Thread: isuzu & LPG

Originally Posted by 1103.9TDI

(snip) We're after as much factual information from as many sources as possible (snip).

Cant agree more, so i'll ask the question again:

At what point do we go from "fumigation" to "dual fuel". This does seem to play a large part in what this is all about.

Are we talking more than say 2% air/LPG or more than 25% LPG/Diesel and at what ratio would any form of preignition / detonation become possible.

I have already guesstimated aprox 150:1 air/diesel at idle and would this have a bearing if fumigated at 0.5% LPG/air (nearly 1:1 LPG/diesel)

This is starting to keep me awake at night trying to nut it out. Should have payed more attention in chemistry.

Lyle.

But that’s the point, I agree with Bush65 post. I don’t agree when Dougal posted that

"The Ricardo Comet is a type of swirl chamber which fits in the head of some indirect injection diesels.
It is not comparable in form or function to the piston combustion chamber in a direct injection diesel. "

they both are the same animal, Bush65 has a much better way of explaining it.

Originally Posted by Sparkie

Cant agree more, so i'll ask the question again:

At what point do we go from "fumigation" to "dual fuel". This does seem to play a large part in what this is all about.

Are we talking more than say 2% air/LPG or more than 25% LPG/Diesel and at what ratio would any form of preignition / detonation become possible.

I have already guesstimated aprox 150:1 air/diesel at idle and would this have a bearing if fumigated at 0.5% LPG/air (nearly 1:1 LPG/diesel)

This is starting to keep me awake at night trying to nut it out. Should have payed more attention in chemistry.

Lyle.

well i don’t have any sources just my experience.

"Fumigation" to "dual fuel". at 7 liters /100 klm i had great power and was backing off the diesel quite a bit to keep it smokeless, just over the 7 ltr point the CT temperature started to rise so I started to back down at that point. at a guess I would say that at the 6ltrs per 100 the LPG was replacing diesel as a fuel.

Pre ignition , forget about it, if a diesel motor running only LPG but still needs diesel injected to start ignition. I doubt that we could get that to happen, besides the lower limit for LPG to burn is 2.1% by volume so that’s not going to happen.

detonation, taking the above into account I doubt that any detonation or Knocking would be because of a low percentage of LPG being present though if detonation was to occur I would think that the lpg wouldn’t help the situation.

Dougal mentioned that oil fumes will detonate, that's expected, but its a poor analogue of LPG. The main components of LPG are simple short chain (3 and 4 atoms of C) hydrocarbons, no double or triple bonds. The effective octane ratio of LPG for SI engines is around 130 making it quite a knock resistant fuel. If it takes substantial power output to generate detonation on my 13:1 comp ratio V8 on LPG then 17:1 without diesel fuel present simply won't cut the mustard. Detonation in the fumigation case may take place AFTER diesel is injected and the remaining LPG/air mix is rich enough. The proper way to test this would be to run a diesel engine hot, then manually operate the shut-off (separate switch?) leaving the LPG running and test for ANY power output, vibration or other sign of combustion. Anyone game?

Originally Posted by clean32

But that’s the point, I agree with Bush65 post. I don’t agree when Dougal posted that

"The Ricardo Comet is a type of swirl chamber which fits in the head of some indirect injection diesels.
It is not comparable in form or function to the piston combustion chamber in a direct injection diesel. "

they both are the same animal, Bush has a much better way of explaining it.

It is not a matter of you choosing to agree or disagree cleanwhatever, as you have chosen to disrespect other posters. I want to asorb your post, in your own words, as It appears you know so much about the 4BD1, you might see your way 'clear' too enlighten us all in your superior knowledge of the difference between an direct and indirect injection diesel engine.

Yes, I think you will find if you look carefully that the handle is 'Bush65' not bush as you describe him. Who knows why, you obviously pay little attention to detail, therefore I think subject to the usual, we should be able to expect a half decent reply regarding the above.

Originally Posted by bee utey

Dougal mentioned that oil fumes will detonate, that's expected, but its a poor analogue of LPG. The main components of LPG are simple short chain (3 and 4 atoms of C) hydrocarbons, no double or triple bonds. The effective octane ratio of LPG for SI engines is around 130 making it quite a knock resistant fuel. If it takes substantial power output to generate detonation on my 13:1 comp ratio V8 on LPG then 17:1 without diesel fuel present simply won't cut the mustard. Detonation in the fumigation case may take place AFTER diesel is injected and the remaining LPG/air mix is rich enough. The proper way to test this would be to run a diesel engine hot, then manually operate the shut-off (separate switch?) leaving the LPG running and test for ANY power output, vibration or other sign of combustion. Anyone game?

You didn't mention which octane scale you're using. This link gives a RON scale of propane as 110. When mixed with butane to form LPG (ratios vary depending on the market) the octane rating will drop further.
WPS:LPG

It directly compares data on petrol, propane and methane.

Units Methane Propane Gasoline

Autoignition point C 538-732 468-494 185

Flammability Limits vol percent 5-15 2.1-9.5 1.4-7.6

Stoichometric A/F kg / kg 17.3 15.3 14.7

Stoichometric A/F m3/m3 9.7 24.6

Research octane number 130 112-125 91-95

The interest here is that methane is considered by stationary diesel engine makers to be a safe dual fuel gas, where propane and lpg aren't. So finding how those gas properties compare to each other gives an idea of why it's not considered safe.

If we compare the heat of compression (adiabatic compression) for different engine compression ratios what do we find?

Using 20 deg C intake at sealevel (101.3kPa). Using 70% efficiency for turbo compression which fits very well with measurements I've taken of boost temperatures in my own Isuzu 4BD1T.

High compression petrol. 11:1 - 491C
Low compression petrol 8:1
- No boost - 400C
- 15psi boost and no intercooler - 614C
- 15psi boost and 60% intercooling - 486C

Built LPG Engine 13:1 - 544C

Diesel 17:1
- No Boost - 637C
- 15psi Boost, no intercooler - 926C
- 15psi Boost, 60% intercooling - 752C
Diesel 18:1
- No Boost - 658C
- 15psi Boost, no intercooler - 954C
- 15psi Boost, 60% intercooling - 776C
Diesel 21:1
- No Boost - 717C
- 15psi Boost, no intercooler - 1,032C
- 15psi Boost, 60% intercooling - 843C

As you can see, even a 13:1 compression spark ignition engine has nothing on a diesel for compression temperatures.

Of course these temps are adiabatic (no heat loss) and for a complete cylinder full take to TDC. Real temps will be lower due to heat loss to the cylinder walls and because ignition starts before TDC is reached.
If you make allowance for heat loss, not getting a full cylinder charge and not getting to TDC the temps start to make sense with the published autoignition points for the fuels used.

Your 13:1 LPG engine's predicted temp of 544C isn't far from the shell MSDS autoignition point of 468C.

A 17:1 intercooled stationary diesel's predicted temp of 752C compares to the autoignition range of methane from the link of 538-732C.

But when you compare lpg's autoignition point to the predicted compression temps of the diesels people run it in, it gets quite scary. Autoignition 468C compared toe predicted temps of 630-1,030C.

Originally Posted by Dougal

The interest here is that methane is considered by stationary diesel engine makers to be a safe dual fuel gas, where propane and lpg aren't. So finding how those gas properties compare to each other gives an idea of why it's not considered safe.

If we compare the heat of compression (adiabatic compression) for different engine compression ratios what do we find?

Using 20 deg C intake at sealevel (101.3kPa). Using 70% efficiency for turbo compression which fits very well with measurements I've taken of boost temperatures in my own Isuzu 4BD1T.

High compression petrol. 11:1 - 491C
Low compression petrol 8:1
- No boost - 400C
- 15psi boost and no intercooler - 614C
- 15psi boost and 60% intercooling - 486C

Built LPG Engine 13:1 - 544C

Diesel 17:1
- No Boost - 637C
- 15psi Boost, no intercooler - 926C
- 15psi Boost, 60% intercooling - 752C
Diesel 18:1
- No Boost - 658C
- 15psi Boost, no intercooler - 954C
- 15psi Boost, 60% intercooling - 776C
Diesel 21:1
- No Boost - 717C
- 15psi Boost, no intercooler - 1,032C
- 15psi Boost, 60% intercooling - 843C

As you can see, even a 13:1 compression spark ignition engine has nothing on a diesel for compression temperatures.

Of course these temps are adiabatic (no heat loss) and for a complete cylinder full take to TDC. Real temps will be lower due to heat loss to the cylinder walls and because ignition starts before TDC is reached.
If you make allowance for heat loss, not getting a full cylinder charge and not getting to TDC the temps start to make sense with the published autoignition points for the fuels used.

Your 13:1 LPG engine's predicted temp of 544C isn't far from the shell MSDS autoignition point of 468C.

A 17:1 intercooled stationary diesel's predicted temp of 752C compares to the autoignition range of methane from the link of 538-732C.

But when you compare lpg's autoignition point to the predicted compression temps of the diesels people run it in, it gets quite scary. Autoignition 468C compared toe predicted temps of 630-1,030C.

Remarkable piece of research, thank you. However there are are a couple of points here:

1. We are talking about fumigation not dual fuel. I would expect that below flammability limits in air (Flammability Limits vol percent 2.1-9.5 ) that LPG would require higher temps to detonate, if it did at all.

2. In my 13:1 engine I only experience detonation at high load: cylinder temps/pressures at the detonation point will be CONSIDERABLY higher than adiabatic compression of air only. Otherwise it wouldn't be accelerating rapidly. It is this which makes me decline to believe compression even in a turbo diesel will produce lpg detonation without diesel fuel burning first, at the levels discussed.