Is my Diesel Mechanic brilliant or a fool?

[Kailua Kid]

Keeping the diesel tank topped off (“pressed up”) during the off-season will reduce the risk and or amount of condensation forming on the internal tank walls, which in turn reduces the risk and or amount of of algae growing in the boundary layer that forms between the diesel oil and the resulting water in the tank. I have never needed to drain the fuel tank of diesel during periods of layup. Rather, if an extended (months or even years) layup is anticipated, I have treated the fuel with a biocide and just pressed up the tank(s). Diesel seems to be more stable than gasoline and seems to retain its BTUs for prolonged periods while stored in a tank below decks. I am not a chemist; my statements here are based only on personal experience, including in the PNW and Alaska.

While I am at it, let me amend my earlier comment on this thread above to say that I do disagree with your mechanic’s reported statement that older Yanmars were built to lower tolerances and so tolerate deterioration longer, or words to that effect. A peek at BoatDiesel.com and or a comparison of service manuals for older and newer Yanmars will show the actual tolerances between all wear surfaces, torque specifications, bearing clearances, valve guide clearances, etc. for these engines. While the tolerances vary slightly from engine to engine, all diesels are built to very high tolerances to withstand the very high (compared to a gasoline engine) compression ratio required for diesel combustion. If through excessive wear, abuse or overheating those tolerances are lost, the engine either fails catastrophically, becomes very difficult to start (especially when cold, because of insufficient compression) and or begins to consume lubrication oil at a higher rate than specified. Usually, except for over heating damage or damage from running the engine without oil, the engine with normal high service-hour wear will just become harder and harder to start when “cold” (meaning when the engine has not been run for several hours and the block has returned to ambient temperature). Many diesels have been known to run for years despite becoming progressively harder and harder to start when cold. Assuming your engine’s cranking system is within specifications as regards cranking speed, if your engine can be coaxed to life when cold after 10 or 20 seconds of cranking, maybe in two 10 second attempts 30 seconds or so apart, my recommendation is use it and don’t obsess over it. In fact, when you run it, don’t “baby” it by running it slower than about 80% of its rated speed whenever conditions allow (assuming the engine is properly “propped,” the prop or “wheel” and the bottom are reasonably free of growth and the cooling circuit is operating correctly). Engines are not mules. We do not do diesel engines any favors by running them slower than they were designed to be run. In fact, they run better in all regards when they are operated 80-90% of rated speed — and last longer when run in their designed working range, which varies from engine to engine based mainly on the ratio between cylinder diameter and stroke (longer stroke, lower speed), with most Yanmars and Volvos being designed to be operated at higher speeds than most other diesels.

[JD-MDR]

[quote="Kailua Kid"]Keeping the diesel tank topped off (“pressed up”) during the off-season will reduce the risk and or amount of condensation forming on the internal tank walls, which in turn reduces the risk and or amount of of algae growing in the boundary layer that forms between the diesel oil and the resulting water in the tank.

I dont ever see any algae or water in my fuel just what looks like rust particles. . I'll get photos this weekend when I work on it.

[fritz3000g]

Keeping the diesel tank topped off (“pressed up”) during the off-season will reduce the risk and or amount of condensation forming on the internal tank walls, which in turn reduces the risk and or amount of algae growing in the boundary layer that forms between the diesel oil and the resulting water in the tank. I have never needed to drain the fuel tank of diesel during periods of layup. Rather, if an extended (months or even years) layup is anticipated, I have treated the fuel with a biocide and just pressed up the tank(s). Diesel seems to be more stable than gasoline and seems to retain its BTUs for prolonged periods while stored in a tank below decks.

Ok that makes sense. I'd much rather leave the tank full in winter than drain it. I've read that diesel lasts a year without stabilizer, and 5 years with it. If I could add biocide and never drain the tank (letting the two filters handle any particulates), that would be much preferred.

[Carl Thunberg]

I wasn't going to respond to this thread until the topic of condensation came up. I've always struggled with this whole concept of condensation in the tank, and maybe it's more prevalent in southern climates. For the record, I'm in Maine. In order for condensation to occur there needs to be a temperature and humidity gradient. Basic chemistry and thermodynamics. That's where I struggle. Where does the temperature gradient come from? It's not like a toilet tank that has a continuous supply of cold water against warm humid air. If condensation in the tank is real, then why can't we solve all the world's water problems by leaving a bunch of empty cans around? Isn't it more likely the fuel is contaminated with water at the fuel dock?

[fritz3000g]

I've always struggled with this whole concept of condensation in the tank, and maybe it's more prevalent in southern climates. For the record, I'm in Maine. In order for condensation to occur there needs to be a temperature and humidity gradient. Basic chemistry and thermodynamics. That's where I struggle. Where does the temperature gradient come from? It's not like a toilet tank that has a continuous supply of cold water against warm humid air. If condensation in the tank is real, then why can't we solve all the world's water problems by leaving a bunch of empty cans around? Isn't it more likely the fuel is contaminated with water at the fuel dock?

An engineering question!

The thermal mass of the diesel fuel in the tank causes the temperature inside the tank to change slower than the surrounding air. So when the outside temperature drops, the diesel fuel temperature drops slower. That means that the wall of the tank will be at a lower temperature than the air inside the tank, creating the opportunity for the tank wall to hit the dewpoint and create condensation.

Because the air inside the tank is at a different temperature than the outside air because of its contact with the diesel fuel, it will also expand and contract at different rates than the air outside the tank, causing a pressure differential which causes air to enter and exit the tank if it's not completely airtight.

If you have a completely airtight tank, this would be less of an issue.

[Carl Thunberg]

While I follow the logic, it seems such a rare combination of events that would trigger actual condensation. I'm not the only one who has doubts.
https://www.yachtsurvey.com/myth_of_con ... _tanks.htm

[sgbernd]

I've always wondered about this as well.

First, if condensation were a problem, every lawn mower, edger, car, etc. would eventually get water in increasing amounts in the tank. I have never experienced this, and have owned and driven cars/ mowers, etc. which were 20+ years old. And generally, cars don't even have water traps. Even on the boat, I've never worried about it, and have checked the water separator every year or two but never found even a hint of water. I always assumed it was because in So. California, the humidity is relatively low, but humidity on the boat is pretty high. A car typically has a vent to allow air in but not out, but the environment, quantity of fuel, etc. is more extreme than a boat and the day/night near-water temperature does not change as much the air temp does far from the water.

Second, if water really condensed and got trapped, you could capture water out of the air by just putting a vented tank, say a 55 gal. drum, somewhere and eventually it would fill with water. Experience shows this is not the case. A dry tank stays dry. Forever.

Thanks for the insight. I learnt something today.

Steve

[ghockaday]

Well, water does get in diesel worst than gasoline. Two very different products. Why do you have a water separator on your diesel anything? I have one on my diesel VW. Water is in the air, Have you ever done this experiment in school. Dig a small hole, put a piece of plastic over it and over a jar, punch a hole in the bottom and in the morning you have water in the jar. I guess enough plastic, holes and jars we could solve the worlds water problem as well.
Anyway here is a nice article to explain that we that believe water gets into partly filled fuel takes due to the greater volume of air exposure. Those of us that have oil tanks for furnesses also have to have the water pumped off the bottom of those takes as well. It is why the pick up is up off the bottom.


Diesel Digest: by Capt. Jeff Werner

Diesel fuel is the lifeblood of a diesel engine. If left unmanaged, diesel fuel quality can degrade rather quickly rendering it harmful, rather than helpful to an engine. Understanding the sources and nature of diesel fuel contamination is the first step in maintaining optimal fuel quality aboard a yacht.

All forms of contamination have their own unique properties that can have an effect on the efficiency and reliability of an engine and its fuel system. Water is the most damaging contaminant found in diesel fuel, and it is the primary cause of additional fuel breakdown. Water can exist in three different states within diesel fuel: Dissolved, free and emulsified.

Dissolved water

Dissolved water, sometimes called entrained water, is the result of diesel fuel being hygroscopic. That means that fuel has the ability to attract and hold water from the environment, whether it is from humid air or condensation on the wall of a fuel tank. And as the fuel temperature in the tank increases, so does the amount of water that can be dissolved and held in solution in the fuel.

Free water

Free water is water that gathers at the bottom of the fuel tank due to its higher density relative to diesel fuel. This can only occur once diesel fuel has become saturated with dissolved water and the fuel temperature is not high enough to hold any additional water. For example, since a diesel engine does not burn all the fuel that reaches the injectors, the remainder is returned to the fuel tank. This unburned fuel has passed through tubing next to a very hot engine and picked up some of this residual heat. Once the returned fuel mixes with the fuel already in the tank, all the fuel in the tank is warmed up, which allows it to absorb more water. By the end of a day’s run, the fuel has more water entrained in it.

While at the dock or at anchor overnight, as the fuel in the tank cools down, the saturated fuel releases this moisture as free water and it falls to the bottom of the tank. The simplest way to detect this is by coating the bottom six inches of a fuel tank dipstick with water finding paste. Then stick the tank. If the paste changes its color, then free water is present.

[Carl Thunberg]

I didn't mean to imply water in diesel isn't a problem. The source of the water is the question. Take a close look at your deck fill. It's flush with the deck, and relies on an O-ring to keep water out. Poorly maintained tipping buckets where the fuel delivery truck off-loads. Lots of sources of water. I just don't think condensation is one of them, at least not in a 20-gallon tank.

[fritz3000g]

Interesting. I looked a little bit and found this thread. https://forums.sailinganarchy.com/index ... on-t-or-f/

About the fifth post someone explains that it's not the condensation of the water in the air that's the problem, it's the water that's already dissolved in the diesel fuel that condenses out of solution when temperature drops. Then when the temperature warms not all of it is re-absorbed because the fuel also takes up moisture from the air that's been pulled into the tank.

I don't know if this is really a problem or not, but it does imply that my initial explanation is incomplete.

[Carl Thunberg]

I'm totally on-board with that explanation. Condensing out of solution is entirely different than "condensation" in the traditional meaning of the word. Phase-change is probably a better choice of terms.

[ghockaday]

Dissolved water, sometimes called entrained water, is the result of diesel fuel being hygroscopic. That means that fuel has the ability to attract and hold water from the environment, whether it is from humid air or condensation on the wall of a fuel tank. And as the fuel temperature in the tank increases, so does the amount of water that can be dissolved and held in solution in the fuel.

Definition of: A hygroscopic substance is able to absorb or adsorb water from its surroundings. Typically, this occurs at or near ordinary room temperature.

Hygroscopicity:

Diesel fuel is highly hygroscopic, meaning it readily absorbs water from moisture in the air. Water contamination in diesel fuel is of particular concern as it can lead to the corrosion of steel components and the promotion of microbial growth.

I intemperate this differently than ya'll do but that's ok. I don't have a diesel anymore. It is is probably worse in the humid South were we have 98 percent humidity often in the Summer time. The above are not my words or words from a chat room but from reliable sources.

Gas is not hygroscopic but the Ethanol is hygroscopic, That is one reason the shelf life of gas is shorter now. Some say 30 days.

I'll let this poor horse lay in peace now.

[Tom Keevil]

I don’t understand the chemistry of this. Gasoline and diesel are both hydrocarbons. The difference is that the molecules in the diesel are larger (have more carbon atoms) than those in gasoline. All of these dissolve only tiny amounts of water, but the smaller (gasoline) molecules dissolve more than the larger molecules. Everyone says that diesel is hygroscopic, but it is less hygroscopic than gasoline (not to mention gasoline with ethanol!). So why are diesel users more concerned about water in the fuel? Are the injectors more easily damaged, perhaps?

On another topic, I am sure that something can grow at the diesel-water interface in your tank, but I have serious doubts that it is algae. Algae are photosynthetic, and the inside of the tank is pretty dark.

[JD-MDR]

Here is the contraption I made to suck the tank dry. I didnt get any crud this time. Last week I did this while I was at anchor. I got a couple of tablespoons of cruddy solid wont try getting technological or philosophical it just looked like coffee grinds The tanks clean enough till I start seeing stuff in the Raycor. The filters can do their jobs

[ghockaday]

On another topic, I am sure that something can grow at the diesel-water interface in your tank, but I have serious doubts that it is algae. Algae are photosynthetic, and the inside of the tank is pretty dark.[/quote]

You are correct. It is bacteria and fungus. Most everyone just calls it algae cause it just what everyone has called.