Plantoil/diesel conversion basics
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danalinscott

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Reply with quote  #1 

(under construction)

Links to archived info...

Pictures of ring coking...


There appears to be quite a bit of confusion by individuals as well as a few conversion kit vendors regarding the link between Injector Tip Coking and Ring/Land/Groove Coking. In order to differentiate between the two types of "coking" I will use the acronyms IC to refer to Injector (tip) Coking and RC to refer to Ring(Land/Groove) Coking in this discussion. I encourage others to do so as well.

It has apparrently been assumed that they are both caused by the same "mechanism" and so that ITC can be reliably used to monitor RLGC. Though the mechanisms are similar in several ways they are different enough that this it is not reasonable to assume that if no ITC is observed that RLGC is not occurring.

The general mechanism by which coking occurrs is fuel ..in this case VO/WVO accumulates in liquid form and slowly "cooks" rather than combustng cleanly. VO/WVO is much more subject to this than petro or bio diesels. So care must be take to avoid accellerated coking in engines using VO/WVO as fuel.

Both IC and RC DO occurr rarely in diesel engeins run on petrodiesel and it is assumed that the mechanisms are the same or very similar whether this is observed in engines fuelled on VO or petrodiesel. The deposts appear nearly identical whether on engines run on petrodiesel or VO/WVO so I believe this is a reasonable assumption as do the diesel engine techs I have discussed this subject with.

Inector Tip Coking
IC is most observed in injecotrs that are "out of spec" due to wear or improper adjutment. These may tend to allow liquid fuel to drip from the injector tip orifices after the ignition cycle has progressed beyond the point where it can "burned off" by the current combustion cycle. The injector tips are hot enough to allow the fuel to carbonize however. And this carbon if not burned off by the heat of the next combustion cycle will tend to accumulate and create a growing accretion/deposit of carbon..or "coke" on the injector tip. In rarer cases the hot gasses of combustion can force thier way into the injector tip itself and create carbon/coke deposits inside the injector tip rather than on the outside.

Both interior and exterior accretions of carbon will eventually cause symptoms which are easily noticable. These include smoke on startup, hard starting, smoke all the time, lack of power and rough running. And the normal "fix" for IC is removal of the injectors and replacment with new/rebuilt injectors. This is relativly easy and inexpensive since injectors are designed to be easily removed fromt he engine.


Ring/Land/Groove Coking

Liquid fuel can also accumulate on cold cylinder walls..be scraped off by the piston rings and carbonize on the rings, in the ring grooves, and on the "lands" above and between the grooves in the piston which the rings are located in. These piston ring/land/groove accretions are much more difficult to notice than IC carbon accretions are though. And they are much more serious in terms of the engine damage they cause.

RC is also much harder to notice than IC since engine performace may degrade very slowly unitil secondary engine damage occurrs. A regular compression value log will show dropping compression values however and is reccomended for all experimenting with VO fuel ...especially in cold climates or with conversions on which the cylinder walls are cool when VO/WVo fuel is introduced into the combustion chamber or in engines that have very cool operating temperatures.

In engines run on petro diesel ring/land/groove coking is usually traced back to long idle times where the cylinder walls cool sufficiently to allow liquid diesel to accumulate on them or in engines in cool climates that have chronically cold coolant temps while operating. When VO/WVO is substituted as a fuel care must be taken to not allow liquid fuel to reach cylinder walls on a regular basis or accellerated (relative to petrodiesel) ring coking will be the result. Unlike injecotors rings are not easily accessed and so replacement/repair can be very expensive. Additionally secondary damage occurs if RC is allowed to progess to the point where carbon has accululated enough to begin ring seizure. This includes piston and cylinder wall damage as well as accellerated bearing wear.

Quote:
Would you please explain WHY RC occurs when the cylinder walls are cold, and not when they are hot?


There are several reasons.

The most obvious is that hot cylinder walls tend to vaporize any fuel which lands upon them...while cold cylinder walls allow partially combusted fuel to condense upon them.

So even if the cold VO fuel is injected properly (which is not likley) upon startup the cold combustion chamber tends to lead to incomplete combustion and condensation of partially combusted VO being deposited as a thin film on the cylinder walls. This thin film is scraped from the wall by the upper piston rings and seeps behind them forming coke accretions.

In an engine up to operaitng temp the cylinder walls are warm, as are the injectors, the head, and the piston. The fuel gasin some heat as it passes through the now hot injecotrs and so the droplets of fuel injected tend to be smaller which promotes more complete combustion as does having the fuel closer to combustion temp. partially combusted fuel is less likely to reach the cyinder walls and if it does it is less likely to condense there.

I have seen pistons which were from VO converted test engines (in person..not on the interenet) which had heavy coke accreations in the rings and upper ring lands. These acretions had formed due not to cold startign on VO but to using VO fuel in engines that had faulty thermostats or had been operated at chronically cooler than normal operating temp. This indicates to me that these acrretions are at least partly due to cold cylinder wall temp.


 
Quote:
Also, is it your experience that "cold" for the purposes of pronounced RC would be any temperature below normal engine operating temperature (as measured by the coolant temp gage)?
Yes. Though the colder the cylinder wall is the faster these accretions tend to form.
Quote:
I would tend to think (w/o any experience, simply intuitively) that the cylinder walls would be one of the first parts of the engine to heat up, thus reducing the likelyhood of RC as you describe it, shortly after initial ignition.

And relative to many parts of the engine this would be true.
The parts of the engine not exposed to the heat/flame of combustion warm very slowly. So the parts of the engine directly exposed to the flame produced by the burning fuel inside the combustion chamber tend to heat up first. These are essentially the piston and the combustion chamber top and sides.

Since the piston is essentially only cooled by the oil pumped to its lower areas and its contact points with the cylinder walls it heats up the quickest and tends to remain the hottest component of the conbustion chamber. In contrast the top and sides of the combustion chamber are cooled by the liquid on the "other side" of the cylinder walls and the passages in the head.

The cylinder walls are designed to efficiently shed heat into the coolant. Due to this they remain much closer to the temperature of the coolant bathing their exterior than the inferno contained within them. This is why it is important to avoid use of VO while the engine is under normal operating temp not only at startup but also during day to say driving.


 
 
 

 

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longwolf

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I have an old 1984 Ford E350 with a 6.9L IDI engine.
Before trying a conversion I want to try and clean the engine as well as I can.

I was thinking of using Marvel Mystery oil because I've seen what it can do for gasoline engines.
Within about 1500 miles the carbon starts pealing off like old paint.

But I did some surfing and learned that it alone may be bad for diesel engines based on a study that is posted all over the web, here's one link to the article.
http://www.thedieselstop.com/forums/f27/id-like-know-what-really-works-197824/#post1559943

But if you look closer at the article, you'll see that just using biodiesel at a 50:1 ratio greatly improves the lubricity of the fuel.
It should improve it way more that the MMO worsens it.

So I'm think that using biodiesel with Marvel Mystery oil in your diesel tank could reduce or even clean up the coking.

I was wondering if you have any ideas on this?

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84 E350 6.9L
danalinscott

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Reply with quote  #3 
Unfortunately up to this point nothing short of re-ringing pistons has proven effective in removing the carbon in ring grooves.

Edit...
This may not be correct.

There is the possibility that a procedure that was developed for the GM Northstar engine MAY work to decoke piston rings/grooves.

Quote:
Cadillac service bulletin 01-06-01-011 deals with oil burning on 1996-'99 Northstar V8s. The cure, says Cadillac, is to do a ring cleaning procedure (seems those long oil change intervals weren't such a good idea after all). Cadillac recommends using GM cleaning kit (P/N 12378545) and Kent-Moore J-45076 induction/evacuation tool to do the job. The cleaner is added into the cylinders through the spark plug holes and allowed to soak the rings for two hours. The cleaner and dissolved crud is then vacuumed out of the cylinders through the spark plug holes, followed by an oil change.  


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longwolf

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Reply with quote  #4 
My experience with MMO was on an old Harley.
I don't remember how well the ring grooves did, but it was amazing how the carbon was lifting off the piston heads.

It was as if the marvel had soaked into the pistons, and under the carbon, and the carbon just could not stick any more.


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danalinscott

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Reply with quote  #5 

Exposed carbon is not that difficult to remove. But for some reason that carbon which is packed into the ring grove is extremely tenacious. If you find a way to remove it without removing the pistons please let us know.


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mark13

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Reply with quote  #6 
I noticed that ring coking is under 'long term problems' but I'm wondering exactly how quickly this issue could occur? I am currently using up a tank of 30% WVO / 70% petrodiesel in my single tank 300SD Mercedes conversion (before I upgrade to 2 tank system) and I'm concerned that my frequent cooler starts may have resulted in ring coking after going through only a few tanks of WVO / petrodiesel mixes (never less than 50% petrodiesel). This morning it was -2 and I did have a bit of a problem starting, but later in the day the car started right away every time. Can you reassure me, or it it possible that coking could have occurred after this relatively short time?

NB: I have a vegtherm inline fuel heater installed, as well as a coolant heated extra filter. Have also been glowing the glow plugs 3x before morning starts.

Mark

danalinscott

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Reply with quote  #7 
Quote:
I noticed that ring coking is under 'long term problems' but I'm wondering exactly how quickly this issue could occur? I am currently using up a tank of 30% WVO / 70% petrodiesel in my single tank 300SD Mercedes conversion (before I upgrade to 2 tank system) and I'm concerned that my frequent cooler starts may have resulted in ring coking after going through only a few tanks of WVO / petrodiesel mixes (never less than 50% petrodiesel). This morning it was -2 and I did have a bit of a problem starting, but later in the day the car started right away every time. Can you reassure me, or it it possible that coking could have occurred after this relatively short time?

NB: I have a vegtherm inline fuel heater installed, as well as a coolant heated extra filter. Have also been glowing the glow plugs 3x before morning starts.

 


Mark,
In your particular circumstance (single tank/blend) I think the simplest way to check for advanced ring coking would be to have a compression test done. By comparing what your current compression values are with what the minimum acceptable values AND optimum (newly run in engine) you should be able to roughly determine how much useful life (maximum) is left in your engine. The "actual" remaining engine life can then be roughly calculated by reducing this by 15-30%.



 Studies in the 1980s and 1990s using unmodified diesel engines found that test engines run on both VO and VO blends (down to 5% VO) developed severe ring coking so quickly (in in less than 200 hours) that they (incorrectly) concluded that neither strait VO or VO/diesel fuel blends were a viable substitute for petrodiesel.

Ring coking begins when you start using VO as fuel. It is for all practical purposes unavoidable. It is a fact of life for those using VO fuel. BUT it can be minimized to the point that it does not significantly shorten engine life.

Since ring coking is caused by leakage of incompletely combusted VO past the piston rings the goal is to eliminate the total amount of uncombusted and partially combusted VO that is allowed to leak past those rings. Since the percentage of uncombusted and partially combusted VO is highest AND the total amount of combustion gasses (which carry uncombusted and partially combusted VO) able to leak by the rings are at their maximum when a cold engine is first started it is at this point that ring coking is occurring at the fastest rate. Both the amount of gasses leaking by the piston rings AND the percentage of uncombusted and partially combusted VO in those gasses decreases as the engine comes to full operating temperature the less one operates an engine on VO or VO/diesel blends during that period the slower ring coking will progress.

Optimally (as far as ring coking progression) one should NEVER use VO fuel or VO/diesel blends when an engine is below full operating temperature. Since most diesel engines tend to run at full operating temperature ONLY when under load one should not use VO fuel or VO/diesel blends during idle or low load conditions. This is the operating regimen that I suggest all my fleet clients use and which all automatic fuel controls in the conversion configurations I design are set to maintain.

This also allows for VERY complete purging of VO from the fuel systems with minimum inconvenience since purging begins whenever the engine is allowed to go to idle. Nearly all conversion kit designers have concentrated on short purge times but unfortunately this almost without fail results in incomplete purging of VO and as a result the initial fuel when the engine is next started is usually a VO/diesel blend.

Piston rings are not perfect seals so there is ALWAYS some blow-by. An engine with new rings initially has a LOT of blow-by until the rings "wear in" during the initial few hours the engine is operated. The least amount of blow-by occurs at this point. But as an engine accumulates hours the piston rings slowly lose their ability to seal as well as they did shortly after the engine was first run. Due to this engines tend to allow more and more leakage/blow-by until compression drops to the point where the engine must be overhauled or replaced.   When petrodiesel is used as a fuel this point is usually signaled when the engine becomes progressively harder and harder to start.

However since since VO fuel mixing with the crankcase oil (due to blow-by) causes the lubricating oil to solidify (due to polymerization)  this warning point may not be reached before the engine is destroyed upon a cold start (due to lack of circulating crankcase oil). This is why I advise that regular compression checks are a wise part of any VO fuel program regimen.

Recently a solvent has been developed that when blended with crankcase oil allows existing  VO based ring coke deposits to be dissolved during normal engine operation. Prior to the development of this additive fleet clients were advised to budget for 20% fewer useful engine hours during ROI calculations. It appears that by using this additive in conjunction with regular compression test logs that modifier may be lowered to 5% or less.

Unfortunately this solvent is not commercially available so as the next best alternative I suggest regularly testing samples of used crankcase oil for any signs of polymerization at every oil change. If you notice ANY signs of polymerization I strongly advise shortening the oil change interval by 20% and saving up for a "ring job". Ignoring lube oil polymerization will almost certainly mean that the entire engine and turbo will require replacing at a much greater cost. Of course if the car is not worth investing in a ring job I suggest simply monitoring crankcase oil polymerization and progressively shortening the lube oil change interval until it becomes too short to bother with.


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Dana danalinscott@yahoo.com
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