Motor Oil 104

I was going to bring up the constant flow pump concept myself as it seemed that nobody else was seeing this (up to now). That was a good pickup. First, it goes back to the principal that doubling the pressure of the same weight oil does not exactly double the flow but it is close. Also doubling the RPM for the same reason does not exactly double the flow but again it is close.

This shows the problem best:

(A) For a 30 wt oil at operating temperature:
RPM....Pressure..Flow
1,000......20 PSI....1
2,000......40 PSI....2
4,000......80 PSI....4
8,000... 160 PSI....8 The maximum flow because of the oil pop off valve at 90 PSI will be 5

(B) For a 30 wt oil at operating temperature
and a higher output oil pump:
RPM....Pressure..Flow
1,000......30 PSI....1.5
2,000......60 PSI....3
4,000....120 PSI....6 The maximum flow because of the oil pop off valve at 90 PSI will be 5
8,000... 240 PSI....12

If we stick with the same weight oil and increase the oil pump output we will increase the pressure and the oil flow too. If we double the oil pump output we will double the pressure and we will double the oil flow.

(C) For a 40 wt oil at operating temperature:
The oil is thicker, has more internal resistance and therefore requires more pressure to get the same flow.
RPM....Pressure..Flow
1,000......30 PSI....1
2,000......60 PSI....2
4,000....120 PSI....4
8,000....240 PSI....8 The maximum flow because of the oil pop off valve at 90 PSI will be 3

(D) For a 40 wt oil at operating temperature
and a higher output oil pump:
RPM....Pressure..Flow
1,000......45 PSI....1.5
2,000......90 PSI....3
4,000....180 PSI....6 The maximum flow because of the oil pop off valve at 90 PSI will be 3
8,000... 360 PSI....12

(E) For a 40 wt oil at operating temperature
with the original pressures
RPM....Pressure..Flow
1,000......20 PSI....0.5
2,000......40 PSI....1
4,000......80 PSI....2 The maximum flow because of the oil pop off valve at 90 PSI will be 3
8,000... 160 PSI....4

The situations (A) and (C) are close to real life, assuming no loss in the system. This is what happens when you change the 30 weight oil to a 40 weight oil in your car:

(A) For a 30 wt oil at operating temperature:
RPM....Pressure..Flow
1,000......20 PSI....1
2,000......40 PSI....2
4,000......80 PSI....4
8,000... 160 PSI....8 The maximum flow because of the oil pop off valve at 90 PSI will be 5

(C) For a 40 wt oil at operating temperature:
The oil is thicker, has more internal resistance and therefore requires more pressure to get the same flow.
RPM....Pressure..Flow
1,000......30 PSI....1
2,000......60 PSI....2
4,000....120 PSI....4 The maximum flow because of the oil pop off valve at 90 PSI will be 3
8,000....240 PSI....8

At 6,000 RPM the maximum rate of flow has been reached with the thinner oil (A). When you go to 7, 8 or 9,000 RPM you do not get any more flow. You only get a maximum rate of 5. The internal forces on the bearings increase but there is no additional flow of oil.

With the thicker oil you reach maximum flow at 3,000 RPM (C). Worse yet is that the maximum flow is now only 3. As we increase RPM to 4, 5, 6, 7, 8, 9,000 RPM we get no additional flow, no increase in lubrication.

Next topic:
“It comes down to the coefficient of friction between surfaces does it not?”

The answer is NO and YES. In a journal bearing it is the FLOW of oil that does it. And the equations show that the separation force between parts is directly related to that flow. The metal parts never touch. The force of a thick oil under extreme pressure however, can make it act as a solid. This will actually cause metallic wear even though there is no metallic contact. Somebody mentioned this before in more detail but I forgot who that was.

On the other hand, a cam lobe is not in a journal. Oil is delivered by simply splashing onto the surface or by oil delivery holes on each lobe as in high performance engines. As the lobe moves oil builds up in front of it. The surfaces then hydroplane as your car tire on a wet road. The parts never touch.

I had a discussion with the head USA Lamborghini tech guy about this. The Murcielago has shims instead of hydralic lifters. I asked how often they needed to be replaced with thicker shims as the miles ring up. He said that the engine has never shown any wear between the cam and shims at 150,000 mile of hard use. He said they would probably never need new, thicker shims. This is under normal use.
In the hot and heavy racing condition new shims may be needed during the life of the engine but not because of wear in the cam or shim. In a hot engine the valve and valve seats wear from the heat and from impact. The valve over time may seats deeper and the clearance from the cam lobe may actually lessen requiring thinner shims. It may also be that in these extreme conditions, there is some cam/shim wear and this compensates for the valve sitting higher. They balance. The Lamborghini mechanics I have spoken to tell me they have tested the clearance in higher mileage cars and never found any to be out of specifications.

More:
I got this from the Amsoil synthetic motor oil web site:

At 150 C, 1,200 RPM 4 ball wear test (not a bearing, just balls rolling in oil from what I can tell)
.....Straight AHR Amsoil 60 weight oil.....wear = 0.39 mm

Same test but with 1,800 RPM instead of 1,200 RPM:
.....20W-50 ARO Amsoil multigrade....wear = 0.39 mm
.....10W-40 XL Amsoil multigrade.......wear = 0.38 mm
.......5W-20 XL Amsoil multigrade.......wear = 0.38 mm

This is a surface lubrication test with rolling balls. All tests were at 150 C. The better results were from the thinner oils at higher RPM. I do not know why. But it does make me think that thicker is not always better. Also, the 60 weight oil was only SJ rated and all others were SL rated.

aehaas

 

Now let us go back to the Ferrari recommended parameters in my 575 Maranello manual. It calls for 75 PSI at 6,000 RPM. The pop off pressure has not been reached. As we now increase the RPM we still get an increase in flow rate. This is what we need and this is exactly what they are recommending. We get our maximum flow at the maximum system pressure, at about the maximum engine RPM of 7,600. There is no bypassing of the oil. All oil pumped goes through the system. There is no waisted BHP pumping oil past the bypass valve back to the oil tank. It is the perfect system.

ali

 

The numbers I used are fairly realistic. They would work for any engine. The flow rate = 1 is whatever your engine flow rate is in liters per minute at 1,000 RPM. So for example, if your engine puts out 1.25 liters per minute at 1,000 RPM then it would put out twice that or 2.5 liters at 2,000 RPM and 5 liters per minute at 4,000 RPM, assuming ideal, laboratory conditions. In reality it is close but not exact.

aehaas

 

Mixing oils.
The best choice is to plan ahead, bring a few cans with you in your travels, particularly if your oil is one that evaporates a lot.

If you are forced to add oil of a different type there are rules.

Use the same type and brand if you can. If you are using Mobil 1 then it is OK to mix different grades but use a close grade when possible. It is not a good idea to mix say 1/2 your oil tank with 0W-30 and 1/2 with 15W-50. This is a no-no.

If there is no Mobil 1 available then use mineral based oils next, preferably Mobil as first choice then any other name brand next.

The last choice is to mix a synthetic of another brand. They specifically say this should not be done.

Good question.

aehaas

 

Finally let us look at a 20 weight oil at operating temperature. We get the same flow out of our constant volume pump but the thinner oil requires less pressure to move through the system. We are going by the rule that we should use an oil that gives us 10 PSI per 1,000 RPM:

RPM....Pressure..Flow
1,000......10 PSI....1
2,000......20 PSI....2
4,000......40 PSI....4
8,000.. ...80 PSI....8

The maximum flow rate has not been reached. If the engine went to 9,000 RPM then the flow would be 9 at 90 PSI, our maximum pressure at pop off. The engine now has 3 times the flow rate as with the 40 weight oil at full RPM. The nozzles at the bottom of each cylinder are spraying 3 times the amount of oil lubricating and cooling this section. Everything runs cooler and the separation forces in the bearings are 3 times higher.

aehaas

ps- Cold engine showing very high pressures because of the thickened oil at startup:

For a 40 wt oil at 75 F at startup:
The oil is thicker, has more internal resistance and therefore requires even more pressure to get the same flow.
RPM....Pressure..Flow
1,000......60 PSI....1
2,000....120 PSI....2 The maximum flow because of the oil pop off valve at 90 PSI will be 1.5
4,000....240 PSI....4
8,000....480 PSI....8
Now the maximum flow comes at 1,500 RPM. This is at idle for some cars. Still the best one can do is to double this flow when the engine is up to full operating temperature around town. This engine requires a thinner oil, like a 20 weight. If you want a multigrade oil then use the 0W-20.

 

I suppose there could be some difference in expansion but that is a lot to me. Maybe one retains more foam. I am only guessing.

aehaas