What´s maximum static compression ratio appliable to LPG? 1

[sraposo]

Sirs,

I need to know what is the typical maximum static compression ratio appliable to LPG.
An Diesel-Cycle engine was converted to Cycle Otto to run on LPG and its original static compression ratio (21:1) was maintained. I believe such compression ratio is highly improper and should cause pre-ignition. I´ve unsuccessfully searched for such information.

Regards

 

[RodRico]

PhD theses are wonderful for all the background, context, and references they provide....

https://minerva-access.unimelb.edu....343/38535/305762_MORGANTI file properties.pdf

 

[CWB1]

You're correct, typically gaseous fueled engines have compression ratios similar to gasoline, ~9-13:1. Aside from dual fueled engines using a combination of diesel and gaseous fuels via compression ignition, I'm not aware of any production fueled gaseous engines running above 13:1.

 

[sraposo]

RodRico, thanks A LOT for your pretty useful suggestion!
I´ve already found some scientific material but none of them was so comprehensive like that!

 

[sraposo]

CWB1, it is relatively easy to find mentions of a compression ratio around 11:1 on engines running on LPG, but until now no document was got presenting a maximum (typical, theoretical) compression ratio for LPG.

 

[BrianPetersen]

Evidently, LPG has an octane rating of 100 to 112 RON depending on the actual blend of gases. Figure on 100 to be safe.

Premium gasoline has a RON of 98 or thereabouts. This is comparable to North American 93 - 94 (RON+MON)/2.

So, it will perhaps take a smidge more compression than a premium-fuel gasoline engine, but not a whole lot more.

 

[enginesrus]

And to put things into perspective, 22:1 CR is what a particular diesel engine manufacture used to run their diesel engine as a multifuel engine. For some that don't know what a multifuel diesel is, it is a diesel engine that can effectively burn gasoline. So done correctly it could work with LPG. But not with a carburetor nor spark plug, it has to be direct injection (like the way a liquid fuel is injected, and timed) and I suppose that would be a bit difficult with a gaseous fuel like LPG.
And yes of course to run LPG in a normal gasoline spark type engine the compression ratio would very closely match what the gasoline engine uses.
You may get away with a bit more CR because the higher octane of LPG.

 

[burbfixer]

I asked ths exact question on Raso fuels propane forum.
There is just not alot of guys on forums who have pushed the limits on this, and but there are a few.
My personal experience, mostly with small cam, iron head Big Block Chevy engines and some Ford and Dodge big blocks is 10.25 is a good, safe 'drive it every day & don't worry about it' number. Some said they had gone higher successfully, but needed to be more cautious with timing, coolant temps, air inlet temps, etc. Usually longer duration cams were also in the mix which keeps with low rpm cylinder pressures from being excessive.
I had a 10.25 396 engine and added a paxton supercharger with 6 psi boost. I had to be much more careful with timing, but it ran great for several years like that.

 

[sraposo]

Hi, folks.

I should update the scenario presented on the opening post:
1) static compression ratio is 16:1. Even so, it seems to be quite high for LPG;
2) to worse a (not so) bit more, engine is turbocharged at (intended) around 1 bar;
3) Engine brand is Scania model DS11.
4) this engine is owned by a client located almost 1000km far. Thus, I have to base myself on a mechanical technician´s reports on what is going on.

My company develops and produces special ignition systems and other electronics for engine control and signal processing. Present case refers to a distributorless ignition system that manages ignition timing and distribution on a wasted-spark approach (costumer chose this. There´s also a fully sequential version of the product). This system uses no toothed fly-wheel, requiring only an electrical pulse per crankshaft revolution that should occur at minimum ignition advance angle required. It´s very simple to install, since all complexity was embedded on control algorithm.

This very engine is my client´s first experience on converting a diesel engine on a spark ignition Otto Cycle one, so he is facing his initial amount of problems on such kind of development. Due his reports of engine malfunctioning that have not been so technically detailed as I need, I have been trying to detect (potential) reason(s) to such engine behavior, at least to realize if it´s due to a fail on our product and/or there´s some non-conforming procedure on installation. It seems that too low ignition timing was set, what may explain frequent little blows (and not always so little) on intake and/or exhaust and RPM never goes beyond 1200 or something around. In a couple of hours other checking tasks will be done. I´ll keep you updated.

Anyway, independently of some problem related to our product, the 16:1 compression ratio raises a suspicion that knocking is or going to be an issue.

Regards

 

[enginesrus]

So what triggers your system? There has to be something on the crankshaft or cam to tell where TDC or firing point is?
16:1 Will be too much for some slow speed engine I think. You could verify a problem with a pressure monitoring system.

http://www.tfxengine.com/SensorsOverall.html

 

[sraposo]

enginesrus,

What trigger the system depends on version of product. There´s version that employs a pair vr sensor-magnet and version that uses an inductive proximity sensor to detect either an unique lobe or notch on pulley crankshaft or fly-wheel. Optionally, a disc/gear fixed on camshaft or to an axle 1:1 attached to camshaft can be used, but two events per revolution are necessary.
This pattern applies to an wasted-spark version system. If a fully sequential one, sensor should be mounted on something related to camshaft to detect 1 event per revolution.

Yes, a very high compression ratio would be specially inconvenient on low RPM, but it´s not the case, since this engine is intended do operate at 1800 RPM.

Sure, a pressure monitoring sensor would be useful to properly evaluate cylinder dynamics (at least) during engine conversion development, but this is a costumer´s choice.

 

[BrianPetersen]

Backfiring through the intake or out the exhaust isn't a compression-ratio-related issue. It's an ignition-timing-related issue ... or a someone-mixed-up-the-plug-wires issue, or perhaps an air-fuel-ratio way out of whack issue. Even with wasted-spark, under normal operation, the extra spark happens near the end of the exhaust stroke, and there shouldn't be enough air and fuel mixture in the cylinder to ignite at that time ... unless the intended ignition a revolution before was a misfire because of the air-fuel ratio or the ignition timing being very far from where they should be.

Were any changes made to the engine at all? Spark plugs were installed where the original injectors or glow plugs were installed? If that's the case, it will have maintained the diesel engine's flat cylinder head and bowl-in-piston high-turbulence combustion chamber.

 

[sraposo]

Hi, BrianPetersen. Below, comments on your remarks.

"Backfiring through the intake or out the exhaust isn't a compression-ratio-related issue."

Me > Sure, CR has nothing to do with those blows. CR was mentioned because it is potentially an issue to cause knocking. Since present malfunctioning is preventing engine of raise RPM, even with no load, an eventual knocking has not been perceived and/or has not occurred yet.
Those (apparent) backfires may be due an insufficient ignition timing as you have already commented. It is going to be checked.



"It's an ignition-timing-related issue ... or a someone-mixed-up-the-plug-wires issue, or perhaps an air-fuel-ratio way out of whack issue."

Me > Yes, improper plug wires connections could cause all that mess, but it was already verified.
Erroneous A/F ratio is one of the possible reasons for engine malfunctioning, but if it´s really happening, it seems to be a secondary factor.



"Even with wasted-spark, under normal operation, the extra spark happens near the end of the exhaust stroke, and there shouldn't be enough air and fuel mixture in the cylinder to ignite at that time ... unless the intended ignition a revolution before was a misfire because of the air-fuel ratio or the ignition timing being very far from where they should be."

Me > Yes, wasted-spark is not a intrinsic problem, but can be an issue when a very little ignition advance and/or higher valves overlap. I think that´s not the case, but the huge CR may obligate to employ a (very) little ignition advance, becoming a problem. Let´s wait for results.



"Were any changes made to the engine at all? Spark plugs were installed where the original injectors or glow plugs were installed? If that's the case, it will have maintained the diesel engine's flat cylinder head and bowl-in-piston high-turbulence combustion chamber."

Me > The only changes we have been told refer to replacing diesel injector with spark plugs. Stock pistons, crankshaft, camshaft, valves, CR ans so on were maintained.
My company is basically a ignition system maker, not a consulting provider, so we don´t interfere on costumer´s project, although, depending on situation, we discretely suggest and/or recommend reconsideration on some project specification.

 

[BrianPetersen]

Aye, so they haven't run into pre-ignition or detonation trouble YET.

 

[sraposo]

Video showing engine malfunction.

https://files.engineering.com/downl...6ec-c27d5ebb1922&file=VID-20180817-WA0025.mp4

 

[BrianPetersen]

So it runs, but roughly, sputtering and missing.

Does your ignition system provide enough spark energy with such high compression? High pressure air puts up more resistance to spark than low pressure air.

What's lambda?

 

[sraposo]

Brian,

"Does your ignition system provide enough spark energy with such high compression?"

Me > The model that is being used provides up to 45kV.
I also think that spark plug gap of 0.8mm may be excessive for such compression ratio.

Several A/F ratio were experimented but failure persists.

 

[enginesrus]

If ignition system is doing its job, and fuel distribution and ratio okay, how about the mechanicals, valves etc?
Is it getting proper fuel flow? So it is a carburetor? It would be easier to be there and fool with a few things to figure it out. A compression or leak down test was done? All the simple things need to be checked.

 

[sraposo]

Hi!

I´ve found two papers that reinforce suspicion that 16:1 CR may explain this malfunctioning:

1) "A DIESEL ENGINE CONVERTED INTO OTTO CYCLE ENGINE: THE INFLUENCE OF THE SPARK ADVANCE ON ITS PERFORMANCE AND ON NOX EMISSIONS"

http://servidor.demec.ufpr.br/reterm/ed_ant/20/artigos/252-2012.pdf

Check on page 2, Methodology, "2. The compression ratio, was set at 9.33:1, even though it was noted, during the experiments, that a smaller rate would do better."

2) "Spark Ignition Engine Performance When Fueled with NG, LPG and Gasolin"

https://www.researchgate.net/profil...ance-When-Fueled-with-NG-LPG-and-Gasoline.pdf

Check on page 10, Conclusions, "The study results reveal that the HUCR for the tested gasoline was 8: 1, for LPG, was 10.5: 1, and for NG was 13: 1. The gasoline bp was higher than that of NG and LPG when the engine was run at a compression ratio of 8:1."

Since my focus is on engine malfunctioning, I´m not considering which CR results the best efficiency, but CR values mentioned in above papers (9.33:1 .. 10.5:1) are pretty below 16:1, so such a huge CR can´t be considered a minor factor.

There´s a new information: it was experimented vary ignition timing and, according costumer, no change happened on engine behavior, what makes me think sparks are happening AFTER combustion have already initiated. In this scenario, what else could make fuel combustion begin BEFORE sparks generated by ignition system but an excessive CR?

 

[BrianPetersen]

Pre-ignition. You've invented a HCCI (homogeneous charge compression ignition) engine.

It may be randomly getting compression-ignition and spark-ignition when the compression-ignition misfires.

I still wonder if the spark is actually making it across the gap ... or if there is adequate fresh charge in the chamber to allow the spark to fire anything.

Where is the spark relative to the combustion chamber?

If it is recessed too far back (spark plug too short), it's conceivable that the area around the spark plug is in a quiet area that is seeing too much exhaust from the last charge to fire the next one.

If it is projecting into the main chamber, it is conceivable that the very high turbulence from the diesel-style combustion chamber is blowing out the fire before it really gets going.