More questions about LPG 2

[rdd48856]

Well I have several LPG questions:

1) On a French study (CFBP) it was found that on some gaseous sequential multiport injections systems CO emissions were higher while on LPG than on gasoline. They justified this as a deliberate strategy to protect valves by richening the mixture on deceleration. Does this make any sense? I read(on your site FranzH) that when burning gaseous fuels richening the mixture actually increases combustion temps, while leaning decreases temps, but I also read on an IANGV report (

http://www.iangv.org/files/IANGVREPpart3final.pdf

) that Nox emissions increases until l aprox. 1.3. Could this mean that temps only start to decrease from this point on.

2) What is the reason for increasing timing ignition advance for lower revs while decreasing for high? A balance between higher flame speeds and higher minimum ignition energy for LPG than for gasoline?

3) Would it be possible to use Liquid LPG injection on dual fuel pilot injection Diesel engines in a similar fashion of Westport cicle?

Thank you in advance for your contributions, specially to FranzH…
J.Tavares

 

[franzh]

You are very welcome.

I would guess that if CO emissions are higher with sequential injection versus single point carburetion, there may be a fuel injection programming problem. The fact is that metering fuel in one location serving 4, 6, or 8 cylinders, not all cylinders will get the same fuel mixture. Some lean, some rich, but the aggregate mixture at the O2 sensor may be close to ideal. With sequential injection, the precise fuel for each cylinder is metered in sequence, no fuel bouncing from one cylinder to another.
Most fuel systems cut the fuel on decel, no sense on using fuel for no work provided. I have not heard of using a rich mixture on decel, especially for valve protection. If this is the case, I can certainly see a CO spike.
You are correct about the NOx dropping, we call it the lean NOx knuckle, or operating in lean burn for economy and NOx control.
Can liquid injection be used for supplemental fuel in a diesel engine? No reason not to, within limits, but the fuel metering MUST be very precise, with minute amounts of fuel used. See the FAQ section at the top of this column for more details on LPG.

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.

 

[rdd48856]

Thank you FranzH for your time...
1)Both (gasoline and LPG) systems were sequential multiport injection(gaseous for LPG), I'm trying to figure out why someone would enrich a gaseous fuel mixture to protect valves...the raising on temps occurs just before lambda 1 or only at lower values?

2)Can you enlighten me a bit about question 2?

3)By Westport cycle I was talking about this:

http://www.westport.com/expertise/westport_cycle.php

Do you think it would be possible in the near future to have a dual fuel Diesel pilot ignition engine with dual fuel injectors like this but working with liquid LPG instead of gaseous natural gas? I must stress that I'm talking about huge amounts of Diesel substitution (about 90%)...

Thank you again...

 

[franzh]

LPG will never reach 90% diesel substitution, on a diesel engine platform (static diesel compression with or without boost.)
The critical compression ratio of propane prevents this. Ever encreasing lean conditions can extend this a bit, but at some point, the mixture would be too lean to be of benefit.

I have seen too many engines destruct when trying to run rich as you would with a liquid fuel engine (remember, no evaporative cooling effects with a gaseous fuel engine, or at least, very limited cooling with heat absorbtion). Liquid LPG may help here, but as the fuel enters the chamber and encounters the high residual temps, absorbing too much heat may have a detrimental effect to the combustion process. You may see a delta 700deg F or more between each cycle.
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.

 

[rdd48856]

Thank you for your answer FranzH ….I read your FAQ too.
Forgive me but there's something I must be missing here...if you are injecting the LPG directly on the combustion chamber (without pre-mix, I'm not talking about fumigation) how can critical compression ratio of propane prevents a smooth and single flame front combustion...if WestPort is doing successfully this with CNG why not with LPG...if LPG can autoignite than we could reduce Diesel pilot injection…Of course there would be specific problems to overcome with the High Pressure Injector and the Common-Rail…
Greetings …J.Tavares

 

[franzh]

To the best of my knowledge, there has been no successful direct injected LPG engine. If the DI fuel is injected on the compression stroke, it will initiate preignition with the diesel, allowing for a significant pressure spike. NG has a higher anti-knock ratio which allows for this process.
There has been some research on Late Cycle Ignition where propane is injected after the diesel ignition cycle, or about 60 deg ATDC. This process has shown some acceptable performance. Combustion pressures are significantly low enough to prevent uncontrollable detonation.

Just because it is a vapor phase fuel at atmospheric pressure doesnt mean that behaves the same at compression or combustion pressures. LPG in each phase has its place, as does CNG, and LNG for that matter, but not the identical place.

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.

 

[rdd48856]

Many thanks FranzH...

 

[j2bprometheus]

Hi-

Westport injects the natural gas into the combustion chamber near top dead center (TDC), along with a small amount of diesel fuel. The natural gas is in a gaseous state at high pressure ( ~ 5000 psia). The diesel fuel autoignites and this helps to ignite the natural gas.

So, with the Westport system, the natural gas cannot autoignite too early since it is not in the combustion chamber.

If LPG were direct injected, in a gaseous state, in the same manner that Westport injects the natural gas, then the engine would run fine.

In order to inject the LPG in a liquid or a gaseous state, the pressure of the LPG would have to be higher than cylinder pressure is near TDC of the compression stroke. While this is possible, it would take some development work.

Westport and others prefer to use natural gas since it is usually cheap compared to diesel fuel.

LPG is more expensive than natural gas.

j2bprometheus@aol.com

 

[franzh]

My esteemed colleague "j2bprometheus":
LPG has a liquid phase/vapor pressure index. At any significant pressure above its temperature stabilized vapor pressure, LPG will condense to a liquid. We now have a fuel with full hydraulic properties, occupying a space 270 more compact than the same calorific value than vapor.
Injecting liquid LPG in the hot combustion chamber will almost instantly lower the combustion temperature (while helping with NOx, it will decrease the diesel combustion flame propagation).
Working with liquid phase LPG in an injector at varying pressures has been the problem, there are few that will handle the pressures at the ms duration needed for operation. Late Cycle Injection tends to overcome this.
Natural Gas and LPG are different animals with totally different physical properties which rarely interchange.
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.

 

[j2bprometheus]

Franz--

Let me clarify a few of points:

1. Since LPG is more expensive than natural gas, very few people would want to use LPG instead of natural gas.

2. Of course you would have to avoid unplanned condensation or vaporization of the fuel, regardless of what the fuel is (think of gasoline powered cars and vapor-lock). Any useful design would have to ensure that the pressure - temperature combination kept the fuel in the desired phase.

3. Could you run LPG in the Westport engine with no changes at all to the fuel system? No, of course not.


4. Once in the cylinder, the LPG would burn just fine in the Westport engine. Might the amount of diesel fuel utilized need to be adjusted to account for latent heat of vaporization? Yes, possibly so.





j2bprometheus

 

[franzh]

j2bpro:
I am not too sure about that. With lpg (propane) at 2500 btu's per cu/ft, and NG at 1000 btu's cu/ft, and add in the CNG requirement for compression, storage, and limited high pressure distribution, the discussion about NG and LPG, which is less expensive, becomes a topic for a new thread. Each fuel has its niche, each filling a need. Combustion being the primary concern, NG certainly takes the top honor, but with the availablity of user accessible lpg far greater than natural gas, and the driving range of lpg versus (about 5 times the distance with the same stored volume load) lpg has been the favorite fuel of smaller transit vehicles.

The Westport design is excellent but it is optimized for natural gas. It will require considerable fuel mapping with lpg, as to time of injection, amount injected, duration of injection, and so on. It is possible, most likely probable, and I would be just short of daffy to think that the engineers at Westport have not yet tried it. Look at it as "when" not "if" they introduce it.
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.

 

[rdd48856]

Thank you again, to both...fuel injection accuracy is increasing every day…second generation common-rail systems are able to make 5 separate injections per cycle (two pre-injections, one main injection and two post-injections)…if you could do the same with Westport injectors and playing with injection parameters for both fuels I’m pretty sure a high degree of Diesel substitution would be possible…. I agree with you j2bprometheus, on the long term NG is the way to go, since we have to diverse our engine fuel supply from oil and more than 50% of world’s LPG still comes from oil. But right now we, on most European countries, have a wide refuelling LPG network and LPG costs half of Diesel.... furthermore increasing oil prices are boosting Diesel and Direct Injection Gasoline car sales...so if LPG Direct Injection (for both SI and CI engines) is not achieved soon, this market will die....

 

[franzh]

Its not the accuracy of fuel injection, its the mechanical ability to function at load/speed/pressure. The computational ability exists to make the fuel injection feasible, but the physical capability of the fuel injectors is lagging. This has been the biggest problem when working with liquid phase injection, plus the fact that LPG is a multiphase fuel makes it all the more interesting.
Depending where you are in the world, and even in the US, LPG production average is about 75% from natural gas, but this depends on demands for the base fuel stock. If a hard winter is anticipated, more LPG is pulled from natural gas (raw crude dedicated for heating oil), but then the back side is that natural gas fuels most of the nations power generation and home heating systems. It is a fast moving see-saw, predicting where and what the fuel ratio and fuel costs will be.
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.

 

[turbocohen]

Liquid phase DI was successfully demonstrated in the late 90's. I stopped the projects my former employer supported due to poor LP fuel quality. The olefin and sulphur deposits killed liquid phase injectors DI much faster than liquid port.
Forget HD-5 and all that nonsense... The LP sold for over the road use needs keep clean and clean up type additives for the same reasons gasoline does. Until the LP providers take care of this problem, LPG is a total waste of OEM resources.

 

[rdd48856]

You're right turbocohen...even with gaseous port injection(and filters to both liquid and gas phases) we have troubles with deposits on the injectors rail...

 

[franzh]

With fuel injection becoming more and more prevelant, fuel quality has returned as a hot topic at LP conferences. The EPA emission control mandate has pushed the paraffins and oil issue back to a front burner. Mechanical systems tolerated the buildup to an extent, but eventually it came to a plugged halt.
Glad for your input, now, all we have to do is get the Gas Processor Association to issue a good motor fuel standard, persuade the propane industry to install and maintain a seperate motorfuel distribution system, and the consumer market to buy a propane powered vehicle.
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.

 

[turbocohen]

Franz, I respectfully disagree with you. Fuel injection is the only way the eagle flies and that has been the way it is for years now. Propane industry is still in the dark ages. Management at the highest levels have consistently failsd the lpg motorfuel market by not adopting fuel additives that are on the market and ready. Forget the gas processors, the fuel at the dispenser needs to be additized before the retailer.. maybe at the bobcat or terminal..
Forget OEM propane vehicles. The propane industry failed to address the most fundamental technical problem that plagues EVERY oem lp injection system whether liquid or vapor phase type and the oem's are not too interested in losing more money. Even the oems tell the propane execs that the fuel quality isnt the issue as much as a lack of adequate fuel with keep clean additives.
Is anyone in the lp arena making motor fuel quality a top priority yet?
Pardon the doom and gloom Franz, the lp industry let my team down for 20 years+ and all I hear now is talk and more education re status quo.

Regards, T

 

[franzh]

Turbo, there is no disagreement, I have been working on this for ages too. As for the rest of your statement, I echo every word. We have had this discussion many times in the past, and as we speak, there are several research programs underway to find ways to mitigate the creation of oils. Some say that the oils comes from the plasticizers in hoses, others say it is the byproduct of heating propane and let the lighter gasses pass out while leaving the long chain (roughly C-16 and heavier) to accumulate. The additive makers also disagree on what is needed to clean the fuel, without affecting the emissions.
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.

 

[turbocohen]

The plasticizers in the hose is easy to solve using engineered plastics that are manufactured for use with LP. Other oils that precipitate or coalesce are troublesome to all fuel systems, especially the more sophisticated fuel injection systems which like gasoline systems are dependent on additives with surfactants, etc that allow olefins and sulphur to pass through the fuel system and burn completely. Heavy H/C's are easy to deal with using off the shelf additives that have been tested extensivly and are marketed for use with LP intended for use as motor fuel. There are a lot of snake oil salesmen out there as others on the forum also speak of and I have heard of most of them.. been contacted by several too. GFI and an additive client I have assisted in the past tested an additive formulation that allows about the worst case LP to pass through a fuel system without negative side effects.

With the exception of excessive glycols there is nothing I have seen in LPG that will not safely pass through a fuel system that has a 10 micron magnetic filter as long as the fuel has adequate deposit clean up and keep clean additives.

The problem with LPG is not technical in any way shape or form. The problem begins with the upper management of LP distributors and retailers who fail to deal with the root problem with their product as used for enginesand the problem ends when management decides if they want to sell a true motor fuel that by nessecity is an additized fuel.

Good luck getting anything done this decade, the top brass in the LP industry by failing to act on this issue have decided to kill many OEM projects simply because the oem has to stand behind their product with a warranty for 1000's of hours or risk recall action. Forklift mfg's are having a hell of a time meeting 5K hrs with LPG and electric looks more promising in spite of the triple up front cost..

Regards, T

 

[aaranyi]

Thanks to the very interesting aspects about these non conventional fuels (LPG, CNG). As I'm a bit conventional I'd put down some drawbacks. I suppose, you are more or less avare of them.

CNG (mainly methane) is a "permanent" gas, meaning its normal storage needs high pressure (more than 1000 psi) or very low temperature (cca. -250 °F) or both. Ensuring these conditions is very costly. Moreover specific gravity of natural gas is very low, so the storage needs tanks of high volumes at the above conditions. And the safety problems. Practically you'd sit on a HE bomb. Of course this problem can be, more or less, solved but again for excess money. Then the safety of supply on the roads, that is to build up a filling station network or modifying the present ones. All the filling stations should solve the above problems but in higher dimensions.

Consequently the CPG may be cheap as heating gas but as a fuel for vehicles, as a whole, must be very expensive. The costs can lowered by some conservationist funds, etc. (I'm not American) but the overall expenditure of the Nation is won't be lower. For immobile engines, turbines it is quite all right.

So the CPG as a fuel for vehicles is a dead racehorse.
But the stupid ideas are dangerous. The more stupid it is the more probable it's realisation.

Of course I don't want to hurt anybody, if accidentally so I beg your pardon.