8 replies to this topic

[flymo]

The other day as I was driving my car (powered by a 1300cc bike engine) around the track and watching my oil pressure gauge, I got to wondering about the relationship between oil flow and rpm, specifically in engine bearings. Can anyone share any insight into the way that oil flow changes with engine speed ?

Oil pumps obviously provide more oil at higher rpm (up to the point that the regulator opens), but I was wondering if this is really necessary. If one had a constant pressure lubrication system, it seems to me that the bearing gap flow would remain quite consistent, since any boundary layer that developed in the oil would become thicker than the gap at quite low rpm.

How do the pressure systems in fancy-pants F1 engines and such deliver oil, and is there much loss to be recovered compared to a garden-variety pump-and-dump-valve system.

[shaun979]

Sorry I don't have any good answers for you, but I recall reading that the BMW V10 engine in the new M5 has an oil pump that varies pressure and/or volume with engine load or engine speed... can't remember the exact details but it is something like that. If anyone can tell how the pump works, that would be great.

[J. Edlund]

Originally posted by flymo
The other day as I was driving my car (powered by a 1300cc bike engine) around the track and watching my oil pressure gauge, I got to wondering about the relationship between oil flow and rpm, specifically in engine bearings. Can anyone share any insight into the way that oil flow changes with engine speed ?

Oil pumps obviously provide more oil at higher rpm (up to the point that the regulator opens), but I was wondering if this is really necessary. If one had a constant pressure lubrication system, it seems to me that the bearing gap flow would remain quite consistent, since any boundary layer that developed in the oil would become thicker than the gap at quite low rpm.

How do the pressure systems in fancy-pants F1 engines and such deliver oil, and is there much loss to be recovered compared to a garden-variety pump-and-dump-valve system.

Oil consumption of an engine is basically quite unaffected by engine speed. If the oil consumption is seriously affected by engine speed, there's usually something wrong.

The typical lubrication system works with a constant oil pressure. But since oil consumption is "constant" and the flow capacity of the pump increase with engine speed, a pressure regulator is needed to keep the oil pressure constant.

Without the pressure regulator the oil pressure would either get too high at high speeds or too low at low speeds.

Originally posted by shaun979
Sorry I don't have any good answers for you, but I recall reading that the BMW V10 engine in the new M5 has an oil pump that varies pressure and/or volume with engine load or engine speed... can't remember the exact details but it is something like that. If anyone can tell how the pump works, that would be great.

I think you have mixed up the water and oil pump here. The BMW engine has an electric coolant pump that can adjust the flow according to the need.

[Hans Derbe]

I think you have mixed up the water and oil pump here. The BMW engine has an electric coolant pump that can adjust the flow according to the need.

I don't know exactly if the M5 engine has a variable oil pump but the 3l I6 has one. The pump has to deliver a high volume flow at low revs for the cam phasers to work poperly together with the valvetronic variable valve train strategies. This design also saves 2kW at max power, compared to a conventional design.

Hans

[shaun979]

Originally posted by J. Edlund


I think you have mixed up the water and oil pump here. The BMW engine has an electric coolant pump that can adjust the flow according to the need.

"This system incorporates two electrically-operated duo-centric pumps which pick up oil from the outer cylinder head and transport it to the main oil sump if lateral acceleration rates exceed 0.6 g. A lateral-g sensor transmits signals to the pumps. The oil pump itself is a continuously variable pump with volume control which delivers exactly the amount of engine oil needed by the engine. This is achieved by the variable eccentricity of the pump’s rotor in relation to the pump casing, depending on the oil pressure in the main oil duct." - BMW M5 press release

Variable volume reduces parasitic losses I guess...

[Supercar]

Originally posted by shaun979


"This system incorporates two electrically-operated duo-centric pumps which pick up oil from the outer cylinder head and transport it to the main oil sump if lateral acceleration rates exceed 0.6 g. A lateral-g sensor transmits signals to the pumps. The oil pump itself is a continuously variable pump with volume control which delivers exactly the amount of engine oil needed by the engine. This is achieved by the variable eccentricity of the pump’s rotor in relation to the pump casing, depending on the oil pressure in the main oil duct." - BMW M5 press release

Variable volume reduces parasitic losses I guess...

Without seeing the picture of this variable pump, it sounds to me like a vane pump that is used in every power steering system.

[Stian1979]

The oil consumtion should be the same as the clerance is the same at all engine speed's.

A lot off marine diesels have electrical pumps running at constant volume whatever rpm and load they have.

Smaler engines has oilpump driven by a the engine itself and the volum is therefor variable.

[persovik]

Oil transport and evacuation from the bearings are quite dependant on engine rpm, and while the lubrication is fully dependant on the film strength of the oil, you certainly want to keep the flow high enough to replace any broken oil-film as quickly as possible. Since the film strength is dependant on temperature, higher loads require higher flow to keep the temperature right.
At higher rpm, liner and piston lubrication and piston cooling becomes more critical, so you want a higher flow to these areas.
In a road-going engine, oil loss through seals, scraper rings and valve guides have to be kept at a reasonable level, say 1 litre/10.000 kms, while 1 litre/100 kms is quite acceptable in a race engine. Therefore, you can have a higher pressure, increasing the loss thru seals as long as you don't blow them. The scraper ring in the piston will be less eficcient at high rpm, giving poorer oil distribution and higher loss. Generally also, higher rpm means higher temperatures and therefore need for more clearance, increasing consumption.
Althoug you would want to open up the lubrication channels in a high rpm engine, and therefore have higher flow at a "normal" pressure, there is extra safety in having both higher pressure and higher flow capacity from the pump.
You want to keep the pump as simple as possible, so it doesn't break or malfunction, and at the same time keep it from drawing too much power, so you tend to end up with a high capacity solution that isn't quite up to providing good pressure at idle after hard running.

[J. Edlund]

Originally posted by shaun979


"This system incorporates two electrically-operated duo-centric pumps which pick up oil from the outer cylinder head and transport it to the main oil sump if lateral acceleration rates exceed 0.6 g. A lateral-g sensor transmits signals to the pumps. The oil pump itself is a continuously variable pump with volume control which delivers exactly the amount of engine oil needed by the engine. This is achieved by the variable eccentricity of the pump’s rotor in relation to the pump casing, depending on the oil pressure in the main oil duct." - BMW M5 press release

Variable volume reduces parasitic losses I guess...

So basically they have two electric scavenge pumps for the heads and a variable displacement pump as pressure pump.

By the way, here's some data from oil consumption measurements, the engine is a 2 litre inline four with 16 valves and 100 kW/180Nm using a SAE 5W30 oil with 3.6 bar and 140 degC. The crankshaft is crossdrilled.

Large end oil flow for one bearing

Half groove main
rpm - oil flow (l/min)
3000 - 0.6
4000 - 1.0
5000 - 1.4
6000 - 1.4
6500 - 0.9

Full groove main
rpm - oil flow (l/min)
3000 - 0.7
4000 - 1.0
5000 - 1.6
6000 - 2.1
6500 - 2.2

Large end oil temperature rise

Half groove main
rpm - temperature rise (degC)
3000 - 11
4000 - 14
5000 - 19
6000 - 26
6500 - 36

Full groove main
rpm - temperature rise (degC)
3000 - 8
4000 - 12
5000 - 17
6000 - 24
6500 - 26

Effect of engine load

Load - Large end (degC) - Total flow (l/min) - Main 1 (degC)
No load - 15 - 2.92 - 9
Full load - 21 - 3.00 - 11

Oil flow though the main bearings are more influenced by the type of groove used than rpm.

Note the decrease in oil flow after 6000 rpm with the partial groove. At this point the pressure caused by the centrifugal forces begin to equalize with the supply pressure and thereby reduces the oil flow to the con rod bearing.