[peppercorn (OP)]

Indeed! Spinney spinney is not always better than upy-downy!
Getting technical now!
I was also wondering if a barco type hammer could be made using a twin-pot diesel engine. Starting & stopping would be more tricky though!

[lyner]

I have a feeling that you would possibly need an FG flywheel, which might make it a bit of a lump to hold!

[peppercorn (OP)]

I have a feeling that you would possibly need an FG flywheel, which might make it a bit of a lump to hold!

FG is?

Providing the compression is high enough no flywheel should be necessary (remember it's a free-piston configuration). With two cylinders one piston would be on the compression stroke whilst the other was on exhaust. It would need some valve gear though.

[lyner]

This might work for one cylinder compressing whilst the other is expanding but I think, with the four stroke system, the first cylinder will be exhausting during the ignition stroke of the other.

[peppercorn (OP)]

I'd invisaged:

To start:
Cyl 1 & Cyl 2 -   Lift up, then close decompresser & let drop.
Cyl 1 -   Squash  Cyl 2 -   Blow
Cyl 1 -   Bang    Cyl 2 -   Suck   
Cyl 1 -   Blow    Cyl 2 -   Squash
Then (repeated):
Cyl 1 -   Suck    Cyl 2 -   Bang
Cyl 1 -   Squash  Cyl 2 -   Blow
Cyl 1 -   Bang    Cyl 2 -   Suck   
Cyl 1 -   Blow    Cyl 2 -   Squash

Open decompresser to stop.

[lyner]

I think I see your idea but don't you need bang and squash to coincide as that's the only way to get the compression: letting the engine do it for you?

[peppercorn (OP)]

Not if you use the tool's weight to compress the air/fuel - although I concede that the tool would be incredibly heavy or the cylinders being seriously undersquare (for higher pressure for equal force).

I see that one piston could drive the compression of the other, but that seems to defeat the object of the design.

[lyner]

The energy used for compression is tiny, compared with the energy from the ignition stroke. After all, mutticylinder, rotary engines do this, effectively.
I can't see a system being popular if the operator had to provide his own compression effort!

[lyner]

I've been thinking. Perhaps the existing ones must be two Stroke engines???
The link seems to suggest it is.

[peppercorn (OP)]

Yes. I'm sure they are (two stroke).
Obviously, it's hardly advanced technology even for its time!

I'm not suggesting the operator has to provide the compression each time just to start then the process then the weight of the hammer takes over.

[lyner]

I remember hearing that wonderful "Doompha Doompha" sound, as a lad. The operator was the coolest one in the team.
Having read more about it, I think you're prob right that two, four stroke halves would be more efficient than the single one. But I'm still not sure of the timing.

[peppercorn (OP)]

I don't know how 'cool' I'd feel operating one of those big ol' things - petrified more like!!  You'd certainly need balls-of-steel (plus toes of steel) to use one of those all day!

I think the timing would be relatively straight forward - just replace the cam shaft with a ratchet.

[lyner]

I was referring to the relative timing of the two cylinders!  []

[peppercorn (OP)]

I was referring to the relative timing of the two cylinders!  []

Is it not just: one goes bang whilst the other sucks clean air - i.e. 180° out of phase?

[lyner]

But, as I said, if you 're talking fourstroke, I think that the phase isn't right the other way round.

I guess it calls for a picture of the workings you have in mind.

[peppercorn (OP)]

Here's how I imagined it (whilst running):



I have illustrated for spark ignition, but Diesel will be the same cycle.
The cylinders are labelled 1 & 2 with each stage shown down the page.
The bang of one cylinder powers the hammer back up (the conn rod fixed to the hammer end) & draws clean air for the other cylinder. The weight over the cylinders works to compress the gas prior to ignition & exhaust the gas for the other cylinder.

[lyner]

Nice pictures. I see what happens.
You get four stroke efficiency and a bang every time.
There is the same trade off as with a big single pot 2stroke or a twin 4stroke motorcycle engine. It can't 'rev' at a different rate- it needs to dumpha dumpha at the same rate.
You will have one explosion each time from a half capacity cylinder instead of the less than full capacity two stroke power stroke.
There is the weight factor. The whole thing can't be much heavier. But the standard model is probably very heavy in any case, so probably not relevant.
What about the symmetry of the load on the machine? It's no longer coaxial as it has two parallel pistons. A lot of highly impulsive loads could be an embarassment to this slick new piece of kit.

[peppercorn (OP)]

Yes. There would be a tendency to rock the operator from side to side. Maybe it would need some kind of half weighted flywheel (although reciprocating not rotating) to cancel the lateral effect.
Ford used off-centre weighted flywheels on their V4 engines, I think.

Maybe this technology would be better suited to braking up material fed in to a stationary device. As I alluded to earlier, the design could partly pulp waste wood or other dry biomass in batches as a step in bio-fuel (or similar process) production.

[lyner]

IS it all appropriate for a device for pounding roads? It is getting a bit like fitting a gunsight to a clubhammer!

Bearing in mind that all the effective compactors use high frequency impulses nowadays, I think that the Jackhammer, charming as it is (and I'd love to play on one), is probably a dinosaur.

[peppercorn (OP)]

Maybe this technology would be better suited to braking up material fed in to a stationary device. As I alluded to earlier, the design could partly pulp waste wood or other dry biomass in batches as a step in bio-fuel (or similar process) production.

Beats using electric motor or 2-stroke driven machines...