TORQUE
Generally speaking, the greater the engine capacity the higher its torque. A high torque allows faster acceleration: the feeling of being pushed back in one's seat is increased. A higher torque is also able to move a greater weight. This is why diesel cars are often used to tow caravans, trailers or other heavy loads.
With diesel engines, the torque is obtained at a low engine speed (between 1,300 and 2,400 rpm according to the car, with modern engines getting close to 1,300 rpm). The force transmitted to the wheels is then at its greatest. A high torque gives good pick-up (there is no need to change down) and good acceleration.
The formula for torque is a force applied at a given distance.
The torque can be compared to a weight-lifter: he raises an enormous weight above his head. The greater the weight lifted, the stronger the weight-lifter.

TORQUE=FORCExDISTANCE

In this case, the force is expressed in Newtons, the distance in metres.

As the torque curve peaks at low revs, it drops off quickly in the high engine revs part of the graph. Another parameter then takes over...

POWER
Power measures work done in a given time. It therefore depends not only on the torque, but also on the speed of rotation of the engine. The faster the engine rotates, the greater the power... until it reached a limit which, for diesel engines begins at between 4,000 and 4,500 rpm. The power of the engine influences the speed of the vehicle.

The power is obtained by multiplying the torque by the speed of rotation of the engine.
Power can be compared by looking at two athletes: the first raises 120 kg in 1 minute. The second raises 120 kg in 30 seconds. In one minute, the second athlete would have raised 240 kg, and is therefore more powerful.



POWER=TORQUE/TIME
or
POWER=FORCExSPEED

Power is expressed in Watts (W) or DIN horsepower (hp). Torque is in Newton meters and the speed in radians per second.
For information, 1 DIN hp = 735.5 W


ENGINE RPM
The engine is pleasanter and more flexible in that part of its curve that lies between the zone of maximum torque and the zone of maximum power. Outside this zone, the engine is not efficient. The best zone is therefore situated between 1,500 and 4,000 rpm: it is within this range that the engine consumes the least fuel.

At very low rpm, the engine consumes very little but cannot produce the required force: it vibrates and grunts and, even when the accelerator pedal is pushed all the way down, the rev counter doesn't move. In general, this is because there is insufficient torque or power.
At high rpm, the engine consumes a lot more fuel than it really needs and wears more quickly: it is necessary to change up a gear or slow down (in general at over 3,500 rpm in 5th gear, diesel passenger vehicles running at over 140 km/h).
The zone between 3,500 and 4,000 rpm should be considered on a diesel as spare capacity (e.g.: when overtaking).

The power of diesel engines increases up to 4,200 rpm, then begins to fall off. Their torque is no longer very advantageous above 3,500 rpm.
The power response of petrol engines is not only more linear, but particularly up to 5,500 rpm (the curves aren't quite right), which is all the same 1000 rpm more than diesel engines. The torque is very low below 2,000 rpm, unlike the diesel engine, which has better pick-up at low rpm ...

When running-in a diesel engine, it is recommended to remain within the 1,800 to 3,000 rpm range: as modern engines are produced with an accuracy of 100th of a micron, they no longer require to be run-in, but this precaution allows the different moving parts to "find" their place.
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Source: extraits du site : http://www.lewebdesconducteurs.com/
Auteur: golfy@free.fr