engine compensator

[Martin Bennitt]

Just reading Peter Hart's 'Aces Falling' and a couple of pilots he quotes refer to the compensator, which seems to be some sort of engine power control. Can

anyone enlighten me in not too technical language, please? Do modern engines have them?

thanks and cheers Martin B

[burlington]

Martin

From what I can remember from one of his talks, I think that this is a gadget that enables the aircraft to fly higher and faster than normal.

I think it is something that reduces the effect of wind drag.

I am sure Peter will correct me and enlighten both of us!

Martin

[Roger Coasby]

Just reading Peter Hart's 'Aces Falling' and a couple of pilots he quotes refer to the compensator, which seems to be some sort of engine power control. Can

anyone enlighten me in not too technical language, please? Do modern engines have them?

thanks and cheers Martin B

I suggest it could be a device to alter the air/ petrol ratio going into the engine cylinders, or it may have been a 'Gate' to increase normal maximum throttle setting to give a temporary boost in combat

Roger Coasby

[centurion]

None of the above

See introduction to US Patent 6234909 - Engine compensator

An engine compensator is a mechanism that transmits rotational loads between an input shaft and an output ring member while also providing compensating rotation of the output ring member relative to an input shaft. Typically, the input shaft is the crankshaft of an engine. This compensating rotation, or "give", is often needed in order to absorb rotational shock loads. In many industries, compensators are desired for their ability to provide smooth power delivery. Compensators are often required to reliably protect transmission gears, sprockets, chains and other drive train elements from failure due to spike loads delivered to the drive train system. This type of failure is of particular concern in the motorcycle industry where large spike loads from the engine are delivered to the drive train during shifting, accelerating, and decelerating. These spike loads are particularly severe in racing conditions where, as often times occurs, the clutch is abused during shifting. This abuse occurs when the clutch is rapidly re-engaged when the engine RPM is significantly greater than the input RPM desired by the drive train or system. The same abuse occurs during deceleration, that is, when the engine RPM is significantly less than the input RPM desired by the drive system when the clutch is rapidly re-engaged.

Approximately during the 1930's the manufacturer of Harley-Davidson Motorcycles developed a highly successful engine compensator which to this day has essentially remained unchanged. The compensator weighs approximately 6 lbs 1/8 oz and was originally designed for V-twin four stroke engines of around 750 to 880 cubic centimeter capacity. These engines initially produced approximately 38 to 40 horsepower. The prior art compensator for these engines comprises a plurality of components, and the compensating action of the mechanism is achieved by the use of belleville springs that are configured to absorb the rotational spike loads. The prior art compensator functions satisfactorily as long as spike loads remain less than about 130 ft-lbs, which is generally the case for 38 to 40 horsepower engines.

Many disadvantages of the prior art compensators become apparent when they are used in drive trains where the horsepower of the engine substantially exceeds 40 horsepower. Today it has become commonplace to modify, for example, motorcycle engines to achieve power ratings of 100 horsepower or greater. As the horsepower increases, so do the rotational spike loads imposed on the components of the drive system. Under high performance conditions, these spike loads can easily exceed 130 ft-lbs. The prior art compensator is unable to consistently and reliably absorb these increased rotational shock loads. The belleville springs in the prior art compensator often break, causing the compensator to slip. When the compensator slips, rotational loads can no longer be transmitted through the mechanism. Attempts have been made to add additional belleville spring washers to the prior art compensator to correct this problem, however the rotational shock loads of these high performance engines still cause spring failure. A common, but undesirable, solution often employed by users to overcome this problem is to weld the compensator mechanism solid. This not only eliminates the desirable compensating action of the mechanism, but also reduces the mechanism to the role of adding additional weight and rotational inertia to the engine. This unnecessary weight and inertia is highly undesirable in high performance applications.

Prior engine compensators that were proposed for use with high performance motorcycle engines generally could not consistently and reliably absorb rotational shock loads in excess of approximately 130 ft-lbs.

Thus, those concerned with these problems recognize the need for an improved engine compensator capable of consistently and reliably absorbing the increased rotational shock loads that are associated with high performance engines.

[burlington]

Well, I knew someone was an expert in all this.

Any chance of a simple, layman's, explanation, in words of 1 syllable?

Thanks Centurion.

Martin

[dfaulder]

What is described above is a mechanism to take out the "snatch" that can arise from engaging two rotating items that are not rotating at the same speed. Motorcycle clutches are an excellent example (the engine rotational speed not matching the drive mechanism rotational speed).

However, I wonder if this is the case for the Original Posting as, as far as I am aware, there is not a clutch mechanism in the drive of a propeller driven aircraft. It is possible that when rapidly changing from climbing to diving, load comes on or off the actual propeller - the "snatch" in this case is between the air-flow and the propeller (the air equivalent of wheel-spin) - and to protect the drive shaft, this sort of mechanism could be useful.

I have found (http://www.motorera.com/dictionary/CO.HTM)

Cold engine compensator

When an engine is cold a richer mixture of fuel is required. The cold engine injector supplies more fuel to compensate for the condensation of fuel against the cold combustion chamber walls and intake manifold

This sounds a bit like what used to be known as "the choke" (see Roger's post #3). Rapid changes in altitude would affect inlet air temperature and I would guess that engines of aircraft would need this sort of mechanism - particularly if making rapid changes in altitude.

David

[centurion]

What is described above is a mechanism to take out the "snatch" that can arise from engaging two rotating items that are not rotating at the same speed. Motorcycle clutches are an excellent example (the engine rotational speed not matching the drive mechanism rotational speed).

However, I wonder if this is the case for the Original Posting as, as far as I am aware, there is not a clutch mechanism in the drive of a propeller driven aircraft. It is possible that when rapidly changing from climbing to diving, load comes on or off the actual propeller - the "snatch" in this case is between the air-flow and the propeller (the air equivalent of wheel-spin) - and to protect the drive shaft, this sort of mechanism could be useful.

I have found (http://www.motorera.com/dictionary/CO.HTM)

This sounds a bit like what used to be known as "the choke" (see Roger's post #3). Rapid changes in altitude would affect inlet air temperature and I would guess that engines of aircraft would need this sort of mechanism - particularly if making rapid changes in altitude.

David

I'm not old enough to speak for WW1 but I do know that it was an issue when the Griffon was built (there were still some around when I did my graduate apprenticeship at RR)

[Rockdoc]

On AJS & Matchless motorcycles built before 1957 the shock absorber, also known as a cush drive, was fitted between the drive shaft and the primary sprocket. The sprocket itself had no direct connection to the output shaft and had a smooth but snugly-fitting bore. On one side were two, large lobes parallel to the shaft. Loosely connecting to them was a sleeve with matching lobes that had a ribbed bore that matched the splines on the shaft. Pressing the two together was a large, strong spring. When the engine fired, the inertia in the drive-train made the lobes on the sprocket and sleeve move slightly against each other so that the sleeve moved along the shaft and compressed the spring.

From 1957 the sprocket was rigidly attached to the drive shaft because the shock-absorber transferred to the clutch. The part that fitted to the gearbox input shaft was made in two pieces and six, hard, moulded-rubbed segments kept the two apart. On a firing stroke the rubbers were compressed, like the earlier spring had been.

I think this is the kind of thing Centurion is describing. Similar devices are also used in connections on rotating industrial equipment because they allow minor misalignments to be ignored. Getting large-scale kit into absolute alignment would be extremely costly so these are a cost-effective solution.

Keith

[johnreed]

I thought that the Prop was directly coupled to the crankshaft, I know it was on a radial engine, or perhapsI'm being a bit thick. They used to use the Prop to start the motor/engine.

John

[apwright]

I was thinking it could be a type of altitude compensator.

As you climb, your fuel/air mixture gets richer ('cos the incoming air is less dense, therefore less oxygen) so the compensator lets more air in to lean it. AIUI modern props have the "mixture control", which does the same thing but by throttling the fuel flow.

Probably wrong though

Adrian

[Tom Morgan]

I know nothing about flying, but......

Imagine a motorcycle where all the power of the engine is transmitted to the back wheel. The engine is trying to lift the bike up onto the rear wheel and make it spin around the rear axle. Normally, there isn't enough power to actually do this, so the energy is tranlated into forward motion. But if the engine is powerful enough, the motorbike will indeed rise up onto the rear wheel and flip over. By using the throttle carefully and by leaning forward to compensate for the upward force, a rider can control it, and the result is a wheelie.

Would a rotary engine be doing something similar, and trying to make the very light and finely balanced aircraft spin round and round, while the engine stays still? If so, any effect would be more noticeable at high revs and might show itself in some degree of instability/steering problem. Perhaps the Engine Compensator was a device to overcome this? This is a blatant guess.

Tom

[Andy Wade]

Just to further complicate things there were also compensators for use during inverted flight which would lead to fuel starvation problems.

Although I'm not sure how much of a problem this would have been for WW1 era aircraft.

The problem with the OP is there's no context relating to this 'compensator' from which to give a reasoned answer.

[Andy Wade]

I know nothing about flying, but......

Imagine a motorcycle where all the power of the engine is transmitted to the back wheel. The engine is trying to lift the bike up onto the rear wheel and make it spin around the rear axle. Normally, there isn't enough power to actually do this, so the energy is tranlated into forward motion. But if the engine is powerful enough, the motorbike will indeed rise up onto the rear wheel and flip over. By using the throttle carefully and by leaning forward to compensate for the upward force, a rider can control it, and the result is a wheelie.

Would a rotary engine be doing something similar, and trying to make the very light and finely balanced aircraft spin round and round, while the engine stays still? If so, any effect would be more noticeable at high revs and might show itself in some degree of instability/steering problem. Perhaps the Engine Compensator was a device to overcome this? This is a blatant guess.

Tom

What you're describing would be an issue of huge propellor torque loading but I think it's unlikely there'd be enough to flip an aircraft, given that the propellor has only the air to bite into, there would be flying surfaces/trimming devices to counter this, but I think they'd be an 'airframe solution' rather than an engine one. Centurion would probably know the answer to this better than me. There's a much more obvious example of this in helicopters where they have a tail rotor to balance out the torque from the main blades. Or contra-rotating propellors/blades. The Chinook is an example of the contra-rotation of the main blades cancelling out the yawing rotational forces on the airframe.

[Tom Morgan]

Andy, the helicopter reference is an excellent example of what I was trying to say. I just wondered if there might be some effect, as the aircraft were so light.

Tom

[centurion]

I thought that the Prop was directly coupled to the crankshaft, I know it was on a radial engine, or perhapsI'm being a bit thick. They used to use the Prop to start the motor/engine.

John

On a Rotary the prop is not conected to the crankshaft but to the engine that spins around the crankshaft. On some Rotaries (notably the Siemens Halsk it could be geared with the engine rotating in the opposite direction) On a Radial it can be connected to the crankshaft but is frequently geared. The same applies on an inline engine.

[David B]

An article in The Aerodrome Forum describes the compensator in aero engines as an Ältitude compensating Carburettor".

Unfortunately the article would not copy and paste but can be found by Googling WW1 aeroengine compensator. If anyone really needs the

article I can type up by hand - having regard to my old fingers.

David

[KONDOA]

Its a bellows operated mechanism to change the mixture by moving the jets on a carburattor and thus compensate for altitude and change in air density as described above.

Roop

[Martin Bennitt]

thanks for all your input. 'Fraid I'm not a lot wiser but I am a bear of very little brain as far as mechanics are concerned. I'm also at work without the book so cannot quote the context, though IIRC one quote talked about using the throttle in conjunction with the compensator. Will look up again when I'm back home and re-post

cheers Martin B

[Andy Wade]

Right then, simple it is. Here's my take on it. Centurion, jump in whenever you feel like it...

For any engine to work, the fuel and air need to mix in the right amounts in the carburettor in order to feed the right mixture into the pistons which then gets compressed and the spark makes it go bang resulting in power to run the engine/propellor. Still with me? Good.

When you go higher (as with an aeroplane) the air gets progressively thinner with less oxygen and the fuel mixture is then too rich. This needs to be compensated for by reducing the fuel ratio with the compensator valve so you don't flood the engine with fuel. If there's far more fuel than air it won't all get used and will either carbon up the engine's innards or come straight out of the exhaust, but either way you get less power for the fuel used. That's a serious waste of fuel given that it's a finite amount you're carrying for the flight and you need the engine to work as efficiently as possible when the hun's on your tail.

Throttling back reduces both air and fuel but the compensator reduces the fuel amount to compensate for the lower oxygen level in the air coming into the carburettor. This way you can set the compensator for higher level flight and use the throttle as normal for more, or less power, depending on what you're doing.

Still with me? You're probably the only one that is...

[Martin Bennitt]

By jove, I think I've got it

thanks very much Andy

have a drink on me, mixed in whatever proportions produce the best result

cheers Martin B

[Rockdoc]

Are we talking about carburettors as such or some other device for mixing fuel and air? A carburettor works by forcing the incoming air through a venturi to generate a partial vacuum that draws fuel through a metering-jet, the fuel being atomised by the air-stream before it reaches the cylinder. That relies on the density of the air, not the proportion of any of the component gases, so unless the density of the air changes at a different rate to the proportion of oxygen with altitude there's no obvious use for a device such as Andy Wade has described. At altitude the carb is less efficient at drawing the petrol through the jet and should keep the mixture close-ish to normal, assuming everything's properly designed of course.

There are other systems for this job that don't rely on a venturi. A chap called Wal Phillips came up with what he called an injector that he developed for use with speedway bikes - in the sixties, I think. In it, the fuel was dribbled into the air stream instead of being pulled so it only worked properly at one throttle position. It was fine for speedway bikes because they're on full throttle or they're shut off but no use at all on a road bike. That kind of use seems as if it would work well with a rotary engine where, I understand, the engine speed could only be varied by cutting the sparks. It's at full throttle so there's no need for the compromises that have to be designed for use on road vehicles. Depending on the way in which the fuel and air were mixed I can see that there could be a need to mechanically vary the fuel supply with altitude. Having no knowledge about how it was done in practice, I'm very interested.

Keith

[Andy Wade]

We don't really know exactly what the compensator is, just that there's this 'engine compensator thingy'. I do of course realise that carburettors are more complicated that I explained but was going for the simple explanation. I'm curious about how this works too but have a rough idea of it's purpose in reducing fuel when the air is thinner/lacking in oxygen.

Unless I'm way off track, in which case I'll just crawl back under my rock.

[Rockdoc]

Andy, no criticism intended. Just trying to expand the discussion a bit.

Keith

[dfaulder]

Are we talking about carburettors as such or some other device for mixing fuel and air? A carburettor works by forcing the incoming air through a venturi to generate a partial vacuum that draws fuel through a metering-jet, the fuel being atomised by the air-stream before it reaches the cylinder. That relies on the density of the air, not the proportion of any of the component gases, so unless the density of the air changes at a different rate to the proportion of oxygen with altitude there's no obvious use for a device such as Andy Wade has described. At altitude the carb is less efficient at drawing the petrol through the jet and should keep the mixture close-ish to normal, assuming everything's properly designed of course.>><<

This would be true if we were worrying only about the density of air, however as I hinted earlier I think there is also a temperature effect. As you go higher, it tends to get colder and when the air going into the engine is cold, do you need a richer mix?

There must be someone with a Private Pilots Licence on this forum (who would have to know?); if not I might have to dig out my 30 year-old thermodynamics notes - problem is they may make as much sense now as they did then!

David

[Andy Wade]

Andy, no criticism intended. Just trying to expand the discussion a bit.

Keith

Oh I know.

I'd really welcome an expert opinion on this as I'm just guessing with my fading memory from my RAF time as an Airframes Fitter. I have extremely vague memories of 'skimming over' the principles and processes of aircraft engines.

Now ask me a question about beer and I'm your man...