The following is my question to the tech department at Superflow Dyno, and is followed by their reply.
Harold, Is the percentage of horsepower loss due to drivetrain friction
the same on a 500 hp engine as it is on a 300hp engine in the same car,
same drivetrain, same chassis dyno, etc.? By the same I mean, if it's 18%
on the 500 HP will it be 18% on the 300 HP. The 500HP & 300HP being engine
dyno numbers. Also, HP readings seem to be closer, chassis dyno to chassis
dyno than they are engine dyno to engine dyno. Does this have to do with
operator input etc.? Thanks, Al
Al,
This question is one that makes me have a good time in answering. It is of common interest to many and unfortunately, it is not addressed enough for folks to get a better understanding of the variables.
The loss across the drivetrain is not a fixed percent. It is a loss that is the same at the same speeds. I have attached some drivel that I have written on the subject. I hope that it is of some use to you.
Although operator influences are of some effect, most variations come from calibration differences and room conditions, exhaust back pressure or exhaust gas recirculation.
Normally speaking, good procedures will produce good results.
Numbers mean something, but only if you know where the numbers come from.
I had answered another question concerning the driveline losses in the following fashion and I think that it certainly applies in answering yours.
"I will attempt to answer them categorically and although I will try to be brief, I am generally not successful to that intention.
The issue of drivetrain losses happens to be a very complex problem and most people that are interested in these types of problems want rapid or simple answers, not complex ones. That is one reason why the fixed % loss (relative to power input) has been so popular over the years albeit an incorrect process of evaluation.
In the first example that I used, based on real circumstance measurements, the loss in the drivetrain was indeed 200+Hp. The major portion of the loss was a very "sloppy" torque converter that had a high percentage of "slip". Racing type torque converters are a mystery in themselves and I do not know how to address that issue other than with what we measured. The issue becomes even more confusing when testing with the converter "locked out" and the coupling becomes just a fluid coupling (according to what has been reported to me from our Customers). What I neglected to state (in a more clear fashion) was that the loss via the drivetrain should be somewhat fixed relative to speed (ie: varies with speed) and temperature reference points those losses are represented by a curve that is not linear with speed.
Almost everything in the drivetrain is a basic power resistor, absorber, or extractor. F=uN is certainly a common way to evaluate simple sliding friction, but as an example, if applied to the tires of a racecar, the coefficient changes with temperature and loading through drivetrain (the tire patch is torque input sensitive) and racing tires can generate a coefficient that is in excess of unity and maximum adhesion is typically at 12-15%(on some as high as 18%) of tire slip (according to race tire manufacturers). The losses for the tires themselves vary for a number of reasons and are important issues when considering total driveline losses. Most racing gearboxes (manual) utilize straight cut gears and as such do not generate the thrust forces on the case that the helical or hypoid type gears generate, but the losses (windage and viscous drag) are a function of friction, speed, and temperature. Additionally, dragging brakes or changes in pinion angle are among the many variables and "power extractors" in the system. The engineers and technicians at the OTC here in Colorado Springs work with exactly the same challenges for the bicycles used in competition (EX: Lance Armstrong's bike used tires that were approx 5yrs old).
The 12 to 20Hp losses that I mentioned were for 4speed gearboxes, the cars were using 9" Ford drive axles, and the power varied on the three race cars that we tested in the excercise from a high of about 500Hp to a low of about 350Hp as I recall. The point that I was attempting to make in my comments was that the gearbox is an absorber and perhaps more of an absorber than choosing the correct gear ratio for the drive axle so the racecar could be raced in direct gear instead of 3d gear.
The power losses for the trucks that I cited were passing on comments from manufacturers, not testing that we had done. However, because the axle that we use in our AutoDyn product is a large industrial axle that is used on some large over-the-road trucks, we have good data on the losses on that type axle. We even found out why it is better to use a synthetic lubricant in the axle assembly. The lube volume is about 5 gallons and with a ratio of 3.9:1, the measured power to roll the system at 200mph is typically about 36+Hp. Standard lube causes substantially larger losses. Not all manufacturers measure the losses in their systems, but we measure both the losses and the inertia (including aero losses) of our chassis dynamometer systems we do not use estimates or calculate what the inertia of the system "should" be. We also provide a method of measuring tire slip by using a non-contact speed sensor to relate to the tire patch to roll interface.
Inputs shared from some professional race teams seems to indicate that Nextel Cup cars and Busch GN cars (an example of a good one) has a loss of about 50 - 60+Hp range at 200mph. That figure is drivetrain loss only and is not including any aero or complete rolling losses although it does include the drive wheels and tire patches. The same car tested at the same speeds with a lesser powered engine has virtually the same power loss in the drivetrain. The power losses plotted vs speed is not linear at all. The friction power that should be considered when evaluating power systems for a reciprocating engine is also not linear with rpm.
Most of the advanced level testing that goes on at some levels of motorsports these days has a very tight evaluation window of acceptance (typically a maximum of +/-1%) in all that they do. Although the same teams have CFD programs, they still rely on testing to refine their engine airflow designs and spend time testing in full-size wind tunnels as well. The basic analysis for driveline losses is applied via testing the engine(s) so that they have a BHp reference (FWHP) curve and when the installed power package is tested, they have the power at the tire patch reference (RWHP) curve. The delta between the two is a driveline loss curve for that particular vehicle. Various corrections are typically applied so that evaluations are done with some standard reference, but in general, comparisons are done in the same fashion."
Regards,
HB2