[Vwdiesel] torque vs. horsepower (was: something worth trading the A4 in for)

[Roger Brown]

David Schwarze wrote:
> What you say below is simply not true.  Horsepower DOES move things. 
> More HP will get you down the road more quickly, regardless of torque.
> 
> The units of HP and Torque tell the story.  Power (hp) is a quantity 
> that you can use along with the weight of the vehicle, friction, 
> aerodynamic drag, etc. to predict how quickly a car will accelerate.  It 
> is a measure of how much work is being done.  Accelerating a car is work.
> 
> Torque is a measurement of rotational force which CANNOT be used to 
> predict how quickly a car will accelerate (why not, you say?  Because 
> "it depends" on the RPM).
> 
> For a real world example, take a sport bike.  That fits your definition 
> of "massive amounts of hp and little to no torque".  Ever race one of 
> those at a stoplight in your diesel VW?  I'm sure with the massive 
> torque of the diesel engine that poor sport bike didn't stand a chance, 
> right?

Torque, via the gearing in the transaxle, it what turns the wheels/tires to push the car 
forward.  More torque, whether in raw form out of the engine or via reduction gearing in 
the transmission equals more force transferred by the tires to the road and more force 
acting on a given vehicle mass (or weight) results in an equal and opposite force (per 
Isaac Newton) called acceleration.  The units of torque are the same as the units of work, 
force over a distance.  And if you take things to finer and finer increments of time (in 
mathematical terms), the RPM factor goes away, acceleration is defined as the derivative 
of velocity with respect to time (dv/dt) and is simply reduces to the mass and the force 
acting upon it.

Power (or horsepower) is in units of work per unit of time.  That will tell your how much 
work an engine can produce over a given period time.  Since engine HP is a function of 
torque and RPM, you can vary both of those factors via gearing, tire size, etc.  And since 
HP is a combination of these two values (torque and RPM) and since in typical internal 
combustion engines torque varies with RPM, it is hard to separate the two factors.

Imagine one engine with a very peaky torque curve with tuned intakes and exhaust made for 
maximum power/torque output at some high RPM.  And then an engine with the same overall HP 
but one with a fairly flat torque vs. RPM curve.  If you use both engines to accelerate a 
vehicle, (assuming fixed gearing) you need to integrate the torque over the range of RPMs 
from beginning to end to get the overall time to speed.  Think of taking the area under 
the HP vs. RPM curve.  A broad flatter HP curve has more area under it than one that is 
shaped like a triangle.  A flatter curve is the result of a torque that builds up at a low 
RPM and holds fairly constant at the RPM rises.  A steeper curve is the result of an 
engine where the torque keeps rising with RPM.

And yes, a 50 HP gas engine in a motorcycle will out accelerate a 50HP diesel engine in an 
automobile.  Why?  Weight.  Put 50 HP behind a 300 lb. bike and put in gearing to drop the 
high revving, low torque engine output down to a usable value for the load, that low 
torque times the gear reduction results in a large torque at the wheel and that torque 
times the lower mass equals higher acceleration.  Put 50 HP behind a 2000 lb. Rabbit and 
guess what, it is slower to accelerate than a 6 times lighter motorcycle.

So it all really boils down to the fact that torque is the fundamental unit and is what an 
engine produces.  RPMs are just the result of what the engine is connected to and what 
that load does in response to the applied torque.  HP is just the derived product of the 
supplied torque and the resultant RPM.

-- 

   Roger