Keith Williams piston engine

[parasite tango]

Guys I came across this question in Keith Williams

Where is oil pressure measured ?
a. on the output side of pressure pump
b. on the input side of pressure pump
c. in the sump
d. On the output side of scavenge pump

From what I studied the oil pressure is measured after is pressurized by the pressure pump so the answer should be output of the pressure pump.

But in the book it says answer is B

I request Keith Williams sir if he can tell us the correct answer.

Thanks

Parasite

[SNS3Guppy]

Who, or what is a Keith Williams?

Oil pressure is sampled at some point in the pressure line downstream from the oil pump. Generally it's not measured at the oil pump outlet or inlet, and generally the oil pump is internal to the engine, or mounted as an accessory.

The location where oil pressure is sampled depends on the engine in question. Oil pressure may be sensed in the flow line from the oil pump, or at some other point downrange. In piston engines, oil pressure is generally sensed from a passage point in the block, while in turboprops it may be sensed along with torque at a point in the propeller gearbox, or elsewhere in the pressurized supply. Turbojet sense works the same way, and may involve more than one pressure sense point. In fact, often a low pressure light is triggered by a different sensor or switch than the oil pressure indication, and a loss of one or the other doesn't necessarily mean one has lost oil pressure, or has an emergency.

Temperature sensing, conversely, is often placed at the oil cooler outlet, or near the pump inlet, though with some systems it's also placed in the oil cooler inlet, too. This can be significant because if temperature is sensed at the oil cooler outlet, one can only see the temperature of oil leaving the cooler. Oil bypassing the cooler with a stuck vernitherm or thermostat won't be sampled, and one may have a false indication of temperature that's artificially low. Sensing in more than one location is also sometimes used.

Oil pressure is generally not measured right at the pump outlet because it doesn't give a true measure of the pressure that the engine is receiving. To protect from excessive oil pressure and to regulate oil pressure, a means must be provided to increase or decrease oil pressure; this is generally done using a pressure relief valve or bypass valve. The way this works is a valve opens at or just after the outlet side of the oil pump to allow excess oil to be sent either back to the oil tank or sump, or commonly back to the inlet side of the oil pump. Measuring oil pressure downstream of this bypass point, then, gives an indication of the oil pressure that the engine is actually receiving or using. That's the pressure that interests you as the operator of the engine.

[barit1]

The pressure of interest is the pressure supplied to the bearings. For most practical purposes, this is the pump outlet pressure. This applies if the oil sumps (bearing boxes, in other words) are at ambient, atmospheric pressure.

But some turbine engines have an internal sump at higher pressure, because of the vent system design. In this case, the pressure difference between the pump discharge and the sump pressure is of interest, because that is the effective nozzle pressure.

When this is the case, the displayed pressure will probably be a "delta P", in other words measured by a differential pressure instrument; Oil pressure to the "high side" and scavenge pressure to "low side".

[SNS3Guppy]

Pressure to lubricate (and cool) bearings is important. Critical, even. If it's turbopropeller engine, pressure to regulate and operate the propeller is also very important.

[Old Fella]

The oil pressure measured is that pressure provided BY the pump and available TO the lubrication system. Oil pressure is monitored and displayed so that any failure of a bearing, such as the main bearings and big-end bearings, which will reduce the resistance to flow and consequently reduce the pressure, will be made known by either a drop in indicated oil pressure and/or warned of by a "low oil pressure" warning light. Scavenge pumps which return oil to the oil tank will/should always have a greater volumetric capacity than a supply pump, to ensure oil available for the pressure pump, the pressure not being critical. Oil sumps are not usually pressurised (except in 2-Stroke engines) and it is volume to the inlet side of the pump which counts. Therfore "(A) On the output side of the pressure pump" is the correct answer.

[SNS3Guppy]

Oil sumps are not usually pressurised (except in 2-Stroke engines) and it is volume to the inlet side of the pump which counts.

Oil tanks or sumps in light piston engine installations are usually not pressurized.

Oil sumps for most turbine equipment are pressurized to reduce foaming and to ensure adequate oil feed at higher altitudes.

[Old Fella]

No argument from me Guppy. My post was in relation to the original question re: Piston Engines. Have a good day.

[keith williams]

Although this thread was started several months ago I did not see it at the time.

Parasite Tango you are correct in pointing out that the book indicates option b as the answer. But the accompanying explanation states that "the pressure is sensed on the outlet side of the pump". The diagram provided with this explanation also shows the pressure sensor on the outlet side of the pump.

This is clearly a case of a typing error in putting option b as the answer. I am about carry out a major update of this book shortly to include CQB15 questions. I will correct this typing error as part of that update.

Thank you for pointing it out.