OzVMX Forum
Clubroom => Tech Talk => Topic started by: Gerard De Ruyter (Twistandshout) on March 17, 2011, 03:25:29 pm
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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?
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Thats exactly what 4 strokes do now. Big bore short stroke = Quick reving horsepower.
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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. :D
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. ;D
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Thats exactly what 4 strokes do now. Big bore short stroke = Quick reving horsepower.
And every one knows the side effect of that hey
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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. ;D 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.
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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).
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Nathan you forgot to add Harley's to that list of agricultural machinery with long strokes.
Grahame
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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.
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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.
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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
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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.
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Aye - I wonder if we've melted Twsty's brain yet?
:D
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... :P
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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.
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got it in one nato, :P
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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 ;D
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OK so where in 'four stroke land' does a 7000/8000hp ,most only two valve per cylinder, V8 Top fuel dragster fit in ? long ,square or short stroke ? :)
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:P I know the bb 454 runs a 4in stroke and 4.25in bore pointing toward slightly over square whilst the 427's ran 4.250in bore and 3.760in stroke so beats me Tim..one is over and one is under and both are/were quite popular drag engines ???
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OK so where in 'four stroke land' does a 7000/8000hp ,most only two valve per cylinder, V8 Top fuel dragster fit in ? long ,square or short stroke ? :)
Come on Tim, you know that's unfair - the drag V8's et al break ALL the rules ;) - and only last about 4 seconds and 2000 revolutions :-\
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All that playing about with bigger valves ,porting , cams and the like is to increase volumetric efficiency.
Tis possible to get 130% ve. A standard car motor [say a 308] used to run about 80%ve when new.
In other words it,s possible to stuff 50% more air/fuel into the same capacity motor with a bit of work [ and a lot of money ]
Or you can just jam a turbo/supercharger on it and supply it with some fuel that carries it,s own oxygen.
Or nitrous.
In the old days the rails were using some wierd rocket fuel that burns without air.
Their 1/4mile times were incredible BUT the fuel was downright dangerous and was banned.
Those tiny remote controlled airplanes running methanol are making up to 1hp per cc.
Imagine. :o
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Tim, if you watched "Gasoline" this week your very question was answered
In short they burn 18 gallons of fuel ( nitromethane - the more you can jam in those pots AND still be able to burn it with the motor hydraulicing is how it works ) in just a tad over 4 secounds, young andrew Cowins top fueler makes 1000Hp per pot X 8 = 8000hp .
pay attention to the unburnt fuel pouring out of the headers just prior to launching.
Firko, these things are awsome ;)
http://www.youtube.com/watch?v=G7g-z43znlc
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At 8000rpm a rail motor is doing 133revs per sec.
The engine would only do 665 revs in a 5sec 1/4 mile run. :)
Sounds like a lot more . :)
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Volumetric efficiency is not as important as combustion efficiency, which co-incidently is around 0.8 in production machines, 0.92-3 in the very best race engines.Volumetric can only fill the cylinder and the cylinder head especialy on a 4T engine.
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In the old days the rails were using some wierd rocket fuel that burns without air.
Their 1/4mile times were incredible BUT the fuel was downright dangerous and was banned.
hydrazine perhaps? or something really nasty, like triethylborane?!
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hydrazine perhaps? or something really nasty, like triethylborane?!
Correct Luke...it was hydrazine. It achieved the wanted HP plus a hell of a lot more but at the cost of turning engines into lethal bombs. They banned it after a couple of drivers were killed by exploding engines.
http://www.dragzine.com/tech-stories/fuel-cooling-ignition-tech/flashback-friday-the-story-of-the-leathal-fuel-called-hydrazine/ (http://www.dragzine.com/tech-stories/fuel-cooling-ignition-tech/flashback-friday-the-story-of-the-leathal-fuel-called-hydrazine/)
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I have been watching this discussion with interest and frustration from a computer away from home since friday last,frustrating 'cause the computer wouldn't let me in!.
my go - I waited a fair while for some-one to mention bmep. {nathan s] Volumetric efficiency, well tuned 2/ commonly achieve better than 120% I believe. Not forgeting megaphone effect on 4/ Then you have turbo and supercharge.
The new CAT Diesel truck engine runs 50 psi turbo!
What about the value of short verses long conrod? [Conrod angle] Plus offset gudgeon.
Then there is constant pressure verses constant volume [this one is interesting when you think about the latest development in direct petrol injection.]
One thing here from jc on the psi on the area of a bigger piston got me thinking.
cheers pancho
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Volumertic figures can only happen over a short rpm range when the resonace is in phase, which means the efficiency will be poor when not in phase.
Never seen the downside from a longer rod, desaxation(there you go firko) is not used that often these days and is basicaly a defacto longer rod.
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There was a trick with chev motors where you put in a longer rod and shorter gudgeon height piston.
Trying to remember why.[ getting old and forget half this stuff] :o
Piston tip over just after tdc on the firing stroke I think , causing loss of hp through friction.
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Volumertic figures can only happen over a short rpm range when the resonace is in phase, which means the efficiency will be poor when not in phase.
Never seen the downside from a longer rod, desaxation(there you go firko) is not used that often these days and is basicaly a defacto longer rod.
Herr Stolk - what is this crazy talk? Please explain "desaxation" !
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Herr Stolk - what is this crazy talk? Please explain "desaxation" !
Gudgeon pin offset
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The term is a very old one from some early engine that had a 'desaxe' layout or gudgeoun pin off set. The YZ 85 is the only engine I know now that still uses this today. Gudgeon is offset to the inlet side of the piston.
With the volumetric thing. resonances and harmonics are only in tune for a few 1000 rpm not across the rev range,
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desaxation(there you go firko)
Nice effort Loz.....now all I have to do is find a use for the word in my day to day work. ::)
Now, get back into my engine! ;D
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didn"t some 6HR guys get busted for boring a kwaka 4 (desaxe),was permissable to overbore,but not all to the front of the barrell ;D, :P
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10+ YZF450 has offset gudegon.
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01+ YZF450 has offset gudegon.
They don't count. ;D
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Umm sorry I am a little familiar with the workings of Nitromethane fuelers :) Just being a serial pest again.... Cheers Tim754 ;)
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01+ YZF450 has offset gudegon.
They don't count. ;D
And I think the YZF450 has its barrel offset from the crank centreline. Trying for the same effect [I assume] in a different way.
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A shorter rod which increases conrod angularity while the piston descends the stroke increases the piston speed which in turn takes advantage of the rapid pressure rise occuring in spark ignition piston engines, resulting in increased torque. Torque plus rpm =horsepower.
Similar situation occurs to a lesser degree in those engines where the gudgeon pin is offset [to quieten things down a bit], if you reverse the piston to put the offset the other way for a power increase. Ask the old "stock car" drivers who got sneeky extra power this way.
Torque plus rpm=horsepower.
This explains to those who wonder why, when one looks at a rather large capacity V8 car engine sees it putting out 200hp, where as a motor cycle high performance engine puts out half that figure from less than a quarter of the capacity.
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what about a 2 stroke then ;D
seeing thats what 90% of us ride ???
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A shorter rod which increases conrod angularity while the piston descends the stroke increases the piston speed which in turn takes advantage of the rapid pressure rise occuring in spark ignition piston engines, resulting in increased torque. Torque plus rpm =horsepower.
Similar situation occurs to a lesser degree in those engines where the gudgeon pin is offset [to quieten things down a bit], if you reverse the piston to put the offset the other way for a power increase. Ask the old "stock car" drivers who got sneeky extra power this way.
Torque plus rpm=horsepower.
This explains to those who wonder why, when one looks at a rather large capacity V8 car engine sees it putting out 200hp, where as a motor cycle high performance engine puts out half that figure from less than a quarter of the capacity.
Think you got yourself all befuddled there pancho, most high performance engines run >2.1 rod to stroke ratio. Aprilia GP 125 and TM/Pavesi 125 shifter kart engines(all over 50HP) run 120 and 118mm rods for a 54.5mm stroke. There isn't much pressure rise as the piston decends.
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Long rods reduce peak piston acceleration, and reduce the angularity of the rod relative to the crank (meaning that the piston is marginally more efficent at pushing on the crank). They also hold the piston near TDC for longer, meaning that the rate of increase in compression pressure is more gradual, and that ignition timing is less critical.
This is all good stuff.
In theory, a long rod can make the engine more likely to detonate, as the additional time the piston spends at/near TDC can prevent the combustion gases expanding as fast as they want (poor explaination, but hopefully it makes sense). In reality, it is very difficult to find anyone who even claims to have experienced this phenonema - if it exists at all, it has far more to do with large bore sizes (4"+) requiring lots of time for the flame front to travel across the combustion chamber.
Long rods also increase the height of an engine, which is rarely desirable but not often a problem.
The trend in the last thirty-odd years for rod:stroke ratio, has been clearly toward longer rods in all sorts of engines.
Four strokes have also had a clear trend toward shorter pistons - the reduced side loading of longer rods has allowed this.
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The trend in the last thirty-odd years for rod:stroke ratio
mmm.. is that because of fuel quality or other factors?
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It's because the longer rod 'works' better. Even a stock Banshee with just the 115mm rod swap feels torquier to ride. Something to consider for your PE ;)
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Longer rod, when retro-fitted, changes valve/port/ign timing, increases dwell around TDC, & in 2T increases crankcase volume (unless compensated by piston w reduce deck height).
I waited a fair while for some-one to mention bmep
Torque, HP & BMEP are mathematically related to ea other by simple formulae. Its hardly rocket science.
Getting back to the threads original question, one of the other effects/benefits of a long-stroke design is more compact/efficient combustion chamber (lower surface-area/volume ratio).
You can only go so far w oversquare 4T designs for better breathing via more valve area & higher rpm, till the size/shape of the combustion chamber becomes a problem (hi surface-area/volume ratio) & you start losing the gains.
In a 2t, its more win-win cos of more compact combustion chamber (lower surface-area/volume ratio) & more cylinder surface area allowing bigger/better ports (already mentioned) in long stroke/undersquare design.
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First of all let me say I enjoy this kind of discussion for the opportunity for me to learn more, I believe that one should never ever stop trying to learn more.
As a 73 & a 1/2 year old I find the older I get the less I know [specially about 2 strokes]
Lozza re.pressure on the piston. As I intimated on an earlier post, spark ignition engines are described as 'constant volume' which as I understand it means the fire goes BANG and the pressure rise is instant and short lived,which is the same as your post re 'not much pressure rise'.
A short rod does have the disadvantage of more severe side thrust on the piston but conversely has a better straight line push on the crankpin.
Added to this is the more rapid descent of the piston taking advantage of the short but rapid pressure increase from combustion. Less detonation?
Going back in history [nothing new under the sun] 350 Gold Star BSA made up oval flywheels to allow a short conrod for a torque increase. More horsepower, from more torque same revs.
Obviously rapid engine bore and piston wear follow.
I believe the main reason engines in England, both car and m/cycles went to small bore - long stroke [to the detriment of good design] was the fact that the gov't taxed engine size on piston area times No. of cylinders for years so manufactures increased stroke to gain capacity.
cheers pancho
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(http://www.perthstreetbikes.com/13829//ccf1117200800000eo9.jpg)
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It's because the longer rod 'works' better. Even a stock Banshee with just the 115mm rod swap feels torquier to ride. Something to consider for your PE ;)
I believe that the 3rd birthday is coming up - any chance of it progressing? ;D
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Your call has progressed in the queue...................... stand by for the nearest available operator................................... ;D
(will get some answers for you)
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Your call has progressed in the queue...................... stand by for the nearest available operator................................... ;D
(will get some answers for you)
I bet I get put through to some kid in Bangalore!
That Irving chap has a way with words. Good stuff.
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All very interesting. Who is this Irving dude who has a name that makes me think he should know what he is talking about?
I found his last sentence interesting if not revealing.
I would love to talk to him face to face about some of his ideas and conclusions that appear to be 'drawing a long bow.'
His paragraph stating ' Maximum cylinder pressure is reached at an early phase of the power stroke, and the longer rod ensures that high cylinder pressure is developed and held when the crankpin is at a greater angle, leading to improved thrust'.
To my understanding, by the time that the 'leverage angle' is at its most productive, effective pressure has dissipated...... Not like a diesel.
not arguing learning.
cheers pancho.
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He's a automotive engineer I dunno if he is any relation to Phil Irving (maybe go the the Rapid Bikes forum and ask), as I said before pancho, Ian has it correct and you have it arse about. The ideal angle for peak cylinder pressure is not long after TDC, having the piston dwell at TDC longer gets greater thrust. I don't think all the F1 engine builder, Aprilia , KTM and HRC all got it wrong.
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I've known Ian Irving for a number of years (although haven't spoken to him for quite a while) and he's indeed a very cluey individual. He used to have a little tuning shop across from the Tollgate Hotel in North Parramatta back in the late 90's. I worked around the corner and used to enjoy dropping in for a beer and a bench race and sucking up his amazing knowledge. He built an impressive big bore GPZ1100 powered Bimota that was as trick as they come in 1995. Jeez that bike was quick......BTW..no relation to Phil
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i forgot all about asking this question till tonite. um. wow. there's some answers there isn't there. while most went right over my head, i sort of get it now - everything interdependant. not in buddhist sense (tho they're on the money there), but design factors. longer stroke gets you more time to get more mixture in and out (and other impacts too), while bigger bore gets the same thru broader ports/bigger valves (and other impacts too). the other impacts relate to torque and hp. and reliability/longevity. and the article by that irving guy....longhand way of saying 'om' i reckon.