Odd number of pistons?

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odd number of pistons??

in my internship at the aircraft accessory overhauling shop, instructor asked us why there are odd number of pistons in all type of pumps used. All pumps, be it engine driven pump, power transfer unit, ram air turbine etc etc(variable delivery pump installed in A320), they all had odd number of piston. The obvious question is why is it so??
So far i think it has something to do with the rotation direction(clockwise or counter clockwise), but that may explain such case in RAT and PTU which are supposed to work as motor also, but EDP always works as pump.
Does it have some design significance or operational significance??
 

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  • #2
mgb_phys
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Not sure that a ram air turbine has pistons. Do you mean blades by any chance?

In things like rotary aero engines which always have an odd number of pistons think about starting it and what would happen if an even number of pistons were all in the same point in the cycle.
 
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brewnog
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Good clue, mgb.
 
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Not sure that a ram air turbine has pistons. Do you mean blades by any chance?
No, they use the same configuration as turbine(well, not turbine because it works more like an engine, a compressed air engine, i guess) and pump. I asked how is it possible to get maximum efficiency for both configuration. Answer was it is a trade off between efficiency, space constraint, money etc etc

In things like rotary aero engines which always have an odd number of pistons think about starting it and what would happen if an even number of pistons were all in the same point in the cycle.
Exactly what i meant, it should control the direction to which the rotor will start.
 
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mgb_phys
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I see, the T part of RAT is just the propeller blades, the hydraulic pump is a piston engine - I assumed you would use an impeller for the pump part.
 
  • #6
brewnog
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The 'odd number' of pistons is not for starting the pump. Typically the pump will be driven by a motor or engine of some kind, so starting the pump turning is not an issue.

Build on that principle, though, to work out why an odd number of cylinders is advantageous for a pump (as well as a radial engine).
 
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The 'odd number' of pistons is not for starting the pump. Typically the pump will be driven by a motor or engine of some kind, so starting the pump turning is not an issue.

Build on that principle, though, to work out why an odd number of cylinders is advantageous for a pump (as well as a radial engine).
Yes right, a pump is driven by an external source, so no point of direction problem.

I was saying that, for a PTU or RAT working as motor, if same numbers of pistons are exposed to outlet and inlet(if the configuration was symmetric), it may rotate in both direction.

A good analogy will be a single phase induction motor without a condenser(capacitor), it can rotate either way simply by pushing, but with a capacitor, we have provided a push(if analyzed by double magnetic field revolving theory, one field goes up and another takes a dip), so motor starts spinning in the required direction

http://www.dutchops.com/Portfolio_Marcel/Articles/Flight Controls/A320_Flight_Controls/A320_Systems.html
see the second last pic, it shows the positioning of various units in green, yellow and blue system for an airbus 320

mgb_phys said:
Not sure that a ram air turbine has pistons. Do you mean blades by any chance?
I guess impeller pumps do not(or can not??) produce such high pressure(upto 3500psi), so piston pumps are used.
 
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NateTG
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mgb_phys said:
In things like rotary aero engines which always have an odd number of pistons think about starting it and what would happen if an even number of pistons were all in the same point in the cycle.
Radial aircraft engines with even numbers of cylinders exist and were used.
Radial aircraft engines have an odd number of cylinders per bank so that they can have a single intake, exhaust and ignition system per bank - a 'boxer' design would require alternating sparks on opposite points on the engine to be balanced. This consideration doesn't translate to most piston pumps, since piston pumps tend to have fixed-position cylinders, and tend to be two stroke machines.


I suspect that I know the answer to the original question. Do you ever deal with pumps that have more than three cylinders?
 
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I have a feeling that the type of pump i am talking about is not clear.

unfortunately i cant find any pic of such a pump, i ll try to make one in inventor.
 
  • #10
turbo
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In the case of radial aircraft engines, you may want to consider the number of cylinders from the point of view of an engineer concerned about vibration and resonance. How might a 7-cylinder engine be superior to an 8-cylinder engine in this regard?
 
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here is a pump(turbine) used in A320. I made only the functional elements, no dimensioning, nothing, it just shows the basic thing.

The grey top is the manifold(inlet and outlet), left arc being outlet and right being inlet(in case of pump, it is reverse for a motor). Brass barrel has 9 cylinders in it, and white ones are the pistons. The grey bottom is the hanger having ball and socket joint(not exactly a ball & socket joint, the sockets is constrained in a small spacing where it can move in the inclined plane) with the pistons. Barrel is given the drive, pistons rock back & forth and also rotate. working is quiet easy. Indeed the odd piston is for starting sense while working as a motor.
 

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  • #12
FredGarvin
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That's usually referred to as a variable wobbler (for the one that can tilt) or a fixed (they are often found in pairs) for a piston pump. That kind of pump is used quite a bit in hydraulics and transmissions. If you look at the A320's engines, undoubtedly they will have a constant speed transmission to which the main generator is attached. That too will have them.

Hey...the new software upgrade has a new picture viewer. I like it.
 
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That's usually referred to as a variable wobbler (for the one that can tilt) or a fixed (they are often found in pairs) for a piston pump. That kind of pump is used quite a bit in hydraulics and transmissions. If you look at the A320's engines, undoubtedly they will have a constant speed transmission to which the main generator is attached. That too will have them.

Hey...the new software upgrade has a new picture viewer. I like it.
Yes the hanger(wobbler) is variable, it maintains a particular stroke length for a particular flow requirement. They work in a pair in a PTU. here is a pic
 

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odd number of pistons??

ank_gl
I think that it basically due to the ripple pattern (torque or flow pattern variation). The fundamental frequency of even number piston devices matches that of the number of pistons. On the other hand, the fundamental frequency of odd number of piston devices is twice the number of pistons. A nine cylinder would have 18 pulses in 360 degrees while an 8 piston has only 8 pulses –with more being better.
Also the amplitude of torque or flow rate variation for odd numbered is much lower than even numbered. A higher number of odd pistons make for quieter unit, less vibration etc..

The following explains how the formulas were developed:
http://edt.missouri.edu/Winter2006/Dissertation/MehtaV-041906-D5101/research.pdf
 
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