Just for fun. Ground up engine build.
Just for fun. Ground up engine build.
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?