Effect of the Ratio Between Connecting-rod Length and Crank Radius on Thermal Efficiency

In reciprocating internal combustion engines, Otto cycle indicates the best thermal efficiency under the same compression ratio among ideal cycles. To achieve this, combustion must take place instantaneously at top dead center, but it is actually impossible. Meanwhile, if a slower piston motion around top dead center was allowed, both the in-cylinder pressure and degree of constant volume would increase, leading to higher thermal efficiency. In order to verify this idea, an engine with a slow piston motion by adopting a large ratio between the connecting-rod length and the crank radius was tested. As expected, while degree of constant volume was increased, thermal efficiency was not improved due to increased heat loss. Further experiments were carried out using a direct injection stratified charge combustion system which allows selective reduction of heat loss, and high thermal efficiency was attained. On the contrary, an engine with a faster piston motion by adopting a smaller ratio between the connecting-rod length and the crank radius attained high thermal efficiency under the quick burn pre-mixed spark ignition combustion.