I know that this has been around before but I always marvel at this novel approach to adjusting the compression. Eccentric crankshaft bushing that can rotated to adjust the compression.

This approach now has a new leash on life in modern automotive engines. I have seen the patent applications a few years ago, so something is brewing for sure.
What engine is your's?

I remember seeing a few engines with an eccentric bushing for the crankshaft so they could change the compression of the engine. It should work pretty good as it would eliminate the need for a variable compression head with its potential problems with the contra piston.

But for four stroke engines the eccentric bushing is a problem because of the camshaft gear setup, unless it used the rear camshaft arrangement. But maybe a belt drive cam setup would work Ok for this too though.

There were several makes of diesel that used this eccentric bushing approach to compression adjustment produced in the late 40s-the French Ouragon, the Norwegian David Andersen and the British Airstar. I suppose the manufacturers thought that by making compression adjustment similar to advancing the ignition timing on an ignition engine there would be greater market acceptance. ......and not forgetting the little British Telco CO2 motor-which alone among CO2 motors-used the same eccentric bush adjustment to alter the engine speed.
From a production standpoint, though it would seem to require much more critical machining of the crankcase than conventional adjustment via a screw in the head...........the degree of eccentricity, the fit of the bushing to the case.....

ChrisM
'ffkiwi'

Please forgive me, but I am curious as to how this eccentric crankshaft bushing works to adjust engine compression. I am a bit naive in regard to some of the technology of those days...

It's quite simple-there is no contra piston, merely a fixed head with a combustion chamber (or even a blind bored cylinder)-the crankshaft is carried in a bush which is eccentric-in other words-the OD and the ID of this bush are not on the same central axis-but offset. This bush can rotate within the crankcase............so if a lever is attached to the bush, the bush can be turned through perhaps a 45 degree arc of movement. This movement-because of the eccentricity-will cause the crankshaft axis to rise (or fall) as a consequence.....since the piston and rod are also attached, it follows that the piston will travel higher (or lower) in the cylinder as well-since the head is fixed, the piston will come closer or further from the head depending on where the lever is set. Simple-we have a way of adjusting the compression ratio.....
The limits are set both by the orientation of the bush within the crankcase-and also by the limits to the lever movement-in some engines it operated in a slot in the casting itself-so the length of the slot limited how much lever movement there was, or in a cutout sector of the crankcase nose-again the length of the arc cutout limited lever movement.

I'll try and put together a diagram and post it later............

ChrisM
'ffkiwi'

Edit: diagram added

Interesting but it precludes the use of any form of races on the crank, unless you really want an engineering nightmare to content with and any form of front induction.

And how does varying the centres interact with liners port timings and that litte carbon ridge that usually forms at TDC?

Does the comp setting drift at all due to the shaft wanting to turn the bushing in the same direction as its rotating plus the hammering it gets along the way?
Cause thats exactly how an impact driver works when freeing up frozen bolts and screws.

Ah so many questions!

Pe'. it is an American Thermite, here's a link.

http://modelenginenews.org/ad/thermite.html

Pe'. it is an American Thermite, here's a link.

http://modelenginenews.org/ad/thermite.html

.....and how many ball raced sideports do you know of?

ChrisM
'ffkiwi'

Probably almost every two stroke lawnmower, weed whacker, and a great number of motorcycles - but sadly no diesels there in that lot.

You have to go marine to do that and if you give me some time I am sure thatI can oblige.

Interesting idea... I definitely can see the machining nightmare involved.

Ok, seriously - as in model diesels?

The only one that springs to mind is Dave Owens SP 2.8.
http://modelenginenews.org/ed.2008.07.html

Racking the memory banks I seem to recall a homebuilt diesel from AM in the early 50s-was it a Trevithick?-that used this form of compression adjustment as well. The effect on cylinder port timing is unavoidable with this approach-but may not be too significant (it will obviously depend on HOW much vertical piston movement the full adjustment movement of the arm generates. Thinking about it another way-I don't think-among my hundreds of diesels-that I have ANY where there is more than 1-1/2 turns of the compression screw needed for adjustment for the smallest and largest running props-including starting. Often its more 1/2-2/3 of a turn. A number of engines use the 1/4-32 thread for the comp screw (Oliver Tigers for one), while quite a few are 5x0.8mm-1-1/2 turns on either corresponds to about 0.047" of vertical movement of the contra-so my earlier guess of 0.05" of total movement from the eccentric bearing setup would appear to be bang on.........

The machining challenge with this eccentric bush is getting the bush-to-case fit bang on-tight enough not to leak primary compression-but not so tight that adjustment via the lever is impossible........the shaft fits will be the same regardless of whether the bearing bush is fixed or moveable-so they won't alter.

While we're on the subject-not too many of you will have seen one of these [see pic]-a John 0.35cc-there is no movable contra in this one either-compression adjustment is achieved by screwing the cylinder up or down in the crankcase (locking being achieved by the knurled ring)...........again, in theory this too will alter the cylinder port timing..........in practice the engine doesn't seem to care...........!

ChrisM
'ffkiwi'

Well there's also the Jena 1cc....utterly conventional.......except for the intake. The point I was making that AFAIK-there were no sideport ballraced engines of any persuasion during the sideport era of say 1930-1950-the era in which these eccentric bearing diesels were developed and marketed-and light weight was actively sought after, so ball races were not even a consideration-on weight grounds alone, let alone costs. The few ball raced sideports ever produced (Taplin twin, and the Jena) date from the mid fifties and early sixties respectively while Dave's effort is only a decade ago .......

ChrisM
'ffkiwi'

and here we are chilluns, -from the depths of my photo archives, the Arne Hende replica of the David Andersen D-A M1 2.5cc of 1950.......

ChrisM
'ffkiwi'

Here are a few more. The repro engine isn't mine, but the others are and they all run well.

Thanks a lot Hobbsy,

It was with great interest that I read the article in the link.
When young, the only diesel engines available were European made, and we all thought that America made glow engines only.
This article proves us wrong.

Does this setup bleed off compression or regulate the volume of the crankcase as a means of control?
I gotta love the simplicity of the older things.

Never mind I got it. Eccentric bushing

which is what this thread is all about.

contra piston in head or cylinder much easier way to to build an engine and of course allow a front intake machined in crankshaft martin