If you put: horsepower per foot pound into Google you get 550 Hertz. I was reading about cars, where the engines are described with the power output and torque in horsepower and pound-feet, respectively, and realized that the ratio was either time or frequency. Interesting, though, that the numeric values for horsepower and torque are similar, but car engines typically don't run at 33000 RPM. -- glen
550 Herts
Started by ●March 16, 2009
Reply by ●March 16, 20092009-03-16
On Mar 16, 2:34�am, glen herrmannsfeldt <g...@ugcs.caltech.edu> wrote:> If you put: �horsepower per foot pound > > into Google you get 550 Hertz. > > I was reading about cars, where the engines are described > with the power output and torque in horsepower and > pound-feet, respectively, and realized that the ratio > was either time or frequency. � > > Interesting, though, that the numeric values for > horsepower and torque are similar, but car > engines typically don't run at 33000 RPM. > > -- glenA common question I pose to my students is what is the difference between energy and torque where they can have units of Newton-meters? I recall seeing something recently about "silly" units such as the horsepower per foot pound. It had quite a few examples. One unit I like is the furlong per fortnight which is amazingly close to 1 cm per minute. Thanks for posting. Clay p.s. I once heard that Nevada had the highest rainfall per capita of all of the states.
Reply by ●March 16, 20092009-03-16
On 16 Mar, 17:31, c...@claysturner.com wrote:> p.s. I once heard that Nevada had the highest rainfall per capita of > all of the states....and it is common knowledge around here that the polar bear is the safest pet. Rune
Reply by ●March 16, 20092009-03-16
On Mon, 16 Mar 2009 06:34:08 +0000, glen herrmannsfeldt wrote:> If you put: horsepower per foot pound > > into Google you get 550 Hertz. > > I was reading about cars, where the engines are described with the power > output and torque in horsepower and pound-feet, respectively, and > realized that the ratio was either time or frequency.If you express torque in N-m and power in Watts, then the difference is in radians/second. Since a radian is a dimensionless* unit it's easy to leave it out.> Interesting, though, that the numeric values for horsepower and torque > are similar,I think that's just happenstance having to do with how fast a car engine will spin, and the customary units** for rating engines. Try the computation on a modern racing model airplane engine, where peak power occurs in excess of 20000 RPM, and your numbers might diverge a bit more.> but car engines typically don't run at 33000 RPM. >_Your_ car engine doesn't run that fast -- I wonder how fast the Formula 1 racers are going these days? * I always get caught up on that -- are radians and napirs really units, or are they just a reflection that some things really don't need units? ** In the US. The international ads and reviews I've seen rate power in kW; I'm not sure what they rate torque as -- Newton-meters (_not_ Joules) are 'correct', but I often see non-engineering specifications for force in grams or kg, so they may rate torque in kg-m. Perhaps they should adopt the 'Bush' as a unit of torque, in honor of GWB's propensity to torque off Europeans. -- http://www.wescottdesign.com
Reply by ●March 16, 20092009-03-16
clay@claysturner.com wrote:> [significant SNIP ;] > p.s. I once heard that Nevada had the highest rainfall per capita of > all of the states. >So if a tree falls in an unpopulated forest ???? <lol>
Reply by ●March 16, 20092009-03-16
Tim Wescott wrote:> On Mon, 16 Mar 2009 06:34:08 +0000, glen herrmannsfeldt wrote: > > >>If you put: horsepower per foot pound >> >>into Google you get 550 Hertz. >> >>I was reading about cars, where the engines are described with the power >>output and torque in horsepower and pound-feet, respectively, and >>realized that the ratio was either time or frequency. > > > If you express torque in N-m and power in Watts, then the difference is > in radians/second. Since a radian is a dimensionless* unit it's easy to > leave it out. > > >>Interesting, though, that the numeric values for horsepower and torque >>are similar, > > > I think that's just happenstance having to do with how fast a car engine > will spin, and the customary units** for rating engines. Try the > computation on a modern racing model airplane engine, where peak power > occurs in excess of 20000 RPM, and your numbers might diverge a bit more. > > >>but car engines typically don't run at 33000 RPM. >> > > > _Your_ car engine doesn't run that fast -- I wonder how fast the Formula > 1 racers are going these days? > > * I always get caught up on that -- are radians and napirsDid you not mean "nepers" ???? WVBR studio engineering once had sign saying "DANGER 1000 NEPERS" OK so I show my age ;/
Reply by ●March 16, 20092009-03-16
On Mon, 16 Mar 2009 11:44:11 -0500, Tim Wescott <tim@seemywebsite.com> wrote:>On Mon, 16 Mar 2009 06:34:08 +0000, glen herrmannsfeldt wrote: > >> If you put: horsepower per foot pound >> >> into Google you get 550 Hertz. >> >> I was reading about cars, where the engines are described with the power >> output and torque in horsepower and pound-feet, respectively, and >> realized that the ratio was either time or frequency. > >If you express torque in N-m and power in Watts, then the difference is >in radians/second. Since a radian is a dimensionless* unit it's easy to >leave it out.I have this discussion with my other gearhead friends occassionally, who often tend to think of optimizing torque and hp separately. Sometimes it takes some effort to get the idea across. It's also confusing, sometimes, since dynomometers typically measure torque and then convert to power by metering the engine rpm. One of the major dyno manufacturers has some sort of twist in their software that if the engine isn't metered it won't spit out torque, but hp, e.g., if somebody dynos a diesel. You need to instrument the rpm to get it to output torque numbers. That's really confused a lot of people (including me, I still don't know how they do that, but I suspect they just make a power estimate using the drum rotation rate rather than the engine rpm).>> Interesting, though, that the numeric values for horsepower and torque >> are similar, > >I think that's just happenstance having to do with how fast a car engine >will spin, and the customary units** for rating engines. Try the >computation on a modern racing model airplane engine, where peak power >occurs in excess of 20000 RPM, and your numbers might diverge a bit more.For torque in ft-lbs and power in hp the general formula is hp = (torque in ft-lbs)*rpm/5252 so the torque and power curves always cross at 5252 rpm for ft-lbs and hp. So, at least for ft-lbs and hp, the numeric values are always in the same ballpark. My race car gets classed largely by weight-to-power ratio, with peak hp used for power. I have an advantage in that my car makes a lot of torque over a broad rpm range (203 ft-lbs peak) but doesn't breath as efficiently at higher rpm so peak power is only 188hp. Since the peak hp number is what matters, I'm a happy camper. ;)>> but car engines typically don't run at 33000 RPM. > >_Your_ car engine doesn't run that fast -- I wonder how fast the Formula >1 racers are going these days?They've been limited by the rules to 19000 rpm for a while, that's going down to 18000 rpm this year. The rpm limit is to help control research and development costs for the teams since the well-funded teams can otherwise get a substantial advantage.>* I always get caught up on that -- are radians and napirs really units, >or are they just a reflection that some things really don't need units?They're a reflection that some measurable things don't have dimensions.>** In the US. The international ads and reviews I've seen rate power in >kW; I'm not sure what they rate torque as -- Newton-meters (_not_ Joules) >are 'correct', but I often see non-engineering specifications for force >in grams or kg, so they may rate torque in kg-m. Perhaps they should >adopt the 'Bush' as a unit of torque, in honor of GWB's propensity to >torque off Europeans.ha Eric Jacobsen Minister of Algorithms Abineau Communications http://www.ericjacobsen.org Blog: http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php
Reply by ●March 16, 20092009-03-16
clay@claysturner.com wrote:> On Mar 16, 2:34 am, glen herrmannsfeldt <g...@ugcs.caltech.edu> wrote: >> If you put: horsepower per foot pound >> >> into Google you get 550 Hertz. >> >> I was reading about cars, where the engines are described >> with the power output and torque in horsepower and >> pound-feet, respectively, and realized that the ratio >> was either time or frequency. >> >> Interesting, though, that the numeric values for >> horsepower and torque are similar, but car >> engines typically don't run at 33000 RPM. >> >> -- glen > > A common question I pose to my students is what is the difference > between energy and torque where they can have units of Newton-meters? > > I recall seeing something recently about "silly" units such as the > horsepower per foot pound. It had quite a few examples. One unit I > like is the furlong per fortnight which is amazingly close to 1 cm per > minute.Doesn't microlight-year/century usually get quoted with that?> p.s. I once heard that Nevada had the highest rainfall per capita of > all of the states.Statistics! Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●March 16, 20092009-03-16
Tim Wescott wrote:> On Mon, 16 Mar 2009 06:34:08 +0000, glen herrmannsfeldt wrote: > >> If you put: horsepower per foot pound >> >> into Google you get 550 Hertz. >> >> I was reading about cars, where the engines are described with the power >> output and torque in horsepower and pound-feet, respectively, and >> realized that the ratio was either time or frequency. > > If you express torque in N-m and power in Watts, then the difference is > in radians/second. Since a radian is a dimensionless* unit it's easy to > leave it out. > >> Interesting, though, that the numeric values for horsepower and torque >> are similar, > > I think that's just happenstance having to do with how fast a car engine > will spin, and the customary units** for rating engines. Try the > computation on a modern racing model airplane engine, where peak power > occurs in excess of 20000 RPM, and your numbers might diverge a bit more. > >> but car engines typically don't run at 33000 RPM. >> > > _Your_ car engine doesn't run that fast -- I wonder how fast the Formula > 1 racers are going these days? > > * I always get caught up on that -- are radians and napirs really units, > or are they just a reflection that some things really don't need units? > > ** In the US. The international ads and reviews I've seen rate power in > kW; I'm not sure what they rate torque as -- Newton-meters (_not_ Joules) > are 'correct', but I often see non-engineering specifications for force > in grams or kg, so they may rate torque in kg-m. Perhaps they should > adopt the 'Bush' as a unit of torque, in honor of GWB's propensity to > torque off Europeans.Going electrical, a volt-second is both the dimension of a voltage impulse, and a unit of magnetic flux. The unit if inductance is the volt-second/ampere. That's too big a mouthfull, so we call it a Henry. Now a coulomb is an ampere-second, and a Farad is a coulomb/volt, so of course the product LC is /sec^2. Work it out! Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●March 16, 20092009-03-16
Tim Wescott <tim@seemywebsite.com> wrote: (I wrote)>> If you put: horsepower per foot pound >> into Google you get 550 Hertz.>> I was reading about cars, where the engines are described with the power >> output and torque in horsepower and pound-feet, respectively, and >> realized that the ratio was either time or frequency.> If you express torque in N-m and power in Watts, then the difference is > in radians/second. Since a radian is a dimensionless* unit it's easy to > leave it out.Yes. If I actually do torque problems I always do the angular velocity in radians/second so that wouldn't come up. In this case, google came up with Hertz (not 1/seconds) and I didn't immediately think about radians. That would seem to me to be a mistake in google. (Hertz implies cycles per second, not anything else per second.)>> Interesting, though, that the numeric values for horsepower and torque >> are similar,> I think that's just happenstance having to do with how fast a car engine > will spin, and the customary units** for rating engines. Try the > computation on a modern racing model airplane engine, where peak power > occurs in excess of 20000 RPM, and your numbers might diverge a bit more.Yes, I presume the units were chosen as they give conveniently sized numbers.>> but car engines typically don't run at 33000 RPM.> _Your_ car engine doesn't run that fast -- I wonder how fast > the Formula 1 racers are going these days?There is another complication that I didn't get into. The HP and torque peaks are often at different RPMs. With graphs, that would be easy to see but in most cases just the peak value is given. -- glen
