Generally speaking, the larger the engine capacity, the greater the torque. A high torque enables greater acceleration: the feeling of thrust pushing you back in your seat is greater. What’s more, a high torque enables you to move much greater weights, that is why diesel vehicles are often used to pull caravans, trailers or heavy loads.
With diesel engines, torque is reached at low engine speed (between 1300 and 2400 rpm depending on the vehicle, while modern engines are closer to 1300 rpm). The power transmitted to the wheels is at its greatest at this point. A high torque leads to good acceleration and pick-up (no need to drop a gear).
The torque formula is a force in relation to a distance.
You could compare torque with a weightlifter: it lifts an enormous weight up to its head level. The greater the weight lifted, the more force the weightlifter has.
TORQUE = FORCE x DISTANCE
Here, the force is expressed in engine capacity, the distance in metres.
As the torque curve reaches its maximum in the lower area of the rev counter, it rapidly decreases in the higher area of the rev counter. Another parameter then takes over the relay.
Power is work achieved over a certain time. It therefore depends on the torque, but also on the engine revolution speed. The faster the engine turns, the greater the power. This continues up to a certain point which, on diesel engines, starts between 4000 and 4500 rpm. The engine power has an influence on vehicle speed.
You can compare power using two sportsmen: The first lifts 120 kg in 1 minute. The second lifts 120 kg in 30 seconds. In one minute, the second athlete would have lifted 240 kg, so he is therefore more powerful.
POWER = TORQUE / TIME
POWER = FORCE x SPEED
Power is expressed in Watts (W) or horsepower (DIN). The torque is measured by engine speed and the speed in radians per second.
1 HP DIN = 1 Watt x 1.36
The engine is at its most pleasant and flexible between the area of maximum torque and maximum power. Outside of this area, the engine lacks performance. The best area therefore exists between 1500 and 4000 rpm: within this range, the engine consumes less.
With low rpm the engine consumes very little, but cannot provide the required effort: it vibrates, complains, and even when you press the accelerator pedal to the metal, the rev counter needle doesn’t rise. Generally, this means there isn’t enough torque or power.
When over-revving, the engine consumes a lot more than it really needs, and wears out more quickly: you need to either move up a gear or slow down (in general, above 3500 rpm in 5th, diesel vehicles run at over 140 km/hr). The 3500 to 4000 rpm area should be considered as a reserve area on a diesel (for overtaking, for example).
Diesel engines give their greatest power up to 4200 rpm, after which performance falls off. Their torque does not improve after 3500 rpm.
A petrol engine gives a more linear power, and above all up to 5500 rpm (the curves are not truly exact), giving nevertheless 1000 rpm more than diesel. Below 2000 rpm the torque is really of no interest, unlike the diesel engine which enables better pick-up at low revolutions…
For a diesel engine during running in, it is advisable to run within a range of 1800 to 3000 rpm: current engines, being machined to the 100th micron, no longer require mandatory running in, but this precaution enables all of the moving parts to ‘settle in’…
Source: Extracts from the site: http://www.lewebdesconducteurs.com/