An inventor has proposed a modern manifestation of an atkinson cycle engine concept to control engine load without throttling and thus improve efficiency at part load. the concept uses late intake valve closing (ivc) which effectively creates an expansion ratio larger than the effective compression ratio. the engine is 4-stroke, spark-ignited, and operates unthrottled. the engine has a clearance volume of 50 cm3 and a displaced volume of 600 cm3 per cylinder. the fresh working fluid is a lean mixture of methane (ch4 ) and air at an equivalence ratio φ = 0.9. the lower heating value of methane is 50 mj/kg. the cylinder contents behave as an ideal gas with constant specific heats (γ = 1.25, r = 0.300 kj/kg-k). the cycle processes are described below:

intake process (5-6) and constant pressure work process (6-1)

for the first part of the process (5-6), the piston descends from tdc to bdc. for the second part of the process (6-1), the piston ascends half-way into the geometric compression stroke while the intake valve is still open. throughout the intake process 5-6, fresh fuel-air mixture is inducted into the cylinder and mixed with the residual gas, while during 6-1 mixture is displaced from the cylinder; the cylinder pressure remains constant at 100 kpa during the process 5-6-1. when the intake valve is closed (state 1), the temperature of the cylinder contents is 440 k.

compression process 1-2

compression is isentropic. cylinder pressure rises as the piston ascends towards tdc. effective compression