I posted this response in another thread, but it was buried near the end and I decided that it should be a subject for a new thread.

We all need to look in another area for the answer to the drive train loss issue... The dirty little secret that no one seems to understand (and the Dyno Manufacturers will not tell you) is that the majority of the losses are not generated in the drive train of the car, the losses are generated in the dyno.

The typical chassis dyno is not an active load applying dyno, they are passive inertial drum dynos. That means that the system can only operate in a transient condition. As the drum is accellerated by the rear wheels of a car the drum is absorbing a portion of your horsepower in the form of inertial energy (called I-Omega losses). These losses are proprotional to the rate at which the drum is being accelerated and its mass (which is huge, approximately 3500#). The faster you accellerate the drum, the greater is the absorbed inertial energy... This is why your loss is greater with a lower rear end ratio. The lower ratio increases rear wheel torque causing the drum to be accellerated at a slightly faster rate and the result is more I_Omega loss absorbed by the dyno drum. With an assumed fixed percentage of loss (15% for MN6, 20% for A4), the result is falsely inturpreted as a slightly lower RWHP rather than a higher loss to the dyno. This is also true with higher HP engines and is the reason that the loss is a PERCENTAGE of total HP generated by the engine.

For those that do not believe this to be true, I suggest a simple test... Put your car on a dyno and do a normal 4th gear dyno run. Then repeat the run with the car in 3rd gear... The result will be a RWHP curve LOWER than the first one... Since the cars engine output is the same regardles of what gear it is in, the increased loss is due to greater inertial absorbtion by the dyno drum... Okay, okay, I can hear it now... "The losses are due to the extra gear set used in third gear and not in the dyno"... So that begs one more test... Conduct another dyno run with the car in 5th gear... Since this will accellerate the drum more slowly, the drum loss will be less and the RWHP will be HIGHER, even with the additional gear set in 5th gear... All this inertial energy is released in the form of heat after the dyno run is complete and the operator applys a huge brake to the drum bringing it to a stop...

As a side note, if it were possible to measure RWHP starting from high rpm and DECELERATING, the result is reversed since the drum would be releasing its energy on the decel giving your car a falsely HIGHER RWHP curve... This is not possible with a passive enertial drum dyno system, but any operator of an active load absorbing dyno knows these facts to be true...

So how much of the total loss is ligitimate drive train losses and how much is lost to the dyno? A typical gear set absorbes about 1.5% (just add up the stages). Bearing frictional losses and other losses will be about 3%... So your (MN6) car in 4th gear should not have more than about 5% actual drive train loss... That leaves about 10% that is generated in the dyno drum for the perceived total of 15%...

The proper way to conduct dyno testing is with an active load absorbing dyno that can measure engine output under constant speed conditions. This is normally done at specific engine speed points that can then be used to plot HP and torque curves for the engine... This method results in accuate hp readings regardless of where it is measured (flywheel or rear wheels)... It is also the reason that the Mustang dyno (an active load dyno) reads different from a passive drum style dyno... If the Mustang dyno were used properly by stopping at discrete speed points to measure steady state data, you would get actual RWHP data including about 5% of actual drive train loss...

How do I know all this? I am a degreed Mechanical Engineer and my job is to dyno test 2000 hp turboshaft helicopter engines every day... And I use an electric dyno to conduct steady state measurements and eliminate the I-Omega effects...
Cheers
Shirl
SD Racing Enterprises

 

Good post! :cheers:

 

Thanks for the post MrEracer, I always wondered where the power disappeared to! :flag :flag

 

that was a great post thanks :cheers:

 

What character was it in literature that said "things are not always what they seem"? In a way, this explanation is a bummer, in that we can no longer add the 15 or 20% to RWHP figures and come up with a big bragging number for flywheel HP. :(

 

Marco383,
Actually your engine is still producing the 15% to 20% more at the flywheel, its just not showing up on the bottom line (dyno graph)... The good news is that you can take your dyno number and add 10% to get a more realistic rear wheel power output (Superflow dyno's only)...
Shirl :D

 

inertia dynos are not designed to "certify" an engine. Their primary function is to accurately give a "before" and "after" mod comparison...
in which inertia dynos are very capable of doing as their results are very repeatable.

It really doesn't matter what the initial HP numbers are for a given car. It is the gain you would get after adding a H/C or Supercharger package. That's why many speed shops use inertia dynos; to determine what gains (if any) certain mods give to a particular car.

Granted you could use an inertia dyno to compare one particular bone stock car to many others that are all bone stock. You would establish a baseline. Again, the baseline isn't to certify an engines output, btut solely to compare it with others to see if they are similar.

.


[Modified by Mike Mercury, 11:34 AM 12/29/2002]

 

I couldn't agree with you more, Mike... The enertia dyno is a great diagnostic and tuning aid. My post was not intended as a slam against these dynos, my intent is to help others understand the physics of what is going on with the assumed drive train losses...

The idea that a car's drive train is consuming 15 to 20% of an engines output is pretty ridiculous... As Jimman pointed out in another post, the heat equivelent of 15 to 20% of an engines maximum output would melt the entire car... The actual drive train loss is still enough to cause serious heating of the transmission and rear end, which in long endurance applications requires coolers to keep things in check...
Shirl
SDRE

 

Thanks for the dyno lesson. :)

 

:cheers: It's people like you and posts like this that make this forum so additictive! :thumbs: :steering:

 

Great post Shirl.. Thanks. And good choice of Corvette year/colors too!

 

Shirl,
I am having a hard time understanding why the interial dyno is understating hp. I'll go through the physics as I understand it and you tell me where I am off track.

The interial dyno drum has a mass and moment of inertia of I. A torque T from the car's wheels is applied to the drum causing the angular velocity w (omega) to increase at a rate of w' (angular acceleration). Expressed as an equation,

w' = T/I angular acceleration is torque divided by moment of inertia

The dyno is just a torque measuring device. Measuring angular velocity of the drum as a function of time yields angular acceleration and the drum's moment of inertia is a known constant. Then torque is just

T = Iw'

The hp at the rear wheels is derived as the product of the torque and the angular velocity. (The torque is usually expressed as the rear wheel torque "referenced" to the flywheel to take out gear ratio effects.)

P = Tw

There are undoubtedly windage and bearing losses in the drum but these can easily be measured and corrected for in the dyno software. I assume that the drum is designed to have a moment of inertia equivalent to the mass of a typical car so that the time period of the acceleration roughly approximates actually accelerating the vehicle under WOT.

As for drive train losses, there are the losses associated with friction such as gear oil, bearing friction, windage etc. These are steady state losses that would be apparent with an active load dyno at a steady RPM. Under transient conditions, such as the passive inertial dyno or drag racing, there is the reaction torque of accelerating all the rotating masses including the crankshaft, cam, harmonic balancer, alternator, blower (for those so fortunate), flywheel, clutch, driveline, transmission, differential, axles, rotors, wheels and tires. This would not show up in a steady state, constant RPM hp measurement. But under acceleration it shows up as a substantial net loss of torque (and consequently hp) at the rear wheels and it varies depending on how hard you are accelerating and what gear you are in. It is real loss measured at the wheels and reduces the linear acceleration of the car. This is why lighter flywheels and wheels are beneficial. I believe this is where the 15% is to be found. It is not in the dyno, but results from making a transient measurement of hp rather than a steady state.

After having gone through all that, I think we are saying much the same thing except that I don't agree that the inertial losses in the dyno are a factor. I see it as the inertial losses in the rotating running gear of the car. But isn't the measurement of hp under transient conditions more representative of how we use our motors?:D

Happy New Year!! :seeya

Alan





[Modified by alc2001, 2:17 PM 12/29/2002]

 

MrEracer. Im trying to understand...

I understand that attributing frictional losses alone to the 15 or 20% figures would be asking alot. But thinking about it, I can see the 15 or 20% losses coming from the engine having to rotate the extra mass after the crank. To me that is where the losses are and would seem, coupled with the frictional losses, to produce a loss at 15 o5 20% depending on the transmission type.

For example, simply adding an underdrive pulley adds approximatly 4% in power by recooping losses the motor has to perform externally. And to me thats very miniscule relative to the engine turning the tranny, rearend, driveshaft, half shafts, wheels, tires with respect to weight, not considering frictional losses, through gear to gear contact, u-joints, and so forth.

Also, does not the intertial dyno compensate in its calculations for the intertial energy absorbtion?

thanks

 

Kewlbrz - Under transient conditions, the torque required to accelerate all the rotating mass including the engine contributes to apparent loss at the rear wheels. I believe that factory flywheel hp ratings are at steady state, constant RPM which accounts only for friction losses.
Alan

 

extremely enlightening. Yes; at a steady RPM you are only maintaining rotational mass; not trying to increase it's rotational speed.
It takes more power to increase the rotational speed of a mass... then to maintain a constant speed.


.


[Modified by Mike Mercury, 6:07 PM 12/29/2002]

 

if that is true that the dyno loses the power then why when i use a g- tech the horsepower is almost the same as a dyno with in 3 hp + or - where is the loss there ?? :confused:

 

All I can say is that it is a good thing that Horsepower is a relative measure and is actually calculated based upon torque and rpm. In other words, it doesn't really matter what the number is, it only matters that when comparing, you are using the same measuring device.

I feel better now. :crazy:

 

In the algorithms designed to compensate.

 

...and then there's SAE and STD-correction factors that I still don't quite understand completely!

 

Thank goodness for racetracks! :cheers: :lol: