KW vs Nm




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Thread: KW vs Nm

  1. #1
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    Default KW vs Nm

    I know that 0,746kW = 1hp
    I know that torque is a twisting motion
    I know that it is fun to drive a diesel car with lots of torque.
    I know that a for eg. a 75kW diesel with 350Nm of torque is quicker than a lets say, 85kW petrol car with say 200Nm of torque.

    The question is why?
    How do you explain torque and kWatts? How do they relate to each other?

  2. #2
    4ePajero Guest

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    Read http://www.fleetwatch.co.za/magazines/Sept2006/57-DIESEL%20ENGINES.htm


    The relationship between power (kW) and torque (Nm) is expressed in the formula -

    kW = (Nm x r/min) / 9554

    or Nm = (kW x 9554) / rpm
    Last edited by 4ePajero; 2008/12/16 at 05:39 PM. Reason: url fixed

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    For the SI unit calculations, the factor is [ 60 / (2*PI) ] * 1000 = 9549
    Kobus

    HDJ81 VX

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    Quote Originally Posted by 4ePajero View Post
    Read here.


    The relationship between power (kW) and torque (Nm) is expressed in the formula -

    kW = (Nm x r/min) / 9554

    or Nm = (kW x 9554) / rpm
    When I click "here" it says "404 not found"

  5. #5
    4ePajero Guest

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    Quote Originally Posted by Dave Robb View Post
    When I click "here" it says "404 not found"
    Fixed.

    Here it is again:

    http://www.fleetwatch.co.za/magazine...%20ENGINES.htm

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    Quote Originally Posted by Dave Robb View Post
    I know that 0,746kW = 1hp
    I know that torque is a twisting motion
    I know that it is fun to drive a diesel car with lots of torque.
    I know that a for eg. a 75kW diesel with 350Nm of torque is quicker than a lets say, 85kW petrol car with say 200Nm of torque.

    The question is why?
    How do you explain torque and kWatts? How do they relate to each other?
    Hi Dave

    A favorite topic of mine. Torque and power are actually two sides of the same coin. We attempt to define the performance of an engine with one point on the power curve : typically 100 kw @ 5000rpm. (max) . This is a fairly poor way of defining that motors characteristics. It is one point only on a curve of an infinite number of points . We can back up the power max with a torque max eg 300Nm @ 2500rpm. This effectively is a second point on the engine characteristic curve and it tells us a lot about the curve : in particular is it flat , does it have grunt at low revs or is it all about screaming high revs and power at 10 000rpm such as with a Wankel rotary.

    The weakness of using either the max power or the max torque figure to define an engines performance is that a typical driver and car actually spends very little time at these two points . So to get an idea of the real engine characteristics you need to determine the engine use (ie racing or off road ) and then look at the power curve in the areas that the car will be spending amost of its time (in F1 racing most of time is spent at 17000 rpm to 19000 rpm whereas in offroad time wil;l be spent at between 2000 rpm and 3000 rpm. ) and look at the power curves over the operating rev range.

    The formula connecting torque and power has been quoted. But what does it mean actually ? An example from the cycling world might be of assistance. ( I am not a cyclist) .
    If it comes to torque input to a cycle I am pretty sure I can match or beat Lance Armstrong . A cyclists max torque output is easy to calculate : Mass (kg ) x 9.8 X peddle stroke (m). In other words the cyclist applies his mass to force the pedal down. 9.8 is a constant , known as g , and .3m is a typical bicycle pedal stroke.
    In my case this will be 110 kg x9.8 x.3 = 323Nm (not too shabby)
    Lance Armstrong , a much lighter and fitter man) will develop less torque , typically 80kg x 9.8 x.3 = 235 Nm (much less than my torque!)
    So why can i not beat Lance in a cycle race ? Simple : Lance develops a lot more power than i do . He can pedal faster while continuing to apply a high torque on the pedals. In my case once i start pedalling I will quickly run out of breath and have to slow down , hence my power drops off rapidly .
    In a physiology lab precise power output , oxygen consumption , blood glucose etc can all be monitored and balanced out . Just as fuel consumption on our car can be balanced out with power outputs.

    Torque in the cyclists case is a measure of how hard can he push down on the pedal , and power is a measure of how hard and how fast can he push down on the pedals. The car engine is an exact analogy where the cyclists legs are the con rods and the pedals are part of the crankshaft.

    Trust that this does not further confuse mtters !

    Cheers Dave

    PS In the example you gave of a 75 kW diesel being fatser than a 85 kW petrol engine , this is because the power curve of the petrol engine will "droop" at lower revs , so that the average power that the disel enginev puts out will be higher than the average power that the petrol engine puts out.
    However supposing both cars were fiteed with very efficient constantly variable transmissions(CVT's) and both engines wer then operated at their respective max power revs , the petrol vecar would be faster than the disel car. This is the potential attraction of CVT's...
    Last edited by pretdave; 2008/12/16 at 07:12 PM. Reason: Add CVT comment

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    Thanks Dave, that sound excellent. I want to read it again to digest it. An illustration like that makes sense to me.

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    Quote Originally Posted by 4ePajero View Post
    Wow that's very informative, thanks 4eP

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    layman's terms....

    Torque = ability to do labor/work
    Kw = tempo/speed at which you can do labor
    Frik Eloff
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    Toyota 2.5D4D 4x4 .... my Platkar vir werk

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    If memory serves me,
    Torque is measured in newton meters, which is the force of 1 newton applied with a lever of 1 meter.
    Power is work divided by time.(work is force applied over a distance). 1 newton applied over meter is 1 joule. And a joule applied in 1 second is 1 watt.

    Hope this makes sense. My understanding is that power equals acceleration and torque is pulling power or the ability to drive up a hill with out loosing speed and having to gear down.

    From what I remember from my petrol days, You could advance your timing and this would give you lots of acceleration(power), but the vehicle would then perform poorly up hills (torque).

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    Simply put Torque is the twisting force applied to the wheels by the engine.

    Power is Torque multiplied by Revs.

    In practical terms Torque is required to accelerate or climb a steep hill without changing down.

    Power is required to reach high speeds.

    So high torque and low power means you are driving a tractor. (You can pull out tree stumps but cant exceed 30km/h).

    Low torque high power means you are riding a super bike, it wont pull a caravan (although some might) but it can get to 300km/h at 19,000rpm

    Torque is measured in Newton Meters (a Newton is a measure of linear force, the Meters is the distance between force couple applied - picture two linear forces the same magnitude but in opposite directions applied to a body where those forces are separated by a distance - this is also called a bending moment). Nm is actually a unit of energy/work same as Joules and kWh. Power is work done per second.
    Last edited by alanB; 2008/12/19 at 09:01 AM.

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    Makes sense thanks Alan, just one question though... how did a turbo diesel win the Le Mans

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    how did a turbo diesel win the Le Mans
    I think you will find that that Audi engine revs to a tad more than the normal bakkie or truck does

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    I've heard this take on the topic once...

    kW = how hard you hit the wall
    Nm = how far you take the wall along with you

    Jacques Joubert

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    kW = how hard you hit the wall
    Nm = how far you take the wall along with you
    Strictly speaking the second part of that should read

    Nm = how far you can push the wall from standstill

    (How far you take the wall with you is more a function of momentum which is velocity times mass of your vehicle) eg a motorcyclist may bounce off while a truck at the same speed stops in your neighbours yard after going through the wall and your house!

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    The other simple difference is that power is measured against time. So it is not only top speed, but how long it takes you to get to that speed.

    Tourque does not have a time component.
    I think that this is the most important difference.

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    Default Torque and power

    Hi Guys

    A point that perhaps requires emphasis is that the power at the driven wheels is restricted to the power output of the motor , less some efficiency losses . eg a 100kW motor can put out a maximum of 100kW less , perhaps, 10kW , at the driven wheels.

    Torque , however, does not have this restriction. eg any motor can put out whatever torque you want at the driven wheels . This is a function of gearing.

    In other words power is not multiplied by gearing but torque is.
    Who was that classical scientist who stated " give me a lever long enough and I will move the world" ? Exactly synonymous. In this case the long lever will multiply the torque applied by as much as the length of the lever allows. The power is of course restricted to one man's power and so the "movement of the world " will be extremely slow (consistent with the relationship that "torque x RPM = Power"or " RPM= Power / torque)

    And yes -a superbike engine fitted to a 4x4 , with the right gearing, will happilly tow a caravan.

    So what does this mean for us in the "real world"?
    Power is actually more fundamental than torque in defining performance. With power you can create whatever torque you require. That is what multispeed gearboxes , and CVT transmoissions and low range transfer boxes are all about.

    So why do so many people give more credibility to torque figures than power ?

    Simply because a high torque figure on an internal combustion engine suggests a flat , even spread of power, meaning that the driver does not have to continually be searching for another gear to get the vehicle motoring. .

    With a good gearbox (lets take for example a 7 speed automatic ex a Mercedes S Class) a high revving ,highly tuned ,high power and relatively low torque motor would operate very effectively off road in an SUV. Of course the high complexity and cost would not be ideal - so we are back again to looking at , for instance, a high torque , low power motor such as the 4.2 diesel in a LC 70. Never under strain and never far off usable torque and power.

    In the final analysis what gives a car performance is the " shove " that the driven wheels apply to the road. This shove multiplied by the wheel radius gives the torque at the driven wheels . In low range first this torque is enormous ( many times greater than the torque rating of the engine)and will allow the car to do amazing things (including , unfortunately, breaking halfshafts ! ), but very slowly. But the ultimate limiter to the max torque that can be applied to the driven wheels remains the engine power because of the relationship "torque at wheels = Max Power / RPM at wheels " . torque at wheels here is the dependant variable and Max Power is a fixed constant. Confusing ?

    Cheers
    Dave

    PS Something that can confuse the issue is the meaning of the word "power" . In common terms when we say a man is powerful , we mean his muscles are strong - he can pick up a heavy load. This is not the same as the scientific or engineering concept of power . The engineer would say that the man is powerful if he can lift this heavy load many times in a minute. The common term used by the layman for "power " is actually closer in concept to the engineers "torque" . Even more confusing ?
    Last edited by pretdave; 2008/12/19 at 09:53 AM.

  19. #19
    4ePajero Guest

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    Dave,

    1.
    Power is actually more fundamental than torque in defining performance.
    I beg to differ. Power is the application of the available torque over a period of time.
    Furthermore your definition of "performance" needs to be clarified. To me, in the SUV/4x4 context, it is getting the job done, which is powering the SUV in a controlled and economical way.
    "Performance" for a racing engine is totally different, of course.

    2. In your whole dissertation above, you ignore efficiency and economy. Your superbike engine revving at 14000 rpm, (with the appropriate gearing) in a 2.5t SUV will use considerably more fuel than a TD revving at 2000 rpm.
    I am convinced the TD will outlast the superbike engine as well.

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    Power is actually more fundamental than torque in defining performance.
    So what you are really mean is "Torque is cheap"? Sorry couldn't resist


    Simply because a high torque figure on an internal combustion engine suggests a flat , even spread of power, meaning that the driver does not have to continually be searching for another gear to get the vehicle motoring.
    Not necessarily true!
    A high performance car may have a torque peak at high revs with very little at low revs, enabling it to be very fast round a race or rally track but difficult to drive around town (you have to rev it pull off without stalling). The advent of variable valve timing such as Honda's VTECH or BMW's VANOS system and electronic engine mapping has helped engineers "flatten" the torque curve of the average petrol engine and get more torque lower down without compromising performance at higher revs. Older super cars for instance were very difficult to drive for this reason.

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