The selection and testing of compression seal and chamber coating materials for the pivotal engine™.
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
Wear and friction tests were conducted on a rotational tribometer to identify material couples for compression seals and chamber coatings to extend the service life of the Pivotal Engine™. Potential materials were identified based on reports in the literature of successful use in similar environments. From the rotational tribometer tests, the best material couple was found to be reaction bonded silicon nitride against a Sulzer Metco F4301 plasma sprayed coating followed by a Total Seal TiN coated seal against the F4301 coating. However, the performance predicted by the rotational tribometer was not realised in a fired engine, where the TiN coated seal wore almost as fast as an uncoated seal. This discrepancy was due to the large differences in operating conditions between the rotational tribometer and the fired engine, particularly the reciprocating motion of the engine. The different operating conditions mean that the results from the rotational tribometer have little or no relevance actual performance in the engine. To overcome the limitations of the rotational tribometer a reciprocating tribometer was designed and built. The performance predicted by the reciprocating tribometer was much closer to the observed wear rates from the engine, particularly with the Total Seal TiN coated seals. Some of the results from the reciprocating tribometer were a direct-contradiction to those obtained on the rotational tribometer. When tested on the rotational tribometer the nitrided stainless steel seal wear rate was lower than that of the Mazda cast iron seal. However, on the reciprocating tribometer the Mazda cast iron seal had a lower wear rate than the nitrided stainless steel seal. From the testing conducted on the reciprocating tribometer the best material couple was found to be Total Seal TiN coated seals on the F4301 chamber coating. However, this material couple cannot be recommended to increase the seal life in the Pivotal Engine™ because at the conclusion of a twenty hour test the Total Seal TiN coating had worn through to the substrate. In addition to testing different materials, different oils were tested while keeping the wear couple constant. The oil that offered the best wear protection to both the seal and chamber coating was Castrol A747. These tests showed that different oils have as much effect as different materials.
Even with the best material combinations (Total Seal TiN seal, F4301 chamber coating) the seal wear rates in the Pivotal Engine™ are much higher than would be expected in other engines. This leads to the hypothesis that the origin of high seal wear in the Pivotal Engine™ is not due to material properties, but to design issues. The most likely problem is distortion of the chamber, which causes high localised seal loadings, and in turn causing high seal wear.