Bore x Stroke

[Motu]

Design an engine optimised for low end torque.

Now....what sort of torque do you want? A loooong stroke engine will give you ''hang on'' ability....it''ll tow start a Pajero up hill without strain.A short stroke engine will give you neck snapping grunt.Both engines give low speed torque and not high power and revs....so what sort of torque turns you on?

Do I have to mention trials bikes again? One clue is high intake gas speed....small ports,small valves,small carbs,small exhausts.Like Ixion says,move the torque curve down.

[Big Dave]

Now....what sort of torque do you want?

So when I whap the throttle at 1500rpm - it immediately wheelstands.

[Ixion]

Torque plays only a part in that. Flywheel weight, fueling, wheelbase, as important, probably more so. Think trials bikes again. But you wouldn't want to ride one very far on the road.

[Motu]

I've owned both long and short stroke high torque engines.The one in my avatar is an M20 BSA,a 500cc sidevalve with 13 hp to pull it's 400lb weight.But it's not about hp - it had so much torque that it could pull a child/adult sidecar with that 13hp.It would drop in speed on a long hill until it hit max torque,and then just hang on forever at that speed.And I've had a 50cc Zundapp run away from me at the lights.That is the traditional long stroke engine everyone is talking about.

I also had a 350cc Bultaco Sherpa T,and I think they were 19hp.These engines started out as a 175,and were just bored out eventually to 325cc,so a very short stroke,and short rod.A motor that would never stop so long as the throttle was open,no matter what the revs or load.So much torque it would pull the front wheel up in 2nd from idle and hold the wheel wherever I wanted it,like the front wheel was controlled by a wire from the throttle.This is the short stroke torque that is not quite understood.

[Big Dave]

Bloody engineers.

If I were to design an engine that would be described as torquey.

The bore and stroke would be described as.........

The conrods world be as ..........I could make them

The flywheel would be as.............I could make it.

The ideal compression ratio would be..............

It would be a ............ cylinder.

Blardy engineers.

[Movistar]

Bloody engineers.

If I were to design an engine that would be described as torquey.

The bore and stroke would be described as.........

The conrods world be as ..........I could make them

The flywheel would be as.............I could make it.

The ideal compression ratio would be..............

It would be a ............ cylinder.

Blardy engineers.

Fill in all blanks with.......BIG!

[Big Dave]

Fill in all blanks with.......BIG!

Big dave doesn't fire blanks....oh, I see.....

[Ocean1]

Fill in all blanks with.......BIG!

Yup. Single most important design consideration. Everything else is just tweaking the curves.

Oh, and BD, you know yer TT is well undersquare?

[Ixion]

Bloody engineers.

If I were to design an engine that would be described as torquey.

The bore and stroke would be described as.........

The conrods world be as ..........I could make them

The flywheel would be as.............I could make it.

The ideal compression ratio would be..............

It would be a ............ cylinder.

Blardy engineers.

Well, you could fill those in with almost anything. You need to be asking about camshafts. Valve angles. Inlet port diameters. Exhaust resonance (want more torque ? Stuff a spud up the exhaust). Valve overlap. Ignition advance. Not to mention more fundamental stuff. Before we move on to th etechnical stuff. In other words, it ain't as simple as that.

But the short one line answer to your question is

"If I were to design an engine that would be described as torquey."

"It would be a side valve"

Enjoy.

[Motu]

"If I were to design an engine that would be described as torquey."

"It would be a side valve"

Or a two stroke - with a firing impulse every revolution,it is a longer sustained push.2 strokes are not always a high revving screamer....they are just as often a low speed grunter.No accident those locos pulling freight are 2 strokes....or those ships hauling containers.

We won't even mention electricity....that would be unfair....they don't even make a noise when they torque....

[cooneyr]

.....the large surface area of the piston applying a lot of pressure,a short sharp shock giving more torque.....

But I don't think the hydraulic analogy is valid. Sure, there is a bigger poiston area. But the VOLUME of gas is the same (all other things being equal, which they never are), so the bigger piston area implies a lower pressure per square inch .

I tend to agree with Ixion - pressure if force / area i.e. more surface area (bigger piston) = less pressure

What somewhat confuses this argument is that bore and stroke can seldom be considered in isolation.....

......The old long stroke singles, f'instance, ALSO had very small valve areas. mainly for the sake of fuel economy.

Do I have to mention trials bikes again? One clue is high intake gas speed....small ports,small valves,small carbs,small exhausts.Like Ixion says,move the torque curve down.

Your both make a good point - stroke and bore are only part of part of the equation. As I understand it high gas speeds (due to small carbies and or small valves) results in better air/fuel mixing hence better combustion pressures and less emissions. The 'restrictions' mean that the engines cannot flow enough air for high rpms though.

So we now have (warning this may be complete shite!)

1. long stroke motors with small valves, due to bore dimensions, relative to capacity with high torque. They have to have high fly wheel inertia to get though the stroke which means they dont spool up very fast though are good luggers. Low power as cant pull high revs.
2. short stroke motors with small valves, relative to capacity with high torque. They have low fly wheel inertia (don't need it to get through the stroke) so the spool up well. Low power as cant pull high revs as cannot flow enough air to pull the rpm.
3. short stroke motor with large valves, relative to capacity with low torque due to low gas speeds hence poor combustion at low rpm, and yet high power as can pull high rpm.

So what we want is a short stroke with low inertial fly wheel and with a variable 'restriction' of sorts to keep gas flow speeds up at low rpms and yet can open up at high rpm to flow enough gas?

Am I about 1/2 right????

Cheers R

P.S. fly wheel inertia is the key not weight.

Edit - Damn it - course it they had to add more stuff - stop being so bloody technical I and M

[Ocean1]

I Am I about 1/2 right????

Perfect.

Sounds simple when you say it quick don't it.

Been a few variable intake devices over the years.

[Ixion]

Or a two stroke - with a firing impulse every revolution,it is a longer sustained push.2 strokes are not always a high revving screamer....they are just as often a low speed grunter.No accident those locos pulling freight are 2 strokes....or those ships hauling containers.

We won't even mention electricity....that would be unfair....they don't even make a noise when they torque....

Of course. I did mention right at the beginning that PROPER bikes etc etc.
And one of the torquiest bikes I ridden is the GT750. The Titan is pretty grunty, too.

[Big Dave]

Am I about 1/2 right????

I'm asking the qvestions, Schtarker!

Nice Job. :-)

I'll communicate with ixion when he lands.

[Ixion]

Landing? Ah, now we are talking about thrust. There is a relationship can be derived between torque and thrust . That introduces some new concepts into our discussion. Coefficient of friction, hysteresis, lots of nice interesting stuff.And quite a bit of pure physics. We'll need some mathematicians as well as engineers. This could be a worthwhile discussion.