Expert advise on diesel propane fumigation

[toupee]

Hi all,

Hopefully someone with a good deal of experience in dealing with lp and diesel systems can clarify things for me here.

First let me say that I work for a propane dealer, so my interest in propane fumigation is from an economic standpoint.
I have seen propane work well in our medium duty trucks and thought I'd try it in a light duty truck.
The truck I am working with is a 05' Dodge 3500 5.9L.
My setup works by using the engine's vacuum to allow propane into the intake via a regulator/converter.
When testing my system I noticed that I could hear a faint bit of detonation in the engine. The noise level was about the same as what my 4 cylinder Nissan Altima makes when I start it cold. Another comparison is that it is only slightly louder than the typical injection rattle that I hear from the truck.
I have not seen any increase in egt's that are noticeably beyond normal (diesel only) levels. I have not witnessed any white smoke , black smoke, or haze out the exhaust. Engine coolant temps are staying normal.
I actually noticed an increase in up-hill power and a substantial increase in gas mileage.
I've read everything I could find on propane injection and I gather that the propane must be pre-detonating because of it's CCR of 12:1? I've found that my engines ratio is 16:1.
What I'm lost on is the evidence of how this is damaging the engine. I would think I would see a decrease in performance or mileage.... or I should be seeing some increase in egt's at least. I generally don't associate knock with good things happening.
Any input would be appreciated.
The system is off at the moment, btw.

 

[catserveng]

We did a good bit of conversions several years ago, both CNG and Propane. Our propane experience was similar to what you have described, including what appeared to be audible detonation. The results of that detonation showed up in oil sampling reports as increased aluminum and iron, and after a couple of engine failures, we stopped doing diesel propane fumigation systems, but continued with CNG systems for some time.

We had several engines that appeared to have no problems, mainly Ford 7.3L engines, with the propane conversions. Virtually every turbocahrged engine we did had almost imediate problems. Also vehicles that had a lot of lugging in their operations, school buses, delivery and trash trucks also did not do well.

Hope that helps.

 

[franzh]

Propane has a different burn rate than diesel, and since propane is fumigated through the air intake system instead of injected at the peak of the compression cycle, the propane can ignite from compression heat. What you are hearing are two or more combustion pressure waves colliding inside the combustion chamber. IMMEDIATELY reduce the amount of propane by whatever means you have or you will have a rather expensive paperweight.

Natural Gas has a higher CCR and longer burn duration than propane and thusly tolerates higher levels of fuel substitution than propane. Engines with higher static compression ratios show the detonation more than engines with low compression ratios.

Franz

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[toupee]

Thanks for the responses.
I have read another thread posted here concerning fumigation so I had heard of the colliding pressure waves. I guess my real area of confusion is how the damage occurrs? I can understand if it is due to excessive heat. But since I haven't seen abnormal egt's is it due to something else? Is it sheer pressure doing the damage? Is the cylinder's internal heat reaching it's peak early enough that it's cool enough by the time the exhaust valve opens to appear to be normal?

 

[Lou Scannon]

For starters the detonation creates very high forces & accelerations beyond what the factory diesel combustion system does. So the base structure and cranktrain components are experiencing higher stresses than they were designed and validated for. Secondly, the turbulence resulting from detonation results in much higher heat transfer into the surfaces that define the combustion chamber. In the case of the cylinder walls & head, this is not critical since they are anchored to jacket water temperature. The piston, however, is much more vulnerable, since it is cooled indirectly. Aluminum pistons, particularly cast aluminum pistons, are especially vulnerable due to their sharply deteriorating material strength with increasing temperature.
I wouldn't rely on instantaneous EGT to tell you whether or not everything is OK. Depending on your sensor and where it is located, it may or may not respond fast enough to indicate anything is amiss during a transient event.

 

[j2bprometheus]

toupee -

When a fuel autoignites, it can be that nearly all of the fuel/air/EGR mixture ignites at about the same time. This can make peak pressures very high -- not good for an engine.

And combustion can be very rapid. This causes high peak temperatures & pressures. But rapid combustion also means that the piston has more opportunity to expand the charge, resulting in lower exhaust temperatures.

I have observed a spark ignition engine on a dyno just starting to knock and the exhaust temperatures actually go down since, after the fuel has burned, the piston has the chance to expand the charge further than it normally would.

I have also seen pistons skuff / partially melt when the engine is knocking heavily.

I haven't read much about the HOW knock damages the engine.

 

[toupee]

Thanks guys,
That's the kind of info I am looking for.
I know the ISB engine is a tank build wise. As far as I know it's all iron, so hopefully no aluminum worries. Seems to me that the first thing to go with the increased pressure would be the head gaskets since they would seem to be the weakest link. I'm probably overlooking something here though.
It's very unfortunate that my system isn't working out. There was a day when our entire fleet ran on propane. Now, none of our new trucks do. Do to lack of interest in propane as a motor fuel from the big companies, it doesn't look like I'm gonna be ordering any new vehicles with propane motors anytime in the future either. Everything I've heard about converting from gas to propane is just like a bad joke, it always costs way too much and you probably won't see a return during the life of the vehicle.

 

[j2bprometheus]

Toupee-

Cummins makes a spark ignition 5.9 liter engine that runs on propane.


But I doubt that anyone puts it in a vehicle.


j2bprometheus

 

[toupee]

j2bprometheus,

I actually looked into that engine. Funny thing is that nobody within 3 states radius of me knew anything about it. I actually ended up being directed to Cummins corporate. At that point I found out that the propane 5.9 didn't make enough torque to lug around one of my delivery trucks and it cost about $13000 if I remember right. No wonder you don't see them in service. Don't quote me on that price. It was ridiculous, I remember that much. I think it's mainly intended for industrial use and it runs off industrial grade propane too.... I think.

 

[franzh]

There are a few new applications now being introduced using the GM 8.1, with Liquid Propane Injection. These are hoss engines with lots of lower end torque, plus a warranty and pretty darn good performance too.

Liquid Propane Injection improves intake air charge density, lowers NOx, and since the application uses the OEM computer and sensors, no "piggy back" electronic cheat modules are installed. The OBD interface is maintained.

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.

 

[Lou Scannon]

There is a mobile version of the Cummins 5.9, which is currently marketed by the Cummins-Westport joint venture as the B-LPG Plus. I believe it is rated at 195hp and somewhere around 400 lb-ft torque. For more info go to cumminswestport.com
As far as I know all versions of the Cummins B series engine, diesel or gas, use cast aluminum pistons. The steel pistons are reserved for the 'C' (8.3L) and larger heavier duty engines (the higher rated versions, that is).
The typical scenario when an engine is subjected to knocking above its design limits is,
piston crown heats up,
loses mechanical properties and starts to erode at the hottest locations
if knock is unabated the top ring land expands to the point that clearance to the cylinder bore becomes zero or negative. At that point the piston will start to scuff, and if power is not reduced quickly a catastrophic failure will ensue.
Whether or not the head gasket fails prior to this point depends on whether it is the weaker or stronger link on the engine in question.
An engine that is exposed repeatedly to non-catastrophic levels of knock may eventually suffer a failure due to cumulative damage to the head gasket, or crank or rod bearings.

 

[toupee]

Thanks again for all the responses guys.
I have talked with a couple of salesmen and heard reports from other companies who have bought the GM trucks with the 8.1 liquid injection. It is a step in the right direction. The problem that I have is everytime I do an economic analysis the $$$'s just don't add up in favor of the propane trucks. That's what's really annoying.
There are currently tax incentives on both the conversions and the fuel... just not enough. Last time I checked the 8.1 conversion kit was about $8k in a truck.
Another issue I have in my area is that no mechanics are left around here that have any understanding of propane powered vehicles... let alone computer controlled liquid injection trucks. They had enough trouble with my old vaporizer system ones.
This is kinda deviating from my original subject matter, but it does shed some light on why the dual fuel idea looks so good to me as a marketer.

 

[marcdeluca]

I have run propane injection on my Duramax for over three years now with great success. The key is to not use so much that you get detonation. At light load, I can burn 3 gal of lp to 1 gal of #2 and not get pinging. When towing, I have to cut it back. My setup is self modulating and has a control valve adjustable from inside the cab to adjust the fuel curve. I also have cng injection on it, and it works better than propane because of the higher octane rating.

 

[Lou Scannon]

"The key is to not use so much that you get detonation"
Agree wholeheartedly. OTOH, how efficiently are you burning the LPG in very lean (i.e. light load) operation? I.e. what is your overall fuel economy in this mode?

 

[marcdeluca]

What I have found is that there is no magic happening. My truck will get 18 mpg at 70 mph on straight #2. #2 has around 140,000 btu/gal, which comes to 7777 btu/mi. If I get 50 mpg on #2 using gas injection, that would be 2800 btu/mi. 7777 - 2800 is 4977 btu coming from the gas. LP is around 91,000 btu/gal. So, that would be 18.3 mpg on lp while getting 50 on #2. My real life experience shows these numbers to be very close to calculated results. The savings comes from being able to get lp for less than 2/3 the cost of #2. The cheaper it is, the more you save. The cng is a better value, but you can't carry much of it, so it isn't great for more than a few hundred miles.

 

[XXXLR8R]

To the OP:

The HPCR engine in your truck uses hypereutectic aluminum pistons with oil jets under the crown for cooling, so it can safely handle 1400*F EGT (pre-turbo) indefinitely, short excursions to 1600*F, and spikes in excess of 1800*F; however, introducing fumigated fuel - such as propane - into the combustion chamber advances the CTD's effective timing below the ECM's "radar", and thus concomitantly the danger of melted piston crowns, broken ring lands and dropped valves... carnage that can occur before dangerous EGT or audible warning.

Especially for the common rails, it's far easier and more efficient to manipulate the fuel map via the injector's duty cycle and the CP3's output to achieve BSFC (and therefore mileage and power) gains.

As the only medium-duty diesel available in a pickup truck, the Cummins is overengineered for the application, which is why they are so popular as test-mules or for rodding.

 

[dcasto]

The new thing is LNG liquified natural gas as the fumigation fuel.