How Should Pistons Be Timed in Differently Angled Cylinders?

[Chantry09]

Im looking at designs for a engine, but I am giving myself a headache over something which is probably incredibly obvious. Ill explain it as simply as I can in text and I've drawn a (terrible) paint image to help explain it a bit better.

There are two cylinders. One cylinder needs to have its piston position 90 degrees offset to the other cylinder. If the cylinders are mounted at 60 degrees to each other, does the crank offset for the second cylinder need to be 90 degrees offset or 30 degrees offset?

 

[xxChrisxx]

What do you mean by the following?

One cylinder needs to have the travel of the piston

 

[Chantry09]

Urgh I am sorry that doesn't make any sense. I've edited it now, I hope it makes more sense.

Its basically a 90 deg phase angle between the two pistons

 

[xxChrisxx]

Urgh I am sorry that doesn't make any sense. I've edited it now, I hope it makes more sense.

Its basically a 90 deg phase angle between the two pistons

You would need a 30 degree offset, to bring it up to the 90.

90 degree V6's have a split/offset crank jounals to allow for even firing, as the natural angle for a V6 offset is 120 deg with a 3 throw crank, and 60 degress with a 6 throw crank.

 

[Chantry09]

Ah see I would have been more inclined to say it still needed a 90degree offset because the position of the cylinder around the circumference doesn't seem to have an affect on the pistons phase.

 

[Chantry09]

Just to also run this past you, when it says a 90 degree offset, I guess that is the same as saying 1/4 offset?

 

[xxChrisxx]

I'd guess so.

 

[Chantry09]

Im really stuck here, I asked my friend and he said this:

...Since cylinder phasing takes place at the crank its not going to matter the position of your cylinders.....you always need a 90 deg. difference so your illustration is not correct. You only see Stirlings with parrallel cylinders with two crank pins 90 deg apart or at 90 degrees to one another with both rods connected on a common crank pin. If you do a 60 deg cyl. spread the rules don't change. If you moved the larger cylinder to 120 degrees from the smaller cyl. on the same radious and your crank connection remains (90 deg.in a corrected illustration). This would just change the position of the piston to higher up in the bore. The relation of the pistons in both bores stays the same, one leads/follows by 90 deg.

Can I get a second opinion on this to make sure I don't do it wrong.

 

[xxChrisxx]

Ok before we carry on, I need to know a lot more information.

What engine are you doing this for?
Can you give it's geometry (block offset, vee?, inline?, number of cylinders etc)
How are you altering the crank angle and do the conrods share a crank pin?, (eg, is it using 1 throw crankshaft or 2 throw?)
What do you mena by engine timing, do you mean then the piston reached TDC?

Also what is the goal of whatever it is you are doing? (even firing, odd firing etc etc?)

I think we're going wrong on nomenclature, as phase, block offset and crank offset are all different things.

 

[Chantry09]

Hi. Its a StirlingEngine. The offset between the crank pin and the centre axle is .625 inches (the travel of the piston is 1.25 inches)

Hope the exert below from a website can clarify:

A4. Power Piston

The expansion and contraction of the rubber can be transformed into power output and the function of the rubber is the same as a power piston. A linkage can be connected to the above-mentioned crank and then the movement of rubber expansion and contraction are transformed into the rotation of crank. The crank section connected to the displacer and that connected to the power piston need to have a fixed phase angle, generally 90 degrees (FIG. 8 and 9). The crank rotation produced by the rubber expansion can contract to provide the force needed by the displacer to move up and down, and extra force can be outputted. It is noted that the displacer does not move by itself and is driven by the crank, and the power source is the power piston.

Why the 90 degrees phase angle
As shown in FIG. 9, when the displacer moves to the top position, the bottom space for heating is the largest, and meanwhile the pressure generated is the largest. When the displacer moves to the bottom position, the top space to be cooled is the largest, and the pressure generated is the smallest. If connecting the crankshaft of the power piston to the farthest position of crank level position, the biggest twisting force can be generated, and meanwhile the crank section connected to the displacer and that connected to the power piston have an angle difference of 90 degrees, which is called the phase angle.

The above conditions are results of operation in static environments. Typically, the phase angle is set to be 90 degrees. When the rotating speed of engine, load, temperature, and gas used are different, the optimal phase angle may be different.

 

[xxChrisxx]

AHHHH I see, it's a stirling. I thought you were talking about something like a V2 or Vtwin where both cylinders are powered and they needed to be timed to be fire correctly.

Yeah I agree with what your friend says.