Dynamometer Tech 101

OBJECTIVE: To educate our customers about our dynamometer including a clear definition of what a dynamometer is, explanation why the dynamometer is an important tool, and technical information of its operation.

Altered Atmosphere Motorsports, Inc. is privileged to have been the first facility with an AWD (All-Wheel Drive) Dynojet Dynamometer in the United States. In this basic training session we are going to learn the inner working of the dyno, its operation, and how to use it. Before we begin I would like to explain the basic function of a dynamometer including its safety, efficiency & accuracy, baseline & successive testing, and its diagnostic capabilities.

What is a Dynamometer?

A chassis dyno, or dynamometer, to be precise is an automotive tool designed to simulate a road in which an automobile is driven. It’s main purpose is to provide a means in which an automobile can be accelerated, cruised, and generally driven while remaining stationary in the confines and safety of a shop floor. A chassis dynamometer is used in this manner to achieve several objectives ranging from performance tuning to diagnostics. The chassis dynamometer employs a simple design consisting of a roller mass specifically weighed to simulate identical physics of a car being driven on a road and measurements are taken through sensors attached to the roller and the vehicle to evaluate performance.

Safety

A technician can safely test and tune a car repetitively without having to drive the car at unsafe and illegal speeds. Many automotive performance tests consist of running the vehicle at wide-open throttle and at high speeds. Even if a technician has access to a test track and professional test driver, the issue of safety still exists. Not to mention the wear on the vehicles tires, brakes and suspension.

Efficiency & Accuracy

A chassis dyno provides an automotive technician a platform in which to repetitively test, evaluate and tune a vehicle in a relatively short amount of time. Using the dyno's load functions, a technician can tune a vehicle across the engines' entire operating range and performance envelope. Modern chassis dynamometers such as the Dynojet 248 operate with cutting edge software technology to provide accurate, efficient, and repeatable testing. A dyno used in conjunction with current electronic engine management systems and tuning software provides a technician or enthusiast a complete toolkit to evaluate, adjust, re-evaluate, and perfect a vehicle’s tuning (running) parameters.

Baseline & Successive Testing

An automotive enthusiast wishing to improve vehicle performance can use a chassis dyno to determine the vehicles baseline performance before any modifications are made. After each stage of modifications are performed, successive dyno tests are conducted to evaluate and determine the effectiveness of the modifications. Leading aftermarket vendors such as Altered Atmosphere Motorsports who are involved in developing performance products use chassis dynos to test their products design and effectiveness in improving performance.

Air-Fuel Option (Wide Band Air-Fuel Ratio System)

Provides real-time air/fuel ratio analysis via a tailpipe sample tube or bung attachment. A/F data can also be graphed along with horsepower, torque, etc. This option is widely used because it gives you a very important reference point in which to monitor your engines air-fuel mixture. Because power comes as a result of the combustion of air and fuel in an engines cylinder, to optimize your vehicles power and ensure greatest reliability the air-fuel options should be considered mandatory. For best gas mileage air-fuel ratios between 14.5 to 15.5 at idle and light throttle are desired. For best power (wide open and/or high throttle) air-fuel ratios on most naturally aspirated engines like to be between 12.5 and 13.5, and 10.5 to 11.5 for turbocharged vehicles.

Diagnosis

Certain symptoms of vehicular problems are many times exhibited only when the vehicle is driven and only during specific speeds, RPM, gear, etc. A chassis dyno can aid a repair technician by simulating the same conditions in which the symptoms are exhibited while the vehicle is driven on the street without the safety and legal issues.

TECHNICAL

About Altered Atmosphere’s Dynojet Dynamometer

Our Dynojet Dyno can accurately measure performance or help diagnose drivability problems in vehicles with anywhere from 20 to 1500 HP at the drive wheels. Dyno runs can be done at speeds in excess of 180mph, providing safe, reliable road testing right in the shop.

With a quick vehicle mounting time, we are able to measure, record, and diagnose performance problems quickly. Energy drawn from the vehicles drive tire is stored in a rotating mass (inertia drum) sized to simulate driving a vehicle on the road. A computer connected to the Dynamometer displays horsepower by measuring drum acceleration. Runs with up to 1500 horsepower are processed into easy to interpret graphics and saved for future reference. The software (WinPEP7) automatically corrects for altitude and atmospheric conditions to producing consistent and easily interpretable power graphs. All tests are stored for future comparison and analysis.

Our dyno can be operated with most FWD, RWD, and AWD (all-wheel drive) vehicles. We actually have two separate dynos that can be run together to accommodate an AWD vehicle. The Model 248x Dynamometer (larger red drum) features 48" diameter, knurled, precision balanced drums, while the Model 224x Dynamometer (smaller black drum) features 24" drums. The drums provide stable contact for the tires eliminating unnecessary slippage and tire deformation resulting in precise repeatable results each run. This testing capability allows a realistic simulation of street or track conditions - highlighting mechanical problems graphically as they would occur and eliminating unnecessary engine and power train wear in actual road use testing.

THEORY OF OPERATON

Our inertia 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, making computer hardware and software 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 (HP) is equal to applying a 550 pound force through a distance of one foot in one second. In real terms, it would take one HP to raise a 550 pound weight up one foot in one second. So to measure horsepower, we need to know force (in pounds) and velocity (in feet per second). Our Dynojet inertia dynamometer measures power according to the terms just described. The dynamometer measures velocity by measuring the time it takes to rotate a heavy steel drum one turn. The dyno 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, (F)orce equals (M)ass times (A)cceleration. Power is coupled to the drum by friction developed between the driving tire of the vehicle and the knurled (diamond shaped) steel surface on the drum of the dynamometer.

Torque

When an object rotates around a point, the object’s 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 one foot radius, is pulled with 550 pounds of force, the resulting force 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 because 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. The correction factor 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 the 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 liter of air and the more air the engine gobbles up during the intake stroke (increasing horsepower potential). 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 is Absolute barometric pressure. RelHg is Relative barometric pressure. Elev is 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 dry bulb temperature. As air is blown over the wet bulb 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 equals 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 (decreasing horsepower potential). 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 liter of air and less air is swallowed during the intake stroke (decreasing horsepower potential). Dynojet’s WinPEP 7 Performance Evaluation Program for Windows) 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 To+460 / 537) - 0.18

Where: To is intake air temperature in degrees F. Bdo is dry ambient absolute barometric pressure.

What does this mean? Our Dynojet’s software will use this formula to accurately display the horsepower and torque of dyno runs made on different days (regardless of weather conditions). These corrected horsepower and torque numbers are referred to as “SAE” and allow you to compare runs of similar vehicles tested on other Dynojet dynamometers across the country.

Because “SAE” is a correction factor the horsepower and torque numbers displayed may be more or less than the vehicle actually made on the day it was tested. The actual power output of the vehicle is called “Uncorrected” horsepower and torque. “Uncorrected” power and torque varies depending on weather, barometer, and elevation. To see the horsepower and torque readings the vehicle actually made you can switch from “SAE” to “Uncorrected” power display.

Hardware & Software

The dynamometer electronics acquire data necessary for power, torque, and correction factor calculations. This includes air temperature, absolute barometric pressure, drum timing, and engine RPM.
During a run, the data is stored in the dyno electronics memory. After a dyno run is finished, data from the dyno electronics, calibration data, and user notes are saved to a file on the computer’s hard disk. When the user selects a run, the data is loaded from the hard disk into computer memory. Data from up to twelve runs can be placed in memory at once. This information can be used for viewing horsepower and torque graphs.

The drum data is used to calculate and display power while viewing graphs. Data can either be viewed as measured (“uncorrected”) or as corrected according 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 traveled 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 straight forward 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. Our Dynojet’s inertia dynamometer has become the industry standard because of its accuracy, repeatability, and design simplicity.

To schedule a dyno appointment for your car, e-mail service@alteredatmosphere.com or give us a call 301.330.8835!