Speed, tractive force and rpm

[Ris91]

hi
when we see torque vs rpm graphs then the rpm there is engine rpm but what about torque..........is it the torque at wheels or torque at engine flywheel?????

Yesterday i saw graphs relating tractive force and speed..............so this tractive force is on wheels and speed is of the vehicle so we can say rpm at wheels....................so how can tractive force at wheels reduce with increase in rpm of wheels

[MagicRat]

Most power and torque measurements for any car model are taken at the flywheel, with all the equipment the engine would normally have if it were in the car. So, the engine is on a dynomometer, and not in the car. But it has the full exhaust system, air intakse system and all accessories attached, to duplicate it's 'in-car' environment.

This is mostly used by manufacturers or testing organizations, when reviewing or reporting on a car design.

However, it's pretty unwieldy to pull an engine out of every individual car that needs testing. Therefore, some dynos are designed for an actual car to be placed on rollers, so power is measured at the wheels. However, it is reported as engine torque, not wheel torque.

This testing is inaccurate. The efficiency of the transmission can affect results, usually by reducing power measurements approx. 25-30%, compared with flywheel testing.

This makes car-to-car comparisons difficult. However, it is good for testing a particular car, to measure changes made due to mechanical changes and improvements.

As for tractive force vs. rpm..... yes, given equal torque, tractive power decreases as wheel speed increases. This is simple physics. Consider how hard your car accelerates in first gear. Now, compare that rate of acceleration in top gear. See the difference? It's essentially the same principle.

In the real world, it's not always so simple. Often, an increase in wheel speed allows an engine to rev higher and become more volumetrically efficient, so torque and/or hp increases, this increasing tractive force as rpm increases... up to a point.

[Ris91]

u said for given equal torque tractive power decreases with increase in speed but .........................if torque is constant why would speed increase.........increase in engine torque increases wheel speed................how will u explain it.....?

secondly, if in case of dyno torque is measured at engine ....... then engine torque and engine speed(rpm) should vary linearly,i.e. they should be directly proportional because consider a fan ..... i give it a torque say t1...........it moves with a certain speed in rpm say n1........now i increase the torque so as obvious it's rpm should also increase so why engines have such inverse curves bw torque and rpm