HP & TQ

[KF6GUB]

Tons of people talk of horsepower (HP) as if it were God. HP is a computed number based on torque and rpm with the constant 5252 thrown in, right? Even a dynometer does not directly measure HP, correct? A dynamometer measures TQ and RPM, right? Especially for the diesel, HP seems to do very little, whereas torque says a lot. So, what's the big deal with HP? Am I missing something? Thx. Jim

 

[SRadke]

Oh... . dang..... here it comes... ..... Scott hunkers down ready for the :-{}

 

[KF6GUB]

Wasn't trying to start a "thread war. " I really want to know. Because HP is a product of the horse-and-buggy days, how is it helpful in today's discussions and comparisons of the force that engines put out? Thx. Jim

 

[SRadke]

I'm not blaming you but there are two distinct points of view on the topic. The gassers have HP and little torque, they like that number. We're out numberd so we use it even though it has less meaning.



-Scott

 

[RustyJC]

Two 350 BHP engines can do the same amount of work per minute. If you think of the low-torque, high-revving gasser as a quick little fellow who can lift 100 16-pound bowling balls per minute onto a 2 foot high table, and think of the high-torque, low-revving diesel as a slower moving, but stronger weightlifter who can lift 10 160-pound boulders per minute onto the same 2 foot high table, then both of them have done 3200 ft-lbs of work in that minute (100 x 16 lbs x 2 feet, and 10 x 160 lbs x 2 feet), but they went about it in completely different ways.



(For reference, 1 BHP = 33,000 ft-lbs per minute, so both expended a little less than 1/10 BHP. )



(For further reference, there are 2 x pi radians per revolution. If you divide 33,000 ft-lbs/min by 2 x pi {approximately 6. 283}, you'll see where the 5252 comes from in the BHP = Q x N / 5252 equation. The 2 x pi function is used to convert rotating work to linear work. )



Edit: In my analogy above, the number of items lifted onto the table per minute equates to RPM; the weight of each item equates to torque insofar as the BHP = Q x N / 5252 equation is concerned.



Rusty

 

[TWilkening]

If it is a combination of torque and rpm, how does horsepower increase when torque sharply decreases after peak. I was look at a hp/tq chart on a 12 valve. Torque takes a dive after peak, but horsepower steadily increases?

 

[RustyJC]

As long as RPM is increasing faster than torque is decreasing, BHP will increase. When the decrease in torque matches the increase in RPM, BHP will peak. When the decrease in torque exceeds the increase in RPM, BHP will decrease.



Rusty

 

[SBall]

Rusty,

Your post (#5) says it all. I love the analogy using the bowling balls as it should clarify the distinction of torque vs HP in performing work. You also follow it up with the equations too. I would urge anyone who is still confused over the issues of torque vs hp, to use your equations and plot some speed-torque curves as published by the engine manufacturers. I'll add too that the guy lifting all those small bowling balls might tire earlier than the heavy lifter, (duty-cycle).

Steve

 

[nickleinonen]

and with that 5252 number, any engine will produce equal torque and HP at 5252rpm... below that, torque will be more than hp, above that hp will be more than torque [if i am remembering things right... ]



so at work my newer GE engines are making 4500hp at 1042rpm, so thats makes 22681. 38 ft/lbs... and the emd engines making 4300hp at 950rpm make 23772. 21 ft/lbs... not bad... emd engine is a 2 cycle too...