Understanding Dyno Power Correction

[Lud]

There seems to be a good deal of misunderstanding about how the "corrected" numbers on a dyno chart are derived and how environmental variables affect performance. I found a good summary here http://www.superflow.com/support/cycledyn-theory.html and the relevant portion is reposted below:


Power Correction Factors

The power output of an internal combustion engine is significantly influenced by barometric pressure, ambient air temperature, and air humidity.

• Lower ambient barometric pressure reduces the density of the air, thus reduces the amount of oxygen filling the cylinder for each cycle, resulting in lower power output. Conversely, higher barometric pressure increases power.
  
• Lower ambient air temperature results in increased density of the air, thus increases the amount of oxygen filling the cylinder for each cycle, resulting in higher power output. Conversely, higher air temperature reduces power output.
  
• Lower air humidity (less water vapor) leaves more room for oxygen per cubic foot of air, thus increases the amount of oxygen filling the cylinder for each cycle, resulting in higher power output. Conversely, higher air humidity reduces power output.

Several Standards organizations have determined methods for estimating engine power under reference conditions. The best known organizations are:

ISO (International Standards Organization), worldwide
SAE (Society of Automotive Engineers), USA
ECE (European Community), Europe
JIS (Japanese Institute for Standardization), Japan
DIN (Deutsche Industrie Norm), Germany

There are power correction standards for gasoline and Diesel engines, for applications in road vehicles, stationary engines, or marine engines, etc. For a motorcycle dynamometer, relevant standards are those generally intended for gasoline engines in road vehicles and those specific to motorcycles.

Power correction standards try to estimate what engine power would be under reference conditions. They cannot actually calculate exactly what power output would be. The greater the difference between the ambient conditions during the test and the reference conditions, the greater the error in the estimate. Most power correction standards include limits on their applicability. This limit is typically +/- 7%. This means if the correction factor is greater than (>) 1.07 or less than (<) 0.93, the corrected power numbers are not officially considered to be acceptable, and the test should be performed again under conditions which are closer to the reference conditions.
For private applications this is less of a problem, and the corrected power numbers are still the best basis for comparisons. However, please keep this into consideration when comparing test results obtained under considerably different test conditions.

Power corrections are only valid for Wide Open Throttle (WOT) tests. You should disregard corrected power numbers for any test performed under partial throttle conditions. The default configurations supplied with the CycleDyn system include power corrections to the following standards: SAE, STP, ECE, DIN.

• SAE -- The SAE standard applied is a modified version of the SAE J1349 standard of June 1990. Power is corrected to reference conditions of 29.23 InHg (99 kPa) of dry air and 77 F (25°C). This SAE standard requires a correction for friction torque. Friction torque can be determined by measurements on special motoring dynamometers (which is only practical in research environments) or can be estimated. When estimates must be used, the SAE standard uses a default Mechanical Efficiency (ME) value of 85%. This is approximately correct at peak torque but not at other engine operating speeds. Some dynamometer systems use the SAE correction factor for atmospheric conditions but do not take mechanical efficiency into consideration at all (i.e. they assume a ME of 100%).
  
• STP -- The STP (also called STD) standard is another power correction standard determined by the SAE. This standard has been stable for a long time and is widely used in the performance industry. Power is corrected to reference conditions of 29.92 InHg (103.3 kPa) of dry air and 60 F (15.5°C). Because the reference conditions include higher pressure and cooler air than the SAE standard, these corrected power numbers will always be about 4 % higher than the SAE power numbers. Friction torque is handled in the same way as in the SAE standard.
  
• ECE -- The ECE standard is based on the European Directives. Power is corrected to reference conditions of 99 kPa (29.23 InHg) of dry air and 25°C (77 F). Friction torque is not taken into consideration at all. In 1995, a new Directive (95/1/EEC) regarding test methods for motorcycles was published.
  
• DIN -- The DIN standard is determined by the German automotive industry. Power is corrected to reference conditions of 101.3 kPa (29.33 InHg) of dry air and 20°C (68 F). With the advent of European legislation and standards, national standards such as the DIN (formerly widely used) are now less significant.

There is a tendency for all those standards to converge. The only worldwide power correction standards at this time are the ones determined by ISO. For internal combustion engines in road vehicles, this is the ISO 1585 standard. The current SAE J 1349 and ECE standards are nearly identical to the ISO 1585 standard.

 

[1BADNSX]

Lud, good primer. The other item many folks misinterpret with dyno numbers is the torque. They usually say they have xxx rear wheel torque. This is not true. The dyno calculates actual drum torque and converts it using the measured drum RPM and engine RPM to torque at the “crank”. If it didn’t, the measured rear wheel torque would change significantly when testing in different gears. So what you really get is crank torque that includes drivetrain losses.

Bob

 

[sjs]

Lud, good primer. The other item many folks misinterpret with dyno numbers is the torque. They usually say they have xxx rear wheel torque. This is not true. The dyno calculates actual drum torque and converts it using the measured drum RPM and engine RPM to torque at the “crank”. If it didn’t, the measured rear wheel torque would change significantly when testing in different gears. So what you really get is crank torque that includes drivetrain losses.

Bob

Yes, thanks Lud. It still didn't clear up a question I asked on a thread in the Forced Induction forum, at least not directly.

Thanks Bob. I guess I've heard that before but not so succinctly and it didn't sink in. So torque numbers are still relevant and comparable between cars etc., but are not as closely related as one might think to HP readings on the same car. Given this information and the fact that HP is really just calculated from torque and RPM, shouldn't we be able to back-calculate with some degree of accuracy the % of mechanical losses represented in the HP figures?

 

[KGP]

The dyno calculates actual drum torque and converts it using the measured drum RPM and engine RPM to torque at the “crank”. If it didn’t, the measured rear wheel torque would change significantly when testing in different gears.

I never even thought about that, but what you say makes perfect sence.

Thanks guys.

 

[Arata]

It does not matter what gear you are in the dyno is measuring rear wheel HP and rear wheel torque.
The drum is a certain known mass, in a higher gear the drum spins faster, but takes longer to get there, in a lower gear it does not spin as fast, but gets there sooner.
I can post some dyno charts of runs in different gears, the HP and torque are the sam, in fact if you do not show the speed, because the dyno is showing only RPM and HP, and or torque, there is no way to know what gear it is in.

Watt calculated that the horse traveled with a force of 180 pounds, and that it traveled at a rate of about 144 revolutions per hour, or about 181 feet per minute. By multiplying the two quantities, Watt quantified the horse’s measurable leverage—i.e., its torque—at 32,580 pound-feet per minute. Converted (and rounded off) to 33,000 pound-feet per minute. Watt had his 1-horsepower equivalent.

Torque formula T = HP x 5252 Divided by RPM

The most common gear to do a dyno run is 4th, in first, the rear wheels accelerate so quickly it is dificult to keep traction , just like on the street, by the time you get to forth, the wheels are accelerating slow enought to maintain traction and have enought load on the engine to give the most usable reading, using 6th gear just puts the maximum load on the engine upping the chance of causing a problem.

 

[sjs]

Hmmm... is there a difference in this respect between dyno types, such as inertia vs eddy current vs whatever you call the hydraulic approach of Dynapack? Not asking for a dissertation on the various types just now (although a link to a good full technical comparison would be nice) but in terms of torque at the wheels or crank.

BTW, I've always seen/heard that the most common gear for testing was the one closest to 1:1 in street cars, for whatever reason.

 

[Arata]

If you are measuring HP or torque at the rear wheels, the maker of the dyno can put in a formula to guess at what the HP or touque would be at the crank, but losses from the rear wheels to the crank are going to be different with different types and weights of drive systems. So if you want accurate crank HP you must measure it at the crank, to much work unless you are building engines and have them out already.

 

[wildbill846]

Lud,

Good explanation. We do quick power computations everytime we fly because ambient conditions can make a large difference in the power output of an engine.
In my helicopter, I can see a 30% difference in power output between flying on a cool winter day in North Carolina or flying in the Iraq desert in the middle of summer.
The dyno corrections appear to be just the same as we use when computing Density Altitude. Pressure altitude (we use 29.92 as standard) compensated for temperature and humidity.

Bill

 

[1BADNSX]

It does not matter what gear you are in the dyno is measuring rear wheel HP and rear wheel torque.

This is why I made the post, most people do not understand it. In a stock geared 5-speed NSX the rear wheel torque in 3rd gear is 27% (1.23/0.967) greater than in 4th gear at any RPM, but a dyno curve will give the same torque as you noted. This is because the dyno corrects the torque to crank values. Corrects for gearing, not mechanical losses as I mentioned above, therefore the value includes losses associated with getting the torque to the rear wheels. It is a crank magnitude of torque, minus losses getting the torque to the rear wheels.

I can post some dyno charts of runs in different gears, the HP and torque are the sam, in fact if you do not show the speed, because the dyno is showing only RPM and HP, and or torque, there is no way to know what gear it is in.

The dyno knows the exact final gearing of the vehicle by taking a ratio of the drum RPM to engine RPM, both are measured.

Bob

 

[sjs]

Well, we still have conflicting theories here. Does anyone know of an on-line technical document covering this topic? I did a quick search without success.

Arata, do you understand what Bob is trying to say, and do you see a flaw in it?

 

[Arata]

With a Dynojet dyno you are acelerating a heavy drum, on the car dynos, I believe it is around 1500lbs.
In third gear you are indeed acelerating the drum faster (time), but the RPM of the drum is slower. in forth gear the aceleration is slower (time) but the speed is faster.
Both of these measurements rate of aceleration (time) and speed that the drum is spinning are measured.
So because the RPM is measured it does not matter what gear you are in, the dyno is measuring HP and Torque at the rear wheel.
HP is a math equation of torque and Torque is a math equation of HP. If you know one and the RPM you can calulate the other.
So if you know rear Wheel HP and you know RPM you know Rear wheel torque.

 

[1BADNSX]

This is my last post on the subject. Arata, please use the specific form of the power equation Power = Torque x RPM / 5252 and prove yourself wrong. If you enter the rear wheel HP and torque as you claim, you will not calculate the rear wheel RPM. You will calculate the crank RPM because the dyno converts the torque back to crank values. Horsepower is always independent of gearing and doesn’t need converted. It does this so everybody can compare their measurements and it makes the torque measurements relatively independent of gear choice (final drive ratio). BTW, the business of using a gear ratio near 1-to-1 isn’t anything magical because this suggestion doesn’t take into account ring and pinion gearing which is significantly different from car to car.

Bob
15 years of experimental testing experience, PhD Mechanical Engineering, Instructor for numerous clubs and BMWCCA Racer

 

[Arata]

Let me see if I am reading this correctly.
If you measure HP at the rear wheel and RPM at the crank and use the standard accepted formula, as you correctly posted it, it becomes crankshaft Torque?

If you enter the rear wheel HP and torque as you claim, you will not calculate the rear wheel RPM.

I cannot find where I said this, and I hope I never said it, if I did it is wrong as you point out.

 

[Arata]

How about calling ie Crankshaft torque as measured at the rear wheel, I am happy with that.
As if it is measured at the crank it will of course be different then if measured at the rear wheel.