Full throttle altitude and WEP at low alt

[Seeker]

I must admit I'm a bit confused with the concept of throttle height.

As I understand it; supercharged aviation engines are limited to boost pressure. Instead of merely ramming hime the throttle lever; a pilot; unlike a supercharged car driver, regulates engine performance by juggling the throttle to ensure the engine doesn't exceed a permitted inlet pressure; say (and these are just illustrative numbers) 12 lbs boost over atmospheric; or 60 inches of mercury; all the way up to the engines critical altitude; where atmospheric pressure drops enough for the throttle to be opened wide; as by this height the blower can't make more than 12lbs/60 inches anyway.

This doesn't apply to turbocharged engines; as turbos have a waste gate or dump valve which releases over pressure (why superchargers don't have this I don't know; just as why I'm unclear why terrestial engines aren't so worried about throttle height).

Anyway; WEP in AH works by boosting manifold pressure. If my Spit is already running at it's maximum of 12 lbs; how do I get away with hitting the WEP button and not having it blow up?

[pasoleati]

Merlins have automatic boost control. This prevents exceeding the max allowable limit below FTH if properly adjusted. Early war American engines didn´t have an ABC, so were prone to overboosting below FTH if the pilot didn´t monitor his boost cauge. An ABC was also lacking in the Hellcat and the Corsair (at least those with -8 and -8W) in neutral blower (i.e. aux stage disengaged).

[pasoleati]

At least the Jumo 211F/J has a bleed valve in the supercharger case to bleed off excess pressure with high RPM and low boost (e.g. diving with throttle closed).

[Captain Virgil Hilts]

Supercharged automotive engines don't deal with the changes in altitude that planes do. Witness NHRA drag racing in the U.S. They go from sea level to about 5K feet. The lowest tracks are Gainesville Florida and a couple of other sea level tracks. The highest track is Bandimere outside of Denver at 5K feet or so. Even so, the supercharged classes change drive ratios on the belt drive for the superchargers to compensate.

High performance aircraft with crank driven superchargers were tuned to a specific altitude range, mostly with supercharger gear ratio. Even two speed two stage superchargers are tuned for specific altitude ranges. Above that range and you don't have enough boost, below that range, you get too much boost above a certain RPM. A highly adjustable supercharger drive able to compensate for all altitude ranges on a crank driven supercharger would be terribly complex, and likely unreliable. Bleeding boost is possible, but brings with it new problems.

And a waste gate on a turbo does not bleed off boost. It merely bypasses exhaust gasses to slow down the turbo to reduce boost. Racers sometimes say the waste gate dumps boost, but that is an oversimplification that some get upset by.

[Seeker]

I understand what you mean by a turbo allowing exhaust to escape; and not boost; but couldn't the same be done with some sort of "slipping clutch" to the blower drive?

(On second thoughts; isn't that how the 109 worked; with a fluid coupling to drive the blower?)

So the difference between terrestial and arial blower installations is that aviation engines were deliberatley designed to be "over boosted" at ground level? I had'nt thought of that.

But to the original question.... There I am in my SPitIX at "full allowed boost" in AH at 10K. Then I hit the WEP button; and the boost goes up. Why doesn't the engine explode?

@ Pasoleati: Any details of this automatic boost limiter? It would seem to negate the concept of "full throttle height".


Thanks for the help guys; I'm very unclear on some of these concepts.

[gwshaw]

Originally posted by Seeker
I understand what you mean by a turbo allowing exhaust to escape; and not boost; but couldn't the same be done with some sort of "slipping clutch" to the blower drive?

(On second thoughts; isn't that how the 109 worked; with a fluid coupling to drive the blower?)

[/b]
That is exactly how the DB variable speed drive worked, and the Allison and P&W variants later in the war as well.

Problem is that a hyrdraulic clutch isn't as efficient as a mechanically locked one, so some of the gains you get are offset by the lower efficiency. Also causes a slight increase in your cooling requirements since you have to cool the oil in the coupling. But overall you get a much smoother power curve than a mechanically geared system.

So the difference between terrestial and arial blower installations is that aviation engines were deliberatley designed to be "over boosted" at ground level? I had'nt thought of that.

[/b]
Yes, the aircraft engines are either altitude rated or SL rated. And you size the blower and gearing to whatever your goal is. Also, even though an engine is SL rated doesn't mean it isn't boosted. Even SL boosted Allisons were boosted to anywhere from about 45 in Hg to 60 in Hg at SL. It just means they were designed to run with the throttle wide open at SL.

But to the original question.... There I am in my SPitIX at "full allowed boost" in AH at 10K. Then I hit the WEP button; and the boost goes up. Why doesn't the engine explode?

@ Pasoleati: Any details of this automatic boost limiter? It would seem to negate the concept of "full throttle height".

[/b]
The ABC acted at the throttle, closing the throttle butterflys restricts the amount of air that gets into the induction system. Basically just a system that automatically limits the throttle to the amount of air necessary for the desired boost. Which is essentially what the pilot had to monitor manually in the US engines.

Greg Shaw

[Tails]

Originally posted by Seeker

(On second thoughts; isn't that how the 109 worked; with a fluid coupling to drive the blower?)

Sounds an aweful lot like the CSD (constant speed drive) you find on the generators on big jets. Or are these completely different animals? Just curious.

[HoHun]

Hi Seeker,

>So the difference between terrestial and arial blower installations is that aviation engines were deliberatley designed to be "over boosted" at ground level?

If you consider "over boosted" to describe an engine that's capable of generating more power than it can survive, that's probably true.

In fact, it's even true for un-supercharged engines. Towards the end of WW1, Germany developed normally-aspirated engines that could not be run at full throttle near sea level. They could only be operated at full throttle above a certain altitude - the "full throttle height", obviously.

The advantage was that they had created a 185 HP engine that above its full throttle height was as good as (say) a 250 HP engine, but a good deal lighter since it didn't actually have to withstand the 250 HP it might have generated at sea level.

Regards,

Henning (HoHun)

[Kurfürst]

Originally posted by gwshaw
Problem is that a hyrdraulic clutch isn't as efficient as a mechanically locked one, so some of the gains you get are offset by the lower efficiency.

Greg Shaw

Technically right, but a rather theoretical lower effiency... the hydraulic clutch caused typically the loss of 5-10 HP at maximum, you barely notice that on a car, let alone on a ~1500 HP engine.

Otoh, the hydraulic clutch meant that up to several hundred horsepower was gained.

In summery, the pros and cons of the hydraulic clutch :

- 5 to -10 HP loss due to slip
+ 200 to 300 HP gain due to improvement in s/c drive control between FTHs.

Besides, most mechanicaly clutched superchargers like that on the Merlin or Griffon for example, also featured some slip so the relative tiny loss with a hydraulic clutch are practically non existant in reality.

[gwshaw]

At min slip there is probably only a 5-10 hp difference between a fluid coupling and a mechanical system. At max slip it isn't nearly as good. There is certainly an advantage to the variable speed drive, but not on the order of several hundred hp. I'm too lazy to figure up mass flow and blower power requirements now, but I doubt that it is even 100 hp between a DB 605A at max slip and an Allison or Merlin 4x or a Merlin XX in lower gear. Despite the Allied engines having much higher pressure ratios.

Greg Shaw

[HoHun]

Hi Greg,

>At min slip there is probably only a 5-10 hp difference between a fluid coupling and a mechanical system. At max slip it isn't nearly as good. There is certainly an advantage to the variable speed drive, but not on the order of several hundred hp.

The British estimated the minimum slip of the DB601A-1 to be 3% and the maximum 20%.

The advantage of the hydraulic coupling is pretty obvious in the AVIA 6/9352 DB601A engine chart: At sea level, the engine gains about 110 HP (going from 880 HP at 3% slip to 990 HP at 20% slip).

At about 5600 ft, the gain begins to decrease because the supercharger speed increases, and at full throttle height, it's down to maximum speed, minimum slip and actually loses a couple of HP compared to a fixed ratio supercharger.

As the maximum power gain is about 12.5%, the hydraulic clutch definitely improves performance markedly - compared to a fixed-ratio single-speed supercharger at least.

If you're thinking of a two-speed supercharger, the advantage of course is not as obvious since the fixed-ratio supercharger, if the fixed ratios are chosen to correspond to minimum and maximum slip ratios of the hydraulic clutch, will actually outperform the variable-ratio engine below low gear full throttle height and above high gear full throttle height.

Still, I'd expect the hydraulic coupling to give a higher integral of power of altitude because it avoids the power minimum above low gear full throttle height.

I believe Allison introduced hydraulic couplings on their late V-1710 variants with two-speed supercharger drives, making the comparison even more complicated :-)

Regards,

Henning (HoHun)