LPG and ignition 1

[treetotreef]

I've purchased a nice new toy with a 4.2 V8, 11.6:1 compression and Prins LPG kit fitted.

As i understand it, these LPG kits take signals from the standard injectors and use that to trigger the lpg injectors (which are controlled from the fuelling maps in the LPG ECU), but don't modify the ignition maps.So even if the Standard ECU has self learning capibility, it's altering the wrong map...

In the interest of fuel consumption rather than max power output, how much scope is there for improvement with ignition advance(assuming that pump LPG is ~110RON)?
Please feel free to correct me if my understanding is wrong...!

 

[Lou Scannon]

My advice is try to find a website for users of this system, to find out more about the algorithms and methods of changing calibrations.

 

[romke]

i doubt whether you may assume LPG to have a RON of about 110. LPG has no fixed composition, the amount of propane and butane in the mix can vary quite considerably from supplier to supplier and with the season and so will the RON. Also, although the RON is usually higher then with gasoline, so is the sensitivity (RON-MON). That means you might not be able to exploit the higher RON to the full because now the MON becomes the limiting factor.

First make sure you get the mixture right - a lean mixture in combination with advanced timing and higher loads may well cause thermal problems with very unpleasant results.

 

[Lou Scannon]

romke, can you define "a lean mixture"?

 

[j79guy]

treetotreef; I have a similar system on my 2003 GMC 4.3L V6, and it is working well. (Technocarb ESIP system.) To address the same question you presented, I purchased HP Tuners VCM Suite, to allow me to alter the ignition map(s), which I have done. Essentially, I added significantly more ignition lead off idle through to maximum operating RPM. (5300)
I kept a close watch on the knock count during the process of sequentially bumping up the timing lead, to make sure I was not running into detonation. Net result, the engine revs cleaner to shift point, and pulls a load easier. I have an accelerometer based performance indicator (Vericom) as well, which backs up my "seat of the pants". The OEM timing curves tend to be conservative, assuming you will be pulling a loaded trailer through Death Valley, with poor gasoline. There certainly is room for improvement over these timing curves, especially, if the quality of propane available to you has consistant quality. Where I'm at, the butanes/pentanes/heptanes ect. are stripped, therefore the quality of the propane is pretty consistant.

romke; The system treetotreef is referring to uses the OEM fuel system sensors, including keeping the system at commanded A/F ratio. (It simply piggybacks the LPG injectors on the petrol injectors, by interuppting their drive pulse to drive the LPG injectors) It will not "go lean" unless there is a system failure, and even then, there is an automatic switch back to petrol if the vapouriser outlet pressure drops to .6 Bar, to help preclude a "lean out".

j79guy

 

[dcasto]

LPG has set standards of composition. Butanes must be less than 1.5% and ethane in the mix can vary from 4% to 6%. In reality, the iso butane is about 1% because iso C$ sells for more, so we don't want it LPG. The ethane is right at 5% because ethane sells for less than LPG.

NOW the wild card, propylene. The standard (look up HD-5) the 5 in the standard means no more than 5% propylene. If the LPG comes from a refinery you'll find 2% to 4% propylene. If the LPG comes from natural, no propylene. In California, they wanted to allow 10% propylene, but I believe we shut that down.

Propylene will cause problems, its high octane, but it doesn't burn completely and you end up with cabon deposites.

 

[franzh]

I've tested lots of distribution grade propane, both commercial and HD-5, and almost no two lots were the same. I have seen commercial grade propane at 98% propane, 1% propylene and the balance N&I butanes, followed by what was advertised at HD-5, at 82% propane, 4% ethane, 6% propylene, and the balance a combination of butanes, CO2, and traces, and no, the lots were not mixed (different locations, different times of the year). I have seen propane as low as 70% and as high as 98%, all from locations selling commercial grade or HD-5 grade fuel.

In the US, there are no legal standards, HD-5 is a voluntary rating, called a "Spec Grade" fuel, although there are no enforcement processes.

This has been one of the biggest problems with Propane as a motorfuel, there are no consistencies. The Gas Processor Association and the American Petroleum Institute may set standards, but its up to the buyer to buy a grade of fuel that is economically saleable, and the seller to sell fuel that is at the minimum requirement of the buyer.

Back to the OP, the Prins system does not alter timing, instead choosing to rely on the vehicles controls to alter timing and using a knock sensor (if equipped). Emissions with timing advancement is questionable. Depending on the vehicle, emissions may or may not be improved. Generally, retarded ignition will lower HC's, keep CO the same, lower Nox, and a hotter exhaust.

Franz

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[patprimmer]

Are the LPG standards you guys are quoting world wide or USA.

The OP does not say where he is from. That normally means he is from the USA (don't ask me why), but he uses the term LPG rather than propane which means he is quite likely outside the USA.

My info is well out of date and dependant on memory, but as far as I know, USA has a gas fuel that is very high in propane and well over 100 octane regularly.

The rest of the world has a mixture with quite a bit of butane and it can only be depended upon to be over 100 octane, but by how much varies widely.

Regards

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[romke]

the LPG i am referring to is the mix you can get in europe. depending on the country you buy it, the mix varies between 60/40 butane/propane and 40/60 butane/propane depending on climate and season and sometimes "gas" is sold that is nearly 100% propane. there are no hard standards and you may occasionally get a mixture outside this ranges. also other components (unsaturates) are not very well standardized and therefore fuel quality may varie quite a bit.

if you have a supply with "consistent" quality, of course you can optimize the engine timing quite a bit better then when you have to allow for occasional changes in fuel composition

 

[treetotreef]

I'm based in the UK, and my LPG supply will be based on both the roadside forecourt (BP/Total etc.) and a wholesale distributer close to work.

Thanks for the advice, it sounds like i need to investigate grade consistency from these suppliers before anything else, and this isn't something i had thought about. with reference to butane/propane percentage mix, which gas is the more favourable to have as a higher percentage?

Thanks also to j79guy, thats exactly what i wanted to hear but i'm trying to be careful and investigate all the avenues as i'm pretty new to gas as a fuel. i assume there is a knock limit as well as a torque limit to LPG, just like petrol, and because its a gas, lean/rich mixtures work in different ways. I'm also curious about spark and flame travel. has anyone here experimented with different spark plugs and found any positives/negatives?

 

[patprimmer]

Propane is the high octane one.

Regards

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[dcasto]

The EPA references the ASTM standards for testing LPG and then the commercial contracts set what values are acceptable. HD-5 and commercial propane indirectly have the ethane composition set by ASTM D-1267 and the butane set by ASTM D1837 or D2163, so thats where the slight variations can come.

The EPA has approached PERC and GPA to use their standards, but no one can find a way or why it would be benificial to set one standard with all the contracts already in place. The EPA has tried to test variability of LPG and they can't seen to find anyway to set standards like they have on motor fuels, sulphur excluded.

GPA is publishing some fractionation or what I'd call pipeline interchange standards so the industry.

Compliance is voluntary, but there are contractual obligations. If you get 70% C3, then sue over contract quality, do not rely on the government. The government doesn't regulate quality, just the outcome of low quality, air pollution. The government doesn't say gasoline must contain 50% this or that, they never will.

NOW, LPG in the turms of C3/C4 mix, its wide open. The commercial specs are based on vapour pressure for safety or equipment reasons. There is no way to get consistant quality, but a peak at the vapour or real presure can tell you the composition and you could adjust your engine.

 

[franzh]

We had one engine study where we were having durability problems. A GC-FIC of the fuel showed 73% Propane, around 15% N&I Butane and a surprising amount of ethane (we think to counter the pressure drop of butane, since the overall 205 psig at 100 F was right on target.) The fuel supplier could not explain the difference but the replacement load was right on target for HD-5.

One problem with HD-5 is that there may be up to 5% of non defined gasses. As long as the vapor pressure is at spec, and the copper strip corrosion test is negative there is relatively little that can be done to enforce the standard.

Franz

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[dcasto]

franzh, the 5% in in HD 5 is no more than 5% propylene, not just anything. Propylene does not make good engine fuel.

 

[j79guy]

treetotreef; Pretty basic advice, I'm sure more experienced fellows here will chime in; In "general", I gap the sparking plugs down to .028". Propane under compression does not ionise as readily as a petrol mixture, thus gapping the plugs down tends to help with ignition misfire. Also, your ignition system as a whole must be kept in top shape, to preclude misfire. You are correct, "lean" air fuel mixtures tend to simply reduce power output, whereas "rich" mixtures tend to make the engine more pronounced to knock. With your PRINS system, the commanded A/F ratio from the vehicle VCM will keep the injected propane at correct ratios, so long as the system was mapped out correctly by the installer. Ideally, this should be checked with a wide band O2 sensor, to verify that the installer did his job correctly, or, if you have access to a good hand held OBD-II scanner that indicates your Long Term/Short Term fuel trims, you can verify if the VCM is "happy" with the current propane injection rates. (LTFT/STFT averaged out should not be more than +/- 6%
There is a whole world of tuning options available to you, should you choose to delve into an aftermarket tuning program. I suggest combing the various fora on the internet for each tuning company, and see which one offers the features you would like to play with, and their level of customer support. You did not mention what manufacturer/model of vehicle you have. Currently I can only offer my past experience with HP Tuners, which originate out of OZ, and for the Holden/GM engines. They are slowly branching out to other marques.
As far as sparking plugs go, you will get all manner of suggestions here. However, over the past 20 years, I've used colder plugs, hotter plugs, ect., all with minimal, or negligible results. Now, I just stick with the OEM recommended plugs and be done with it. Again "in general" the flame front speed of propane mixtures tends to be somewhat slower that a corresponding petrol mixture. Thus, engines tend to be a bit happier with more initial, and low RPM ignition lead. However, once the RPM climbs, the lack of propane latent heat of vapourisation, as opposed to petrol, tends to favour less total timing. Of course, there are many other variable at play here, which all contribute to optimum ignition lead.

j79guy

 

[franzh]

DCasto;
For HD-5, as you mentioned, the -5 means that there is a maximum of 5% propylene. If you read the spec, it also gives a percentage of other products which works out to a minumum of 90% propane. If you take the worst case scenario (which we seem to have here in Texas), 90% propane with 5% propylene, that gives up 5% of unknown gases.

Toss in the minimum vapor pressure requirement and the ASTM copper strip corrosion test, and that leaves a pretty well non-defined total product. Pity, since the original spec for HD-5 came at the request of the transportation industry to standardize motorfuel grade propane. Its hard to dictate what comes out of the tailpipe if one cannot dictate what comes in the front end!

At a recent query, about 15% of the total LPG manufactured in the US made its way to the local distribution for heating and motorfuel use, and only about 15% of that actually ends up in motorfuel applications. We just dont carry a big enough stick to require a more stringent grade of fuel for OTR.

Franz

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[dcasto]

Franz, but we know what the unknown gases are, they are either ethane or iso/normal butane. They can't be methane because a .25% methane raises the vapour pressure 10 psi. Pentanes in the mix lower the RVP by 5 psi per 1%

Mont Belvieu Eth Pro Norm Iso Pen+
- July 1, '08 138.18 188.84 227.68 228.68 300.52

Look at economics propane is $1.8884/gallon, why would they put $2.28/gallon butanes and they would maximize ethane. All with in limits of 208 RVP.

On problem is stratification in tanks. Over time the ethane will be on top and the butanes on the bottom of a stationary tank, that is more of a problem.

Finally, if you want better quality, you can buy refrigeration grade propane, its at least 97.5% propane the 2.5% can only be C2, C3=, and iC4.

I voted no to the HD-10 initive in California because I understand what propylene does to motor fuels.

 

[franzh]

One issue in the HD-10 proposal was to allow the additional percentage of Butane and Propylene which was outlined in the Cummins study. Cummins did not find any significant problems when running on HD-10. Some of the other components we found were N2, O2, SO2, water (we surmise that this may be mostly from the tanks being open for a while and not properly purged) and other C-6's and higher.

Stratification is indeed a problem on stationary tanks, thank heaven for heat convection, at least it keeps it somewhat in circulation.

For one engine study, we purchased 1000 gallons refrigerant grade propane, and it cost close to $8.00 gallon, but it was certified at 99.97% pure with no cross contamination in dedicated transports.

Your comments are right on, thanks for your input.

Franz

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[Teza37]

Hi, This is my first post here. In Aust we don't seem to have any regulations regarding mixtures, butane is usually in a higher percentage than propane although around the top end of Queensland it's all propane as only 1 truck is sent to fill heating gas tanks and autogas tanks and heating gas must be propane.
When setting vehicles up to run on LPG only on the dyno initial advance was between 12 - 15 deg, 22 - 25 deg by 2,000rpm and a max of 26 deg by 2,500deg.
Air fuel was 15.1 : 1 any richer lost power, any leaner caused increased exhaust emissions and heat eventually leading to engine overheating.
If we bought propane in the large household gas cylinders the upper timing marks were the more common but for what was available from the autogas bowser it was generally in the lower ranges.
On dual fuel vehicles I used the AEB timing reprocessors, a number of diferent ones are available but the translation of operating instructions leaves something to be desired.
I did some experimenting with liquid injection at the ports using Haltech after market computers but it was outlawed by the authorities so I gave up on that idea.
Don't know if this helps any.
Terry