Timing map for LPG

[web]

its good to know the mixtures dont have to be perfect though. apparently stoic for LPG is 15.5:1 so i will try and get some numbers off the o2 sensor and see how close i can get it.

There's one catch with that. I don't know what AFR meter you'll be using, but 15.5:1 is the actual mixture by mass, and most AFR meters really measure lambda, then calculate what the mixture by mass on petrol must have been. If you tune for 15.5:1 in a wideband made for petrol only, you'll be running lean!

So when the O2 sensor tells the gauge "we're at stoich" (lambda=1), the meter will display 14.7, assuming you're running on petrol. Using the same gauge on LPG, it will be off. To tune a different fuel, preferrably work with lambda values. Stoich is lambda=1.00 on any fuel. Or tune for 14.7:1, ignoring that the actual mixture by mass on LPG is different from what the gauge is showing. Keep in mind a narrowband O2 sensor is not much more than a "stoich detector" from exhaust gas O2 content - and a wideband is the same, only equipped with a pump cell to measure how far off from stoich you are.

It has no idea what fuel you're running, it only knows how far off from stoich you are. In other words: it knows your lambda value, but not the "by mass" AFR.

Note: some techedge DIY-WBO2 meters can be modified to show the correct numbers on LPG - or simply to show lambda values that will be correct whatever fuel you throw at it.

See http://www.techedge.com.au/vehicle/wbo2/1v5/WB15-um.pdf for a handy table listing lambda values, petrol AFR and LPG AFR.

[web]

this is correct. the Bosch Automotive Handbook (maybe more "fact" than the internet) shows LPG burns 45% slower @ atmospheric pressure and a certain temp. as temp and pressure rise the mixture will burn quicker so that seems to agree with the idea of bringing all the advance in nice and quick cos it needs it most down low.

I'll be looking for that book on ebay, it's hard enough to find solid info on LPG, further confused by the various mixtures all sold by the name "LPG".
So they say at low pressures it burns slower, and at high pressures/temperatures it burns faster... than at low pressure, or faster than petrol too?

To me, this doesn't yet explain much, as both at low and high RPM, the same compression + temperature rise happens in the compression stroke. It happens more times per second, but it doesn't get much hotter or anything like that. If LPG burn accelerated a lot at high pressures, enough to require timing to be retarded, it would definitely need retarding at low RPM as well, because with everything happening slower, peak compression and heat lasts longer at low rpm than it does at high rpm, and the burn would complete way before TDC !

Given that everyone seems to agree it needs advance at least at low rpm, I guess burn doesn't accelerate enough at high pressure/temps to "overtake" the burning of a petrol mixture.

So... unless I'm overlooking something there, I'm still in the "advance everywhere" camp but open to other evidence to the contrary =)

[oldcorollas]

faster compression = higher temp rise no? cos more of the compression energy goes into heat than pressure... etc etc... gas laws blah blah
non-adiabatic compression? i forget the correct terms

[andrew_mx83]

my old man was a consultant writing factory service/workshop manuals etc for a number of the big manufacturers, he has some great references on hand.
i just got handed down the GT-HO supplement for the XW service manual the other day
(edit: web not sure if ur from australia but if u dont know what a GT-HO is google XW falcon and then google XW GT-HO....such a beautiful car)

i did not read the bosch artice myself, he just paraphrased it for me but i would not think that even at higher pressure LPG would burn quicker than petrol but is maybe getting closer?
the general concensus seems to be more advance low down and less up top than petrol....so maybe it does?


either way its running like a pig atm

[oldcorollas]

which burns faster when hot.. liquid or gas?

edit: surely there are a bunch of SAE papers on this?

[web]

faster compression = higher temp rise no? cos more of the compression energy goes into heat than pressure... etc etc... gas laws blah blah
non-adiabatic compression? i forget the correct terms

Adiabatic compression (= without losing heat to the surroundings) always has the same result, irrespective of speed. So the question is... is it adiabatic or not.

At very low rpm, you'd have both slow compression leaks and some cooling (losing heat to piston and walls) to worry about, both making end temperature and pressure lower. That's why compression meter readings improve if you pull ALL plugs to increase cranking rpm. Cranking is definitely non adiabatic, losing both heat (to the walls) and pressure (to leaks).

But increasing rpm to improve compression meter readings works only to a point. So I'd agree pressure and temperature would be higher at 500rpm than at 200rpm, but if we're comparing 5,000 to 2,000... I believe both can be approximated by adiabatic compression quite accurately. Both are too fast to lose substantial heat or pressure, so end temp/pressure for the same charge should differ very little.

[oldcorollas]

increasing RPM to improve compression readings is more to do with the late closing of the intake valve no?
..
re adiabatic... maybe this helps
http://www.lptmc.jussieu.fr/users/lesne/MEMPhys.pdf

edit: not trying to lead you on wrong path here...
the supposed reason they need different advance amounts, is because of differences in burn speed.
petrol is liquid, and even when injected/atomised, it remains mostly as small liquid droplets which burn out to in, and the fuel is poorly mixed thru the air. plus petrol are long molecules..
LPG is gas. it is well mixed with the air, and smaller molecules = burn easier = less advance.

so the question is, why are burn speeds different at low engine speed, not high engine speed.
(edit: or i'm totally off track )

[amichie]

For petrol to go from a liquid to a vpour it needs to absorb energy from its surroundings (latent heat of vapourisation). This is not an instant process. If the process is incomplete it will slow down the burn process and require more advance.

In the case of LPG it is done in the gas converter prior to being drawn into the engine.