bore vs stroke

[Gerard De Ruyter (Twistandshout)]

i've often wondered something about jugs (the metal type....on this occassion....) - i can understand why bigger bore means more power (thru more gas/air capacity), but why does longer stroke do the same thing?  or does it??  why isn't it best to have as wide a bore as possible and a short a stroke as possible?

[head]

Thats exactly what 4 strokes do now. Big bore short stroke = Quick reving horsepower.

[vmx42]

Boy Twisty you ask the simple questions don't you? There are so many chickens and eggs in that little doosy that you could start a poultry farm and omelette restaurant and still have both chickens and eggs left over.  

i can understand why bigger bore means more power (thru more gas/air capacity)

I assume you mean increased capacity [not just bigger bore] is an obvious path to more power.

why isn't it best to have as wide a bore as possible and a short a stroke as possible?

Modern, high revving racing engines do just that. But there is a limit on how quickly you can get enough mixture into the cylinder when you have an extremely short stroke [with resultant high revs]. Not to mention the problems of incomplete combustion due to the high flame speed required.

In a modern F1 engine, revving to 18,000 rpm - the cylinder has to be filled and emptied 150 times per second. At this speed the pistons are experiencing acceleration loads of up to 8500g. The conrod  stretches by about .33 of a mm whilst slowing down the piston. The piston is experiencing loads including combustion approaching 6 tonnes. The valves have a mass of only 50g but because they are opening and closing 150 times a second they are experiencing  10,000g and have an 'apparent' weight of 500kg each. Add in the extreme temperatures and you have a living hell that somehow survives for 1500 klms. Amazing stuff.

I would suggest you get hold of some of Kevin Camerons [tech editor at Cycle World] writings to get a better understanding of your question. It is fascinating to put together little pieces of the puzzle in your mind - and just when you think you are getting somewhere, some new development comes along and makes the impossible, possible.

Good luck
VMX42








P.S. and be careful you don't get your 'jugs' mixed up. That could be painful.  

[LWC82PE]

Thats exactly what 4 strokes do now. Big bore short stroke = Quick reving horsepower.

And every one knows the side effect of that hey

[Lozza]

i've often wondered something about jugs (the metal type....on this occassion....) - i can understand why bigger bore means more power (thru more gas/air capacity), but why does longer stroke do the same thing?  or does it??  why isn't it best to have as wide a bore as possible and a short a stroke as possible?

Big bores (like big ports) don't equal big power. The bore to stroke ratio is determined by rpm, the greater the rpm the shorter the stroke.This is mainly to do with mean piston speeds, or the maximum rpm the rod/bearing/piston can handle(about 25 m/s), shortening the stroke reduces the mean piston speed.That is a generaliseation which mainly applies to 4T engines. Two strokes don't play by those rules. You will find 2T engines have universaly  square or under square dimensions, modern 125's have 54 X 54.5 (old dimensions were 56 X 50), 250's have 66.4 X 72(old 250's have 72 X 64), all for a good reason a smaller bore and longer stroke give a longer cylinder with more room for better port shapes and sizes.To get the correct durations on big bores the ports have to be tiny, negating any rpm advantage.
F1 and modern sport/dirt bike 4T engines are massively over square this allows rpm and room for large valves that the engine needs to breathe at high rpm.

[Nathan S]

2-stroke or four, Twisty?

Like Lozza said, they are almost opposite to each other.

In a 4-stroke, the simplest explaination of why a super-short stroke isn't ideal, is to remember that the stroke is a lever - and a long short stroke turns the force of combustion into more torque at the crank...
Its damn near impossible to get decent low down grunt from a short stroke engine (which is important to the owner of a Landcruiser, TT500 or similar piece of agricultural machinery).

[mustanggrahame]

Nathan you forgot to add Harley's to that list of agricultural machinery with long strokes.
Grahame

[JC]

2-stroke or four, Twisty?

Like Lozza said, they are almost opposite to each other.

In a 4-stroke, the simplest explaination of why a super-short stroke isn't ideal, is to remember that the stroke is a lever - and a long short stroke turns the force of combustion into more torque at the crank...
Its damn near impossible to get decent low down grunt from a short stroke engine

Nathan, I think you'll find thats old-school tho't now. The Honda trials bikes of the 70's got plenty of grunt from very over-square short stroke engines.

[Nathan S]

Yes and no.

The Honda S2000 is a long stroke motor, but still has a higher redline than any other production car.

Engineers can make anything work, if they're given the time and money (eg: Porsche 911s go around corners despite the engine placement), but the fundamentals are still the fundamentals.

I believe the short stroke trials bikes were about making the engine as physically short as posible - a design compromise.
If the engineers hadn't been constrained by that compromise, they'd have build a longer stroke motor for sure. (Actually, they'd have probably built a 2-stroke....).
The other way of looking at it, is that if you turn a short stroke motor into a grunter, then you've 'wasted' the rev potential of having a short stroke - its kinda like building a rev-happy motor and then fitting a tiny carby and a pipe that's tuned for bottom end power.

[Lozza]

Interesting as Gas Gas has 3 configurations of 250/300 engines, the 'standard' 250 mx 66.4 X72 a trials version which is 72 X 64 and a version for 250 National kart racing 64 X64mm and some Pampana variant The 300 trials has a similar short stroke version. I dunno how hard the trials version revs to but the kart engines rev to 10,500 and do that all year(different story going to 11,000 though) make 67HP at the wheel. The mx version revs to 9-9500

[JC]

The long stroke = more leverage = more torque in 4Ts is a bit of a layman's furfie which sounds good but is up the creek.

The reality is that (real) torque is proportional to how much air/fuel mixture you can get in there & how often you ignite it. (You get more valve area in a 4T w big bore oversquare design & bigger ports in 2T w longer stroke undersquare design, therefore more air/fuel with opposite configuration.)

The reason long stroke 4T engines typically give power at lower revs is that their limited valve area limits their volumetric efficiency to (relatively) low rpm so their power is at lower rpm. They simply can't breath enough for higher rpm.

Its not that their long stroke gives more leverage & therefore more torque, cos torque is a product of combustion pressure acting over the area of the piston for the stroke of the piston. Its not just the stroke that affects it. So a long stroke is offset by smaller piston area for the pressure to act on in an undersquare design, while short stroke is compensated by more piston area for the pressure to act on in oversquare design.

Beyond that, where the torque/power peak is produced is largely a matter of tuning.

[Nathan S]

Aye - I wonder if we've melted Twsty's brain yet?
 

I'm just going to drop the term "BMEP" into the discussion now... and then run away because I don't have time to begin to discuss more than about 0.001% of the little that I think I understand properly...

[GMC]

I think that most long stroke motors tend to carry large flywheels as they have to carry the piston further until it reach’s its next explosion.
This flywheel weight can sometimes be interpreted as torque.

Trials bikes would have short stroke motors not for high revving capabilities but for instant throttle response.

[matcho mick]

got it in one nato,

[Lozza]

The amount of mixture you have is irrelevant as in a 250 be it 2T or 4T you will only get 250cc +head volume inside the engine at any intake cycle. Torque is force X distance you can only increase force (combustion pressure)so much, distance can be easily varied.Pressure across the piston doesn't have as great affect as when the peak pressure acts on the crankpin, rod length also plays it's part