Silicon Nitride Turbocharger Rotor for High Performance Automotive Engines 900656
Toyota Motor Corporation has mass-produced turbochargers with silicon nitride ceramic rotors since October, 1989. Those turbochargers have been introduced into Celica and MR-2 which are Toyota sporty-type passenger cars. The designing of ceramic rotor was carried out in order to ensure the strength and durability of the component as well as to obtain the same aerodynamic characteristics as in the metal rotor. A moment of inertia was reduced by 60% using ceramic rotor which improved turbocharger response. The ceramic rotor was joined to metal shaft by new method which compensated problems in both shrink fitting and active brazing methods. High temperature strength of silicon nitride material was improved by controlling the amount of sintering additives and sintering conditions. The ceramic injection moulding was employed to mass-produce rotors with complicated shape, applying optimun binder compositions and moulding conditions. The stresses applied to ceramic rotor during rotation were simulated using finite element analysis (FEA) in combination with fracture speeds in a spin testing. The actual fracture moment was photographed with high-speed camera to detect the regions where the failure initiated. Various durabilities and resistances for foreign objects damage (FOD) of ceramic rotor were evaluated using a burner type test facility which was able to introduce foreign objects into ceramic rotor. Those testings proved that ceramic rotor showed enough strength and reliability under various conditions in engine operation. The turbocharger with ceramic rotor reduced response time in boost pressure by 30%, resulting in an improvement of engine performances on acceleration response and maximum power out put (165kW).
SINTERED SILICON NITRIDE is suitable structural material for a turbocharger rotor due to its light weight and good mechanical strength at high temperature. When a metal rotor is substituted by sintered silicon nitride, more than about 50% reduction of inertia moment can be possible, resulting in a considerable improvement of engine response. Ceramic materials have, however, disadvantages of low fracture toughness and strong dependence of strength on various micro-defects in sintered bodies. Therefore, both desigh and processing methods must be developed to apply ceramic material to turbocharger rotor. The designing of ceramic rotor must be carried out so that ceramic rotor has the same aerodynamic characteristics as in metal rotor and enough strength and reliability.
As for processing of ceramic rotor, the joint should have sufficient strength and durability under rotor operating conditions. It is also important to reduce cost for joining process. Improvement of high temperature strength of silicon nitride is essential, since high strength is favorable to design ceramic rotor which shape can be similar to that of metal rotor that expects better turbocharger efficiency. Forming of ceramic rotor in mass-production must be controlled in order to reduce the internal defects in complex shaped sintered bodies.
It is important to ensure the strength and reliability of ceramic rotor by several evaluation methods including a simulation and analysis of fracture behavior under more sivere conditions than in usual durability testings for engines or vehicles(1).