By RICHARD RUSSELL
Thursday, July 28, 2005 Page G18
One of the least understood measurements or statistics involving the automobile is that of torque versus horsepower. Both are important, but for different reasons.
Think of torque as a measured amount of work, and horsepower as the ability to do that work over a period of time.
Let's go to the 18th century and James Watt.
After inventing the steam engine, he figured it could do the work of a number of horses, the common means of work at the time. But he reasoned nobody would pay the newfangled gadget any attention unless they could see the advantages.
He saw a particular market for his new device in helping pump water out of mines. The common method of doing so at the time was to harness a horse to a lever connected to what was essentially a crankshaft. As the horse walked in a circular path it turned the pump.
Watt could see the benefits of proving his engine could do the work of several horses. He conducted some simple tests on a horse operating a gear-driven mine pump and found it could lift a 550-pound weight one foot in one second or 32,580 pound-feet per minute. He rounded this off to 33,000 and published that as the equivalent to the power of one horse - or one horsepower. Another way of putting it is a single pound-foot of torque.
A little further ciphering to calculate the rate of movement relative to producing a single pound-foot of torque arrived at 5,252 revolutions per minute. This gives us a formula we can use to calculate horsepower from measured torque.
In an engine, pistons going up and down cause a crankshaft to go around. To measure the output of an engine we need to calculate the "twisting" force produced by that rotating crankshaft.
Thanks to Watt, we know that a single pound-foot of torque generated at 5,252 rpm is equivalent to one horsepower. By converting the twisting force of torque into working units, we arrive at horsepower. Horsepower equals the maximum amount of measured torque, multiplied by the rpm at which it occurs, divided by 5,525.
Okay, so what has that to do with our automobile? Everything.
Torque or that twisting force is the push in the back when you step on the gas. Torque occurs from idle and increases with engine speed, peaking at fewer engine revolutions than horsepower. A vehicle will accelerate hardest at its torque peak in any gear and less so after engine revolutions exceed that number. Torque is what you feel. Horsepower is an esoteric measurement used by ad agencies.
Torque rules. Or does it?
In contrast to the torque curve, horsepower rises rapidly with engine rpm and continues to climb well past the torque peak. So we have and appreciate torque for its contribution in getting our vehicle under way from rest -- the more the merrier.
And, through the use of a transmission, that torque is effectively multiplied, to a decreasing amount with each succeeding gear, allowing us to accelerate faster or tow a heavier load. But, while torque is relevant at low speeds, horsepower tells us how well the engine performs at high speed. Maximum torque generally occurs in the 2,000-3,500 rpm range and maximum horsepower often in the 5,000-6,000 rpm range.
As engine revs rise and torque falls off, horsepower builds and takes over. Engineers can tune these two curves through a variety of systems including intake and exhaust systems and valve timing.
Generally, forcing more air into the engine through a supercharger or turbocharger brings more torque at lower rpm, while designing an engine capable of very high revolutions allows increased peak horsepower.
Think of it this way. If you want to pull a heavy load or maximize acceleration of a heavy vehicle from rest, you want lots of torque.
If it's speed you're after, look to horsepower which is really a calculation of the engine's ability to produce work at higher rpm.
rrussell@globeandmail.ca
SOURCE - The Globe and Mail, Canada's National Newspaper
Thursday, July 28, 2005 Page G18
One of the least understood measurements or statistics involving the automobile is that of torque versus horsepower. Both are important, but for different reasons.
Think of torque as a measured amount of work, and horsepower as the ability to do that work over a period of time.
Let's go to the 18th century and James Watt.
After inventing the steam engine, he figured it could do the work of a number of horses, the common means of work at the time. But he reasoned nobody would pay the newfangled gadget any attention unless they could see the advantages.
He saw a particular market for his new device in helping pump water out of mines. The common method of doing so at the time was to harness a horse to a lever connected to what was essentially a crankshaft. As the horse walked in a circular path it turned the pump.
Watt could see the benefits of proving his engine could do the work of several horses. He conducted some simple tests on a horse operating a gear-driven mine pump and found it could lift a 550-pound weight one foot in one second or 32,580 pound-feet per minute. He rounded this off to 33,000 and published that as the equivalent to the power of one horse - or one horsepower. Another way of putting it is a single pound-foot of torque.
A little further ciphering to calculate the rate of movement relative to producing a single pound-foot of torque arrived at 5,252 revolutions per minute. This gives us a formula we can use to calculate horsepower from measured torque.
In an engine, pistons going up and down cause a crankshaft to go around. To measure the output of an engine we need to calculate the "twisting" force produced by that rotating crankshaft.
Thanks to Watt, we know that a single pound-foot of torque generated at 5,252 rpm is equivalent to one horsepower. By converting the twisting force of torque into working units, we arrive at horsepower. Horsepower equals the maximum amount of measured torque, multiplied by the rpm at which it occurs, divided by 5,525.
Okay, so what has that to do with our automobile? Everything.
Torque or that twisting force is the push in the back when you step on the gas. Torque occurs from idle and increases with engine speed, peaking at fewer engine revolutions than horsepower. A vehicle will accelerate hardest at its torque peak in any gear and less so after engine revolutions exceed that number. Torque is what you feel. Horsepower is an esoteric measurement used by ad agencies.
Torque rules. Or does it?
In contrast to the torque curve, horsepower rises rapidly with engine rpm and continues to climb well past the torque peak. So we have and appreciate torque for its contribution in getting our vehicle under way from rest -- the more the merrier.
And, through the use of a transmission, that torque is effectively multiplied, to a decreasing amount with each succeeding gear, allowing us to accelerate faster or tow a heavier load. But, while torque is relevant at low speeds, horsepower tells us how well the engine performs at high speed. Maximum torque generally occurs in the 2,000-3,500 rpm range and maximum horsepower often in the 5,000-6,000 rpm range.
As engine revs rise and torque falls off, horsepower builds and takes over. Engineers can tune these two curves through a variety of systems including intake and exhaust systems and valve timing.
Generally, forcing more air into the engine through a supercharger or turbocharger brings more torque at lower rpm, while designing an engine capable of very high revolutions allows increased peak horsepower.
Think of it this way. If you want to pull a heavy load or maximize acceleration of a heavy vehicle from rest, you want lots of torque.
If it's speed you're after, look to horsepower which is really a calculation of the engine's ability to produce work at higher rpm.
rrussell@globeandmail.ca
SOURCE - The Globe and Mail, Canada's National Newspaper
