A dynamometer that can either drive or absorb is called a universal or active dynamometer. In addition to being used to determine the torque or power characteristics of a machine under test MUT , dynamometers are employed in a number of other roles. In standard emissions testing cycles such as those defined by the US Environmental Protection Agency US EPA , dynamometers are used to provide simulated road loading of either the engine using an engine dynamometer or full powertrain using a chassis dynamometer.
In fact, beyond simple power and torque measurements, dynamometers can be used as part of a testbed for a variety of engine development activities such as the calibration of engine management controllers , detailed investigations into combustion behavior and tribology. In an engine dynamometer, water flow, proportional to the desired applied load, creates resistance to the engine.
A controlled water flow through the inlet manifold is directed at the center of the rotor in each absorption section. This water is then expelled to the outer dynamometer body by centrifugal force. As it is directed outward, the water is accelerated into pockets on the stationary stator plates where it is decelerated. The continual acceleration and deceleration causes the dynamometer to absorb the power produced by the engine.
Through this transfer of energy the water is heated and discharged. An integral component of a dynamometer is its data acquisition system. The system is typically comprised of two units, a Commander and Workstation, connected by an Ethernet cable. The Commander, a desktop computer operated by Windows-based software, issues commands to the Workstation, a touch-screen operated unit housed in a rugged industrial enclosure.
The Workstation operates the precision load and throttle control systems, collects the data, and sends it to the Commander to be processed, stored and analyzed. If a gear is selected that puts the engine speed somewhat higher than the engine speed at which the engine torque peak occurs, that numerically lower gear will result in greater torque multiplication and the rear wheel torque will be greater even though the engine torque will be somewhat less than its maximum value.
This will be true so long as the torque curve remains reasonably flat above its peak value. Even with engines that have a pronounced peak in the torque curve, the torque curve will be essentially flat for some distance near the peak.
If the gear ratios are properly matched to that torque curve and the wheel speed is within the normal operating range, it will always be true that the acceleration will be greatest when the engine speed is higher than the engine speed at which the torque peak occurs. This begs the question of how the acceleration is related to the power. That relationship is slightly complicated by the coupling of several facts.
Power is the rate of change of the kinetic energy, kinetic energy depends on the square of the velocity, and it is not intuitively obvious whether the velocity should change most rapidly at the same engine speed where the square of the velocity changes most rapidly. However, by combining two of the identities equations that were presented near the beginning of this article, we can derive an identity that describes acceleration as a function of power, velocity and mass. Rearranging the terms to isolate acceleration, we get an identity that describes acceleration as a function of power, velocity and mass:.
This formula provides us with a couple of useful facts. For one, it tells us that for a given power and mass, the acceleration decreases as the velocity increases, which is consistent with the fact that kinetic energy increases as the square of the velocity. Probably of greater interest to most readers is the fact that for a given velocity and mass, the acceleration is directly proportional to the power.
There are two distinct and meaningful consequences of the proportionality between acceleration and power. First, at a given speed, the acceleration will be greatest when the gear selected is such that the power at the associated engine speed is the greatest among all the gears. Second, given any two vehicles with identical mass to include the mass of the rider , the one with the more powerful engine will exhibit the greatest maximum acceleration, regardless of which one produces the most torque.
To be sure, the power to weight ratio determines the vehicle's maximum acceleration, which of course is why that ratio is frequently quoted. The simplest way to assess what the acceleration can be at any given wheel speed, is to convert that wheel speed to the equivalent engine speed for each gear, and then look at the power curve to find which of those engine speeds yields the most power.
You can also answer this question from the standpoint of rear wheel torque, but then after looking up the engine torque for each of the engine speeds, you have to turn back around and multiply those engine torque values by their corresponding reduction ratios in order to find the rear wheel torque for each gear.
You'll get the same result either way; if you don't, then at least one of the two graphs is in error. However, the torque method requires more computational work as compared to simply looking at the power curve, so why would anyone want to do that?
Just for grins, let's consider an actual example. The Yamaha shop manual gives the primary, secondary, and gear-specific reduction ratios for the five gears. I want to find the optimal road speed for shifting from 1st gear to 2nd gear. I worked out the value of the multiplier for converting the road speed in mph to engine speed in rpm.
That multiplier is 60, and the primary and secondary reduction ratios are already factored in to that, but I still have to multiply by the gear-specific reduction ratio. At 50 mph, the engine speed will be rpm in 1st gear more or less depending on the accuracy of my measurement of the wheel radius , and looking at the dynamometer chart, I read about hp for that engine speed. That's very close to the peak power, but the peak is located a little higher, just shy of rpm.
Therefore, I expect that I should go beyond 50 mph in 1st gear before shifting to 2nd gear. At 60 mph, the engine speed will be rpm in 1st gear, which is just off the chart because when you cross rpm you're into the red zone, but I can visually extrapolate the curve and estimate hp, well below the maximum hp. The error in my measurement of the wheel diameter might be the reason for the engine speed being in the red zone at 60 mph in 1st gear, but that error won't effect the essential comparative result, so I will ignore the red zone.
I can't know whether I should shift to 2nd before reaching 60 mph, until I check what the power will be in 2nd gear at 60 mph and confirm that it is more than hp. At 60 mph in 2nd gear, the engine speed will be rpm, and the chart indicates about hp, which is slightly less than the hp that I'll get at 60 mph if I remain in 1st gear.
Therefore, if this chart is correct and subject to the accuracy of my measurement of the rear wheel diameter, the implication is that for purposes of maximum acceleration, I should wait until I reach a speed slightly higher than 60 mph before shifting from 1st to 2nd.
But what happens if I compare the engine torques? At 60 mph in 1st gear, at rpm, the engine torque has dropped to about 60 lb. At 60 mph in 2nd gear, at rpm, the engine torque is about 85 lb. If I were to believe that the acceleration is greatest when the engine torque is greatest, then I would conclude that I should shift well before I reach 60 mph, probably somewhere in the neighborhood of 50 mph.
But let's see what happens when we convert those engine torque values to rear wheel torque. When 60 lb. When 85 lb. In other words, I have slightly more rear wheel torque at 60 mph in 1st gear than I do in 2nd gear, which suggests that I should wait until I reach a speed slightly higher than 60 mph before shifting from 1st to 2nd, which agrees exactly with the result that I got when I compared the power!
Let's consider the question of what determines the engine torque, and in doing so, let's be careful to distinguish the instantaneous torque from the average torque through a full rotation of the crank.
The quantity of work done during one complete rotation of the crank is fully determined by the integral of the instantaneous torque over that complete rotation. The average torque over the rotation is the integral of the instantaneous torque divided by the angular distance, so it follows that the average torque over the crank rotation effectively determines the quantity of work done over that crank rotation. I have of course simplified matters by adopting an engine that has but a single cylinder.
Now, if it were possible to increase the average torque over a crank rotation simply by lengthening the stroke and the crank throw, then it would be possible to arbitrarily increase the work, the power, and the acceleration simply by lengthening the stroke! Talk about your free lunch! The work performed by an engine during the movement of the piston through a single power stroke is determined by the quantity of energy released by the combustion of fuel and by the compression ratio the compression ratio determines the thermal efficiency.
Any desired compression ratio can be achieved for a given stroke, by shortening the space between the piston face and the cylinder head. Since the length of the stroke does not fundamentally determine either the compression ratio or the amount of energy released in the combustion, the quantity of work performed during a single crank rotation must be independent of the length of the stroke.
Since the average torque over the rotation likewise determines the quantity of work performed, it follows that the average torque over the rotation must also be independent of the length of the stroke. The torque applied to the crank depends on the force as well as the crank throw distance. The force that the gas exerts on the piston face is proportional to both the gas pressure and the area of the piston face.
As the stroke is make longer, for a given displacement, the piston face area is made proportionally smaller, and the force exerted on the piston face is made proportionally smaller. Thus, the effect of increasing the stroke length is cancelled by the coupled effect of reducing the piston face area.
Here, we see that even the maximum instantaneous torque applied to the crank will be independent of the stroke if the stroke variation is subject to a constraint on the displacement. To the extent that a long stroke engine happens to produce more engine torque at low engine speed as compared to a short stroke engine, the reason for this is at best indirectly related to the length of the stroke.
Rather, it can only be due to a difference in volumetric efficiency at that lower engine speed, i. Instead of the long stroke causing the engine to produce more torque at low engine speed, the truth of the matter is that the long stroke and its associated greater piston speed and piston acceleration limit the engine speed. As such, it only makes sense that the design characteristics such as the valve lift and duration be optimized for greatest volumetric efficiency at the lower engine speeds where that engine will always be operated.
Because of that specific optimization, you would expect that such an engine should be capable of producing more torque at those low engine speeds than an engine that is not similarly optimized for low engine speed. The power and acceleration will come on a little sooner off the line, and this sort of engine will be able to increase its output from low output to maximum output more quickly, because the rpm range through which it must be accelerated to reach its peak power will be smaller.
However, power determines acceleration, and power depends on the engine speed, so an engine of this sort is inherently incapable of achieving the same power or acceleration as compared to a high rpm engine of similar displacement, at any road speed.
Even "off the line", if a short stroke, high rpm engine is mated to a gear box with 1st gear set adequately low, the short stroke, high rpm engine will out-accelerate the long stroke, low rpm engine, anytime, anywhere. Torque and Power Home. All rights reserved. End of the article. Main Menu Home. Dyno Photo Album. My Home Page. Contact Me. Site Map. Privacy Policy.
Horsepower and Torque. Horsepower and Torque Everyone knows race engines make big horsepower and torque, but what do those terms mean? Without rpm, torque is useless. Torque and Power Home Five Popular Urban Myths about Torque and Power Debunked by Tom Barber Anyone who spends much time on motorcycle and car related Web forums knows that there is a fair amount of debate on whether the torque or the power of an engine is the true indicator of the vehicle's maximum acceleration.
Myth 1: Dynamometers only actually measure torque. Power is an abstract quantity that can't be measured, and must be calculated from the torque. Torque Multiplication There is one more important theoretical concept that we have to discuss, that being an important consequence of the relationship among power, torque, and angular velocity as given in the last equation above.
Metrics of torque and power; dynamometer theory Okay, at last we're ready to talk about the metrics of torque and power, and about how dynamometers work. Inertial dynamometers vs. When it comes to understanding how your engine is performing, torque and power are key data points to assess. Torque is an effective measurement when seeking to quantify the true mechanical torque on the driveline component, such as a driveshaft or a coupling.
The measurement is typically used to either validate the design of the component or to troubleshoot failures, such as those caused by damaging torsional vibrations. Power measurements, on the other hand, are typically most important when quantifying the performance of the equipment driving the shaft such as an engine. Torque is a measurement of twisting or rotational force. The more torque an engine produces, the greater its ability to perform work. Horsepower is defined as the rate of doing work, or how quickly work is accomplished.
A horsepower value tells you how much work your engine is capable of, in a certain time frame. That value is dependent on both torque and RPM. This test works usually by connecting the output shaft of an engine to a set-up that applies a resistive load.
As the resistive load is applied, the dynamometer measures both the torque and the speed applied by the engine. The end result is an engine performance curve, which graphs torque, speed, and power.
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