|Institution:||Washington State University|
|Keywords:||Mechanical engineering; Compliant Combustion Chamber; Four Stroke Resonant Operation; Lumped-Parameter Model; Micro and Mesoscale; Small Scale Internal Combustion Engine; Variable Compression Ratio|
|Full text PDF:||http://hdl.handle.net/2376/5166|
In this study, work on developing a small scale internal combustion engine operating on a four-stroke principle is conducted. To mitigate the parasitic losses, a complaint engine concept is proposed in which the piston-cylinder assembly is replaced by a flexible cavity. This approach offers various advantages, such as mitigating friction and blow-by losses, fuel-flexibility, and a simpler design. The study encompasses mathematical modeling, feasibility analysis, and testing of a prototype engine. A physics-based lumped-parameter model is developed to predict the engine performance for a range of equivalence ratios at various load conditions in both open and closed cycle operations. The open and closed cycle results showed similarities. For instance, both the cycles showed that for a fixed load, increasing the heat input results in increase of stroke length and efficiency. However, the predictions from the cycles differed numerically in terms of efficiency, operating frequency etc. For example, the indicated thermal efficiencies obtained from open and closed cycles were 38.4% and 47.6%, respectively. It has been realized that when modeling the compliant engine in a more realistic way, an open cycle would be more appropriate. The open cycle results highlight some unique features of the complaint engine. For instance, the engine volume excursions are found to be dependent on the heat input or equivalence ratio and load condition. To ascertain the feasibility of a practical engine, super compliant structures fabricated from metals and, or composites that can sustain extreme pressures and temperatures are evaluated. In addition, a prototype engine is realized to practically demonstrate the compliant engine concept. The engine is motored to estimate the parasitic losses at resonant operation, and the results are presented in the form of an energy flow diagram. The results showed that friction losses associated with the flexible cavity are less than that in a piston-cylinder assembly. An example simulation based on open cycle with practical engine parameters showed that the ratio of friction work to indicated work is about 8%; less than targeted 10%. This result reinforces that a compliant resonant engine has the potential to outperform the contemporary engines at small scale.