90 deg-- 180 deg crank

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racerfrank
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90 deg-- 180 deg crank

Post by racerfrank » Wed Jan 27, 2016 8:22 pm

I have read somewhere that crank pin journals set at 90 deg apart ensures self starting and reversing and that setting them at 180 deg makes for a smoother running engine at higher speeds, but may not self start.

Has there ever been a twin cylinder built with a crankshaft arrangement other than described above? Pros--Cons?
When I assemble my crank if I miss 90 deg by say +or- 5 deg is this going to be a problem?

Just some thoughts running through my head today.

Frank
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Lopez Mike
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Re: 90 deg-- 180 deg crank

Post by Lopez Mike » Wed Jan 27, 2016 9:01 pm

In no particular order:

Being off by a few degrees isn't an issue as long as the valve timing on that cylinder is in the correct relation to that cylinder. That said, it isn't that hard to get very close to 90 or 180 degrees.

There are many compromises in getting the balance "better". I say better because all engine with a very few exceptions have basic theoretical and practical issues.

Certain layouts such as an opposed twin or a 90 degree twin can be set up to be fairly nicely. The rest are a compromise between shaking in various directions.

A 180 degree parallel twin balances out most of the vertical masses but has significant remaining side to side motion due to the weight of the lower half of the con rod. This mass can be compensated to some degree with balance weight on the crank but any balance weight rotating at the same speed as the crank will simply convert unbalance in one direction (say, up and down) to unbalance in another direction (side to side). The 180 twin has a residual rocking action fore and aft. Also, most people find the constant kicking of the flywheel to get it to start and reverse to be a pain in the hind end. You might as well have a single in that matter.

Most. if not all, twins are 90 degree for the self starting and reversing advantage. Not hitting the dock trumps any vibration issue. They have smoother torque related vibration due to having four power strokes per revolution rather than two as with the 180 engine. As to balance, most engines add crank balance equal to around 50 to 70 percent of the reciprocating masses. That gives a decent ratio between jumping up and down and leaping sideways. In practice, most us spend little time at the top limit of r.p.m. so some thrashing about isn't a great big issue. Stout motor mountings to solid stringers with the loads spread over a decent area of hull does wonders for smooth running. My boat is a serious offender in almost all areas. Junk stringers and a lightly built junk plywood hull. And still I'm only irritated by engine motion and vibration when I'm showing off and racing.

There are sophisticated solutions involving balance shafts running at multiples of engine speed which are in use by some motorcycles. The riders, of course, are hanging on to the engine through the handlebars and footpegs and vibration is a serious issue often dealt with by a fair bit of trial and error at the design and prototype stage. Even heavy weights at the end of the handle bars.

There are other great solutions if you are completely deranged. Six cylinder inline engines are very smooth as are various arrangements of twelve cylinders. Off to Bedlam We go!
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barts
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Re: 90 deg-- 180 deg crank

Post by barts » Wed Jan 27, 2016 9:18 pm

racerfrank wrote:I have read somewhere that crank pin journals set at 90 deg apart ensures self starting and reversing and that setting them at 180 deg makes for a smoother running engine at higher speeds, but may not self start.

Has there ever been a twin cylinder built with a crankshaft arrangement other than described above? Pros--Cons?
When I assemble my crank if I miss 90 deg by say +or- 5 deg is this going to be a problem?

Just some thoughts running through my head today.

Frank
There's nothing that requires a particular phase relationship between cylinders. There are questions of balance in higher speed engines, but the engine will work in any case. Compounds can be at any desired angle; the receiver size may need to increase but the result is the same.

You will hear odd cylinder offsets, though; it's hard to miss a 60 V twin motorcycle, for example, and my Super Tenere has a 270 (firing) degree spacing on its cylinders.

If balance is wanted, a heavy weight moving 180 degrees out from the piston and then balancing the rotating parts completely work well; single cylinder engines (e.g. large Briggs & Stratton) are done this way. Dual balance shafts on either side of the crankshaft also work well....

- Bart
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Lopez Mike
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Re: 90 deg-- 180 deg crank

Post by Lopez Mike » Wed Jan 27, 2016 9:23 pm

The more I read the posts on this topic, the more I like my single.

If wanted a twenty-first century power plant I would have an electric boat powered by a fuel cell.

Of course my personal object of veneration is a properly maintained African Queen so you might keep that in mind when reading stuff I post.
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Re: 90 deg-- 180 deg crank

Post by barts » Wed Jan 27, 2016 9:36 pm

Lopez Mike wrote:The more I read the posts on this topic, the more I like my single.

If wanted a twenty-first century power plant I would have an electric boat powered by a fuel cell.

Of course my personal object of veneration is a properly maintained African Queen so you might keep that in mind when reading stuff I post.
Which is why I'm building a big single w/ perhaps 20G of piston acceleration. The balance weights will balance perhaps 50% of the piston weight; simple
and easy.

If I were worried about balance, dual counter-rotating weights taking half the piston forces between them and the other half via the crankshaft weights would work nicely... all sideway components are gone, and there's no rocking couple. The only weight left to balance is the con rod... but it doesn't look much like a steam engine :).

- Bart
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Re: 90 deg-- 180 deg crank

Post by DetroiTug » Thu Jan 28, 2016 1:51 am

Setting them at 180 would essentially be the same as a single. Lots of work to build two cylinders/valve linkages etc with no real advantage.

Setting them at 90 provides a huge advantage. The engine will self start and deliver smooth even power - same as a V8 4cycle. Reversing is 100% reliable.

Being off a few degrees isn't really going to change anything much, but that's pretty easy to avoid. A trick I use and you probably know it already. Clamp a V-block to the shaft and use it's sides to square off the table for 90 degree orientation in set up. Use a hard square off the table surface or a cross-test level.

For indexing the keyway in a bore: Locate the center of bore on X and Y in the mill, then move to key location, then plunge a 1/4" endmill (same size as key)straight down with the quill to make semi-circle (step in to it .005" per plunge to keyway depth). Then just locate the broach and bushing to that. Works really well and your broach will like it too. :) Trying to scribe layout for a broach and get it right is about impossible.

-Ron
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Re: 90 deg-- 180 deg crank

Post by Lopez Mike » Thu Jan 28, 2016 2:19 am

When I do a locomotive I use a broaching jig to make all of the driving wheels the same. Opposite the crank pin or near the crank pin. Then I use either an end mill or preferably the right size milling cutter to cut the keyways in the axles. I do the axles between centers with a dividing head. I have yet to blow one. When assembled in the frames with the side rods installed there is no binding.

Dirty secret:
In the case of a locomotive with suspension and going around corners, the next thing we do is open up all of the clearances and undo all of our wonderful precision work. A Locomotive left all tight will bind, jump the track, have poor adhesion and generally be a pill to operate.

Weird thing about steam locomotives. All over the world, the right side runs 90 degrees in advance of the left side. No exceptions as far as I know EXCEPT the Pennsylvania R.R. in the u.s.a. which was left ahead of right. I have no idea why. Historical stuff I suspect.

What I don't know and expect to learn on here shortly is whether there was any standard with stern wheel boats. Were they built with the right crank ahead of the left? Were there regional or national differences?
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Re: 90 deg-- 180 deg crank

Post by fredrosse » Thu Jan 28, 2016 4:13 am

Ah, the joy of a direct drive walking beam single engine. 62 RPM maximum speed, no counterweights, no vibration.
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Re: 90 deg-- 180 deg crank

Post by Lopez Mike » Thu Jan 28, 2016 4:16 am

And the sound is great. Like a draft horse pulling hard.
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Re: 90 deg-- 180 deg crank

Post by barts » Thu Jan 28, 2016 4:32 am

I'm finding the 5.75" x 5.75" engine large enough.... at 63 rpm, it would need to be 15" or so bore and stroke to make the same kind of power at the same BMEP. I think I'll stick to a prop :).

- Bart
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