(2) Restraining of Rise in Impeller Temperature
a) Compressor Pressure ratio and Impeller Temperature
Generally, the turbocharger compressor impeller is made of aluminum alloy featuring a high resistance to heat. Aluminum alloy, though given the aging treatment at about 200゜C for more than 10 hours in the manufacturing process for refining and strengthening, undergoes creep deformation when subjected to high stresses at elevated temperatures of about 150゜C and above. With the compressor pressure ratio at the level high enough for the aforementioned engine scavenging pressure of 3.7 to 3.9 bars, the compressed air discharged from the compressor impeller would exceed 200゜C in temperature. When exposed to such a high temperature for a prolonged period of time during operation, the impeller would suffer the decline in its strength.
The mechanical properties of aluminum alloy as the impeller material, therefore, impose limits on the maximum pressure ratio and operating time that the turbocharger can continuously offer.
b) Cooling of Compressor Impeller
The alternative to the aluminum-alloy compressor impeller, which cannot be expected to last long in the continuous operation at high pressure ratio and high suction air temperature, is the impeller in titanium alloy. Being light weight and high strength, the titanium-alloy compressor impeller can be incorporated into the turbocharger of ex-
