Rod Length Ratio_____Effect of connecting rod length relative to stroke length

Discussion on Stirling or "hot air" engines (all types)
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samandress
Posts: 6
Joined: Wed Jan 27, 2016 5:18 am

Rod Length Ratio_____Effect of connecting rod length relative to stroke length

Post by samandress » Sun Apr 03, 2016 2:58 pm

The rod ratio is the relative lengths of the stroke vs the connecting rod between crank and piston.
i have never head any mention of this specific quantity within stirling or model engine discussions.
so i thought i would put forward my view and see what the group consensus was.

Image

There is no definitive rule for what the ratio should be, however as a guide i found this quote (referring to IC engines)
"The connecting rod is between roughly 1.4 and 2.2 times the stroke length" (1.6 is common for cars, often higher for racers)


There are various arguments for and against a qualitatively "long" or "short" rod,
people can google if they're interested in exploring the tradeoffs between the two.

But my conclusion is that a "long" stroke has the features suited to stirling engines
especially the small static ones that are the mainstay of this forum.
as shown in the example picture a long stroke has lesser lateral forces on the piston reducing friction and vibration, high priorities.
a longer rod also means that the pistons (inc displacer) will have greater dwell time at top and bottom which suits the stirlings operation,
and should increase efficiency, particularly at high rpm.

The downsides being that being long also makes it taller which can have practical disadvantages.
the increased dwell time also means increased acceleration going from top to bottom. which may limit rpm from higher pumping losses
it also means more care needed to keep the displacer light.

But the question remains... how does this information transfer into the stirling model world,
post what ratios you use in your models and if you have any experience in varying it.
is there an optimal or recommended rod ratio for a stirling?

Regards,
Sam

samandress
Posts: 6
Joined: Wed Jan 27, 2016 5:18 am

Re: Rod Length Ratio_____Effect of connecting rod length relative to stroke length

Post by samandress » Sun Apr 03, 2016 3:17 pm

DEFINITIONS (automotive)

Short Rod -- Min Rod/Stroke Ratio -- 1.60 Max Rod/Stroke Ratio -- 1.80

Long Rod -- Min Rod/Stroke Ratio -- 1.81 Max Rod/Stroke Ratio -- 2.00

Ian S C
Posts: 2221
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Rod Length Ratio_____Effect of connecting rod length relative to stroke length

Post by Ian S C » Sun Apr 03, 2016 10:54 pm

I don't know much about the ratios, I just make the con rod as long as I can fit into the design, and to limit length on displacers, the Scotch yoke is a good idea.
Ian S C

samandress
Posts: 6
Joined: Wed Jan 27, 2016 5:18 am

Re: Rod Length Ratio_____Effect of connecting rod length relative to stroke length

Post by samandress » Wed Apr 06, 2016 12:13 pm

yes i am thinking of trying a scotch yoke further down the line,
but given the normal crank set up is the standard it seems odd to have a glaring gap in community knowledge.
making the ratio excessively long doesn't really hurt most of the time,
but there are plenty of times you'd want to keep it minimal, such as pressurized designs, moving designs or simply wanting it not to be taller than it is wide!

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