Abstract
Keywords:
Knocking, Thermal efficiency, Heat loss, Piston-crank mechanism, Compression ratio, SI engine
Abstract
A new combustion method of high compression ratio SI engine was studied and proposed in order to achieve higher thermal efficiency, comparable to that of CI engine. Compression ratio of SI engine is generally restricted by the knocking phenomena. A combustion chamber profile and a cranking mechanism were studied to avoid knocking with high compression ratio. Because reducing the end-gas temperature will suppress knocking, a combustion chamber was considered to have a wide surface at the end-gas region. However, wide surface will lead to large heat loss, which may cancel the gain of higher compression ratio operation. Thereby, a special cranking mechanism was adapted which allowed the piston to move rapidly near TDC. Numerical simulations were performed to optimize the cranking mechanism for achieving higher thermal efficiency. An elliptic gear system and a leaf-shape gear system were employed in numerical simulations. Livengood-Wu integral, widely used to judge knocking occurrence, was calculated to verify the effect for the new concept. As a result, this concept can be operated at compression ratio of 14 using a regular gasoline. A new single cylinder engine with compression ratio of 12 and TGV (Tumble Generation Valve) to enhance the turbulence and combustion speed was designed and built for proving its performance. The test results verified the predictions. Thermal efficiency was improve over 10% with compression ratio of 12 compared to an original engine with compression ration of 10 when strong turbulence was generated using TGV, leading to a fast combustion speed and reduced heat loss.