Bore x Stroke

[Big Dave]

Would someone care to explain the differences in characteristics between a square bore and a long stroke motor?

And why it has an effect?

[FJRider]

The square bore (short stroke...bore width is the same as bore stroke) means(generally) more firing strokes per revolution(faster reving multi cylinders) as in contrast to the long stroke (usually [V] TWINS) which fire less often (slow reving)
I think thats how it goes...
The firing pattern on the twins is a whole different kettle of fish...Ask a Ducati-ite...

[popelli]

large bore = large valves = ability to move more air and make more power

shorter stroke means lower piston speed = ability to rev higher = more horsepower

longer stroke = greater leevrage on crank = more torque

[Owl]

Wouldn't a square bore be considered a long stroke motor in this day and age?

[JMemonic]

Popelli has it basically right

Long stroke, or under square is generally (in automotive terms) an older design now and rarely used, it will be low revving with high torque, one of it disadvantages is the need for a heavier flywheel to carry it through its cycle. In this engine the diameter of the bore of the cylinder is less than the distance the piston travels through a cycle. I am not sure if I remember right but this design could possibly hark back to steam.

Square bore. The diameter of the bore of the cylinder is equal to the distance the piston travels through a cycle. Revs higher than a long stoke, and has a reduced flywheel requirement. Still a lot of torque at lower revs.

Short stroke or over square, the diameter of the bore of the cylinder is greater than the distance the piston travels through a cycle. These engines will rev and fast, quick to come up to speed due to having basically no flywheel requirement due to their higher speeds, think Ferrari vs a Massey Ferguson both are good examples of the extremes of these engines.

Modern motorcycles have been square or under for ever, well most brands some have until recently been stuck in their agricultural heritage.

[JMemonic]

Wouldn't a square bore be considered a long stroke motor in this day and age?

No it does not have to have a long stroke to be square, small piston small stroke square motor, none the less thinking .

[davejenknz]

Popelli has it basically right

Long stroke, or over square is generally (in automotive terms) an older design now and rarely used, it will be low revving with high torque, one of it disadvantages is the need for a heavier flywheel to carry it through its cycle. In this engine the diameter of the bore of the cylinder is less than the distance the piston travels through a cycle. I am not sure if I remember right but this design could possibly hark back to steam.

Square bore. The diameter of the bore of the cylinder is equal to the distance the piston travels through a cycle. Revs higher than a long stoke, and has a reduced flywheel requirement. Still a lot of torque at lower revs.

Short stroke or under square, the diameter of the bore of the cylinder is greater than the distance the piston travels through a cycle. These engines will rev and fast, quick to come up to speed due to having basically no flywheel requirement due to their higher speeds, think Ferrari vs a Massey Ferguson both are good examples of the extremes of these engines.

Modern motorcycles have been square or under for ever, well most brands some have until recently been stuck in their agricultural heritage.

Think you've got your squares mixed up.

Short Stoke = over square
Long Stoke = under square

http://en.wikipedia.org/wiki/Stroke_ratio

[sels1]

One wonders why BD is up at 1:30am pondering these things!
But thanks guys, I have added a bit to my knowledge now too

[merv]

The square bore (short stroke...bore width is the same as bore stroke) means(generally) more firing strokes per revolution(faster reving multi cylinders) as in contrast to the long stroke (usually [V] TWINS) which fire less often (slow reving)
I think thats how it goes...

Hey best you modify your thinking slightly because the number of firing strokes per revolution is exactly the same - either its a two stroke and fires every revolution or its a four stroke and fires every second revolution - that's in each cylinder. Now sure, add more cylinders (I think that's what you were trying to say) and you get more firing strokes per revolution.

However, that's not the point of Dave's question re square versus long stroke as some older four cylinder bikes weren't as short stroke as they are these days.

Much as has been said above, traditionally long stroke motors had good torque at low revs due to the longer crank throw and greater lever arm, but couldn't rev hard because of piston speed limitations. Square or short stroke motors in comparison had less bottom end torque but could rev harder and make more power.

These days the fantastic world of electronics and some design changes have improved a lot of this as I have raved on many a time about the bottom end torque of my WR250F. Its is a very short stroke engine but pulls like the bejeesus at low revs due to having a very short piston which allows for a very short con rod which changes the angle of the dangle to the crank and then the electronics, using throttle position sensors and the like, takes care of the rest. The modern 4 cylinder bikes are similarly as short in the stroke with all the whizz bang stuff including fuel injection.

This has opened up a whole new world in terms of engine wear and need for maintenance which is one of the trade offs but to a degree is being overcome by better materials and oils with less friction.

So I'm saying these days the engine characteristics are altered greatly over previous experiences but generally what still applies is the long stroke motor can't rev as hard and make as much power but the old adage that short stroke engines couldn't pull at low revs has somewhat gone.

[avgas]

Yep what others have said has been my understanding.
Bore = capacity = big bang = power
Stroke = waving either the short or long end of the stick (conrod) = engine momentum = torque.

Easiest way i thought of it was if you wanted to throw a bucket and the bucket got bigger everytime - you get slower and require more effort in each throw (bore)

If you sweep the floor, and shorten you stroke (cut broom handle in half) then you can do more strokes for less effort over a shorter period of time.

The fun starts when you have big bore, short stroke V8's....... good for drifting as the rpm curve states that hp kicks in nice and low, where the engine accelerates quickly

[Ixion]

It gets more complicated on proper bikes what don't have all those cams and valves and other unnecessary stuff.

Because the valves are holes in the cylinder walls, the length of cylinder wall available acts as a control on the port timing.

[Big Dave]

One wonders why BD is up at 1:30am pondering these things!
But thanks guys, I have added a bit to my knowledge now too

Nights are for work - days are for play.

[Blackbird]

Just to add to the erudite comments, "longer stroke" motors have higher piston speeds and put more stress on componentry like con rods and pistons. Most of this stress occurs as the piston/rod decelerates and accelerates around top and bottom dead centre. The limiting mean piston speed from memory in the days when the British twins reigned supreme was about 4000 ft/min. This will have improved a little with forged components, better alloys etc, but not hugely so; hence the move to shorter strokes to reduce piston speed whilst increasing power.

A really good example was when I drag raced a supercharged 350cc Triumph twin. The supercharger allowed it to breathe more freely so it would rev more easily. The higher revs added significant extra stress and I was forever having the piston crowns detach at the oil control ring. I'd reached the strength limits of the standard stroke motor, so I machined a short stroke crank and married it to a modified 500cc (bigger bore) barrel to give a very over-square 350. This dropped the piston speed but I also lost torque - the essential ingredient of drag racing. Months followed of testing different flywheel weights, cam timing, boost and upping the percentage of nitro in the methanol until it was competitive again.

So there you are.... a practical demo of engine design 101

[cooneyr]

......flywheel weight.....

Thesis time . . .

While fly wheel weight is a factor in this - more relevant is the length of the stoke and what that does re accelerations of the piston. If the stoke is long (as a result of a large big end crank offset) the larger the velocity of the piston at mid stroke compared to a short stroke engine (for the same given rpm). Now as rpm is involves a unit of time and velocity over time is acceleration, as the velocities increase (i.e. long stroke) the accelerations increase.

Imagine this - at TDC or BDC the piston is stationary and then has max velocity at mid stroke hence it must accelerate from stationary to max velocity to stationary for one stroke. As mentioned above the higher the max velocity the higher the accelerations.

We all know that to accelerate fast takes more "effort" than accelerating slowly. This "effort" is lost power in an engine. Hence a short stroke engine will have more power than a long stroke engine all other things being equal. For the same reason a short stroke engine will increase in revs much faster and can pull higher revs (as the mid stroke velocity is less at certain rpm than a long strong motor). As I understand it the max piston velocity is one of the parameters that influences red line rpm (along with valve characteristics etc etc).

As the other fullas have mentioned that long stroke = more torque. This is because torque = force (fuel explosion) x distance (crank to big end off set). Unfortunately due to high piston velocity this also means less rpm (or less usable rpm as the effort to accelerate the piston becomes to significant at higher rpm).

It is also worth noting that power = torque x rpm.

Traditionally long stroke = torque but not very high rpm hence using the above relationship not much power, short stroke = high rpm and lower torque but lots of power and square = is a compromise between the two. The net result (think of formula 1 cars here or a 250 4 cylinder pulling 17000 rpm) is the more rpm the engine will pull the more power, however we must keep piston velocities down to achieve more rpm hence we must have short stroke.

Modern short stroke engines somehow managed to develop reasonable torque but I dont understand this much. It might be more to do with more efficient fuel mixtures and ignition timing as much as anything.

Cheers R

Disclaimer - I'm a civil eng not a mech eng so I could be taking a complete load of rubbish but I dont think so.

Doh - Blackbird bet me too it.

[Big Dave]

Cool - I had a pretty good handle on it - that Ixion is on another planet - not actually a planet - that is.

I was reconciling/writing the Goldwing.

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More impressive is how it goes.

Its Liquid-cooled 4-stroke, 12-valve, SOHC flat – 6 cylinder engine has a
Displacement of 1,832cc with a rather square 74 x 71 Bore and Stroke and a compression ratio of 9.8:1.

It’s got some ‘schtonk’. It puts out 87kw (117hp) at 5,500rpm, but that isn’t the important figure. The 167nm of torque at 4,000rpm, is.

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'Rather square' is too subjective?