Originally Posted by Barracuda
As the general consensus here has been, driveline losses are neither a fixed amount nor a fixed percentage, but do decrease as a percentage with increased engine power. I'll try to explain - to the best of my knowledge and ability....
Driveline losses are a VERY complicated thing, with numerous components and equations of physics coming into play. But, in general, the driveline losses can be broken down into inertial and frictional losses. I'll leave the equations out for now, but...
Inertial Losses - this is the power lost in accelerating the driveline components. Since chassis dynos are transient (i.e. accelerating), unlike steady state engine dynos that are used to certify the engine, inertia comes into play. Just like it takes torque to accelerate your car, it takes torque to accelerate the driveline components. Take the driveshaft for example... It has a rotational inertia based on it's diameter, mass, and design (i.e. solid vs. hollow). A car with a heavier, larger diameter, hollow crankshaft will have higher inertial losses (for this single component) than a car with a lighter, smaller diameter, solid driveshaft. The question here, though, is for the same car (with the same driveshaft) and different engine power levels (along with assumed higher torque levels), do the inertial losses change. The answer is YES. That is because the driveline components are accelerating at a different rate. The work done in accelerating a driveshaft from 0 RPM to max speed is the same. But Power = Work/Time. So to go from 0 RPM to max speed quicker, it takes more power. If you measured RWH with a constant acceleration dyno, then the inertial losses would be the same regardless of engine HP. Otherwise, if the dyno accelerates quicker with higher engine HP (which the ones I know of do), you will see higher driveline losses from driveline inertia.
So, while these inertial losses are not a fixed value with varying engine HP, are they a fixed percentage? Not likely. The acceleration will not double with double the horsepower. It will depend more on torque and frictional losses (which also change - see below) than on engine HP. So the net effect will be an increased amount, but reduced percentage as engine HP increases. (NOTE: I realize Torque will increase the same percentage as HP if you assume the same peak HP engine speed. I did not make that assumption.)
Friction - Some elements of driveline friction will increase with increased engine power (i.e. torque). Others will not. To keep it simple, lets think of the gearbox. There is a certain amount of frictional losses related to the gears turning in oil. The magnitude of these losses are dependent upon oil type, viscosity, temperature, level of fill, rotational velocity, etc.. It is not affected by engine power (or torque). The force required to accelerate the gears through the oil may be affected with different levels of acceleration (as in the inertia description above), but there is a component of friction that will be unaffected by engine power. The friction due to gears meshing, on the other hand, will be affected by engine output. As you put more torque through the driveline, the contact force between meshing gears will increase, which increases the friction.
So, as some elements of friction go up with engine power (and torque) and other do not, then it is clear that this component of driveline losses is neither a fixed value or percentage.
Hope this helps.
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