Supercharger high gear at low alt?

[=X51=VC_]

I noticed the other day I dived in a Lagg-3 from 3k to 500m and forgot the supercharger gear in high. Should this have overboosted my engine or was there a safety valve to prevent this? 100% throttle and MP didn't go above rated maximum. Similarly in La-5 I've at least once flicked the supercharger into high gear by accident at low alt, nothing happened.

[JtD]

There is a safety, it commonly is called automatic boost control or automatic manifold pressure regulator or something along these lines.

 

Your throttle lever is not directly linked to the throttle valve, which controls how much air (and thereby boost) is let into the engine. With a regulator in between, you typically get a feedback loop from the boost after the throttle valve, which works on an aneroid, which contracts when the boost goes up, and expands as boost goes down. The whole thing is carefully set up and tuned on the ground according to the specifications of the engine. It will make sure that the boost that goes into the engine is what is selected by the pilot via throttle lever position, independent of outside conditions. So actual manifold pressure is a result of throttle position set by the pilot plusminus throttle corrections made by the aneroid.

 

Even faulty operation of a supercharger would typically be compensated by this regulation. It's still pretty hard on the engine (increased risk for detonation), which also delivers less power this way.

[Finkeren]

JtD got it right (as usual) but in terms of the sim itself running in high gear at low altitude seems to do little other than slightly lower your power output.

[=X51=VC_]

Fair enough, that was going to be my next question about what the consequences are in game. It's not particularly hard to remember to change gear but good to know. Thanks!

[=TBAS=Sshadow14]

Agreed

Also Most of those old Superchargers were pushed very hard to the point of max Volumetric efficiency where spinning any faster mean air is heated to much and pressure output drops and hot air increases chance of detonation and in gear 1 on deck they are operating @ 90-95% Maximum flow (or almost as much as the unit could make unrestricted - CFM)

Once you start to climb the supercharger spins at that same speed but the air thins out and that speed is no longer good enough and to maintain the same CFM the gear is changed and rpm is increased and now in the thinner air it produces the same CFM as it did on deck in gear 1 but this is still 90-95% of its max abilities,

So even if you dive down the gear 2 will still only produce the same CFM as supercharger cannot make much more,
But the Parasitic horsepower load on the engine will be high resulting in less net HP in most cases.

You have to remember that Superchagers are different from Turbochargers in the fact a supercharges is belt/gear/shaft driven and requires engine HP to run the air pump (charger)
Turbochargers are run from purely wasted energy in Exhaust gases and only give HP gains never any losses.
So say if your unboosted engine is 400HP and you bolt on a 250HP supercharger that takes 25HP to drive off the belt then your total NET HP will only be 625HP not 650,
By adding gears you can manipulate this Parasitic HP lose and also increase the life of supercharger as higher rpm in 2nd gear @ same flow on deck will shorten its life of course.

So gear 1 might keep the Charger rpm at 14000rpm @ at cost of say 200hp,
and gear 2 at 14000rpm @ at cost of say 300hp

HP numbers are random examples and not meant for yak or any plane in game

Edited January 31, 2018 by =TBAS=OccludedLight14

[=X51=VC_]

So the parasitic HP loss is modelled but the knocking and possible engine damage isn't?

[=TBAS=Sshadow14]

i dont know whats modeled or not properly
i was talking about the real world application

[Field-Ops]

This type of engine damage is hard to determine if it is modeled ingame, and if so would be almost pointless as the damage to the supercharger would exceed a full tank of gas to see the damage effects. Which we do not have airfield re-arm and re-fuel to see damage like this, we just get a new plane and/or end the mission.

[ACG_pezman]

Turbos have less power at low rpm whereas super chargers don’t have that issue. Just saying, turbos aren’t all sunshine and lollipops.

[ZachariasX]

Fair enough, that was going to be my next question about what the consequences are in game. It's not particularly hard to remember to change gear but good to know. Thanks!

You should always check your manifold pressure gauge when operating the throttle. When when you hit high pressures with lever only half way, then it is a sign that you should switch.

 

You can expect to lose up to 20% of power by forcing the blower to run at high speeds. You know what that means if trouble is inbound.

 

What is however a good idea sometimes, is remaining at low gear at higher altitude, even if it is way above critical altitude of the low blower. As long as you still reach power setting for cruise, you significantly save fuel and gain range.

[ZachariasX]

Turbos have less power at low rpm whereas super chargers don’t have that issue. Just saying, turbos aren’t all sunshine and lollipops.

That would be almost at idle. At „low“ rpm, like 2000 rpm, the supercharger works fine. You can really up your mileage that way as well. It is just a very bulky item.

[AndyJWest]

...

Turbochargers are run from purely wasted energy in Exhaust gases and only give HP gains never any losses.

...

A turbocharger inevitably increases exhaust back pressure: you can't use all the 'wasted energy' without some loss.

[=TBAS=Sshadow14]

A turbocharger inevitably increases exhaust back pressure: you can't use all the 'wasted energy' without some loss.

Thats why turbo cars are loud..

 

Best is no exahust at all except where gas scavenging is needed like on WRX unequal length headers,

but for most turbo application a zero back pressure open exhaust is best.

 

but you are right gas passing through the turbo is restricted by the turbine wheel itself but gas flowing through the needed High back pressure exhaust on a naturally aspirated engine is much more restricted.

 

My point what if you take an engine with 200hp output and bolt on a turbo that will make 350HP your car will make 350HP

There is no Notable HP cost to run turbo off exhaust as the gains outweigh the loses .

 

Maybe 1% power loss from back pressure

25-750% power increase from adding turbo

[JtD]

In terms of ww2 fighter aircraft, the main turbo losses come from loss of exhaust thrust. Late war supercharged aircraft achieved thrust values of ~1500N at speeds of ~200m/s, which are 300kW. At the same time, the engine (reduced by propeller efficiency) generated maybe 1200kW or 6000N of thrust. So 20% of the total power/thrust would come from the exhausts. You'd near completely lose this thrust with a turbo. For these losses, i.e. 300kW, you can power a pretty big supercharger.

 

Which leads to the fact that turbocharged aircraft are more effective in low speed flight, while supercharged aircraft might be more effective in high speed flight.

[Finkeren]

In terms of ww2 fighter aircraft, the main turbo losses come from loss of exhaust thrust. Late war supercharged aircraft achieved thrust values of ~1500N at speeds of ~200m/s, which are 300kW. At the same time, the engine (reduced by propeller efficiency) generated maybe 1200kW or 6000N of thrust. So 20% of the total power/thrust would come from the exhausts. You'd near completely lose this thrust with a turbo. For these losses, i.e. 300kW, you can power a pretty big supercharger.

 

Which leads to the fact that turbocharged aircraft are more effective in low speed flight, while supercharged aircraft might be more effective in high speed flight.

I’m always amazed that the exhaust provided so much thrust. You could almost say that high-powered WW2 aircraft were 20% jet powered.

[=TBAS=Sshadow14]

The lost thrust would not matter from the turbo as depending on size of turbo fitted the power gains would be anywhere from 25% to 500%
So minus 20% thrust from exhaust but +25-500% power from the turbo turning that 20% thrust in exhaust into lots more HP.

Turbo charged engines are the most efficient of ALL internal combustion engines.
All noise and heat pumped out exhaust is also lost power. to make the sound you must convert fuel energy into sound.

In terms of car turning
there is about 15% loss in Exhaust gas velocity when passing through Turbo Hot side (exhaust turbine)
When the turbo reaches max boost and the wastegate is opened exahust gas bypasses the turbine and flow is more like 95% of that without turbo on it.
so once max boost is achieved with turbo then exhaust thrust would be like 17% more power.


 

 

The Supercharger: Pros And Cons

Obviously, there are pros and cons to a supercharger as well.

First, the pros.
Superchargers have no lag, they boost an engine at low RPM, they run at cooler temperatures than turbos, and they're relatively cheap in comparison to turbos (those turbines can get really expensive).

Now for the cons.
Superchargers are inefficient, because they require quite a bit of engine power to turn. That makes superchargers less fuel efficient than turbos. And finally, because they use a system of pulleys and gears to turn, there are more parts that can break.

High speed racing PISTON planes are all turbocharged and supercharger not good enough and boost can only be controlleed by gears. not by a blled valve or wastegate.
 

Edited February 1, 2018 by =TBAS=OccludedLight14

[ZachariasX]

In terms of ww2 fighter aircraft, the main turbo losses come from loss of exhaust thrust. Late war supercharged aircraft achieved thrust values of ~1500N at speeds of ~200m/s, which are 300kW. At the same time, the engine (reduced by propeller efficiency) generated maybe 1200kW or 6000N of thrust. So 20% of the total power/thrust would come from the exhausts. You'd near completely lose this thrust with a turbo. For these losses, i.e. 300kW, you can power a pretty big supercharger.

 

Which leads to the fact that turbocharged aircraft are more effective in low speed flight, while supercharged aircraft might be more effective in high speed flight.

 

20%? AFAIR, the Spitfire gained 13 mph @ 20k ft (334 -> 347) when those instruments were commonly fitted to Merlins:

 

 

Well, when going from 1A to 1B in the case mentioned.

 

And that 13 mph increase is equivalent to theroretical power invrease like 1000 hp vs "1200 hp"?

 

Also, I'm not so sure about that "rocket" with a gas speed of 200 m/s. Basically one could assume it accellerates (assuming 2800 rpm) 700 times 27 x 1.6 g of air per second, that is roughly 30 kg of air accellerated to 200 m/s. As a rough comparison, it would have to be a correspunding impulse to accellerating a 3 ton aircraft for a speed increase of 2 m/s. It is a push, but not that much of a push.

 

Or is it really that much higher? Why?

 

I am positive that at high altitute nothing beats the turbocharger. For a reason a 109 cannot even dream of following a P-47 in a zoom climb at altitude. Having to install this complicated and expensive thing is not trivial however. It made planes as fat as a P-47 or as fancy as the P-38. The relative simplicity of the supercahrger plus the design advantages it offers for the aircraft structure and aerodynamics is another topic. The supercharger is simpler, lighter and less efficient. Find your sweet spot.

[JtD]

power gains would be anywhere from 25% to 500%

I'm sorry, we're still talking about WW2 aviation engines, not modern race cars. A typical example for WW2 engines might be the P&W R-2800, which came both supercharged, and turbocharged. If you compare the variants, the turbocharged engine (as installed in the P-47) produced about 2300hp at 56" boost. The supercharged engine (F4U, F6F) produced about 1900 in low gear at the same 56" boost. This is the typical 20% difference one might find in these types of engines. The exhausts of the P-47 run through the entire aircraft, and exit at the rear with near zero thrust production. As typical for the time.

The turbo had a higher full throttle altitude, which would give it a 50% benefit up high, but at sea level, the power difference was less than 10%.

 

20%? AFAIR, the Spitfire gained 13 mph @ 20k ft (334 -> 347)

Which requires 13% more power (speed relation cubed). And it isn't really fast. The effect gets larger at higher speed (at least linear).

 

The Jumo213a produced more than 1700N of exhaust thrust, when the engine delivered 1900hp. 1900hp are maybe 1200kW at the propeller at high speed, rather less. High speed, in a Fw190D-9 would be 190m/s, which means 1700N of thrust equal 323kW. In this case, more than 25% of the aircraft propulsion comes out of the exhausts.

The Jumo213a is the most extreme case I have figures for, but you can see that the 'partial jet power' idea is pretty much spot on.

 

The BMW801D needed about 300PS to drive the supercharger in 2nd gear, and produced about 1000N of exhaust thrust. On an aircraft going in excess of 800km/h, the exhaust thrust would amount to more power than eaten by the supercharging. It would be pretty hard for a turbo to compete with that.

Edited February 2, 2018 by JtD

[ZachariasX]

 

 

The Jumo213a is the most extreme case I have figures for, but you can see that the 'partial jet power' idea is pretty much spot on.

Impressive indeed.