LPG with low % propane ignition timing 1

[ale944]

Hi!
I have porsche 944 turbo converted on lpg gaseous injection.
I live in Serbia, Europe, and in my country compositon of lpg is mostly butane (70-90% butane).
Now, I would like to modify a ignition timing map (bosch motronic ml 3.1), to make my engine more efficient/powerfull,
but i found on web many oppose data....
My dilemma is, on higher revs/load, should i advance timing just like on lower revs (3-6 deg) compared to 95 oct gasoline,
or should i maybe even retard it ?
I think i found on this forum that for cng and propane timing should be retarded on higher revs, but what about butane ?

 

[franzh]

Generally, LPG fuels require a little more advance than with petrol at lower engine loading/speeds. If you have access to a rolling road chassis dynomometer, that would be the best way to map the timing, short of removing the engine and performing a perfect ignition map.

At several RPM and road speed points, generate a map of the current timing specifications. Set the timing until there is either a knock detection or a reduction in torque, or no more increase in torque. This is called "torque limited ignition timing". If the engine experiences knock or detonation, the limit is reached, called "knock limited timing".

Remember that this map setting will be for those set points you use and will not be perfect for all conditions.

I have seen as much as 10 degrees crank additional needed at lower speeds, while as much as 3 to 5 degrees reduced at wide open full load, but it will be different on every engine type. What works for you will NOT work for another type of vehicle.

Franz

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[dcasto]

What a wild mix. You know there are two butanes, iso and normal. Between the thre componets the vapor pressure and octanes can vary quite a bit so mapping would be difficult.

Tell us how it woks out. I'm curious.

 

[ale944]

Thanks for the answers.
Unfortunately, I don't have access to any dyno... so I guess I will have to do some experiments based on "web knowledge"
I have LED knock monitor, and to my suprise , there was no sign of knock with lpg on 1 bar boost with ultra lean mixture, car almost stalled, but there was no knock.. while on petrol in similar conditions LED makes a light-show in my car ... so there will be no knock limited timing, just torque.
I have attached my ignition timing map if anyone interested, with worksheet with original (bbr starchip) map and several maps i will make experiment with...

http://files.engineering.com/getfil...584-4806-8817-f709a7df20e7&file=Mape_944T.xls

...one more question: I've seen many so called "LPG timing advance devices", which advance timing for pre-set value.
are they advancing it with this value in all operating regimes (loads, speeds) or they have some "algoryhtm" within them which makes greater advance on lower rpm or something like that?

 

[franzh]

Timing advance devices are intended to boost combustion pressure on low compression ratio engines (low octane petrol). What you find while playing with your air-fuel mixtures is a RICH mixture will induce detonation while a lean mixture will reduce it. This happens with a dry gas fuel since there is no evaporative cooling effect of the fuel as opposed to a liquid fuel engine.

If you couple the digital LED knock sensor with a wide range Lambda meter, you should get a relatively decent knock and air-fuel map. Dont forget that advancing timing until you hear a knock is not always the best timing. At some point, the torque will reach its maximum level due to engine and pressure design limitations.

One thing else, watch your exhaust temps when reducing and advancing timing, they are opposite from a liquid fuel. You may find that reducing timing to eliminate knock will increase exhaust temps and boost pressure faster than leaning the fuel mixture.

Franz

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.