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Kickstart
The Oracle
Joined: 04 Feb 2002
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 Posted: 19:27 - 22 Jul 2011  Post subject: Power And Torque |
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Hi
Seems to be some confusion over the difference between power and torque.
In basic terms torque is the turning effect of a force while power is the rate of work. In practical terms power = torque * rpm, but taking units into account:-
Power (hp) = (torque(lb/ft) * 2 * pi * rpm) / 33000.
One thing that comes from this is that on a graph of power and torque against rpm, with both power and torque given in imperial units (and sharing a common axis) the 2 lines will cross at 5252rpm. Quite often you will see a dyno run where this doesn't happen, which either means the power and torque units are not aligned (ie, the line for 50lb/ft of torque is not the same line as 50hp) or that someone has fiddled the figures to make them look good.
Things get a bit more confusing when an engine with a flexible power delivery is described as torquey, and with the specific amount of torque given in support of that. It is entirely possible to have an engine producing high torque over a narrow rev range, which would be anything but flexible. Common example of this is a performance 2 stroke engine, which few would describe as flexible but which for their capacity have pretty decent peak torque figures. Probably a better measure of how flexible an engine is would be the difference between the rpm for peak torque and peak power.
As power is torque * rpm, if you want more power you have a choice of producing more torque or revving further (or both, which is difficult). Increasing torque significantly is quite difficult on its own without just increasing the engine capacity, while getting an engine to rev further is fairly easy. However having an engine that produces reasonable torque at high rpm often means tuning it in a way that loses low down torque; hence a less flexible engine. The steps you feel in the power delivery are changes in the torque and so a power band is really the range where torque has jumped up. If you are riding (say) a 2006 GSXR1000 you will feel the rate of acceleration fall away after about 10000rpm even though peak power isn't until 12000rpm. Even if torque is falling away, as long as it does so slowly power will still be increasing. With a turbo diesel you will often get peak torque at pretty low rpm with it then falling away quite dramatically yet power still increases. Despite power increasing what you feel is acceleration tapering off as torque falls away.
You actual acceleration is down to the amount of thrust you can get minus any drag (ie, wind resistance and rolling resistance) all divided by the weight. Thrust is basically torque * gearing (slightly confusing as it is the gear ratio that is important, and a lower gear has a higher ratio as far as this goes ; ie number of driven teeth / number of driving teeth). If you use a higher gear you thus lose thrust. However you might land up being in a lower rev range where the engine is producing more torque at the crank to compensate.
And example of thrust falling away can be shown with a restricted 125. The following graph is for the thrust at the back wheel in each gear for a restricted 6 speed Cagiva Mito, showing from 3100rpm to 10000rpm in each gear. As you can see thrust falls away quite rapidly (peak torque is at about 6750rpm - pretty much the same rpm as peak power). While the bike will rev out to 10000rpm for just under 30mph in first the torque has fallen away so much that it has about the same thrust as it has at 3100rpm in top gear at the same speed. It actually has 3.5 times the engine torque at 3100rpm that it has at 10000rpm, yet needs about 3.5 times higher gearing to do the same road speed at 3100rpm compared to 10000rpm which cancels out the increase in engine torque.
https://www.alfa-pages.co.uk/TempPicture/CagivaMitoRestrictedThrust.jpeg
From this it sounds like power is meaningless, and to an extent it is. However to have good power it means producing useful torque at higher rpm. If you can rev an engine further then it means you can use the low gears for longer, hence maintaining rear wheel torque (and thrust). If you have 2 engines, one making 100lb/ft everywhere and running at 5000rpm to do 50mph then it will have exactly the same thrust (and assuming equal weight / aerodynamics / etc, the same acceleration) as another bike making only 50lb/ft everywhere but using 10000rpm to do 50mph. Power is relevant as it is taking into account the gearing that you can use.
An non engine idea that might help show the difference between power and torque is someone turing a bolt. The longer the bar they are using the more torque they can apply but the further they have to move to do it. If Joe Bloggs uses a 1m long breaker bar a fair amount of torque can be applied fairly quickly. If instead he uses a breaker bar that is, say, 1000 metres long he can apply far more torque to the bolt (1000 times as much) but to turn the bolt one full turn he will need to move a long way (about 4 miles) hence the rate of work (ie power) hasn't increased.
All the best
Keith
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Frost
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Posted: 20:14 - 22 Jul 2011  Post subject: |
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Where were you when we had people claiming their rs 125 could do 125mph?!  |
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Kickstart
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Teflon-Mike
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| Posted: 20:53 - 22 Jul 2011 Post subject: |
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Put simply, what we 'feel' as 'Power' isn't power, nor torque, nor even rear wheel thrust. What we 'feel' is the rate of change of acceleration forces on our body...and that is very deceptive.
Used to do a neat 'trick' with my Cota trials bike; it has a secondary fly wheel on the end of the crank to damp crankshaft acceleration for tractability... wip it off, and the bike would accelerate faster, because of the lower fly wheel inertia, and people would truly believe I had removed some kind of 'power restrictor'!
Nope, bike mad as much power and thrust with or without the second fly wheel, and its actual 'mass' made little odds to the acceleration felt, engine just picked up more rapidly!
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WannaBeDude
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| Posted: 21:08 - 22 Jul 2011 Post subject: |
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Did you just say put simply 
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Kickstart
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Kickstart
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| Posted: 22:17 - 22 Jul 2011 Post subject: |
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Hi
A normal dyno works by timing how quickly a heavy drum takes to change speed. From the time and the weight of the drum you can work out the power used to accelerate it.
Add a flywheel into the drive train somewhere (whether that is on the end of the crank or just a heavy tyre) it is more weight to accelerate. Hence accelerates the drum more slowly which means less power seen to accelerate the roller.
Brake dyno is working off how much force required to stop the roller accelerating. As it doesn't actually change speed the weight of the roller is irrelevant
All the best
Keith
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| Posted: 18:21 - 23 Jul 2011 Post subject: |
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| Kickstart wrote: | A normal dyno |
Inertial Dynomters are probably the more 'usual', I wouldn't say 'Normal'. They have become more common, since computers have become more accessible, and it's been possible to use 'cheap' electronics to measure the speed, and change of speed of a rolling road, and therefore calculate the change in inertia, to derive a power reading. This has made it practicle for them to be installed in service workshops, and used to set engines up.
Measuring "instanteniouse" power, most often at the rear wheel of a vehicle, they are very useful for measuring power in the 'Dynamic' state, of changing load and throttle, mimicking an engine in use on road or track, and are very useful for fine tuning or 'setting up' an engine. However, they are notoriously inaccurate.
They rely on the computer making an awful lot of very fast readings, and doing an awful lot of calculations on the results to obtain a result, and with so much measurement inaccuracy, and possible calculation error, results lack both precision and repeatability.
Brake Force Dynometers, work very differently, they measure the actual shaft force or torque deliverd to a 'Brake' holding the engine in the 'Steady State" at constant RPM.
Directly measuring the force, under a steady state, the measurements and readings are far more accurate and repeatable, and measuring directly 'real' quantifiable 'units' of time and force, require less, and less complicated mathmatics to achieve a power reading.
Brake Force Dyno's are more often used in development labs, to valuate prototype engines, or perform the 'qualification' tests to recognised standards, to obtain 'quoted' power ratings and specific fuel consumption, etc.
Worth noting, people often talk about 'Rear Wheel' power, or 'Crank Shaft Power'.
Is you measure the power of a bike engine on a rolling road (normally an inertial Dyno) you are measuring the power delivered to the rear wheel.
As Keith & I were quipping, on a rolling road 'inertial' Dyno, the power 'reading' is calculated from the acceleration of the Dyno's Inertial Drum. If you power that drum from the rear wheel of a bike, the engine has to accelerate the mass of the engine's alternator, the clutch basket, and all of the shafts in the gearbox, before accelerating the chain, and its own large 'flywheel' rear wheel and tyre. Accelerating so much mass BEFORE it can accelerate the Dyno Drum, you would reasonably expect a lower power reading than if you drove the Dyno Drum directly off the crankshaft.
So, take a standard ZX6R; Quoted as having 100 DIN HP, and run it up on a rolling road, likely to find it only delivers say 80bhp, at the rear wheel.
How much of that is actual transmission loss, and how much is 'experimental error' in not just the measurement on the rolling road, but the manufacturers 'standardised' power measurements, probably NOT on that actual motorcycle.....
Think about this one; Manufacturer measures the power of a number of production engines, on Brake Force Dynometers, in a Lab, and 'standardises' results to provided quoted power.
Their measuremnts will have an experimental accuracy; but you also have the tolerences on the actual engine parts and manufacturing process.
Lets say, engine has 'quoted' bore and stroke of 63mm x 48mm to give a cylinder capacity of 149.5cc (x4 to give 598cc total engine displacement)
Lets put a standard engineering tolerance of +/-0.025mm on both dimensions (Actually, tolerances in the crankshaft would be 'compounded' by that on the big end journal, and the main journal, and the stroke, and then multiplied by 2, as the crank rotates, but lets keep it simple!)
The 'actual' capacity of a production bike off the line could be anywhere between, 597.7cc and 599.3cc.
Power = Combustion Pressure x Cylinder Displacement x Number of Cylinders x Crank-Speed.
You imedietly have a 1/4% margin of error JUST for the capacity tolerance. But you also have Cylinder Pressure in there, and that is effected by how much 'Charge' you get into the cylinder, and th Compression Ratio.
Imagine a few more tolerences; Comprssion ratio is set by the volume of the combustion chamber... that has a tolernce; there's then a tolerence for the length of the con rod, the crown height of the piston, the height of the cylinder, and the 'squish' in the cylinder head.... all of these tolrences can have a huge difference on the ACTUAL compression ratio..... now add a few more for the size of the valves. Bigger valves flow more charge, more lift lt's more in too, so tolerances there can also effect how much power you get... then we are into how accurately the cam timing has been set, the tappet clerances, and beyond, into ignition timing and carburation set up.
This is all taking the discusion off into the realms of 'Blue Printing', which is, where an engine builder will attempt to exploit the declared manufacturing tolerences and service limits to 'tune' and engine for more power, selecting and matching components, and 'stratgically' wearing out, parts to build an engine that is 'legally' 'Standard' and conforming to manufacturers declared specs, but in fact, produces FAR more power than anything legitimatly coming off a production line!
(Eg; easy to get more power from an engine, fitting a camshaft with wilder timing and more valve lift. But, if class regulations prevent you using anything other than a 'standard' cam... you can get wilder timing, and more lift by slecting a cam that has com out the factory on 'upper limit' and has most metal in the lobe, then grinding it to a wilder profile, within the wide 'band' provided by production tolences and service limits)
Anyway, back to our showroom ZX6R, that delivers 80bhp to a Dyno! Factory 'claim' a notional 100bhp at the crank, BUT, with manufacturing tolerances taken into account, could 'actually' deliver anything from about 95 to 105, though on a 'win some loos some' basis, where an advantageous cam gets fitted to an engine with less compression, etc... MOST will be 'around' or close to Claimed Nominal.
So, to find out EXACTLY what our ZX6R is making at the crank, we can 'Back Motor' the Dyno and measure how much power it takes to turn over the gearbox at any particular speed, then ADD that to our power figure.
We are now into an area of Dyno Opration called 'Calculation Correction', and it's a 'Dark Art', and yet another HUGE area of exprimental and calculation error.
Back motoring a bike in neutral, you will get the tranmission losses up to the crank shaft, so you wont get the losses of the altrnator or other ancilliaries, AND in neutral, you will be gtting a large back motoring loss due to viscous drag in the gearbox oil, that you didn't get when engine was motoring the Dyno....
To SOME degree, you can get readings and corrections that are 'close enough', but depends what you want them for.
If you are sticking an after market exhaust on your ZX6R, and using the Dyno to set up the jetting, numbers DONT actually matter that much, all you want, is to know what the shape of the power curve was BEFORE you fitted pipe, and then after fitting the pipe, whthr you have made any flat spots, and swapping carburettor jets, can you fill them in again, so that before and after, you have actually got a net gain in power some where.
IF you want, 'Bragging Rights', though, getting a 'Dyno Shoot-Out' chart that says your 100bhp Ninja ONLY actually makes 80bhp, dosn't sound so impressive!
Hence 'correction factors' will be added to give a figure that is 'aproximatly' representative of the declared crankshaft power, measured to standards by manufacturer.... some oprators may do some back motoring to calculate this, but in many cases, they us simple 'fudge factors'.....
Manufacturers 'claim' 100bhp from a 'stock' ZX6R, a 'show room' ZX6R, delivers a measurement of 80bhp, on the rolling road, 20% 'down', so whevever dynoing a ZX6R... multiply results by 1.25... that would bring 'show room' bikes 80bhp up to 'quoted' 100bhp, and a piped example reading 83bhp, up to 103.75... for round numbers 104.... so pip that 'gives' an extra 3 'measured' bhp, gets flatterd to 4bhp.... BUT.... bike that was actually measured at 80bhp, need not be the bike that recorded 83 with a pipe..... what was THAT bike making before the pipe?
THIS is the world of the 'Rolling Road' Inertial Dynometer.
See a print out, "Rear Wheel BHP".... well, yes, that is probably where the power was measured, BUT, what 'Correction" has been added to make that figure 'Comperable' to Quoted crank figures? And how accurate is the Dyno, how repeatable the results?
Intresting anomoly. Inertial 'Rolling Road' Dyno measures the instanetnious power as the engine is accelrated, so a lighter or heavier flywheel or drive line will raise or lower readings. Brake Force Dyno, measuring actual force in the steady state, will measure same reading, whatever reciprocating mass is on the drive line, becouse without it changing speed, it has no effect on the calculation.
Turbo Charged Engines, suffer 'Turbo Lag'. At tick over, engine dumps little exhaust gas into the turbine, so it dosn't start spinning, or making boost. As the throttle is opened, more charge allowed into the engine, more exhaust it makes, starts to turn turbine faster, making a bit of boost, that adds to charge going into engine, that adds to exhaust gas coming out, and all that adds to the boost.
Put a Turbo Charged engine on a Brake Force Dyno, and take a power reading at a fixed engine speed. You would run the engine up to speed, clamp the throttle, then increase brake force, which would load the engine, and make it slow down... like driving up a hill without opening the throttle. So you open the throttle a bit, to get the revs back where you want them, then put some more load on the brake, revs drop, open throttle, etc etc etc, until you have the throttle wide open, and you cant gt the revs back up by opening it any more, so you have to take a little load off the brake, and find the balence point.
This is how you do it on a Brak force Dyno, for just about any engine; two stroke, four stroke, super charged, turbo charged, petrol, diesel, whatever.... BUT, getting the engine to "steady state" revs unchanging, brake force balenced, you can let the thing run forever, and readings shouldn't change.
IF it's turbo charged, that means you are getting 'full' boost. Any 'Lag' has long been encountered, and a constant gas flow through the turbine achieved.
So, GSXR1100 engine, with turbo kit. To get the power curve, you run it at tick over, and measure the power, up revs 500rpm, measure power, etc at incremnts all the way to the red line...
And you get a pretty neat power chart, that tells you the 'peak' power at every engine speed.
Run the same engine up on an inertial dyno, measuring instantniouse power, as you accelerate through each rev speed... chances are, that the Turbo will constantly be in 'lag' and you will never get a reading close to what you would on a Brake Dyno.
OK, so you have a Turbo Charged Gixer... what you using it for in the real world, and what is important to you? Probably a Pro-Street bike, you are putting the thing up the strip, you want to know how it accelerats... you don't give too hoots how much power it COULD make, is you waited ten seconds between each engine speed to let the Turbo 'catch up'.....
But... on the strip, bike will accelerate at a rate proportional to how fast it over comes drag... on an inertial Dyno, it will accelerate in proportion to the moment of inertia of the Dynbo Drum....
Light drum, and it will accelerate very fast, heavy drum, it will acclerate very slowly... its STILL not that represntative....
AND, to increase 'accuracy' and give the Dyno a chance to make more readings, Inrtial Dyno's will tend to have a havier drum, and quit likely, that drum will have a mass that takes perhaps twenty or thirty seconds to accelerat from tickover..... in the real world, your Gixer Pro Street Bike, will have launched off the line, reved wildly, and mad two gear changs through the rev range, in a third the time, it would have to accelerate an inertial Dyno.....
Dyno's and power readings are 'nice' flashy science, BUT, like power itself, as Keith was pointing out, often very mis understood, and the Dyno Results CAN be utterly meaningless.
Point of the Flywheel comment. On a Brake Force Dyno, my Cota would provide a reading of about 10bhp, no matter whether the secondary flywheel was fitted or not. On an inertial Dyno, likley that without the flywheel, would get a more 'flattering' figure, even though the engine is making no more force in the cylinder.
But either way, the 'expression' of power we 'Feel' when riding a bike, is only very 'loosely' associated with the 'actual' power the engine might deliver.
Our bodies do not feel 'force' particularly well. If I wrapped a bandage around your arm, I could apply quite a lot of force to your skin. Spread over a wide area, you would probably not say it was uncomfortable. If I prodded you with my finger, I would probably apply a lot LESS force, in fact even 'pressure' the force per square inch of skin, than the bandage did, but you'd grumble it was 'uncomfortable' or even hurt.
Because what your nerves are responding to ISN'T the actual force, but the DIFFERNCE and CHANGE in force.
Going round a corner, I can gt a car to generate a LOT of sideways G-Force. 80mph on a fast country road, that force applied gently, is no where near as 'dramatic' as chucking a car into a tight roundabout... the 'sudden' change in force makes it 'feel' a lot more than it really is.
Sam applies to bikes, and probably the best illustration of the MIS understanding of power, is when people fit after market exhausts, and from the riding 'experience' express how much 'more' power the bike has, and how much 'harder' it accelrates.
Dyno charts often show, that a pipe change will give perhaps only a 3% gain in 'peak' power, and working from thrust ratio's that xtra power could NOT give as 'huge' an improvement to accleration as rider suggests. And split timed Drag runs, often bar that out, showing that quite frequently, piped bikes accelerate SLOWER than stockers.
The anomaly is explained by peak power gains, vs mid rang losses.
Fitting after market pipe, for a 3bhp gain at peak power rpm, you will often loose perhaps 2bhp lower down.
Some times you will get more gain, over a wider range of revs than you loose, though more often not.
BUT, riding the bike, what you 'feel' is the bike climbing a steeper ramp in the power curve, the power ramping 5bhp more over the same rev range, than it did before; the 2 'lost' bhp, plus the 3 'extra'. giving a more 'sudden' and dramatic change in forces, more 'noticable' to our 'dull' perception.
Which is an awful lot of pretentious symantics. I was once told, bottom line is, when the Flag Drops, the Bullshit Stops....
Which, as a teenager, sat in the pit wall, with a stop watch, I learned was only actually a momentary reprieve! Soon as everything was packed away, and possibly even before, of 30 starters, only 'one' would b saying 'I told you so', and 29 'runners up', would be, with varying degrees, brandishing the bullshit that excused their NOT finishing first! However...
On track, what floats to the top, is that the bloke with the most 'power' doesn't ALWAYS win. In fact, bloke with the 'best bike' dosn't always win.
Old arguments, always about about speed vs handling, and lighter but the bottom line is, what 'wins' is the 'best' overall 'blend' of all of the individual elements.
Power is only ONE factor, and on that INFLUENCES acceleration and speed. Dosn't actually control it.
Bike with a big peak power figure, COULD be fast, but only if it can USE that power. In the two stroke days, engine builders could get incredibly carried away porting motors for ever more 'peak' power, but unless they gave the engine a wide enough 'spread' of 'usable' power to get at the peak power, and the engine had gar ratios that could put that power to work, bike could be utterly unreadable, or simply dog slow!
So power DELIVERY is as important as peak power; and THEN you have to have the gearing to make that power delivery most 'useful', and THEN you need to have the handling to b able to exploit it...
But, one thing I learned VERY early on, sat in that pit wall..... if you want a faster bike, fit a faster rider!
The bike is ONLY ever as 'good' as the human sat on top of it...
But MORE.... you can have a great rider, and a great bike, they have to work in harmony.
Its the OVERALL package, and how well EVERYTHING works, as a WHOLE, that governs how 'Fast' it goes, or how 'well' it performs.
Dyno's, power readings, IF you know what they 'mean' can give an insight... but they are very, VERY long way from 'The Whole Story'.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?' |
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| WannaBeDude |
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WannaBeDude
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| Posted: 18:25 - 23 Jul 2011 Post subject: |
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Get a room !!
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| Pete. |
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Pete.
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| Posted: 18:36 - 23 Jul 2011 Post subject: |
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War & Peace Mike.
Try breaking it down a bit because a lot of that info isn't relevant, some isn't even accurate.
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WannaBeDude
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| Posted: 19:24 - 23 Jul 2011 Post subject: |
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As soon as i saw Kicks post, i knew Tef was gona go on one, and i bet kick did too 
Tefs great, but ffs, i don't learn fuck all from his posts. 
Caus i don't read them, see sig.
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| Teflon-Mike |
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Teflon-Mike
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| Posted: 19:47 - 23 Jul 2011 Post subject: |
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| jjdugen wrote: | A few half explained ideas about Torque & Compression Ratio |
Power = Combustion Pressure x Cylinder Displacement x Number of Cylinders x Crank-Speed.
Power is rate of work done. Work done is Force x Distance.
Force we have in an engine is the Force on the piston, provided by combustion pressure.
Combustion Pressure x Piston Area, provides the force on the piston.
'Torque' is force x Distance (of leverage)
In the engine, its the force on the piston, (Combustion Pressure x Piston Area) transmitted by the econ rod to the crank shaft. Crankshaft 'leverage' is the distance between the big end journal center and the main journal center, and that is 1/2 the stroke
Hence if you do some maths, you get:
Torque = Combustion Pressure, x Piston Area x 1/2 Stroke
Piston Area x Crank Stroke = Cylinder Displacement, and with a few constants of proportionality, you get the simplified derivative of Power I have quoted ion bold.
'Torque' is an imaginary commodity, and a mathematical convenience.
Its forces that makes things move, and forces, distances and times are 'base' units we can directly measure.
Hence we CANNOT measure EITHER 'Torque' nor 'Power', what we measure is FORCE, and DISTANCE to get 'Torque', or 'work', and we measure TIME to get 'speed' and derive 'Power'.
If we back up, Torque is a base expression of combustion 'pressure' and what you are saying, clumsily is that you get THAT from cramming charge into the cylinder... true.... and from how hard we squash it with the compression ratio... also PARTIALLY true.
The effect of compression ratio on power is perverse. Within the simple algebra, it 'sort' of acts as a combustion pressure multiplier, but it's not a simple correlation. Increase C/R and for the same engine with same charge, combustion pressure will 'tend' to increase. But its a fickle variable, on which there are many contributory factors.
Too high a C/R and you risk detonation, or self combustion of the charge 'Knock'. This produced incredibly high 'instantaneous' combustion pressure, as detonation will tend to ignite a lot of charge, in a very short space of time, when the piston is close to Top Dead Centre and combustion chamber volume smallest.
However, with the piston at TDC, there is next to no 'leverage' on the crank shaft, and virtually no movement of the crank shaft, so that 'pressure spike' is to all extents and purposes useless.
When we talk of 'Combustion Pressure' we are generally referring to an 'idealised' averaged pressure over the entire power stroke, known as BMEF, or Brake Mean Effective Pressure, which is essentially an 'averaged' pressure that would give the same 'work done' (torque or power) as the dynamically changing pressure and leverage throughout the combustion stroke.
As Keith aluded too, commenting on my mention of two stroke C/R's in his thread on Octane Rating; There are two 'Compression Ratio's
CALCULATED C/R is the Combustion chamber volume, divided by the Cylinder Displacement.
EFFCTIVE C/R is the actual compression ratio, of how much charge you get in the cylinder and the volume you squash it into.
If you have a two stroke, with holes in the cylinder wall, half way up the bore, piston will push charge out the holes as readily as it will squash it up into the combustion chamber. So you could have a Calculated C/R of 15:1 but if the exhaust port is open until 90deg before TDC, you wont start squashing anything into the combustion chamber until it closes, and the amount of charge you ar likely to start squashing, half a full cylinder, so instead of compressing 125cc of 'Cylinder Displacemnt' into 8cc, you only squash perhaps 62cc into it, and get an effective CR of 7.5:1...
BUT, depends on the trapping efficiency...
First, did you get a full cylinder of charge into the pot to begin with? And then, how much of that 'escapes' through the closing ports, as the piston rises.
Possible if you don't have a full cylinder fill to begin with, you wont get such a high 'effective' C/R, but if the ports close quick enough, you might be able to trap more than the 'half' you'd hope to.
And you have similar effects with the cam timing of a four stroke, and the 'overlap' of the inlet and exhaust valves.
And THEN matter than max power only occurs under full throttle conditions, when you aren't artificially 'choking' flow into the cylinder, limiting the charge volume to modulate 'power'.
Engines designed for 'normal' use are NOT expected to run very long, at full throttle, at constant speed... they'll be accelerating through speeds, at full throttle, or holding constant speed at part throttle.
Hence a 'road' bike might actually survive a higher calculatd C/R because MOST of the time, running at part throttle, the short cylinder fill is giving a much lower 'effective' CR.
Quite common for modern 'road' engines to run comparitively high calculated or quoted C/R's for this reason, to improve combustion efficiency at part throttle.
Race engines, expected to run, with the throttle pretty much wide open, or compltely shut, the whole time will rarely be 'short filling' and need a calculated CR that is possibly lower, to account for the fact their 'effective' CR will more often be closer to the calculated.
Your ideas of 'Torque' and big twins with high compression ratio's and long strokes, are 'quaint', but illustrate the misconceptions that abound.
The a twin doesn't have 'torque' because it has a longer crank stroke and there fore a longer lever. Its a simple and rational idea, but it simply isn't true, if you dig into the detail.
Also depends on many other things how useful a higher C/R is, and what effect it has on 'power'.
On a conventional power trace, 'peak torque' generally coincides with the highest BMEF... highest cylinder pressure
This is where you get the highset combustion pressure from your charge. And it occurs as a 'maxima' in the torque trace for a number of reasons, but significant one is the burn speed of the fuel, and at 'that' engine speed, in 'that' combustion chamber, at 'that' effective compression ratio, you get a 'burn' that thee engine captures most energy from.
Power increases with revs to peak torque, at which point, any further increase in power comes from increased revs, at the same or reducing combustion pressure. If power continues increasing, it is generally because the increase in revs adds more power than is lost from pressure drop.
Increasing CR under these circumstances, CAN to some degree compensate a little for combustion pressure drop, BUT managing that is all down to optimisation.
IF you set the base, or calculated C/R too high, you could risk the engine detonating at peak torque and not running beyond; you could compensate for that retarding the ignition or ritchning mixtrure strength to get past; or you could revise cam timing or porting so that you 'short fill' at that engine speed, perhaps over sizing ports and valves or stretching cam duration, to get higher flow at higher rpm, and allow the engine to 'short fill' from too low charge inertia, or back flow from overlap, at that sped, introducing a 'flat spot' in the rev range, forgoing efficiency, for power....
Its a complicated interplay, where the C/R is only one variable, granted a significant, and important one, but one where the relationship between CR & Power is only 'loosely' proportional, and influenced by MANY other factors.
What you are talking about as 'Torque' is the 'Thrust' the engine delivers to the back wheel, which was what Keith was explaining.
Makes no odds whether you get that thrust from a big cylinder making one big bang at low revs or a lot of little cylinders making lots of small bangs at higher revs, or how you modify the force produced at the piston tops with levers or gears to get it to where its 'moves stuff' at the back wheel.
And what you are 'percieving' as 'Torque' in terms of the engines willingness to 'shove' or provide thrust, is the engine's 'Power Delivery', how readily it will deliver a 'change' in acceleration. The point I was making about having a lighter or heavier fly wheel, or putting a hole in the mid range to gain a point or two of 'peak' power, fitting an after market pipe.
And one of thee elements of power dlivery or 'engine character', and where low reving 'torquey' feeling engines get that feeling, is that where an engine can offer more 'variation' in deliverd power at a fixed engine speed, by opening the throttle, you often get a more 'instantiouse' increase in delivered thrust than an engine that has to increase engine sped to deliver it.... and again, MOST of that 'percieved' feeling of 'torque' is coming from the change in delivered acceleration, and NOT from the power the engine delivers, when the throttle is opened, but from the power is ISN'T delivering, when the throttle is closed....
Back to my 10bhp thirty year old piston ported trials bike! This is NOT a powerful motorcycle, but 250cc engine tuned for lots of low down power, like a tractor, it doesn't rev very high, or make much power, but its power delivery is almost liniar.
At tick over it probably offers 2/3bhp. Probably revs to no more than 7,000rpm, where its making around 9bhp, with its 10bhp peak at about 6ooo, and a 'ramp' that lifts from probably 5bhp at 2ooo to that peak.
So, bimbling along a gulley, 4mph-ish, at maybe 3K revs on a wiff of throttle, turn and point the thing up a hill, and crack the throttle, engine takes on load, and without changing engine speed, power probably tripples, in the time it takes me to open the taps.
That 'feels' pretty 'Powerful', and impression is of an engine with a lot of 'low down torque'.... utter rubbish, it's a 250cc engine making less power and less torque, than a 100cc commuter bike.... BUT, in such a low state of tune, power DELIVERY, povided by choking the engine on the throttle can give far more 'dramatic' response than an MX bike, that has to actually accelerate, through a wide rev range to provide the same change in delivers thrust.
It ISN'T even more 'responsive', the bike dosn't 'accelerate' on the throttle, it simply changes 'load' at the same engine speed, more easily.
And back to what 'wind races'.... see above! Power and power delivery are only small factors in the greater 'whole'
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Teflon-Mike
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| Posted: 20:08 - 23 Jul 2011 Post subject: |
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| Pete. wrote: | War & Peace Mike.
Try breaking it down a bit because a lot of that info isn't relevant, some isn't even accurate. |
I suspected you may have issue with bits; aware of War & Peace, hence some 'simplification' by my convoluted thinking, least wise, rather than inaccuracy!
Salient points:
 Two kinds of Dyno.
Brake, measuring force at fixed speed
Inertial, measuring acceleration of fixed 'load'
Both have advantages for different situations, but results can vary wildly, and aren't always comparable.
'Power' and 'Dyno Traces' are a very small snap-shot on 'engines' less still on entire 'bike' (or car) 'Performance
Can be interesting, and can give useful insights if you know what you are looking at, the limitations of the 'data' and can interpret it to some better use.
Correlation between measured 'Power' (whatever the Dyno) and how it expresses itself in what we 'feel' riding a bike, is very 'loose'...
We don't 'feel' power, we cant even (Directly) measure the stuff, it's an 'imaginary' commodity.
What we can measure to 'calculate' power, is force, distance and time.
And what we 'feel' as an expression of power is more often the change in acceleration forces, a function of MANY variables, of which delivered power is but one.
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?' |
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Teflon-Mike
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| Posted: 20:42 - 23 Jul 2011 Post subject: |
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| colin1 wrote: | | Kickstart wrote: | In basic terms torque is the turning effect of a force while power is the rate of work. |
That's actually a little muddled, as torque is to rotation, as force is to speed. |
Not really, and actually Keith's simplification is probably more accurate than your own.
A Torque is the rotational 'effect' of TWO opposing forces, seperatd by a sistance of offset, forming a 'couple'.You don't need a pivot or a fulcrum.
| colin1 wrote: | What makes torque different from a linear force, is that movement is about a fixed point, and movement is perpendicular to that point. |
THAT is an utter 'muddle' FORCE is NOT 'movement', it may cause movement, but it is not, in itself a movement.
THIS is a fundemntal precept, and if you dont understand it, you cant udnerstand ANY Newtonian physics.
If I decided to do a 'chin up' on my kitchen door frame, I'd be suspending about 80Kg from the door frame.
Force = Mass x Accelration Acceleration due to gravity is roughly 9.81m/s2 So, the door frame would be experiencing a 'force' of about 780KN
Unless it was rotten, door fram wouldn't move. I could get Snowie swinging from same door frame, and she would apply a load of maybe 600KN to it, the door frame still wouldn't move.
Force ONLY creates motion when there is no 'opposing' reaction to the applied force, or as in th case of a couple, there is an 'offset' between oposing forces.
Force is a 'liniar' commodity, and its pretty scientifically accurat to dscribe Torque as the Rotational 'effect' of forces.
Saying that torque is to rotation, what speed is to force is an utter scientific nonsense!
A force is a force is a force! Speed is th rate of change of distance. A function of base units, TIME and DISTANCE, which ar utterly unrelated!
Force is a Force is A FORCE...
Time is time is TIME
Distance is Distance is Distance
They are all, primary, basic newtonian expressions of commodities that are NOT functions of ANYTHING else, but themslves.
They are the basic building blocks.... which is all GCSE 'Physics', and I'm surprised that you suggest people misuse the term 'Torque' as defined by Newtonian Physics when you clearly dont understand the basic defanitions of the units to which you refer!
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?' |
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ace-card
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| Posted: 23:45 - 23 Jul 2011 Post subject: |
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| Teflon-Mike wrote: |
A Torque is the rotational 'effect' of TWO opposing forces, seperatd by a sistance of offset, forming a 'couple'.You don't need a pivot or a fulcrum.
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Except, if you had no pivotal reference point/fulcrum, then the couple would not exist. Take for example a pirate ship steering wheel (   )......when the pirate is turning the wheel right, the left arm pushes up whilst the right pushes down.....this is a couple and the pivot is in the middle. It is the distance from the perpendicular force being applied by each arm, to this said pivot that is important in determining the applied torque by each hand, and thus the resultant couple.
What I think colin is getting at when referring to movement is that the force applied is responsible for the turning moment about a given point, and as we are talking about pistons, conrods and cranks, your analogy of doing chin ups on a door frame, whilst technically correct, is splitting hairs. The only point at which force to a piston would not result in movement is if the engine were seized! Also, if you were to suspend yourself from your door frame, you are applying a force downward only, and this would not create a turning moment. So again, whilst correct, is actually irrelevant.
Also Mike, why did you start talking about speed, time and distance?
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Old Thread Alert!
The last post was made 12 years, 280 days ago. Instead of replying here, would creating a new thread be more useful? |
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