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2 Cycle Oil Test Summary
by Elson Shields
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Summary Comments Oil Mixes and Carbon Formation:

These tests were designed to simulate the effects of long-term high engine temperatures on the formation of carbon from the different two-cycle oils tested. The target temperature was 220 degrees F. However, it was only achieved on a couple of engine runs. A more typical temperature was in the 190-200 degree F range. In order for these tests to be truly scientific, each engine run would have to be replicated on a different engine and the order of the oil mix runs randomly selected. The only thing close to science about these results is that they were run by an individual who makes a living as a scientist. The cost of this trial was estimated to be between $2500-$3000. It was conducted in support of an ongoing research program utilizing large RC aircraft to conduct aerial sampling for various projects. I am happy to share the results with you.

After conducting the tests, the real question in my mind is whether the engine temperature within normal running parameters actually has any influence on the formation of carbon at all. It seems to me that the temperature of combustion of a fuel mix over an extended period would not change very much regardless of the temperature of the engine. Testing this idea would require another series of engine runs which won't happen anytime soon in my shop.

The applicability of these tests to your engines depends on your style of flying and the temperatures at which your engines run. Cool engine temperatures (100 degrees and below) cause little carbon formation. Therefore, carbon formation is related to the engine temperature and the composition of the oil.


Interpreting the results:
As you look through the pictures, you will notice 4 different patterns of carbon formation on the piston. Amsoil generally coated the entire piston, Mobil and a couple of others coated the majority of the piston leaving some portions clean, Pennzoil deposited carbon in a single stripe and Belray deposited carbon in a couple of spots. I don't know the explanation of the deposition patterns or the importance of the patterns.


Bel Ray Oil mixture and Amsoil Oil mixture:
I believe that I have an explanation about the initial increased temperatures and increased fuel consumption with Bel Ray H1R.

For the test engine to turn the propeller at 6000 rpms, a very similar amount of energy was required from each oil-gas combination. Surprisingly, all of the oil-gas mixes except 2, required only very minor adjustments of the high speed needle to achieve peak rpms. Therefore, the energy per unit volume of all of the oil-gas mixes except two were very close. The two oil-gas mixes which required a major adjustment of the high speed needle were Belray @ 32:1 and Amsoil @ 100:1. Both of these mixes required that the high speed needle be opened between 1/8 and 3/16 of a turn. Therefore, these 2 mixes had less energy per unit volume than the other 8 mixes. Since these two mixes are very different, what is the explanation?

I believe the evidence strongly suggests that on the 8 mixes which only required minor adjustment of the HS needle, combustion of the oil contributed a significant amount to the energy required to turn the prop. These mixes also had significant carbon accumulation.

In the case of Belray, the oil did not combust and did not provide any energy to the task of moving the propeller. Therefore, the HS needle needed to be opened to allow more gas into the engine to provide the energy to spin the prop at 6000 rpms. The lack of carbon formation with this oil also supports the suggestion that the oil was not burning and adding energy to the fuel. The high cylinder temperatures observed early in the run was from a lean run. Opening the needle valve was required to allow enough gas into the engine to readjust the mixture to the proper ratios. Since the oil does not combust, the fuel requirements of the BelRay oil mix is increased by around 15%. In other words, you need to carry around 15% more fuel on your airplane for the same flight time. This is not much of an issue for the average modeler but it may be a real issue for me. If I burn off a gallon of fuel per flight (typical of some flights), I will have to carry an additional 20 ozs of fuel if I choose to use Belray h1R (an extra 1.5 lbs of fuel load).

In the case of Amsoil 100:1 mixed at 100:1, there is so little oil in the mix, the combustion of the oil added very little to the energy needed to turn the prop. The HS needle had to be opened to allow in the extra required gas to turn the prop at full speed. The presence of carbon with this mix indicates that the oil was burning but was in insufficient quantities to provide much power.

Finally, please remember that this test looked at carbon formation but did not examine the ability of the oil mixes to lubricate.

Several individuals have ask me to conclude which oil I would use in my "high engine stress" flying in the summer of '05. Remember, this is my biased opinion.

1) I would never use Amsoil 100:1 mixed at 100:1. There is no decrease of carbon formation and I noticed an increase of piston scuffing on the test engine. Even though I have over $20,000 of engines in my shop which I did not purchase on my own money, I still think it is "nuts" to shorten the life of an expensive engine from limited lubrication for the benefit of airplane cleanup.

2) If you don't mind scraping a little carbon every couple of years, any of the oils mixed 32:1 to 50:1 are a good choice.

3) For my net-pulling planes which really work hard, I plan to switch to Pennzoil air cooled mixed at 20:1. This oil mix was rich enough to provide a lot of lubrication at these high engine temperatures and caused minimum carbon formation.

4) For my fleet of 12-3W-50 powered planes which run full throttle of 35 min but don't pull as draggy of sampling devices as the net planes, I will use either Pennzoil air cooled at 40:1 or 50:1 or Amsoil mixed at 40:1 or 50:1 (I have the Amsoil on hand).


Break-in and Pre Carbon Formation

The pictures are of 2-3W 50s which were broken in on the engine stand using Lawnboy ashless at 32:1 for four hours at reduced rpms (2500-4000) and reduced temperatures and Amsoil 50:1 for 1 hour at 4000 rpms (5 hours total). The engine on the right looks like carbon formation but it wipes clean as indicated by the engine on the left. The second picture shows that the carbon-like formation is very heavy oil. This oil could easily change to carbon with high engine temperatures.
Lawnboy and Amsoil after 5 Hours running Engine Lawnboy and Amsoil after 5 Hours running


Sub - 100 Degree Running

The picture is of the piston of the test engine (3W-42) in which 3 gallons of Mobil 1 MX2T was burned while keeping the engine temperature around and below 100 degrees F. The lower temperature was accomplished by using a small propeller and reduced rpms. It required 11 hrs of run time to burn 3 gallons of fuel. As you can see, there is little carbon formation. The carbon which is formed is more like a varnish. Carbon formation is linked to engine temperature along with oil composition.
Sub 100 degree, 11 hour run


Test Parameters

Test engine: 3W-42 (1/2 of an 85) which was well broken in. The cylinder was
removed and the piston, ring and inside of the cylinder was cleaned of
carbon. The picture of the piston is a typically cleaned piston before the
start of each test. The engine was reassembled. This process was repeated
before the start of each new oil test.

I think everyone should respect the toughness of the test engine. It is
hooked to an immovable object, runs full bore with too large a prop for 6
hrs straight with temperatures near the acceptable maximum. This engine has
churned through 30 gallon of gas/oil mix, run for 58 hrs and never missed a
beat. It is running just as strong as when I started. I think one could call
this test, engine abuse.
Engine Test Environement Engine before Oil Testing



Amsoil Dominator 50:1 at 50:1. Gas was 93 octane.
The engine was run for 6 hrs and almost 3 gal of fuel was burned. Engine
temperatures fluctuated 210-220 degrees. Outside temperature was 30-32
degrees. Engine rpm with a 24 x 10 prop was 6000.

After 6 hrs of running at full throttle, the piston was covered with brown
hard carbon 5-10 thousands thick. A 1/2 inch band running from the intake
port to the center of the piston was shiny clean otherwise the entire piston
was covered with carbon. No carbon was found on the ring, ring groove or
inside the dome of the cylinder. This carbon buildup was very similar to
Amsoil Saber 100:1 mixed at 50:1.
Amsoil Dominator 50:1 - Cylinder Amsoil Dominator 50:1 - Piston



Amsoil Dominator 50:1 oil mixed at 20:1. Gas was 93 octane.
The engine was run full throttle for 6.5 hrs. During this time period, 3
gallons of fuel was burned. For the first 2.5 hrs, the engine was propped
with a 22 x 10 prop and the head temperatures fluctuated around 180 degrees
F. With this prop, the engine turned 6200 rpm. In an effort to raise engine
temperatures to the 220 degree F range, the prop size was increased to 24 x
10 for the final 4 hrs of the run. Engine rpm were 6000 and head
temperatures fluctuated around 200 degrees F. Outside temperature was 30-32
degrees F.

In 6.5 hrs of running, there was significant carbon formation on the top of
the entire piston and in the entire dome of the cylinder. There was no
carbon formation on the ring or in the ring groves. The carbon had a
brownish cast and was very hard. Carbon layer was in the range of 10-15
thousands thick.
Amsoil Dominator 20:1 - Piston



Amsoil Saber 100:1 mixed at 50:1 - gas was 93 octane
The engine was run for 6 hrs and almost 3 gal of fuel was burned. Engine
temperatures fluctuated around 220 degrees. Outside temperature was 30-32
degrees. Engine rpm with a 24 x 10 prop was 5900-6000.

After 6 hrs of running, there was a light carbon coating on the entire
piston, which scraped very easily. The dome of the cylinder was the same.
There was no carbon formation on the ring but the top of the ring has a
slight brownish cast. This reduced carbon over oil #1 may be a result of
higher engine temperatures (20 degrees F) or the oil itself. It was
interesting to note that the initial engine temperature shot up to 235
degrees F after running 15 min and then cooled down to the 220 degree F
range.



Amsoil 100:1 at 100:1. Gas was 93 octane.
The engine was run for 6 hrs and almost 3 gal of fuel was burned. Engine
temperatures ranged between 190-205 degrees F. Outside temperature was 30-32
degrees. Engine rpm with a 24 x 10 prop was 5680-5900. This drop in engine
rpm was very obvious. Either the engine is finally showing the wear of the
long accumulated runtime or the 100:1 oil mix did not give the ring seal of
the higher oil mixes or both.

After 6 hrs of running, there was a carbon coating on the entire piston
which was quite hard. The carbon was harder and slightly thicker than the
Amsoil Saber 100:1 mixed at 50:1. Hard carbon was also forming on the
cylinder head just above the exhaust port. Two spots of carbon were also
forming on the ring. Piston scuffing on the skirt started showing up after
this 6 hr run on 100:1. Scuffing was on both the intake and exhaust sides of
the piston. This may be due to the low oil or just to the 54 hrs of
accumulated time and the wearing of the wrist pin and bearings. I will be
interested to see if the engine rpm return in test run #10 when we go back
to the high oil ratio mix.

The photos are representative of 50:1 and 100:1.
Amsoil 100:1 - Cylinder Amsoil 100:1 - Piston



Mobil 1 MX2T mixed at 32:1. Gas was 93 octane.
The engine was run for 6 hrs and almost 3 gal of fuel was burned. Engine
temperatures fluctuated 210-220 degrees. Outside temperature was 43-45
degrees. Engine rpm with a 24 x 10 prop was 5880-5910 down from 6000 with
most other oils.

After 6 hrs of running at full throttle, the piston was covered with hard
black carbon 5-10 thousands thick. A 1/2 inch band running from the intake
port to the center of the piston was shiny clean. The piston edge close to
the intake port had a 1/8" clean area and the piston edge next to the
exhaust port also had a clean band 1/8" wide. Otherwise the entire piston
was covered with carbon. Carbon buildup also was starting on the upper edge
of the ring and in the cylinder dome. This carbon buildup was very similar
to Amsoil Saber 100:1 mixed at 50:1 and Amsoil Dominator mixed at 50:1. The
only difference was the color of the carbon. Mobile carbon was black and
Amsoil carbon had a brownish cast. Both carbons were very hard.
Mobil 1 MX2T - Cylinder Mobil 1 MX2T - Piston



Mobil 1 MX2T + injector cleaner
The final test was to take Mobil 1 MX2T which formed carbon in the previous
test and mix 2 oz/gal of injector cleaner into the gas/oil mix to see if the
injector cleaner had any effect on carbon formation as suggested by one
modeler.

We ran the engine for 3.5 hr at full throttle. Engine rpm returned to the
5900-6000 rpm level indicating a better ring seal that Amsoil 100:1 at 100:1.
With the air temperature 24 degrees F, we were only able to achieve cylinder
temperatures of 190-195. In spite of the lower temperatures, we still had
significant carbon formation very similar to when Mobil was run without the
injector cleaner and the cylinder temperature was higher. After 3.5 hrs, we
pulled the plug and looked with a flashlight. Since we could see carbon
formation on the piston, we stopped the test, pull the cylinder and took
pictures.


Remember, this carbon formation is just in 3.5 hrs rather than 6 hrs.
Mobil 1 MX2T + injector cleaner - Cylinder Mobil 1 MX2T + injector cleaner - Piston



Pennzoil air cooled at 20:1 - gas is 93 octane.
The engine was run for 6 hrs and almost 3 gal of fuel was burned. Engine
temperatures fluctuated 215-220 degrees. Outside temperature was 30-32
degrees. Engine rpm with a 24 x 10 prop was 5900.

After 6 hrs of running, the 1/3 of the piston next to the intake port was
shiny clean. A band of carbon equal to about 1/2 of the area of the piston
ran down the middle of the piston in line with the boost ports in the front
and back. The thickness of the carbon band was 5-10 thousands and it was
black carbon and soft. The area directly in front of the exhaust port was
shiny clean. The dome inside the cylinder was without any carbon buildup.
There was no carbon formation on the ring or ring groves in the piston.
Pennzoil air cooled - Cylinder Pennzoil air cooled - Piston



Pennzoil TWC-3 outboard synthetic mixed 32:1. gas was 93 octane.
The engine was run for 6 hrs and nearly 3 gal of fuel was burned. Engine
temperatures fluctuated around 200 degrees F. Outside temperature was 24-26
degrees F. Engine rpm with a 24 x 10 prop was initially 6130 rpm for the
first hour, 6060 for the second hour then settled to 6000 for the remainder
of the run.

After 6 hrs of running, the carbon formation looked a lot like the Pennzoil
air-cooled. The 1/3 of the piston next to the intake port was shiny clean,
and a band of carbon equal to about 1/2 of the area of the piston ran down
the middle of the piston in line with the boost ports in the front and back.
The thickness of the carbon band was 5-10 thousands and it was black hard
carbon. The area directly in front of the exhaust port was shiny clean. The
dome inside the cylinder was without any carbon buildup. There was no carbon
formation on the ring or ring groves in the piston.

While the piston and cylinder look great, this oil mix really plugs up
exhaust systems with carbon.
Pennzoil TWC-3 - Cylinder Pennzoil TWC-3 - Piston



Belray H1R mixed at 32:1, gas is 93 octane.
The engine was started and tuned slightly rich as in past tests. The engine
temperature soared to 235-240 degrees within 5 min of running. The mixture
was richened a bit more and the engine temperatures dropped back into the
220 degree range for the remainder of the run. However, the engine burned
the 3 gal of fuel in 5.25 hrs compared to a small amount of fuel remaining
after 6 hrs of run time. Outside temperature was 42-48 degreed F. Engine
rpm with a 24 x 10 prop was 5900.

After 5.25 hrs of running at full throttle, there was almost no carbon on
either the piston or within the dome of the cylinder. The little bit of
carbon present on the piston was two small spots in front of the boost ports
close to the piston edge. The carbon was black and very soft. Otherwise, the
piston was shiny clean. The cylinder dome was also shiny clean. This is the
best looking oil tested to this point with lots of oil throughout the engine
and no carbon buildup.

This mix requires about a 15% increase in fuel consumption.
Belray H1R - Piston



I would like to thank Elson Shields for his hard work in running these tests.
Hobbyist like Elson continues to make the hobby informative and enjoyable.
Geistware


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Updated March 1, 2005