Author Topic: Superchargers on models....can they work?  (Read 8379 times)

Offline Intheshed

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Superchargers on models....can they work?
« on: April 12, 2016, 10:05:15 AM »
Having a discussion elsewhere about the use of a supercharger on a model.
My contention is that if you reduce the volume of the blower by the cube of the scale, say, 1/4, you get a device 64 times smaller and therefore likely to do little for performance except perhaps to mess up the fuelling and possibly even have a negative effect.
What think the experts?

Martin

Offline Bluechip

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Re: Superchargers on models....can they work?
« Reply #1 on: April 12, 2016, 10:30:27 AM »
There was some experimentation carried out ages ago on 2-strokes written up in M.E.

I eventually    :old:  remembered the author's name and did a quick search. [ Hollick ].

http://www.itech.net.au/modelengineer/index.php

( The specific search link didn't work, looks like you'll have to put it in yourself. )

Dave

Offline Jasonb

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Re: Superchargers on models....can they work?
« Reply #2 on: April 12, 2016, 10:52:17 AM »
But would the engine also only be 1/64th the capacity so the ratio of blower volume to capacity would be the same for full size as it would for the model?

I suppose there are also two types of model, the scale one that mimics the original supercharger and looks right, this will run for display and not be put to work so does not really matter what effect the supercharger has. Then there is the performance model engine that is likely to have a non scale looking supercharger that does increase performance but would look out of scale if upped to full size.

I see some of the 1/5th scale off road models are now sticking blowers onto their engines. I think the power increse speaks for itself

« Last Edit: April 12, 2016, 11:00:12 AM by Jasonb »

Offline Ian S C

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Re: Superchargers on models....can they work?
« Reply #3 on: April 12, 2016, 12:18:46 PM »
Dean Clarke who ran a thread on making a Supercharged V-12 Two Stroke in the ME site also posted on this site, if you look on (P) 2 of Your Own Design you will find it.  I think he plans to build a flat 12 (? using up the spare parts).
Ian S C

Offline steamer

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Re: Superchargers on models....can they work?
« Reply #4 on: April 12, 2016, 12:40:37 PM »
But would the engine also only be 1/64th the capacity so the ratio of blower volume to capacity would be the same for full size as it would for the model?

I suppose there are also two types of model, the scale one that mimics the original supercharger and looks right, this will run for display and not be put to work so does not really matter what effect the supercharger has. Then there is the performance model engine that is likely to have a non scale looking supercharger that does increase performance but would look out of scale if upped to full size.

I see some of the 1/5th scale off road models are now sticking blowers onto their engines. I think the power increse speaks for itself


It's interesting that they use a centripetal supercharger.   it's response is very non linear! ( You BRM lovers know this all too well!) , however, 2 strokes on tuned pipes put the power out in a non linear fashion as well, so it would seem that the result is ideal....    Our team manager is a nationally ranked 1/5 scale driver, I'll toggle him on this subject....but I won't know when I'll get a response...stay tuned...  8)
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Offline pgp001

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Re: Superchargers on models....can they work?
« Reply #5 on: April 12, 2016, 01:07:08 PM »
I am sure the plan is for this one to be fully working, it is on Mike Sayers superb model Bentley engine, I cut the teeth on the internal gears for him on my Mikron hobbing machine as they had to be backlash free.





Phil


Offline Intheshed

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Re: Superchargers on models....can they work?
« Reply #6 on: April 12, 2016, 02:25:19 PM »
Hmm, that Bentley is quite some piece of work, but will that scale blower work AS a blower on a scale engine.  My point about the reduced volume was in connection with the fact that you can't scale nature.  The air and fuel molecules are not reduced by scale and so are likely, maybe (I don't KNOW) to be affected differently by the supercharging.  The thing on the 1/5th engine is designed (with no pretense to scale appearance or size) to suit that engine and clearly does so very well, but then it's not a scale engine is it?  It's a glorified weed whacker basis with tune up goodies. It's a full sized engine in its own right.  Very different from a scale V8 running a scale blower.

Martin

Offline steamer

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Re: Superchargers on models....can they work?
« Reply #7 on: April 12, 2016, 02:29:58 PM »
Hmm, that Bentley is quite some piece of work, but will that scale blower work AS a blower on a scale engine.  My point about the reduced volume was in connection with the fact that you can't scale nature.  The air and fuel molecules are not reduced by scale and so are likely, maybe (I don't KNOW) to be affected differently by the supercharging.  The thing on the 1/5th engine is designed (with no pretense to scale appearance or size) to suit that engine and clearly does so very well, but then it's not a scale engine is it?  It's a glorified weed whacker basis with tune up goodies. It's a full sized engine in its own right.  Very different from a scale V8 running a scale blower.

Martin

Agreed.....something to ponder, is the clearance on the "Scale" super charger is probably nearly identical to the clearance on the full size one...so I imagine leakage and blow by may be more prevalent on the scale one ....
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Offline Jasonb

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Re: Superchargers on models....can they work?
« Reply #8 on: April 12, 2016, 02:40:51 PM »
Yes the Bentley and the RC engines are the two extreams of the model engine hobby where only one would really be bothered about how the blower actually performs in terms of a useful power increase.

As Steamer says its more likely that clearances and friction losses will affect a scale application than the actual air/fuel mix. I doubt that Mikes engines will get dyno tested but quite possible someone has done some tests with a blown and normally aspirated model V8

J

PS Good article by Mike on making scale model radiators in ME mag that is currently in the shops.

Offline Jo

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Re: Superchargers on models....can they work?
« Reply #9 on: April 12, 2016, 02:49:02 PM »
I have every confidence that Vixen's Supercharger will work: http://www.modelenginemaker.com/index.php/topic,5142.msg98385.html#msg98385

Mike Sayer made that radiator a few years ago it has been to a fair few Guildford shows that I know of. We are not expecting him down this year  :(

Jo
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Offline pgp001

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Re: Superchargers on models....can they work?
« Reply #10 on: April 12, 2016, 05:30:54 PM »
On Mikes other Bentley that is finished and running, he only runs it from one of the two carbs because of similar scaling nature problems. It runs too rich on both carbs.

Phil

Offline lohring

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Re: Superchargers on models....can they work?
« Reply #11 on: April 12, 2016, 06:31:00 PM »
It depends on whether you want a scale blower/supercharger or to add performance to an engine.  In my opinion, most scale blowers/superchargers suffer severely from scale effects, especially in the clearances.  However, there is an exception with piston and vane superchargers.  They are capable of moderate to high pressures with good efficiency in small sizes.  Design for low friction is the main issue.  Some very successful model 4 strokes use under piston compression for supercharging.  Supercharging can work for two strokes as well, but the exhaust back pressure needs to be raised  along with the intake pressure to keep the mixture from blowing out the exhaust port.  A tuned pipe can be designed to do this.  Bigger turbocharged two strokes do this with the exhaust turbine.  The 4 cylinder turbocharged engine pictured develops over 800 hp with nitrous injection.  See below for two and four stroke applications. 

Lohring Miller

Offline Vixen

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Re: Superchargers on models....can they work?
« Reply #12 on: April 12, 2016, 07:16:33 PM »
Do supercharger work on models? The short answer is YES.

Supercharging is defined as forced induction of the fuel/ air mixture into the engines cylinder(s). There are three types of supercharging currently being used with model engines.

The R/C flying community have supercharged engines like the YS140 FZ. This type of engine pumps mixture into the cylinder using crankcase compression, similar to a 2 stroke engine. The performance gains are modest.

The 1/5 scale model car racing guys employ centrifugal compressors to force the fuel/ air mixture into the cylinders to gain additional power, as we have seen in the earlier video clips. Centrifugal compressors can be made to work at scale model sizes, it is even possible to use the compressor rotor from a small turbo charger. All centrifugal compressors produce pressure proportional to RPM squared, i.e. the power gains only occur at high revs

The third class of supercharger is the positive displacement or Rootes type compressor. The Rootes type blower commonly adorns high power dragster inspired V8 engines. The Blower Bentley being painstakingly built by Mike Sayers and the two-stage supercharged Mercedes Benz W165 engine being constructed by Mike Tull (AKA Vixen) are other examples of positive displacement superchargers being built for true scale model engines. The Roots blower pumps the fuel/air mixture from the low end of the rpm range to the top end, obviously the power gains will be higher at the highest speed. For those, fortunate enough to have driven a road car equipped with a Rootes blower, can testify to the extreme smoothness and effortless power of the blown engine.

The positive displacement Roots blower is basically a gear pump. A fixed amount of air is displaced with each revolution of the pumps gear wheels. In the case of a Rootes blower the pump gears may have as few as two or three gear teeth (lobes). The lobes need additional fine tooth gear wheels to keep them synchronized and so prevent metal to metal contact. In full size supercharges, where the emphasis is on outright performance, the clearance between the compressors lobes is kept as small as possible (a few thou) to prevent blow by losses.

Most, if not all, scale models of supercharged engines are built for display rather than racing competition. When they are demonstrated on their display stands they generally do so without and external load. The authors two-stage supercharged Mercedes Benz W165 engine is a 1/3 scale replica of an engine which achieved maximum power at 2.6 atmospheric pressure, that's +24 pounds of boost. Clearly, this is far more boost pressure than is needed to run the model engine on a display stand. It is not connected to any load, so is doing no work. Therefore I have deliberately increased the lobe clearances in my two stage compressor to make it less efficient and so produce less boost. The inefficient compressor provides work for the engine and the lower boost pressure make the running the engine more manageable and controllable (I hope!!!)




As far as scale is concerned, the W165 engine is being built at a large 1/3 scale, which implies a swept volume of 3 x 3 x 3 = 27 of the original. This is a photo of the 1/3 scale two stage supercharger sitting on top of the full size engine. There are only two surviving W165 cars in the world so this was a rare occasion. It sure looks small compared to the real thing

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Offline Intheshed

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Re: Superchargers on models....can they work?
« Reply #13 on: April 12, 2016, 07:36:30 PM »
Tuned pipes with a turbo on the end of them?  Doesn't the turbo cause a back pressure that gives problems?  I guess not if that can develop such power, although nitrous seems to help most engines!

The Mercedes engine model looks very promising, but it seems that only the non-scale centrifugal(petal) has any real effect on performance.  The scale end of things being more for show it seems.

Fascinating subject.

Thanks for all the info. gents.

Cheers,
Martin

Offline steamer

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Re: Superchargers on models....can they work?
« Reply #14 on: April 12, 2016, 07:37:56 PM »
No turbo chargers that I know of in the scale sizes....but definitely superchargers....ie mechanically driven by the engine not from the exhaust gasses
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Offline lohring

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Re: Superchargers on models....can they work?
« Reply #15 on: April 12, 2016, 09:45:41 PM »
"Tuned pipes with a turbo on the end of them?  Doesn't the turbo cause a back pressure that gives problems?"

Back pressure is only a problem in four strokes where the piston has to do work by pushing out the exhaust against  the back pressure.  In a two stroke the intake and exhaust processes take place around bottom dead center with almost no negative piston work.  Exhaust back pressure is essential to keep the mixture from blowing out the exhaust. 

A tuned pipe acts like a turbocharger with no moving parts.  By raising and lowering the pressure at the exhaust port it can do the same thing at a higher pressure with the turbocharger in the picture.  Below is a graph of what a tuned pipe alone does to the pressure at the exhaust port.  The high pressure pulse at the exhaust port opening is used to first create a low pressure wave that helps fill the cylinder then returns a high pressure pulse that supercharges the cylinder just before the exhaust port closes.  In this example the low pressure is .7 x 14.7 or 10.2 psi.  The boost pressure is 2.2 to 2.4 x 14.7 or over 30 psi.  That's usually better in a small engine than any mechanical supercharger.

Lohring Miller

Offline steamer

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Re: Superchargers on models....can they work?
« Reply #16 on: April 12, 2016, 11:23:18 PM »
Yes....the key term there being "Acts like" a turbo charger..

Dave
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Offline Intheshed

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Re: Superchargers on models....can they work?
« Reply #17 on: April 12, 2016, 11:27:44 PM »
I'd love to say I understood all that, Lohring, but alas, it's beyond me.  I have never understood the finnessing of 2 strokes.  I'm just in awe of any hydroplane driver who, with his nose a few inches off the water can operate the adjustable tuned pipe length with one foot and the adjustable trim of the engine with the other.  The torque difference of lengthening and shortening the pipe out of a bend and on a straight is noticeable.
Thanks for taking the trouble to explain the principles.

Cheers,
Martin

Offline steamer

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Re: Superchargers on models....can they work?
« Reply #18 on: April 14, 2016, 01:03:22 PM »
Hey my Team Manager Matt got back to me.....no they don't use  the RB supercharger racing off road anyway...."more power isn't always what you want"....and I know from experience...he's right.

He thinks the amount of abuse that the engine would see would far outweigh the value of the extra power....to finish first, first you must finish....
He thinks RC drag racing though...you might have something!...

Dave
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Offline lohring

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Re: Superchargers on models....can they work?
« Reply #19 on: April 14, 2016, 04:01:04 PM »
I'd love to say I understood all that, Lohring, but alas, it's beyond me.  I have never understood the finnessing of 2 strokes.  I'm just in awe of any hydroplane driver who, with his nose a few inches off the water can operate the adjustable tuned pipe length with one foot and the adjustable trim of the engine with the other.  The torque difference of lengthening and shortening the pipe out of a bend and on a straight is noticeable.
Thanks for taking the trouble to explain the principles.

Cheers,
Martin

Constant pressure supercharging in two strokes is easier to understand and has been done a very long time.  Two examples are the Napier Nomad https://en.wikipedia.org/wiki/Napier_Nomad and the Rolls Royce Crecy https://en.wikipedia.org/wiki/Rolls-Royce_Crecy  An explanation of the research that went into the Crecy can be found in Sir Harry Ricardo's very readable book "The High Speed Internal Combustion Engine".  I have a .pdf copy of the chapter on his aircraft engine two stroke program, but it's too big to post here.  Sir Harry's explanation of the engine's development is the basis of my understanding of two stroke supercharging.  The book is available but is expensive.  Sometimes reprints show up at a reasonable price. 

Lohring Miller

Offline lohring

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Re: Superchargers on models....can they work?
« Reply #20 on: April 14, 2016, 04:23:39 PM »
The 1931 edition of The High Speed Internal Combustion Engine available on line does not contain information on two strokes.  That information is in the 1953 edition, the one I have.  The Ricardo company has a reprint available at a reasonable price.  It's one of the most worthwhile books on the development of internal combustion engines.
http://estore.ricardo.com/shop/the-high-speed-internal-combustion-engine/

Lohring Miller
« Last Edit: April 14, 2016, 04:41:28 PM by lohring »

Offline petertha

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Re: Superchargers on models....can they work?
« Reply #21 on: April 14, 2016, 07:37:50 PM »
The R/C flying community have supercharged engines like the YS140 FZ. This type of engine pumps mixture into the cylinder using crankcase compression, similar to a 2 stroke engine. The performance gains are modest.

I was an active RC pattern junky through the 2S to 4S to 4S-SC engine evolution period, gosh late 80's & early 90's and YS was my weapon of choice. I remember when the first 'AC' (air chamber = the boost box add on) started showing up on YS engines. The power difference was very noticeable on same displacement, but of course its all relative to the real world application you have it bolted into. I was going to guess 20% improvement. Probably there are some old spec sheets out there in internet land that quoted power/rpm figures. Nobody I knew had a model dyno, but we could inference guestimate it by comparable propeller size/rpm. I seem to recall a 0.90 CI AC would pull about the same as non AC 1.20. Link below seems to corroborate +/-. Of course they were constantly evolving. AC's had different sized valves & you felt the power peak at slightly different rpm ranges, but anyway the power increase was for sure a reality.
http://www.rcgroups.com/forums/showthread.php?t=735276

Offline jadge

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Re: Superchargers on models....can they work?
« Reply #22 on: April 15, 2016, 11:39:51 AM »
The Ricardo company has a reprint available at a reasonable price.  It's one of the most worthwhile books on the development of internal combustion engines.
http://estore.ricardo.com/shop/the-high-speed-internal-combustion-engine/

Lohring Miller

Thanks for the heads up, I've read the Ricardo autobiography but didn't know about the technical reprints. I am visiting Ricardo at Shoreham next month for a hybrid vehicle project meeting so I'll see if I can buy a copy while I'm down there and save on postage.

Andrew

Offline Vixen

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Re: Superchargers on models....can they work?
« Reply #23 on: April 15, 2016, 03:42:24 PM »
I ordered my copy of the Ricardo High Speed Internal Combustion Engine book yesterday. I could not find an excuse to visit them in Shoreham, so I have paid the postal charge instead. I have wanted to add this book to my collection for some time. Thanks for pointing it out Lohring.

Mike
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Offline Greg Haisley

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Re: Superchargers on models....can they work?
« Reply #24 on: April 15, 2016, 05:59:19 PM »
Some info many may not know.

For every lb. of boost 7% more HP is generated.

The intake stroke also pushes on the crank with this added pressure verses drawing in the air. With the added push from the pistons on the intake stroke the forced induction engines produce more torque and rev up a lot quicker. I know I had a 4-71 blower on a stock 350 SBC. Man would that mild engine come to life with the blower on it.

Just thought I'd pass this info along.
Cheers,
Greg Haisley

Offline lohring

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Re: Superchargers on models....can they work?
« Reply #25 on: April 15, 2016, 08:46:38 PM »
Some info many may not know.

For every lb. of boost 7% more HP is generated.

The intake stroke also pushes on the crank with this added pressure verses drawing in the air. With the added push from the pistons on the intake stroke the forced induction engines produce more torque and rev up a lot quicker. I know I had a 4-71 blower on a stock 350 SBC. Man would that mild engine come to life with the blower on it.

Just thought I'd pass this info along.

Roots style blowers don't have any internal compression so the temperature of the intake charge as well as the efficiency deteriorates quickly with increasing boost pressures.  A Lyshom blower is much more efficient at higher pressures and is what some manufacturers use on supercharged engines these days.  That style compressor is also what rotary air compressors like those from Sullair and Ingersoll Rand use.  See http://www.lysholm.us/superchargers.php

Again this only applies to four stroke engines where the boost pressure is applied to the piston for a full down stroke.  It won't help a two stroke where the piston is near BDC for most of the intake cycle. 

From Ricardo's 1945 high power tests on an aircraft two stroke engine:
BMEP 204 psi, rpm 2750, intake 11 psi, exhaust 2 psi, for a difference (supercharge) of 9 psi; 100 octane fuel; all pressures gauge pressures
BMEP 302 psi, rpm 3500, intake 33.5 psi, exhaust 11 psi,  22.5 psi difference; the maximum power from 100 octane fuel
BMEP 321 psi, rpm 3500, intake 36 psi, exhaust 12 psi, 24 psi difference;  100 octane with water injection

The power developed from these BMEPs would be twice that of a four cycle engine supercharged to the same BMEP (brake mean effective cylinder pressure).  The power from the exhaust pressure and flow was considered to be more than enough to drive a compressor developing the intake pressures.  At the time the gas turbine was a much more promising technology so the development of military high power piston engines stopped.  Fast forward 70 years to
to see what modern batteries and electronics can do with turbo/supercharging.

Lohring Miller

Offline jadge

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Re: Superchargers on models....can they work?
« Reply #26 on: April 21, 2016, 11:39:24 AM »
I've ordered my copy of the Ricardo book, I think it's the 1968 version. Rather than muck about online I emailed the CEO at Ricardo (we worked together some years ago) and he put me in contact with the information services manager. With his help I've ordered the book and marked it for collection when I visit. That's 5.95p saved that I can spend on something else - more tools!

Andrew

Offline Jo

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Re: Superchargers on models....can they work?
« Reply #27 on: April 21, 2016, 11:57:24 AM »
That's 5.95p saved that I can spend on something else - more tools!

I love the male logic to spending money :ROFL:

Jo
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Offline Vixen

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Re: Superchargers on models....can they work?
« Reply #28 on: April 21, 2016, 11:59:08 AM »
Andrew,
My copy arrived by post yesterday.  Looks like it will be a long slow read. Ricardo makes a good point in the introduction; the commercial success of a new design depends on many factors including chance and circumstances. Therefore the best design does not always succeed.

Mike
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Offline Vixen

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Re: Superchargers on models....can they work?
« Reply #29 on: April 21, 2016, 12:00:51 PM »
Jo,
I suppose the female logic would be------more castings

Mike :LittleDevil:
It is the journey that matters, not the destination

Offline jadge

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Re: Superchargers on models....can they work?
« Reply #30 on: April 22, 2016, 09:59:35 AM »
Mike: I'd agree with Ricardo - certainly in electronics it is often not the best technical solution that is successful. The market speaks, but sadly the market is sometimes seduced by a good sales pitch.

Andrew

Offline jadge

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Re: Superchargers on models....can they work?
« Reply #31 on: April 22, 2016, 10:04:16 AM »
I love the male logic to spending money :ROFL:

Jo

Well I dunno, seems logical to me? On the other hand I am down to two aeroplanes at the moment, so instead of more tools may be I shoud buy another aeroplane.  :LickLips:

Andrew