Horsepower, RPM, and Supercharging

[Zipper730]

I know engines have a range of RPM's they work at and I know mechanically driven superchargers are geared to the shaft and spin at a given gear ratio, ratios, or range of ratios; I also know that superchargers produce disproportional compression when their RPM is ramped up.

The thing is how do you end up producing full RPM without over-boosting the hell out of the engine? I know the throttle isn't all the way forward when you're taking off and is run progressively further forward until critical altitude is reached () and that sort of thing.

How does boost get varied independent of engine RPM. It's probably something I should have asked right away when joining, but it somehow never popped into my head.

The only real knowledge I have with reciprocal engines doesn't involve manifold pressure: It's not a measurement on cars, there's RPM, the gear you're in (if you don't have an automatic transmission), and your speed. In the US we don't have to get our license on a stick to be able to drive a stick, we can learn on anything and the DMV favors people to drive an automatic (you have less items to work against you for not shifting smoothly). Once you got the license, I actually did learn how to drive stick to a degree (meaning I drove my mother's older car, and sometimes acted as a designated driver because I don't drink), but the car I ended up with had an automatic transmission (the biggest difference is you have an extra pedal, and you shift at 2000-3000 RPM).

That all said, pushing the gas-pedal produces an increase in RPM: You'd figure pushing the throttle forward would increase boost and RPM.

 

[wuzak]

I know engines have a range of RPM's they work at and I know mechanically driven superchargers are geared to the shaft and spin at a given gear ratio, ratios, or range of ratios; I also know that superchargers produce disproportional compression when their RPM is ramped up.

The thing is how do you end up producing full RPM without over-boosting the hell out of the engine? I know the throttle isn't all the way forward when you're taking off and is run progressively further forward until critical altitude is reached () and that sort of thing.

How does boost get varied independent of engine RPM. It's probably something I should have asked right away when joining, but it somehow never popped into my head.

I think you've answered your own question.

It's the throttle. It controls teh mass air flow through the supercharger and, consequently, the boost.

That all said, pushing the gas-pedal produces an increase in RPM: You'd figure pushing the throttle forward would increase boost and RPM.

That's not actually true.

If you are driving along a flat road at 50mph and then the goes into a climb, you will have to increase the throttle to maintain speed, and for a fixed gear ratio (manual or auto with torque converter locked), rpm.

Opening the throttle admits more air and fuel into the engine to make more power and torque, but will only lead to an increase in speed and rpm if the power is in excess of the load. If the power is less than the load the rpm and speed will drop.

 

[pbehn]

Don't they have a waste gate?

 

[wuzak]

Don't they have a waste gate?

Turbochargers use a wastegate to adjust the amount of exhaust going through the turbine, which controls the speed and boost.

Mechanical superchargers don't have that.

Modern turbos can also have a blow-off valve, which releases excess boost after the compressor. They could do that with a mechanical supercharger, but I don't believe they did in WW2.

 

[pbehn]

Turbochargers use a wastegate to adjust the amount of exhaust going through the turbine, which controls the speed and boost.

Mechanical superchargers don't have that.

Modern turbos can also have a blow-off valve, which releases excess boost after the compressor. They could do that with a mechanical supercharger, but I don't believe they did in WW2.

How did they set and main maximum boost, on say a Merlin?

 

[wuzak]

Throttle plate on the carburetor.

 

[Shortround6]

There was an air pressure capsule with piston the compared the pressure in the capsule to the outside air and was linked to the throttle plate (or equivalent) so that the throttle could only be fully opened when the pressure difference reached a certain value. as the pressure difference decreased (lower altitude) the mechanism slowly restricted the throttle opening until it reached a preset limit when the pressures were equal.
That is the basic set up, there were often additional bits and pieces to allow overriding (WEP) throttle openings.
Some early American aircraft had no such mechanical restriction and while it allowed the pilots to use much higher pressures in combat it also allowed for some dangerous overboosting in an emergency situation, pilot rams the throttle all the way forward and then gets occupied trying to to do an evasive maneuver or recover from a strange attitude and by the time he can look at the pressure gauge and adjust the throttle the engine has gone well over even any common WEP pressure.

 

[wuzak]

You are describing automatic boost control?

 

[Deleted member 68059]

The thing is how do you end up producing full RPM without over-boosting the hell out of the engine?

This is all very thoroughly described here:

https://youtu.be/Pm4SaAnPtYI

...and with respect to some different throttling methods even more thoroughly described here:

https://youtu.be/HwALos8CwJ0

 

[pbehn]

I really do hope there was some means to control it, if not all I have read about max boost and WEP and max climb must be horth thit.

 

[MIflyer]

There was an article about the RAF finding some Tomahawk I's still in their crates and deciding they may as well put them togther and use them for something, specfically for training bomber gunners. They put them toghter and stuck some relativley inexpericed pilots in them. The first one pushed in the thgrottle and went screaming off, impressed with the P-40's perfor,ance, then then ... "Bam!" The Allison blew its top from over boosting. In "Winkle" Brown's excellent descriptions of flying various airplanes he always notes how that with the early Allison's you had to be careful not to overboost; the Merlin the Brits were used to had a manifold pressure regulator.

I read where when the Japanese hit the PI, a P-40E pilot ran to his airplane, shoved in the throttle, and took off, desperately trying to build up some speed and altitude. Then he looked at his manifold pressure gauge and was distressed. It only showed something like 10 inches; obviously his engine had a bad problem. But as it turned out the gauge actaully was showing something like 70 inches. The needle had gone all the way around past the 60 inch mark and began a new circuit of the dial.

 

[Zipper730]

So basically, to summarize...

1. Superchargers designed for altitude are, effectively oversized
   • This takes more horsepower off the shaft to drive it
   • For them to break even (the pressure produced adds enough HP to compensate for the HP taken off to drive it), you'd over-boost the engine at sea-level
   • The engine runs innately with a horsepower reduction at low-altitude for the given manifold pressure
   • That said, as the altitude goes up, they can keep producing full manifold pressure well above what a naturally aspirated engine would do (or an engine with a sea-level supercharger).
   • All altitude superchargers are sea-level superchargers if the maximum manifold pressure could be raised high enough... (yeah, I'm taking that to the nth degree...)
2. Airflow is generally reduced via a butterfly-valve
   • The movement of the throttle adjusts the butterfly-valve
   • This gives enough air to the engine to maintain the right RPM, without over-boosting
   • The butterfly-valve effectively imposes additional losses because of the fact that it forces the supercharger to run at a higher pressure ratio (unsure I grasp)

 

[Zipper730]

As for the Swirl-Throttle set-up: This reminds me a little bit of a variable guide vane used on a jet-engine. It seems to reduce the throttling loss to nearly zero, which makes me wonder if the Russians were the first to think this up...

 

[wuzak]

So basically, to summarize...

1. Superchargers designed for altitude are, effectively oversized

Yes. If the supercharger was designed to give the rated boost at sea level, then the engine would perform like a non-supercharged engine, but with more power. The exception is if the engine was used in conjunction with a turbo designed to provide sea level conditions up to its critical altitude.

• This takes more horsepower off the shaft to drive it
• For them to break even (the pressure produced adds enough HP to compensate for the HP taken off to drive it), you'd over-boost the engine at sea-level

Yes, the supercharger will take more power to drive than a lower altitude rated supercharger because it is putting more work into the air.

Not sure what you are trying to say.

• The engine runs innately with a horsepower reduction at low-altitude for the given manifold pressure
• That said, as the altitude goes up, they can keep producing full manifold pressure well above what a naturally aspirated engine would do (or an engine with a sea-level supercharger).

Yes, for a throttle and geared drive system like the Merlin and V-1710. May be different for a supercharger with variable speed drive.

Yes, that's the whole point of supercharging.

• All altitude superchargers are sea-level superchargers if the maximum manifold pressure could be raised high enough... (yeah, I'm taking that to the nth degree...)

Um, what?

A sea-level supercharger is one where the critical altitude or full throttle height is sea-level. An altitude supercharger is where the critical altitude or full throttle height is above seas level, at some nominal altitude.

If we use the British term, boost is the pressure above sea-level atmospheric pressure. Thus +18psi is 18psi above standard sea-level pressure no matter at what altitude the engine is. Maximum boost at maximum rpm or max continuous rpm (3,000rpm and 2,850rpm, respectively, for most Merlins) gives positive boost up to very high altitudes.

Cruising settings, as for most economical, use low rpm and boost, which can go slightly negative boost at altitude.

So basically, to summarize...

1. Airflow is generally reduced via a butterfly-valve
   • The movement of the throttle adjusts the butterfly-valve
   • This gives enough air to the engine to maintain the right RPM, without over-boosting
   • The butterfly-valve effectively imposes additional losses because of the fact that it forces the supercharger to run at a higher pressure ratio (unsure I grasp)

This is a supercharger map for a modern centrifugal supercharger (like those used in most supercharge WW2 engines).

Vortech Superchargers

The bottom axis is mass air flow
The vertical axis is pressure ratio.
The curved lines left to right are constant compressor rpm.
The curved lines down to up (plus the loops) are lines of constant efficiency.

Depending where you start on the map, decreasing mass flow can increase or decrease the pressure ratio.

For WW2 engine superchargers it can only have reduced teh pressure ratio, otherwise boost would have been higher.

If anybody has a good map of a Merlin, or any other, supercharger, that would be good.

[Note: modern superchargers and turbocharger maps for car applications will often use volumetric air flow rather than mass air flow]

 

[Deleted member 68059]

For WW2 engine superchargers it can only have reduced teh pressure ratio, otherwise boost would have been higher.

Hmmm try not to confuse the pressure RATIO of the compressor with the subsequent ABSOLUTE pressure available to the engine, a slightly higher pressure ratio
acting on a considerably lower ABSOLUTE inlet pressure (caused by inlet throttling at constant compressor speed) can provide lower ABSOLUTE boost pressure.

Your request to see WW2 compressor maps is satisfied in the 2nd video I posted here:



and...here



Sadly the audio and video quality is rubbish - but luckily the actual information is not. Sorry I dont know why its decided to put huge
thumbnails of my face this time when it didnt before... anyway.

 

[wuzak]

Hmmm try not to confuse the pressure RATIO of the compressor with the subsequent ABSOLUTE pressure available to the engine, a slightly higher pressure ratio
acting on a considerably lower ABSOLUTE inlet pressure (caused by inlet throttling at constant compressor speed) can provide lower ABSOLUTE boost pressure.

Yeah, forgot about the pressure loss across the throttle.

 

[Zipper730]

Yes. If the supercharger was designed to give the rated boost at sea level, then the engine would perform like a non-supercharged engine, but with more power. The exception is if the engine was used in conjunction with a turbo designed to provide sea level conditions up to its critical altitude.

Understood

Not sure what you are trying to say.

If I had a sea-level supercharger, it would take off a certain amount of horsepower to drive it, but it would, presumably boost the pressure enough to raise horsepower beyond the amount that the driving of the impeller took off.

If the engine could be boosted hard enough, eventually you'd hit that point, the key word is "if you could boost the engine that much". My father was fond of saying "if the Queen had balls, she'd be King".

Um, what?

A sea-level supercharger is one where the critical altitude or full throttle height is sea-level. An altitude supercharger is where the critical altitude or full throttle height is above seas level, at some nominal altitude.

It's a joke. It was just taking the idea of boost to it's logical conclusion.

For example if I could maintain 52.8" MAP to 5900 feet with one stage of supercharging; then with two stages in low gear, you'd be able to go to 14000 feet, with less horsepower, but overall, more speed as you'd be chopping through thinner air; should one be able to spin those two stages in high gear, possibly something like 23800 feet would be possible on the existing manifold pressure, with even less horsepower (yes this is lifted off the F4U, though the figures might be a tiny bit off here and there). Should I increase the maximum manifold pressure, the critical altitude would go down because I can't produce that pressure at higher altitudes in the thinner air, but it can be produced in thicker air.

That said, if the engine could be boosted hard enough, eventually you would reach some comically absurd manifold pressure that would be impossible to achieve at any altitude but sea level because the air is too thin at any other altitude (Yeah, I know, lame).

This is a supercharger map for a modern centrifugal supercharger (like those used in most supercharge WW2 engines)

What I find strange is that many engine charts often seem to list BHP as a straight line right on up to the critical altitude rather than revealing the pumping loss. I'm not sure why airplane performance graphs don't show that kind of thing -- you'd think it'd be important.

Depending where you start on the map, decreasing mass flow can increase or decrease the pressure ratio.

I was working along what Calum posted, he said something about raising the pressure ratio.

Hmmm try not to confuse the pressure RATIO of the compressor with the subsequent ABSOLUTE pressure available to the engine, a slightly higher pressure ratio acting on a considerably lower ABSOLUTE inlet pressure (caused by inlet throttling at constant compressor speed) can provide lower ABSOLUTE boost pressure.

I have a hunch I'm probably totally misinterpreting this (and I haven't really slept in almost 24 hours), but the butterfly valve closing, does that produce an area change like a bell-mouth? If so I could imagine that an undersized bell mouth would produce a pressure gain but not enough air...

Sadly the audio and video quality is rubbish

I can understand you just fine

 

[Deleted member 68059]

What I find strange is that many engine charts often seem to list BHP as a straight line right on up to the critical altitude rather than revealing the pumping loss. I'm not sure why airplane performance graphs don't show that kind of thing -- you'd think it'd be important.

Depends which system their using.....

A turbocharged plane can alter the compressor shaft speed to provide the correct boost level without throttling (by using a wastegate) - similary
any engine with a swirl-throttle has an ALMOST flat BPH/altitude graph because the throttling losses (even with constant speed compressor geared
to the crank) are so low.

If the engine has a mechanical supercharger geared to the crankshaft, and has no swirl-throttle, it MUST show a "sawtooth" pattern (depending on
how many gears it has, obviously one peak per gear, so basically 1 or 2 in nearly every case).

 

[MIflyer]

I understand that with the BF-109, which used a fluid coupling drive that eliminated the specific supercharger gear shift points, at sea level the supercharger actually absorbed more power than it added. One BF-109 pilot said he was sneaking up behind a couple of P-38's at low altitude when they spotted him, poured on the coal, and pulled away from him at a rate he found to be absolutely incredible, "They just disappeared!"

An A-36A in the Med shot down an FW-190 that was being ferried to an operational unit. When the German pilot met the American who got him he asked, "How in the world did you catch me?" Well, the A-36A best performance was set up for about 5000 ft; there probably was not much it could not catch down low.

On early P-38's the turbos were known to come apart at high boost. They added a shield on each side to protect the pilot from turbo shrapnel and changed the V-1710 supercharger gear ratio to get more pressure out of the mechanical supercharger. What this change did to performance I do not know.

 

[pbehn]

That said, if the engine could be boosted hard enough, eventually you would reach some comically absurd manifold pressure that would be impossible to achieve at any altitude but sea level because the air is too thin at any other altitude (Yeah, I know, lame).

If you create enough boost then combustion occurs as soon as fuel is introduced. You can then dispense with the cylinders and other stuff and concentrate on refining the development of your jet engine.