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Model Engines - Great new discussion topic

 
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lhodgson



Joined: 13 Jul 2003
Posts: 1
Location: Cincinnati Ohio 45242

PostPosted: Thu Feb 26, 2004 14:25    Post subject: Model Engines - Great new discussion topic Reply with quote

Welcome to all the AEHS members who have Big Iron engines, as well as those of us not lucky enough to have a R-2800 or Allison built to 12" = 1 foot scale.

For model builders, the challange is designing and build running model engines from 1/5 to 1/3 size. The challenges are many : Suitable design plans, finding suppliers of model spark plugs, ignition systems, various methods for making piston rings, tooling for making all the necessary fins, friendly material suppliers and the challenge of cam design.

There is a large and growing hobby of home machinist who build engines with everything from light equipment to sophisticated CNC equipment.

The innoviation and chaftsmanship is amazing.

The AEHS has graciosly allows us this forum to coordinate and exchange ideas from different builders the world over.

In the past many authors have published their experience in Strictly IC, and in Home Shop Machinst, Machinist Workshop and other model magazines and newsletters. Hopefully this design and fabrication experience can find a home in this forum.

Lets hear from everyone, post your questions and replys freely, we can all learn from each other, and make more, and better model engines.

This forum is for the beginner as well as the experienced model maker.
No question is to basic.

Looking foward to seeing fellow engine makers at the Convention in July.


Lee Hodgson
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gravery



Joined: 15 Oct 2004
Posts: 11
Location: Le Chesnay, France

PostPosted: Fri Oct 15, 2004 14:58    Post subject: Reply with quote

Hello,

I am a newcomer and apologize for my english which is not my mother tongue. I am a fan of various mechanical achievements, from the 'Big Boy' steam locomotives to the last piston aero engine (the reason I joined the AEHS).
I read with a great interest, to say the least, the construction of the Rolls Royce 'Eagle 22' of Mr Hares. I understood the cooling system (water ways, hoses and rad) if reduced to the same 1/5 scale, can't cope with the heat dissipation needs of the engine (or did I misunderstood?). Could someone explain me why?
I don't know the RPM speed at which the original engine produced its peak power (3200 bhp) but is there a way to know the rpm/power developed by the model? Thank you very much in advance.

Guillaume Ravery
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rhavemann



Joined: 13 Jul 2003
Posts: 9
Location: carefree AZ.

PostPosted: Sat Oct 16, 2004 11:26    Post subject: cooling of minature engines Reply with quote

Hello Guillaume Good to read your question, my comment would be ..
The smaller the size of the engine, the larger the surface to volume
Ratio becomes..Volume being the total cylinder displacement ,surface being the area exposed to radiate combustion and frictional heat.
So I guess little engines are easier to cool than big ones. Laughing
Can any one out there finish this off??
_________________
Bob Havemann, The Wrenchmann
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ctomlinson



Joined: 07 Dec 2003
Posts: 2

PostPosted: Tue Oct 19, 2004 09:41    Post subject: Reply with quote

gravery wrote:
Hello,

I am a newcomer and apologize for my english which is not my mother tongue. I am a fan of various mechanical achievements, from the 'Big Boy' steam locomotives to the last piston aero engine (the reason I joined the AEHS).
I read with a great interest, to say the least, the construction of the Rolls Royce 'Eagle 22' of Mr Hares. I understood the cooling system (water ways, hoses and rad) if reduced to the same 1/5 scale, can't cope with the heat dissipation needs of the engine (or did I misunderstood?). Could someone explain me why?
I don't know the RPM speed at which the original engine produced its peak power (3200 bhp) but is there a way to know the rpm/power developed by the model? Thank you very much in advance.

Guillaume Ravery
Quote:


Hi this is my first attempt to use this forum. I am going to attempt to past in a previously written word doc. If it succedes that will be a milestone. I suspect it will not manage to reproduce all the superscript and \\\\\greek text. If thats the case I`ll have another go.



I will try to keep this short and simple! The following comments are general but aimed at my Napier Deltic engine (not an aero engine) and Barry Hares Rolls Royce Eagle 22. The problem with building working small scale models of IC engines is all to do with maths and physics of scale; thermodynamics.
Generally, when refering to scale we talk of the ratio of linear dimensions, i.e for my 1/8th scale Deltic I reduced all the linear dimensions by a factor of 8. However we normally measure the size if IC engines by their capacity. The reduction in capacity of a model is the cube of the scale factor. My Deltic model is 1/8th scale and therefor has a capacityof 1/512nd (8 cubed). The prototype is 88ltrs, the model approx 160cc.
It is reasonable to expect, all things being equal (which they definitely are not) the potential power from the model to be proportional to the capacity. The single factor that governs the power output from any IC engine, whatever it`s size is it`s ability to consume (pump) or to be force fed air. Unfortunatly for us model makers one cannot scale air; air is air! It`s molecular structure/pressure/density ratios are fixed, we cannot make it "thinner"
For my deltic engine all the inlet and exhaust port dimensions are accuratly scaled from the prototype which runs between 1.5k and 2k rpm. I suspect the same goes for Barry`s Eagle. Our little model engines have the potential, maybe, to turn over the scale factor faster due to the reduced reciprocating masses, this would require much more radical timings, i.e lead/lag/overlap. The prototype Deltic produces approx 18bhp/ltr at approx 1.6k rpm. Mechanically the model should be capable of 100bhp/ltr at around 15k rpm (16bhp). However there is little prospect of that.
There are similar problems related to cooling; firstly scaling and secondly, like air one cannot scale water, water is water! As the volume of a cylinder is reduced the surface area of that volume reduces by a lesser amount. Therefor the heat transfer to the engine structure is greater. At first sight this would indicate that it would be easier to cool the model engine but that will only be the case if you can supply adequate volumes of the more viscous(in scale terms) water.
To illustrate this problem we can look at the ratio of volume to surface area fo a series of "combustion chambers". To make it a little simpler we can make those combustion chambers spherical and work in inches. The engines in question were built to imperial dimensions and I guess most of the readers are American.
The following table relates to spherical combustion chamber diameters that approximate to the prototypes and models in question and starts with a reference one of 6inches which happens to have the ratio of 1 : 1.

Diameter Surface area Volume Ratio
inches Square ins Cubic ins

Reference 6.0 113.1 113.1 1 : 1

Deltic/Eagle 5.0 78.54 65.45 1.2 : 1

Eagle model 1.25 4.91 1.023 4.8 : 1

Deltic model 0.625 1.227 0.128 9.6 : 1

You have probably noticed that the ratio increases by the csale factor.
In the case of the deltic model I have increased the longitudinal pitch of the cylinder centres to allow a greater volume and easier flow of water around each liner. I have also provided an individual cooling circuit for each cylinder arranged as three parallel sets of six manifolds with provision to force feed the water pump at 4bar to help stop low pressure flashing of at the liner centres.
I do not know what, if anything Barry has done to the cooling system of his Eagle. I hope he will correct my assumptions and give us some more details.
These small model engines do not often have to work hard, although often called upon to give demonstrations, seldom run for more than a minute.

`til next time, best regards Clen.


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gravery



Joined: 15 Oct 2004
Posts: 11
Location: Le Chesnay, France

PostPosted: Tue Oct 19, 2004 10:52    Post subject: Reply with quote

Clen, Bob,

Thanks a lot, I am interested to know what are the current limitations on Mr Hares's 'Eagle'. Are they the same than on your Napier engine (I guess yes...)
Thank you anyway
Best regards.

Guillaume

PS : is your Napier model a two stroke diesel as the original?
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ctomlinson



Joined: 07 Dec 2003
Posts: 2

PostPosted: Wed Oct 20, 2004 17:27    Post subject: Reply with quote

Guilluame,
I believe the potential problems with the Eagle 22 of barry Hares are less severe than mine as it is to a larger scale.
My Deltic model is an opposed piston 2 stroke as the prototype but it is not a diesel. It is virtually impossible to get an engine of this scale to run as a compression ignition engine. It is the old problem of surface area to volume ratio. The heat losses are so great that the compressed charge is not hot enough.
My engine is spark ignition and I intend to fuel it with Propane gas. That way I stand some chance of getting a combustable mixture where the spark plugs are. If this engine runs and gets anywhere near it`s potential it will need more than 30k sparks a minute.
If anyone is interested I can elaborate on the problems of fueling and igniting this engine.

I notice that the table in my previous message does not appear in the form it "left here" The headings: Diameter in inches, Surface area in Square inches, Volume in Cubic inches and Ratio should be set out above the 4 columns of numbers.

Clen.
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