Problems concerning LPG 2

[rdd48856]

As some of you already know I’m moderating the only LPG forum in Portuguese. There are a few recurrent problems arising on lpg bi-fuel cars.
1)Damage head gaskets. Can this be caused by know higher LPG burning temperature? How to avoid this? If thermostat temperature is lowered, the heat transferred to the radiator is about the same, right? What about increasing water pump speed? If the injection system is delivering the same energy(as fuel) to the engine what is the real cause behind this increasing in burning temp.? Incorrect ignition timing, only? Does the increased load on the water pump (water has to circulate through the vaporizer) plays a role, too?

2) Valve Seat Recession. This is a major problem. Some engines(Honda VTEC and Toyota VVT-I, as an example) are very sensible. One user had to replace his CRV engine valves just after 40000km! It seems this can be reduced by using a graphite based fluid, delivered on the inlet manifold. You can see an example here

http://forum.autogas.co.pt/download.php?id=60

Comments are welcome. A problem with this delivering device is that it can’t deliver properly on turbo engines…any suggestions to overcome this?
Thank you…
rdd48856

 

[franzh]

In the US, I cannot recall any problems with damaged head gaskets on engines running LPG. I personally have owned and currently drive several vehicles in excess of 100,000 miles on LPG, and they are hard miles. What is happening to your head gaskets, you do not specify. I assume they are burning between cylinders. See below:

My experience with engines that have exhibited valve recession is improper fuel mixture control and retarded ignition timing. Retarding timing to reduce detonation increases exhaust port and valve temperatures, contributing to exhaust valve recession. When I mention retarded timing, I am not implying that the timing has been retarded, but not recurved or remapped to take advantage of LPG octane qualities. This can be as much as 10 degrees, but it all depends on individual engine specifications. There is no "one curve fits all" situation.

Top end lubricant such as you describe defeats the purpose of using LPG, and since it is not a controlled process, the ability of this product to achieve emission certification is next to impossible.

Again, if the fuel mixture is properly maintained with first quality electronic mixture control devices, I have seldom seen any problems with an engine.

Also, at one time, I was a forum advisor for gaseous fuel vehicles, closed for about 5 years.
Franz

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[EngJW]

LPG is a dry fuel and cleaner burning so it does not build up protective depsosits on the valve seats like gasoline does. Typically, and industrial engine will use Stellite faced valves for LPG.

I don't know where the information comes from that LPG burns hotter. I have heard the claim but never seen any documentation and I'd like to have it for my files if there is any. Of course, a gaseous fuel would not have the cooling affect of a rich gasoline mixture at full power.

Any heat problems could be localized within the chamber or around the exhaust seat. Changing the coolant pump flow might not make any difference, providing the engine itself is not overheating.

 

[franzh]

Uh, hang on a second.
Its not the protective deposits that save the valve seats. Lead disappeared here in the US in 1975. LPG does not burn hotter, but its combustion characteristics allow for different requirements in compression or ignition timing. Longer burn duration with the same ignition mapping as gasoline leads to the burn sequence still in process when the exhaust valve opens, torching the valve and seat. Modern valve seats are induction hardened, or hard material inserts are installed.
You are correct about the liquid fuel evaporative effects not being present in propane. I have leaned the fuel mixture out considerably at full power on many, many engines, to keep the exhaust temps low. That is the theoretical process of lean burn anyway.

Franz

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[rdd48856]

Thank you, both, Franzh and EngJW for your answers. About head gaskets. Yes they are burning between cylinders and the evidence comes from the abnormally elevated occurrences mentioned by forum users. I’m trying to find a solution for this, but as EngJW wrote it may be localized heat(may this explain damage to head gaskets?). I don’t believe (although I can’t explain why) the reason for “hotter burning” is incorrect ignition mapping only. A French LPG bus manufacturer(MAN engines) had problems with high exhaust temps and I’m sure they were using correct ignition maps.
Now the valves. All I’m going to say refers to gaseous sequential multiport LPG injection. IMPCO’s owned Italian manufacturer BRC is demanding its clients to sign a disclaimer note about installation of kits on cars know to have fastened valve recession problems. They say this problem can’t be solved by setting time right, in fact they only suggest the use of time advance processors for CNG(I don’t know if this has something to do with the fact that our LPG is mostly butane). As for now the only solution(other than open the engine to change valves and guides) I came across is to use the top end lubricant. They clam miracles on preventing valve recession (you can see a test here

http://www.flashlube-europe.com/technical/fltest02.pdf

) and although I don’t trust to much on someone who is selling a product I think it will help. They also refer an emission reduction. On Europe the only reason for using LPG is an ecological one and I don’t believe that a product used on a 1:1000 proportion and made of hydrocarbons and graphite would increase emissions so that much. Now comes the third problem. How can I modify the releasing device to work with turbo engines. By the way lean burn on closed-loop wouldn’t be a solution because it increases Nox(detected by European OBD as a major fault) and the other major LPG problem: the increased voltage needing(which lead to spark plug wires insulation breaks).
Thank you again…