Why more torque doesn't generate more acceleration
Lexicon> Letter D> Torque
Definition: a physical quantity that indicates how strongly a rotary movement is driven
Categories: Basic concepts, engines and power plants, physical fundamentals
Formula symbol: M.
Unit: N m
Author: Dr. Rüdiger Paschotta
How to quote; suggest additional literature
Original creation: January 16, 2015; last change: 05.08.2020
Rotating movements, like straight (linear) movements, are driven by forces. However, the strength of such a drive cannot be specified as a force, since the effect of a force acting on a lever on the rotary movement also depends on the so-called lever arm. The length of the lever arm is the distance of the point of application of the force from the point of rotation, if the force acts perpendicular to the line connecting the point of rotation and point of application. Otherwise, the distance between the line that runs through the point of application in the direction of the force and the pivot point is used (see Figure 1).
The unit of measurement for torque is the newton meter (N m), i.e. the product of a newton (unit of force) and a meter.
The torque M.which his attacking power F. with the lever arm r is defined as the product of force and lever arm: M. = For r. Mathematically, it is practical to define the torque vector as the so-called cross product of the force and radius vector, but this will not be discussed further here. Often only the amount of torque is of interest in technology anyway.
Drive torque of a motor
Most motors transfer the generated mechanical energy to a rotating shaft. They drive this shaft with a certain torque. The transmitted mechanical power then results as the product of torque and angular velocity: P. = M · ω. Here, the angular velocity ω is the change in angle (measured in so-called radians, not in degrees) per unit of time. For example, if a car engine generates a torque of 100 Nm at a speed of 3600 / min, this corresponds to an output of 100 Nm · 2π · 60 / s ≈ 38 kW. (One revolution corresponds to an angle of rotation of 2 π, and minutes must be converted into seconds, the basic SI unit.)
The torque that can be generated by a motor generally depends on the speed of its drive axle. The torque curve, d. H. the dependence of the torque on the speed depends very much on the type and construction of the motor. Some examples of this:
- With an internal combustion engine (e.g. Otto engine or diesel engine), the highest torque is usually achieved at medium speeds (see Figure 2). At low speeds it drops, mainly because the working gas then loses a lot of heat to the walls of the combustion chamber within a (then long) work cycle and the pressure can also be reduced through leaks. At high speeds, on the other hand, the gas exchange reaches its limits; the degree of delivery drops significantly.
- Some electric motors can generate roughly the same torque at all permitted speeds; it is Z. B. proportional to the electrical current in the rotor winding, and its value is limited by the tolerable power loss in the winding. When operating with constant electrical voltage, on the other hand, there is often an approximately linear decrease in torque with increasing speed, because the related current intensity decreases due to the increasing induction voltage.
- In other electric motors (so-called series motors), the torque is very high at low speeds and decreases sharply with increasing speed. This is due to the fact that the current intensity in the rotor winding and the excitation winding decreases sharply with increasing speed.
- With other electric motors, namely synchronous motors, power output is only possible at a very specific speed, which is determined by the mains frequency used and the number of poles. Asynchronous motors can be operated in a wide speed range, but the maximum torque is just below the synchronous speed, but disappears when the synchronous speed is reached.
As shown, the torque characteristics of an electric motor depend very much on its design. General claims that electric motors have a particularly high torque or a particularly flat torque curve are therefore incorrect.
Effective medium pressure
In an internal combustion engine, the torque is closely related to the so-called effective mean pressurepme connected. This is defined in such a way that the mechanical energy emitted by the engine per work cycle corresponds to the product of the effective mean pressure and displacement. Energy losses due to friction in the engine and also in ancillary units of the engine are already taken into account; the effective mean pressure is thus slightly lower than the actual mean pressure on the cylinder (the inner mean pressure). The peak pressure shortly after ignition, on the other hand, is several times higher.
Torque and power
At the engine speed at which the maximum torque is reached, the power generated is not yet maximum. The speed can then be increased, initially without the torque dropping significantly. Only at significantly higher speeds does the torque drop so much that the power also drops.
Internal combustion engines for vehicle applications are often to a maximum elasticity optimized towards, d. H. on a torque curve that is as flat as possible. A variety of methods can be used for this, such as variable valve timing.
With some engines, the torque curve is completely flat in a certain area. This can be the result that the torque is artificially limited by the engine control - for example, in order not to exceed the maximum permissible load on the crankshaft drive.
Internal combustion engines that are supposed to deliver particularly high performance with a limited displacement (e.g. for racing cars) are often optimized for very high speeds. The torque that can be generated is limited by the resilience of the materials, so that only increasing the speed remains. Of course, this also increases certain burdens, so that this strategy is also limited.
Influence of a gear
A gear is a device with which the rotation of, for example, the axis of a drive motor can be converted into slower or faster rotation for an application. The torque also changes here. If there is no energy loss (e.g. due to friction) in the gearbox, the power consumed and output must be identical. This means that a power output at a lower speed must be associated with a correspondingly higher torque. In reality, certain energy losses occur, which is why the torque output is somewhat reduced.
Acceleration of a vehicle
When a vehicle is driven by an engine through a manual transmission, the acceleration of the vehicle that can be achieved by the engine is essentially proportional to the torque (and not to the power) as long as a certain gear of the transmission is used. There is then a fixed relationship between engine torque and driving force for the vehicle, which depends on the gear selected.Within a gear, it depends on the torque, with a free choice of gear, however, on the power.
However, it should not be concluded from this that the acceleration capacity of a vehicle is only determined by the torque of the engine. Namely, if a lower gear is selected, the same engine torque causes a higher driving force for the vehicle. For a given speed of the vehicle, the acceleration becomes maximum when the maximum engine power is reached.
When accelerating a vehicle from 0 to 100 km / h, for example, the engine runs through a certain speed range in each gear. The time required for this acceleration therefore of course does not depend on the maximum, but on the average power. That is why it is beneficial if the engine is as good as possible elastic works (see above), i.e. can provide high power and high torque over a wide speed range.
The often found information z. B. the torque of the motor of an electric car or an electric auxiliary motor in a hybrid drive, as long as it is not known what gear ratio this motor works with. Specifying a service would make much more sense here.
Questions and comments from readers
Here you can suggest questions and comments for publication and answering. The author of the RP-Energie-Lexikon will decide on the acceptance according to certain criteria. In essence, the point is that the matter is of broad interest.
If you receive help here, you might want to return the favor with a donation with which you support the further development of the energy dictionary.
If you would like personal feedback or advice from the author, please write to him by email.
By submitting you give your consent to publish your entries here in accordance with our rules.
See also: power, power, motor, electric motor, combustion engine, effective medium pressure
as well as other articles in the categories basic concepts, engines and power plants, physical principles
- What is a book used for?
- What options do I have after graduation
- Which is the largest riven in India
- What is free wicca
- How do I fix genetic hair loss
- What is the most reliable lightweight tent
- When did you get your dog
- Why isn't my iPhone 6s charging?
- Is quartz cheaper than granite
- What is Sin Theta
- Stem cell research is unbiblical
- What is Surya Shakti
- Toothpaste is inherently acidic
- Is considered to be a creative art of writing
- Which books are considered classics and why
- How can i draw a flower
- What Research In Motion did before blackBerry
- How is quartz sandstone made
- Who was Frigg's father in Norse mythology
- Are there celebrities with royal titles?
- MPC candidates can study MBBS
- What are control factors
- Has almond milk yogurt probiotics
- What is Indus Valley in India