Internationally renowned professor Predrag “Pega” Hrnjak passed away on August 31 at the age of 70. He was a regular speaker at IOR international conferences and recipient of the IOR J&E Hall Gold Medal for Innovation in 2021.

He joined the University of Illinois MechSE department in 1993 (then Mechanical and Industrial Engineering), earning recognition as Stoecker Faculty Fellow and Distinguished Research Professor, and serving as the Director of the Air Conditioning and Refrigeration Center (ACRC), an Industry/University Cooperative Research Center founded by the National Science Foundation.

Pega’s direct impact on the next generation of scholars was truly extraordinary. He mentored nearly 120 students to graduate degrees and welcomed close to 100 visiting scholars to his research team. His students have gone on to leadership positions in industry, national labs and public service, and to careers in academia—all around the world.

Pega was internationally recognized for his technical leadership and his career has spanned a remarkable range of activities and accomplishments. In the early days of the ACRC, while building his own research program, he assumed lead responsibility for design and construction of state-of-the-art laboratory facilities. His academic research ranged from fundamentals of refrigerant-lubricant mixtures and the dynamics of developing two-phase flow, to optimizing design of components and systems around the properties of various refrigerants and their compatible lubricants. During the CFC phaseout, Pega was asked by an international consortium of automakers to lead the first carefully controlled side-by-side comparisons of several new generations of A/C systems using HFC, HFO, hydrocarbon and carbon dioxide refrigerants.  Based on the results of his multi-year project, automakers were able to double the efficiency of their R134a systems by adopting many of the component technologies (e.g. internal heat exchangers; ejectors) developed for the competing systems.

In 2003, he founded Creative Thermal Solutions, and what began as a “garage business” grew under his tireless leadership to the state-of-the-art, 100,000 sq. ft. research facility it is today. Between the ACRC and CTS, Pega managed to incubate a critical mass of enthusiastic researchers, engineers, and support staff of nearly 100 people working simultaneously on the sustainable HVAC&R technologies he was so passionate about. At CTS and Illinois, he was engaged in deeply impactful research on a wide range of natural refrigerant projects, from low-charge ammonia to ejectors for transcritical CO2 to some of the early CO2 mobile air conditioning systems.

Pega’s recognitions span decades, including and Fellow of the American Society of Mechanical Engineers, Fellow of the American Society of Air Conditioning and Refrigeration Engineers, Fellow of the Society of Automotive Engineers, the 2008 IEA Ritter von Rittinger Award, the 2011 IIR Gustav Lorentzen Medal, the 2012 Institute of Refrigeration J&E Hall Medal, and the 2019 ASHRAE Louise and Bill Holladay Distinguished Fellow Award. In 2012, he was named Academician of the Academy of Engineering Sciences of Serbia.

Pega’s boundless energy was directed toward helping others reach their full potential, professionally and personally, and his impact was profound. He will be deeply missed.