Silly oil temp. test

[FowVay]

I had some idle time on my hands today so I thought I'd try something with my lawn mower. I was curious how two different viscosities would react in a air cooled engine with regard to oil temperature. I really didn't expect what I encountered. Are there any good explanations for these results?

I have a 5 HP Briggs engine. I ran the engine for 10 minutes with Shell Rimula SAE 30 oil in the sump. It uses 20 ounces. The outside air temperature was 75ºF. The oil temperature after 10 minutes was 212ºF.

I drained this oil and put in 20 ounces of Castrol 5W-20. I ran the engine for 10 minutes. the oil temperature after 10 minutes was 222ºF.

I drained this oil out and put in 20 ounces of Shell Rimula SAE 30 once again. I started it and ran it for 10 minutes. The oil temperature this time was 207ºF.

Why did the oil run hotter with 5W-20? Was the 5w-20 actually holding more heat from the metal and therefore causing a cooler running engine? Or was the SAE 30 actually keeping temperatures more in line?

 

[Pablo]

I just wonder - in what I consider to be a "loose" engine - if the SAE 30 (ignoring the whole heat transfer part) with a better film strength, kept the frictional heat lower???

 

[Gary Allan]

I would tend to think that the 5w-20 was absorbing more of the heat.

..on the other hand ..the properties of the 5w-20 may have caused more heat to be generated.

This is a ball bearing application ..not a pressurized oiling system so

 

[Brett Miller]

A 5hp briggs has pretty loose clearances and either the flywheel side crank bearing is just an aluminum bushing or on alot of engines the crank runs on aluminum bushing on both sides.

[ May 31, 2004, 09:03 PM: Message edited by: Brett Miller ]

 

[Blue99]

In a splash lubricated air cooled engine, the oil actually helps cool the engine thru heat transfer. The 5W-20 oil runs hotter due to the better heat transfer qualities of a lighter viscosity oil.

At 222F, the 5W-20 temp is elevated 5% and the actually internal engine head temp, if it was measured, would show a decrease, since the lighter viscosity oil is taking away more heat.

As a side note, Briggs approves a 5w-30 synthetic oil (thin, when compared with SAE 30)for use in all air temps above 40F.

 

[Last_Z]

So, the question is.....where does the heat absorbed by the oil go?

There is no oil cooler or heat exchangers of any kind in these motors. Assuming the extra heat in the oil was absorbed from the metal; such heat has nowhere to go but back onto the metal.

 

[FowVay]

These last two posts are exactly what I was thinking. Today after work I'm going to do this test again except instead of checking the oil temp I'm just going to check the temperature of a spot on the cylinder.

As LastZ said, if the lighter oil is absorbing more heat where is it going to go? In order to be a functioning cooling fluid it would have to be able to dissipate the heat into another medium someway. None of these little engines have cooling fins on the sump and they only blow air across the cylinder and head via baffles and ducts mounted around the cylinder.

If the cylinder temperatures are measureably lower then I might run 5/20 this summer just for curiosity sake. If cylinder temps. are higher as the oil temps were then I'll just stick with the SAE 30.

 

[Bill]

Hi Fowvay,

When you say the temperature outside was 75°F,
were you performing this test in the garage
with the garage door open?

Or was this test done in the yard?
If in the yard, was there any wind?
The lawnmower is air cooled and a wind would
cool off the lawnmower,
maybe there was more wind during part of this test?

Just an thought.

 

[unDummy]

Anyone ever measure the "fan output" of the lawnmower blade? Theres your cooling fan!

I also think that the thinner oil would rev higher with all else equal. Could a mildly higher rpm cause an increase in temp?

Was the original sae30 test with old oil(already sheared down) when compared to the final SAE30 run(fresh unused oil straight from the bottle)?

 

[Blue99]

quote:

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As LastZ said, if the lighter oil is absorbing more heat where is it going to go?

---

The metal surfaces of the sump.

The hot spot in the engine is the 300F + temps of the combustion chamber. The oil is transferring heat from the components exposed to the actual combustion temps when it contacts the base of the piston and lower cylinder walls.

BTW Fowvay, an interesting experiment & keep us updated with the results!

 

[metroplex]

The lighter oil isn't doing a good job protecting that air cooled motor. I think the main reason why they spec SAE 30 for those motors is because of its shear resistance and durability under extreme conditions. I'd say the lack of an oil filter coupled with its reliance on air cooling in such a small package running around on your lawn would count as fairly extreme.

No one uses 5W-20 in gas engines in the desert.

 

[FowVay]

Bill, I tried to make this as standardized as I could. I had the mower in my driveway and the air was relatively still throughout the whole exercise. The SAE 30 was fresh as was the Castrol 5W-20. When I put SAE 30 back into the engine after the 5W-20 run I just reused the stuff that I drained out from the first run.

As for the mower running at higher speeds, the governor is controlled by a flap that sets its position based on air blown from the fan. If the fan spins faster then it produces more airflow and therefore causes the governor to reduce the throttle opening and bringing the engine speed back down.

I like the idea of the thinner oil being able to remove heat from the combustion section and transferring it to the case halves.

While I was fooling around I did stick the thermocouple on the spark plug and read a temperature of 260ºF and I stuck it inside of the muffler where the exhaust gases were 430ºF. I didn't really think to measure the head and record the temperature with the two different oils.

I know that it isn't scientific but it's fun to mess around with this stuff. I have a ancient tiller that has a old briggs engine that was put on the tiller in approximately 1977. It doesn't see much use and is actually in relatively good condition. It has a decal on the engine that says 5W-20 is approved for use below 32ºF. I was shocked and didn't know that they had such oils back then.

 

[XS650]

quote:

---

Originally posted by FowVay:

I like the idea of the thinner oil being able to remove heat from the combustion section and transferring it to the case halves.

While I was fooling around I did stick the thermocouple on the spark plug and read a temperature of 260ºF and I stuck it inside of the muffler where the exhaust gases were 430ºF. I didn't really think to measure the head and record the temperature with the two different oils.

---

If the 0W-20 is removing more heat from the top end of the engine, it will also be losing more heat through the cases. Net result will be cooler oil not hotter. You 0W-20 is running hotter because your engine isn't as happy with such thin oil.

Stick with the therocouple under the spark plug. It gives agood indication of head temperature and any error will be fairly consistant, which is what you need for comparisons.

 

[jason07-1zzfe]

Why would the heat transfer only work in one direction. I'd have to agree with Xs650

Higher oil temps are not a good thing.

 

[T-Keith]

If this is a Flathead style engine, there won't be any oil in the head. In fact there won't be much, if any oil by the combustion chamber at all.

-T

 

[FowVay]

Yes, it's a flathead design. The only oil in the cylinder areas is what is splashed on the cylinder wall by the little slinger thingy on the connecting rod.

I'll play around and see what temperatures I can come up with on the cylinder head. If I don't get any temperature change then I'm going to buy a quart of SAE 50 and see what it does.

I wonder how diesel fuel would do as a crankcase lubricant?

 

[Jay]

What is your variability? If you run the engine 3 times with the same oil, what temperature variations do you have?

 

[XS650]

quote:

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Originally posted by Jay:
What is your variability? If you run the engine 3 times with the same oil, what temperature variations do you have?

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A very good comment.

FowVay, if you do the three or more runs with the same oil, be sure to record the ambient air temperature too. Since your engine has no thermostatic temperature control, the difference between air temp and oil temp will be more significant than oil temp alone for the purposes of your tests.

 

[Blue99]

quote:

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If the 0W-20 is removing more heat from the top end of the engine, it will also be losing more heat through the cases.

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Yes, exactly the reason that the temperature of the oil in the sump is running higher. The oil is transferring heat to the sump surfaces and is generating an elevated sump temperature.