downunder

It seems from another thread that Clarence Lee is perpetuating the myth that oil carries heat away from the engine through the exhaust and helps cool the engine. Now I'll agree that it plays a miniscule part, after all any oil that comes out is hot so it must do something. But how much? In my opinion very little and here's my reasoning. I'm going to work in metrics because it's so much easier.

The critical thing is the rate of oil flow. The more oil that passes through the more heat it can pick up. So how fast does oil pass through an engine? Well let's try for some ball park figures. There's no point trying to be too accurate because of variations in % oil in the fuel and state of tune (rich/lean).

Assume there's a 300cc (10 ounce) fuel tank, an engine that will run for 10 minutes at an average of 10,000 rpm on that tank before a dead stick. And the fuel has 20% oil. OK, that gives 60cc of oil to last for the 100,000 revs that engine will turn in those 10 minutes.

Now, the maximum amount of oil that can come out the exhaust per cycle (and carry heat away) is limited by what goes in with the fuel. In this hypothetical case it's the 60cc divided by the 100,000 revs turned which is .0006cc of oil/rev. For the Americans, that's about 20 millionths of an ounce and if my maths are right that's the same as a drop of oil 1mm in diameter (or just over 1/32"). That seems way too small to carry away any appreciable heat.

I'd better add that if it comes out as oil then latent heat of vaporisation doesn't enter in to it.

Let the arguments begin

mpascual

My Feedback: (1)

I agree with your calculation, not with your conclusion, so then why use NITRO ?
0,0003 cc. of nitro / rev. at 10% fuel .

Hobbsy

My Feedback: (102)

20 millionths of an ounce sure does mess up a plane.

JimTrainor

Could there be secondary reasons why it is important. Such as oil increasing the heat transfer rate between the piston and cylinder thus helping to cool the piston? The oil is vital to heat conduction between moving parts but not part of the ultimate tranfer of heat out of the engine - which would be due to exhaust gasses and convection?

AirGar

My Feedback: (7)

BWAAAAAAAA HA HA...good one Hobbsy...dat is toooo funny!

Gary

gubbs3

My Feedback: (1)

That's an interesting peice of info but lets put it into more appreciable terms.

If there's 60cc of oil that runs through the engine in ten minutes that's 6cc every minute. If you're running 20% fuel that is 6cc out of 30cc that is dedicated to pulling heat out of the engine.

And yet again if you were to richen up the mixture so you would run out of fuel in 8 minutes rather that 10 that would be 7.5cc/minute. So...every minute, you could fill the cylinder with oil to cool the engine.

Sounds pretty significant to me.

Rudeboy

The oil comes goes in as a liquid... and comes out as a gas... because it evaporates during combustion...

Put 60 cc's of oil in a pan, heat it until it starts to smoke... now push in the timer and wait until all the oil is gone...
I bet it is going to take a while!

Now, of course not all the oil evaporates during combustion... a large part of it is just "mist-ified" (don't know how else to put it ), but I still think it plays a significant role.

Somebody should put an oxygen sensor in an exhaust, run different fuel blends through the engine (without nitro, just varying oil contents), tune the engine every time until the oxygen sensor reads the same... and measure the head temp every time... that should clear it up...

Jim Thomerson

OK, I'm running a diesel, with 33% castor oil. The exhaust is cool enough that I can stick my finger in to see what color oil accumulates on my finger (the things we diesel people worry about!). If I do the same thing with a glow engine with any percentage of any kind of oil, I start screaming and hollering and my finger gets red and blisters.

Why this is so is not relevant to this discussion, but it seems clear that the oil coming out of the diesel is not taking a bunch of heat with it.

Jim

Hobbsy

My Feedback: (102)

One difference with the Diesel is that the combustion process is finished before the exaust port opens and the exhaust has already started to cool, the glow engines combustion is still on going when the exhaust port opens, hence the blue flame that comes from the glow exhaust port wehn running an unmuffled glow engine in the dark.

William Robison

My Feedback: (3)

DU:

Yes, there is a very small amount of oil going through the engine with each complete cycle. But something is carrying heat out. If it were not so the exhaust would be at ambient temperature.

Bill.

Ross Kean

There are a number of ways that heat gets out of an engine:\

1) Convective cooling of air over the head
2) Adiabatic expansion of the compressed gasses (somewhat countered by the heat gain of compression)
3) Physically carried out by the hot exhaust gasses
4) Evaporative cooling of the methanol in the carb
5) Carried out with hot oil
6) Converted to energy which is actually propelling the airplane (eventually lost to air friction).

In view of the expected magnitude of the other sources of heat loss along with the relatively small volume and small heat capacity of liquid oil (can't consider heat of vaporization), my wild-***** guess is that heat transfer (heat removal) due to physical carry out with the oil is not going to be very large - certainly not in comparison to the heat energy contained in the original alcohol/nitromethane.

I am willing to listen to anyone who has actually put numbers to all of the various factors but until I see some real numbers, the oil thing falls into the category of Urban Legend!

Ross

Sport_Pilot

The exhaust of my car is hot also, yet there is no oil. Obviously if oil is heated and goes out hotter then some heat is taken out by the oil. But I doubt it is enough of a factor either way to choose one oil over another.

DarZeelon

Metals, copper and iron more than aluminium, have a lower specific heat capacity than liquids.
Water and brass for instance; the water (4,216 J/kg Deg. C) has roughly ten times the heat capacity of the brass, which is an alloy of iron (440 J/kg Deg. C) and copper (385 J/kg Deg. C).

This would mean is an ounce of water contacts an ounce of brass, the brass cools down by 10 degrees, for every 1 degree the water heats up.

See [link=http://www.apo.nmsu.edu/Telescopes/SDSS/eng.papers/19950926_ConversionFactors/19950926_MProperties.html]this table[/link].

Oil is not water and the specific heat values for Castor oil is around 1,800 J/kg Deg. C. Not as effective as water, but still 4.5 times higher than brass. It will still reduce the heat of the metal it contacts, by more than you can imagine.

Methanol, BTW, has a specific heat value of 2,547 J/kg Deg. C, so it is even more effective than the Castor oil.

Sport_Pilot

OK where did you find the heat capacity of Castor oil? Did you find the capacity of synthetic oil? If so which has the higher heat capacity? I suspect nitromethane has a much higher heat capacity than methanol since it is denser than water.

DarZeelon

Hugh,

You can look down the table [link=http://www.engineeringtoolbox.com/24_151.html]here[/link] for the heat capacity of Castor oil. It is in KJoules, so I converted to Joules.

I did not find the number for synthetic oil, since I don't actually know the type of oil discussed.

Since it is a specific value, i.e. per kilogram, nitromethane will be no different. It is all per kilogram, irrespective of the density.

Take a look at nitro prices [link=http://www.worldwideracingfuels.com/catalog_c30758.html]here[/link]. It sounds too good to be true. Only $30 a gallon; less in larger amounts.

Red B.

O.K. here goes:

The heat power dissipated by heating the oil (not considering the heat of vaporization) and emitting it with the exhaust is:

power = heat capacity * density * oil flow * temperature increase

The heat capacity of castor oil is 1.8 J/g/K and the density is 0.98 g/cm3.
In you example the oil flow is 60 cm3 / 10 minutes = 60 cm3 / (10*60)s = 0.1 cm3/s.
If I assume that the exhaust gas temperature is approx 700 deg. centigrade above the ambient temperature (average for full size race car engines running on methanol) the heat power disssipated will be

Power = 1.8 J/g/K * 0,98 g/cm3 * 0.1 cm3/s * 700 deg. centigrade = 123 Watt

This may seem like a lot but consider this:
Assume that the power output of the engine is 1 b.h.p (approx. equal to 735 W if I remember correctly). Typically the efficiency of an ordinary combustion engine is not above 20% and thus 735 W / 20% = 3675 W of heat need to be dissipated in order to produce 1 b.h.p power output.

Thus the castor oil carries away only 123/3675 = 3.3% of the total heat generated by the engine! Obviously other processes are much more important for carrying heat away from the engine.

/Red B.

jessiej

Interesting info, Dar. Thanks!

jess

DarZeelon

Red,

I was not saying he engine is primarily cooled by the Castor oil, but it contributes.

Not all the unused energy of the engine becomes heat. Some does, and despite the relatively small cooling fins, they remain cool enough to keep running.

Sport_Pilot

Yes but despite that dense materials tend to have a higher specific heat, though there are a lot of exceptions.

Sport_Pilot

Thats without shipping, unless you can ship it commercial it will be about $50 per gallon.

smokingcrater

from a physics standpoint, 100% of the energy produced becomes heat, so out of the total energy available (in the fuel), it either goes one of two places: out the exhaust as raw fuel, or to contribute to heat somewhere. (techincally even the prop spinning heats the air)

(with a small disclaimer, a substantial portion of the energy goes to making noise also, which is another form of energy, just not energy in the same spectrum as the other mentioned)

Sport_Pilot

rkramer,

Ah, Ah, Ah!

Lets not forget about entropy! Not all energy is recoverable!

downunder

OK, let's play with those figures. One gram of oil will absorb 1.8 Joules for every degree C rise in temp. Castor has almost the same SG as water so our .0006cc of oil weighs .0006 grams. This means that it takes 0.0011 Joules to raise it's temp 1 degree C. If this droplet gets 200C (392F) hotter then it's absorbed 0.22 Joules of heat. If 400C (752F and not likely!) hotter then it's absorbed 0.44 Joules.

Now the heat energy released by burning methanol is rather considerable and in keeping with Dar's metrics it's about 23.2 million Joules/Kg. To shortcut the maths, each droplet of methanol per cycle releases 464 Joules. If the engine is 30% efficient this leaves 325 Joules as waste heat or roughly 1000 times more than the oil can carry away. A good reason to use cooling fins

Sport_Pilot

Very true, but on any engine most of the heat goes out the exhaust. Not just the oil but the combustion gases, this includes gas four cycle engines which has very little oil going out the exhaust. But we don't want the heat to go out the exhaust, or the cooling fins if we can help it. The more heat going toward spinning the propeller the more efficient the engine will be. I doubt our glow engines are even 20% efficient.

smokingcrater

this topic is more heated then the old castor vs synth debate! [8D] (sorry, late in the week, couldn't resist that one!)