Oil pressure on a TO30?

Larry

If oil passes bearings quickly (high mass flow rate in terms of mass per second or m(dot)), the temperature rise (dT) of the oil will be smaller, but the net temperature drop (the 'work' done by the oil or 'Q') will be the same or better than for oil flowing more slowly.

The reason for this is that the heat coefficient (Cp) for the oil will only allow heat to be absorbed at a given rate. Varying the rate the oil flows at will only change how hot the oil gets - slow flow, oil gets hotter (and so does bearing) - fast flow, oil stays cooler, but heat removal is better (bearing stays cooler) because of a greater temperature differential between the oil and the bearing.

Increasing oil pressure will reduce oil mass flow rate, and so will make bearings run hotter.

The engineering notation for this, is:-

Q = m(dot) x Cp x dT

In this instance, we can change m(dot), and the change in oil temperature (dT) between oil arriving at the bearing, and leaving, but Cp remains a constant, and Q must satisfy a minimum or better value if the engine is to survive.

Another way to think of it is that you can use a small nuke to boil water in a kettle in a fraction of a second, or achieve a kettle of boiling water with a cooker flame in 15 minutes. The same amount of work (temperature change) is done to the same fluid (same Cp) in each case, but the energy flow rate (the Q) has changed for the same mass.

Hope this helps.

I know this isn't the place to ask this question but it is pretty much the same as the oil pressure/heat transfer question. Please believe me this isn't making fun of your answer which makes sense, in fact should be common sense. Maybe you can shed some light on it.

For many years I've wondered whether when it is raining do you get wetter if you walk to the barn or if you run to it? I almost got divorced over this one because when I posed the question to my wife she decided I'd finally gone over the edge.

I think if you walk, you're only occupying a very small space at any given instant and will absorb only that amount of rain. If you run, you'll occupy more space in that same instant than if you walk and any rain in that space will be absorbed by your body. So, it doesn't matter whether you walk or run you'll get just as wet. Almost the same as your oil pressure/heat absorption problem. No?

I really ain't nuts it's a valid question.

Jimbo

I was perusing the forum for information on oil pumps, pressure, etc for my 1951 TO-30 and came across your "Do I get wetter if I run or walk in the rain?" question. I realize that you posted this long ago but figured since I didn't see an answer I'd throw in my two cents.

This question was posed to us in college Physics. The answer, as it turned out after a fairly short derivation by the Prof, was RUN. It basically came down to an assumption of a constant rate of flow of the rain and the time spent within the flow. If I take less time, I am less wet. I've never forgotten the answer although I'm not sure I could recreate the proof. I'm sure there was at least on integral in it. So good luck, and stay drier by running.

BTW...I always walk regardless of the proof unless it's really a down pour.

gsd

Firstly, it isn't my theory, it is a function of one of the three immutable laws of thermodynamics which go something like this:-

1. Nothing gets hot all by itself.
2. Nothing gets cold all by itself.
3. If you put something hot next to something cold, the cold thing will get hotter, and the hot thing colder, until they reach a state of equilibrium.

Also, I should have called Cp the Specific Heat Capacity, not the heat coefficient - the coffee hadn't kicked in.

To avoid a lecture in thermodynamics (probably not fun or interesting unless you are an engineer) think instead of sitting in your office on a hot day. You have a fan in front of you set to speed 2 - but you still feel hot.

Do you turn it down to speed 1 to cool down more, or up to speed 3? The exact same principle applies to nice cooling oil flowing through a bearing in an engine, but you can get to the point where the oil is flowing so fast that it (a) doesn't reach all the places it should so (b) won't absorb the heat it is required to. Similarly, if oil flow is too low, the oil does not remove the heat fast enough.

Now to answer your question on why engine manufacturers don't all use 150 psi pumps in their engines - it's a balancing act between pressure (since you want the oil to be forced through all the important parts) and flow - which you need to remove the waste heat from an engine.

It is possible to have high pressure and low flow, or high flow and low pressure, or a mixture of the two - but flow and pressure are two completely different things.

There is a rough rule of thumb that states that the flow from a pump rises as a cube of the energy needed to drive it - so doubling the flow does a lot more than double the power required, and you need 8 times more energy.

Hold that thought - and now consider installing a monster oil pump in your engine - how much power (energy) is going to be left to drive the car? A manual transmission will absorb up to 5/8ths of your rated power output - so if your oil pump takes another chunk, performance of your car or tractor will drop fast!!!!

I am running on memory here, but I think something like 30% of the calorific input (from the petrol/gasoline) to an engine is wasted as heat to oil, exhaust, water in the cooling system, and radiant heat, and another few percent is lost to driving the water pump, alternator, oil pump, mechanical fuel pump (if fitted), a/c will rob another 5bhp or so, etc.

Finally - "How will faster flowing oil absorb more heat since the contact time is less?". Yes, the unit contact time of each small 'chunk' of oil is less and each 'chunk' therefore absorbs less heat, but you have more 'chunks' of oil passing per unit time, so the net effect is the same (or better if you include heat exchange theory, where the delta T needs to be higher for more effective heat transfer).

You will just have to take my word on this - it's the way thermodynamics works - it's funny stuff, but it works - just look at the average jet engine - pure thermodynamics is giving you the power needed to lift a jumbo off the ground.

Nuff said on this, I think. Sorry for the long posting.

Another way to look at this...place two identical pans on two identical burners on a stove cooktop. place twice as much water in one as the other. Heat them for awhile. With calibrated thermometers in each check the temperature. Are they the same temperature? No! Does each pan contain the same amout of heat? Except for minor differences of heat loss, they should.

Jimbo

Unfortuantly, or fortuantly I understood everyword and James is correct.

Jeff
MA-Aero/Asto Eng.

Crazy, isn't it, we spend all week playing with clean bits of metal, and relax at weekends playing with really filthy bits of metal that should have been pensioned off years ago.

The whole oil system thing to me is simple. Oil is used to reduce friction. We want just enough oil pressure to force oil between the bearing surfaces. Any excess pressure could be generating heat in the oil itself as it is forced through the pump and lines/channels. I'm sure the oil is involved with heat transfer, but that should be minimal compared to the engine coolant. Hopefully the oil transfers heat out through the oil pan and through the block to the water channels faster that it gains heat from the pistons and bearing surfaces. Unfortunately, my little blurb doesn't set an exact value for oil pressure. In my opinion 5 to 20psi would be adequate.

I just bought a TO-20. It has an external oil filter on the right side of the engine. Anybody know what Fram(or other) filter number I need to replace it?

Then again, you might have a diesel, Standard, Perkins, or other flavour.

As to what oil filter, this will be dependent on which engine you have - and the info you need will be in the archives, somewhere, along with loads of other useful stuff.