engine compensator

[NigelS]

A search in the 'Flight' Archive between 1909-20 using 'compensator' as the search term gets just two results: one is irrelevant, the second Click will be of interest as it relates to the results of an examination (by the RAE) of a 200hp High compression Mercedes engine recovered from A Fokker D.7 biplane brought down in France in June 1918 'the first Mercedes to be fitted with an altitude compensator carburettor'. The second page of the article has a sectioned drawing of the carburettor.

NigelS

[Rockdoc]

I wish I could see one as I'm having trouble getting my head round what's happening! Looking at Figure 4, you can see the float on the right of the carburettor body and the petrol feed to the jet running horizontally left-right from the base of the float chamber. The jet mounting rises vertically along the centre-line of the body and lies within a venturi - the two curves in the diagram that would form a throat. It looks as if the air comes in from the bottom of the body and rises vertically past the jet outlet. That, as you'd expect, is the same principle as any other carburettor.

I think that the "sliding air valve" regulates the flow of air to the engine downstream of the venturi. If I'm reading the description correctly, at altitude the engine runs rich because the air is able to create a vacuum that pulls more petrol through the jet than there is oxygen to burn it. You can't add air before the venturi or the effect would be worsened so the compensator allows the pilot to add air after the venturi to weaken the mixture. I don't get the impression it was automatic, just an additional movement on the throttle available to the pilot.

Keith

[NigelS]

I don't get the impression it was automatic, just an additional movement on the throttle available to the pilot.

I don't know enough about the finer points of carburation to comment, but I guess it's a sign of the times: today we wouldn't expect anything other than compensation - in whatever field of engineering it's being used - to be applied automatically; in the early part of the 20th century they would, I imagine, have been very grateful to have had the ability to apply compensation manually however crudely it might have been implemented.

NigelS

[Andy Wade]

And at that time, this was the state of the art. Looks like I was at least in the general area with the guess about it being some kind of a fuel compensator for altitude conditions. I'd like to see a picture of what the control apparatus looked like. I'm assuming some kind of ratcheted or friction lever, either on, or next to the throttle.

[grantowi]

Back in the 1980's a chap called David Vizzard - who was a wizz at modifying the old A series engines - decided to drive a mini around the world, he talks about having to set the choke at about half mast (half in, half out) when driving normally in order to be able to push it fully in when he was climbing over the big mountain ranges because of the lack of oxygen at that altitude.

Same sort of thing required on a carb driven plane i would have thought

Grant

Edit - from wikipedia - Too much fuel in the fuel-air mixture is referred to as too rich, and not enough fuel is too lean. The mixture is normally adjusted by one or more needle valves on an automotive carburetor, or a pilot-operated lever on piston-engined aircraft (since mixture is air density (altitude) dependent).

[salesie]

As an ex-REME Aircraft Technician (Airframes & Engines) I think the "secret" lies in the following phrases from the article:

"...it is impossible to run the engine all out on the ground...uncover extra air holes, thus weakening the mixture to such an extent that the engine stops if the throttle is fully opened on the ground...Duplex horizontally opposed air pumps...A double acting air pump of the usual plunger type is now fitted in place of the standard Mercedes air pump used hitherto on these engines."

If, as stated earlier, fuel were reduced at altitude then the power would drop off even more dramatically than the loss of power caused by the thin-air alone i.e. the mixture level would be maintained but volume would be greatly reduced. The only way to maintain power and/or give a higher rated altitude to any aero petrol-engine is to introduce "extra air" into the induction manifold, which has been done for many years now by supercharging and/or turbocharging.

So, it seems to me that the duplex air pumps are early attempts at, or alternatives to, super/turbocharging, and in this system when the throttle is moved beyond its ground position at altitude then the extra-air from the pumps, delivered through the extra air-slots mentioned in the article, compensates for the thin-air thus maintaining the air-flow through the venturi and therefore the mixture level, so volume is maintained which gives a higher rated altitude.

This is similar in practice to turbochargers fitted with hydraulic waste-gates that open wider with altitude and thus increase boost and which lock in position to avoid sudden loss of power at altitude if hydraulic system failure occurs - the pilot in such an instance has to throttle back manually as his altitude decreases (same as with the example in the article), but not because the mixture would become too weak and the engine stop as in the mercedes air-pump example but because the extra-boost with thicker air at lower altitudes would literally blow the engine apart. The old Sioux (Augusta Bell 47G with a Lycoming petrol engine) was fitted with a hydraulic waste-gate and a Benbow-valve to lock the waste-gate if the hydraulics failed, though the British Army ones which I worked on very rarely flew above a few hundred feet so it was never an issue from what I recall.

Hope this makes sense?

Cheers-salesie.

[Martin Bennitt]

sorry I've been away -- fully tied up today with coverage of the Polish president's air crash.

Both brief references in Peter Hart's book are from DH4 pilots, therefore not a rotary engine.

One Lt W.E. Johns, whose name may be familiar, describes being chased by a flight of D VII's and says "Toying with the compensator, I climbed to 21,000 feet, but the coloured

gentlemen were stil with me."l

The other says, "I closed the throttle and compensator with difficulty, as my left arm was almost useless."

make of it what you will. Glad to have interested a number of people

cheers Martin B

[Ghost]

Nearly cracked it. But what volume is maintained?

Alan

[salesie]

Nearly cracked it. But what volume is maintained?

Alan

I assume your question is aimed at me, Alan.

To maintain full power at altitude then the volume of fuel, in correct proportion to the amount of air going into the cylinders, has to be the same as if on the ground. The correct air/fuel mixture (around 14.5:1), called the stoichiometric mixture, is crucial to performance but so is the volume of this mixture i.e. when opening the throttle in a petrol engine (whether it be an aircraft, a car or any other type) the engine sucks in more air through the inlet manifold in proportion to how wide the throttle is opened, this automatically increases the airflow through the carb's venturi thus increasing the vacuum across it (even though there is more air travelling through it, it is at greater speed thus decreasing the air pressure) which in turn sucks more fuel from the carb's main jet, and this metering maintains the stoichiometric mixture whilst greater volumes of fuel/air go into the engine to increase revs and thus power. More fuel is needed to move to higher rev levels (higher power) than at idle speed - you don't get owt for nowt.

Mixture level and volume are crucial to engine power levels. So, the only way to maintain power at altitude is to introduce extra air to compensate for the thinner air, this extra air compensates for loss of volume at altitude. Supercharged, turbocharged, and air-pumped aero-engines have greater rated altitudes than normally aspirated ones.

Cheers-salesie.

[Ghost]

Not aimed at anyone in particular, just trying to figure out how this compensator worked. Carburettors are not used in my field, something to do with petrol being dangerous.

I have enjoyed following this thread, watching how it has developed. I did get slightly confused on some of the explanations where terminologhy is a bit wrong, but we are getting there.

Fuel needs a theoretical ammount of oxygen to burn perfectly.

Stoicheometric combustion, may be defined as theoretical combustion, we used to call it perfect combustion.

14.5:1 seems about right for a petrol engine, but all fuels are different, and the value vares widely 14.5:1 relates to one fuel only (or group of similar fuels) . Hydrogen requires far less O2 than carbon, about a third as I recall.

Today engines or other controlled processes in which fuel is burnt, do not operate with stoicheometric/theoretical/perfect combustion. An ammount of air in excess of theoretical is added to the combustion process, to ensure complete combustion of the fuel under all reasonable variations of the combustion, and thus prevent unburnt fuel from exiting the combustion chamber, which would cause safety and maintenace issues, as well as inefficiency. This extra air is aptly named excess air.

As the earlier mercedes engines (without compensator) had a sigle air pump, it is reasonable to assume that the pump/pumps were used at all times, and that the engines were indeed compressed. The only method of altitude compensation on the type of carb illustrated would be dilution of the fuel mixture, ie allowing extra air air into the combustion chamber whilst maintaining the same fuel supply for the required power.

As I read it once the engine begins to lose power, a movement of the throttle lever past the normal ground maximum stop, allows the extra ports to open. However, that would render a condition of zero throttle control when using altitude compensation. I would have thought two seperate controls, so that throttle/speed control could be maintained, possibly by the use of the rotational lever atop of the carburettor cylinder.

Alan

[grantowi]

I would have thought two seperate controls, so that throttle/speed control could be maintained, possibly by the use of the rotational lever atop of the carburettor cylinder.

Sound a bit like a description of a choke and an acelerator cables

Grant

[salesie]

As I read it once the engine begins to lose power, a movement of the throttle lever past the normal ground maximum stop, allows the extra ports to open. However, that would render a condition of zero throttle control when using altitude compensation. I would have thought two seperate controls, so that throttle/speed control could be maintained, possibly by the use of the rotational lever atop of the carburettor cylinder.

Alan

You may be confusing aircraft and cars, and petrol with diesel engines Alan.

Unlike cars, aircraft operate with very little throttle control, the throttle tends to be at 90%+ all of the time when off the ground, and when flying as high as rated altitude the throttle is usually 100% open, otherwise the aircraft wouldn't be up there. And petrol aero-engines are tuned to the top-end of the rev range, that's why they sound so awful when idling and at very low revs - normal operating range is 90%+ throttle, below this tuning is off by varying degrees from nearly in-tune down to well-off tune at ground idling speed.

The ideal stoichiometric mixture may be theoretical for many fuels, but it is crucial to the performance of petrol engines to have the correct ratio within very tight limits. In petrol engines, just a tiny variation in mixture ratio, either way, sends the engine out-of-tune and just a touch further away from the ideal then the engine will stop - so not so theoretical when it comes to petrol engines; maintaining the correct ratio is crucial throughout the engine's full operating range.

In a diesel engine, the air-fuel ratio does vary i.e. no throttle butterfly in the inlet manifold (apart from the odd design) thus giving a full air-charge even at idling speed (which is why diesels deliver higher torque than equivalent sized petrol engines at lower revs), and to rev-up and increase power in a diesel engine then more fuel is introduced, thus the air-fuel ratio varies throughout the operating range, therefore a theoretical ideal-mixture ratio is apt when discussing diesel engines.

Back to petrol aero-engines. In the article about the mercedes engine, it says that extra air-holes are exposed when moving the throttle beyond ground maximum - I doubt if this is an all-or-nothing operation, I should imagine there is a degree of graduation when opening the extra-air holes, between maximum ground and full throttle, thus giving the pilot at least some degree of control when operating in this throttle range.

In post #32, Martin B gives us two descriptions of the throttle and compensator being two separate controls:

1) In the first description, the pilot infers that he was only able to climb to 21,000 feet by "toying with the compensator" - as already stated, the only way to increase rated altitude is to supply extra-air to the engine. This description, therefore, tells me that the compensator he refers to is a manual control to provide extra-air to the engine from some kind of air compressor (super/turbocharger or air-pump).

2) In the second one, the pilot says he closed the throttle and compensator with difficulty because his left arm was almost useless. The pilot would only be closing the throttle to either cruise at an altitude lower than rated (between max and around 90% only) or to come into to land. Either way, it seems he was lowering his altitude, which tells me that the compensator is indeed an extra-air device i.e. as I said in an earlier post about the Sioux and its Lycoming engine, if keeping the compensator fully open and thus staying at full boost when losing height then the thicker air lower down would almost certainly cause catastrophic damage to the engine - it would take the engine way over its maximum designed power level, it could literally blow apart.

From this, I'm pretty certain that the 'engine compensator' referred to is an early way for the pilot to give the engine extra-air and thus gain a higher rated altitude. If it were a mixture control device (found on many aero-engines and used within a very tight range) then it wouldn't give any realistic assistance in achieving higher altitude. It is just possible that it is part of an early constant speed unit (CSU), used to vary propeller pitch, which adds to or lessens engine loading, but I seriously doubt it. The only way to achieve any meaningful higher rated altitude is to introduce extra-air into the inlet manifold.

Cheers-salesie.

[Ghost]

Salesie, nice description. I had not considered that fixed wing aircraft maintained near to max revs at altitude, helos however I know through my passenger experiance do.

Only point I would care to make is that Stoicheometric combustion is the calculated theoretical combustion value of any fuel. In my previous post I was using the word may as an introduction to a defination. Theoretical and perfect being less highbrow then stoicheometric, although the same thing, usage may have changed in generations.

I was unable to find that article on the carb again, but did manage to come up with this nice little article on the same subject which gives more insight to the mercedes carb, along with some others of interest.

see post #15 Altitude Compensating carburettors

The interactive map of airfields/squadrons is nice tool for finding locations.

Alan

[David B]

I am quoting in part from a late 1930's book I have on aeronautical engineering, which states that as a supercharger is mechanically driven from the crankshaft it is

always in action and is therefore capable of giving a mixture delivery pressure at ground level far in excess of requirements. If too much mixture is forced into the engine

the combustion pressure will be too high, power developed will be greater than designed and that serious damage to the engine will result.The primary object of the super

charger is to counteract the drop in atmospheric pressure resulting from increasing altitude so that below about 14000 feet the supercharger could deliver excess pressure.

In order that the supercharger may only deliver the correct pressure special automatic safeguarding devices are fitted to the engine.

I am wondering therefore, is the compensator a device to lower the supercharger pressure when coming to a lower altitude. Of course this would only apply if the

Mercedes engine was supercharged in the first place.

Incidentally a diagram of a throttle control shows only three positions cruising/rated/take off, ignoring the off position of course.

David

[grantowi]

This is from a review of the PC game Rise of Flight which came out last year, the planes are based on actual WW1 aircraft - Nieuport, SE5a, Albatros DV and Spad XIII are mentioned as being flown by the reviewer.

All the controls in the cockpits are animated, and I spent a little while just watching all the wires and cables moving as I manipulated the controls When you cock the machine guns (yes, you have to cock them before you fire them!) you see the cocking lever move. I loved the Albatros DV cockpit where your view out the front is straight over the engine, with all the rocker gear animated and clattering away.......

..... also features full engine management too - you can't open the throttle wide and forget about it (well you can, but you'll soon find yourself listening to nothing more than the wind whistling by your cockpit). You have to manage the power, RPM, radiator and fuel mixture. In yet another flight I put my aircraft into a high speed dive and when I pulled out my engine came to a grinding halt because I had oversped the RPM in the dive."

The bold is mine, could this be our compensator

Grant

[salesie]

see post #15 Altitude Compensating carburettors

Alan

This article basically answers the question, Alan, nice find.

It tells us, unequivocally, that the British during WW1 only ever used mixture control in service, prepared to accept a drop of 50% in performance at altitude compared to low altitude performance (use bigger engines and maintain the mixture ratio whilst losing volume) - so the compensator in question is a mixture control device not the extra-air method the Germans favoured (as demonstrated by the mercedes example given earlier, this extra-air of course being the real answer to maintaining performance at altitude).

Never entered my head that they'd trade such a huge drop in performance for altitude, I couldn't see beyond the mantra of maintaining performance - just goes to show that these were not just highly skilled and brave combat pilots (on both sides) but in many ways they were also daring pioneers/test-pilots.

It seems that British compensators were indeed mixture control devices in those days - I stand corrected.

Cheers-salesie.

[Martin Bennitt]

Well, there we are then.

Glad to have raised an interesting question and produced some interesting answers. I think even I can understand basically what is meant. Excellent link, Alan, I shall return to that site

thanks and cheers all

Martin B