In the interest of improving the efficiency of the internal combustion engine, many alternatives to the traditional 4-stroke engine have been proposed. One promising alternative is the 5-stroke engine, which adds an additional expansion stroke to the traditional 4-stroke engine. By adding an additional expansion stroke, the combustion gas can be further expanded with increased work output for the same heat input. The extra expansion stroke, i.e. the 5th stroke, is accomplished in a separate cylinder operating on a 2 stroke cycle. This can be readily accomplished by adding one expansion cylinder with two combustion cylinders operating 360 degree off-phase, creating an engine with 3 cylinders. Previous works have investigated the interactions of the engines operating points and design parameters. Few have, however, included in-cylinder heat transfer and mass lost due to blow-by, which can have a significant effect on the thermodynamic performance of such an engine. In this paper, the air-standard cycle analysis is performed for a 5-stroke engine to obtain the indicated thermal efficiency and power output over a range of operating points and design characteristics, including engine RPM, compression ratio, overall expansion ratio, expansion cylinder clearance volume, and transfer port volume. The results are compared with those of a baseline 4 stroke engine. This analysis is accomplished by an air-standard thermodynamic model for both engines with heat release function with heat transfer and mass loss for both the combustion cylinder and the expansion cylinder. The results indicate increased thermal efficiency and power output over the baseline 4 stroke engine, depending on the engine RPM and overall expansion ratios.