Castor oil fuel versus sythetic oil fuel

Editors Note: Mechanical Engineer, TimBle (his RCUniverse.com handle) has worked in Product Engineering for both Lubricants and Fuels for 2 major petroleum companies over the past 11 years. Prior to his petroleum career he worked 4 years in the Aerospace industry.

What is the difference between castor and synthetic rc fuels? Does it matter which one you use and when you use it? And why has there been raging debate over which one is better? Control Chat interviews Fuels and Lubrication Specialist, TimBle, about these long standing and often heated questions.

What is the primary difference between Castor oil and synthetic oil?

Castor Oil is a vegetable fatty acid based oil, similar to many other vegetable oil derived oils from the beans of plants.

Synthetic oil can be derived from nearly any source e.g. crude oil, vegetable oils, petroleum wax, pyrolysis of waste materials. Synthetics are termed such due to the altering of the base structure to create a set of molecules that retains advantages or improves on them but minimises the disadvantages of a simply processed oil. Castor Oil would be considered a simple processed oil.

Why use Castor oil?

Castor oil has been used as a lubricant for decades. Its chemical structure allows it to polymerise at high temperatures to form a sticky wax type material often referred as castor varnish. This wax still has lubricating properties. In the event of oil starvation the wax still separates the metal surfaces for a short time.

Why use synthetic oil?

Synthetic oils typically offer high film strength without the wax formation. Wax formation can be undesirable in ringed engines where the wax build up can result in ring sticking which will lead to a ring failure and subsequent engine failure. Synthetic oils also tend to keep the insides of the engines cleaner which results in more consistent combustion.

Can you switch from running an engine on a fuel containing Castor to a fuel containing synthetic? Or vice versa?

Yes, Castor oil is compatible with all Synthetics, especially the Poly glycols and Di-ester types used as engine oils for RC engines. When switching from castor oil to synthetic lubricant, it is not atypical for an engine to pick up rpm as there is less viscous drag with synthetics than there would be with castor oil. What also often occurs is  removal of the castor varnish due to the high solvency of the synthetic oils.

Is one oil  better to use with a 2-stroke engine? 4-stroke engine?

There is a lot of debate around this question in RC forums, often resulting in the closing of threads because people are really passionate about what works for them. There is also a lot of misinformation around synthetics. When considering an engine oil for an rc engine, the first thing to consider is the mechanical design. Is the engine an ABC/ABL type engine or does it have a compression ring?

If it is of the former type i.e. ABC/ABL, then it is wise to use some castor oil in the fuel. These engines are typically 2 stroke engines. However there are also 2-stroke engines that are ringed engines and this is typically where the debate rages into fisty cuffs and degrades into a “hand bags at dawn” type disputes.

Two Stroke methanol engines are quite tolerant of the lubricant because they are oil lubricated but fuel cooled. In the hot cylinder environment, the fuel evaporates leaving the lubricant behind to do its job.  This lubricant needs to be able to separate the ring from the liner effectively. You will be surprised to know that with a ringed engine the type of oil is less critical because the lubrication comes from the ring riding an oil wave much like a water-skier being pulled over water.

This is called hydrodynamic lubrication. In ringed engines, the oil can escape  behind the ring resulting in effective oil flow under ring to maintain that hydrodynamic wave. In an ABC type engine the oil is pushed away along with the piston, also hydrodynamic but the distribution of pressure in the oil is different. In the ABC type engines, the castor is beneficial here since its highly polar nature allows for it to cling very strongly to the walls of the cylinder ensuring you have that one layer thick oil film to provide separation.

Four strokes are typically ringed engines so the above applies to them as well. However we also need to consider where that oil feed comes from; the top of the piston or is it recirculated oil?

Now we know that 4-strokes draw fuel and oil in from above so lubricating the bore below the piston crown is tough. Hence it is even more important to use an lubricant that can flow past the compression ring i.e. an oil that does not get continuously more viscous and forms wax, but one that is stable under increasing temperature and pressure. Synthetics meet that need. Some 4 strokes recirculate the lube oil and reintroduces the oil via the crankcase. These are a little more tolerant of different lubes but essentially high quality stable viscosity oil is best.

Some people have argued that Castor oil is better than synthetic oil. Historically this argument stems from the manufacturing process of rc engines. Why is this?

If we consider the demographics of the RC hobby, I think you’ll find it s is mostly 40 to 60 something’s who are most active. When these folk started in the hobby, manufacturing techniques where not dominated by CNC (Computer Numerical Control)  machinery but by manned lathes and mills. Engines were mostly cast  and liners were not very accurate. These components were not “close fit” as they are today. The use of castor oil would result in a varnish layer on the components that effectively took up some of the tolerance and provided sealing and hence compression.

Today engines are not made this way. CNC allows millions of components to be produced within a very tight tolerance and very little deviation of dimensions from the intended specification. Modern Rc engines being close fit, actually benefit from as little varnish as possible coating engine parts.

The other dogma driving castor oil is that somehow some folk have it in their heads that castor is still in widespread use today in automotive lubricants. This is simply not true. Although vegetable oils are continuously being developed and reviewed for suitability, they see less than 1% volume use in modern motor vehicle engine oils.

Castor blend fuels often coat the inside of an engine with what is called a ‘varnish’. What is this and is it bad?

The castor varnish is essentially a polymer of castor formed by the oxidation of the castor oil. It still has lubricating properties hence people feel that this is a good thing. However in modern engines where tolerances between parts are much finer, the buildup of castor varnish can lead to a degradation in engine performance.

What is one benefit to using Castor?

It is fairly easily available over the counter and it lubricates well enough.

What is one benefit to using synthetic?

Synthetics oils tend to be more thermally stable at the operating temperatures of RC engines and they don’t leave  residue  behind in the engine.

Is mixing your own fuel recommended for a beginner?

Unless one can ensure a reliability of supply and quality of the components, I would advise against home blending. Varying fuel quality will lead to constant fiddling with mixture settings when what a beginner needs is stick time, not needle time.

What is the best fuel advice you can give to a beginner?

Pick a reputable brand and learn to work with your engines on that brand. Ensure that you keep Nitro, and oil content constant in the fuels to minimise the variables when tuning the engine. It matters not whether you opt for fuel with a little castor, a lot of castor and or full synthetic, just keep using the same fuel blend until you are comfortable with understanding your engines “moods”.

3 Comments

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