LPG tune/fuel mixtures in turbo application, any suggestions?

[ONDGAS]

ok i know its a bit broad but maybe some of you would have a opinion.
im running a turbocharged 6cyl engine on LPG, compression is at 8.2:1 and fuel mixtures are between 10.8 to 11.3 under full boost which is at 18-20 psi.

im in australia so the engine in question is a 250ci (4.1 ltr) pushrod 6cyl, with a cast piston and standard conrods. i used to run the same setup in a daily driver at 16psi(and 20psi at the track frequently) and a front mount air to air intercooler and never had any issues but now with the boost set at 20psi and a water to air intercooler i have bent a rod. timing is reasnobly safe and now detonation signs on sparkplug or cast piston but the big end bearing showns signs of slight detonation.

to cut a extremly long story short, do you guys think that the fact that LPG enters the engine in a reasnobly "hot" state compared to petrol that it would be causing high combustion chamber temps that would therefore lead to detonation? if timing is right and air fuel ratio is safe, where is my detonation coming from?

 

[Alchemi]

LPG's knock resistance goes right down with temprature, perhaps some water injection after the turbo??

 

[franzh]

Its running too rich. A vapor fuel does not have the evaporative charge cooling effect of a liquid fuel. Try leaning the mixture and see how that works with detonation. This is a very common issue when people assume an engine has to run rich when operating on vapor fuels.

Franz

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[Lou Scannon]

Of course, leaning it out (i.e. going leaner than stoich) will reduce power proportionately. However, in principle, the power reduction can be compensated by increasing boost pressure. I say in principle, because turbomachinery and intercooling sized for a stoich application may not be up to the task for a lean application, depending on how far lean you need to take it. So pulling off a successful lean calibration may require a new, larger turbo, intercooler, etc.

 

[franzh]

Thats true but by this posters comment at 10.8 to mid 11's AF ratio, there is too much fuel, not enought air. Running it closer to stoich is not lean running, its pulling the chemical balance of combustion closer to its ideal area.

Running an engine at this rich AF ratio deprives the engine of air by displacement.

Otherwise, your comments about lean running are right on. Given the choice, I would rather run lean burn than rich burn.

Franz

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[Lou Scannon]

Franz,

I'm skeptical that running richer than stoich moves you toward detonation (i.e. on a vapor fuel). What is the basis for your statements to that effect?

 

[PLeon]

Running lean will overheat your piston and rod - it does bent rods. It's not easy to tell even looking at the piston.

 

[PLeon]

If you're sure that cylinder is not running lean, you are experiencing local overheating. The water doesn't flow fast enough at some narrow point and it's boiling creating steam pockets. This makes local overheating worse and causes detonation. It's hard to troubleshoot.

You may run straight antifreeze if your cooling system has the extra capacity. It doesn't boil and you won't get steam pockets.

 

[Lou Scannon]

PLeon, are you speaking of liquid fuels (gasoline, alcohol) or vapor fuels (LPG, natural gas)?

 

[franzh]

Hemi:
I have spent more hours (days, weeks) in an engine dyno lab than I care to count, and one thing we watched more than anything else was the AF ratio in direct correlation to exhaust temp. When running an engine in rich mode on a vapor fuel, there is no evaporative fuel cooling (we have discussed this on this forum multiple times). Up to the point of rich misfire, which I feel this engine is dangerously close to, adding fuel will increase the combustion temps.

One thing to remember, that reducing ignition timing on a turbocharged engine may NOT reduce detonation since delayed ignition timing results in increased exhaust temps and increased turbo pressures, and this too can cause increased detonation. It is very easy to reduce detonation by quickly leaning the fuel mixture.

Many people will mistakenly richen the fuel mixture on a vaporfuel engine trying to reduce detonation and pre-ignition while the opposite is true. Remember, a vapor fuel behaves VERY differently under boost than a liquid fuel. Conventional theory with liquid fuels does NOT apply with vapor fuels. More than one engine has been lost on the dyno.

Franz

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Lou Scannon]

I'm following you when you're on the lean side & moving toward stoich.
Can you explain the mechanism when already rich of stoich, how going richer increases combustion temps? Where is the additional energy release coming from?

 

[turbocohen]

Would you guys do yourselves a favor and read Franzh posts. He speaks from experience. Optimize timing and stoichiometric AF for durability and power. End of thread.