and other minor fractions. Of all these fuels, propane has the longest burn duration, and the highest octane, and the highest resistance to ignition. The other fuels tend to diminish the combustion quality of propane. If the combustion chamber is largely open, the CCR can go a bit lower, if it is smaller, it can go higher. Small chambers will combust the fuel faster during the maximum crank-angle leverage point.

On a variable compression ratio lab engine, the chamber volume is altered during operation until the engine begins to knock, thusly determining the actual compression ratio.

Franz

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[automotivebreath]

The highest compression ratio that can be designed into a propane engine depends on many variables as mentioned before. It's important to understand why. As this chart displays, the ignition temperature of LPG is much higher.

Also remember the energy content of propane s lower than gasoline. Compared to gasoline the energy content of propane is 74%.




,

 

[vintageAP]

"automotivebreath" - If I'm interpreting your chart correctly then the higher ignition temp of LPG (because of the octane I'm assuming) is the reason that the pre-ignition point is higher and is functionally the reason a higher static compression ratio can be used to get the best efficiency from a LPG engine. Please advise if I am not interpreting this correctly?

"patprimmer" - By those numbers 12:1 seems to be a safe place to start to build a high-efficiency LPG engine. Given that a real-world use would take into account that the fuel would be bought from different sources and vary in quality in those fractions I'm thinking 12:1 would be a very good starting point. Feedback?

 

[automotivebreath]

Detonation is the spontaneous combustion of the remaining fuel/air mixture in the chamber during combustion. The initial combustion at the spark plug is followed by a normal combustion burn. Excessive heat from the flame, hot engine parts, inlet gas temperature and building pressure can cause the end gas in the chamber to explode. With the high ignition temperature of LPG; elevated combustion pressures are possible.

 

[patprimmer]

vintageAP

It depends where in the world you are. You did not say, but reference to Propane rather than LPG suggests USA.

I believe that the Propane sold in the USA is fairly pure high octane stuff. The LPG sold in many other regions contains quite a lot of Butane and is substantially lower octane. I would expect that 12:1 might be OK in the USA. 11:1 is the max I would recommend as safe under all conditions in all engines in NON USA regions



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[rdd48856]

Propane doesn't have isomers....insaturates (propene and butene)as well as waxes are the ones you should be worried about, as they tend to decrease easily octane rating.

 

[GunMuse]

Ok let me see if my thinking is right here. While propane has a lower energy rate it is more stable than diesal according to the chart?

Since Compression makes the burn-rate(explosion) grow expotentially at what point of compression does LPG surpass the 87 octane with nice 9.5 to 1 ratio?

http://www.prnewsnow.com

 

[GunMuse]

Ok One follow up on this before someone answers. I have a couple of propane powered trucks.

The delivery system seems very "crude" at best. It would seem to me that a MFC valve would be better suited than a "power valve".

There doesn't seem to be anyway to really achieve fuel mileage from LPG automatically allowing for elevation and driving conditions.

I have been buying up the propane powered trucks because, frankly they are super cheap. I bought a 94 3/4 ton chevy with a 5 speed for $400 nobody at these farm auctions wants to go near propane because of its low power. I think that LPG could achieve 30 MPG

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[turbocohen]

11:1 is safe. LP in the USA is not consistently free of heavy's which is a metering accuracy issue so you need some margin for cylinder to cylinder distribution errors.

The bigger issue is ignition system capability. 11:1 takes its toll on gasoline ignition coils, wires, plugs and other goodies.

I run a stock 6 liter LS-2 Chevy at 10.9:1 that runs ok on 87 octane gasoline but at reduced output. It scoots on bp Ultimate and is slightly improved with liquid phase injected lp.. about .2 sec faster 0-60. (comparison measured with 40lb more vehicle weight after changeover to LP system)

 

[JBlack]

VintageAP-
"Am I still looking for the same aproimate A/F ratio - 14.7?"


I believe that the stoichometric A/F ratio for propane is ~15.3

 

[GunMuse]

Liquid phase injected. Wouldn't that defeat one of the benefits of LP by injecting it in liquid form.

I would suggest a custom computer to play with the timing on the fly if your going to be switching between Gas and LP on compression over 10.5:1.

As I was saying it looks to me that the efforts to use LP have been crude at best. From its fuel delivery to any sort of addon's to increase power for LP.

I would like to try LP in a 2 stroke setup with an injecter setup (Like Diesal)and a lubrication system like convention 4-strokes. 4 valve heads with and evactuaded exhaust system.

Unfortunately only college kids seem to be able to get grants to use their "experience" to help us all.

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[turbocohen]

Atchooly one of the advantages of metering lp as a liquid with the metering tip located in the stock fuel injector port is the latent heat of vaporization. The recalibrated oem ECU with tweaks for LPG was a gift from a friendly Canadian GM guy whom I made a duplicate LS-2 lpi injection system for.

The mass air flow meter aint a liar.. Higher charge density + higher octane equivalence = (fill in the blank).

Truth is the retrofit was damn easy. The LPI injectors have 10% less static flow than the stock gasoline ones did but they can open up to 600psi. And the fuel pump delivers 4bar boost pressure above static tank pressure while the engine cranks over to facilitate .5 second hot restart. The calibration mods were not complex.. primarily modified the authority limit to allow closed loop all the way to wot, eliminated cold start enrichment, reduced accel enrichment just a tad and reassigned an open channel for fuel rail pressure and temp compensation.

Runs the same as with the stock gasoline system but goes like a scalded hog.

As for college kids.. I sponsored many of them. About 60 teams so far. A lot of these kids become engineers and some are a real asset to alternative fuels hobbyists.

 

[franzh]

Turbo;

Those college engineering competitions were fun, werent they? Many of the students we mentored have moved on to good positions at OEM's, research labs, and to teaching. I get mail from them all the time thanking me for the direction we provided.

Natural Gas Vehicle Challenge's
Propane Vehicle Challenge's
Ethanol Vehicle Challenge
Formulae SAE

Franz

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