supercharger/turbocharger questions

[downwind]

hi all,

have some question on the above;

SUPERCHARGERS
1. what is the pressure in the absolute manifold indicution pressure system given in?

2. what is the boost pressure of the of the induction system given in?

3. how can vibration be reduced in a internal supergcharger?

4. what does the manifold pressure gauge messure in a internal supercharger?

5. With an internal supercharged engine at a constant RPM the initial increase with altitude is due to?

6. When the power lever is fully advanced at sea level, the throttle butterfly of an engine with a sea level boosted internal super charger would be?

TURBOCHARGERS
1. During normal advancement of the throttle of a turbo charged engine, the manifold pressure rose excessively, this could have been caused by?

2. What will be the affect on turbocharger operation of a significant leak downstream of the exhaust gate?

3. The manifold air pressure of a turbocharger engine is controlled by?

5. Turbochargers are prone to bootstrapping this tendancy is reduced by?

6. When a turbocharger engine with a automatic waste gate control is stopped the waste gate will be?

7. On engine starting the waste gate will be in what position?

8. A turbi charger is operating at full throttle an d at sea level the turbocharger RPM will be?

9. The waste gate of a turbo chargerd engine sticks in the descent this may cause?

10. Aircraft fitted with a turbocharged engine driving a constant speed propeller is climbing at constant RPM and MAP. cylinder head temperatures can be expected to?

thank you

[camlobe]

Hi downwind.

Seeing as no-one else has replied yet, I'll see if I can be of some help. If I have understood your questions correctly, my answers will hopefully make some sense.
Superchargers

1 + 2. Absolute manifold pressure i.e. ambient pressure plus boost pressure, and induction boost pressure, are indicated in recognised units. These units will depend on convention.

What this means is, absolute manifold pressure, or indicated manifold pressure, will normally be given in Inches of Mercury if you are following American guidlines, and Pounds of Boost if you are using British. WWII American naval fighters used Inches of Mercury (" Hg) while Spitfires and Lancasters used Pounds of Boost. Both systems told the same story, just used different units of measurement.

Therefore, at sea level, an absolute manifold pressure of 40 psi would be 81"Hg or 25 lbs boost, both equivelant to 25psi above atmospheric ambient pressure.

You can use any recognised unit you like for your own consumption e.g. hectopascels, Atmospheres, Bar etc.

2. With reference to 1 above, boost pressure is indicated in the same units. It must be borne in mind that 0 Pounds of Boost is not the same as 0 Inches of Mercury. With an engine at rest, the indicated manifold pressure will be ambient. A quick cross check with your altimeter subscale will assist in judging the accuracy of your boost gauge, especially if it is calibrated in Inches of Mercury.

At rest, your manifold pressure guage should indicate (approximately) 30 Inches of Mercury, or 0 Pounds. (remember, 0 Inches of Mercury is vacuum - ignore anything you may have seen in a car).

3. Centrifugal supercharger impellors, such as those fitted to Pratt and Whitney, Wright and Rolls Royce engines, are dynamically balanced by the removal of metal from the 'heavy' side. This is specialised task and most definately not a DIY job.

4. The manifold pressure gauge indicates the pressure between the outlet of the supercharger impellor and the cylinders. i.e. the pressure in the inlet manifold.

5. I must apologise as I don't fully understand the question. Could you please indicate what you need to know. e.g. do you mean 'how does the engine vary boost with changes in altitude'.

6. For most supercharged engines the answer is 'not fully open'. To explain this further, engines are supercharged for one of two reasons.
a) if the engine is 'modern civil application' e.g. Lycoming or Continental, then the primary purpose of supercharging is to allow the engine to develop full power at altitude.
b) if the engine is 'military front-line application', then there was a second reason as well. This was the need to produce more power for a given weight, cubic capacity, and aerodynamic cross-section, thereby retaining an operational advantage. The Rolls Royce Merlin is an excellent example of this, the last military varient producing double the power of the initial version without increasing the cubic capacity of 27 litres, or 1649 cubic inch displacement.

With the throttle control lever (power lever) fully forward, the height at which the throttle valve would be fully open (known as full throttle height) was dependant on the application of the engine. If the engine was designed to be operated at relatively low altitudes such as maritime patrol, then full throttle height would be around 3000 feet. If the engine was designed for high altitude air superiority, then full throttle height could be in excess of 20,000 feet.

Therefore, a high-altitude engine at full chat at sea level would have its throttle valve perhaps less than half open, whereas a low-altitude engine at full chat at sea level may have its throttle valve 85% open.

Other than some of the Reno racers, I cannot think of a supercharged aeroengine that may have their throttle valve fully open near sea level.

Turbochargers or as they are correctly known - turbo-superchargers

1. The potential causes are totally dependant on the method of boost control. Some turbocharged applications have no automatic controller, and are totally dependant on pilot (or engineer) hand-to-throttle lever management. e.g. Continental TSIO-360 series installed in Piper Arrow and Seneca II aircraft.
a) For non-automatic controlled applications, the operator allowed the throttle lever to advance too far.
b) i) for automatic controlled applications, the oil-pressure-operated wastegate controller device has failed, e.g. Lycoming TIO 540 series in Piper Chieftain aircraft, or
b)ii) the wastegate has stuck in the closed position, allowing the turbocharger to deliver unregulated full boost.

2. In this case, there would be no detrimental effect on the turbocharger system as the leak is downstream of (i.e. after) the turbocharger and the wastegate. This scenario would be the same as shortening the length of the exhaust pipe. The potential fire risk, however, is another story, depending on the installation. Non of this would really be an issue for the P38 Lightening as its turbocharger is external.

3. As a primary method of control, a bypass device such as the wastegate. A manifold pressure relief valve is often fitted between the compressor outlet of the turbocharger, and the cylinders of the engine. This device, however, is not regarded as a method of control, but a protection device to prevent overboosting of the engine should the normal control system fail.

4. ??

5. Sorry, I shall have to look into this aspect further before replying.

6. With the engine at rest, the wastegate will normally be in the fully closed position, as there is no sensing of near or overboosting.

7. The wastegate will normally be in the fully closed position. Refer to 6 above. The wastegate will normally start to open, dumping excess exhaust energy, as the turbocharger controller senses the approach of the pre-set maximum value of boost.

8. The turbocharger RPM is dependant on a number of criteria, such as diameter of impellor and turbine, and total mass flow capability. This is a bit of an open-ended question. A general answer would be 'less than maximum design speed'. The reason for this answer is, the engine will not be at full throttle height. At full throttle height, the exhaust back pressure will be greatly reduced, and the throttle valve will be fully open, thereby allowing the turbocharger to reach its full design speed.

9. With the engine producing power at low altitudes, the turbocharger will be potentially over-productive. To prevent over-boosting, the wastegate will allow surplus exhaust energy to bypass the turbocharger. At higher altitudes, where the air is considerably less dense, the wastegate will close off allowing the turbocharger to deliver the selected boost.

If the wastegate sticks in this closed off position, decreasing altitude increases air density. With the wastegate not opening to counter the effect of higher density air, the engine will overboost unless the throttle lever is gradually closed (a manual method of boost control).

10. If everything is constant i.e. MAP, RPM and indicated air speed, (i.e. engine load and cooling air flow) then the only true variables will be outside air temperature and air density. The thinner air density would equate to less mass air flow cooling for air cooled engines. However, this is countered slightly be decreasing ambient air temperature. If there are no automatic cowl flaps fitted, the cylinder head temperature may rise slightly. If the airspeed is allowed to reduce in the climb, then the cylinder head temperatures will rise considerably.

I hope the above is of some help.

camlobe

[Cessna Master Beta]

5. Turbochargers are prone to bootstrapping this tendancy is reduced by?

"The differential pressure controller reduces the unstable condition known as 'Bootstrapping' during part throttle operation . "

"Bootstrapping , then is an undesireable cycle of turbocharging events causing the manifold pressure to drift in an attempt to reach a state of equilibrium"

Thats from some random notes when I was studying for a CASA turbocharging exam. Most is very historical .

When you reach an altitude when the wastegate is fully closed the engine could also be prone to bootstrapping because the turbo is unregulated and hunting for a correct setting

[Mark1234]

superchargers, question 5: I believe refers to the slight increase in power seen up to FTH in a supercharged system due steady upper deck pressure and reduced atmospheric pressure (reduced backpressure effectively) improving the efficiency/flow.

Turbos: 6 - to the best of my knowledge, it depends completely upon the nature of the installation - there was a thread on this subject here not so many months ago.
Turbos: 7 - as 6.

Most of this stuff is in standard texts for license study, I also suspect from the nature of the questions they're straight out of an example paper or similar. Suggests a slight lack of application on behalf of the OP.

[downwind]

Camlobe,

To repharse q5 of supercharers; it is:

With an internal supercharged engine at a constant RPM the initial power increase with altitude is due to? Whoops forgot to put POWER increase.

And I have searched for answers, I am doing a ATPL license exam for a pilot thank you guys for all your help it is 1st class, really appriciate it.:)

[Mark1234]

So, downwind - my answer for 5 is what you're looking for - the reduced atmospheric pressure as you increase altitude reduces the exhaust backpressure and improves the cylinder filling.

Not being a smartass; I note you're locale is australia, assuming you're studying here, a chunk of this is covered in the aviation theory centre PPL/CPL books, the first book I think, I'd give you a reference, but I can find every volume but the one I need, sorry!

[NutLoose]

Some of these may help to read

Key Reprints - Lycoming (http://www.lycoming.textron.com/support/tips-advice/key-reprints/index.html)