IC Engine Scaling and Design

[Kawka777]

Hi All,

There are a ton of you that build scale engines based on full size drawings of IC engine like the RR Merlin, Dusenberg Straight 8, Wasp Series Radials, etc.

I am wondering how you handle scaling full size plans down to your desired scale?

How on the world do you handle the carburetor if it is naturally aspirated? Surely scaling down everything will mess with your air/fuel ratio of the original design?

Do you maintain the same compression ratio or do you modify it with cylinder diameters/strokes?

I suppose valve design and timing would all be laid out in the drawings, but do you need to modify valve diameters to maintain proper airflow?

It’s incredibly impressive what you all do!!

-Mike

 

[GreenTwin]

I can't comment on IC engines, but for steam engines, I often upsize critical components, such as the crankshaft diameter, slightly.

Sometimes I beef up the flywheel rim and spokes too.

Slender flywheel rims and spokes can get too thin when you scale them down.

I have a fastener chart on the wall, and I select the closest fastener size that still has sufficient strength for the application.
Fasteners sometimes have to be increased in size slightly.

If gasket areas get too thin, sometimes I enlarge them slightly.

Crank throws and crank pins can scale down too small too, and sometimes I bump them up a size.

Otherwise I copy old steam engines exactly, with scaled ports, valves, eccentrics, timing, etc.
One has to be careful that the ports don't get so narrow that they cannot be machined.
For a small scaled steam engine, you may have to reqwork/enlarge the ports, and that changes the valve, eccentric, etc.

The IC guys will have to chime in on that topic.

.

 

[stevehuckss396]

Carburetor, depending on how small you go may just end up resembling the original but be a simple design. My V8 looks like a holley 4 barrel but is just a simple drum style RC Carburetor.

The compression ratio I brought down to 5.5:1. Don't need the high compression for a display and desk top runner. No sense adding all that load and wear to the crank shaft, rods, and pistons.

Valves can scaled down directly. Sometimes the diameter may have to be reduced if things get a little too close together. Sparkplug size and placement is a big factor.

 

[Kawka777]

I can't comment on IC engines, but for steam engines, I often upsize critical components, such as the crankshaft diameter, slightly.

Sometimes I beef up the flywheel rim and spokes too.

Slender flywheel rims and spokes can get too thin when you scale them down.

I have a fastener chart on the wall, and I select the closest fastener size that still has sufficient strength for the application.
Fasteners sometimes have to be increased in size slightly.

If gasket areas get too thin, sometimes I enlarge them slightly.

Crank throws and crank pins can scale down too small too, and sometimes I bump them up a size.

Otherwise I copy old steam engines exactly, with scaled ports, valves, eccentrics, timing, etc.
One has to be careful that the ports don't get so narrow that they cannot be machined.
For a small scaled steam engine, you may have to reqwork/enlarge the ports, and that changes the valve, eccentric, etc.

The IC guys will have to chime in on that topic.

.

I can't comment on IC engines, but for steam engines, I often upsize critical components, such as the crankshaft diameter, slightly.

Sometimes I beef up the flywheel rim and spokes too.

Slender flywheel rims and spokes can get too thin when you scale them down.

I have a fastener chart on the wall, and I select the closest fastener size that still has sufficient strength for the application.
Fasteners sometimes have to be increased in size slightly.

If gasket areas get too thin, sometimes I enlarge them slightly.

Crank throws and crank pins can scale down too small too, and sometimes I bump them up a size.

Otherwise I copy old steam engines exactly, with scaled ports, valves, eccentrics, timing, etc.
One has to be careful that the ports don't get so narrow that they cannot be machined.
For a small scaled steam engine, you may have to reqwork/enlarge the ports, and that changes the valve, eccentric, etc.

The IC guys will have to chime in on that topic.

.

Green Twin,
It sounds like the steams are a little simpler to convert, relatively speaking!! Any reason to upsize the crankshaft other than ease of machining??

At work, I get to help keep a 1916 William Todd 1000t Steam Slab Shear alive and reliable.

I made a 1/22 scale non-functioning scale model of it for an old engineer that is near retirement. I had to use #0 machine screws for the normal 1 1/4” bolts that are all over it, which still looked a little big because of the large heads. I can totally see how fasteners become an issue real fast.

I would imagine dealing with the porting of the throttle valve on that would be a nightmare and would definitely need to scale a lot of that up.

Definitely considered making a working model of that… maybe a future rainy day endeavor.

 

[Kawka777]

Stevehucksss396,
Awesome pics! Makes sense to keep the compression ratio low!

If you redesign the carb for a simpler version, is there a rule of thumb for combined piston displacement vs carb size?

 

[GreenTwin]

If the crankshaft gets too small, it will bend.

.

 

[TonyM]

Very often function has to take priority over scale, as do material choices. Often fasteners are an approximation unless you are machining every nut and bolt. Although a lot depends on the scale you are trying to achieve. The smaller the scale the more difficult the choices and compromises.

 

[stevehuckss396]

Stevehucksss396,
Awesome pics! Makes sense to keep the compression ratio low!

If you redesign the carb for a simpler version, is there a rule of thumb for combined piston displacement vs carb size?

No rule of thumb. I usually start out with a hole that I know is too small. The engine will start but struggle to gain RPM. Drill open a little more and it will come off idle but struggle to reach 2000RPM. Them keep doing that until it's what I need. I started out with a .093 hole and ended up at .130 or something like that.

 

[Lloyd-ss]

Take a look at post #2 from Ken I in this thread. A lot of good math about scaling.

https://www.homemodelenginemachinist.com/threads/reduce-engine-size.31971/#post-334859

 

[peterl95124]

Mike, this is a topic I really enjoy researching and thinking about

compression ratio should be in the 6~8 range for easy starting and running of a model on regular unleaded regardless of what the original had, modern autos running on regular unleaded have 9~10 but in a model there's nothing to be gained, we're not trying to dyno test our engines and get good milage! Similarly you don't scale oil pressure, but various modelers have their own favorite regulator settings, Jerry Howell uses 20 PSI, George Trimble 30 PSI, and Paul Knapp 40 PSI (I'm aiming for Paul Knapp's 40 PSI).

similarly valve timing should be for easy starting and running, we're not trying to get great MPG or optimize power and torque, all my engines use 130-deg three-arc cams (I'm guessing effectively 120-deg after tappet clearance) with zero overlap, and lift = 1/4 valve diameter.

my starting point for my Merlin and Duesenberg were technical drawings that I photo(Xerox) enlarged or reduced as necessary to match my desired cylinder bore (1" for the Merlin, and 15/16" for the Duesey) and took all measurements from them.

things like head bolts scale perfectly, the tension on them scales with the head area and the strength of the bolts scales with their cross section area. Many parts of the engine scale this way, though I generally leave my crankcase and oil pan with much thicker walls by not milling out so much on the inside.

the scaling laws indicate problems when there are weird exponents/powers in the equations, this comes out in flywheel inertia (smaller scale engines need larger than scale flywheels), springs (my valve springs use thinner wire and less turns than scale). Ken notes that torsional strength also doesn't scale (in the same way that flywheels don't scale), smaller engines need larger diameter shafts than scale. The full size Merlin suffers from not stiff enough cam shafts, so I should have enlarged mine for both reasons, but didn't and so far so good, keeping my fingers crossed, making new cam shafts is a LOT of work...

to get anywhere near the power-per-cubic-inch of the full size a model needs to be run at the scaling factor times the RPM, IE a quarter scale model should run at four times the RPM. this same scaling factor works for centrifical superchargers. but I never actually run my engines this fast.

Since we're not delivering power to a drive train I just scale the gears and don't worry about them, but it would be interesting to investigate the forces and such in the Merlin's supercharger gears and propellor reduction gears some day.

things that are influenced by Reynolds Number don't scale if you cross the laminar/turbulent boundary, but don't know that anyone has ever tracked a problem down to this in a model engine.

One issue that I don't have a clue about is cooling, how much coolant surface area is needed, how much coolant flow rate is needed. One thing I do know is that every working model Merlin is under cooled and over heats, mine included, Argh!

Spark ignition also doesn't scale, you still need a minimum spark energy to ignite the mixture regardless of its volume, so we mostly use full size coils hidden inside wood box display stand (I use moped coils, they're pretty small but still do the job).

then there are lots of rule-of-thumb that I build to (many gleaned from "Aircraft Engine Design", Liston, 1942), like piston-cylinder clearance per inch of bore, etc...

HTH,
Peter.

 

[GreenTwin]

Excellent and very useful/relevant information Peter.

Thanks very much for taking the time to post that info.

Pat J

.

 

[Lloyd-ss]

Peter, I second what Green Twin said!

 

[ajoeiam]

https://ia801506.us.archive.org/23/...5.205332/2015.205332.Aircraft-Engine_text.pdf

If one is interested . . .

 

[GreenTwin]

Excellent book !

Thanks much for that.

Pat J

.

 

[mrehmus]

It is very difficult to get an exactly scaled engine to run. A case in point is the famous Duesenberg car model that now resides in the Sherline Museum. The straight-eight engine was exactly scaled and would just pop but not run. After a talk with Jerry Kieffer, the engine was reworked and it did run. Here is the article from issue # 15 of Model Engine Builder magazine by Jerry Kieffer.

 

[Lloyd-ss]

It is very difficult to get an exactly scaled engine to run. A case in point is the famous Duesenberg car model that now resides in the Sherline Museum. The straight-eight engine was exactly scaled and would just pop but not run. After a talk with Jerry Kieffer, the engine was reworked and it did run. Here is the article from issue # 15 of Model Engine Builder magazine by Jerry Kieffer.

WOW! Great article. So much to learn!

 

[DKGrimm]

It is very difficult to get an exactly scaled engine to run. A case in point is the famous Duesenberg car model that now resides in the Sherline Museum. The straight-eight engine was exactly scaled and would just pop but not run. After a talk with Jerry Kieffer, the engine was reworked and it did run. Here is the article from issue # 15 of Model Engine Builder magazine by Jerry Kieffer.

I had missed this one for some reason. Thanks.

 

[peterl95124]

It is very difficult to get an exactly scaled engine to run. A case in point is the famous Duesenberg car model that now resides in the Sherline Museum. The straight-eight engine was exactly scaled and would just pop but not run. After a talk with Jerry Kieffer, the engine was reworked and it did run. Here is the article from issue # 15 of Model Engine Builder magazine by Jerry Kieffer.

Exhibit A for why all my engines are about 1" bore !!!


Also Jerry describes a cold-process for rings, same as what I use, and mine do not pass the light test, they have to be lapped in a dummy cylinder, because as machined they are perfectly circular but don't have uniform spring pressure against the cylinder wall after being taken off the machining mandrel, they always end up with light showing on both sides of the gap, usually extending almost half way around, so lots of wall pressure right at the gap which if installed in the real cylinder will score it, No Bueno, hence lap in dummy cylinder, with lots of removal, rotate, re-insert so the dummy cylinder does not get scored all in one place.

 

[mrehmus]

Exhibit A for why all my engines are about 1" bore !!!

Bore has a lot to do with it but one of the main problems with smaller-scale engines is the lack of rotational inertia. Heavy flywheels or the equivalent are important. And then there is the problem of the carburetor. I've yet to find any definitive design criteria (i.e. real equations) that lead us to the proper size of the carburetor for any given displacement of an engine.

 

[peterl95124]

Bore has a lot to do with it but one of the main problems with smaller-scale engines is the lack of rotational inertia. Heavy flywheels or the equivalent are important. And then there is the problem of the carburetor. I've yet to find any definitive design criteria (i.e. real equations) that lead us to the proper size of the carburetor for any given displacement of an engine.

Mike, it surprises me that carburetors work at all ! [1], when I was sizing a model airplane engine carb for the Merlin I looked into carbs for 60 sized engines (each Merlin cylinder was pretty close, about 77 sized), but I ran into the fact that 60 sized model airplane engine carbs varied in bore from 7.5mm to 9.5mm or more, probably the larger ones were from Schnuerle ported engines.

so my scientific-wild-ass-guess for a rule of thumb for carbs would be 1/3 of the cylinder bore as a good starting point. its based on bore rather than displacement because smaller engines run higher RPMs and larger engines run slower RPMs, but they all hit peak power at about the same max piston linear speed, so air speed through the carb would be the same across different engine sizes if it were scaled with bore rather than displacement. with the caveat that just like Schnuerle ported engines need larger carbs, so to do four-valves-per-cylinder engines.


anyway, this is all just midnight musing, don't take it too seriously !


[1. bernoulli/venturi effect draws fuel out through the spray bar in proportion to the air speed, so the mixture stays nearly constant because the venturi size is fixed. but the air velocity needs to be high enough to atomize the fuel <--- that I think is the governing factor in a carburetor rule-of-thumb if there is one.]