Designing an engine

[Fabrickator]

If you're dropping a steel cylinder into an aluminum block, you will need at least .004" interference fit. Aluminum expands at 7 times the rate of steel and anything less could become loose when/if the block heats up. You would heat the block slowly to 375* and then drop the cylinder in it.

 

[mu38&Bg#]

You'll want to double check your figures for thermal expansion coefficients. 6061 aluminum is about 23um/m-°C and 1018 steel is about 12um/m-°C. .004" press on a 1" bore is going to be a problem.

Model aircraft engines are built with steel liners in aluminum blocks with slip fits, but the liner has a flange which is clamped by the head. Do a search for press fits and work with that. Depending on how carefully you bored the block it may be tricky, tapers cause problems. Lapping would help fix problems. If you don't have a .0001" bore gage, you may want to make the liners to size then lap the bores to fit the liners. It will be hard to make liners to fit an unknown bore size when the fits need to be in the tenths range.

 

[the_one]

hey guys I was just wondering how to figure out the max PSI that would be in the combustion chamber

 

[barnesrickw]

That's a tough one. Compression ratio and thermal expansion of whatever fuel, and the air to fuel ratio may come into play.

 

[gbritnell]

the-one,
Why would you need to know this?
Gbritnell

 

[the_one]

so i could run a FEA

 

[Niels Abildgaard]

Ricardo say peak pressure in psi is roughly 100 times compression ratio for a spark ignited with satisfactory valves.

 

[the_one]

Thank you Niels

 

[gbritnell]

Pardon my ignorance but what is FEA?
gbritnell

 

[stevehuckss396]

Pardon my ignorance but what is FEA?
gbritnell

Hello George!

I think its part of Solidworks where stress on parts is checked to make sure there is not going to be any failures.

 

[Metal_slicer]

Hello Guys,

I have been away for a long while. I have been busy looking at techniques and learning as much as I can. I have been looking into cam cutting and I think I have figured out how to do this using Cogsy description from another thread. It took me a few tries in Viacad, but I think I got it. See attached image:

 

[Metal_slicer]

Ok, I don't have much to share but I have included some pictures of my practice valves as well as a trial piston which I may remake because i'm not sure if the second ring groove will interfere with the wrist pin.

I went to home depot today and bought a box of assorted springs for just over 4 bucks. I cut the springs to length and then ground the ends flat. I can always use the other springs for other projects or for the throttle linkage. I have also made a pair of stainless steel exhaust pipes to be soldered to a mounting bracket later.

 

[gbritnell]

I'm looking at the picture of the valve springs and at first glance they look waaaay too heavy. What is the diameter of the wire? You should be able to get away with a wire size of around .032-.036.
gbritnell

 

[Metal_slicer]

I'm looking at the picture of the valve springs and at first glance they look waaaay too heavy. What is the diameter of the wire? You should be able to get away with a wire size of around .032-.036.
gbritnell

Not sure. I will have to get the calipers on them but my workshop is in the barn way out there and I kicked off my shoes for the night. I wanted to use a spring strong enough to prevent valve float and to keep the valves closed nice and tight.

 

[Metal_slicer]

Well you got me curious so I had to go take a look. The springs in the picture measured .047" I have some .038" and .025" available. If I use the .047" springs, what would happen? To much resistance and wear?

 

[Longboy]

Your scale is close to what I build Metal. On my first gas scratch build, I first tried .031 springs and could feel the resistance on a hand turnover and engine would not run. Redid with .023 springs and my engine turned over easy. I don't believe you will have a chance with the "47's" on this displacement. You want enough spring to move the valves to a certain RPM without adding to the internal load of the sum of your engine's moving parts. Dave.

 

[Metal_slicer]

Hello Guys,

Ok I have 2 more sets of springs that I could use.

.023 and .019 thick springs.

Which would I have the best luck with?

 

[petertha]

I'm no expert & maybe others can chime in if I'm off base.The engine I'm building from plans has wire dia = 0.8mm = 0.031" which is similar spring values & length layout as a commercial RC engine (OS-FS-61) at 0.027" & Edwards 5-cyl radial at 0.030". I was interested in how much force this arrangement translated into lifting the valve off its seat, ultimately driven by the cam ring & pushrods through the rocker assembly. I think the general method goes something like this:

- go to calculator website like link below (looks a bit different than at the time I took the screen grabs, but appears to yield the same values).

- define a compression spring by: wire diameter + od + free length + number of coils. Of course it has to fit any dimensional constraints of your engine. Example spring OD sits semi submerged in a head hole or ID must clear valve cup stem diameter.

- the calculator spits out a tabular listing of force values for incremental compression lengths starting from zero force at uncompressed (free) length to max travel (solid) height. I plotted these for visual reference.

- using the graph you can eyeball the corresponding forces at valve closed & valve open spring compression lengths. In this 0.028" dia wire example it might be ~2 pounds whereas a similar dimensioned 0.035" dia wire might be ~ 4 pounds. Now what is low or high or desirable relative to our little engines I will leave to others to comment on, but I think the forces themselves are in the ballpark. Of course you can achieve similar target forces with different combinations of lengths, coils, wire type etc. all as dictated by your own engine layout.

http://www.planetspring.com/pages/compression-spring-calculator-coil-calculator.php?id=compression

 

[petertha]

Edwards & Ohrndorf radials, head/valve/spring examples

 

[Metal_slicer]

Hello Guys,

Started to machine the valve cover and nothing but disasters, well maybe more bad luck than disasters. I started to drill the bolt holes and noticed the drill bit screeching and then locked up. The chips got clogged. Restarted the CNC code and more problems with the drill bit clogging. Tried one more time then then the Z axis lead screw became loose so the bit stayed in the hole. I had to tear down the z part of the mill and fix the problem. When Finished I changed the drill bit to something sharper and then manually drilled the holes and was able to get through that operation. :wall:

The cheap china bits are ground horrible and the 6061 aluminum was sticking to the tip causing virtually no cutting action. A small bit at .112" needs to be sharp! I wasted so much time fixing the mill and setting up several times that
I got tired of it all and called it the day. Tomorrow I will be back at it again.