4 cylinder opposed layout

[petertha]

I'm contemplating an opposed 4 cylinder for a future build. Screen grabs show some layouts from drawings I have of various displacements. Not that I'm attempting to model as a semi-scale at this point, but looking at the the FS Continental reference, it has what I would loosely call 'slightly' staggered cylinders. Maybe its more of a visual thing but I prefer this over the wider offset cylinder stagger.

The Jung (largest engine) has 2 connecting rods sharing the same CS throw. Whereas the Ohrndorf, Peewit & OS layouts all have dedicated CS throws. The Peewit has a bit of L on the small end of connecting rod because the rod is offset relative to piston axis.

I'm kind of favoring the shared throw layout. Naively, seems like 2 less journals to machine & get right LOL. From a model standpoint, are there any other factors to consider? Balance maybe? Would bottom end journal sizing be any different between shared or dedicated? Are there any other model engines you have come across I should look at?

[steamer]

Well   Horses for courses.    Either layout will make a running engine obviously.    What do you intend to do with the engine?     

Dave

[RReid]

Another option you might consider is shared journals with a "knife & fork" conrod arrangement. This would allow the opposing cylinders to be directly inline, no stagger.

[petertha]

Either layout will make a running engine obviously.    What do you intend to do with the engine?     
Dave

Well I'm a longtime RC guy but strangely I don't have as much affection to building & flying scale models vs other RC facets. Never say never, but I suspect it will power a test stand for fun & enjoyment, maybe the occasional show. At least that's the plan for my radial. So I do want it to run well with a reasonable prop, idle, transition & not have mechanical issues. This might be my crack at integrating spark ignition vs glow but probably unrelated to rod layout issue. I guess I was more wondering out loud why a particular model designer chose an arguably more complex 4 throw CS with wider cylinder offset vs a 2 throw with narrower stance, when they are all somewhat mimicking FS aircraft engines from the exterior look. Maybe because the FS did have a 4-throw CS & managed to squeeze the cylinders together whereas the smaller scales just don't shrink to scale the same way without other tradeoffs?

Hmm.. I just thought of something, optimal firing order vs throw position maybe?

Now that I think of it, I may have some drawings of Satra engine which looks pretty close to FS layout. I'll check out hw he went about it.

[crueby]

The offset ends on the Peewit are very interesting, I have never seen that before. No advice to offer, but I will be watching along, looks like it will be a fun journey.


 

[Vixen]

Hello petertha

There more to this than just the aesthetics of large or small amounts of cylinder stagger.

The four throw crankshaft gives the 'Boxer' configuration which allows the two opposed cylinders to fire simultaneously. Which, like a good strong coffee first thing in the morning, can reduce the shakes.

The cylinders of the two throw crankshaft MUST fire in sequence.

There are a number of cylinder firing orders possible with either crankshaft configuration, some have more balancing/ vibration issues than others. It would pay you to research these and then decide which to follow through for your design.

The OS FF-320 engine is interesting. The joint between the two crankshaft halves is another variation on the Schilling built up crankshaft technique. OS use two half cylinders (D shapes) to transmit the torque and the two bearings to achieve the alignment.

The flat four layout was always seen as the logical follow on to the the small five cylinder radial for light aircraft.

Good luck

Mike

[petertha]

Yes I'm going to focus in on the firing order side of things now. I suspect it was an important if not main driver behind the dedicated CS throw per rod, at least on opposed 4 layouts like this. Maybe it will reveal why the shared throw configuration is less common even if the CS appears a bit simpler. To be continued when I have something to show. The cylinder offset was more of a cosmetic thing. It just made me wonder why the FS Continental type seemed more closely aligned despite independent throw. But the devil is probably in the relative sizing details which may or may not scale down to a model, at least without other tradeoffs.

I believe the Satra opposed 4 also had a CS mid section split 'D' joint held captive with a bearing. When I find the sketch I'll post.

[Jasonb]

Malcom Stride did do a few articles in his IC Topics column just before the Bobcat design was published that discussed various crank and firing options with regards vibration etc.

You could also look at mason's Mastiff that has the central two cylinders sharing a single but longer crank pin. the end two cylinders have their own short crank pin, no plan but the section will give you the layout

[petertha]

Here is the Bruce Satra 0-440 4-cyl opposed CS. Turns out it is also a shared throw configuration (or whatever the proper term is). Also shows detail of the CS joint previously mentioned.

[Vixen]

Have you made a search for the Puma flat four, built by Double top, on this forum?

Mike

[petertha]

Thanks for link Mike, I will review. Here it is for others

https://www.modelenginemaker.com/index.php/topic,9156.0.html

[Vixen]

There is a lot of discussion about various firing orders. But beware... you also need to know how the cylinders were numbered. Some are numbered left to right. while others are numbered front to back. or vice-versa

Mike   

[crueby]

There is a lot of discussion about various firing orders. But beware... you also need to know how the cylinders were numbered. Some are numbered left to right. while others are numbered front to back. or vice-versa

Mike   

I ran into that on my old Mazda - the firing order was front to back in some of Mazda's documents, back to front in others!!   

[steam guy willy]

Hi ,Is this because of the Coriolis effect

Willy