Kinda long but it's the shortest explanation I can give. . .
> Two means to the same end. I just can't remember where the 1hp
> = 550 ft-lbf/s come from. Thought you would.:-)
I remember. The 550 is actually 552.59. 1 horsepower is 746
watts, 1 lb. ft. is 1.35 n*m. 746 / 1.35 = 552.59.
> > Q2: YES. If you get the same torque at higher RPM, then you can
> > use shorter gears at the same vehicle speed ('cuz the engine revs
> higher) and that means you get a bigger gear ratio multiplier
and
> > more torque at the wheel. Basically, when you can rev higher you
> > can use shorter gears without sacrificing vehicle speed. The
> > overall push of the car is the product of the gear ratio with the
> > crankshaft torque, so both are equally important for
> > acceleration.
> >
>
> NO, as you worded your blanket statement. Of course, once you take gear
> ratios, wheel diameter, drivetrain losses, etc. into account it is possible to
> *tune* them for optimal acceleration with a given powerplant. Stating that the
> same torque at a higher RPM produces more acceleration is false (all else
> remaining equal).
This is WRONG (though it is a common misperception). RPM is not
simply an arbitrary factor that has to be optimized in gearing
(transmission, wheel size, etc.). Getting the same torque at
higher RPM means you're producing more power and you can do more
work in less time. That means you can get more acceleration.
Example: Let's suppose we already have a 3,000 lb. car and we are
deciding on what kind of engine and transmission to put into it.
Here are our choices:
Engine A: 100 ft. lbs. of torque flat from 1000 to 3000 RPM,
redline at 4000 RPM.
Engine B: 100 ft. lbs. of torque flat from 2000 to 6000 RPM,
redline at 8000 RPM.
Engine B uses gear ratios that are twice as short as Engine A, so
both have the same shift points at the same speeds -- say, 1st to
30 mph, 2nd to 60 mph, etc. Regardless of which engine you pick,
your car will have the same shift points and both engines have a
flat torque curve.
Which do you pick? If you pick A, I'll pick B and whup you in a
drag race. At any given vehicle speed, Engine B can get twice as
much torque to the wheel of the car. Why? Because it revs twice
as high, it uses gear ratios twice as short, so it has twice the
torque at the wheel of the car. At every speed B has twice the
acceleration of A. That's because torque at the wheel of the car
is equal to crankshaft torque multiplied by gear ratio. The
crankshaft torque is the same but the gear ratio of B is twice as
big. That gives you twice the torque at the wheel of the car.
Now you might say, "I'll just give engine A shorter gears". That
won't work because you'll hit your shift points sooner. Let's say
you give Engine A the same gear ratios as B. Now we have the same
acceleration in 1st gear. But you have to shift to 2nd gear at 15
MPH, at which point you lose half your acceleration. From 15 to
30 mph I pull ahead rapidly because I'm still in 1st gear
accelerating at twice your rate. I get to 30 before you do and I
hit 2nd gear. Later on, you get to 30 mph and you have to hit 3rd
gear now. By now you're a shrinking dot in my rear view mirror.
Of course we can make this a lot simpler by thinking about
conservation of energy. Both cars have the same mass, so they
have the same kinetic energy at any given speed. The engine that
can produce that amount of energy in less time can accelerate the
car to that speed more quickly. Engine A has 57 horsepower and
Engine B has 114 horsepower. End of story.
> As you well know, real world acceleration depends only on
> how much torque is being transferred at the wheel to the ground.
True. The same crankshaft torque at higher RPM enables you to get
more torque to the wheel at the ground.
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