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Old 13th February 2015, 10:03   #31
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Default Re: Understanding Power, Torque, Gearing and Top Speed

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Originally Posted by R2D2 View Post
Thanks very much Harbir.

This has got to be one of the best, if not THE best thread I've read on TBHP in a long time. As a technology freak and former technician (worked in a workshop soon after graduation repairing & tuning bikes/cars) your lucid explanations worked like a charm in dispelling some oft repeated myths.

Look forward to more insights from you.

Indeed, a very informative thread. I totally agree with R2D2. And special thanks to Mod - Samurai, for letting me know about this thread (sorry for not searching before posting the thread).
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Old 13th April 2015, 17:31   #32
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Is there a primer on what is torque and what is horsepower here on TBHP?
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Old 13th April 2015, 17:50   #33
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Default Re: Why do cars struggle to climb a gradient (slope)?

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Is there a primer on what is torque and what is horsepower here on TBHP?
Torque is the tendency of a force to rotate an object about an axis and horsepower is a unit of measuring power.

This may help. http://www.team-bhp.com/forum/techni...top-speed.html (Understanding Power, Torque, Gearing and Top Speed)

There is a PDF in this thread that makes things very clear.

Last edited by navsjab : 13th April 2015 at 18:11.
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Old 13th April 2015, 18:09   #34
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Default Re: Why do cars struggle to climb a gradient (slope)?

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[b]Torque[b/] is the tendency of a force to rotate an object about an axis and horsepower is a unit of measuring power.

This may help. http://www.team-bhp.com/forum/techni...top-speed.html (Understanding Power, Torque, Gearing and Top Speed)

There is a PDF in this thread that makes things very clear.
So how many appreciate the fact that engines produce torque which can be measured, and that horsepower is only a calculated number? An interesting tidbit is that torque and horsepower are always equal at 5252 rpm.
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Old 14th April 2015, 00:03   #35
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Default Why do cars struggle to climb a gradient (slope)?

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So how many appreciate the fact that engines produce torque which can be measured, and that horsepower is only a calculated number? An interesting tidbit is that torque and horsepower are always equal at 5252 rpm.

SI unit of torque is Nm. SI unit of power is Nm/s or watt. Both can be measured in various ways. Never heard about your tidbit, where does that come from?

Jeroen

Last edited by Jeroen : 14th April 2015 at 00:07.
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Old 14th April 2015, 01:17   #36
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Default Re: Why do cars struggle to climb a gradient (slope)?

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SI unit of torque is Nm. SI unit of power is Nm/s or watt. Both can be measured in various ways. Never heard about your tidbit, where does that come from?

Jeroen
What is the formula for calculating horsepower? Only torque is measured, and horsepower is calculated from it.
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Old 14th April 2015, 08:19   #37
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What is the formula for calculating horsepower? Only torque is measured, and horsepower is calculated from it.

When you hook an engine up to a dynometer it measures two parameters, the Force in Newton and the RPM. The RPM gets converted into linear velocity meters per second. So that gives you Nm/s or Watt (ie Power)

So you measure two different parameters and through a simple calculation you get the result. The dynometer will show you constantly, force, RPM and power.

Think of it like this speed: Speed is the measurement of distance travelled divided by the respective unit of time. Subsequently converted into the relevant units e.g. Kmh, or mph, or m/s. Nobody would say you cant measure speed only calculate it. Its the exact same thing for power.

You could argue its semantics, which is fine, but then we need to apply it across the board and many parameters we think can be measured will need to called calculated.

Power can also be measured in different ways then using a dynometer. On larger diesels such as used in power generation and on ships, they use so called PV diagrams. See http://en.m.wikipedia.org/wiki/Pressure_volume_diagram

In my navy days we used these indicator tools and there you could claim that you would calculate the power. Once you had obtained the PV diagram you had to measure the surface of the enclosed PV graph and subsequently through some simple formulas calculate the power for each respective cylinder.

These days all done electronically. Again, I would call that measured. Because it involves the measurement of several different parameters and some calculations to give you the end result.

Another anorak fact, the latter measurement is known as iHP indicated Horse Power and is a true measure of the power developed in the cylinder whereas on the dynometer you measure at the wheels of the car or shaft of the engine. You will often see it referred to as brake horsepower. The brake being the dynometer.

On your 5252 rpm rule, look at the formulas as floating on the net. Now, what happens when you convert those to proper SI units? Is the magic number still 5252?
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Old 14th April 2015, 11:52   #38
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Default Re: Understanding Power, Torque, Gearing and Top Speed

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On your 5252 rpm rule, look at the formulas as floating on the net. Now, what happens when you convert those to proper SI units? Is the magic number still 5252?
I was intrigued by this so looked it up. Turns out that the "magic number" is simply 33,000/2π = 33,000/6.28 = 5252 (approx)

33,000 pound feet/min is the actual power generated by a horse as calculated by James Watt , i.e. one horse power. As you rightly said, the units of power and torque and not dimensionally identical. But since RPM has a unit too (min^-1 or per minute), the actual formula is:
Quote:
HP = TORQUE x RPM 5252
Simply put: Power measured in HP is equal to Torque measured in lb-feet when the engine rotates at 5252 rotations per minute.

Ain't physics fun!
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Old 14th April 2015, 12:27   #39
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Default Re: Understanding Power, Torque, Gearing and Top Speed

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Ain't physics fun!
Yes, it sure is.

Now try and build the same formulae with SI units and see what happens! The answer is more or less in the wike page you refererred to.

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where is power in watts when is torque in newton-metres, and is angular speed in radians per second. When using other units or if the speed is in revolutions per unit time rather than radians, a conversion factor has to be included.

So what happened with the magical 5252 when you go metric?

So do "imperial engines" work differently from "metric engines"

Jeroen

Last edited by Jeroen : 14th April 2015 at 12:35.
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Old 14th April 2015, 13:26   #40
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Default Re: Understanding Power, Torque, Gearing and Top Speed

some further food for thought on this 5252 thingy and the claim that at this RPM the torque equals power.

Check your car manual for its power torque diagram and see if the two curves intersect at 5252 RPM and let us know.

Alternatively, you can just google "torque power diagram" and you will get a whole bunch of them. Now try and find the one where the torque curve will intersect with the power curve at 5252RPM. (the solution is partly in the units)

Now some further thought; would this formulea (the one with 5252) only work for combustion engines? Or could we apply it to say gas or steam turbines and or electric motors as well?

I can vouch for one thing; the 'metric' formulea applies universally! What's wrong with the "imperial" one, if anything?

Jeroen

Last edited by Jeroen : 14th April 2015 at 13:29.
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Old 14th April 2015, 17:11   #41
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Default Re: Understanding Power, Torque, Gearing and Top Speed

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some further food for thought on this 5252 thingy and the claim that at this RPM the torque equals power.


Jeroen
It's not a claim, it's a Mathematical Fact for a certain set of units.

If you study Undergraduate Engineering in the US, one class is dedicated to "Unit Conversion" where you will discover that there are "Rule of Thumb" conversion factors in almost all disciplines of engineering.

It's really not that bamboozling.

Is S.I. better than Imperial? Becomes trivial when at the end of the day you are just crunching numbers, as long as they are correct

Cheers
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Old 14th April 2015, 19:02   #42
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Default Re: Understanding Power, Torque, Gearing and Top Speed

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It's not a claim, it's a Mathematical Fact for a certain set of units.

If you study Undergraduate Engineering in the US, one class is dedicated to "Unit Conversion" where you will discover that there are "Rule of Thumb" conversion factors in almost all disciplines of engineering.

It's really not that bamboozling.

Is S.I. better than Imperial? Becomes trivial when at the end of the day you are just crunching numbers, as long as they are correct

Cheers
What people forget when touting the SI units is that anything converted into a rate with a time unit becomes just as bad (or good) as Imperial units. 60 seconds, 60 minutes, 24 hours, 7 days, 365 days. Huh? Why should rotations of the crankshaft be in terms of revolutions per minute?
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Old 14th April 2015, 19:53   #43
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Default Re: Understanding Power, Torque, Gearing and Top Speed

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Is S.I. better than Imperial? Becomes trivial when at the end of the day you are just crunching numbers, as long as they are correct
Better is often in the eye of the beholder. To a certain extent it is what you are used to.

There are a couple of things that SI has going for it over imperial or any other unit system:

You have just one unit of measurement for each physical quantity
It's a decimal system so fraction are easily expressed is decimal notation
No conversion factors
Scalable by prefixes.

For instance, lets take length:

Non-SI:
inch, foot, yard, mile, fathom, rod, furlong, league, mil, pole, perch, hand, link, chain
SI:
m, hm, km

To fully comprehend the metric system all you need to remember is the following:

To measure the same range of the same quantities in the metric system, only the following units can occur:

Length: meter
Area: square meter (are)
Volume: cubic meter (liter)
Mass: gram (metric ton)
Force: newton
Pressure: pascal
Energy: joule
Power: watt
Temperature: kelvin (degrees Celsius)

combined with none or one of the following prefixes:

micro, milli, centi, deci, deca, hecto, kilo, mega

In a non-SI system you need to be able to tell the following apart:

Length: inch, foot, yard, mile, fathom, rod, furlong, league, mil, pole, perch, hand, link, chain
Area: square inch, square foot, square yard, acre, square mile, township, square fathom, square rod, square furlong, square league, square mil, square pole, square perch, square hand, square link, square chain
Volume: gallon, liquid quart, dry quart, liquid pint, dry pint, fluid ounce, teaspoon, tablespoon, minim, fluid dram, gill, peck, bushel, cubic inch, cubic foot, cubic yard, cubic fathom, cubic rod, cubic furlong, cubic mile, cubic league, cubic mil, cubic pole, cubic perch, cubic hand, cubic link, cubic chain
Mass: pound, apoth. pound, ounce, apoth. ounce, dram, apoth. dram, grain, spoth. scruple, pennyweight, short hundredweight, long hundredweight, short ton, long ton
Force: pound, ton
Pressure: pounds per square inch, pounds per square foot, pounds per square yard, pounds per acre, pounds per square mile, pounds per township, pounds per square fathom, pounds per square rod, pounds per square furlong, pounds per square league, pounds per square mil, pounds per square pole, pounds per square perch, pounds per square hand, pounds per square link, pounds per square chain, tons per square inch, tons per square foot, tons per square yard, tons per acre, tons per square mile, tons per township, tons per square fathom, tons per square rod, tons per square furlong, tons per square league, tons per square mil, tons per square pole, tons per square perch, tons per square hand, tons per square link, tons per square chain
Energy: calorie, inch-pound, foot-pound, yard-pound, mile-pound, fathom-pound, rod-pound, furlong-pound, league-pound, mil-pound, pole-pound, perch-pound, hand-pound, link-pound, chain-pound, inch-ton, foot-ton, yard-ton, mile-ton, fathom-ton, rod-ton, furlong-ton, league-ton, mil-ton, pole-ton, perch-ton, hand-ton, link-ton, chain-ton
Power: horsepower, inch-pound per second, foot-pound per second, yard-pound per second, mile-pound per second, fathom-pound per second, rod-pound per second, furlong-pound per second, league-pound per second, mil-pound per second, pole-pound per second, perch-pound per second, hand-pound per second, link-pound per second, chain-pound per second, inch-ton per second, foot-ton per second, yard-ton per second, mile-ton per second, fathom-ton per second, rod-ton per second, furlong-ton per second, league-ton per second, mil-ton per second, pole-ton per second, perch-ton per second, hand-ton per second, link-ton per second, chain-ton per second
Temperature: degrees Fahrenheit

Science only uses SI really.

There are quite a number of very well documented disasters that happened because engineers got their numbers muddled up doing endless conversions. Much less likely to happen when you use SI units. See http://www.cnn.com/TECH/space/9909/30/mars.metric.02/

Quote from this article:

Quote:
Lorelle Young, president of the U.S. Metric Association, said the loss of Climate Orbiter brings up the "untenable" position of the United States in relation to most other countries, which rely on the metric system for measurement. She was not surprised at the error that arose.

"In this day and age when the metric system is the measurement language of all sophisticated science, two measurements systems should not be used," Young said.

"Only the metric system should be used because that is the system science uses," she said.
The reason you might have these conversion courses in the USA really says it all. Countries where the SI system is properly implemented and accepted have no use for conversion courses. Here an interesting reflection on the USA where, at least in theory, they use both system.

http://blogs.plos.org/scied/2013/01/...ific-literacy/

So, call me lazy, but I will go with SI units any day!

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Originally Posted by VCheng View Post
Why should rotations of the crankshaft be in terms of revolutions per minute?
Because rotations per annum would be quite a large number.

And not all engine rotations are expressed in RPM. It just happens to be a convenient way for most rotary engines.

Jeroen

Last edited by Jeroen : 14th April 2015 at 20:13.
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Old 14th April 2015, 20:29   #44
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Default Re: Understanding Power, Torque, Gearing and Top Speed

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........
Because rotations per annum would be quite a large number.

And not all engine rotations are expressed in RPM. It just happens to be a convenient way for most rotary engines.

Jeroen
We are not talking about large marine engines, are we?

The point remains that any measurement, including SI, that has any unit of time associated with it becomes as bonkers as the Imperial units.
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Old 14th April 2015, 20:41   #45
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Default Re: Understanding Power, Torque, Gearing and Top Speed

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So, call me lazy, but I will go with SI units any day!
Phew!! Did you miss any units?

With a post like that, I think "lazy" is the last thing you should be calling yourself.

Me, on the other hand, am well and truly lazy, and will wait for the world to fight it out. Just let me know who won so I know which one to use.


Quote:
Originally Posted by Jeroen View Post
Science only uses SI really.
Jeroen
I shudder at the unfathomable depth of Supremacy and/or Ignorance in that statement for me to want to dive into that.

Cheers
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