Superchargers- how do they work?!?

[<simon>]

Howdy all,

I apologise in advance for asking what must seem a stupid and easy question.
Thing is, i dont exactly know how superchargers work. Especially when certain engines have a "2 stage supercharger" etc.

Could someone give me a bit of a lesson? I know the basics of an engine but not a lot when it gets more complicated

Thanks
Simon

 

[Clave]

OK, I will have a go...

When you are sucking air into the engine to mix with fuel for combustion, the amount of air depends on the ambient air pressure. This means you are limiting the power you can get from each piston stroke, because you can't physically get any more air into the cylinders. As you fly at higher altitude, the air pressure is less, so your aircraft actually loses power as you climb...

The 'fix' is to run a very high speed pump which actually compresses the air before it's sucked into the cylinder. Simply put, it is a fan, driven from the engine itself, which sucks air in at 'ambient' pressure and boosts the pressure to above the normal range. More pressure means more oxygen, which means more fuel can be burned, and more power produced. So the supercharger will normally produce more power at all altitudes, but is specially effective high up, as it counteracts the effect of low pressure.

The two-stage supercharger has different gearing settings, and can be switched between different levels of boost. It's important to note that there are negative effects of supercharging along with the power benefits. These are: Overheating - this is the main problem, and can lead to the fuel igniting before it is supposed to. The second is power loss - driving the supercharger unit itself consumes engine power. This is a minor effect though, compared with the huge boost of power that the supercharger brings.

A quick note about Turbochargers:

A turbocharger or 'turbo' does exactly the same thing as a supercharger, the only real difference being that the turbo is driven by exhaust gas instead of mechanical gears or shafts. If anything is called 'turbo' and is say a razor, computer processor, vacuum cleaner, etc. this is basically a lie. Turbo means turbocharger, and if there's no internal combustion engine involved, then the term is meaningless BS...

 

[seesul]

The two-stage supercharger has different gearing settings, and can be switched between different levels of boost. It's important to note that there are negative effects of supercharging along with the power benefits. These are: Overheating - this is the main problem, and can lead to the fuel igniting before it is supposed to. The second is power loss - driving the supercharger unit itself consumes engine power. This is a minor effect though, compared with the huge boost of power that the supercharger brings.
/QUOTE]

Perfectly explained!
was the system (speaking about WW2 engines) that switch from the 1st stage to the 2nd and back always automatic or sometimes manual? Which kind of system did it use? Electromagnetic clutch? Had the pilot a chance to change the boost pressure manually?

Thanx!

 

[Haztoys]

A supercharger is driven by the motor so it starts given power at low rpm's

Turbo's ... need the exhaust to get some speed up to get the turbo turning ...so theres some "lag" down low...

Super's give power ..but cost power some to run...And a turbo uses the wasted exhaust gases to make power..


You can control a super better and EZ'er smoother power ... A turbo is harder to control and has a big step in power when the power comes on ...But no lose in power to run the turbo... Alot of super's were hooked to the back of the motors to get a drive point to drive them ... And turbo's like in the P-38 could be but behind the wing and exhaust piping run back to turn a turbo...

Did any country use turbo's but the US..."I" was "thinking" that most used super's...

 

[KrazyKraut]

I think so too. Germany tried to develop high altitude Fw 190s using turbocharged BMW 801s and DB 603s but instead chose the supercharged Jumo 213 for what became the Fw 190 D. Apparently the turbocharged BMWs were very expensive and the "piping" (don't kill me for using this word) would've run across the fuselage and wings (too little space inside the small fighter?).

Was the DB 603 in the Ta-152 C turbocharged? I can't remember, but i think not.

I often wonder, could you say, as a rough generalization, that superchargers offer the simpler solution but are inferior (performance-wise) to the more complex turbochargers at really high altitudes?

 

[Karl Sitts]

OK, I will have a go...

When you are sucking air into the engine to mix with fuel for combustion, the amount of air depends on the ambient air pressure. This means you are limiting the power you can get from each piston stroke, because you can't physically get any more air into the cylinders. As you fly at higher altitude, the air pressure is less, so your aircraft actually loses power as you climb...

The 'fix' is to run a very high speed pump which actually compresses the air before it's sucked into the cylinder. Simply put, it is a fan, driven from the engine itself, which sucks air in at 'ambient' pressure and boosts the pressure to above the normal range. More pressure means more oxygen, which means more fuel can be burned, and more power produced. So the supercharger will normally produce more power at all altitudes, but is specially effective high up, as it counteracts the effect of low pressure.

The two-stage supercharger has different gearing settings, and can be switched between different levels of boost. It's important to note that there are negative effects of supercharging along with the power benefits. These are: Overheating - this is the main problem, and can lead to the fuel igniting before it is supposed to. The second is power loss - driving the supercharger unit itself consumes engine power. This is a minor effect though, compared with the huge boost of power that the supercharger brings.

A quick note about Turbochargers:

A turbocharger or 'turbo' does exactly the same thing as a supercharger, the only real difference being that the turbo is driven by exhaust gas instead of mechanical gears or shafts. If anything is called 'turbo' and is say a razor, computer processor, vacuum cleaner, etc. this is basically a lie. Turbo means turbocharger, and if there's no internal combustion engine involved, then the term is meaningless BS...

Simon, wasn't a silly question at all. I certainly learned something!

 

[SoD Stitch]

You can control a super better and EZ'er smoother power ... A turbo is harder to control and has a big step in power when the power comes on...

...

Actually, you can control a turbocharger to some degree by varying the amount of exhaust that goes out the wastegate; the wastegate is usually controlled by an exhaust valve that regulates how much of the engine's exhaust goes to the turbo, and how much simply gets dumped overboard (hence the "waste" part of wastegate). With the wastegate closed, 100% of the exhaust goes to the turbo; with the wastegate open, only a very small percentage of the exhaust goes to the turbo. IIRC, on the original turbosuperchargers (that's what they called them originally), the wastegate was controlled manually by the pilot, but there were too many problems with overboosting (and the consequent destruction of a very expensive engine!), so an automatic system was devised (don't know all of the details on that; I'm sure someone else here probably knows a lot more than I do about this).

But no lose in power to run the turbo...

And, actually, there is a small penalty for a turbo; the amount of exhaust backpressure with a turbo is somewhat greater than in a normally aspirated engine, so the piston has to work a little harder to get the exhaust out of the cylinder. However, compared to a supercharger, this is negligible so, in essence, you are almost getting "something for nothing".

 

[Old Wizard]

...
And, actually, there is a small penalty for a turbo; the amount of exhaust backpressure with a turbo is somewhat greater than in a normally aspirated engine, so the piston has to work a little harder to get the exhaust out of the cylinder. However, compared to a supercharger, this is negligible so, in essence, you are almost getting "something for nothing".

Nowadays there are there are some ingenious folks who turn turbo-chargers into jet engines!

 

[HoHun]

Hi Stitch,

>However, compared to a supercharger, this is negligible so, in essence, you are almost getting "something for nothing".

Hm, if we're talking high-performance aircraft, that's not really accurate. The turbo-supercharger exploits the energy that remains in the hot, fast-moving exhaust gases expelled from the engine, sure - but the piston engines of the era were usually equipped with jet exhausts that exploited the same energy to gain additional thrust.

At high speed, this thrust gain would be considerable - it could add the equivalent to 500 shaft horse power to an engine in the 2000 HP class. The penalty was much less extra weight than required for a turbo-supercharger, and some additional backpressure just as with the turbo-supercharger.

The turbo-supercharger's strengths were fuel economy for the cruise and very high altitude operations, while mechanically-driven superchargers with ejector exhausts were lighter and had an advantage to high speeds.

As a quick rule of thumb, one could say that the turbo-supercharger was better suited for bomber aircraft and the mechnically-driven supercharger better for fighter aircraft. However, that's just summing up the general tendency - there were a lot of successful aircraft types that violated this rule

Regards,

Henning (HoHun)

 

[seesul]

...

Actually, you can control a turbocharger to some degree by varying the amount of exhaust that goes out the wastegate; the wastegate is usually controlled by an exhaust valve that regulates how much of the engine's exhaust goes to the turbo, and how much simply gets dumped overboard (hence the "waste" part of wastegate). With the wastegate closed, 100% of the exhaust goes to the turbo; with the wastegate open, only a very small percentage of the exhaust goes to the turbo. IIRC, on the original turbosuperchargers (that's what they called them originally), the wastegate was controlled manually by the pilot, but there were too many problems with overboosting (and the consequent destruction of a very expensive engine!), so an automatic system was devised (don't know all of the details on that; I'm sure someone else here probably knows a lot more than I do about this).



And, actually, there is a small penalty for a turbo; the amount of exhaust backpressure with a turbo is somewhat greater than in a normally aspirated engine, so the piston has to work a little harder to get the exhaust out of the cylinder. However, compared to a supercharger, this is negligible so, in essence, you are almost getting "something for nothing".

Today´s turbos have also a control system called variable geometry...the exhaust chamber volume of the turbocharger is automaticaly changed by the control system so in case of need- smaller volume= higher exhaust gas speed=higher turbine rpm=higher boost pressure, bigger volume=lower gas speed=lower rpm=lower boost pressure...

 

[<simon>]

Wow! What a reply!

Thanks for that everyone!
Another piece of knowledge I can hopefully pass on to others

Thanks again
Simon

 

[seesul]

does anyone have a drawing or picture of the supercharger drive on a B-17 (R-1820 engine)? I´m curious how did it work as the engine was in the vertical position and the supercharger in horizontal position so they didn´t have common axis...on my opinion...
On the engines with the supercharger mounted on its back it´s clear...

 

[red admiral]

A supercharger can be thought of as an air pump. They are usually centrifugal type for aircraft engines, apart from the axial one on the Napier Nomad. The air enters at the middle of the supercharger impeller where it is flung outwards and gathers speed. At the outer edge of the impeller is a diffuser ring which slows down the air and increases the pressure and density before it is fed to the cylinder.

A two (or more) speed supercharger has different gearing which alters the speed at which the impeller rotates. This gives different pressure ratios which correspond to different full throttle altitudes. This is why graphs of hp/altitude usually have a saw tooth shape with one or more peaks. The German DB series engines used a hydromatic system instead of gears which doesn't give the saw tooth form as the transmission is continuously variable - but it is less efficient than a simple gear so sucks a bit more power out of the engine.

A two (or more) stage supercharger is more efficient than a single stage, especially when high pressure ratios (= high altitude) are needed. It is more efficient to split the compression up into multiple stages. The extra stage to the engine adds weight however so it isn't a simple case.

The US type of turobcharged setup used both a turbocharger and a supercharger. Essentially it can be thought of as a two stage supercharger, but only one of the stages draws power from the engine. Air enters the turbocharger first where it is compressed and then sent along a duct to the supercharger which is mounted behind the engine where it is compressed again before being fed to the cylinders.

There was also a US two stage supercharged setup that featured a clutch so that one of the stages could be decoupled at lower altitudes. I'm not so keen on this as it adds length and weight but has no real gain.

As a rule of thumb, the turbocharger is best suited for aircraft below around 350mph and the supercharged setup above this. The turbocharger removes the energy from the exhaust stream to provide energy for compression. In an unsupercharged engine this can be used with ejector exhausts to directly give thrust. The ejector exhausts on the Spitfire XIV gave an extra 47mph (maximum) so their effect is quite considerable.

 

[dairwin]

...ejector exhausts on the Spitfire XIV gave an extra 47mph (maximum) ...

Can you quote a reference/source for this? I would like to read more about it. Thanks.

 

[Micdrow]

does anyone have a drawing or picture of the supercharger drive on a B-17 (R-1820 engine)? I´m curious how did it work as the engine was in the vertical position and the supercharger in horizontal position so they didn´t have common axis...on my opinion...
On the engines with the supercharger mounted on its back it´s clear...

Here are a couple of diagrams from the B-17 erection and maintance manual. Hope they help

 

[seesul]

Wow, thanks for the perfect drawing Micdraw!
So as I can see this is the turbo supercharger so in fact turbocharger!!!
Don´t know why but I was always told that b-17 didn´t have turbocharger but mechanically driven supercharger. This picture shows something else- a pure turbocharger....with a waste gate system for the boost pressure control and intercooler for the power increasing...wow, what a technology back in 40´s...
Thanks again mate!

 

[HoHun]

Hi Seesul,

>This picture shows something else- a pure turbocharger....

If you have a look at the Supercharger Flow Diagram, you can find a mechanically-driven Supercharger there as well: Follow the intake airstream though A, B and C, and right after the fuel injection, you'll see a part labeled "impeller". This impeller is the wheel of the mechanically-driven supercharger, also called "engine stage" in US terminology since it was part of the basic engine. In the schematic drawing, it's located right on the back of the crankcase and appears to be driven directly by the crankshaft, but I'm not sure this is entirely accurate - I think it might be driven by a geared transmission in order to reach the high speeds usually require for centrifugal superchargers.

Regards,

Henning (HoHun)

 

[seesul]

Thank you for your note, haven´t noted it.
Yes, you´re right there´s a mechanicaly driven turbine but I can´t see any RPM/pressure control system on it. Should it be a supercharger it would cause overpressure in the air intake at high engine RPM. Maybe stupid thought but it could be device for a better mixture distribution into the cylinders.
But anyway, the main part that is on the lower part of the wing is turbocharger.
Think I´ll print out this drawing and will ask some crew member from one of 3 B-17 beeing displayed in Duxford...I´m leaving for Duxford tomorrow...

 

[seesul]

Henning,
here is the text I got from Jim Peters, a former engineer of a B-17, WW2 vet, few months ago...that´s an excellent explanation. I´ve forgot I have it and found it now...

Roman :......There are three types of superchargers.... One is belt driven that in effect pumps air at an increased pressure to the carburetor. Second is an internal so-called supercharger that by rotating with the engine distributes the air/fuel mixture to each individual cylinder more or less evenly, which is it's purpose. Third is the exhaust driven turbosupercharger, which consists of two wheels mounted above each other,where both rotate on a common shaft. This third type has the exhaust driving a turbine wheel which , when the exhaust gasses flow through the turbine wheel, causing it to spin...at the same time rotating the compressor section which pumps compressed air to the carburetor, also there is an item called a waste gate, which when open....bypasses the exhaust past the turbine wheel, but when closed causes both the turbine wheel and the compressor to spin...the B-2 type had a top RPM of 23,500 RPM and the B-22 had a top RPM of 26,000 RPM...both were used interchangeably. This waste gate is controlled,so that the exhaust gasses,pass through the turbine wheel in varying pressures, causing the turbine wheel to spin faster and faster as the waste gate is closed gradually. This turbosupercharger is compact....with only about an inch (25cm) seperating the turbine wheel half from the half that compresses the air going to the carburetor, and the turbine wheel which operates at 1500 degrees F,from the air compressor which is drawing in and compressing the up to -60degrees F. going to the carburetor. In efffect, all this does is fool the engine into thinking it is operating at sea level pressure..14.7 Inches Manifold Pressure...(37cm), when it is operating at an altitude of 30,000 ft, where the pressure is down to 1.5 inches of Manifold pressure or 4 cm. Therefore the actual pressure being delivered to the carburetor is at roughly 12 inches of manifold pressure or 30cm. Does this make sense to you ? Control System....the early B-17 through the "F" model was operated by hydraulics where an engine driven hydraulic pump on the two inboard engines, supplied hyd pressure to operate the waste gate, but was troublesome in that the lines and fittiings were subject to damage. With the "G" model (and also the later B-24s) the hydraulic system was replaced by an all electric controlsystem which then had a electric motor closing the waste gate, along with a pressuretrol which sensed the alititude, an overspeed governor which did not allow overspeeding of the turbine wheel, and where the control system had what was known as a Wheatstone Bridge Circuit, whereby any unbalance of an input would cause the system, after making the change to automatically balance itself. The cockpit control was called a TBS for TurBo control, where one control knob controlled all four engines simultaneously. There was also an amplifier for each engine, a junction box for each engine, a waste gate motor for each engine, a pressuretrol for each engine, all conncected by the one circuit called a Wheatstone Bridge, again where either the cockpit control, pressuretrol (altitude) or overspeed governor would automatically maintain the setting of the cockpit control. The WW II aircraft operated on a 24V DC electrical system. and the turbo control system operated on a 400 Cycle 115V AC system...this 400 cycle 115 V was supplied by an Inverter which was an electric motor driving a 115v 400 cycle generator, on the same shaft. The B-17 landing flap position indicator was also operated by this same 115v 400 cycle supplied by the Inverter. My scanner is not working or I would scan a photo of an exhaust driven turbosupercharger. I hope you can understand this explanation. If not, please ask. Jim

_____________________________

James S. Peters Sr. T/Sgt
B-17 Flt Engr, 27 missions
99 BG, 348BS, 5th Wing, 15th AAF
Tortorella, (Foggia#2), Italy
My Tour was from 12/03/44-06/19/45
M/Sgt USAF (Retired)

 

[Heinz]

Great info guys!