Just for fun. Ground up engine build. 4

[RoarkS]

Howdy all,

I'm a mechanical that lives at a machine shop all day. If I really wanted to I could make literally anything in that shop.
I've set my sights on an engine. What better to say I'm a mechanical engineer than design and build a ground up engine... only problem... Engines have a lot of chemistry in them too.

So this is what I have my sights on... a 150-200 hp @ 2400 rpm 7 cylinder NA SI radial. Yes, I would like to some day put it on an airplane (so weight is of concern), but if it never gets that far, it's okay. I just need something to keep me busy for the next year or so as the lady and I save up for a house.

I dug up my Norton and Mott Machine design books, purchased Heywood and Taylor ICE books, and a VW engine shop maintenance book... even found a very cool radial engine balancing paper from the 30's that's apparently still gospel. That being said and reading all this theory has left me a little interested in real world problems:

1)From what I can tell octane rating is at best a rough comparative estimate (seasonal blends, atmospheric changes temp, dry bulb, wet bulb, humidity, dew point blah blah a million other variables it seems) as to what pressure will cause the fuel to detonate. Obviously since hydrocarbons only have so much energy, obviously the higher the chamber pressure before ignition the better. So... Is there a table somewhere that has a rough estimate of octane numbers to ignition pressures? Or... Since other engines that seem to work well for my application run an 8:1 compression ratio running 100LL (av gas) and 93 octane mogas... should I just leave it at that and hope it works?

1.2) Secondary to that... I'm having a hard time trying to figure out what Aluminum alloy aside from A242... which no one I know can get in billet let alone cast... I've been thinking about using by FEA skills to thin-wall some other material in hopes to keep heat rejection at acceptable levels, possibly get rid of multiple thermal expansion values... Steel would be ideal if I can get it to work, brass would look awesome... not so great endurance limit, great thermal props... but that's beside the point. How thin I can make it is up to max cylinder pressure (p3). Which I'm not sure how to nail down not having a better understanding of how to deal with octane ratings.

So I plugged the numbers for naturally aspirated, isooctane, at 8:1... I ended up with about 230 psi at P2, and P3 was right about 1030 psi. Does that sound safe? I'm still playing with safety factor, but I think I'm okay with a SF of 1.2x for a good bit of ground testing. Anyone at Lycoming care to speak up? lol.


2) Leaded fuel... aside from it's octane enhancing characteristics which hopefully will be contained to my first question... Okay so really all I can seem to find is that with no lead = harder valve seal material. Is that it? The go to reference is apparently from 1954 "Iron or steel similar to the valve". Older Radials I've personally made replacements for had bearing bronze guides and seats. I'm guessing I can still run 100LL (Low Lead) in an engine with these "harder" seats?

3: Head design. The engine is going to be a pushrod, 2 valve per cylinder. It's just lighter that way. I'm thinking a domed head. Parallel valves would be nice, but I'm using a cam ring so its not like I'm going to get to entirely mitigate valve guide side loads anyway. That's all possibly useless information, but it might mean something to someone smarter than I. That being said... What's the deal with dual ignition (two spark plugs)? When I do a magneto check when flying I loose 50 rpm killing one of the plugs. Is it simply allowing the combustion process to happen faster...thus I gain more RPM... or is there a P2 pressure increase I need to be aware of over my calculation in 1.2?

Thanks ahead of time! Sorry if some of the questions are simple... this isn't my area of training. Yet.

Bonus: After reading the other thread about forced induction basically eliminating concerns about manifold design... that's pretty tempting.. especially seeing as how I think Rotec (only current mfg's of radials) do it for that reason. I'm not wanting to turbo, or for that matter turbo-normalize my system, I would rather have any % reliability over performance, but lets say I ran a straight vane impeller directly of the crank inside of the induction housing (its round on a radial) how much boost are we talking to get rid of worrying about geometry?

 

[tbuelna]

Is a TV analysis necessary for a crankshaft that is one throw only?

A single throw crank for a 7 cylinder radial is still a rotating structural component subjected to input forces of varying magnitude and frequency. This crank has structural modes that can couple with input forces just like any other crank. Obviously a short single throw crank has higher torsional stiffness than a longer crank with multiple throws would have, but that does not mean there are not any TV issues with it.

@RoarkS- If you have good machine skills, I would recommend machining your A/C heads/cylinders from 2219 aluminum bar. 2219 has excellent high temp strength and is readily available.

 

[GregLocock]

I meant worth bothering with, not claiming it doesn't have TVs at all.





Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[kjoiner]

RoarkS,

Here is a good source of inspiration for you.

http://www.craftsmanshipmuseum.com/Tomlinson.htm

Kyle

 

[RoarkS]

Well, I have managed to collect a good bit of reference information on the subject. I'm thinking that the range of horsepower that I am wanting to play 150-200 may be in a range that would not justify much concern past a 1st order vibrations. Even found one source quoting that engines smaller than large 2800 cu double row engines, a good set of motor mounts should make most 2nd order vibrations "unobjectionable." I can't see myself using a constant speed prop just yet, So I think I will design around a fixed pitch wooden or composite prop so as to help keep down things wanting to fool with the TV of my crank. Besides a good CV prop is nearly $5k!

However. After going thru Hartog... I'm entertained at the complexity of the methods they went through to approximate the various mass properties required. I think this is where solidworks is going to shine.
I should be able to at the least use solidworks to investigate the true MOI properties of my designed system along with torsional stiffness of the crank... much like this fella:

http://thebloughs.net/understanding-crankshaft-counterweights/

but... I'm confident I can at least take a stab at some dynamic motion also, and hopefuly forego the separation of the rods into concentrated masses about the pins.

The only thing that I think may be stretching my ability would be input of firing pressures on the pistons. It appears that there are fairly well known models for cylinder pressure that are scale-able that would make good approximations. It would just be a matter of timing them in solidworks to see what I end up with. I know if I could do that, I could do a pretty good job of "emprically" changing the model to achieve the best "perfectly rigid" balance possible... It's after that, I'm not sure how powerful the simulation software is going to be dealing with a dynamic torsional analysis of the crank. I may have to do some learning there.

But even still... Looking at older (smaller) engines such as the Warner, Ranger, Wasp Jr., and Kinner I'm not yet convinced that it's an issue. Even the famed ABC Dragonfly was nearly 1400 cubic inches. (I'm thinking 500 cubic inches is going to be my upper limit.) This seems to be written by someone pretty smart... and those engines did not have any TV equipment as far as I know->

http://thebloughs.net/work/wp-conte...W.B.-Richards-The-Vintage-Airplane-8_1979.pdf

Ahh... even better As I've been in the blueprints of this thing... didn't know there were engineering reports I could get my hands on...->

http://thebloughs.net/work/wp-conte...W.B.-Richards-The-Vintage-Airplane-9_1979.pdf

I'm very interested in the Chilton style damper and definitely play with it. The mechanism appears to be fairly simple, but I will have to figure out if it's added complexity is of value.

More to come!

 

[Tmoose]

RE: ABC Dragonfly and crank torsional resonance -

FWIW one of the very first Google hits was this dedicated thread on one forum -

http://www.theaerodrome.com/forum/showthread.php?t=14432

Then, 5 hits down was this -

http://www.realclassic.co.uk/books/granvillebradshaw.html

" The worst problem was crankshaft resonance and failure, which in those days was not well understood or easily corrected, and was somewhat unexpected on a short, single row radial. "

 

[RoarkS]

EHudson... WOW! I just got a copy of "Aircraft Engine Design" by Liston... What a wealth of knowledge I'm just getting into it... but the information in the appendix alone! I can actually draw an SAE #10,20,30,40,50 prop hub! sill looking for a #1 flange but that may even be too old for this book. The material selection... AMAZING book so far.

My particular copy is a 1942 copy from the Chrysler Engineering library. Just check out how amazing the library tag is! (see attached)

Anyways...

A few things of interest I was reading about how the R2800 crank is assembled with a thru bolt and face splines. A few things... what is used to safety that bolt? I can't imagine just a lock nut. Then anyone have a good bit of knowledge of why I might go out of my way to make it an involute spline profile? With modern machinery wouldn't it be just as easy to do a filleted 45° design or perhaps just put dimples in both sides that would hold ball bearings?

Thanks!

Oh one last edit for the evening... So the mention of the ABC Dragonfly... It was nearly 1600 cubic inches but made just over 300 horsepower. My guess is the engine I'm wanting to make will have a much higher power density. It goes without saying that I will look at the various vibration sources throughout. but I think it would be a safe bet that TV is probably has a higher correlation to power than cubic inches than I had assumed in an earlier post.

 

[RoarkS]

Radials slave rods have fairly different crank pin paths... thus accelerations for each cylinder would be different...

Does the rate at which a fuel air charge is compressed have any bearing on its combustion?
The lady already turned out the light for the evening, so I guess I won't be reading any books about it until tomorrow.

For reference...
Here is an older set up I drew up a while ago... this is the one that sent me down the rabbit hole. It has an evenly spaced slave pins on the master rod at a constant radius about the main. It is far more along than my current set up, but I was just curious what I could do with it as a baseline until the other is ready.

I have it being driven at design max RPM. 5 and 7 fire in sequence so I was curious to look at them... 1.25X higher acceleration with a bit longer dwell time at TDC... scary. 650g's vs 520g's that's pretty snappy, but they are only going about 51 ft per second max that's not as fast as I thought it was going to be... I was half expecting supersonic velocities.

Look at that double peak in #5! I cant wait to see what happens when I add them all together to see what's happening. No wonder these things are not made anymore... but ya sure can't beat the sound.

I wonder how much fun could be had with a programmable ignition...

 

[MikeHalloran]

You want some fun, you might want to try building a
Marchetti (8 cylinder radial 4-stroke) engine or a
Fairchild-Caminez (4 cylinder radial 4-stroke) engine.

Both use conjugate cams and massive bellcranks instead of a crank and master rod, and have good primary balance because of symmetry.

I don't think a radial will benefit particularly from a programmable ignition, but they need a strong spark to fire the lower plugs, typically fouled by drained oil.




Mike Halloran
Pembroke Pines, FL, USA

 

[EHudson]

Those 'splined' crankshaft couplings might be just a bit more involved than you might guess.

Tonights homework is to check out curvic and Hirth couplings. The old American radials were curvic couplings.

 

[RoarkS]

Check this out!

http://www.geartechnology.com/issues/1186x/Back-to-Basics.pdf

geez... what hasn't been made in Rochester NY!?

It's all about knowing the name of what to look up! The Hirth couplings are much like the M14 rear sight adjustment knobs. The lead machinist and I were talking that maybe if we put a fillet on the form tool that we might like that idea the best. But... The curvic coupling looks pretty interesting. I will have to read all of the link to figure out what's so great about it. Thanks again!

Edits as I'm reading: Curvic claims that they can make interchangeable parts even on turbo-machinery! Again, the lead machinist fella and I were discussing if we would have to send the unit out to be precision ground once it's assembled... looks like maybe not.

 

[MikeHalloran]

What hasn't been made in Rochester for a while is money.
... at least not like in the days when Kodak was printing it.


Mike Halloran
Pembroke Pines, FL, USA

 

[RoarkS]

Lol! I worked with a bunch of RIT guys who were happy to get away. Never been there. Too cold. I wouldn't make money there either.