Comparison of Bench Test Methods to Evaluate Heavy Duty Coolant Thermal Stability

The past 10 to 15 years have seen a dramatic change in heavy duty coolants and cooling system maintenance practices. Controversy exists about the relative merits of newer organic acid (OAT) coolants and more conventional products, especially in the area of thermal stability. Coolant life has been extended from two years/240 K miles to at least five years/600 K miles. It is not uncommon for the same charge of coolant to remain in the cooling system until engine rebuild. Further, there has been an equally significant increase in the coolant service intervals. Reinhibition of the coolant was once tied to the oil change interval at 15 to 25 K miles. Now this additive addition has been extended in many cases to one year/150 K miles to two years/300 K miles. • Along with these dramatic increases in coolant life and service interval, strategies to reduce exhaust emissions such as EGR have increased and will continue to increase coolant temperatures. Sorting out coolant stability issues in the field is both expensive and time consuming. Further, it is very difficult to control a field test so as to obtain reliable data. In this environment, a bench test method that can quickly simulate high temperature, severe field service conditions is of vital importance. This paper compares four bench test methods as far as their ability to sort out thermal stability issues based on results from five coolants representing different additive packages and glycol qualities.