In the drive for greater combustion efficiency for light-duty vehicles, engineers are increasingly aided by faster and higher-fidelity sensors, computers, and algorithms. An evolving sensor that appears ready for high-volume use in light-duty vehicles and passenger cars is the turbocharger speed sensor.
Currently, most engine controllers derive the speed of a turbocharger from maps that correlate speed as a function of pressure ratio compared to mass air flow, or MAF. While useful, they may not be as accurate as one would like.
“Compressor maps derived on a test bench do not necessarily match real-world conditions,” explained Jonathan Tigelaar, Project Lead Mechanical Engineering for Jaquet Technology Group AG, a supplier of turbocharger speed sensors. “One reason is airflow is different,” he noted. “The other is the geometry of the engine and turbocharger configuration is different in its final installation compared to the test-bench conditions. That same turbo will have a different map depending on the engine installation.”
It also is difficult to cover the entire range of the map, so the “edges” may not be well characterized – or even tested. The danger designers face is overspeeding the turbo based on incorrect readings from either the pressure sensor, the MAF sensor or both. To compensate, engine developers add safety margins, reducing the maximum efficiency of the turbo and the engine.
“Correct speed sensing protects from overspeeding," Tigelaar explained. "It can also help match the speed rates of bi-turbos to reduce or eliminate NVH from mismatched turbos. Finally, it also refines the algorithm for switching within multi-stage turbos.”
First marketed in 2000, turbocharger speed sensors now are common on large diesel engines used in off-highway or commercial vehicles, with about 11 million in use today, according to Tigelaar. One of those diesel OEMs claimed a yield of 30% more horsepower from the engine by successfully reducing the safety margin and more efficiently using the full range of the installed turbochargers.
“We are ready for light-duty and passenger car, gasoline and diesel engines. Since 2009 we have used these on high-end passenger cars,” Tigelaar said. Sensor cost is less than a MAF and more reliable over the the sensors' lifetime, he added. Thus, deriving MAF from a speed and pressure-sensor combination might mean more accurate control of turbos that also is more cost-effective.