Mercedes Power Unit Hardware & Software

[PhillipM]

These units will be hotter than 130 out of the bearing, the operating bulk temperatures are in the 130 region.

[godlameroso]

I am afraid that is incorrect - oil viscosity is exactly what determines hydrodynamic lubrication. Oil flow is what carries heat away from the bearing, maintaining viscosity and implicitly - the oil film thickness.

130 C sounds about right for oil leaving bearings - around 120 C I think is the texbook design value.
Typically a very simplistic way of looking at a bearing is to calculate the Hersey number, estimate friction then estimate frictional power loss at a given speed. Then assume a 60-40 split between heat carried away by oil and heat conducted into metal. This, together with the pressure drop caused by bearing clearance gives you a good starting point to estimate oil supply requirements.

Are you sure about that? I know I'm just a humble wrench jockey so I could be wrong, the reason you don't want to floor the engine during start up is precisely because oil cannot flow well when it is so thick. I was under the impression that film strength is the last line of defense as far as lubrication is concerned, and viscosity and film strength are not necessarily the same things. And that anti-wear additives mean more as far as film strength is concerned than simple viscosity.

Very sure - no mass flow terms in reynold's equation.

The reason you don't floor it hot is because there is no oil whatsoever in the bearings. The pressure drop across the lube system is also high at low temp high viscosity so you risk blowing filters.

Other reasons for warming up the engine are related to the transient thermal effects such as pistons growing faster than the bores or exhaust manifolds heating up faster than the heads, etc.

Floor it cold you mean. And yes the second the oil pump starts sending oil through the system, it's getting in the bearings, at least 3 psi or most road cars wouldn't start, but then again, oil pressure does not equal flow. Otherwise yes I agree, aluminum expands much faster than iron liners, the expansion rate is even more critical with eutectic liners.

[godlameroso]

These units will be hotter than 130 out of the bearing, the operating bulk temperatures are in the 130 region.

That goes for all ICE's oil temperature at the bearings is roughly 30-40c hotter than the sump temperature.

[Phil]

No pictures of the Mercedes PU in the back of the Force India running around half a lap without its engine cover around Sochi FP2 yet?

[PlatinumZealot]

Apparently as much as 5 liters of oil are consumed in a race distance. And I thought my track car burned a lot of oil at a little less than a liter in 1,500km. 5 liters is essentially the entire sump of an average 4 banger, and most Mercedes road cars only take about 8 liters total.

Do you believe that number?

The engine is dry sump. The housing and oil channels have to have a certain volume of oil for proper hydraulic behavior of the oil.
The oil coolers have a set volume too.
Then the oil reservoir, oil tank, separator whatever you want to call it... It has to have a minimum volume of oil to work properly. Now this is the only part with variable oil levels. Then ask your self this question is the oil tank at least 5 liters in size? (1.3 gallons?) Yes it looks like it can be. but it would have to be probably three times that size for the engine to burn 5 liters of oil and still have enough oil in the reservoir. And The Oil tanks we see in Mercedes and Ferrari are not that big. So I don't believe that 5 liters figure. If you had said 1 litre I might.

[godlameroso]

Apparently as much as 5 liters of oil are consumed in a race distance. And I thought my track car burned a lot of oil at a little less than a liter in 1,500km. 5 liters is essentially the entire sump of an average 4 banger, and most Mercedes road cars only take about 8 liters total.

Do you believe that number?

The engine is dry sump. The housing and oil channels have to have a certain volume of oil for proper hydraulic behavior of the oil.
The oil coolers have a set volume too.
Then the oil reservoir, oil tank, separator whatever you want to call it... It has to have a minimum volume of oil to work properly. Now this is the only part with variable oil levels. Then ask your self this question is the oil tank at least 5 liters in size? (1.3 gallons?) Yes it looks like it can be. but it would have to be probably three times that size for the engine to burn 5 liters of oil and still have enough oil in the reservoir. And The Oil tanks we see in Mercedes and Ferrari are not that big. So I don't believe that 5 liters figure. If you had said 1 litre I might.

I remember reading that 5 liter figure somewhere, not quite sure where. Total capacity is probably around 15 liters.

[PhillipM]

I'd be amazed if it was, that's a lot of extra weight to lug around, the old N/A motors used to be about 4 litres.

[gruntguru]

5 litres consumption sounds about right to me. Assuming they use about 140 litres for the race, 5 litres is about 3.5% of fuel used. Although that is on the high side, you need to remember that a normal race engine with the same power would use about 240 litres of fuel and 5 litres would be 2% of fuel used.

[PlatinumZealot]

This consumption takes place on the cylinder walls right?

[gruntguru]

Oil loss would be at the usual locations - compressor seal, valve guides, piston rings, turbine seal crankcase ventilation system. Of these, only leakage past the turbine seal and exhaust valve guides would not participate in the combustion.

[J.A.W.]

Oil loss would be at the usual locations - compressor seal, valve guides, piston rings, turbine seal crankcase ventilation system. Of these, only leakage past the turbine seal and exhaust valve guides would not participate in the combustion.

Wouldn't exhaust back-pressure due to the turbo - tend to push oil up the valve guides, overall?

[PlatinumZealot]

The valve seals are above the valve guides abut i guess you have to allow some wetness at the lip of the seal.. Despite a slight vacuum in the cylinder head there might be a pumping action under the valve seals as the vavles reciprocate?

What i notice about valge seals is that when the valve is pushed upwards the lip of the seal "opens" up a bit to allow oil to enter. So during the cycles oil is on the stem. Now on the other side of the seal the seal lip is like a check valve and the same up and down motion might be pumping pop down the valve stem.

[godlameroso]

The valve seals are above the valve guides abut i guess you have to allow some wetness at the lip of the seal.. Despite a slight vacuum in the cylinder head there might be a pumping action under the valve seals as the vavles reciprocate?

What i notice about valve seals is that when the valve is pushed upwards the lip of the seal "opens" up a bit to allow oil to enter. So during the cycles oil is on the stem. Now on the other side of the seal the seal lip is like a check valve and the same up and down motion might be pumping pop down the valve stem.

Most valve stem seals have a metal ring around the top of the seal, which forces the hole in the seal to act like a squeegee, very little oil if any should end up on the guides.



I'm pretty sure those puffs of oil we see the Mercedes blow out when they sit and idle for a while are from the turbo seals, and not leaky valve guides. You don't want oil seeping past the seals and through the guides, when that oil cokes up from combustion heat it makes the valve seats less effective at sealing compression. Especially with direct injection, although not much you can do about it in this instance, given the high oil consumption, perhaps a lot of it is getting burned pre-turbine. Would make sense if the goal is to keep the exhaust gases hot, what better way to do it than to burn oil to do so.

[PhillipM]

Actually even with those springs there the seal lip itself is specifically designed to allow a small amount of oil past to let the seal run on a film of oil.

[gruntguru]

Of course the guides themselves need oil lubrication. It gets there by reciprocation, regardless of adverse pressure.