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Torque and Cars
Name: Bjørn Einar B.
Status: Student
Age: 20s
Location: N/A
Country: N/A
Date: N/A
Question:
Torque measured from a car engine, what does it tell me?
I tried to figure out the difference from my physics book, but I'm not
able to relate the theory from the book to the real world. I know that
torque is N*m (not Joule) and is a measurement of how much
force that is used for rotation, and that power is J/s.
As far as I understand, one can calculate the acceleration of
the car by knowing the mass of the car and the power of the engine. Why
should one care about the torque? I've asked salespersons and so on, and
they brag about the high torque in the turbo-diesel engines. I believe I
can feel the difference in the torque from diesel-engines to ordinary
gas-engines, the diesel engines feel more powerful, but I can not explain
why.
Replies:
As you note, torque is the twisting force the engine applies to the
crankshaft. In SI units, the power of the engine is measured as the torque
times the rotational speed. In US customary units, we measure engine output
in horsepower and torque in foot-pounds. A horsepower equals 550
foot-pounds per second. Two engines with very different torque
characteristics can have exactly the same horsepower since one horsepower
can be generated by moving one pound 550 feet or moving 550 pounds one foot,
as long as it is accomplished in one second. The high torque engine would
be rotating proportionally more slowly than the low torque engine at the
same power output, but twisting the crankshaft harder.
Theoretically speaking, this should not make any difference in maximum
acceleration since the gearing of the transmission can be designed to match
the torque characteristics of any engine. Practically speaking, however, a
high torque engine will have faster response to the accelerator pedal from a
lower RPM. A low torque engine may require the driver to shift into a lower
gear to attain higher RPM's before maximal acceleration can occur. So, in
real world driving between two engines of the same power, the higher torque
engine will respond more rapidly to the accelerator. Keep in mind, too,
that the manufacturer's ratings only give the peak torque and power figures.
Diesels tend to produce pretty much the same torque at all RPMs and will
give good throttle response.
But, small displacement sports cars like the
Honda S2000 require the engine to be revved to very high speeds to produce
much power and tend to produce proportionally lower torque and power at low
RPMs. To quote a friend of mine who test drove an S2000, you have to drive
it like you stole it to make it go fast. Of course, for sports car
enthusiasts, this is considered fun. Unless you're looking for a sports
car, I'd recommend that you not worry about the manufacturer's ratings and
buy what feels good to you during the test drive.
Andy Johnson
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Update: June 2012
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