0
I also asked the same question but could not find satisfactory answer, so I decided to try smacking this myself. Note that this discussion is limited to petrol engines. This may be a bit long but here it goes.
It's a bit theoretical but it is important to know "the basics" prior to further discussion. "Torque" is actually the amount of "work" done (in Newton.meter or Joule). It is a function of how much Newton of "force" being exerted on the engine's piston during expansion in "combustion stroke". If you refer to Otto / Diesel Cycle Temperature-Entropy (T-s) diagram, engine output can be increased at higher pressure ratio (volumetric energy from air density) or higher firing temperature (thermal energy by adding more fuel). So, more air + more fuel = more power..
What is power then? Well I simply view "power" as the "rate of doing work" (Joule/sec or kW). Hence, for the same amount of torque, the power increases at higher RPM (Power = Torque X Rotational Speed). Agree?
OK, now let's discuss on the power / torque vs RPM curve usually supplied by car manufacturers. In naturally-aspirated (NA) engines, initially the torque slowly increase, peaks at certain speed (say 4000 RPM) and started to decline beyond that point. As we've understand that torque and power are related, this torque dip reduces the power increase slope and eventually diminishes the power at a later speed (say 6500 RPM). This happens when the depleting of torque is more significant than increase in engine speed. Make sense, right?
In NA engines, I think the torque started to reduce because of 2 main reasons. Firstly, limitation is on the "intake valve lift" that controls the openings of air passage into the cylinder. At higher RPM, more air is required but the flow is "choked" by this constant opening area. This is the reason why some engines are designed with "variable intake valve lift" to cater for various RPM ranges i.e. small openings at low RPM (for fuel efficiency) and larger openings at higher RPMs (to sustain or even increase torque and power).
Secondly, the air into the cylinder is always at ambient pressure regardless of engine RPM. The amount of air drawn into the cylinder solely depends on the negative "suction" created by the downward movement of the piston during intake stroke. At higher RPM, constant atmospheric inlet pressure could not catch up with rapid cylinder volume expansion from faster piston movement. This results in lower air density available for combustion (although can be compensated with higher fuel consumption).
This is where forced-induction engines come into the picture. In turbocharged or supercharged engines, air pressure at the intake is compressed to slightly above atmospheric pressure, meaning there is a "pushing" of air into the cylinder to complement the "pulling" by the piston during intake stroke. This extra help enables peak torque to be achieved much earlier (say 1500 RPM). Another benefit is sufficient amount of presurized air can be supplied at a wide span of engine speed (say 1500 to 5000 RPM) and constant torque value can be achieved within this range. This flat torque profile is what you referred as "torque plateau". Using this technique, the power output rate can be rapidly increased across the RPM and "higher peak power" can be achieved with a relatively small engine capacity. all this just by playing with the torque value!
-
Wiki User
∙ 11y ago
Add your answer:
Earn +20 pts
Why it is necessary to mention reference point while defining torque?
If you choose a different reference point, the numbers for torque will be different. Calculations will still work out, though - but a reference point must be chosen, and used consistently.
Why torque is zero when force is applied to point that passes through pivot point?
Torque is defined as the product of the distance from the pivot point, times a force, times an angle function. If any of the three factors is zero, the product is zero. In this case, the distance from the pivot point.
Why it is difficult to shut the door from a point near to hinge?
It is difficult to shut the door from a point near the hinge because the hinge acts as a pivot point, causing the door to swing in an arc. When you try to close the door from near the hinge, your force is applied at a smaller distance from the pivot point, resulting in a larger torque required to overcome the resistance and close the door.
Is it possible to calculate the torque acting on a rigid body without specifying the axis?
Not at all possible. Torque defined as the moment of the force about a point or an axis of rotation. Torque tau vector = radius vector x Force vector. Radius is to be measured only from a given point or given axis. Hence axis in very important
Does torque and work are equivalent?
Torque is rotational force, while work is energy transferred between objects. For example, a screw experiences mostly torque as you twist it around its axis, while a nail experiences mostly work as it is driven straight into the wood.
Is the Grand Canyon a plateau?
The Grand Canyon is a "gorge" or "canyon" if you wish. Hence the name Grand Canyon. However, there are two "plateau's" on either side of the canyon which could be considered part of the GC, from a certain point of view.
Is the freezing point of a pure solvent a plateau or a continuous downward trend?
it has a plateau.
Example of a plateau?
The Colorado Plateau Ennedi Plateau Blake Plateau
What is the highest elevated point on the Asian continent?
The highest plateau in Asia is The Tibetan Plateau. Believe it or not, it is the highest plateau in the world.
When water is heated the temperature eventually reaches a constant value and forms a plateau on the graph what does the plateau indicate?
The boiling point.
Why is the horsepower rating and the torque ratings of cars given at different RPM's. What do these ratings mean when say the h.p. is given at a certain RPM and the torque is given at a certain RPM.?
I believe that is the maximum horsepower and torque and at what RPM they occur
What was the turning point for the battle of Iwo Jima?
The seizure and control of the Motoyama Plateau marked the turning point .
What is the point where growth patterns level off called?
The pattern is said to plateau.
What is the difference between max torque and breakdown torque of a electric motor?
Max torque and breakdown toarque are the same...,.the point at which a motor will stop/stall.
How do you used torque wrench?
A torque wrench is like a regular wrench - but with an indicator added. This indicator will tell you when you have tightened the nut/bolt to a certain degree, either by clicking or by a dial showing a certain value. You get the required torque setting from a manual or maintenance handbook.
Why it is necessary to mention reference point while defining torque?
If you choose a different reference point, the numbers for torque will be different. Calculations will still work out, though - but a reference point must be chosen, and used consistently.
What do the plateau natives drink?
They drank water and it is possible that they drank blood of a certain animal if they there was a famine....
