Sensor technology drives accuracy of diesel engine testing - Technology - Power Test Inc.'s dynamometers

Diesel Progress North American Edition, Feb, 2002 by Diane Shaughnessy

Today's diesel engines must meet the stringent environmental requirements of legislators and regulators, while satisfying the performance demands of OEMs and end users. To guarantee top engine performance under a full range of operating conditions and to ensure overall engine reliability and conformance, a full test facility is required.

Power Test, Inc., Menomonee Falls, Wis., provides a complete engine testing solution for commercial, government and military applications. Power Test manufactures dynamometers for the entire diesel industry including engine and chassis dynamometers and compatible data acquisition systems. A dynamometer applies a load to a running engine, allowing engineers to determine the engine's torque, horsepower, safety and potential trouble spots.

Dynamometer Technology

In the Power Test dynamometer, water flow, proportional to the desired applied load, creates resistance to the engine. A controlled water flow through the inlet manifold is directed at the center of the rotor in each absorption section. This water is then expelled to the outer dynamometer body by centrifugal force. As it is directed outward, the water is accelerated into pockets on the stationary stator plates where it is decelerated. The continual acceleration and deceleration causes the dynamometer to absorb the power produced by the engine. Through this transfer of energy the water is heated and discharged.

An integral component of a Power Test dynamometer is its data acquisition system. The system is comprised of two units, called Commander and Workstation, connected by an Ethernet cable. Commander, a desktop computer operated by Windows-based software, issues commands to Workstation, a touch-screen operated unit housed in a rugged industrial enclosure. Workstation operates the precision load and throttle control systems, collects the data, and sends it to Commander to be processed, stored and analyzed.

Workstation's success, and therefore the data acquisition system's accuracy, depends on its ability to correctly measure data in the dynamometer tests. Central to these measurements is the precision of its pressure sensors, which measure airflow in the intake manifold, oil pressure and other fluid pressures. "The operator is interested in different pressures of fluids," said Power Test Project Engineer Allen Bergst. "So having the capability of bringing in pressures while you are running the engine is very important." For this essential task, Power Test uses pressure transducers from Setra Systems, Inc., Boxborough, Mass.

Sensor Technology

Setra Systems is a designer, developer and manufacturer of high accuracy electronic pressure sensors and other electronic measurement instruments. Setra's products are used by process and manufacturing industries, federal agencies, and research laboratories. Setra Systems derives its accuracy from its patented capacitive transducer design, coupled with its proprietary circuitry, which produces a strong signal that generates an accurate output.

In a typical Setra configuration, a compact housing contains two closely spaced, parallel, electrically isolated metallic surfaces, one of which is a diaphragm capable of slight flexing under applied pressure. These firmly secured surfaces are mounted so that a minute change in applied pressure alters the gap between them, creating a variable capacitor. The resulting change in capacitance is detected and converted to a proportional high-level analog signal.

Sensors Meet Demands

Power Test uses Setra's model 209 industrial/OEM pressure transducer because of its ability to measure accurately in rugged conditions. The model 209 transducer withstands mechanical shock and vibration, thermal shock, corrosion, and other extremes found in harsh testing environments of dynamometers.

Bergst said Setra's sensors are ideal because of their flexibility. "We needed versatility in the pressure ranges that we were sensing," he explains. Setra customizes the model 209 transducer to Power Test's specifications because the engine tester often requires uncommon ranges. The model 209 transducer has a standard range of zero to 50 psi or zero to 100 psi for low pressure measurement, but Power Test required sensors with ranges of(15 psi, zero to 2 psi, zero to 20 psi and 5 to 100 psi. Setra Systems Product Manager Jean Collins said customization of the model 209 transducer is well within Setra's capabilities. "The transducer's capacitive design allows for simple modifications to meet our customers' wide range of needs, so we were more than equipped to help Power Test with its request for uncommon pressure range measurement."

The dependability of Setra's design has led to the development of pressure transducers that fit the unique requirements of Power Test's engineers. "Setra has been very good to us as far as its ability to produce a specialty range sensor to meet our specifications with fast delivery time," Bergst stated. He added that "Setra was the only company we found that had a wide range of pressures all within the same compact package." This enabled Power Test to seek only one manufacturer for all its sensors.