Best compression ratio for propane?
Best compression ratio for propane?
(OP)
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.
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.





RE: Best compression ratio for propane?
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.
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RE: Best compression ratio for propane?
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
RE: Best compression ratio for propane?
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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RE: Best compression ratio for propane?
- 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.
RE: Best compression ratio for propane?
Mike Halloran
Pembroke Pines, FL, USA
RE: Best compression ratio for 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.
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RE: Best compression ratio for propane?
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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RE: Best compression ratio for propane?
Also remember the energy content of propane s lower than gasoline. Compared to gasoline the energy content of propane is 74%.
,
RE: Best compression ratio for propane?
"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?
RE: Best compression ratio for propane?
RE: Best compression ratio for propane?
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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RE: Best compression ratio for propane?
RE: Best compression ratio for propane?
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?
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RE: Best compression ratio for propane?
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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RE: Best compression ratio for propane?
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)
RE: Best compression ratio for propane?
"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
RE: Best compression ratio for propane?
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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RE: Best compression ratio for propane?
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.
RE: Best compression ratio for propane?
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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RE: Best compression ratio for propane?
Here's a side note question. "Why is propane cost increasing its not tied to the cost of a barrel of oil but seems to flucate with it?"
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RE: Best compression ratio for propane?
RE: Best compression ratio for propane?
As Turbo stated, for many years, propane and butane was flared off, the cost to transport was more than the product sold for. That is no longer the case.
LPG is a cost break-even for refineries now, as the product usage is growing. It is the worlds third most widely used motorfuel and one of the most versatile fuels (home and business heating, cooking, industrial and recreation uses, motorfuel).
Gee, sounds like a marketing spiel, doesnt it?
Franz
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RE: Best compression ratio for propane?
It can also be used to make various intermediate products that are then used to make a very wide range of plastics and chemicals.
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RE: Best compression ratio for propane?
My original thought was to use propane in the gaseous state using a standard-type mixer replacing a carburetor. I'm intrigued though with the thought of LPG injection. This may present some disadvantages, but I'm also seeing some distinct advantages that are desirable.
Modern FI gasoline engines run a fuel pump pressure of about 35lbs (varies for make and system but let's use that as a number) and regulate the fuel rail pressure with a regulator. Am I oversimplifying here or could I just feed LPG to the regulator and regulate the pressure to the correct PSI (whatever that is determined to be) using all of the OEM system intact? Would a gasoline fuel injector not handle LPG in the same manner if the supplied fuel pressure was where the compensation was made?
This would certainly make things easier on a late model and using the feedback functions would be sweet.
RE: Best compression ratio for propane?
RE: Best compression ratio for propane?
I've seen gasoline injectors blow themselves apart at 125 psig.
The gasoline injectors do not flow enough volume for liquid LPG use, hence the necessity for specialized injectors (re Turbocohens previous life).
Franz
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RE: Best compression ratio for propane?
OK, Turbocohens injectors. These replace the gasoline injectors in a FI LPG conversion? Are these readily available? Will they run with the signal supplied by a typical modern FI control circuit?
RE: Best compression ratio for propane?
RE: Best compression ratio for propane?
It takes a bit more moxie to open the LP version of Deka II when the fuel tank temps are elevated during a hot start after a 20 minute hot soak. These are modified gasoline injectors with higher operating pressure capability and additional attention to low leak rate due to the smaller molecule of lp.
The parts needed to make a lpfi system are on the road in asia and the EU but on the shelf in the US. Problem is the LP industry here is not in step with the EPA and CARB. If they adopt the relavent fuel quality standards THAT ARE ALREADY REQUIRED for gasoline retailers then the maybe the US market will get so called taxi tested OEM lpfi injectors and pumps.. The future availability of proven LPG liquid phase port injection hardware depends on the LP marketers taking the responsibility for deploying fuel detergents. Since auto use is relatively minimal except for forklifts, there is no united front to establish requirements for using LPG motorfuel additives.
Here is a link to a page with the lp injectors I put on the GTO LS2 : ht
and a pic of the pump is here http://www.vialle.nl/
RE: Best compression ratio for propane?
We sure are off topic by this point, arent we?
Franz
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RE: Best compression ratio for propane?
My ecu switchpoint voltage is initially .380mv after 7 sec and .460mv hot.
RE: Best compression ratio for propane?
Isn't the sheer fact that propane is a huge improvement over gasoline enough some paper pushing weasal has to create a new standard on how to restrict engine performance before the technology is released.
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RE: Best compression ratio for propane?
After all, I could take a current gasoline engine and get a measurable improvement in efficiency if I didn't have to meet EPA. Presumably there is some social mechanism that ensures that the EPA target is meaningful.
Cheers
Greg Locock
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RE: Best compression ratio for propane?
In fact by doing so you will have decreased the amount of toxins produced per gallon of fuel consumed. Truly greener thinking anyway.
You will consume less fuel overall. Truly conservative thinking and better for national defense.
You will have engineered an advancement that can be improved upon by someone else who may gain experience in the field of engineering by seeing your idea's in action and actually improve upon them.
Most engineers are more tinkerers than inventors and as such need to see odd ball idea's actually work to think up improvements.
Lawyers in congress and especially those "who seek to save the enviroment" want to "engineer" a result instead of seeking a standard.
I have a 350 chevy with no smog equipment, no cat convertor, straight up a 1960's style engine build but with all the modern equipment and thinking and it Passes epa. But then again its technically illegal because I didn't drain the power by adding all that garbage required by law back onto the engine.
I am not saying that propane shouldn't meet a standard. But that is not what this guys role will be. He will be setting up regulations on how to licence the guy who does the install(creates schools to get the licencing from). Getting a list of "mandatory equipment" written into law.
Anything beyond setting a standard for air quality, any attempt to "require a device" is just an attempt to kill the use or graft money into the grant system. In a free society we don't tell others how to achieve greatness we just set the bar and see who gets over.
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RE: Best compression ratio for propane?
Take a million of your smog producing devices down to LA, and see what LA's inhabitants think of the soup of HC, CO and NO2 they produce.
Tailpipe emissions /are/ a real world concern.
Cheers
Greg Locock
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RE: Best compression ratio for propane?
HD-5 does not guarantee the elimination of heavy ends in fuel, just a vapor-pressure, propylene, and corrosion standard.
Franz
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RE: Best compression ratio for propane?
Put your car on a set of rolls and at least run an ole 3 bag ftp cycle. Just tryin to keep this thread honest.
Respectfully, Tubinator
RE: Best compression ratio for propane?
By the way, thanks to organizations like the EPA to force the automakers to develop the high efficiency engines we have today, like the ones that get mid 20's mpg in a 4000 lb car, engines that last 200,000 miles instead of the 60's and 70's junk that was worn out at 75,000 miles.
Franz
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RE: Best compression ratio for propane?
Every 60's car ever built would fail an 06 even without starting it lol.
RE: Best compression ratio for propane?
By 60's style- Meaning - No smog pumps, no cat convertor. Stateing this didn't mean running a 40 year carb and low tolerance engine.
And you guys totally missed the point.
If you make a LAW that requires a cat convertor you are saying that there is ONLY one way to accomplish something.
The EPA in the USA has become an industry that gets a specific tweak into law to force the use of a specific patented item for mega bucks.
The point I was trying to make is that Goverment should set standards to achieve not laws on how to achieve them.
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RE: Best compression ratio for propane?
RE: Best compression ratio for propane?
If I invented an engine tomorrow that met or exceeded all requirements it would still be Illegal to sell it because it didn't have a cat or oxy sensors. See the problem? Society sets a goal and yet when you achieve that goal
Millions of dollars worth of state and federal testing are required to profit from doing so. Laws have to be rewritten, that means a couple of years go by while it goes into a research committee of lawyers who will pander for money to allow my idea to be an exception or part of new law.
My end all point that your not getting is by just meeting or exceeding the standard is not enough and it should be.
Anyone want to guess how many millions in legal fees chrysler just went through to put a 4 cylinder diesal in their new Jeeps. Not because meeting the standard was hard but because the law didn't have exceptions for 4 cylinder diesals, EPA laws that were written to PREVENT the sale of 4 cylinder diesals in the 80's(Remember the BMW imports that threatened jobs recovery in the US.)
EPA standards on propane will most like just increase the cost to the consumer to a point that its not profitable to research propane engines. Without profit there is no progress. Enviromentalists will sacrifice all profit without regard to "Good science"
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RE: Best compression ratio for propane?
Nope, wrong again. If you developed an engine tomorrow that would meet upcoming emissions standards and continue to meet them without degradation, you would NOT need an O2 sensor or catalyst. The catalyst is needed to convert the residuals after combustion that came from a fuel that is varying in quality across the country but still within standards, plus an engine that is constantly changing its performance profile, the O2 sensor is there to keep the catalyst happy.
There are NO laws preventing a 4 cylinder diesel, just to ensure that the resulting tailpipe standards are where they need to be. The EPA does NOT get a kickback from any company for doing so, just aint happening.
Legal fees? Hmm, anytime anything is developed and introduced, the legal eagles get involved. Thats just part of the progress. I agree that a diesel engine is needed and sorely missing from the US shores, but blame the public for their perceptions rather than the lack of good products worldwide. The reason more diesels are not for sale is not the manufacturers, its the lack of sales.
As for increasing the cost of fuel due to the EPA? If a finite fuel standard were to be introduced that everyone would use, I only see fuel sales increasing because the engines that use that fuel need a good reliable consistent fuel, more engines, more fuel, happier customers.
The price of fuel today is more governed by OPP (Other Peoples Problems) than the US regulations.
Lastly, we are WAY off topic for this thread, and no real place for a political discussion, I get enough of that at work! Care to redirect this thread back to an engineering discussion?
Franz
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RE: Best compression ratio for propane?
High compression DI LPG numbers aint better than 07 diesel. It is looking so good that I will bet diesel may make it into the indoor forklift market.
RE: Best compression ratio for propane?
RE: Best compression ratio for propane?
Use an long life platinum plugs one or two heat ranges colder than suggested for gasoline and reduce plug gap .05-.08" otherwise coils and other lectronics let smoke out. By using Denso iridium plugs the ignition coils and high tension wire will last a long time because they require less voltage under load than old fuddy duddy plugs.
ht
RE: Best compression ratio for propane?
RE: Best compression ratio for propane?
RE: Best compression ratio for propane?
It will go a full ratio higher than any petrol you can afford to buy and is widely available.
If it pings at 10:1, something else is not optimised.
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RE: Best compression ratio for propane?
RE: Best compression ratio for propane?
I would suspect your spark timing and plug heat range might be optimised for petrol rather than LPG.
To get the most from any fuel, an engine should be built with that one fuel only in mind.
A smaller bore alloy head engine will tolerate more compression than an iron head 454.
Regards
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