Author Topic: Superchargers on models....can they work?  (Read 8532 times)

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

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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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Damned ijjit!

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

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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
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

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
It is the journey that matters, not the destination

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