In reciprocating internal combustion engines, Otto cycle indicates the best thermal efficiency under the same compression ratio. To achieve this, combustion must take place instantaneously at top dead conter, but it is actually impossible. Meanwhile, if 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 slower piston motion by ideal constant volume comcustion was tested. It was revealed that the thermal efficiency could not be improved nevertheless an increase in degree of constant volume and lower pumping loss. Numerical analysis deduced that increased heat loss cancelled out the effect of the higher degree of constant volume and that faster piston motion around top dead center rather achieves an improvement of thermal efficiency in a case where rapid combustion was realized.