How come G37 torque is low?
10-13-2009, 01:56 AM
#48
Here, people. Read this article if you'd like to really, really understand the relationship between power (HP) and torque.
Plato and Socrates Discuss Torque, Power and Acceleration
- Thomas Barber, March 25, 2007
Plato: Dude, nice toga. Say, I’ve just been reading up on torque and power. Torque, it seems, is the rotational equivalent of force in straight-line motion.
Socrates: To fully appreciate what that means and get us off on a solid footing, let’s start with a quick look at the familiar equation: F = M x A. This equation tells us that whenever an object’s present velocity is changing, the acceleration is given by the ratio of force to mass. Manifestly, the greater an object’s mass, the greater the force needed to yield a specified amount of acceleration. Mass also determines how much kinetic energy an object contains when it is moving at a given velocity. In rotational motion, that familiar equation is replaced by a similar equation: Torque = Moment-of-Inertia x Angular Acceleration. The moment of inertia determines how much torque is needed to yield a given amount of angular acceleration, as well as how much kinetic energy an object contains when spinning at a given angular velocity. When an ice skater in a spin brings his or her arms in closer to the torso, there is no loss of kinetic energy, and the observed increase in angular velocity reveals that the moment of inertia has been made smaller.
Plato: To find the torque associated with a straight-line force, you multiply the force by the length of the lever arm, which is always measured along a line of direction that is square to the direction of the force. Whenever I tighten a bolt, if I vary the length of the lever arm, the force that I sense in my hand and arm will change, yet the torque doesn’t change unless the amount of friction in the threads changes. Whenever I think about overcoming friction, I think about work and power.
Socrates: To find the work associated with any steady force, you multiply the force by the distance covered. Whereas work is cumulative over time and distance, power is the measure of how quickly work is being performed, instantaneously in time. Work and energy are truly the same concept, so the measure of how quickly work is being performed, is also the measure of how quickly energy is being spent. If you turn the crank of a well to lift a 1-lb bucket at a steady rate of 1 ft/min, you are doing work at the steady rate of 1 ft-lb/min, which is simply the product of the force and the velocity. If the radius of the spool is 1 ft, then the torque applied to the crank by the 1-lb bucket, will be 1 lb-ft. In each complete rotation of the crank, the bucket will move a distance equal to the radius of the spool multiplied by twice pi. It follows that the work associated with a specific amount of torque, for one complete rotation, may be found by multiplying the torque by twice pi. The calculation of power from torque and rotational speed is similar. The following chart summarizes:
straight-line motion
rotational motion
work
force x distance
torque x number of rotations x 2 x pi
power
force x velocity
torque x angular velocity x 2 x pi
The expression for power in rotational motion reveals that you always multiply by the same constant value (2 x pi) to calculate power from the product of torque and angular velocity. Note, though, that this assumes that angular velocity is measured in complete rotations per unit of time. You could just as easily measure angular velocity using a smaller angular distance, such that you would have to multiply that smaller angular distance by twice pi in order to yield one complete rotation. If you measured angular velocity using that smaller angular distance (which is known as a “radian”), the expression for power would be simply the product of torque and angular velocity, i.e., you would not multiply by twice pi to calculate power. Hence, the business of multiplying by twice pi is equivalent in effect to converting from one unit of measure to another, and it is correct to say that power is simply equal to the product of torque and angular velocity.
Plato: If the bucket is raised at steady velocity, its kinetic energy will be steady. Only its potential energy will be changing, and the power will be simply the static weight of the bucket multiplied by the steady velocity. It is easy enough to measure instantaneous velocity when the velocity is steady, but in real-world scenarios, doesn’t it get more complicated?
Socrates: A common approach to measuring the power of an engine is to use a regulated brake to hold the engine steady at the desired speed. You have to measure the force that resists the pull of the engine on the brake, so that you can deduce the engine torque from that force and from the lever arm, which you also have to figure out. And, of course, you have to measure the engine speed. Dynamometers of this sort are known as “brake dynamometers”. Conceptually, you could implement a brake dynamometer of sorts by applying the engine to the task of lifting an elevator car, using a continuously variable transmission in the coupling. The CVT would allow you to stabilize the speed of both the engine and the elevator car at any desired engine speed. To deduce power, you would multiply the elevator’s steady velocity by its weight, and as with brake dynamometers in general, those measurements would be unaffected by the engine’s inertial moment. The other common approach is to hitch the engine to a massive drum that spins freely. As long as the increase in the kinetic energy of the drum is the only energy sink, the power will be given by the instantaneous rate of increase of the drum’s kinetic energy, which can be deduced from the drum’s moment of inertia and its instantaneous angular acceleration. Dynamometers of this type are known as “inertial dynamometers”. The angular acceleration can be measured with the help of an accelerometer, or deduced from closely spaced measurements of time and angular distance. The drum’s moment of inertia can be measured separately, or calculated from its dimensions and the density of its substance. The increase in the kinetic energy of the engine itself is an energy sink. The measurements are influenced to a degree by the engine’s moment of inertia, and they reveal, to a degree, the ability of the engine to quickly increase its work output. As such, measurements taken on an inertial dynamometer give a more realistic picture of an engine’s actual performance on the road. For purposes of ordinary performance tuning on a test bench, that sort of accuracy isn’t particularly beneficial, whereas the ability to keep the engine running steadily for extended periods can be beneficial.
Plato: I read somewhere that to measure power, you measure torque and then you deduce power from torque. That supposedly demonstrates that power is just an abstraction of torque.
Socrates: Clearly, there are various ways to measure power independently of torque. Moreover, the notion, that power is less real than torque, has no meaning or interpretation that is capable of being confirmed experimentally. As far as the orthodoxy and methodology of empirical science is concerned, notions of that sort are meaningless.
Plato: I should be able to measure the power of my mare, by measuring how quickly she is able to lift a large bucket of water from my well. If I adjust the amount of water such that the velocity is steady, the actual force will be equal to the weight of the bucket. That way, I won’t have to measure the actual strain in the rope, and of course, it will be easier to measure the velocity.
Socrates: In the future, a fellow by the name of James Watt will determine that his horse is able to perform work at an ongoing, instantaneous rate of 33,000 foot-pounds of work per minute. If you measure torque in lb-ft and rotational speed in rpm, and you want to express the power in hp, you can use the conversion factor: 1 hp = 33,000 ft-lb/min. The value of twice pi is 6.283, and that divided by 33,000 is about 1/5252. So, as long as torque is measured in lb-ft, rotational speed is measured in rpm, and you want to express the power in hp, you can take a short cut and divide the product of torque and rotational speed by 5252.
Plato: Does that mean that torque and power are equivalent at 5252 rpm?
Socrates: Nope. Torque and power are distinct properties, with each being analytically related to acceleration in its own special way. The value 5252 is merely an artifact of the English system of measure, and that value is not the least bit special if another system of measure is used. In most of the world, torque is expressed in Newton-meters (N-m), and power is expressed in Watts or kilowatts (kW), which we use for electrical power. The engine speed where torque in lb-ft and power in hp coincidentally take on the same numerical value, happens to fall within the operating range of most engines, so on dynamometer plots, it is convenient to use a single number scale for both torque in lb-ft and power in hp. When that is done, the two curves will cross at 5252 rpm.
Plato: But, it seems that torque should determine acceleration, so why does power matter?
Plato and Socrates Discuss Torque, Power and Acceleration
- Thomas Barber, March 25, 2007
Plato: Dude, nice toga. Say, I’ve just been reading up on torque and power. Torque, it seems, is the rotational equivalent of force in straight-line motion.
Socrates: To fully appreciate what that means and get us off on a solid footing, let’s start with a quick look at the familiar equation: F = M x A. This equation tells us that whenever an object’s present velocity is changing, the acceleration is given by the ratio of force to mass. Manifestly, the greater an object’s mass, the greater the force needed to yield a specified amount of acceleration. Mass also determines how much kinetic energy an object contains when it is moving at a given velocity. In rotational motion, that familiar equation is replaced by a similar equation: Torque = Moment-of-Inertia x Angular Acceleration. The moment of inertia determines how much torque is needed to yield a given amount of angular acceleration, as well as how much kinetic energy an object contains when spinning at a given angular velocity. When an ice skater in a spin brings his or her arms in closer to the torso, there is no loss of kinetic energy, and the observed increase in angular velocity reveals that the moment of inertia has been made smaller.
Plato: To find the torque associated with a straight-line force, you multiply the force by the length of the lever arm, which is always measured along a line of direction that is square to the direction of the force. Whenever I tighten a bolt, if I vary the length of the lever arm, the force that I sense in my hand and arm will change, yet the torque doesn’t change unless the amount of friction in the threads changes. Whenever I think about overcoming friction, I think about work and power.
Socrates: To find the work associated with any steady force, you multiply the force by the distance covered. Whereas work is cumulative over time and distance, power is the measure of how quickly work is being performed, instantaneously in time. Work and energy are truly the same concept, so the measure of how quickly work is being performed, is also the measure of how quickly energy is being spent. If you turn the crank of a well to lift a 1-lb bucket at a steady rate of 1 ft/min, you are doing work at the steady rate of 1 ft-lb/min, which is simply the product of the force and the velocity. If the radius of the spool is 1 ft, then the torque applied to the crank by the 1-lb bucket, will be 1 lb-ft. In each complete rotation of the crank, the bucket will move a distance equal to the radius of the spool multiplied by twice pi. It follows that the work associated with a specific amount of torque, for one complete rotation, may be found by multiplying the torque by twice pi. The calculation of power from torque and rotational speed is similar. The following chart summarizes:
straight-line motion
rotational motion
work
force x distance
torque x number of rotations x 2 x pi
power
force x velocity
torque x angular velocity x 2 x pi
The expression for power in rotational motion reveals that you always multiply by the same constant value (2 x pi) to calculate power from the product of torque and angular velocity. Note, though, that this assumes that angular velocity is measured in complete rotations per unit of time. You could just as easily measure angular velocity using a smaller angular distance, such that you would have to multiply that smaller angular distance by twice pi in order to yield one complete rotation. If you measured angular velocity using that smaller angular distance (which is known as a “radian”), the expression for power would be simply the product of torque and angular velocity, i.e., you would not multiply by twice pi to calculate power. Hence, the business of multiplying by twice pi is equivalent in effect to converting from one unit of measure to another, and it is correct to say that power is simply equal to the product of torque and angular velocity.
Plato: If the bucket is raised at steady velocity, its kinetic energy will be steady. Only its potential energy will be changing, and the power will be simply the static weight of the bucket multiplied by the steady velocity. It is easy enough to measure instantaneous velocity when the velocity is steady, but in real-world scenarios, doesn’t it get more complicated?
Socrates: A common approach to measuring the power of an engine is to use a regulated brake to hold the engine steady at the desired speed. You have to measure the force that resists the pull of the engine on the brake, so that you can deduce the engine torque from that force and from the lever arm, which you also have to figure out. And, of course, you have to measure the engine speed. Dynamometers of this sort are known as “brake dynamometers”. Conceptually, you could implement a brake dynamometer of sorts by applying the engine to the task of lifting an elevator car, using a continuously variable transmission in the coupling. The CVT would allow you to stabilize the speed of both the engine and the elevator car at any desired engine speed. To deduce power, you would multiply the elevator’s steady velocity by its weight, and as with brake dynamometers in general, those measurements would be unaffected by the engine’s inertial moment. The other common approach is to hitch the engine to a massive drum that spins freely. As long as the increase in the kinetic energy of the drum is the only energy sink, the power will be given by the instantaneous rate of increase of the drum’s kinetic energy, which can be deduced from the drum’s moment of inertia and its instantaneous angular acceleration. Dynamometers of this type are known as “inertial dynamometers”. The angular acceleration can be measured with the help of an accelerometer, or deduced from closely spaced measurements of time and angular distance. The drum’s moment of inertia can be measured separately, or calculated from its dimensions and the density of its substance. The increase in the kinetic energy of the engine itself is an energy sink. The measurements are influenced to a degree by the engine’s moment of inertia, and they reveal, to a degree, the ability of the engine to quickly increase its work output. As such, measurements taken on an inertial dynamometer give a more realistic picture of an engine’s actual performance on the road. For purposes of ordinary performance tuning on a test bench, that sort of accuracy isn’t particularly beneficial, whereas the ability to keep the engine running steadily for extended periods can be beneficial.
Plato: I read somewhere that to measure power, you measure torque and then you deduce power from torque. That supposedly demonstrates that power is just an abstraction of torque.
Socrates: Clearly, there are various ways to measure power independently of torque. Moreover, the notion, that power is less real than torque, has no meaning or interpretation that is capable of being confirmed experimentally. As far as the orthodoxy and methodology of empirical science is concerned, notions of that sort are meaningless.
Plato: I should be able to measure the power of my mare, by measuring how quickly she is able to lift a large bucket of water from my well. If I adjust the amount of water such that the velocity is steady, the actual force will be equal to the weight of the bucket. That way, I won’t have to measure the actual strain in the rope, and of course, it will be easier to measure the velocity.
Socrates: In the future, a fellow by the name of James Watt will determine that his horse is able to perform work at an ongoing, instantaneous rate of 33,000 foot-pounds of work per minute. If you measure torque in lb-ft and rotational speed in rpm, and you want to express the power in hp, you can use the conversion factor: 1 hp = 33,000 ft-lb/min. The value of twice pi is 6.283, and that divided by 33,000 is about 1/5252. So, as long as torque is measured in lb-ft, rotational speed is measured in rpm, and you want to express the power in hp, you can take a short cut and divide the product of torque and rotational speed by 5252.
Plato: Does that mean that torque and power are equivalent at 5252 rpm?
Socrates: Nope. Torque and power are distinct properties, with each being analytically related to acceleration in its own special way. The value 5252 is merely an artifact of the English system of measure, and that value is not the least bit special if another system of measure is used. In most of the world, torque is expressed in Newton-meters (N-m), and power is expressed in Watts or kilowatts (kW), which we use for electrical power. The engine speed where torque in lb-ft and power in hp coincidentally take on the same numerical value, happens to fall within the operating range of most engines, so on dynamometer plots, it is convenient to use a single number scale for both torque in lb-ft and power in hp. When that is done, the two curves will cross at 5252 rpm.
Plato: But, it seems that torque should determine acceleration, so why does power matter?
10-13-2009, 01:57 AM
#49
Next half:
Socrates: Power matters because at any point in time, acceleration is proportional to the rate at which the engine is performing work. Engine torque tells you how much work is performed over any specific interval of crankshaft rotation, but does not tell you how quickly the work is being performed. It is of course possible to deduce acceleration from the engine torque using other information such as the overall gear ratio and wheel diameter, but that doesn’t change the pertinent and useful fact that at any point in time, acceleration is proportional to power. Recall that power is equal to the product of force and velocity. If you turn that around, it says that force is equal to power divided by the (non-zero) velocity. If you substitute that expression for force into the familiar equation that relates force, mass, and acceleration, you get this:
acceleration = power / (mass x velocity) =>
acceleration = engine_torque x 2 x pi x engine_speed / (velocity x mass)
Hence, given the vehicular velocity that is applicable to some point in time, the acceleration that you get, for a given amount of engine torque and a given mass, depends on the engine speed. Of course, if the ratio of engine speed to vehicle speed is given, as it effectively is while the gear ratio is held constant, acceleration will then vary according to the engine torque. (Note that if you plug a set of values into that equation to calculate acceleration, in order to get proper units of measure for acceleration, you need to use lbf instead of lb for the force component of the torque. 1 lbf is the force of gravity on 1 lb of mass: 1 lbf = 1 lb x 32.2 ft/s^2 = 32.2 ft-lb/s^2 = 4.45 N.)
Plato: But, if acceleration is proportional to power, why does acceleration track with the engine torque curve as the engine speed and the vehicle speed increase in a given gear?
Socrates: The perception of a contradiction, between the fact that wheel torque tracks with the engine torque while the gear ratio remains fixed, and the fact that acceleration is proportional to power, is a false perception. The equations reveal that the proportionality between acceleration and power is different at different vehicle speeds. The acceleration that you get for a given amount of power decreases as the vehicle speed increases, yet, at any point in time, acceleration is proportional to power, and depends as much on engine speed as on engine torque.
Plato: What does this mean from the perspective of gear selection strategy?
Socrates: Whenever you change gears, as long as you are quick to avoid any significant loss in vehicle speed during the up-shift, the proportionality between power and acceleration will be steady across the up-shift. Hence, in order for acceleration to be steady across the up-shift, power must be steady across the up-shift, which means that the engine torque must increase to compensate for the drop in engine speed. If the throttle is held open so that actual power follows the engine’s power curve, the engine speed must transition between two equal-power points on opposite sides of the power peak. Note that shifting such that power will be steady across the up-shift, and shifting such that you are always using the gear that yields the greatest power, are two different ways to describe the same optimal strategy.
Plato: What would happen if the engine torque were to be held steady across the up-shift, i.e., you kept the engine speed within the flat region of the engine torque curve?
Socrates: The acceleration would drop abruptly at the up-shift, matching the drop in engine speed. Let’s look at it another way, and let’s take a quick side trip that may help to put the significance of power into better perspective. In an electrical transformer, any increase in voltage between the primary and the secondary windings, must be accompanied by a compensating decrease in current. Power is equal to the product of voltage and current, and as the saying goes, “power in is power out”. That saying applies as well to the physics of mechanical motion. Except for the energy losses due to friction, the product of torque and rotational speed will be the same at the wheel as it is at the engine, and as it is anywhere else that you measure it along the drive train. You want the wheel torque to be steady across the up-shift, and since the wheel speed will also be steady at the up-shift, the product of torque and rotational speed will be steady at the up-shift, not only at the wheel, but at the engine as well. That, of course, means that the engine torque must increase to compensate for the drop in engine speed.
Plato: Okay, but given two vehicles that are identical except for the engines, the one with the greater peak engine torque will still exhibit greater peak acceleration in each gear, right?
Socrates: If the vehicle with greater peak power is allowed to use a different final drive ratio, then by shifting its engine torque peak to lower vehicle speed, the corresponding wheel torque will increase. Thus, the vehicle with greater peak power may exhibit greater peak acceleration in each gear, even if its peak engine torque is less than that of the other vehicle.
Plato: Well, there are still certain benefits to emphasizing torque in lieu of power, aren’t there?
Socrates: Certain effects, such as improved acceleration from a full stop and less frequent shifting, are the result of a comparatively flat, uniform spread of engine output, starting at comparatively low engine speed. It makes perfect sense to attribute such effects to a de-emphasis on peak power. However, logically speaking, torque and power are not opposites, and it does not follow from the fact that you have de-emphasized peak power, that you have emphasized torque. Of course, if there exists some other justification for the practice of equating the engine’s low-speed performance to torque, that will also constitute justification for equating a de-emphasis on peak power to an emphasis on torque, never mind that torque and power are not opposites. At the wheel, the affinity between low rotational speed and torque is quite genuine, owing to the fact that the transmission is used to exchange rotational speed for torque. But this effect does not apply to the engine. The practice, of equating engine performance at low and moderate engine speed exclusively to torque, seems to derive essentially from the fact that the peak engine torque occurs at a lower engine speed than does the peak power. This seems a weak justification when you consider that the peak engine torque reveals the engine performance accurately at only a single engine speed. That engine speed is often above the midpoint of the engine’s operating range, and no matter how low the actual engine speed, the actual performance depends partly on the engine speed, and is fully revealed by the actual power.
Plato: What else?
Socrates: Many people seem to believe that the full explanation, for why longer stroke generally means improved low-end performance, is simply that by increasing the effective lever arm (the crank throw distance is one-half of the stroke distance), you increase the torque. For whatever reason, they don’t realize that if it were that simple, the improvement in engine torque would be uniform over the operating range, which would not explain why the performance improvement is specific to low engine speed. They have somehow gotten the idea that any change, that directly improves engine torque, will automatically favor lower engine speed. Clearly, it isn’t that simple. If you increase the stroke while keeping the volume displacement constant, the piston surface area will decrease, which will nullify the effect of the increased lever arm, since the force depends on the surface area of the piston face. Engine torque corresponds to the amount of energy spent over any specific interval of crankshaft rotation, and that amount of energy depends on the amount of oxygen used. It follows that the variation in engine torque with engine speed reveals the variation in the amount of air captured per individual intake stroke. Cylinder shape interacts with the duration of the intake stroke to influence the amount of air that is captured on the intake stroke. When the cylinder is made long and skinny, the effect is to increase the amount of air captured at low engine speed, and to decrease the amount of air captured at high engine speed. Note also that if the relationship between stroke and torque were as direct as the naïve explanation suggests, you could get free energy just by making the cylinder long and skinny.
Plato: I need to go get measured for a new toga, but before I run along, I’d like to know what you think about the various claims that engine torque is the true indicator of engine performance.
Socrates: Those sorts of claims have to be interpreted to mean that you are always supposed to get the same acceleration for a given amount of engine torque, no matter the engine speed at which that much engine torque is delivered. There simply is no other meaningful, tangible interpretation of those claims. Yet, as we have already seen, wheel torque depends just as much on engine speed as it does on engine torque. Anyone who is not convinced of that, need only discover for themselves that at any of the various vehicle speeds where the transmission will permit you to choose between two equal-torque points on opposite sides of the torque peak, the acceleration will be dramatically greater in the lower of the two gears. It is logically dubious to infer, from the fact that the peak power does a poor job of revealing the engine’s performance at low and moderate engine speeds, that torque is the true indicator of engine performance.
Socrates: Power matters because at any point in time, acceleration is proportional to the rate at which the engine is performing work. Engine torque tells you how much work is performed over any specific interval of crankshaft rotation, but does not tell you how quickly the work is being performed. It is of course possible to deduce acceleration from the engine torque using other information such as the overall gear ratio and wheel diameter, but that doesn’t change the pertinent and useful fact that at any point in time, acceleration is proportional to power. Recall that power is equal to the product of force and velocity. If you turn that around, it says that force is equal to power divided by the (non-zero) velocity. If you substitute that expression for force into the familiar equation that relates force, mass, and acceleration, you get this:
acceleration = power / (mass x velocity) =>
acceleration = engine_torque x 2 x pi x engine_speed / (velocity x mass)
Hence, given the vehicular velocity that is applicable to some point in time, the acceleration that you get, for a given amount of engine torque and a given mass, depends on the engine speed. Of course, if the ratio of engine speed to vehicle speed is given, as it effectively is while the gear ratio is held constant, acceleration will then vary according to the engine torque. (Note that if you plug a set of values into that equation to calculate acceleration, in order to get proper units of measure for acceleration, you need to use lbf instead of lb for the force component of the torque. 1 lbf is the force of gravity on 1 lb of mass: 1 lbf = 1 lb x 32.2 ft/s^2 = 32.2 ft-lb/s^2 = 4.45 N.)
Plato: But, if acceleration is proportional to power, why does acceleration track with the engine torque curve as the engine speed and the vehicle speed increase in a given gear?
Socrates: The perception of a contradiction, between the fact that wheel torque tracks with the engine torque while the gear ratio remains fixed, and the fact that acceleration is proportional to power, is a false perception. The equations reveal that the proportionality between acceleration and power is different at different vehicle speeds. The acceleration that you get for a given amount of power decreases as the vehicle speed increases, yet, at any point in time, acceleration is proportional to power, and depends as much on engine speed as on engine torque.
Plato: What does this mean from the perspective of gear selection strategy?
Socrates: Whenever you change gears, as long as you are quick to avoid any significant loss in vehicle speed during the up-shift, the proportionality between power and acceleration will be steady across the up-shift. Hence, in order for acceleration to be steady across the up-shift, power must be steady across the up-shift, which means that the engine torque must increase to compensate for the drop in engine speed. If the throttle is held open so that actual power follows the engine’s power curve, the engine speed must transition between two equal-power points on opposite sides of the power peak. Note that shifting such that power will be steady across the up-shift, and shifting such that you are always using the gear that yields the greatest power, are two different ways to describe the same optimal strategy.
Plato: What would happen if the engine torque were to be held steady across the up-shift, i.e., you kept the engine speed within the flat region of the engine torque curve?
Socrates: The acceleration would drop abruptly at the up-shift, matching the drop in engine speed. Let’s look at it another way, and let’s take a quick side trip that may help to put the significance of power into better perspective. In an electrical transformer, any increase in voltage between the primary and the secondary windings, must be accompanied by a compensating decrease in current. Power is equal to the product of voltage and current, and as the saying goes, “power in is power out”. That saying applies as well to the physics of mechanical motion. Except for the energy losses due to friction, the product of torque and rotational speed will be the same at the wheel as it is at the engine, and as it is anywhere else that you measure it along the drive train. You want the wheel torque to be steady across the up-shift, and since the wheel speed will also be steady at the up-shift, the product of torque and rotational speed will be steady at the up-shift, not only at the wheel, but at the engine as well. That, of course, means that the engine torque must increase to compensate for the drop in engine speed.
Plato: Okay, but given two vehicles that are identical except for the engines, the one with the greater peak engine torque will still exhibit greater peak acceleration in each gear, right?
Socrates: If the vehicle with greater peak power is allowed to use a different final drive ratio, then by shifting its engine torque peak to lower vehicle speed, the corresponding wheel torque will increase. Thus, the vehicle with greater peak power may exhibit greater peak acceleration in each gear, even if its peak engine torque is less than that of the other vehicle.
Plato: Well, there are still certain benefits to emphasizing torque in lieu of power, aren’t there?
Socrates: Certain effects, such as improved acceleration from a full stop and less frequent shifting, are the result of a comparatively flat, uniform spread of engine output, starting at comparatively low engine speed. It makes perfect sense to attribute such effects to a de-emphasis on peak power. However, logically speaking, torque and power are not opposites, and it does not follow from the fact that you have de-emphasized peak power, that you have emphasized torque. Of course, if there exists some other justification for the practice of equating the engine’s low-speed performance to torque, that will also constitute justification for equating a de-emphasis on peak power to an emphasis on torque, never mind that torque and power are not opposites. At the wheel, the affinity between low rotational speed and torque is quite genuine, owing to the fact that the transmission is used to exchange rotational speed for torque. But this effect does not apply to the engine. The practice, of equating engine performance at low and moderate engine speed exclusively to torque, seems to derive essentially from the fact that the peak engine torque occurs at a lower engine speed than does the peak power. This seems a weak justification when you consider that the peak engine torque reveals the engine performance accurately at only a single engine speed. That engine speed is often above the midpoint of the engine’s operating range, and no matter how low the actual engine speed, the actual performance depends partly on the engine speed, and is fully revealed by the actual power.
Plato: What else?
Socrates: Many people seem to believe that the full explanation, for why longer stroke generally means improved low-end performance, is simply that by increasing the effective lever arm (the crank throw distance is one-half of the stroke distance), you increase the torque. For whatever reason, they don’t realize that if it were that simple, the improvement in engine torque would be uniform over the operating range, which would not explain why the performance improvement is specific to low engine speed. They have somehow gotten the idea that any change, that directly improves engine torque, will automatically favor lower engine speed. Clearly, it isn’t that simple. If you increase the stroke while keeping the volume displacement constant, the piston surface area will decrease, which will nullify the effect of the increased lever arm, since the force depends on the surface area of the piston face. Engine torque corresponds to the amount of energy spent over any specific interval of crankshaft rotation, and that amount of energy depends on the amount of oxygen used. It follows that the variation in engine torque with engine speed reveals the variation in the amount of air captured per individual intake stroke. Cylinder shape interacts with the duration of the intake stroke to influence the amount of air that is captured on the intake stroke. When the cylinder is made long and skinny, the effect is to increase the amount of air captured at low engine speed, and to decrease the amount of air captured at high engine speed. Note also that if the relationship between stroke and torque were as direct as the naïve explanation suggests, you could get free energy just by making the cylinder long and skinny.
Plato: I need to go get measured for a new toga, but before I run along, I’d like to know what you think about the various claims that engine torque is the true indicator of engine performance.
Socrates: Those sorts of claims have to be interpreted to mean that you are always supposed to get the same acceleration for a given amount of engine torque, no matter the engine speed at which that much engine torque is delivered. There simply is no other meaningful, tangible interpretation of those claims. Yet, as we have already seen, wheel torque depends just as much on engine speed as it does on engine torque. Anyone who is not convinced of that, need only discover for themselves that at any of the various vehicle speeds where the transmission will permit you to choose between two equal-torque points on opposite sides of the torque peak, the acceleration will be dramatically greater in the lower of the two gears. It is logically dubious to infer, from the fact that the peak power does a poor job of revealing the engine’s performance at low and moderate engine speeds, that torque is the true indicator of engine performance.
10-13-2009, 02:10 PM
#53
I remember reading this yrs ago in Car and Driver -- People buy HP but its TQ that they love. In layman terms high tq cars are the best to drive for everyday dart and squirt traffic but high reving up high hp is untouchable for bombing when in 3rd and 4th gr. I love C&D for including top gear accelaration numbers. I think this more than anything accurately reflects everyday driving senarios where flooring it and downshifting isn't always necessary.
10-13-2009, 02:53 PM
#54
People continue to not read.
Torque is a direct function of displacement and volumetric efficiency.
Since VE is roughly the same for all NA cars, it is safe to say that for our purposes, torque depends completely on output.
Is it a coincidence that ALL naturally aspirated cars on the market today make about 75 ft/lbs of torque per liter of displacement? No.
Torque is a direct function of displacement and volumetric efficiency.
Since VE is roughly the same for all NA cars, it is safe to say that for our purposes, torque depends completely on output.
Is it a coincidence that ALL naturally aspirated cars on the market today make about 75 ft/lbs of torque per liter of displacement? No.
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