isuzu & LPG

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Originally Posted by Bush65

Some posts above have asked about rack travel during idling, etc.

I think there might be some misunderstanding of what is happening. The control rack does not move each to a start of injection position. The plungers reciprocate in their barrels (driven by a camshaft) and have a pre-stroke to the position where injection starts.

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I must of been half asleep that day, the question was badly written, I'll try again.

The fuel rack controls the rotation of the plungers in their sleeves and in turn when the spill port in the plunger is exposed, controlling injection volume.

What I was trying to figure out, and have now realised, is the relationship between rack position and injection volume. Initially I was reading the graph (I have the same manual) as RPM vs injection volume, and in that case it makes little sense. Now I have my brain switched on, its reading RPM vs rack position, and from there we can calculate injection volume.

So after looking at a plunger, the ratio of rack position / plunger rotation angle and fuel delivery volume would appear linear, so:

If 9.5mm rack = 8cc / 1000st (approx)
and 11.0mm rack = 78cc / 1000st
so 1.5mm rack = 70cc change in volume.
so 0.02mm rack = 1cc change in volume.
so 9.34mm rack = 0cc / 1000st (approx)

Going by the graph that means that if you increase the idle speed by around 200RPM the governor will cut the fuel completely. Also explains why these engines are so difficult to stall, go below 400RPM (engine) and the fuel rack goes to almost full fuel.

Is this correct?, seems very sensitive to me.

Back to what this has to do with LPG:

In relation to A/F ratio, how does 162:1 air/diesel compare to 0.4% lpg? I'll assume 200:1 air/LPG but thats a guess only.

If the rise in RPM by injecting LPG at 0.4% causes the IP to reduce the diesel by half, that goes to 324:1 air/diesel and still 200:1 LPG, more gas than diesel.

Still don't have a clue what this will cause the engine to do, any ideas?

I'm interested in this stuff but I should really stick with electrics, normally only need ohms law :cool:

Lyle.

oops, double post:angel:

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Originally Posted by Sparkie

In relation to A/F ratio, how does 162:1 air/diesel compare to 0.4% lpg? I'll assume 200:1 air/LPG but thats a guess only.

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We have
4.5 l/min of lpg which is 0.00938 kg/min
1092 l/min of air, which is 1.29 kg/min
Diesel at 150:1 A/F ratio is 0.0086 kg/min
Air/LPG mass ratio = 137:1

LPG/Diesel mass ratio = 1.09:1
At 100:1 A/F ratio it becomes 0.72:1

Do you propose the LPG is more likely go bang with less diesel burning?

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Originally Posted by Dougal

Do you propose the LPG is more likely go bang with less diesel burning?

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As I mentioned previously, after pestering the guy who installed the system on my mates Defa, he indicated the ratio between diesel / LPG was more important than LPG / air. This was about all he was willing / able to tell me.

But it does, in a way, agree with what a few of the proponents of these sytems are saying.

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To understand what is really going on have a look at or look up "propane decomposition under pressure" that will sort of explain all.

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Havent had a look at this myself, but the gist is that the two fuels either mix and burn together, or at least components of them do at the normal low levels of LPG / Diesel.

If the calcs I did for idle diesel / LPG ratio are correct, then maybe at the higher ratio (more LPG than diesel) something goes pear shaped.:eek:

Hopefully someone can explain this. HINT HINT

Lyle.

I'm not going to pretend I understand whats being written in this thread but I do follow that some believe that LPG will detonate before Diesel is injected.
Now this is a laymans question, but the flammability of LPG at atmospheric needs 2.1% to 9.6% air ratio or it wont ignite.
Wouldn't this apply regardless of pressure?
Are the volumes discussed here within those limits?

Quote:

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Originally Posted by Sparkie

avent had a look at this myself, but the gist is that the two fuels either mix and burn together, or at least components of them do at the normal low levels of LPG / Diesel.

If the calcs I did for idle diesel / LPG ratio are correct, then maybe at the higher ratio (more LPG than diesel) something goes pear shaped.:eek:

Hopefully someone can explain this. HINT HINT

Lyle.

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As it seems that every thing I post gets bagged out of hand.
I have tried to explain what is going on in the combustion chamber when LPG is added and that LPG isn’t burning, that all the assumptions or what is generally known about LPG as a fuel is not applicable when used in fumigation.
basically because LPG isn’t LPG for very long. It very quickly ( quickly when over 900 Kevin) reorganizes its self into methane and hydrogen with a few others tossed in for good measure. sheesh you can even get acetylene.

Besides the argument now bores me, I know what I am getting and I am happy. There is just to much misinformation posted and some very bad assumptions, for what reason? I don’t know! Maybe just Ego? the net effect is that this thread is of no value to anyone who wishes to make an informed decision based on other members experiences since those with no experience seem to have the loudest voice.

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Originally Posted by DeeJay

I'm not going to pretend I understand whats being written in this thread but I do follow that some believe that LPG will detonate before Diesel is injected.
Now this is a laymans question, but the flammability of LPG at atmospheric needs 2.1% to 9.6% air ratio or it wont ignite.
Wouldn't this apply regardless of pressure?
Are the volumes discussed here within those limits?

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correct yes yes

What happens is that the LPG brakes down into other flammable gasses that will assist the diesel to burn more completely. In this thread along we have one poster stating the diesel is completely burnt in the combustion process >90% and we have an attachment to an article stating that <>75% is about the norm. In short the unburned fuel ( is wasted). is not burned because its to 1. To cold, 2 to larger droplets to be burnt though, 3 is far from the flame front.
The addition of LPG 1. Assists the diesel to ignite at a colder temperature, 2. has less "solids" so assists in moving burnt matter away from large ( relatively speaking) droplets of diesel allowing the burn to consume more if not all of the droplet. 3, speeds up the flame front so that diesel frutherest from the ignition point has more time to burn.

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Originally Posted by clean32

As it seems that every thing I post gets bagged out of hand.
I have tried to explain what is going on in the combustion chamber when LPG is added and that LPG isn’t burning, that all the assumptions or what is generally known about LPG as a fuel is not applicable when used in fumigation.
basically because LPG isn’t LPG for very long. It very quickly ( quickly when over 900 Kevin) reorganizes its self into methane and hydrogen with a few others tossed in for good measure. sheesh you can even get acetylene.

Besides the argument now bores me, I know what I am getting and I am happy. There is just to much misinformation posted and some very bad assumptions, for what reason? I don’t know! Maybe just Ego? the net effect is that this thread is of no value to anyone who wishes to make an informed decision based on other members experiences since those with no experience seem to have the loudest voice.

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I know this thread got a bit personal for a while, hopefully that is over and we can get back to the science.

If I personally have insulted you then I apologise, this was never my intention.

As you suggest, there seems to be a lot of misunderstanding of whats going on inside an engine fitted with fumigation, you seem to have a good handle on this and I hope you will explain it to me, as although I dont have it fitted to my vehicle, and may never do so, I would like to understand how it works, as do a few others if I'm not mistaken.

Many people have these systems fitted and seem quite happy with them, my mates 300tdi Defa being the one I have been using as a reference, and it done 50,000km with no detremental effects we can detect. (see earlier post for details)

So in reference to some of the recent questions:

Does the ratio of LPG / Diesel affect combustion? and would having more LPG than diesel, as Dougals and Bee_Uteys tests at idle indicate, cause a problem.

At what percentage would adding LPG cause compression ignition if it can?

Does it significantly speed the rate of ignition?

Do you have the results of the oil analyses back?

Lyle.

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Originally Posted by Sparkie

So in reference to some of the recent questions:

Does the ratio of LPG / Diesel affect combustion? and would having more LPG than diesel, as Dougals test and Bee_Utey(I think) test at idle indicate, cause a problem. .

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Defiantly but then you have left the world of fumigation and entered the world of duel fuel.

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Originally Posted by Sparkie

At what percentage would adding LPG cause compression ignition if it can? .

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yes it can, temperature ignition is no mystery see DeeJay's post, he’s correct there are a few other variables but that’s the guts of it.

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Originally Posted by Sparkie

Does it significantly speed the rate of ignition? .

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yes and no, air/diesel/ compression /temperature /turbulence are all variables.

or should I ask what do you call significant?


About 1deg depending. i should say 1 deg of timeing for example

Focus on the complete burn and increased midrange rather than over all KW ( leave that to the domain of the forced induction) chasing detonation is just barking up the wrong tree.


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Originally Posted by DeeJay

I'm not going to pretend I understand whats being written in this thread but I do follow that some believe that LPG will detonate before Diesel is injected.
Now this is a laymans question, but the flammability of LPG at atmospheric needs 2.1% to 9.6% air ratio or it wont ignite.
Wouldn't this apply regardless of pressure?
Are the volumes discussed here within those limits?

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Hi Deejay.

No the volumes that detonation is being experienced with are not within those values. The LEL (lower explosive limit) and UEL (upper explosive limit) that you've posted are for igniting a cold gas with a hot spark.
This is a very different situation to heating and squeezing a gas mixture until it self-ignites.

I've got detonation at idle at 0.4% by volume (0.7% by mass) fumigation, this is under 1/5th of the LEL.