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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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