"An In-Depth Look At the Dynojet Chassis Dynamometer".

taken from:

"An In-Depth Look At the Dynojet Chassis Dynamometer".

GM HIGH-TECH PERFORMANCE

A dynamometer tests the power of an engine or powertrain. It measures output shaft or wheel speed and, in some cases, torque and time. Apply laws of physics to that data and power output can be calculated. Modern dynos have computers to do the math and provide the results either on a computer monitor, in printed form or in a data file.

A chassis dynamometer accommodates an entire motorcycle and allows operation of its powertrain while the vehicle remains stationary. The motorcycle is attached to the dyno with tie-down devices and its drive wheels sit on a metal roller. The roller is connected to whatever measuring system the dyno uses to test drive wheel output.

The chassis dynamometer was developed in 1938 by Clayton Industries. It was termed a "hydrokinetic" dyno because it used a water brake to absorb power. Since then, chassis dynos have been built with water brakes, oil brakes, eddy-current brakes, generators and other devices which actively absorb power.

While brake-type chassis dynos are great pieces of equipment, most are expensive, a good part of which is the power absorption system and its controls. Another problem with some brake-type chassis dynos is they lack the accuracy and repeatability demanded by many performance-aftermarket manufacturers and tuners.

Dynojet Research of Belgrade, Montana, invented the "inertia" dynamometer in 1989 to test motorcycles. An inertia dyno differs from a brake dyno in several ways: 1) it has no active power absorption device 2) it's more accurate, 3) it's less expensive, 4) it's easier on the vehicles being tested and, 5) it's easier to use. Some of the technology that made an inertia dyno feasible was the personal computer's ability to make rapid computations.

Because the inertia dyno idea is so simple and cost-effective and Dynojet has had success marketing the concept, and other manufacturers have recently introduced similar products.

PHYSICS LESSON

At the core of a Dynojet is a metal drum riding on antifriction bearings. The drum weighs about 900 lbs, is two feet in diameter and has a knurled surface to enhance traction. Because 900 lbs. takes time to slow once the dyno run is complete; the drum can have brakes, operated via a button on the dyno's controller.

A key principle of the Dynojet is: the drum's inertia acts as a sort of passive power absorption device. "Mass equivalent" is a term engineers and physicists use to quantify the difference in inertia of a mass in linear or, more properly, "translational" motion and one in rotating motion. The mass equivalent of a rotating drum is quite different than its mass for translational motion so the weight simulated by the drums when rotating is different than their actual weight.

During manufacturing, Dynojet Research figures the mass equivalent of each drum to four places and bearing drag to five places.

Those proprietary figures are figured into the computation the dyno computer makes. If the mass equivalent of the drums is known and the rate at which a vehicle's drive wheels accelerate the drums is accurately measured; then the "thrust force," in pounds, at the rear wheels can be computed with a high degree of accuracy.

A combination of two laws of physics, force equals mass times acceleration and work equals force times distance, gives us this equation: W=m X a X d. "W" is the work (in pound-feet) the rear wheel is doing, "m" is mass equivalent (the drums), "a" is acceleration (increasing drive wheel speed) and "d" is distance (drum circumference). Once we have the work, we can find horsepower. One horsepower is 550 pounds-feet of work done in one second, so we divide the work number by the length of time measured, then divide the number we get from that by 550. To simplify: we get horsepower by multiplying the mass, acceleration and the distance, then dividing that product by time multiplied by 550. This can be expressed by: hp=(m X a X d) / (t X 550).

Torque can be figured by multiplying the horsepower by a constant, 5252, then dividing that product by the speed at which the thrust force was measured. Generally with rear wheel numbers, axle ratio is not considered in the torque computation. For comparison purposes, this makes more sense. The computer factors out the axle ratio by using engine speed data in the torque derivation.

In the real world, the measurements and computations are not quite that simple, but the complex methods Dynojet Research uses to apply these laws of physics and their mathematics to accurate measurement of rear wheel power is a proprietary secret.

COMPUTERS MAKE IT EASY

While all this math and physics stuff may bring back horrid memories of high-school, computer power makes it quite simple and this simplicity is the beauty of inertia dynamometers.

OK. You ran your bike on a dyno and its computer spit out a bunch of power and torque numbers. In fact, you have two sets: uncorrected, or "raw," numbers and corrected numbers. Each dyno is subjected to different atmospheric conditions. To facilitate comparison, there has to be a "standard" to which dyno results conform or are "corrected."

The DynoWare measures and inputs all variables relating to the correction process. The software does the corrections based on the atmospheric data.

CANS/CANT'S AND
PLUSES/MINUSES

A Dynojet can give you the rear wheel power and torque output of your bike. Well-duh! A test run can be done in two ways: 1) wide open throttle in one gear or 2) wide open throttle through multiple or all gears.

Besides simplicity, accuracy and repeatability, a great feature of inertia dynos is they get performance data with less abuse to the vehicle than does a brake dyno. A Dynojet can be used to evaluate the effects of modifications. It can be used to calibrate fuel injection systems and spark curves. It can be used to diagnose engine problems, such as detonation or mismatches between various modifications. It can help in solving driveline noise and vibration troubles. It can be used to evaluate the parasitic losses caused by different types of transmission and rear axle lubricants.

There are a few things a Dynojet can't do. Most importantly, it can't duplicate a drag strip, thus it cannot directly predict a bike's ET and speed in a pass, however, it can predict trends in a bike's performance. That is, if the bike runs well on an inertia dyno, it's most likely going to run well on the racetrack.

An inertia chassis dynamometer can be a tuner's secret weapon. It also can tell much about manufacturers' claims about products. It's simplicity, accuracy and repeatability make comparing and contrasting different performance modifications an easy and sometimes very revealing process.

Taken from: DynoJet Dynamometers Performance Evaluation Program, Version 4, User's Manual

Theory of Operation

Dynojet's inertial dynamometer is a measuring device for recording and displaying power and torque of an engine. Its method of measurement is a direct implementation of the definitions of power and torque. Correction factors assist in the comparison of these measurements under various test conditions. Computer hardware and software is necessary to obtain, interpret and display the data.

Power

Power in mechanical terms is the ability to accomplish a specified amount of work in a given amount of time. By definition, one horsepower is equal to applying a 550 pound force through a distance of 1 foot in one second. In real terms, it would take 1 HP to raise a 550 pound weight up 1 foot in 1 second. So to measure horsepower, we need to know force (in pounds) and velocity (in feet per second).

Dynojet's inertial dynamometer measures power according to the terms just described. It measures velocity directly by measuring the time it takes to rotate a heavy steel drum one turn. It measures force at the surface of the drum by indirectly measuring its acceleration. Acceleration is simply the difference in velocity at the surface of the drum from one revolution to the next. The force applied to the drum is calculated from acceleration using Newton's 2nd law, Force = Mass x Acceleration.

Power is coupled to the drum by friction developed between the rear tire of the motorcycle and the knurled steel surface on the drum of the dynamometer.

Torque

When an object rotates around a point, its speed of rotation depends on both an applied force and the moment arm. The moment arm is the distance from the point of rotation to where the force is being applied. Torque is the product of the force and the moment arm. For example, if a rope, wrapped around a drum of 1 foot radius, is pulled with 550 pounds of force, the resulting torque is 550 foot-pounds.

The torque on the dyno's drum can be calculated by multiplying the force applied by the drum's radius. However, engine torque is not equal to drum torque since the gearing through the drive train changes the moment arm. The change in the moment arm is proportional to the ratio of engine speed to drum speed. Therefore tachometer readings are necessary to calculate and display engine torque.

Correction Factors

The calculation of horsepower or the accuracy of our dynamometer is not dependent on the location or conditions during the measurement. The performance of the internal combustion engine is however sensitive to atmospheric conditions, especially air density and air temperature. To compare power measurements taken at different times or places, it is necessary to compensate for differing atmospheric conditions.

Correction Factors are used to compensate engine horsepower measurements for differences in operating conditions during engine testing. The typical correction factor (CF) is calculated based on the absolute barometric pressure, air temperature and water content of the air used for combustion by the engine under test. It attempts to predict the horsepower that would be developed if the engine were tested at sea level under standard pressure and temperature conditions.

Absolute barometric pressure is a measure of how hard air molecules are being pushed closer to one another. The unit of measurement is typically inches of mercury (inches Hg). The more pressure, the more molecules there are in a litre of air and the more air the engine gobbles up during the intake stroke. Absolute barometric pressure is equal to relative barometric pressure only at sea level. Relative barometric pressure is reported at airports and by weather barometers. A good approximation for converting relative barometric pressure to absolute barometric pressure is:

AbsHg = RelHg – Elev

1000

Where, AbsHg = Absolute barometric pressure. RelHg = Relative barometric pressure

Elev = test location elevation in feet above sea level

Water content is calculated from the ambient wet and dry bulb temperatures. Dry bulb temperature is normal room temperature. Wet bulb temperature is always less than or equal to the dry bulb temperature. As air is blown over the wet thermometer the water evaporates and cools the thermometer. The dryer the air, the cooler the wet thermometer indicates. If the ambient air is saturated (humidity is 100%), very little water evaporates and the wet bulb temperature is equal to the dry bulb temperature. These measurements are then converted to partial pressure in inches of mercury and used in the correction formula. Water vapor displaces oxygen and reduces the amount of combustion air ingested during the intake stroke.

Hardware and Software

Air temperature is the temperature of the air entering the intake system of the engine under test. In some cases this is ambient air temperature but in other cases the intake air is significantly heated by the engine and is different than ambient air. Heat tends to spread air molecules apart. So as temperature increases, there are less molecules in a litre of air and less air is swallowed during the intake stroke.

Dynojet's PEP (performance evaluation program) software uses the SAE's latest correction formula (June 1990). This formula assumes a mechanical efficiency of 85% and is much more accurate than earlier formulas at extreme conditions. The formula used is:

CF = 1.18 x 29.22/Bdo x vTo + 460/537 - 0.18

Where:

To = Intake air temperature in degrees F

Bdo = Dry ambient absolute barometric pressure

With the exception of wet and dry bulb temperatures, all parameters necessary for power, torque and correction factor calculations are measured by the electronics contained on two circuit boards.

The "Pickup Card" is located under an aluminum cover at the rear of the dyno. It measures ambient barometric pressure and detects each drum revolution. The intake air temperature probe, remote switch and tachometer pickup all connect to this card.

A cable connects the pickup card to the" PC Card" in the computer. The PC card converts the temperature and pressure analog signals to digital data. It also measures the speed of the drum by timing signals from the pickup card with a crystal controlled clock. These time measurements are accurate to better than .001 %, making this dynamometer extremely accurate and repeatable. During a dyno run, the run data is stored in the PC card's memory.

The PC card is controlled by the computer through the software package," Performance Evaluation Program " or PEP. PEP prompts the user for wet and dry bulb temperatures and any notes the user wishes to attach to the run. The program also tells the PC card when to begin and end acquiring run data.

After a dyno run is finished, data from the PC card, calibration data and user notes are saved to a file on the computer's hard disk. Each run file is saved in a directory structure composed of "Make", "Model" and "Name" which organizes the data for quick and easy retrieval. When the user selects a run, the data is loaded from the hard drive into computer memory. Data from up to three runs can be placed in memory at once. This information can be used for either viewing horsepower graphs or racing bikes.

The drum data is used to calculate and display power while viewing graphs. Data can either be viewed as measured ("uncorrected") or as corrected to standard atmospheric conditions.

The drum data can also be used for determining speed and distance traveled in a simulated race. When not correcting to standard atmospheric conditions, the vehicle speed is equal to the drum speed and distance travelled equivalent to the number of drum revolutions times drum circumference. The relations are no longer equal when correcting to standard conditions. Both speed and distance are then predicted from the corrected acceleration of the drum.

Conclusion

By accelerating a massive drum, measurement of power and torque is straightforward and accurate. The use of correction factors allows comparison of those measurements under various test conditions. The use of modern computers provides an economical yet logical method of obtaining and viewing the information. Dynojet's inertial dynamometer is fast becoming the industry standard because of its accuracy, repeatability and design simplicity.

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