Torque and horsepower

[hobz]

Have a look at this page: http://www.howstuffworks.com/horsepower1.htm

 

Just before the "Torque" infobox, it states: "[...] convert torque to horsepower you simply multiply torque by rpm/5,252."

 

After the infobox, there is a curve rpm/(hp & torque) which clearly doesn't demonstrate a linear relationship.

I am guessing that the torque goes down (as the rpm goes up) due to the fuel mixture not exploding fast enough as the piston already is moving down due to flywheel inertia. Is this correct?

 

If this is so, then the increase in hp as rpm goes up is less efficient that when the torque is high, right?

[J.C.MacSwell]

Have a look at this page: http://www.howstuffworks.com/horsepower1.htm

 

Just before the "Torque" infobox, it states: "[...] convert torque to horsepower you simply multiply torque by rpm/5,252."

 

After the infobox, there is a curve rpm/(hp & torque) which clearly doesn't demonstrate a linear relationship.

I am guessing that the torque goes down (as the rpm goes up) due to the fuel mixture not exploding fast enough as the piston already is moving down due to flywheel inertia. Is this correct?

 

If this is so, then the increase in hp as rpm goes up is less efficient that when the torque is high, right?

 

 

This is true at any given time, or point on either curve. The power is the torque times the angular velocity.

 

The graph shows the horsepower the engine is capable of at a given rpm, and separately the torque the engine is under at the given rpm and horsepower, but with no numbers to indicate the actual torque (the scale on the right indicates horsepower only).

[hobz]

I see. So why does the torque decrease?

[J.C.MacSwell]

I see. So why does the torque decrease?

 

 

 

At any given rpm those are the maximums for torque or power.

 

At higher rpm the engine cannot continue to maintain greater torque. Only reducing the load will allow the engine to achieve higher rpm, so the torque must fall off. The power output continues to rise due to the greater rpm, until that reaches a maximum as well. At highest possible rpm (not shown, it's off the graph) there is no torque or power output at all. (though you could get some instantaneously from the momentum of the engine)

[hobz]

At higher rpm the engine cannot continue to maintain greater torque.

 

Why is that?

[J.C.MacSwell]

Why is that?

 

There is friction, and other mechanical efficiency limits, but also the pistons are "running away from" the forces that are powering them. It's much harder to push with force against something that won't resist, and it's the force on the pistons that produce the torque.

[hobz]

That seems to agree with my speculation on the piston/flywheel intertia.