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Oil cooler is needed because of the high pressure the fuel injection system needs to see during acceleration. The HPOP makes pressure around 4,000 psi during high fuel demand. That makes a ton of heat in addition to the normal heating the oil sees.
Compressing a liquid doesn't really add much heat compared to compressing a gas (feel the head of an air compressor after it's been running a while). I think more of the heat comes from the oil going thru the turbo and the piston squirters.
Even non HEUI diesels have pretty large oil coolers. My diesel shop was telling me that Duramax's have undersized oil coolers and frequently run into oil temp issues under heavy load.
Compressing a liquid doesn't really add much heat compared to compressing a gas (feel the head of an air compressor after it's been running a while).
That's quite right, compressing a gas will raise the temp according to Charles Law.
And liquids don't compress very much at all.........so there little or no heat added due to compression........
But in a hydraulic system there is considerable heat generated in the hydraulic pumps(due to friction) ........ Aircraft hydraulic systems use a heat exchanger to cool the fluid. They're frequently placed in a fuel tank since the fuel is the coolest liquid available.
In many big aircraft there's a minimum fuel level to operate the hydraulic system on the ground. (for example, B737 (3000psi system) with an approx 10,000 lb capacity main fuel tank, min fuel in that tank is 1675 lbs[approx 260 gal] just to run 1 hyd sys)
Originally Posted by Rusty Axlerod Oil cooler is needed because of the high pressure the fuel injection system needs to see during acceleration. The HPOP makes pressure around 4,000 psi during high fuel demand. That makes a ton of heat in addition to the normal heating the oil sees.
Absolutely right!
Our 6.0L hydraulic system is an up to 4000 PSI system. Although the engine generates a fair amount of heat on it's own, the pump operation adds a LOT of additional heat to the oil requiring a pretty robust oil cooler.
I'm curious though, why BPD doesnt offer one with a fan ?
Probably because theirs is mounted to the radiator. It was mounted behind the cab under the truck for example, you would want a dedicated t-Stat controlled fan.
I'm not sure about the principals of operation but we've been using these things for years now. It's powered by a small turbine engine. In addition to 90KVA of 400hz AC output, it does 110v, and DC output, air for starting another engine, and a serious hydraulic panel.
The hyd panel has two methods of changing pressure, one is a three position electric switch. Center off, momentary UP and DOWN. I believe this changes the stroke on the pump somehow (or operates a bypass circuit inside the pump). There is also a mechanical regulator just behind the output connection on the panel. I know this thing will run for hours without overheating if you adjust the pressure to 3000psi with the switch and have the regulator set to 3,100 but if you run the pump up to 3,100psi and pull it back down to 3,000 with the mechanical regulator, it gets real hot, real quick. Again, I'm not up to speed on the principals at work here but the heat is generated AT that mechanical regulator, and is enough to overheat the whole system in just a few minutes of operation.
That's quite right, compressing a gas will raise the temp according to Charles Law.
But in a hydraulic system there is considerable heat generated in the hydraulic pumps(due to friction) ........ Aircraft hydraulic systems use a heat exchanger to cool the fluid. They're frequently placed in a fuel tank since the fuel is the coolest liquid available.
In many big aircraft there's a minimum fuel level to operate the hydraulic system on the ground. (for example, B737 (3000psi system) with an approx 10,000 lb capacity main fuel tank, min fuel in that tank is 1675 lbs[approx 260 gal] just to run 1 hyd sys)
Absolutely right!
Our 6.0L hydraulic system is an up to 4000 PSI system. Although the engine generates a fair amount of heat on it's own, the pump operation adds a LOT of additional heat to the oil requiring a pretty robust oil cooler.
Cheers,
Rick
Thanks for the info! My background is in centrifugal pumps for building heating/cooling where we are only producing 50-100psi and the heat load is negligible. Not 4000psi with very little flow (comparatively). A little different!
Curious, how do they keep the fuel temp down on the planes? Or is it just a matter of so much mass of fuel that the heat load is insignificant? Also, I guess during flight the wings are out in the whatever very cold temperature the air is at 35,000 ft so the fuel is constantly being cooled anyways.
I'm not sure about the principals of operation but we've been using these things for years now. It's powered by a small turbine engine. In addition to 90KVA of 400hz AC output, it does 110v, and DC output, air for starting another engine, and a serious hydraulic panel.
The hyd panel has two methods of changing pressure, one is a three position electric switch. Center off, momentary UP and DOWN. I believe this changes the stroke on the pump somehow (or operates a bypass circuit inside the pump). There is also a mechanical regulator just behind the output connection on the panel. I know this thing will run for hours without overheating if you adjust the pressure to 3000psi with the switch and have the regulator set to 3,100 but if you run the pump up to 3,100psi and pull it back down to 3,000 with the mechanical regulator, it gets real hot, real quick. Again, I'm not up to speed on the principals at work here but the heat is generated AT that mechanical regulator, and is enough to overheat the whole system in just a few minutes of operation.
the heat is generated AT that mechanical regulator, and is enough to overheat the whole system in just a few minutes of operation.
A fair amount of heat would be created at the regulator. There is a WHOLE LOTTA friction in that regulator.
The most heat would be generated in the pumps.(from same same friction) If you put together a hydraulic line with a simple gate valve on it, put 3000 psi of pressure into one end and only open the valve a little bit, the valve itself would get pretty warm just from the flow of fluid!
Originally Posted by FiveOJester
Curious, how do they keep the fuel temp down on the planes? Or is it just a matter of so much mass of fuel that the heat load is insignificant?
Partly, For example, in that B737 example I mentioned, you have a little less than 10,000lbs of fuel in each wing tank. (approx 1300 gallons) That is quite a heat sink.
Also, I guess during flight the wings are out in the whatever very cold temperature the air is at 35,000 ft so the fuel is constantly being cooled anyways.
That's right!.........The fuel "heat sink" above, is in contact with the upper and lower outer skin of the wings (wing tanks) and outside temps at cruise altitudes are routinely -45C (or less)
............The fuel is so cold that any water in the fuel usually ends up frozen and can clog fuel filters(there's even an idiot light for it [FUEL FILTER BYPASS]) . The fuel is passed through a fuel heater that is heated by engine oil. (not unlike our oil coolers)
The intention is that any (small amounts of ) water is allowed to get to the engine where it is "burned".
The engine oil is also cooled by fuel). It may sound counter intuitive but in some jet engines, the procedure for high oil temperature, is to increase thrust. (more fuel flow=cooler oil)
