Ceramic Valve Analysis, Reliability and Test Results

An evaluation of engine valves made of solid ceramic materials for use in an uncooled DI diesel engine was conducted. Two candidate ceramic materials were chosen: yttria partially stabilized zirconia (PSZ) and sintered silicon nitride (Si₃N₄). The evaluation was undertaken in three phases. First, a finite element analysis (FEA) was conducted for both steady state and transient conditions. Then the finite element analysis results were used in a Weibull ceramic failure theory analysis which yielded predictions of the probability of failure of the valves. These predictions indicated that under severe analytically imposed cooldown conditions, the PSZ valve would have a probability of failure of 100%, whereas the Si₃N₄ valve experiencing the same conditions would fail less than two times per ten million. The final phase of the evaluation was conducted when valves were fabricated from silicon nitride and tested in an engine. The testing was conducted in a single cylinder uncooled DI diesel engine. Engine operation consisted of steady state part load and full load conditions, and severe transient warmup and cooldown conditions. After 26 hours of testing, the Si₃N₄ valves underwent fluorescent penetrant inspection which indicated no signs of failure.