VALIDATION OF AN EXPERIMENTAL METHOD FOR PEAK TEMPERATURES EVALUATION ON A RR MERLIN XX HEAD

Aluminum alloy properties are hugely influenced by temperatures and stress gradients. In piston engines, temperatures vary slowly when compared to operating pressure and stress. Therefore, averaging the temperature values is a valid assumption. This paper compares the experimental head temperatures of a Rolls-Royce-Merlin-XX-head with the ones of a NACA paper on a very similar Merlin-Packard-V-1650-7. This experimental method in based on residual hardness measurement on a head of a RR Merlin that crashed in Italy during WWII. The first part is to define the “working life” in hours of the engine. A few samples from a “cool” part of the head give the initial “thermally intact” hardness. Then the hardness of a few samples from a known temperature part is measured. In our case, it is the part of the head that is directly exposed to coolant. The time interval that gives a residual hardness equal to the one of the part of the head that is directly exposed to coolant gives the engine working life. For this purpose, a set of “cool part” samples are kept in at the maximum constant temperature of the coolant (135 degrees C). Then, a few specimens are kept at higher temperatures and are extracted from the oven at regular time intervals. The residual hardness is measured on these specimens. A further set of specimens is subjected to random thermal cycling to verify that the alloy hardness reduction is influenced by the time at high temperature and not by the thermal cycle history. In this way, a correlation between residual hardness and temperature is obtained for the specific engine alloy. It is then possible to measure the residual hardness of various points of the head and to obtain the maximum temperature reached in a specific point. In general, experimental tests have confirmed the cost-effectiveness of this approach. The NACA TM 2069 data and the ones measured with this method show an extremely good correlation. It is then possible to affirm that, also for the alloy used for the Merlin head, a modified Hiduminium RR50, this method of test is valid. Other positive tests were performed in the past with the much more common AlSi9 alloy used for the head of FIAT-1900jtd-8V automotive engine. The method of the residual hardness is old and has met several critics in the scientific community. This paper demonstrates that, for at least a few aluminum alloys, it is still valid. It is a very inexpensive method to evaluate temperatures in new engines using disposed units with a known load history and time. The results are precious to verify that the simulation results used in the head design led to reasonable results. In this way, the development time of new engines can be significantly reduced.