Hi,

after getting WB I did some WOT pulls and found very strange think:

-while driving on pump gas, AFRs are fine: 12 at full boost going to 11 by red line. No knock.
-while driving on LPG, I found the AFR being 14 at full boost and holding 14 by redline!!! Very occasional 1 knock count.

I tried to reconfigure the LPG injection to do the same AFR like pump gas, but I was getting 3-4 knock count consistently from full boost to redline.

So I guess leaner is better. But is it possible that different fuel would need to run so much leaner???

 

Stoich for LPG is 15.5:1 rather than 14.7:1 so your wideband is not calibrated for LPG. It's a little richer than what you are reading I think but not much. LPG deff has a higher base octane than petrol but I'm not sure of an actual octane rating.

 

That may be true, but LP's stoich on a normal wideband will still be 14.7

 

How do you regulate the gas pressure? Does it increase with boost?

 

Yes,
all of this is correct, but 14:1 at red line without knock at 18-19 psi still surpiseses me.

 

Vaporizer tries to increase gas pressure with manifold pressure, and when it cannot keep up, it recalculates (increases) LPG IPW according to gas pressure decrease.

 

Well, looks like I have found discussion explaining the issue:

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

 

I dont know about vapour fuels being different to regular fuels, but I doubt much changes...

I would be careful with what your doing - your getting into dangerous territory..

What AFR does LPG produce most power and when does it drop off, what about peak temp etc?

 

Well, I studied a lot today :-) and it seems that using vaporized fuels is much different from liquid ones. According to forums interested in LPG, highest power should be in 13.5:1 and everything richer only produces excessive EGT. During cruise it is recommeded using lean mixture - should produce less heat and better fuel efficiency.

Everything I have read today shows that vaporized LPG behaves in matter of EGT as an opposite to liquid gas.

I have found a chart of earth gas (similar to LPG) and EGT looks like this:

lambda 1 -> 750 Celsius degrees
lambda 0,87 -> 850 Celsius degrees

 

what about flame propagation and stability?

we dont run 11.5:1 on pump for the cooling affect, but more to stabalise the mixture so that it doesn't pre-ignite...

I would imagine the same goes for LPG.

is it 100% LPG or are you mixing with pump?

is it just for cruise or for boost as well?

 

interesting thread. what lpg kit do you have?

 

100% LPG, using www.hlpropan.cz injection/ECU kit. Startin engine on gas, after ECT reaches 20C switches to LPG.

Looks like with LPG there is not pre ignition happening even at leaner mixtures.

Temperatures above are measured on non turbo car under full load.

 

hi,
to my knowledge - LPG needs more advanced timing than compared to pump. Because of longer burning cycle. that's why it creates more temperature when running richer or running on the same timing as pump.

also thats why regular engines with lpg kits, after 30000 miles needs valve service.

 

Well, what I have found, it only needs more timing in lower RPM (below 3000), then it burns even quicker than gas and needs less timing. Can you confirm that?

 

I guess you are right. mostly people install lpg for economical reasons, not performance. so most knowledge is from there.

But that link confirms, what you said.
http://www.eng-tips.com/viewthread.c...=204854&page=1

How is car behaving with lpg?