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

 

[Lou Scannon]

If you're suggesting that the engine has been running in pre-ignition the whole time, that's possible, but pre-ignition is such a violent and unstable process that I doubt the customer's engine could take it as a steady diet. Furthermore, the initiation of pre-ignition tends not to occur at a consistent crank angle, but rather, the "timing" so to speak, will jump around, resulting in unsteady torque and other parameters.
Typically, if we talk about a transition from normal spark-ignited combustion to pre-ignition, while holding external conditions steady, pre-ignition will come and go, at first, then, due to heat build up in the combustion chamber resulting from pre-ignition, it will become persistent, and will tend to occur at progressively more advanced crank angles, until the fuel is cut or the engine is destroyed.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[sraposo]

Brian,

LOL! No, I've NOT invented a HCCI!
So, if changing (a lot) spark ignition timing makes no difference on engine behavior, it´s not (totally) wrong to think that something else is igniting fuel.

YES! I still consider that spark plug gap may be exceeding ignition system output capability. In about an hour it will be known if this gap is really improper.

I´ll check plug position relatively to chamber.

 

[sraposo]

Hemi,

Yes, your considerations on a not chaotic engine operation under pre-ignition due over compression are sensible, but the fact that altering spark ignition timing made no change on engine behavior raises a suspicion that something is starting ignition before sparks. Do you also think so?

Maybe I haven´t made it clear that engine is working for the first time after being converted to Otto Cycle and only functioned for a couple of minutes each time, while is being analyzed what is causing the problem.

 

[sraposo]

Brian,

I´ve just received information that spark plug nose is 85mm far from chamber surface. Plug was threaded in a tube and its opposite extremity is screwed on engine head:

https://files.engineering.com/downl...ção_joel_-_Screenshot_2018-08-19-11-41-37.png

Fresh air-fuel mixture has to run into this 85mm tube and burnt gases has to run to same length back...

I´ve never seen such a way of mounting a spark plug, but my costumer said that it is a solution employed in other gas-powered commercial engines.

 

[BrianPetersen]

Well, okay then.

After a successful firing event, that tube will have exhaust in it, which will pretty much stay put through the exhaust and intake strokes due to lack of exposure to charge motion in the main part of the cylinder, then on the next compression stroke the exhaust residual in that tube will be pushed to the end where the spark plug is. Obviously there will be some mixing with charge during this compression process, but the tendency will to be to have a lot of residual exhaust in that tube squeezed to the end where the spark plug is. So maybe on that next stroke there's enough fresh mixture to fire, maybe not. High probability of a misfire. Then on the one after that, probably enough mixing happens that it will probably fire on the one after that - then the cycle repeats.

If combustion happens, it will take a significant (and probably variable, depending on exhaust residual) number of crank degrees to make its way down that tube (against the travel of further gases being pushed in). That means at best, it's going to want a lot of ignition advance. But a lot of ignition advance means there will be even less chance for fresh mixture to get into that tube far enough to be exposed to the spark at the time that the spark happens (well before TDC, well before the point of maximum compression).

Is that tube exposed to coolant jackets on the outside? I tend to suspect not; the injector body wouldn't have been. If by some miracle someone figures out how to make this fire regularly, that tube will get darn hot ... and could very well start operating as a glow plug.

Spark plugs can indeed be mounted inside a tube ... but facing the other way (spark end out, ignition coil end in) and with a long spark plug cap running the length of the tube ... or these days, a coil-on-plug assembly mounted inside that tube.

 

[sraposo]

Hi, Brian.

You´ve taken your time to detail the probable gases dynamics in the pipe and I agree with you. That´s what I´ve thought when I was told about this weird plug mounting.

Probably, the tubes were not thermally considered and I don´t know what metal was employed. So they are exposed to a very high temperature and huge pressure. Would some plug be expelled like a bullet someday?

Yes, a plug could be mounted inside a tube, since electrodes end were aligned with chamber surface, exactly the opposite of what was done.

Costumer told me he has already started the reasonable/necessary modifications: reduce CR to something around 10:1 and mount spark plugs on head, making electrodes end leveled with chamber.

 

[sraposo]

Sirs,

My original intention was to know the practical maximum CR for LPG because of what seemed to me something related to pre-detonation. At that time, costumer suspect that ignition system was mistaking ignition order and/or varying erroneously ignition timing, what was proven not to be true afterwards.

Maximum CR seems to be an information that little people have...

Indeed, as a ignition system provider, these mechanical issues are not related to my company, but usually some informal support are required by our costumers and the possible help, in a limited and responsible way, is provided, specially if something direct or indirectly related to ignition.

But, unexpected and kindly you all have engaged on this topic to help solving problems on a converted engine that presents misconceptions, not all known at the time this post was created. Thanks a lot to you all!

Changes are still being done on engine and maybe today there will be some news. I´ll update you on this.

 

[NickJ67]

Interesting thread.
Seems like the mechanical aspects of the "conversion" haven't been very thoroughly thought out!

The only diesel engines I've come across running lpg without significant mechanical modification (usually different pistons to lower the CR and loose the swirl bowl), have been in a dual-fuel context where the diesel injection system and some diesel are used to time and ignite combustion.

The engine survival here is due to (very) short run times and that it is running unloaded and presumably heavily throttled, so dynamic compression is lower. Open the throttle and add boost and I reckon things will go bad very quickly.

Optimum compression ratio will depend a number of things and one of those will be the combustion chamber dynamics. The current pistons being designed for CI and fuel injected into the bowl is unlikely to help. Taking the (probably) less than ideal combustion chamber design and desire to run with boost, I would think CR needs to be well under 10:1.

The spark plug placement is just amazing and probably explains alot of the erratic running due to frequent misfires and/or very slow combustion in the tubes that seriously alters the timing that the main chamber actually sees. May actually be helping the engine survive in this context!

As regards CI providing at least some of the ignition events, how easily does the engine start? Might expect it to put some serious load on the starter due to wanting to almost run backwards.

Pretty clear the OPs ignition system is not to blame. Would be very curious to know what is done to get this working properly in the end.

Nick

 

[BrianPetersen]

The compression ratio of that engine (as-is) may be appropriate for natural gas (methane-ethane), but not LPG (propane-butane).

 

[sraposo]

Hi, NickJ67.

Well, this thread became interesting in a unorthodox way... it was only intended to discover (not yet achieved) what´s the practical maximum CR for LPG, but due several surprising discoveries not all told here and not always fully related, it turned into a thrilling story.

One more unveiled fact is that CR is indeed 21:1, as stated in first post, but this time this information comes from measurement of head concavity volume, piston concavity volume and head gasket thickness and basic calculation.

So, if spark plugs were not mounted on those so long pipes under so huge compression, maybe this engine would be severely damaged now... and also fortunately turbocharger haven´t boosted intake air because all this mess prevented engine RPM from rising.

Fortunately realized that some modifications were vital to make engine work and do that in a safer way: CR was cut down to 12,8:1 (why this value, I don´t know) and head was prepared to make spark plugs electrodes end be leveled to chamber surface. No spark plug pipes anymore!

It´s expected engine cranks tomorrow afternoon.

 

[BrianPetersen]

^ That was achieved in a hurry given the extent of changes necessary to reduce compression ratio.

For someone (like me) more familiar with motorcycle engines than utility power generation ... a suspicious hurry.

 

[Lou Scannon]

Engine development by trial and error is a pretty strange and futile activity in this day and age. No aspersions to the OP; you have gone above and beyond with support that an engine developer worthy of the name should never need from a supplier.
I've been down that road before and my take is what the project needs is a system integrator that is qualified to develop a spark-ignited heavy-duty engine, from top to bottom. I can think of a long laundry list of development and validation topics that need to be covered before the engine would be ready for sale to an end customer, but I don't see the point of putting it here. The fact that your customer is struggling to just make the engine run makes me shudder.
Let me close with my list of the 6 phases of a project:
1. Excitement; euphoria
2. Disenchantment
3. Despair
4. Search for the guilty
5. Punishment of the innocent
6. Distinction for the uninvolved


"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[enginesrus]

In the video the way it ran didn't sound like it was complaining about the compression ratio. The pop or after fire was the problem, could have been the long spark plug tubes misfire from dilution like someone mentioned. Or a fueling problem.
Even assembly error mechanical problem. I hope you come back here with a lot of good details about what was discovered and what was done.

 

[gruntguru]

I have worked on a somewhat similar project. A few comments.
- You need high percentage propane LPG (HD5). Butane and propylene in the gas blend reduces octane number.
- The turbo boost pressure will probably need to be reduced. The original diesel engine probably runs 50% or more excess air so when you richen it up to stoichiometric or richer for propane it will make too much power, heat, combustion pressure etc.
- It will need throttling to get correct part-load mixture. Hopefully the intake valve stems have adequate seals.
- Doing this properly would involve custom pistons to produce correct CR, combustion chamber shape, squish etc.

je suis charlie

 

[sraposo]

BrianPetersen,

Costumer owns equipment for machining parts. They reduced CR by removing material from piston top,measuring new concavity volume and calculating PMS/PMI volumes ratio. It took a couple of hours.

 

[sraposo]

hemi,

Good engineering is for (preferably) good engineers with proper resources (money, time and knowledge of target application). When at least one of these factor is missing, good engineering is not the expected result.

It´s reasonable that a supplier, specially if a maker, provides technical support not only its product but also on application issues related to the product. But when support on other subjects are needed, dark clouds appear on the horizon and it costs nothing to innocents being blamed, as you wisely said, and those innocents have to prove they are not guilty.
Dealing with costumers, or more properly, dealing with people, exposes you to non-fully-reasonable interactions, then it´s necessary to approach as flexible and pro-active as you can to contribute for an harmonious relationship.

 

[sraposo]

enginesrus,

At the time that video was shot, the only known (potencial) issue was (apparently, but probable) excessive CR. No comments about spark plug pipes, too narrow spark plugs electrodes gap, improper fuel controlling had been told.
Ignition system was the first to be put under suspicion, since costumer believed that his engine was properly modified and mounted. As time passed, surprising facts better explained why engine was working that way, but the question that motivated this thread has remained (clearly) unanswered, posing as a potential issue.

 

[sraposo]

gruntguru,

Although I´m not graduated in Mechanics (I´m MSc in Electronics and BSc in Computer Science, but a long time developing products for ICE), I clearly see your comments. A good product is not a twist of fate. There´s a difference, maybe huge, between an engine working as efficiently as possible and an engine working in some way. So, this engine, although corrections already done and those being made, should run acceptably, not as good as possible, since it lacks refinements.

 

[enginesrus]

sraposo you said it will run today ! Any news?

 

[sraposo]

enginesrus,

Not yet. At present moment engine is being rebuilt and it´s expected that it will work tomorrow morning.
CR now is 11:1, which will not cause problem with natural aspiration, since there are aspired engines that run on 93 octane gasoline with CR 13:1. As LPG has 110..112 octane, engine will work ok and probably will admit some turbocharging.
Don´t miss next thrilling episodes of this season!