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Propylene glycol (PG) based engine coolants have been tested in both heavy-duty and automotive service. Results of comparison tests between PG base engine coolants and traditional ethylene glycol (EG) base coolants are presented in this summary paper. The detailed testing program included laboratory testing for corrosion protection, freeze protection and heat transfer performance, engine testing for corrosion protection, low temperature performance and heat transfer, and extensive fleet testing in both automotive and heavy-duty applications. The testing has proven that PG is an excellent base fluid for modern engine coolants, providing the necessary heat transfer properties, freeze protection, boil-over protection and, when adequately inhibited, corrosion protection.

Propylene glycol (PG) has been demonstrated to be a viable alternative to ethylene glycol (EG) as a base fluid in engine coolants using a variety of laboratory tests designed to simulate on-the-road vehicle conditions. However, actual vehicle testing has been primarily limited to heavy duty diesel applications and the specialized protocol required for high temperature desert evaluations. A light-duty propylene glycol coolant has been evaluated in a 1 year, 48,280 kilometer (30,000 mi.) test using a 15 vehicle fleet composed of compact, mid-size, and sport-utility vehicles. Mileage was accumulated at an average rate of 177 kilometers (110 mi.) per day using a combination of highway, suburban, and city driving routes. Corrosion inhibitor depletion rates and metal coupon weight loss data are presented as are photographs of cooling system components removed from the vehicles at the conclusion of the test.

Propylene glycol (PG) is gaining acceptance as a viable base fluid in engine coolant applications. PG formulations are currently available which rival the performance of those based on ethylene glycol (EG) and have the advantage of being safer products. However, concerns have been raised that because of its higher viscosity, PG might impair vehicle operation in cold weather. This paper examines the low temperature performance of PG base coolants as compared to those based on EG using a combination of laboratory bench scale and actual vehicle testing. No measurable differences were observed between the coolants at temperatures as low as -40°C.