Originally Posted By: Virtus_Probi
Shannow nailed it early on...
The equation I found is
HP = (Torque)X(RPM)/5252
for English units. This basically jibes with Srt20's numbers (I think his first result should be 152 HP, close enough).
But, basically horsepower is proportional to torque and RPM.
You see lots of diesels with monster peak torques but more modest peak HPs because their torque peaks early in the RPM band...I'm sure there's a good explanation for this, but I don't know what it is.
In engineering, power is defined as the rate at which work is done.
In the English system, Horsepower = torque (in ft*lbs) x rpm / 5252. Horsepower is a unit of power that was created by James Watt in the early 1800's to advertise how much work his steam engines could do in a given amount of time. He was building engines for pumping water out of coal mines, and the pumps were originally powered by horses pushing a turnstile. He measured the amount of force that a horse could exert on the turnstile, and how fast he was going, and got the values of 181 pounds force, 180 feet per minute speed. Multiplying these, Watt got 32580 ft*lb/min, which he rounded to 33,000 and called it a Horsepower.
In order to get Horsepower from ft*lbs torque and revs/minute engine speed:
Say you have an engine running at 2000 rpm while producing 400 ft*lbs of torque.
2000 rev/min x 2pi radians/rev x 400 ft*lbs x 1 HP / 33000 ft*lbs/min = 152 HP
Note that the constant of 5252 is obtained by dividing 33000 by 2pi.
Basically, diesels make a lot of torque at low engine speed because there is no limit to the cylinder pressure that can be generated by the combustion process, unlike spark-ignition engines, which are limited by detonation. Diesels don't generally make a lot of power at crankshaft speeds beyond 4000 rpm because the relatively slow combustion process limits the amount of fuel that can be burned while the piston descends. I look at diesel engine performance as the inverse of gasoline engine performance. You can make the same amount of power, but you do it by producing high torque at low speed. If you need to make more power, increase the torque output. The negative byproduct of this design ethos is that diesel engines end up being heavy for the amount of power produced because they must be built to withstand high cylinder pressures.
Does this all make sense, or have I just succeeded in shedding a lot of dark on the subject?
Shannow nailed it early on...
The equation I found is
HP = (Torque)X(RPM)/5252
for English units. This basically jibes with Srt20's numbers (I think his first result should be 152 HP, close enough).
But, basically horsepower is proportional to torque and RPM.
You see lots of diesels with monster peak torques but more modest peak HPs because their torque peaks early in the RPM band...I'm sure there's a good explanation for this, but I don't know what it is.
In engineering, power is defined as the rate at which work is done.
In the English system, Horsepower = torque (in ft*lbs) x rpm / 5252. Horsepower is a unit of power that was created by James Watt in the early 1800's to advertise how much work his steam engines could do in a given amount of time. He was building engines for pumping water out of coal mines, and the pumps were originally powered by horses pushing a turnstile. He measured the amount of force that a horse could exert on the turnstile, and how fast he was going, and got the values of 181 pounds force, 180 feet per minute speed. Multiplying these, Watt got 32580 ft*lb/min, which he rounded to 33,000 and called it a Horsepower.
In order to get Horsepower from ft*lbs torque and revs/minute engine speed:
Say you have an engine running at 2000 rpm while producing 400 ft*lbs of torque.
2000 rev/min x 2pi radians/rev x 400 ft*lbs x 1 HP / 33000 ft*lbs/min = 152 HP
Note that the constant of 5252 is obtained by dividing 33000 by 2pi.
Basically, diesels make a lot of torque at low engine speed because there is no limit to the cylinder pressure that can be generated by the combustion process, unlike spark-ignition engines, which are limited by detonation. Diesels don't generally make a lot of power at crankshaft speeds beyond 4000 rpm because the relatively slow combustion process limits the amount of fuel that can be burned while the piston descends. I look at diesel engine performance as the inverse of gasoline engine performance. You can make the same amount of power, but you do it by producing high torque at low speed. If you need to make more power, increase the torque output. The negative byproduct of this design ethos is that diesel engines end up being heavy for the amount of power produced because they must be built to withstand high cylinder pressures.
Does this all make sense, or have I just succeeded in shedding a lot of dark on the subject?
