Oil tank pressure

Sean

The oil tank pressure is relative to the atmospheric pressure.  The incoming pressure that drives the oil from the crankcase, from blow-by gas pushing into the crankcase by normal ring leakage, will always keep the pressure over the oil in the tank just slightly higher than ambient.  There is always some light pressure in the top of the oil tank that requires venting.  

In regards to very high altitude this causes oil pressure drop relative to standard oil pressures you see at or near sea level.  High altitude drones sometimes use a device on the oil vent line to trap some of this venting air to keep oil pressures in a more normal range.  These are switched on (I have not seen an automatic one as yet) starting around 20 to 25k MSL.  This maintains the pressure over the oil in the tank to flow better and you see it in less drop of oil pressure compared to without such a device.  The one i am thinking of maintains about 5 psi in the oil tank over ambient. 

Cheers

Thanks RW,

Your answers are as I expected, both in the uncontrolled (standard) & controlled venting (high altitude) of crankcase pressure.😈

Just one note. i'd watched the Carbon Cub altitude record video and they speak of little oil circuit modification to compensate for low oil pressure at altitude.

From previous post in rotax owner it seems that the pressure loss effect with altitude shows only at high rpms:

At altitude there is a strange combination full throttle - high rpm for climbing but very low power  - low map due to the low ambient pressure. Even for a turbocharged engine at 100% throttle over a certain altitude (it seems for 916is around 24000 Ft)  map goes down.

But low map means less mean combustion pressure and therefore less blow by gases.

So we have high work of oil pump (high rpm) but low scavenge/return of oil in the tank and less tank pressure.

This can be the mechanism even i do not understand why it is required a slight oil tank positive pressure for correct oil circuit operation

hypothesis: excessive oil suction from the pump (high rpm) with little scavenging/return can lead to depression in the tank with difficulty in oil suction (also increased by the reduced atmospheric pressure - the tank is not hermetic)

Hi Nicola

The turbo can only compensate till the air density is such that it can no longer spin fast enough to build MAP.  The result is at about 5000 to 6000 meters (16404 to 19685 feet) you start to lose MAP as the compressor wheel is too small for the air density to keep the MAP in balance.  The Oil tank pressure, basically ambient, starts to drop with altitude.  

"A Google check shows this:The air pressure at 7300 meters, about 24,000 feet is approximately 5.7 PSI (pounds per square inch). This is significantly lower than sea-level pressure (about 14.7 PSI) due to the decrease in the weight of the air column above. " 

Given this pressure loss over the oil in the tank this will show as a drop in oil pressure of that relative pressure (14.7 -5.7 gives you a oil pressure loss of 9 PSI).  For this reason high altitude drones, and special flights like the record flights done in recent weeks, use a valve to maintain an additional (approximate) 5 PSI over the oil to compensate.  As you can imagine the oil pump cant pull much suction, it pushes much better than pulls oil.  The pump speed is governed by the crankshaft, turning at ½ half crankshaft speed driven off the camshaft end.  

Cheers

Thanks for your knowledgeable replies.

Now I understand the Rotax gear pump's pressure depends on the pressure of the oil it draws in. So I also understand the use of an added valve on the oil tank vent for very high altitude usage.

At this point, however (and since we're talking about the cub 916is record attempt), doesn't altitude also affect turbo oil recovery from scavenge pump (which also depends on the ambient air pressure in the oil tank below the turbocharger)? 

And turbo oil sump position is well below turbo oil recovery pump...