Oil dilution for cold starting

[Spits2]

Spitfires had an Oil dilution button on port side of Frame 10 from MkI.Not sure if prototype had it in 1936.

 

[Crimea_River]

Mosquito too, to the immediate left of the Navigator, next to the fuel cocks.

 

[Simon Thomas]

This paper also has some good information on dilution, as well as a number of other topics related to aero engines.
The author must have been a bit of a wag, let me quote some of the last page.

Inquisitive people frequently ask embarrassing questions as to just how much the presence of a radiator actually slows down an airplane. The Materiel Division made one very interesting test to determine the answer to this question but, as often happens, the value of the data became lost in the rush of progress, and the airplane upon which the tests were made is now obsolete. Unless the trends can be anticipated by at least three or four years, tests of this kind are discouragingly useless.

The main gas tank was filled with water and the latent heat of vaporization of this water, with the aid of an internally located heat exchanger, provided admirable cooling of the Prestone for a flight of 15 to 20 min. duration, and gave the fascinating appearance of an aerial steam locomotive.

 

[Jorge Carlos]

I am often surprised how long the oil change intervals are. Operating the Bf 109 G with DB 605 A, I changed the oil on every pre-winter lay-up and engine inhibit, so that the engine sat with new oil all winter, so that was an oil change at every 20 hours, a far shorter period than the original servicing requirement, but even so, the combustion soot load in the oil was considerable.
Dispersant oils are a factor, with some engines still needing non-dispersant oils.
The German procedure was very much the same as described, dilution fuel cock turned on for a defined time according to planned temperature on a warmed engine before shut-down.

Eng

Could you tell me what type of lubricant you are using and what viscosity? Does the DB 605 engine require lubricants with dispersants or not? Would it be possible to use a 20w 50 mineral lubricant?

 

[RichardSuhkoi]

Does data exist on the original WW2 German oil? I'm sure it was synthetic. A lubricants engineer could look at the original, look at elastomer materials in use, and probably optimize an oil. Improved shear strength and breakdown resistance as well as anti foam might (for example) allow a lower viscosity and maybe reduce oil losses (energy losses). Reduced foaming might reduce bearing scuffs. Gear wear, ball and roller wear, Babbitt wear can all be balanced by an oil guy. Oil consumption at 20 hours a year is probably a non issue. Do you do oil analysis? That might also help the lubricant engineer determine advice. So many people store with old, acidified oil, happy to hear you store with fresh.

 

[Engineman]

Could you tell me what type of lubricant you are using and what viscosity? Does the DB 605 engine require lubricants with dispersants or not? Would it be possible to use a 20w 50 mineral lubricant?

Hi,

The original German DB 605 A specifies "Rotring", which was a mineral 100 grade aviation oil. I do not think dispersant oil was available. We used Mil grade 100 dispersant with 20hr annual oil change which was more than twice the original requirement of 50 hrs or more.

Eng

 

[Engineman]

Does data exist on the original WW2 German oil? I'm sure it was synthetic. A lubricants engineer could look at the original, look at elastomer materials in use, and probably optimize an oil. Improved shear strength and breakdown resistance as well as anti foam might (for example) allow a lower viscosity and maybe reduce oil losses (energy losses). Reduced foaming might reduce bearing scuffs. Gear wear, ball and roller wear, Babbitt wear can all be balanced by an oil guy. Oil consumption at 20 hours a year is probably a non issue. Do you do oil analysis? That might also help the lubricant engineer determine advice. So many people store with old, acidified oil, happy to hear you store with fresh.

The original "Rotring" (red-ring) was a mineral oil, but as the war progressed, the German versions became synthetic ethylene polymerized products. These oils were generally regarded as inferior to the Allied aviation oils . The Germans did suffer with oil foaming, and on the DB 605, cold weather oil requirements were met with oil dilution.
We did our own oil examination of filtered oil residue on oil changes but not chemical analysis. The soot load on these big engines is so high that very regular oil change is wise. A modern installation would be best with a micro filter, but that would have to be a major modification and is not required on an original museum aircraft.

Eng

 

[Simon Thomas]

Some info on dispersants vs straight mineral oil.

I don't have an aero engine, but if I did it would not be running on straight mineral oil unless mandated by the FAA in the limitations section of the manual.

 

[MiTasol]

Regular oil dilution also removes pretty much the same stuff as dispersants do - and more effectively in my experience. Using both you get a far cleaner engine.

Using oil dilution on an engine that has done over 100 hours and has never had dilution can cause engine failures though. On a 300 hour engine I had to do multiple dilutions and oil changes to get all the sludge and carbon lumps out of the system before the next flight. Short engine run on new oil, pull (R-1340) screen remove lots of carbon and crud, change oil, repeat.

When the first dilution was carried out, immediately after receipt from the previous operator, removing the oil cooler and oil tank drains resulted in a tar like mixture draining for the first few seconds and getting the screen out was a major operation due to the amount of carbon in it. A significant part of the crud would have been airframe accumulations as the aircraft had never been diluted by the previous operator.

 

[Engineman]

Regular oil dilution also removes pretty much the same stuff as dispersants do - and more effectively in my experience. Using both you get a far cleaner engine.

Using oil dilution on an engine that has done over 100 hours and has never had dilution can cause engine failures though. On a 300 hour engine I had to do multiple dilutions and oil changes to get all the sludge and carbon lumps out of the system before the next flight. Short engine run on new oil, pull (R-1340) screen remove lots of carbon and crud, change oil, repeat.

When the first dilution was carried out, immediately after receipt from the previous operator, removing the oil cooler and oil tank drains resulted in a tar like mixture draining for the first few seconds and getting the screen out was a major operation due to the amount of carbon in it. A significant part of the crud would have been airframe accumulations as the aircraft had never been diluted by the previous operator.

There are several complications with oils for different engines and circumstances in vintage engines. Certainly, at the bottom line, some engines are extremely particular about the exact oil that is used. The viscosity is often dependent on the ambient temperature but other limitations can be because of material compatibility, like rubbers and certain metals and often it was the internal temperature and duty considerations in the mechanical parts of the engine.
With the basic straight mineral oils, sludge, gum and compacted carbon or lead from fuels was usual. The point about oil dilution acting to disperse the deposits was included in some DB operating guidance where a max limit was placed on operation without dilution being used. A point about the big old engines is that they did not have micro filtering of the oil. The DB 605 A/B had a "self cleaning" filter (the type with a ratchet operated brush to wipe the filter plates) that had 0.2mm gap pitch, so small particles could still circulate. Coarser mesh screens covered the scavenge pump intakes, to protect from larger bits getting into the circulation. The later DB 605 D (1944) introduced a larger finer oil filter matrix of 0.15mm gap but it was not self-cleaning and the filter unit had to be removed and cleaned every 25 hours operation, a considerable task.
Oil dilution was used by the Germans for cold start operation and it did enable starting and operation of the engines in extreme cold. However, incorrect oil dilution use would expose the engine to risks from excessive or insufficient dilution.
Our operation of the DB 605 A was always in temperatures that did not require oil dilution and using modern ashless dispersant oil, there was no build-up of sludge or lacquer inside
the engine.
As an aside, the original oil-change period for the DB 605 seems high, considering the debris load and the simple oil in use during WW2. The modern ashless dispersant oil with its anti-foam and corrosion inhibitors etc, certainly gave a better lubrication package IMO.

Eng

 

[MiTasol]

You say There are several complications with oils for different engines and circumstances in vintage engines. Certainly, at the bottom line, some engines are extremely particular about the exact oil that is used and I totally agree.

I remember way back in my apprenticeship days we discussed a TEAL Solent flight from Sydney to Auckland. The ground staff had topped off the oil tanks on the Solent in Sydney using the usual 100W grade oil used on the R-1830's in the Sandringham's and Catalina's also operating at Rose Bay instead of the special Aeroshell 100U oil the Solent needed for its sleeve valve Hercules engines. One engine shut down in flight, three "nursed" home and all four changed in Auckland was the result. The 100U oil contained a higher level of anti-foaming ingredients as well as other chemicals to ensure the sleeves got properly lubricated.

 

[Simon Thomas]

The metallic dispersants were very important to the operation of the sleeve valve radials, due to the relatively high piston ring land temperatures. I understand that the 100U used a barium sulphonate dispersant, which was essential for keeping the piston ring lands free from excessive deposits. The rate of deposit is influenced by high temperatures, the length of time at high temperatures and the state of the dispersants. The dispersants get less effective during operation as they accumulate more particles.
One of the other additives in 100U was tricresyl phosphate (TCP) which was an extreme pressure additive. Together they work by the barium sulphonate keeping the ring friction forces to a level that the TCP can handle the rod loads.

So to keep a sleeve valve engine running merrily, it requires:
The right oil (Note 1 below)
The oil changed often enough to replace the dispersants before they loose effectiveness.
The dispersants being able to absorb the deposits as much as is practicable.
Sufficient cooling to keep the deposit build up in the ring lands at an acceptable level where the oil flow is so low that the dispersants struggle to absorb all the deposits.

The last two point are a function of airspeed, power, oil age and cooling. Most of the time it is fine. If those parameters allow build ups behind the rings to occur then it is a matter of time before the engine behaviour slowly deteriorates until a cylinder seizes and the rest will not be not far behind.

Note 1:
The 2022 AAIB report into the G-RNHF Sea Fury accident notes that in 1965 the MOD conducted a trial of an ashless dispersant oil, and found "increased rates of piston ring failures, piston ring gumming and head ring failures occurred".
In the same report it states: "Correspondence between the MOD and the engine manufacturer in 2015 re-confirmed the recommendation to use W100 in Centaurus engines." This is an ashless dispersant oil, that the 1965 trial proved caused failures.